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Appropedia:Slow Sand Filter staging area
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- 1 Licensing
- 2 Progress
- 3 Content
- 4 Usage
- water test results
- Water Color Improvement
- Water Clarity
- Slow Sand Filter FAQ
- Float Valve Installed
- First Flush Diverter
- Illustrations and Drawings
- Sand Sizes
- Magnified Sand Size Examples
- Literature Cited
- Slow Sand Filter Blog
- Contact the Author
| Introduction -
The focus of this website is the summary of an ongoing study of the design, operation, and construction of small scale biological water filters using sand and gravel as the filter media. Chemicals used for drinking water treatment such as chlorine, or ozone that produce toxic cancer causing byproducts are not used in these filters. Before any water from a non-public utilty monitored slow sand filter is used for drinking on a regular basis, it is HIGHLY recommended that a UV filter be installed permanently in the water line to the point of use. Also, keep in mind that "Backwashing" a biological sand filter will destroy it.-
The content of this site will change considerably as the study continues. -
* NEW * GOOD NEWS FOR RESIDENTS OF WASHINGTON STATE:note 2 As of October 9 2009, catching roof water is no longer a crime! The department of Ecology in Washington state (the D.O.E.) has finally clarified the law so as to allow individuals to catch rainwater for their own use. (from this preceeding link, click on the links at the D.O.E. site on the upper right hand side of their page that say "new" highlighted in yellow.) -
* NEW * Tests to determine total petroleum hydrocarbon pollutants in roof water from a composition roof are now available 2010-01-05: Water from the composition roof, diverter, and filter 1 system has gone in for testing and we have the results. The tests have shown that Total Petroleum Hydrocarbons (heavy oils and Diesel together) are significantly reduced by the slow sand filter down to less than 2 parts per ten million. Read the details below: See a YouTube video of the roof water filter here
So far work has been done on 7 different filter configurations. Three separate small slow sand filters are involved:-
* NEW * HYDROCARBONS (TPH) IN ROOFWATER ARE SIGNIFICANTLY REDUCED BY A SLOW SAND FILTER SYSTEM The most recent tests on fiter 1 show that a slow sand filter system of this design is capable of significantly reducing TPH (Total Petroleum Hydrocarbon) contamination in the water that passes through it. In this case the pre-filter system contamination is 2.9 milligrams per liter in the winter from water directly off of the roof. When we are talking about drinking water, this is an unacceptable level for use as a drinking water source without proper filtering. The slow sand filter system described here, which is in use with filter 1, has been in operation for 2 years without changing or cleaning the filter media. This system reduces these petroleum hydrocarbons from the roofing material (heavy oils), and (possibly) from air pollution (Diesel), down to less than one tenth of a milligram per 1 liter of water each (2 tenths of a miligram total); and this is when the filter is LEAST effective at 32 degrees F. This exceeds theMTCA Method A cleanup level for TPH in ground water in Washington state 54 (page 4) by a factor of 10. The tests also show that petroleum hydrocarbons are removed by the first flush diverter to a level of less than 1 part per million (this exceeds the MTCA Method A requirment for groundwater which is 1mg/L). Also we know from the tests that over 40 percent of the hydrocarbons are Diesel. We do not know exactly how much hydrocarbon comes from local air pollution, which is significant in this area. The test for how much is due to the roofing material and how much is due to air pollution will have to wait. Funding is limited, and these tests are extremely expensive. The US EPA considers petroleum hydrocarbons (oil and grease) one of 46 other "non-priority pollutants" 55. There are not any "official" specific regulations for levels of TPH contamination in water set by the EPA. One of the reasons is that there are hundreds of compounds in petroleum and it is extremely difficult to identify each one and the effects each has on living organisms . We do know that there are between 5 and 10 million metric tons of oil entering the marine environment every year (as of 1986), and that oil spills are lethal to most forms of life. In spite of that, the EPA has yet to establish strict guidlines for TPH in water; and the studies they have done fail to show toxcity to humans. However, concentrations of some individual elements in TPH pollution above .001 mg/L have been shown to be harmful to aquatic life. 56 (p 203-206), 57 (p 220)
In addition to removing TPH contamination, Filter 1 removes coliform bacteria from 30,000 cfu per 100 ml down to 10 cfu per 100 ml (log credit of 3.477).This exceeds the Washington state Class AA (extraordinary) surface water requirements for the absence of coliform by a factor of 5, and this is when it is operating at minimum efficiency in 32 degree (Fahrenheit) weather.
1. The first filter (filter 1 2007-08-13) configuration was put together with very casual attention to detail and operated with only "human" power (10 gallons of water was added manually each day). This was done deliberately to verify the validity of this question: Can a working biological sand filter be put together with mostly recycled material at very low cost (under 25 dollars) using construction grade sand and average skills? The answer - partially. The test results showed no fecal coliform bacteria from stagnant roof water runoff but some coliform present. This water would be ok for everything except consumption and better than water straight from a "rain barrell".
2. The second configuration of filter 1 2007-10-09 was the addition of graded sand (.15 mm effective size) to the top layer of this filter and ungraded "fine" sand to the bottom layer. These were the only changes to the filter detail. This filter removed 99.9 percent of all coliform bacteria (including ecoli and fecal coliform) and removed more of the color from the roof water runoff - still not ok for drinking on a regular basis but 99.9 percent better than the rain barrel water and much safer. With only close attention to the detail of the size of the top layer of sand, the filter is now functional.
3.The third configuration of filter 1 2008-02-12 was the addition of a first flush diverter to the system. This allowed for significant improvement of turbidity (as determined by visual inspection) and some reduction of the bacteria (as determined by laboratory tests) before the water was put into the filter. Still the same result - removal of 99.9 percent of all coliform bacteria.
4. The fourth configuration of filter 1 2008-02-12 involved the addtion of a small dc powered pump to recirculate the water through the filter. This was an attempt to keep the biofilm alive without having to manually add water each day. The dc pump is operated with a very small amount of power and can be run by a solar panel. This worked. The filter still removed 99.9 percent of all coliform. As of 2008/12/01, this configuration of Filter 1 has been in continuous operation for 14 months without disturbing the biolayer (schmutzdecke). The flow rate is 3 liters per hour as of 2008/12/01. The reason for this long undisturbed run is to determine if a slow sand filter becomes more efficent at removing contaminants as it ages. The compromise would be in flow rate: significantly reduced flow rate to obtain significantly increased water quality. This filter uses water from a composition roof as its source. This water should contain unacceptalble amounts of hydrocarbons from the roof surface. The question is: will the aged slow sand filter remove enough chemicals and pathogens to produce potable water? Update 12/18/2008: Two samples of water were taken from this filtering system (the fourth configuration of filter 1) just before the freezing weather hit: Filtered water and unfiltered water. The filtered water had been through the slow sand filter and the unfiltered water was taken from the storage container that provides the raw water source for the filter.(filter 1) The samples were put into two seperate containers which were identitical and were cleaned and sterilized. These containers were put in a building where the temperature was 32 degrees F. The filtered water froze and the unfiltered water did not freeze. It is a known scientific fact that water containing dissolved substances freezes at a lower temperature than water with less dissolved substances. (this is why salt is put on roads to "thaw" out the ice - the salt slowly lowers the freezing point of the ice and causes it to "melt".) Since the filtered water froze and the unfiltered water did not freeze (the surrounding temperature was not low enough to freeze the unfiltered water - it's freezing point was lower than the filtered water - for some unknown reason), did the filter remove dissolved chemicals? A test will go in as soon as money, time and weather permit.
5. Filter 2 2008-05-24 is 12 inches deeper than filter 1 with more graded sand (.15mm effective size), and the water input is from a shallow well. This filter uses the dc pump modification and is fully functional - it removes 99.9 percent of all coliform bacteria and the water has no roof chemicals in it. This is potable water with one (albeit only slight) caveat: chemicals or pollutants present in the sand will be in the output water. This condition can be checked with a thorough water quality test, if no poisons are found in the water then it is potable and probably the best water around.
6.Filter 3 2008-10-09 is now in the process of becoming fully operational. This filter contains all NSF/ANSI 61 - AWWA 100 approved (the two standards for potable water) sand. This means we know there are not poison chemicals in the sand. The top layer of sand is .25 mm effective size and the bottom layer is .35 effective size, however these sands have known uniformity coefficients of less than 2. The container and all the pipe are also ok for potable water. A test will be done on the water in about 3 weeks. (3 weeks from Sept 19 2008). As of Sept 23 the flow rate is 38 liters per hour (approximately 10 gallons per hour.) 2008/11/07 Update: An excessive amount of alge has formed on top of the sand in this filter. This has drastically slowed down the flow rate - from 10 gallons per hour to 3.6 gallons per hour. It only took 4 weeks for this alge to build up. The cleaing process, however is very simple. This is a description of what was done ( it is called "wet harrowing"): I removed the top of the filter and GENTLY ran my hand over the top 1/4 to 1/2 cm of sand until all the surface of the sand had been agitated. The water became cloudy with fragments of alge. This part of the water was drained off. There was an almost immediate change in the flow rate, although I would recommend plugging the output while the filter is being cleaned. I installed a drain pipe on the top of the filter so water can be run through the input baffle pipes while the excess cloudy water is drained off. When the water draining off became reasonalbly clear, the top cleaning drainpipe was closed and the filter was allowed to resume operation. The flow rate measured 48 hours after this procedure was 28.965 litres per hour (7.65 gallons per hour). The biolayer must have time to re-grow however, so the water will not be safe for several weeks.
7.2009/02/17: Added a float valve to filter 3.
2008/11/10 Update: The test that went in 2008/10/29 has come back on filter 3. All coliform bacteria are removed by the filter.
2008/12/18 Update: Due to extremely cold weather (for this location) all the filters have completely frozen and stopped operation. The temperature has been below freezing since 2008/12/12
2008/12/27 Update: New information from the results of research regarding the viability of composition roofing material for water collection are posted on the FAQ page.
2009/01/16 Update: The filters are now running again. The below freezing weather lasted until the first few days in January 2009. In this location, temperatures were below 20 degrees for nearly one week, and dropped to below 15 degrees F for several days. There was 16 inches of snow on the ground. All the filters were inactive and frozen for 3 weeks. Pipes were cracked and brass valves were destroyed. All the pipes were wrapped with insulation, but that did not help because the water inside the filters froze. In climates where below freezing temperatures occur, these filters MUST be kept from freezing.
2009/01/26 Update: The filters have frozen again. There was an inch and a half of snow last night. Below freezing weather finally froze all the filters as of today. Operation has been sporadic for the past nine days - night temps have been near or below 32 deg F. .
2009/02/08 Update: Both filter 2 and filter 3 were damaged because of the cold weather. They have been disassembled checked and put back together. This meant carefully removing all the sand and gravel from each filter cleaning the container, replacing cracked drain pipes and putting the sand back in. Each filter contains approximately 900 lbs of sand and gravel. This is VERY labor intensive work. The re-assembled filters are now running. Filter 2 has been running since Feb 3 2009 and has a maximum flow rate of 72 litres per hour (19 gallons) with 20 cm (7.5 inches) of water covering the surface of the sand. Filter 3 has a maximum flow rate of 90 litres per hour (23 gallons) with 16.5 cm (6.5 in.) of water covering the surface of the sand and has been restarted again on Feb 13 2009.
2009/03/11 Update: All the filters except filter 3 with the float valve have stopped operation, are frozen and have been for 3 days ( this is the fourth day ). Record low temperatures here were below 19 degrees F. and there has been a total of 5 inches of snow at this location in the past week.
2009/03/13 Update: All the filters are now back in operation.
2009/04/07 Update: Filter 3 flow rate: .75 litres per minute (45 litres per hour) with 16.5 cm water depth over the top of the sand. (11.89 gallons per hour) Filter 2 flow rate: .71 litres per minute (42 litres per hour) with 16.5 cm water depth over the top of the sand. (11.25 gallons per hour)
2009/12/08 Update: All filters are frozen. The temperature has been below freezing for five days, and below 18 degrees F the past two nights.
2009/12/22 Update: All filters are now in operation.
2009/12/27 Update Temperatures the past 4 days have been below freezing at night (25 degrees F), however the water in the filters has not frozen.
Clean water is necessary for life, without it people cannot live. As the population of any given area increases the demand for clean water increases. At the same time more water becomes unsuitable for human consumption. Aquatic life, including many species of fish that support human life also suffer. To prevent human suffering and conflict, clean water must be freely available to all people. In most areas of the world today clean water is not freely available to anyone. In fact, in most parts of the world clean water is avaialble only for those who can afford it. Polluted water is the only option for billions of people. This must change. Clean water must be freely and unconditionally available to all people. If it is not, there will be dire conseqences that will make our current warfare over petroleum resources seem insignificant. The solution to water availability lies not in expensive corporate distribution systems, but in individual or small scale water filtration systems, and the knowledge of how to build and safely maintain these environmentally sustainable systems to provide uncontaminated water.
note 1: Water can be contaminated by many different things: inorganic chemicals, organic chemicals and compounds; and many different types of viruses and bacterium and it is impossible to test for all of these contaminants without access to expenisve elaborate testing facilities. This website and this study must not be interpreted as a cure-all for the ills of water supply problems. The water that goes through these filters is NOT being consumed, and they are NOT part of the plumbing in the dwelling unit nearby; and furthermore, consuming water that goes through any filter is NOT RECOMMENDED without full knowledge of the source of the water and EXACTLY what may or may not be in the source water; and a complete EPA approved test on the water before and after any filtering system used to determine the safety of the water. That said, it must be recognized that even public water supply systems are subject to contamination, 22 40,41 although these systems are continuously monitored for harmful substances by highly qualified technicians. Biological sand (slow sand) filtering is not used in most public water supply systems. Slow sand filters are very efficent at purifying water (they actually remove pathogens) and they are the best and most efficent way of removing cryptosporidum (Beaver Fever) cysts from water without adding chemical poisons. Most public water supply systems in the U.S. do NOT use slow sand filters, they use Chorine or Ozone combined with highly complex mechanical filtration systems to remove particulate matter and inactivate and / or remove pathogens. However, chlorine and Ozone react with organic matter in water and produce cancer causing substances: trihalomethanes (from chlorine), halacetic acid (also known as Acetic acid, dichloro; bichloracetic acid; DCA; dichlorethanoic acid; 2,2-dichloroacetic acid; dichloroethanoic acid; kyselina dichloroctova or Urner's liquid) (also from chlorine) 51 and formaldehyde and ketones (from ozone)23, 24 that are still in the water (at low concentrations) when it arrives at the consumer's plumbing and are VERY harmful if consumed over long periods of time even at low concentrations. Bromide (present in ground water) and chlorine added to kill bacteria plus sunlight equal bromate - a cancer causing chemical. This situation occured in 2007 in Los Angeles. The Sliver Lake and Elysian Resovoirs had to be drained, refilled and black plastic balls put on top of the water to keep sunlight out.42
note 2 Why is this "good" news? I wasn't going to get political here, but this is so important and teaching is, after all, a political act (Paulo Freire) . . . . Four reasons stand out. One; enough water for survival should be free for everyone. It is as necessaary for life as air - that one is sort of a no brainer. Two; if enough people in the city catch rainwater in rain barrels, the load on the storm drains is reduced considerably - and there is less chance of the drains overflowing and spreading contaminated water into the environment before it is treated. This is good news for everyone, as less pollution is washed into streams, rivers, lakes, and oceans during a significant rain event. Three, in the summer, water can then be conserved by using the stored water for flower gardens, lawns, and if filtered by a slow sand filter, vegetable gardens; thereby significantly reducing the load on the public water supply. Four (and probably the most important) it appears as though government is finally getting a clue. . . hopefully I'm not wrong about this. Good job Wa. state gov., thank you! Tax dollars well spent this time!
And, speaking of political stuff, who is funding this site, and why is it on the internet? There are no multinational corporations, environmental groups, universities, or political groups involved in the funding of this site. The funding is private, unconditional, and limited to maintaining the domain name and hosting, and paying for the epa certified tests; and purchasing parts used to build the filters. The website coding, physical work and research are done without any monetary compensation. Every effort is made to assure that unbiased results based on factual evidence and professional testing are presented. Hazards are noted. No guarentees are offered or implied. This information is here because knowledge of how to obtain clean water, which is necessary for life, should be available to all persons. If any outside funding, or donations are offered, the conditions will not change - the funding must be unconditional.
IMPORTANT: Most racoons carry a type of parasitic roundworm called Baylisascaris procyonis 43, 44, 45, 46, that causes very serious illness in people. If there are racoons in your area be aware that your yard may be contaminated. Children are highly at risk. DO NOT use water from a rain barrel, pond or creek unless you are absolutely certain that it is not contaminated. Read the literature cited from the link above (43, 44, ,45) on this issue completely. You have been warned.
Note: In the past seven or eight months websites describing biological sand filters / slow sand filters have been appearing with instructions recommending BACKWASHING a biological sand filter when the flow rate slows down. Forcefully backwashing a slow sand filter, particularly a small slow sand filter with layers of different sizes of sand, will destroy it. The flow rate is slowed by a buildup of material on the TOP FEW CENTIMETERS of sand. Simply gently agitating this layer and DRAINING OFF the cloudy water is all that needs to be done. These well meaning people have apparently confused rapid sand filtration with biological sand filtration. There are small scale biological sand filters that have been carefully engineered to allow reverse flow of water to clean the filter, but the sand bed is not "fluidized" as it is in the process of "backwashing". 52
| Page design by Perpetual PC's
water test results
Test results from the new filter May / June / October 2008:
This is the first test in May. The filter has not fully developed the biological layer.
This is the second test received on June 3, 2008. The new filter is now working.
This is the most recent test on the new filter
This is the first test on the new filter design with AWWA / NSF approved sand
Below are the test results from the original filter:
Water Sample test results post-filter August 8 2007. BEFORE the .15mm sand was added. Not using a first-flush diverter on the roof water source.
Water sample tests pre-filter and post-filter Oct. 18 2007. AFTER the .15mm sand was added. Not using a first flush diverter on the roof water source.
Water sample tests pre-filter and post-filter for Feb 2008. AFTER the .15mm sand was added. Using a first-flush diverter on the roof water source, and the recirculating pump system wherein raw water is added once a day in amounts less than 5 gallons, with as many as 3 consecutive days where no raw water is added and temperatures are at or below freezing (32 degrees F)
Below are the most recent tests on the "old filter" also called filter 1. These are the tests that show how much petroleum pollution comes from this compostion roof and how much is removed.
So far, what conclusions can we draw from all of the test results?
Most importanly please note that only E Coli, Fecal coliform, coliform bacteria, and petroleum hydrocarbons have been shown to be removed by these filters in this situation. ANYTHING ELSE could be present in the water and the only way to determine what is present would be to do a very expensive test for all possible contaminants. Water from different areas will contain different substances. Also, only filter 1 has been sufficiently challenged by large numbers of microbes, and successfully removed them. Although there is a preponderance of research documenting capabilites of pathogen removal far beyond what has been shown here, one cannot safely make the assumption that these filters will work well beyond the parameters shown here. You have been advised.
- The tests show all coliform bacteria are removed: E Coli, Fecal coliform, and coliform in the filter design with .15mm sand added.
- The addition of the first flush diverter and/or the cold weather, or a combination of cold weather, more water, and the diverter have (apparently?) reduced (but not eliminated) the amount of coliform bacteria in the water taken from this roof.
- The biological activity of the filter does not completely stop in weather at near freezing temperatures; and after several days of inactivity while frozen, the functionality of this filter, in this situation, was not adversely effected.
- A first flush diverter will not safely remove all bacteria from water taken from this roof.
- Near freezing temperatures will not eliminate all coliform bacteria from the water taken from this roof.
- It is not safe to consume unfiltered/unpurified water taken from this roof even though there is a diverter in the system.
- From a bacteriological standpoint, the water from this slow sand filter is safe to consume in summer temperatures, fall temperatures and winter temperatures in this area - providing the filter is properly operated and maintained, and not completely frozen.
- We do not know the nature of all chemical pollution of the water from this filter.
- With respect to chemical pollution other than petroleum hydrocarbons, more filtering/testing would be necessary to confirm that the water from this filter / supply system is safe to consume over long periods of time.
- 0We regard to health, it is prudent to err on the side of caution and assume that water from a roof is not safe to drink without filtering and testing.
- We do not know from the tests what viruses are present or not present in the post-filter water.
- Since coliform bacteria are removed, viruses called "Phages" or "Bacteriophages" that occupy coliform may also removed; but we do not know for sure.
- Since the test for pathogens did not specifically include tests for all viruses; if the water is to be used on a regular basis for drinking, the use of a uv filter is highly recommended.
- Coarse sand is not as effective as fine sand in the removal of bacteria.
- Some of the pre-filter samples do not contain large enough amounts of coliform bacteria to aggressively challenge filter 2 and filter 3.
- A slow sand filter will remove petroleum hydrocarbon pollutants from roof water runoff taken from the roof in ths study.
- A first flush diverter will also remove petroleum hydrocarbon pollutants from the same water.
- This slow sand filter still works in freezing temperatures (0 deg C).
- The system in this study will improve the quality of roof water from this roof to exceed class AA standards for coliform bacteria presence by a factor of 5.
- The numbers of coliform in roof water from the roof in this test can vary considerably, from none present, to thousands present.
- The slow sand filter does not accumulate and pass on to the output water, the hydrocarbons that are present in the input water that has been flowing through it daily for 2 years.
- .e level of hydrocarbons (TPH - Total Petroleum Hydrocarbons) present in the filter output water exceeds any standards now set for TPH contamination in ground water.
NOTE 1: WARNING! Racoons carry parasites (Baylisascaris procyonis) often called racoon roundworms, that are usually harmless to the racoons. These roundworms each produce millions of eggs which can persist in the environment for years and can cause serious illness in people. Chlorine, alcohol or ozone will not kill them.43, 44, 45, 46 Rooftops may be contaminated by these parasites' eggs and rainwater may pick them up and deposit them in rainbarrels.Even though a functioning slow sand filter should remove these eggs, (they are larger than 50 microns) this study has not tested for the removal of these parasites' eggs.
Water Color Improvement
Illustrations of how water color can be improved by a slow sand filter and a first flush diverter
| The filter that this water has passed through has been in operation for 1 year (as of 2008-12-01) without deliberately disturbing the biolayer (schmutzdecke) for cleaning purposes, with the exception of disruption from freezing in December 2008 and January 2009. During the sub-freezing temps, which lasted through December and January, the biolayer was compeletly frozen. This filter also has a charcoal filter hooked to the output. -
These pictures are of water that is run through the filter on an ongoing basis during the rainy season (fall, winter, spring). Simply dumping a bucket of cloudy water into a filter and watching clear water come out is not the same thing. It takes at least several hours for the water that goes in to a well designed slow sand filter to get filtered and show up at the output. The first flush diverter removes concentrated chemical pollutants from the water so the slow sand filter can work on the biological contamination. See the larger versions of these images below.-
The picture above (picture zero - roof water) is water from a composition roof that has NOT been through a first flush diverter. The bucket is a 5 gallon bucket.
The picture above (picture one - pre-filter water) is water from a composition roof that has been through a first flush diverter. The bucket is a 5 gallon bucket.
This is water from the same composition roof as the pre-filter water in picture one, that has been through the same first flush diverter mentioned in the picture one caption after it has been through a small slow sand water filter.The bucket is the same 5 gallon bucket as in picture one.
llustrations of how water clarity can be improved by a slow sand filter
What are these pictures? In pictures A and B We are looking to see if the slow sand filters being tested become more efficent over extended periods of time, and if there is any improvement between filtered and unfiltered water. This test DOES NOT show pathogens in the water. Water can be very clear and still contain harmful bacteria. Because the water being filtered here by the slow sand filter has a very low turbidity (but still contains pathogens before filtering) we are using a small low power red laser beam to check water clarity. The laser passes through the water and a photo is taken of the light reflecting from particles in the water, producing what looks like a red "line".
A reliable epa approved commercial 10 stage water filter is used as a basis for comparison as shown in pictures C and D. Water before it passes through this 10 stage filter is known to have more particles in it than it does after it has passed through the filter. The pictures show a difference; and thus we know this is a reliable method of checking the quality of water with respect to particulate matter content.
It is necessary to let the bubbles in the water disperse before checking, or the light will be reflected from bubbles and particulate matter. The intensity of the color of the line is directly proportional to the concentration of particles in the water - in other words more particulate matter in the water will produce a more intense colored line and will indicate poorer water quality. This is a qualitative test - not quantitative. This is NOT a scientific "turbidity test" and no "numbers" are produced. This test demonstrates improvements in clarity resulting from more efficient removal of particulate matter from water by the ( slow sand ) filters being tested.
Slow Sand Filter FAQ
Frequently asked questions regarding slow sand filters (note 1) (note 2-F) Note: In the past seven or eight months websites describing biological sand filters / slow sand filters have been appearing with instructions recommending BACKWASHING a biological sand filter when the flow rate slows down. FORCEFULLY BACKWASHING A BIOLOGICAL SLOW SAND FILTER WILL DESTROY IT. These well meaning people have confused rapid sand filtration with biological sand filtration , and backwashing with reverse flow. In a biological sand filter, the flow rate is slowed by a buildup of material on the TOP FEW CENTIMETERS of sand. Simply gently agitating this layer and DRAINING OFF the cloudy water will restore the flow rate. In some filter designs, water is allowed to flow "backwards" (at nearly same rate as it flows in normal operation ) through a biological sand filter without "fluidizing" the sand bed, but this is not the same thing as backwashing. Backwashing fluidizes the sand bed, reverse flow does not. 52 (p 5-11).
- What is a slow sand filter?
- How does a slow sand filter work?
- Are slow sand filters safe?
- What are the disadvantages of a small slow sand filter?
- What are the advantages of a small slow sand filter?
- How do I keep air pockets from forming when buillding a small slow sand filter?
- How do I tell if the water is too muddy or silty to be filtered by a slow sand filter?
- What is an NTU?
- How much do small slow sand filters cost?
- What kind of sand and gravel do slow sand filters use?
- What size of sand and gravel do slow sand filters use?
- How deep does the sand have to be in a slow sand filter?
- What is "Uniformity coefficient"?
- What does "log credit" mean in reference to pathogen removal in a slow sand filter?
- Where can I find more information about slow sand filters?
- Can roof water be made potable (suitable for people to drink) by a slow sand filter?
- What is a first flush diverter?
- Can I purify water from a creek, stream or pond using a slow sand filter?
- What is "wet harrowing"?
- Will a slow sand filter remove industrial pollution?
- What is the difference between a biosand filter and a slow sand filter?
- Can I purify roof water with a slow sand filter?
- Where can I get plans for a slow sand filter?
- Can I safely water my vegetable garden with roof water that has been purified by a slow sand filter?
- Can I use a slow sand filter to purify tap water?
- I have seen videos of slow sand filters in operation that show very dirty water being poured in and clean looking water flowing out. Is this realistic?
- How is a slow sand filter "cleaned"?
- What is the difference between rapid sand filtration and slow sand filtration?
- What is COD (Chemical Oxygen Demand)?
- Will a slow sand filter remove poisons in water from toxic algae Blooms?
- How fast does water move through a slow sand filter?
1. What is a slow sand filter? A slow sand filter is sometimes referred to as a "Biosand" filter, or a biological sand filter. All three of these names refer to a water filter that works using biological action in sand without adding any chemicals to the water. These filters are good examples of sustainable technology. They can operate without the use of electricty or petroleum based fuel and can be made from mostly recycled materials. A slow sand filter consists of a container with a system of pipes with holes drilled in them covered by about 6 inches of gravel, in turn covered by 3 feet of sand. Water is allowed to flow over the top of the sand and flow slowly down (because of the pull of gravity) through the sand and gravel to the pipes on the bottom. The water then flows back up (due to hydraulic pressure) through one output pipe to the level of the input water. After about 3 or 4 weeks a biological layer forms on the sand that traps and destroys harmful bacteria and viruses. back to top
2. How does a slow sand filter work? When a slow sand filter is first put into operation or after it is "cleaned", a living "community" of aquatic aerobic, predatory microscopic organisms grow in the top 5 to 10 cm of wet sand and form what is called a "Schmutzdecke" or "biolayer" made of exocelluar polymers (complex proteins and carbohydrates) and living organisms consisting of diatoms, algae, bacteria, and zooplankton. This sand and biolayer must always be under oxygen rich water, and it is very effective at mechanically filtering very small particles out of the water flowing through it. Also, the living organisms in the biolayer literally "eat" pathogens in the water that get caught in the biolayer from a process know as "biological flocclation" (they stick to the biofilm). Some filtering also occurs because of the physical action of the sand below the biolayer. And additionally, the organisms in the biolayer produce substances that are toxic to pathogenic viruses and bacteria found in the water flowing through the filter. 58 (270-273; 424-432), 59, 60 Water must not flow through the filter faster than the biological action occurs in the Schmutzdecke. The organisms in the biolayer are also present at lower levels in the filter sand but they are not as numerous and don't form a biofilm. In small versions of slow sand filters the drainpipes at the bottom connect to a (usually pvc) pipe that runs out and up to an outlet several inches above the top of the sand. This way water drains slowly and never leaves the surface of the sand exposed to open air (this will kill the biolayer very quickly). The action of water seeking its own level is a key part of the filtering operation as it helps to regulate the speed and pressure at which the water passes through the sand. Although slow sand filters have been in operation since the early 1800's and have been studied extensively by modern scientists, the complete extent of the biological activity enabling their operation is not fully understood yet; however extensive tests have shown that slow sand filters remove viruses, bacteria, and chemicals as well as, and in some cases better than, modern complex filtering systems (slow sand filters are the most effective way to remove "Beaver fever organisms" from water). back to top
3. Are slow sand filters safe? They can be quite safe but should be used with caution and sufficient knowledge of their operation. The effectiveness of a slow sand filter depends on the conditions in which they operate; and on the degree of pollution in the water to be filtered. Water must be allowed to run through them for at least 3 weeks before any is consumed. All parts of the filter must be kept clean - contamination can come from anywhere outside the filter. Water must be kept over the sand always, and the filter must be allowed to run again for 3 weeks after cleaning before consuming any water from them unless "wet harrowing" is used. Slow sand filters are able to provide very good safe water free from biological pathogens. This is a well established, although not well known, fact. Beaver fever (cryptosporidium) cysts, viruses, and coliform bacteria can all be removed from water by slow sand filters at rates often exceeding 99 percent13 30. It is common knowledge that because of world wide industrialization and lack of attention to the problems of pollution, both "tap" water and rainwater can contain varying amounts of harmful substances, depending on the water treatment process and geographical location. Biological sand filters can remove some harmful substances from water, and most certainly improve the quality of water that is run through them in all respects, but the exact nature of their ability to remove all industrial pollution from water has not been well established. However, ability of carbon filters to remove chemicals from water has been well established; and the addition of a high quality carbon filter to the output of a biological sand filter would be a good idea. Do not consume water from a DYI (do it yourself) slow sand filter unless the water has been tested and shown to be of acceptable biological and chemical quality; and it is monitored and tested regularly by someone who understands its operation. Also a uv filter on the output after the carbon filter should be used. back to top
4. What are the disadvantages of a small slow sand filter? They do not remove 100 percent of the color or odor from water. They require reatively non-turbid water to function properly.
They cannot function if they are frozen. Water must be added regularly and the sand must stay covered by water continuously. During construction care must be taken to prevent air pockets from forming in the sand - this will foul the filter and require removing the sand and removing all anaerobic bacteria and odors. Industrial pollution is only partially removed from water by these filters. These filters must be allowed to "ripen" (have water flow through them for at least 3 weeks) before they will purify water. They are VERY heavy (1000 pounds plus) and much thought must go into where they will be located - it is almost impossible to move them once they are set up. back to top
5. What are the advantages of a small slow sand filter? They are the best way to remove beaver fever bacteria from water.
They can remove over 99 percent of harmful bacteria from water.
They can remove from 91 to 99.999 percent of viruses from water.30 They improve water clarity. They do not require the addition of chemicals to function. They can be designed to function using only the power of gravity.
They can last for many years with only occasionally adding more sand to the top layer. They are inexpensive. They do not pollute. They are sustainable technology.
They do not produce harmful byproducts as do chlorine and ozone purification processes. Maintenance is simple and easily understood. Their construction is easily understood. They can be built from locally available materials. They can operate in remote areas where no electrical power or petroleum energy is available. They empower the individual. The theory of operation is easily understood. back to top
6. How do I keep air pockets from forming when buillding a small slow sand filter? Fill the container with water FIRST - then add the gravel and sand slowly waiting for all the bubbles to stop before putting in more sand. Have all the drain pipe and the ouput (outlet) pipe installed and tested to be sure it works and does not leak. Your last step should be adding the gravel and sand. back to top
7. How do I tell if the water is too muddy or silty to be filtered by a slow sand filter? This is not an easy question to answer because there are some variables: How long has the filter has been running? How long will cloudy water be used in the filter? What is the turbidity level in NTU's (Nephelometric Turbidity Units) of the water, and does it change? Generally, if you can't see through a drinking glass of the water well enough to read your name printed 1 cm high on a white note card it is too silty to use in a slow sand filter. Water that is almost clear will probably work. Muddy water - that is water that is opaque - will not work in a slow sand filter. The accepted maximum turbidity level in NTU's for continuous operation is 20 - this would be water in a glass that looks slightly cloudy - almost clear. Less than 5 NTU's of turbidity cannot be noticed, and 200 NTU's of turbidity is the maximum that can be tolerated by a slow sand filter - this would be water that looks like a glass of non-filtered fresh squeezed lemon juice (without the seeds). The ideal turbidity would be less than 10.12 A good example of what turbidity levels look like can be found here. back to top
8. What is an NTU? This is a measurement of the ability of light to pass through water. A detailed explanation is beyond the scope of this FAQ, but a good explanation of NTU's and how to calculate them can be found here. back to top
9. How much do small slow sand filters cost? Very little or nothing. A small slow sand filter can be built for under 100 dollars. back to top
10. What kind of sand and gravel do slow sand filters use? It is best to use sand and gravel that is NSF/ANSI 61 or AWWA 100 approved. If this is not available, know EXACLTY where the sand and gravel come from and EXACTLY what is in it - and what is NOT in it. Be certain there are no industrial chemicals in the sand and /or gravel. Things like lead, mercury, asbestos, zinc, petroleum derivitives, insecticides, pesticides, or storm runoff residue from road surfaces may be in sand, or on gravel. Be certain the sand is washed clean of any organic material, clay, mud, or slit. The sand should be washed until the water draining from it is clear. Play sand that has been sterilized can be purchased at most home improvement stores. It will work if the sand is fairly uniform in size. Builder's sand may work, but if the grains are too coarse and the size is not uniform (see faq 11) it may not work satisfactorily; also be certain chemicals have not been added if builder's sand is used. The following website has listings of NSF certified sand suppliers and processing facilities: http://www.nsf.org/Certified/Common/Company.asp?Standard=061 Two of the companies listed at this nsf.org site have supplied the sand used in the filters described in the study described on this website They are: MANUFACTURERS MINERAL COMPANY in Washington state and Cemex in Californa. They are wholesale. You will need to find a hardware store, or building supplier near you to order products from them. back to top
11. What size of sand and gravel do slow sand filters use? The most important information to know is that coarse sand does not filter as well as fine sand; and, fine sand offers more resistance to the flow of water than coarse sand. Between .35 mm and .15 mm effective size with a uniformity coefficient of less than 2 is the desired (and most commonly used) range of sand sizes. 13 however all the sand in a layer should be the same effective size. The smaller effective size should be in the top 30 or 40 cm layer of sand. The gravel on the bottom should be large enough to not pass through the holes in the drain pipes and small enough to prevent sand from seeping into them. The drain pipes should be covered by at least 6 inches of gravel. Generally, the smaller the effective size of the sand used the better. If the effective size is too small however, water will not flow through the filter fast enough. If the effective size is too large the biolayer will not form effectively and the filter will not purify water. back to top
12. How deep does the sand have to be in a slow sand filter? The minimum depth of sand is 30 inches and some municipal filters are 48 inches or deeper. If the sand is coarse (.35 mm effective size or larger) increase the depth accordingly. There is a balance to be maintained. Most slow sand filters have layers of sand with the smaller effective size on the top 30 or 40 cm layer. back to top
13. What is "Uniformity coefficient"? The uniformity coefficient of sand is defined as a ratio: the size at which 60 percent (by weight) of a sand sample passes through a sieve (in other words 60 percent of the sand is finer than a given size) divided by the size at which 10 percent of the same sample (by weight) passes through a sieve (10 percent is finer than a given size). A UC of 1 indicates all the particles are the same size. As the number goes up the size differentiation becomes greater and the quality of the sand becomes less desirable for use in a slow sand filter. back to top
14. What does "log credit" mean in reference to pathogen removal in a slow sand filter? "Log credit" means: The log10 (the log relative to base 10) of the number of organisms per unit volume before filtering minus the log10 of the number of organisms per unit volume after filtering. For example 1000 beaver fever cysts before filtering per 100 ml of water as compared to 1 beaver fever cyst per 100 ml of water after filtering is the log10 of 1000 minus the log10 of 1 or 3 - 0 or 3. A very good filter. The corresponding percentage would be 999/1000 or 99.9 percent removal. back to top
15. Where can I find more information about slow sand filters? There is a website here that has much more info about these filters. Also, the "Literature Cited" link on the upper left side of this page has links to other sites and some scientific studies on biological sand filters. back to top
16. Can roof water be made potable (suitable for people to drink) by a slow sand filter? 4,14,15,16,17,18,19,20,21 Under some conditions - possibly - but EXERCISE EXTREME CAUTION. If there are racoons present, keep in mind that they carry roundworms (Baylisascaris procyonis) that produce millions of tiny eggs which are deadly if ingested by humans.43, 44, 45 Although infection is rare, it has happened and there is no cure. The eggs are 50 microns in size, so most commercial water filters will remove them easily. A properly operating slow sand filter should be able to remove them. The best action is to consider water from a roof off limits until the entire area is decontaminated and the racoons are permentantly removed from the area.
Before doing anything more with water from a roof surface, have the surface material tested for asbestos. Roofing applied before 1980 may have asbestos embeded in it. Do not drink or even use water taken from this type of roof - asbestos is a known cancer causing material. If the roofing has asbestos in it DO NOT USE THE RUNOFF FROM IT FOR ANYTHING, and note there is likely asbestos in the soil around the house if there are any areas of damage on the roof. Composition roofing installed by reputable contractors after 1980 will probalbly not contain asbestos, as it was banned from use after 1980 in the U.S. .
Do not drink water taken from cedar shake roofs under any circumstances - there are naturally occurring tannins in cedar which can be harmful18. In addition to tannins, manufactured cedar shakes contain added wood preservatives, (some of which contain arsenic, copper, and other powerful poisons) that are extremely harmful19,20,21,31. Also any roof surface that has lead, copper, tin, or zinc is not suitable for potable water, although the slow sand filter will remove some of these chemical contaminants and nearly all of the biological contaminants from the water.
Use a first flush diverter (see FAQ 17 below).
Have an EPA certified laboratory test the water for zinc, lead, copper, chromium, arsenic, asbestos, and tin.
Metal roofing with the brand name "Galvalume" or "Zincalume" is used in many areas to provide chemical free water for harvesting and consumption. These metals are alloys covered by non-toxic baked on enamel finishes. If they are not coated or if they are allowed to rust, they can contribute toxic metal compounds to runoff water.31. Also there is a roof coating available called CIMtm Industrial Membrane that is NSF 61 certified available from SEALPRO, Inc.; 150 Dow Street, Manchester, NH 03101 1-800-732-5776 .
New information as of 2010/01/05: Some composition and tar roofing (also referred to as asphalt roofing) may be ok for water harvesting. There are, however, a number of caveats that must be considered:
First, total Petroleum hydrocarbons (TPH) were found to be present at 2.9 mg/L of water, in the winter at temperatures of 32 degrees F in tests on water from the roof used in the study on this website. The slow sand filter (filter 1) in the study on this website removes those petroleum hydrocarbons from roof water down to less than one part per ten million - an amount probably harmless to healthy individuals. The TPH working group has done extensive research on Total Petroleum Hydrocarbons. The reference listed here has extensive information on this subject. Mentioned in this study is as much as 7 mg/L as toxic and as little as .01 mg/L as toxic in specific types of hydrocarbons.53 (pp 30,34,45,66,69) Since the slow sand filter takes the total to below the .01 mg/L, we can probalby safely use the water in this case. It must be noted, however that this test was done in the winter months when there is likely to be much less hydrocarbon pollution present than there would be in the summer.
Secondly, asphalt is soluble in water above 20 degrees centigrade (68 degrees F), so if your roof surface is above 68 degrees F when water is collected from it, err on the side of caution and assume the water collected at this temperature contains hydrocarbons from the roofing material. Below 20 degrees centigrade there will not likely be enough dissolved hydrocarbon compounds to be of concern (asphalt has been used to seal reservoirs used for drinking water 17 (p.518)).
Additionally, some composition roofing has chemicals embedded in it, and roofing manufactured before 1980 may contain asbestos. Chemicals, such as zinc or copper are there to prevent the growth of moss. This type of roofing will not be suitable for any kind of water collection and should be removed before it poisons the entire yard. If there is moss growing on your roof, that is a good sign. If your roofing was manufactured after 1980 it probably does not contain asbestos.
Furthermore, in many areas the dust from the air pollution may pose more of a threat than the chemicals from the roofing material - as the sun heats up the roof, hydrocarbons become soluble in water and mix with the dust to form (possibly) even more toxic pollutants.
Another consideration is the coloring in the aggregate covering composition roofing. It can contain heavy metals (which produce the color) such as chromium, copper, or lead. Avoid green, or white colors. Brown colored roofing usually has iron which may not be as harmful as other metals.
Also, the majority of water soluable pollutants in the runoff from most composition roofing come from the pieces of roofing that break loose due to normal aging. These pieces are usually washed off the roof during a moderate rainfall event; however they may remain in the gutter, so they must be removed before using any of the water that runs through the gutter. 15,16,17
Another issue that must be considered is the diversity of complex hydrocarbons, sulfur compounds, and nitrogen compounds present in asphalt. The exact nature of the combination of these compounds depends on the source of the asphalt; so different roofing manufacturers' products will not contain the same Asphalt contaminants.17 (p. 518) The study quoted on this website on roofing materials pertains only to most roofing currently sold on the west coast of the U.S. .
A first flush diverter with a very large disgard amount such as 25 or 30 gallons per downspout (assuming at least 4 downspouts) per rain event on a 2000 square foot roof would be advisable. After the high volume disgard first flush diverter is installed, at the very least, have the water tested for Polycyclic Aromatic Hydrocarbons, (PAH - Polycyclic Aromatic Hydrocarbons) and / or Hydrocarbons (TPH - Total Petroleum Hydrocarbons) (along with the standard EPA water quality tests which should include tests for copper, lead, zinc, iron, phosphates, nitrates, nitrogen compounds, and chemical oxygen demand) and do not use water from a composition, tar or asphalt roof if the temperature of the roof is above 68 degrees F. 4,14,15,16,17,31 For example; during the summer months the water from the roof in this study is of very poor quality. However, water collected during the rainy season in the fall, winter and spring is of very high quality . back to top
17. What is a first flush diverter? This is an absolutley necessary part of a roofwater harvesting system and will keep concentrated toxic chemicals out of the slow sand filter. A first flush diverter (sometimes called a "roof washer") is a device that manages the flow of rainwater into a harvesting system. It diverts the first flow of runoff from a roof, after a dry spell, away from a roof water harvesting system. There are many different designs, but the idea is the same - prevent water that contains dissolved pollutants such as: dust, vehicle exaust particles, animal droppings, and particulate matter due to poor air quality from entering the water storage system. The runoff from the first rain after several months of a dry spell will contain concentrated highly toxic materials including harmful bacteria. A diverter usually has a "temporary storage chamber" that fills with the most polluted water and then seals itself off to allow the fresh rainwater to flow into the storage system while slowly emptying this "temporary storage chamber" so it will be ready for the next rain event. back to top
18. Can I purify water from a creek, stream or pond using a slow sand filter? Yes, but again use extreme CAUTION: Know the exact origin and path of the water in the creek or pond and always ALWAYS have the water tested before consuming any. Also, be aware that many racoons carry roundworms (Baylisascaris procyonis) that produce millions of tiny eggs which are deadly if ingested by humans.43, 44, 45 (see faq 16) These eggs can survive for years in harsh environments. Although infection is rare, it has happened and there is no cure. The eggs are approximately 50 microns in size, so most commercial water filters will remove them easily. A properly operating slow sand filter should be able to remove them. Also, urban and suburban areas have many chemical pollutants that flow into creeks and ponds; and many of these substances can be very harmful. If you are in a rural setting and you are sure that the creek or pond is not anywhere near populated areas your worst problem will probably be biological contamination, and the slow sand filter will be able to purify the water. A slow sand filter can remove 99.9 percent of harmful organisms from water. back to top
19. What is "wet harrowing"? This is one way to "clean" a slow sand filter. The surface of the sand is gently agitated to stir up the biolayer. The cloudy water is drained off and the filter is then allowed to run for several days to re-establish the biolayer. back to top
20. Will a slow sand filter remove industrial pollution? A slow sand filter will remove at least half and up to 90 percent of some chemical pollution at levels up to 100 times greater than recommended maximum allowable levels 12. A properly functioning slow sand filter is capable of remvoing Diesel contaminants present in water from less than 1 mg/L down to less than .07 mg/L, and heavy oil contaminants from less than 1 mg/L down to less than .095 mg/L. Tests for the presence of chemical pollutants are very expensive. back to top
21. What is the difference between a biosand filter and a slow sand filter? Nomenclature - mainly. The name "Biosand Filter" refers to an invention by Dr. David Manz. Typically a biosand filter is smaller than a slow sand filter and may operate intermittantly. A slow sand filter usually operates continuously and can be quite large. Slow sand filters are typically used in municipal water supply systems, whereas biosand filters are usually for individual use. The biological action is similar in both types of filters. A slow sand filter can be large supplying thousands of gallons per day, or small supplying 5 or 6 gallons per day. back to top
22. Can I purify roof water with a slow sand filter? A slow sand filter can purify water harvested from a roof, but the water should not be consumed unless the roof is made of metal with a non-toxic baked on enamel finish; and a first flush diverter is used. In some cases a composition roof may be able to provide water suitalble for comsumption - see faq 16. back to top
23. Where can I get plans for a slow sand filter? Search on the internet under "slow sand water filter" or "Biosand Water filter" or look at the drawing on this website. Also read about their theory of operation - it is not that complicated and does not take a college degree to understand how to put one together and operate it successfully. back to top
24. Can I safely water my vegetable garden with roof water that has been purified by a slow sand filter? Probably, depending on what type of roof you have (read faq number 16 above) and what wildlife is present in your area. As for chemicals that leach from a roof surface, the key concept here is "bioaccumulation". Plants that are regrown each year from seed won't accumulate much poison, unless you have an unusually polluted environment. Fruit trees or perennials like berries may accumulate poisons over time. Use a first flush diverter and have a metal roof with a non-toxic coating to get the purest water. Any other type of roof (excluding possibly some types of tile roofs) will add toxins in proportion to their age and manufacturing. It is also important to note that many carcinogens can be present in extremely small concentrations and only a lab can detect them - and as they build up over time (months or years) in tissue they become quite toxic. The toxins in roofing are varied and expensive to identify. There are coatings available that will vastly improve the quality of water taken from a roof (see number FAQ 16 above). Also, if you have a "carport" or seperate garage, just install suitlable metal roofing on that building and your filtered water will be ok to water a garden. If you intend to have a new roof put on your house have a non-toxic metal roof installed - they will last 50 years or longer, stand up way better in a storm than composition or cedar shakes, will not poison your yard, will give you non-toxic rain water, can be recycled, they are fireproof, and can be installed over old composition roofing. From a biological standpoint, more caution is advised. Racoons carry parasites (Baylisascaris procyonis - sometimes called racoon roundworms) that are usually harmless to the racoons. These roundworms each produce millions of eggs which can persist in the environment for years and can be deadly to humans. Rooftops are often contaminated by these parasites' eggs if racoons are present. Water may pick up these microscopic eggs and deposit them in rainbarrels. Ingestion of the eggs of Baylisascaris procyonis will cause severe illness and there is no cure. Young children are particularly susceptable. Although this problem is rare, the consequences warrant extreme caution. If you have ANY racoons anywhere near your roof you must assume your roof is contaminated. This study has not tested for the removal of these parasites' eggs and although the eggs are 50 microns in diameter,45 a slow sand filter may not remove them unless it is operating correctly. Watering a vegetable garden with unfiltered water from a rainbarrel that is contaminated with the eggs may result in infection, severe disability or fatality. DO NOT WATER VEGETABLES WITH WATER STRIAGHT FROM A RAIN BARREL - NO MATTER WHAT KIND OF ROOF YOU HAVE. 43, 44, 46 back to top
25. Can I use a slow sand filter to purify tap water? Maybe. It will depend on how much bacteria is in the tap water, and how much chlorine is present in the tap water. If your water comes from a public water supply system, it is unlikely that there will be enough living "good" organisms to start a biological layer in a sand filter ( that is assuming your public water supply is completely filtered and purified ). There must be living microbes in the water to form the biological layer. The physical action of the sand will do some filtering, but a 10 stage carbon filter will be much better at removing harmful chemicals from your tap water. If, however, there are even a small number of living microbes in your tap water, it is likely that a functional bioligical layer will eventually form; but it may take several months for the layer to mature. If you are on a non-public water system ( a private well ) or you get your water from an open stream or irrigation ditch, then a slow sand filter will probably work. back to top
26. I have seen videos of slow sand filters in operation that show very dirty water being poured in and clean looking water flowing out. Is this realistic? Yes, but with several caveats. Too much turbid water in will clog the fiter very quickly. A slow sand filter gets better at filtering as the biofilm (schmutzdecke) matures and will do a very good job of filtering out more than just pathogens , however at the expense of flow rate. Water typically takes some time (depending on the size of the filter) to pass through a slow sand filter. The water that is poured in is not the same water that immediately flows out. It can take anywhere from minutes (inefficient purification) to hours (much more efficent purification) for water to flow completely through a slow sand filter. Also a small charcoal filter on the output will help eliminate odors and colors from the output water. back to top
27. How is a slow sand filter "cleaned"? Cleaning methods include wet harrowing, (described above in faq 19) or physically removing the top 2 or 3 centimeters of sand (usually done on larger filters with the water level significantly reduced). DO NOT FORCEFULLY "BACKWASH" A SLOW SAND WATER FILTER ( force water to flow backwards - up through the sand from the bottom drain at any pressure - high or low ). There are some filter designs that use a cleaning method similar to backwashing, but the flow does not disrupt the sand layers and the design has been carefully engineered, tested and shown to be functional. Simply hooking up a pressurized water source to the bottom of a biological sand water filter and forcing water backwards through the filter ( as shown on several youtube videos) is not the same thing and will most likely destroy the filter. In a biological sand filter (slow sand water filter), the top few centimeters of sand contain living organisms and a biological mat. The biological mat thickens and small particles are trapped in and on top of this mat. This is what slows down the flow. If you backwash the filter you will mix all the debris throughout the filter as gravity allows the material from the top to settle down throughout the depth of the sand and this will plug the filter for good. Unlike a rapid sand filter, backwashing disrupts and may very well destroy the biological component and the physical structure of the lower layers of a biological sand filter. There is considerable biological activity below the top layer that does not significantly impede the flow of water, yet is part of the biological filtration process.33 (p 17, 93) If you are using a small slow sand filter with different sand sizes in layers, backwashing may mix the sand layers and significanlty change the operating parameters of your filter. It is possible to allow water to flow down through the filter more rapidly by opening a valve at the bottom of the filter (utilizing only the pressure due to gravity) to wash out turbidity without seriously disrupting the filter strata. When this is done care must be taken to be certain the surface of the sand on top is not exposed to air. back to top
28. What is the difference between rapid sand filtration and slow sand filtration (biological sand filtration)? Rapid sand filtration does not intentionally use a biological process, (some beneficial bacteria may grow between backwashing events 33 (p 18)) its design uses strictly physical filtration. Very few disease causing organisms are inactivated. Viruses are not removed. Water, that has had coagulant added, is forced through coarse sand under high pressure resulting from depth of water and gravity or added external pressure. These filters require cleaning every several days, or more often, by "backwashing" - forcing purified water or air through them from the bottom drain pipes in a reverse direction of normal flow. Chemicals such as chlorine or ozone must be added to the water to kill harmful bacteria and viruses. Large volumes of purified water are required for backwashing, and large volumes of toxic sludge are produced as a result of daily cleaning events called "backwashing". Rapid sand filters are not practical as a water purification method for individual use. Rapid sand filtration is not sustainable technology - it requires continuous significant energy and chemical input and is a highly complex man-made technological process which requires constant monitoring. Rapid sand filters can supply very large volumes of water in a timely fashion typically needed by large cities with populations in the millions of people. To discuss this and read more follow this link to the slow sand filter blog
Biological sand filtration takes place through biological action, and some physical action. It is much slower than rapid sand filtration and requires more area and much finer sand in layers of progressively coarser sand towards the bottom of the filter. Layers of "good" organisms form in the sand. The thickest is in the top 2 or 3 cm of sand, however beneficial organisms are present at depths of up to 40 cm.33 (p 22) The organisms in the top 2 or 3 cm form a mat of organic substance called a "schmutzdecke" which physically filters water while the bacteria in this schmutzdecke, along with the other bacteria present at lower depths in the sand, actually consume and / or inactivate up to 99.9999 percent of all pathogens (including viruses) present in the water that passes through the filter. Even though some slow sand filters have been designed to be cleaned using water forced through the top 20 cm of sand, this is NOT backwashing - the bottom layers are left undisturbed. These biological filters will be ruined if they are backwashed. They are referred to as: slow sand fiters, or biosand filters or biological sand filters. These filters are sustainable technology. They require no chemical input and very little energy input and do not require constant monitoring. They are the single most effective filter for removing cryptosporidium (Beaver Fever) cysts from water (rapid sand filtration combined with chemical disinfection is not as effective - it takes dangerously toxic amounts of chlorine or ozone to kill the spores). The material resulting from cleaning is biologically non-toxic and can be used for fertilizer. If wet-harrowing is used, the sand rarely requires replacement.33 (p 17, 93) 25(p4), 26(p103-104), 27(p268-273)
29. What is COD (Chemical Oxygen Demand)? Chemical Oxygen Demand is a measurement of the amount of organic material in water.32
30. Will a slow sand filter remove poisons in water from toxic algae blooms? In some cases, yes; but this is still being studied and there are differing varieties toxic algae found in most parts of the world. There are 7 different toxins associated with toxic algae blooms: Microcystins, Nodularin, Anatoxins, Saxitoxins, Cylindrospermopsins, Lyngbyatoxin a, and Aplysiatoxins. Slow sand filters have been shown to remove (biodegrade) Microcystins. Some algae (cyanobacteria) are only toxic at certian times and others are highly toxic. Only highly skilled persons with access to a laboratory can positively identify toxins, and toxic algae types. DO NOT EVER USE WATER FROM A SOURCE THAT CONTAINS ANY VISIABLE SIGNS OF ANY TYPE OF ALGAE, UNLESS YOU ARE ABSOLUTELY CERTAIN THAT IT IS NOT TOXIC. DO NOT GUESS. YOU HAVE BEEN WARNED. Toxic algae is seldom present in sub-surface fresh water, or in fresh water that is moving.34,35,36,37,38,39
31. How fast does water move through a slow sand filter? A slow sand filter is a dynamic device and will change over time. The amount of time it takes water to flow through a slow sand filter will depend on the size of the sand, the depth of the sand, how long the filter has been in operation after initial start up or after the most recent cleaning, the depth of water above the surface of the sand, the area of the sand surface, and the turbidity of the input water. The filters described in this study have flow volumes that vary from 3 litres per hour ( after 1 year of running without wet harrowing ) to 45 litres per hour ( after 5 months of operation without wet harrowing ). The sand in these filters described here typically holds 50 percent water by volume - approximately 100 litres. So the simple math tells us that it takes anywhere from about 2 hours (almost too fast) to about 30 hours ( more than slow enough ) for the water to flow through the filters described here. This is approximate time. More water on top of the sand will increase the rate of flow and less water will decrease the rate of flow.
The way many studies refer to the flow rate is in meters per hour (m/hr) (1 cubic meter = 1000 litres ) rather than liters per hour (L/hr): (cubic meters of water flow) divided by (unit time in hours or seconds) divided by (square meters of filter bed area) 47 (p 5)
or: m3 / 1hr / m2 or: (doing the math) m / hr
or meters per hour.
Acceptable rates are between .1 and .3 m/hr The above described filter with 45 litres per hour throughput has a "flow rate" of .184 m / hr
An explanation of the mathematics and theory to accurately describe the rate of flow at any given point in a slow sand filter is beyond the scope of this faq. Wikipedia has information on Darcy's law which describes the flow of water through sand and might be a good place to start. Oklahoma state university has more detailed information about Darcy's law Keep in mind the different layers in a slow sand filter have differing resistances to the flow of water.
Think of this: whatever is on your roof will wash off in the rain and onto and into the soil near your house. If your garden is near any downspout and if there is any lead, arsenic, copper, PAH contamination, or chromium on your roof the plants in your garden will be able to take those chemicals into their roots, and dust from the soil will be on the leaves. 48, 49, 50 The more time that passes the more concentrated the chemicals will become in the soil around your house. ALLOWING THE DOWNSPOUTS TO DRAIN INTO THE COUNTY OR CITY WASTEWATER SYSTEM IS ABSOLUTELY UNACCEPTALBE - PERIOD. The combination of a first flush diverter and a slow sand filter will take a considerable amount of that pollution out of the water that runs off of your roof - but not all of it; and some of the chemicals from your roof water runoff may biodegrade. Also, whatever pollution that is in the rain (and it can be a considerable amount depending on where you are located) that falls on your vegetalble garden will accumulate in the soil. In addition to this the dust that builds up on your roof in a dry spell mixed with runoff will be very toxic and extremly concentrated in water from the first rain event following the dry spell. If you use a diverter (which can be built for under 75 dollars), much of this toxic liquid will be held back, and more of the toxic chemicals will be removed by the slow sand filter.
note 2-F: The information used in this FAQ was taken from experience working with small slow sand filters and from information noted by following the Literature cited link at the top left of this page.back to top
Flow Control Float Valve
Slow sand filter float valve flow control device in filter 3 The pvc pipe is 1/2 inch. There are .040 inch holes drilled on the bottom of the pipe to allow the water to disperse without disturbing the sand surface.
The float valve limits the volume of the flow of input water so it matches the volume of the flow of water through the filter. As the filter "ripens" its flow rate will slow down. The float valve will, by its nature of operation, slow the flow of water down equally.
First Flush Diverter
First flush diverter: After a dry spell, this device diverts the first most polluted flow of rainwater from the roof away from the water harvesting system. The recommended minimum rate of diversion is 1 gallon per 100 square feet of roof surface. This diverter uses 2 gallons per 100 square feet.
New information as of 2009-08-10: The first major rainfall in 10 weeks just occurred: .7 inches of rain in 12 hours. After the 55 gallon space in the diverter storage space was filled, the operation was monitored for several hours at one downspout draining a 1000 square foot surface area. It was necessary to quickly drain 20 gallons to allow more washing to occur. The 55 gallon space was NOT sufficient to satisfactorily provide clean water, although the most polluted runoff was kept out of the system. Without the diverter, the runoff would be of limited use and would quickly impede the flow of water through the slow sand filter. The roof surface has been dry for 10 weeks, and additionally, from 2009-07-26 to 2009-07-31 temperatures here (in the shade) were well into the upper 90's and above 100 deg F for 3 consecutive days (all time record high temps for this area) and the roof is a composition roof. The diverter had been totally emptied and cleaned on 2009-08-09 (yesterday) and the gutters and roof surface had been cleared of leaves, small loose roofing particles, small twigs, and other organic substances that had accumulated during the dry spell. At least 75 to 100 gallons of diversion should be available in this situation (extended dry periods during the summer). It should be noted that this location is heavily wooded and there are numerous large 100 year old fir trees and maple trees surrounding the roof area, and additionally the roof has moss covering 30 percent of the area being monitored. Note change to output pipe size as of Nov. 15 2009
| first flush diverter-
First flush diverter drawing-
Note: the output to storage pipe is designed for light to moderate-
rainfall events. In areas where very heavy rainfall occurs, the-
pipe should be at least 1 1/2 inch diameter; see first flush diverter-
photo below. (1 1/2 inch pipe allows 24 liters per minute flow). -
Drawing of float seal device-
First flush diverter with float seal (external link)
2009-08-10 Water samples from the roofwater harvesting system. Each container is 16.5 cm high by 9 cm in diameter and contains 1/2 litre of water. (1 inch = 2.54 cm), (1 gallon = 3.785 litres)
slow sand filter drawing Slow sand filter drawing Note:In addition to smaller gravel (1/4 inch instead of 3/8 inch), a 2 inch layer of coarse sand is recommended immediately on top of the 1/4 inch pea gravel.
New illustration: The addtion of a float valve makes the filter more convenient to operate and does not require electricty or petrol to operate.
drain pipes This is the drainpipe assembly with the caps installed. It is just sitting on top of the container for the sake of taking the picture. The output of the drainpipe assembly is hooked to a "bulkhead fitting" sometimes called a "bulkhead adapter". This keeps the output pipe from leaking where it passes through the filter container.
drain pipes This is how the drain pipes look when they are installed in the container. This container is 40 inches deep.
bucket connection There is a short piece of 1/2 inch pvc that hooks this up to the baffle assembly.
bucket connector This is a close-up of the threaded bushing and threaded male adapter. Only 1 o ring is shown but 2 are needed - one on the inside of the bucket and one on the outside of the bucket.
inside of bucket A 3/4 inch spade bit was used to drill a hole in the bottom of the bucket. The male adapter and threaded bushing were then installed with a number 16 o ring on each side of the bucket. The adapter and bushing are 1/2 inch.
ball valve This ball valve is used to set the flow to the same rate as the output of the filter.
complete filter The container used for the filter came with a top with 1 hole in the center. Another "vent" hole was added on each side to allow air (oxygen) to circulate through to the water in the filter. The "vent" holes are covered by screens. This illustration shows the baffle pipes hooked up through the top. They are just "friction-fit" into the adapter that has been installed in the bottom of the bucket. The small holes in the baffle pipes are drilled with a number 50 drill bit. They allow the water to slowly flow into the filter. This keeps the surface of the sand and the biofilm from being disturbed. wet harrow drain This is the drain that was added to allow the wet-harrowing water to drain off. Note the screen in the vent hole on top of the lid near the bottom of the bucket. The hole for this screen was drilled with a 3/4 inch spade bit. complete filter The completed filter hooked up to the automatic recirculating pump. Please note:This is not a "production" model and is not intended to be aesthetically pleasing - it is strictly funtional. sand washing drain Open this valve to allow water to flow through the filter rapidly to wash the sand. Keep in mind that this valve, if open all the way, will let water flow through the filter too fast for purification. back to faq page
recirculation pump This is the pump used to recirculate the water. The pressure switch was added for depenabiltiy and pressure adjustment flexibilty. The flow is controlled by the ball valve shown above. motor specifications A close up of the motor. Note it is a 12 volt DC motor. It draws 7 amps for about 10 seconds every 2 minutes. This motor would be easily powered by a solar panel/battery combination. DANGER! do not use a 120 volt ac motor in this fashion. This motor is outdoors under cover but still not safe with 120 volt operations (either ac or dc). The surrounding area is usually wet which adds to the conductivity of the "ground" connection which greatly increases the likelyhood of a dangerous shock. Always, ALWAYS, use a fuse of appropriate amperage in your circuit. You have been warned! baffle hook up This is the baffle before cleaning. Note the small piece of 1/2 inch pvc at the center. That is how it connects to the bucket through the top. It just frictions in. It won't matter if it leaks slightly because the water will end up in the filter as it flows through the air vent.
Illustrations of sand sizes
| sand size examples-
Sand size examples larger version (slow download - large file size)
The following website has listings of NSF certified sand suppliers and processing facilities:-
http://www.nsf.org/Certified/Common/Company.asp?Standard=061 Two of the companies listed at this nsf.org site have supplied the sand used in the filters described in the study described on this website They are: MANUFACTURERS MINERAL COMPANY in Washington state and Cemex in Californa. They are wholesale. You will need to find a hardware store, or building supplier near you to order products from them.
Magnified Sand Sizes
Illustrations of sand sizes large view
sand size examples Sand size examples
1. Presence of Noroviruses and Other Enteric Viruses in Sewage and Surface Waters in The Netherlands W. J. Lodder and A. M. de Roda Husman* Microbiological Laboratory for Health Protection, National Institute of Public Health and the Environment, Bilthoven, The Netherlands.
- Corresponding author. Mailing address:
Microbiological Laboratory for Health Protection, National Institute of Public Health and the Environment, P.O. Box 1, NL-3720 BA Bilthoven, The Netherlands. Phone: 31 30 274 4325. Fax: 31 30 274 4434. E-mail:
The following abstracts were accessed: November 23, 2007 and would be very good reading especially for understanding the biological action in a (slow) sand water filter:
3. Biological and Physical Mechanisms in Slow Sand Filtration Haarhoff, J; Cleasby, JL IN: Slow Sand Filtration. American Society of Civil Engineers, New York. 1991. p 19-68, 11 fig, 10 tab, 54 ref.
Slow Sand Filtration: Influences of Selected Process Variables Author(s): Bellamy, William D.; Hendricks, David W.; Logsdon, Gary S. Citation: Journal AWWA, Vol. 77 Iss. 12, December 1985, Page(s) 62-66
Bacterivory by a chrysophyte in slow sand filters Monroe L. Weber-Shirk* and Richard I. Dick School of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853-3501, USA Received 1 September 1997; accepted 1 June 1998. Available online 25 February 2000.
4. Information on roofing material: A review of Methods for the Manufacture of Residential Roofing Materials. Hashem Akbari, Ronnen Levinson, and Paul Berdahl. Heat Island Group Lawrence Berkeley National Laboratory, Berkeley, Ca. 94720. A report prepared for: California Energy Commission PIER Program. June 2003.
5. Read more about viruses here: freedrinkingwater.com/water-education2/87-water-disinfection2.htm
6. Use the information on this page and on this site at your own risk. The Author assumes no responsibility whatsoever for any damages of any kind as a direct or indirect result of the use of any information on this website. The information provided here is free and published with the intent of sharing experience, and is not provided as an absolute solution to anything. This is a work in progress. Mistakes will likely be found. We reseve the right to remove this content or change it at any time we choose. You have been advised.
7.Mechanisms of inactivation of hepatitis A virus in water by chlorine dioxide: Jun Wen Li Corresponding Author Contact Information, E-mail The Corresponding Author, Zhong Tao Xin , Xin Wei Wang , Jin Lai Zheng and Fu Huan Chao Institute of Health and Environmental Medicine of Tianjin, 1 Da Li Road, Tianjin City 300050, People's Republic of China Received 15 April 2003; Revised 7 November 2003; accepted 13 December 2003. Available online 4 March 2004.
8.Information on virus removal: www.moh.govt.nz/moh.nsf/0/5A25BF765B400911CC25708F0002B5A8/$File/07-viruscompliance.doc
9. viruses that are not killed by chlorine in swimming pool water (caliciviruses): www.cumc.columbia.edu/dept/ps/2007/mid/2006/transcript_02_mid37.pdf
10. The Turbidity Tube: Simple and Accurate Measurement of Turbidity in the Field: Written April 2006 for the requirements of CE 5993 Feild Engineering in theDeveloping World and FQ 5770 Community Planning and Analysis. Elizabeth Myre and Ryan Shaw M.S. Candidates. Department of Civil and Environmental Engineering; Master's International Program. Michigan Technological University.
11. SUSTAINABLE DRINKING WATER TREATMENT FOR SMALL COMMUNITIES USING MULTISTAGE SLOW SAND FILTRATION; by Shawn A. Cleary. A thesis presented to the University of Waterloo in fulfillment of the thesis requirement for the degree of Master of Applied Science in Civil Engineering Waterloo, Ontario, Canada, 2005 Â© Shawn A. Cleary 2005. page 31,32. uwspace.uwaterloo.ca/bitstream/10012/926/1/scleary2005.pdf accessed Sept 27 2008.
12.Toxicant and parasite challenge of Manz intermittent slow sand filter G. Palmateer, D. Manz, A. Jurkovic, R. McInnis, S. Unger, K. K. Kwan, B. J. Dutka Environmental Toxicology. Volume 14, Issue 2 , Pages217 - 225. Copyright © 1999 John Wiley and Sons, Inc.
13. Logan, A.J.; Stevik, T.K.; Siegrist, R.L.; RÃ¸nn, R.N. 2001. Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters. Wat. Res. Vol. 35, No. 18, pp.4359 - 4369.
14. Concise International Chemical Assessment Document 59 First draft prepared by Ms Joann A Wess, Dr. Larry D. Olsen, and Dr. Marie Haring Sweeny, National Institute for Occupational Saftey and Health, Cincinnati, Ohio, USA. World Health Organization Geneva, 2004 http://www.who.int/ipcs/publications/cicad/en/CICAD59_AsphaltWebVersion_2004_08_04.pdf
15. The Contribution of particles washed from rooftops to contaminant loading to urban streams. P.C. Van Metre, B.J. Mahler. US Geological Survey, Research and INvestigations, 9802 Exchange Dr., Austin TX. 78754-3898. Chempsphere www.elsevier.com/locate/chemosphere
16. House Roof Runoff: Is It as Clean As We Think? Jennifer Gadd and Paul Kennedy. Kingett Mitchell and Asociates http://www.kma.co.nz/downloads/PDFs/Publications/House%20Roof%20Runoff%20-%20Gadd,%20Kennedy.pdf
17. Determination of Polycyclic Aromatic Compounds in Asphalt and In Correspoinding Leachate Water. A.J. Kriech, J.T. Kurek, L.V. Osborn, H.L. Wissel, B.J. Sweeney. Heritage Research Group, Indianapolis, Indiana, USA. Polycyclic Aromatic Compounds, 22:517-535, 2002.
19.Western Wood Preservers Ltd. 26035 - 31B Avenue, Aldergrove, BC V4W 2Z6 - Telephone: (604) 857-1900 - 856-7779
20.Using CCA Preservative-Treated Lumber in Gardens and Landscaping. Publication Number: 8128 Inventory Type: PDF File Language: English ISBN-13: 978-1-60107-307-5 Copyright Date: 2004 Length: 8 pp. University of California http://anrcatalog.ucdavis.edu/Items/8128.aspx
22.Corso PS, Kramer MH, Blair KA, Addiss DG, Davis JP, Haddix AC. Cost of illness in the 1993 Waterborne Cryptosporidium outbreak, Milwaukee, Wisconsin.
Emerging Infectious Diseases. (serial online) 2003 Apr. Date Cited: 2009-01-03.
Available from: URL: http://www.cdc.gov/ncidod/EID/vol9no4/02-0417.htm
23.The significance of algae as trihalomethane precursors Graham, NJD | Wardlaw, VE | Perry, R | Jiang, Jia-Qian RESERVOIR MANAGEMENT AND WATER SUPPLY--AN INTEGRATED SYSTEM. pp. 83-89. Water Science and Technology (Water Sci. Technol.). Vol. 37, no. 2.
24.Identification of New Drinking Water Disinfection by - Products from Ozone, Chlorine Dioxide, Chloramine, and Chlorine Journal Water, Air, and Soil Pollution Publisher Springer Netherlands ISSN 0049-6979 (Print) 1573-2932 (Online) Issue Volume 123, Numbers 1-4 / October, 2000 DOI 10.1023/A:1005265509813 Pages 95-102 Subject Collection Earth and Environmental Science SpringerLink Date Monday, November 29, 2004 S. D. Richardson1, A. D. Thruston Jr.1, T. V. Caughran1, P. H. Chen1, T. W. Collette1, K. M. Schenck2, B. W. Lykins Jr.2, C. Rav-Acha3 and V. Glezer3 (1) National Exposure Research Laboratory, U.S. Environmental Protection Agency, 960 College Station Rd., Athens, GA 30605, USA (2) National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 26 W. Martin Luther King Dr., Cincinnati, OH 45268, USA (3) Research Laboratory of Water Quality, Israel Ministry of Health, 69 Ben-Zvi St., Tel-Aviv, 61082, Israel http://www.springerlink.com/content/v07603q640222762/
27.Water Technology An Introduction for Environmental Scientists and Engineers. Second edition. N.F. Gray Ph.D., Sc.D. Department of Civil, Structural and Environmental Engineering. Trinity College, University of Dublin. Elsevier Butterworth-Heinemann 2002.
28. Export of atmospheric mercury from Asia.
Daniel Jaffe, Eric Prestbob, Phil Swartzendruber, Peter Weiss-Penzias, Shungo Katoc, Akinori Takamid, Shiro Hatakeyamad, Yoshizumi Kajiic; Interdisciplinary Arts and Sciences, University of Washington-Bothell, Bothell, WA 98011 8246, USA. Atmospheric Environment 39 (2005) 3029-3038. Received 11 October 2004; accepted 2 January 2005. (available online at www.sciencedirect.com)
29. Long-range transport of Siberian biomass burning emissions and impact on surface ozone in western North America. Dan Jaffe,1 Isaac Bertschi,1 Lyatt Jaegle,2 Paul Novelli,3 Jeffrey S. Reid,4 Ã‚Â´ Hiroshi Tanimoto,5 Roxanne Vingarzan,6 and Douglas L. Westphal4. GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L16106, doi:10.1029/2004GL020093, 2004 Received 26 March 2004; revised 12 June 2004; accepted 19 July 2004; published 20 August 2004.
30.Technologies for upgrading Existing or Designing New Drinking Water treatment facilities. EPA/625/4-89/023 March 1990 300048WU.PDF pages 38,39,40,41. virus information is on page 41. accessed March 11 2009. search for "slow sand filters virus removal" at: http://www.epa.gov/nscep/
31. Roofing Materials' Contributions to Storm-Water Runoff Pollution. Shirley E. Clark, P.E. D.WRE, M.ASCE; Kelly A. Steele, A.M.ASCE; Julia Spicher, A.M.ASCE;
Christina Y. S. Siu; Melinda M. Lalor; Robert Pitt, P.E. D.WRE, M.ASCE; and Jason T. Kirby, A.M.ASCE.
DOI: 10.1061/ ASCE 0733-9437 2008 134:5 638
33. Slow Sand Filtration. L. Huisman, Professor of Sanitary Engineering, Department of Civil Engineering, Technological University, Delft, Netherlands. W.E. Wood, F.I.C.E. Formerly Chief, Community Water Supply, World Health Organization , Geneva. World Health Organization, 1974, ISBN 9241540370 www.who.int/water_sanitation_health/publications/ssf/en/index.html accessed 2009-04-03.
34.Environmental Health Assessment Guidelines."Cyanobacteria in Recreational and Drinking Waters". Prepared by: Environmental Health Unit, Queensland Health, GPO Box 48 Brisbane Q 4001 Telephone: +61 7 3234 0938 August 2001 Cyanobacteria in Recreational and Drinking Waters Page 20. Accessed: 2009-05-10.
35. "TOXIC CYANOBACTERIA BLOOMS; A Field/Laboratory Guide". Dr. M. A. Crayton, Biology Department: Pacific Lutheran University Tacoma, Washington 98447. Funded by: Office of Environmental Health Assessments, Washington State Department of Health Olympia, Washington 98504-7846. 1993. Toxic Cyanobacteral Blooms Page 1-19. Accessed 2009-05-10.
36."Removal of microcystins by slow sand filtration" Gesche GrÃ¼tzmacher, Gabriele BÃ¶ttcher, Ingrid Chorus, Hartmut Bartel German Federal Environmental Agency (Umweltbundesamt, UBA), P.O. Box 330022, D-14191 Berlin, Germany. Environmental Toxicology See Also: Environmental Toxicology and Water Quality Volume 17 Issue 4, Pages 386 - 394 Published Online: 25 Jul 2002 Wiley Periodicals, Inc. Funded by: German Ministry for Education and Research (BMBF); Grant Number: 02WT9852/7. Abstract accessed: 2009-05-10.
37. "Degradation of Microcystin-LR through Biological Sand Filters". Lionel Ho, Daniel Hoefel, Christopher P. Saint, and Gayle Newcombe. Pract. Periodical of Haz., Toxic, and Radioactive Waste Mgmt. Volume 11, Issue 3, pp. 191-196 (July 2007) Issue Date: July 2007. Permalink: doi:10.1061/%28ASCE%291090-025X%282007%2911:3%28191
Based on wikipedia:Template:Doi. Abstract Accessed: 2009-05-10.
38.Woods Hole Oceanographic Institution: http://www.whoi.edu/redtide/page.do?pid=15777 Accessed 2009-05-10
http://www.whoi.edu/redtide/page.do?pid=15776 Accessed 2009-05-10
39. Cyanosite: (receives support from the Department of Biological Sciences at Purdue University):
http://www-cyanosite.bio.purdue.edu/cyanotox/cyanotox.html Accessed 2009-05-10.
40. "Cryptosporidiosis: An Outbreak Associated with Drinking Water Despite State-of-the-Art Water Treatment". Susan T. Goldstein, MD; Dennis D. Juranek, DVM, MSc; Otto Ravenholt, MD, MPH; Allen W. Hightower, MS; Debra G. Martin, RN; June L. Mesnik, BA; Sean D. Griffiths, BA; Angela J. Bryant, BS; Rick R. Reich, BA; and Barbara L. Herwaldt, MD, MPH. Annals of Internal Medicine; 1 March 1996 | Volume 124 Issue 5 | Pages 459-468
http://www.annals.org/cgi/content/full/124/5/459 Accessed 2009-05-11.
41. North Battleford, Saskatchewan, Spring 2001 - Waterborne Cryptosporidiosis Outbreak. Dr. Rob Stirling, Dr. Jeff Aramini, Dr. Andrea Ellis, Gillian Lim, Rob Meyers, Manon Fleury, Dr. Denise Werker, Document ID; 12102 Language: EN. Health, 3475 Albert Street, Regina, SK S4S6X6, Canada http://www.publications.gov.sk.ca/details.cfm?p=12102 Accessed 2009-05-11.
42. "Bromate in Los Angeles Water - Suspected carcinogen may lead to new monitoring regulations nationwide". Jyllian N. Kemsley, Associate Editor. Chemical and Engineering News. December 24, 2007 Volume 85, Number 52 p. 9. Photochemistry http://pubs.acs.org/cen/news/85/i52/8552notw4.html Accessed 2009-05-11.
43. Eberhard ML. Nace EK, Won KY, Punkosdy GA, Bishop HS, Johnston SP. Baylisascaris procyonis in the metropolitan Atlanta area. Emerg Infect Dis 2003 Dec. Available from: URL: http://www.cdc.gov/ncidod/EID/vol9no12/02-0795.htm Accessed 2009-07-06
44. Patrick J. Gavin, Kevin R. Kazacos, and Stanford T. Shulman . Baylisascariasis. Clinical Microbiology Reviews, October 2005, p. 703-718, Vol. 18, No. 4. Accessed 2009-07-06. http://cmr.asm.org/
46. Shafir SC, Wang W, Sorvillo FJ, Wise ME, Moore L, Sorvillo T, et al. Thermal death point of Baylisascaris procyonis eggs. Emerging Infectious Diseases. Volume 13, number 1. 2007 Jan. Accessed 2009-07-09 Available from http://www.cdc.gov/ncidod/EID/13/1/172.htm
47. Huisman, L. (1986). Slow sand filtration. Delft, The Netherlands, Delft University of Technology, Department of Civil Engineering http://www.irc.nl/docsearch/title/109507 accessed 2009-07-28
48. Connecticut Department of Public Health. "Growing Eating Fruits and Vegetables in the Newhall Neighborhood of Hamden." Jan 2004. www.newhallinfo.org/.../HAMDEN_VEGETABLE_UPTAKE_FACT_SHEET.pdf accessed 2009 08 04 page 2,3
49. "Gardening on lead and arsenic contaminated soils" College of Agriculture and Home Economics. WSU cooperateive extension. 1999. cru.cahe.wsu.edu/CEPublications/eb1884/eb1884.pdf accessed 2009 08 04
50. The Myth of Protected Preservatives "The chemicals in pressure-treated lumber will not affect adjacent soils or plants" Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University http://www.puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/index.html accessed 2009 08 04
51. National Toxicology Program. US Department of Health and Human services. abstract: http://ntp.niehs.nih.gov/index.cfm?objectid=A597C921-F1F6-975E-7D73DF11FE4A0470 Toxicology Studies of Dichloroacetic Acid (CAS No. 79-43-6) in Genetically Modified (FVB Tg.AC Hemizygous) Mice (Dermal and Drinking Water Studies) and Carcinogenicity Studies of Dichloroacetic Acid in Genetically Modified [B6.129-Trp53tm1Brd (N5) Haploinsufficient] Mice (Drinking Water Studies) NATIONAL TOXICOLOGY PROGRAM, P.O. Box 12233 Research Triangle Park, NC 27709. April 2007. NTP GMM 11 NIH Publication No. 07-4428. National Institutes of Health. Public Health Service. U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES. Complete study (pdf): http://ntp.niehs.nih.gov/index.cfm?objectid=912DF933-F1F6-975E-708693C5249E525F accessed 2009-08-06.
52. New Horizons for Slow Sand Filtration. Dr. David H. Manz, P. Eng. Published in the Proceedings of the Eleventh Canadian National Conference and Second Policy Forum on Drinking Water and the Biennial Conference of the Federal-Provincial- Territorial Committee on Drinking Water, Promoting Public Health Through Safe Drinking Water,
April 3 â€“ 6, 2004, Calgary, Alberta, pp 682 â€“ 692. accessed 2009-09-19. http://www.manzwaterinfo.ca/documents/New%20Horizons%20for%20Slow%20Sand%20Filtration%20Full%20Paper.pdf.
53. The Association for Environmental Health and Sciences,TPH Working Group Series on Line!. Volume 5 pages 30, 34, 45, 66, 69. Volume 5: Human Health Risk-Based Evaluation of Petroleum Contaminated Sites: Implementation of the Working Group Approach Donna Vorhees, John Gustafson and Wade Weisman 1999, pp.98 ISBN 1-884-940-12-9
http://www.aehs.com/publications/catalog/contents/tph.htm accessed 2010-01-06
54. PERIODIC REVIEW. Glacier Park East; Facility/Site ID #: 349. 1408 US Highway 2 Leavenworth, Washington 98826. Central Region Office TOXICS CLEANUP PROGRAM December 30, 2008 http://www.ecy.wa.gov/programs/TCP/sites/glacierParkE/Glacier%20Park%20East%20PR%20report.pdf accessed 2010-01-08. See page 4 section 2.3 cleanup levels chart. accessed 2010-01-08
55. EPA 440/9-76-023 National Recommended Water Quality Criteria United States Environmental Protection Agency. Office of Water.
Office of Science and Technology 2009 (4304T) http://www.epa.gov/waterscience/criteria/wqctable/nrwqc-2009.pdf
accessed 2010-01-09 477 pages p203-208
56. The Gold Book 1986 http://www.epa.gov/waterscience/criteria/library/goldbook.pdf
57. The Red Book http://www.epa.gov/waterscience/criteria/library/redbook.pdf 534 pages p210-223 (the pages are numbered by hand and do not match the page number given in the pdf reader software which was shown as page 242-256 on the system used when accessed) accessed 2010-01-09
58. Water Technology An Introduction for Environmental Scientists and Engineers. Second Edition. N.F. Gray PHD, Sc.D. Department of Civil, Structural and Environmental Engineering. Trinity College, Universty of Dublin. Elsevier Butterworth Heinemann. 2005.
P. 317 - 318 viruses. p. 270 - 273 slow sand filters p. 424 - 432 biofilm
59. Visualisation of the microbial colonisation of a slow sand filter using an Environmental Scanning Electron Microscope Esther Devadhanam Joubert; Balakrishna Pillay Electronic Journal of Biotechnology ISSN: 0717-3458 Vol. 11 No. 2, Issue of April 15, 2008 Â© 2008 by Pontificia Universidad CatÃ³lica de ValparaÃso -- Chile Received August 28, 2007 / Accepted December 6, 2007 http://ejb.ucv.cl/content/vol11/issue2/full/12/ Accessed 2010-01-18
60. The relationship of Algal Exocellular Polymer to Biological Flocculation. Joseph L. Pavoni, Mark W Tenney, Wayne F. Echelberger Date of Original 1971 Conference Title Proceedings of the 26th Industrial Waste Conference Conference Front Matter (copy and paste) http://earchives.lib.purdue.edu/u?/engext, 19214 Extent of Original p. 957-974 Series Engineering extension series no. 140 Collection Engineering Technical Reports Collection, Purdue University Repository Purdue University Libraries Rights Digital object copyright Purdue University. All rights reserved. Date Digitized 2009-06-25 http://e-archives.lib.purdue.edu/ accessed Jan 18 2010 12:31 pst.
Links to other sites with information on biological sand filters:
Slow Sand Filter Blog
Don’t take water for granted March 19th, 2010
I guess this will be kind of a rant. Sorry, but it must be said, besides I have pneumonia now and have to keep from doing a lot of physical work. People are just too darn complacent. Somebody has to fire people up. So here goes . . . . . After following the access logs for the water filter websites for several years, I have found an interesting trend. More than half of the hits are from areas outside of the USA, or from areas where there are currently issues with water in the USA. ( I’ll call these people the “enlightened” ). For example, in the US state of Georgia, there are places where water is no problem, and places where there is “drought”. These people need water NOW. However, to urban and suburban people in “the developed world” with access to the internet, access to clean water is not really a big life threatening issue. It is philosophical or theoretical – an abstract concept that should be argued in academic discussion groups, and their “rain barrels” are painted stylish novelties to be outdone by competitive neighbors. ( I’ll call these people “clueless” ) Here’s the deal: Clean, pathogen free, poison-free water is a human right, not to be bought or sold as a commodity. The sooner the developed world gets that through their collective thick heads, the better off we will all be. And furthermore, there is a bigger picture here. The picture of life. Our excellent science ( I’m not trying to mock science it is a VERY good thing – it is just misused and abused ) has not yet given us all the details about life; and this fact is brought to light by the mysterious operation of aerobic life forms that exist in water and particularly in a slow sand water filter. Our science can show us that they work to purify water, but scientists still don’t know all there is to know about all the microscopic life that makes these “filters” purify, yes that is purify – remove pathogens from – water. ( Public water supply systems only sterilize water – the dead pathogens are still there when you drink it along with all the toxic byproducts created when chlorine, or ozone turn the dead organic compounds into carcinogens ). Wetlands purify water in a similar fashion to slow sand filters. And, yes ( as the “clueless” will chant ) over a period of tens or, hundreds or thousands of years, the dense rock underground will physically filter water, but that is of no use to us NOW. Biological purification of water takes only days and it works.
For city dwellers this all may be hard to understand. Water is all around. Drinking fountains are all over the place. If you’re not on a well, you just “pay the water bill” and drink on, getting the water to you is someone else’s problem ’cause that’s what you pay for. WRONG. DEAD WRONG. The water bill is nothing but a token. Those of us who get our water from a well understand this. Your water supply is CRUTIAL, expensive and a BIG DEAL. Water is absolutely necessary for life, without it we die quickly, as a result there are tremendous guaranteed profits to be made selling water. Just look at the “bottled water” industry. This is compounded by the undeniable fact that disease is easily transmitted by water, and because of this, water supply purity MUST BE assured. This sort of adds credence to the old saying “you never want a drink till the well runs dry”. Now, the “clueless” will correctly say: “There’s no problem, the earth is huge and there is enough water to last forever, besides, it evaporates and that cleans it. It spends time in the oceans then evaporates to rain and that cleans it. We don’t need biological cleaning. Sunlight kills all viruses and bacteria in the rain clouds anyway.” The problem is that man is way too anthropocentric. The “earths” problems are not the same as “human” problems. Plastics break down in, well, several hundred to several thousand years – a problem to us but not really to the earth. Radioactive material breaks down after 30 or 40 thousand years, a huge problem to us but, not to the earth. Water polluted with heavy metals may eventually clean up but not in time for people to use it. Even petrochemical pollution will eventually break down – not a big problem to the earth, but to people – deadly. We can deny our way to the ultimate hedonistic gratification culture. Science has become so complex that it is easy and convenient to hide motive for profit in complex fact based research writing.
People die without water in 4 days. People die from terrible diseases if they drink water with pathogens in it or if they drink water that is contaminated by industrial or chemical pollution which can come from indeterminate sources.
Now all this water will very likely clean up in several thousand years, but you see that’s not going to cut it. We need the water NOW. Biological sand filters work and if monitored they will work much more efficiently over all ( they are sustainable technology ) than anything man has yet come up with. Don’t agree with me? PROVE I’M WRONG. When those of you who are not aware of the importance of water become aware – will the amazing little microbes that so willingly eat pathogens be gone? Are you willing to risk that? Science DOES NOT KNOW EVERYTHING ABOUT THESE BENEFICIAL MICROBES, or what may eventually destroy them. Think about it folks. Happy drinking – and those of you in the city – well – enjoy your carcinogenic cocktails. Cheers! ( or you could call your local or state legislator and demand change – naaaa. . . . that makes too much sense. Sorry, but I had to say this.)
Harvesting Rainwater from a composition (asphalt) roof January 7th, 2010
New information is available about rainwater harvesting from composition roofs. The slow sand filter described on slowsandfilter.org has been shown to remove petroleum hydrocarbons from water down to less than 1 part per ten million by weight; and this is when it is operating at its LEAST efficient rate of purification (at 32 degrees F). Also, petroleum hydrocarbons are present in roof water from the composition roof in the study in potentially harmful amounts if consumed over a long period of time (2.9 parts per million by weight). If a first flush diverter is used the, concentration is reduced to almost acceptable levels (less than 1 part per million by weight). The hydrocarbons may, in fact, be originating partially from local air pollution, because diesel fuel traces were found along with heavy oils. The implication here is that composition roofing is indeed a viable means of collecting rainwater, if proper filtering is used. A first flush diverter can be built easily with some pvc and recycled containers, and a slow sand filter can also be put together for minimal cost using recycled containers.
Another important fact uncovered is that rainwater from a roof may indeed vary WIDELY in the amount of Coliform bacteria present. The study has shown that water from a roof can contain about 60 cfu/100ml of coliform bacteria (relatively harmless) to 50,000 cfu/100ml (very likely to contain harmful pathogens and not safe to use), and that a slow sand filter will vary somewhat in its ability to remove pathogens; and also that a first flush diverter will remove a considerable number of the pathogens, but not enough to be safe. The slow sand filter has been shown repeatedly, by other studies (see the lit cited page on the above mentioned website) to remove coliform, ecoli, and fecal coliform to a safe level – from none detectable, in the best case, to 10 cfu/100ml in the worst case. In all cases the water from the filter is very non-turbid and should work well in conjunction with a UV filter followed by an epa approved point of use tap water filter.
Cleaning (maintainence on) a (biological) slow sand filter August 15th, 2009
A biological sand filter (slow sand filter or Biosand filter) is maintained or cleaned by actively managing only the top 5 cm of sand. This top layer is either removed and replaced when the flow rate becomes unacceptable (typically 4 to 6 months) with clean sand or “wet harrowed” meaning the sand is gently agitated causing partial break up of the biological surface. A rapid sand filter is maintained and cleaned by forcefully injecting water backwards through the entire sand bed every 2 or 3 days. Backwashing a biological sand filter will destroy it and may result in people getting very sick if water from it is being consumed. A Frequently asked questions (FAQ) on biological sand filters can be found here.
The terminology used to describe water filters that uttilize sand and gravel to filter water can be confusing. A “slow sand filter” uses sand and biological methods to purify water. A “rapid sand filter” uses sand to filter water. Rapid sand filters do not make use of biological methods, although some biological action may take place between backwashing (cleaning). A “Biosand filter” is a modified slow sand filter, invented by Dr. David Manz. A Biosand filter uses biological and sand filtering and is designed to operate intermittantly. A “sand filter” is a general term used to refer to a water filter that uses sand as the filter media.
Effective rapid sand filtration requires that chemicals and coagulants be added to the water. The coagulants attract particles including some bacteria. When the water with these coagulated pieces flows through sand, the coagualted particles are physically blocked by the sand. The water that emerges must be treated with chemicals such as chlorine, or ozone because most of the harmful bacteria, viruses and protozoa are still in the water. A rapid sand filter fills up in several days and must be cleaned by “backwashing” – forcing air and water back through the sand with high pressure. This breaks loose the coagulated contaminants. The resulting muck must then be drained off and disposed of. This material is hazardous waste. The bacteria is not killed, it is contained. When the chlorine or ozone is added to the water from a rapid sand filter the water is sterilized but not purified. Sterilized means the pathogens are inactivated, but their remains are still in the water. Purified means the pathogens are removed from the water.
A slow sand filter, and a Biosand filter work much differently than rapid sand filters. To explain this it is helpful to think about biology class. To purify means to remove, to sterilize means to inactivate. From a biological standpoint, rapid sand filter systems sterilize water, biosand filters and slow sand filters purify water. All water, with the exception of distilled water, will have organisms in it. The most pure water will have very few organisms. Most of these organisms are harmless to people. When water passes through sand due to only the force of gravity, these organisms, that live in oxygen rich water, will cling to the sand particles and start to feed on other organisms. In about 3 to 4 weeks a living layer of organisms forms in the top 5cm of sand. This layer continues to grow and will eventually spread throughout the filter with most of the activity concentrated in the top 5 to 10 cm of sand. Harmful organisms are “eaten” by this biological layer, which is often called the “Schmutzdecke” which is German for “dirt cover”. This results in purification. Eventually the layer on top gets so concentrated that it slows down the flow of water. To restore the flow of water, this biological layer must be “moderated” to allow more water to pass through at a higher rate. This is done in one of two ways. Either the top 2 cm of sand can be removed, or the surface can be “wet harrowed”. “Wet harrowing” means gently disturbing the biological layer and then allowing the resulting cloudy water to drain off. This is not backwashing. The lower layers of sand and organisms remain un-disturbed. Aside from biology, there are physical properties of a slow sand filter. As water and gravity act on the sand, a compaction and settling occur, which results in the sand becoming a “filter cake” which increases the effectiveness of the physical filtering properties of the system. Backwashing a slow sand filter will destroy this property and disrupt the biological action in the lower areas of the filter – in other words backwashing a slow sand filter or a biosand filter from the bottom of the filter will destroy it. Perhaps the confusion on this method comes from the fact that some large slow sand filters use a form of backwashing to effectively wet harrow the filter. Water is forced through the top 20 centimeters ( 8 inches ) of the 4 foot deep sand bed to break loose the top layer of biological activity. The bottom 3 feet 4 inches is left completely undisturbed. This is not the same as what is used on a rapid sand filter. In a rapid sand filter the entire sand bed is forcefully disrupted regularly every 2 or 3 days. There is a huge difference here. Constructing a small slow sand filter, in a 5 gallon bucket or a 55 gallon barrel and then backwashing it to clean it is just plain stupid. If the water from this situation is being used for consumption, people will most certainly get very sick.
First; my humble apologies for being so slow to moderate comments. Vast improvement is in order. We have updated the software that runs the blog ( as of July 24 2009) which went perfectly. Moderating should be quicker now.
The slow sand filter works using biological processes. Water passes through a layer of sand and after about 3 weeks a layer of biological activity forms in the upper 2 to 3 centimeters of the sand. The bacteria in this layer sometimes called the “schmutzdecke” break down harmful bacteria such as “beaver fever cysts”, all coliform including the most deadly and the resulting water out of the filter is safe for consumption. A lively discussion about these filters would be very encouraging. Anyone who has comments, questions, or experience with these filters is encouraged to leave comments. The website http://www.shared-source-initiative.com/biosand_filter/biosand.html documents a year and a half of experience building and operating 2 of these filters built from mostly recycled materials. All tests so far have shown they work very well. Also the website http://www.slowsandfilter.org has more detailed information and an FAQ page.
Contact the Author
About the author:
The author is a graduate of the University of Washington (class of 2006) with an interdisciplinary degree in Environmental Science and has been authoring web pages since 2001. Additionally, David has 28 years of professional experience in industry including 25 years restoring antique clocks and 3 years of experience as an electronics technician; plus two and a half years of experience designing, building, and testing functional slow sand filters used to purify roof water, surface water and shallow well water.
IMPORTANT: None of the water that goes through any of the filters described on this website is being consumed. The tests tell us what is NOT in the water, not what IS IN THE WATER. Anything can be in water. If you have questions, the author will try to answer them, however: USE THIS INFORMATION AT YOUR OWN RISK. The Author assumes no responsibility whatsoever for any damages of any kind as a direct or indirect result of the use of any information on this website or through any kind of correspondence with the author. The information provided here is free and published with the intent of sharing experience, and is not provided as an absolute solution to anything, nor does the author purport to be an expert at anything. Any email correspondence with the author is also covered by the preceeding statements. This is a work in progress. Mistakes will likely be found. The author reserves the right to remove this content or change it at any time. You have been advised.