Water defluoridation

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[edit] Overview

Naturally occurring in the earth’s crust, fluoride can often be found in the worlds water supplies. While innocuous and even beneficial in small amounts, when fluoride is consumed in excess by humans and animals it can cause varying degrees fluorosis. In an effort to combat this public health problem, various technologies have been explored in both the developing and developed world.

[edit] Fluorosis

[edit] Occurrences of Fluoride and Fluorosis

There are numerous fluoride “belts” throughout the world where groundwaters contain unsafe levels of fluoride. These belts span over 14 countries in Africa, 8 countries in Asia, and 6 countries in the Americas that all having water considered unsafe by the World Health Organization (WHO).[1]

1000px-Groundwater-fluoride-world svg.png

Regions of the World with Significant Levels of Fluoride in the Groundwater.


This fluoride typically occurs in the earths crusts as fluorspar(CaF2), rock phosphate, cryolite (Na3AlF6), apatite(Ca5(PO4)3F), mica, and hornblende.[2] Fluorine is the most electronegative element and is extremely reactive, rarely found outside of its ionic form, fluoride.[2]The fluoride then leaches off of these minerals and into the groundwater. The harmful effects of fluoride have been increasing worldwide. Though not exclusively a developing world problem, it is felt much more acutely in these regions as the infrastructure to deal with fluoride is often not equipped to deal with this problem. As the population of the developing world continues to climb, people are forced to move into fluorotic areas.

Fluoride can also contaminate ground and surface water from man-made causes such as mining and the use of certain pesticides.[2]

One of the regions of the world most affected by fluorosis is East Africa, specifically, the East African Rift Valley. Possibly because fluorotic minerals are often carried by water, it is more common to find fluoride rich soils in lowlands and valleys. (Conversely there is typically less fluoride in nearby highlands.) This phenomenon coupled with the high fluoride volcanic rocks in the East African Rift result in significant amounts of fluoride in the Rift Valley.[3] With over 61% of East African water sources having more than 1 mg of F /l (the recommended amount), 20% having more than 5 mg/l, and 12% having over 8mg/l. [2]

[edit] Causes of Fluorosis

It is common in the developed world to intentionally put fluoride into municipal water systems. It has been proven that with 1.0 mg of F/l(dependent on a few other factors) the overall instances of dental health in a community can be greatly improved by the added protection fluoride gives to tooth enamel[2]. In fact, fluoride in water is not considered toxic until it reaches concentrations of 250-450 mg/l.[4] However, because much of the fluoride consumed is retained by the body, it still has a cumulative effect when consumed in far smaller concentrations resulting in fluorosis. Though necessary for the body to function and proven to be helpful in smaller doses, the WHO recommends the ingestion of no more than 4.0 mg of fluoride per person per day.[5] Though much intake of fluoride comes from food, it has been shown that the majority of occurrences of fluorosis come from the consumption of water with excessive amounts of fluoride. [6] Thus, the WHO limits fluoride concentrations in drinking and cooking water to 1.5 mg F/l [2]. This limit is considered incomplete by many. It has been suggested that the optimum amount of fluoride in drinking water is approximately 0.5-1.0 mg/L. [1] Because it is not so much the concentration of the fluoride that is of concern, but rather the total fluoride consumed, it has been suggested that a “sliding scale” of acceptable levels of fluoride should be used based on the average maximum temperature. [3]

Maximum Mean Temperature of Region (oC) Maximum Recommended Concentration of Fluoride (mg F/l)
0 2.1
10 1.3
20 0.9
30 0.7
40 0.6
50 0.5
60 0.4


Given this, it has been noted that those who live in hotter/humid climates and/or labor outdoors are far more likely to develop symptoms of fluorosis than those who do not.[7] This is because they consume far more water than those in other regions and lifestyles. Studies have also shown that children are typically the most affected by fluoride as their developing bones and teeth are more susceptible to the effects of fluorosis. In fact, it has been shown that the amount of fluoride consumed in one’s first year of life has more impact than any other phase of life.[8] Other factors that have affected the severity of fluorosis in individuals are altitude of residence, nutritional status, and use of dentifrice.[9] It is estimated that about 60% (80-90% for infants) of fluoride ingested in person’s body is retained while the rest is primarily expelled through urine.[2]

There are instances in which fluoride can be inhaled through the burning of coal. Fluoride inhalation can cause significant health problems, this problem is most evident in China. [2]

These problems are exacerbated in East Africa by the fact that the common foods used for weaning children are often fluoride rich. [10] Additionally, “Magadi” (Na2 NaHCO3H2O) is a salt used heavily in cooking that is typically collected from lakes or earth. Magadi is typically formed in the region’s fluoride rich bodies of water and is a major source of fluoride intake for the peoples of the region.[5]

[edit] Dental Fluorosis

Dental fluorosis is by far the most common manifestation of over-consumption of fluoride. It is visible by white, yellow, and brown streaks on the teeth, characteristic of the hypoplasia and hypocalcification.
Severe Dental Fluorosis

[11] This damage is more than cosmetic, as it tends to be associated with painful "cavity-like" feelings. Additionally, there are social stigmas against those suffering from fluorosis. It had once been postulated that men were more disposed to suffering from dental fluorosis than women, however, it is now believed that this inference was incorrect, and that women are more likely to try to hide the effects of fluorosis.[7] While all teeth are affected, the incisors (especially the maxillary incisors) and permanenet molars are often the teeth most affected by fluorosis. It is speculated that this is because these are the first teeth to develop.[12]

[edit] Skeletal Fluorosis

Child with skeletal Fluorosis
Though it generally takes far more time, and higher concentrations (typically over 10mg/l [2])to develop, skeletal fluorosis is far more severe than its dental counterpart. Though not initially obvious to diagnose, skeletal fluorosis can be detected early on radiologically.[11] Skeletal fluorosis is characterized by deformation of bone structure. Movement of the spine, pelvis, and joints become increasing arduous as fluoride deposits collect on ligaments and tendons and within the bones themselves.[11] Skeletal fluorosis to the point of crippling is not uncommon.
Skeletal fluorosis

Fluorosis is non-curable[13], thus efforts should be directed toward prevention and attempting to alleviate some of the symptoms.

[edit] Neurological complications

There are increasing accounts of the neurological affects that fluoride can have on the body. It is suspected that these complications are caused by fluorides effects on the spine and compression on the spinal cord.[14] Studies have shown that high levels of fluoride can cause headaches, isomnia, and reductions in the IQs of children.[15]

Fluorosis has significant economic impacts in the developing world. In addition to fluorosis removing people from the workforce, water supply programs have thrown away significant finances while providing costly boreholes that become useless upon the discovery of the toxic levels of fluoride that they contain. [16]

[edit] Perceptions and Education on Fluoride and Fluorosis

Because fluoride does not cause water to have any abnormal, taste, and odor, it is difficult to determine if water has significant fluoride concentrations. Because of this and the cumulative nature of fluoride as a toxin (that is, the results of consumption are not immediate), many peoples do not automatically connect water consumption to fluorosis. The effects of the disease have been attributed to a wide range of sources by different peoples including genetics, infection, and diet. [17](One people group in Kenya believe that fluorosis is caused by eating of potatoes that are too hot.)[17]

Therefore, there is a great need for education on and awareness of fluorosis in fluorotic areas and to connect water consumption to the symptoms of fluorosis. In many of these areas, little priority is given to water defluoridation because water sources are scarce enough that peoples are not concerned with water quality. [17]And because fluoride consumption has no immediate health effects and defluoridation methods are generally more time and money intensive than other water treatment types, there is generally a lack of motivation on the part of the people to be concerned with defluoridation.[17] Even when defluoridation methods are used, the lack of immediate results is a hindrance in encouraging the continued use of defluoridation.[18]

[edit] Measuring Fluoride

Because the presence of fluoride in water is tasteless, odorless, and color less, there is no way to identify the concentration of fluoride in a body of water without the use of instrumentation.[19] The use of a fluoride selective electorde is often considered the most reliable way of testing water for fluoride, though this method is difficult to do outside of a lab setting. The electrode consists of a lanthanum fluoride crystal (LaF6) that experiences a electro-potential when in the presence of fluoride ions.[19] Several methods have been developed for testing fluoride concentrations in the field. Most of these are colorimetric tests that result in fluoride interacting with chemicals and dyes such as the SPADNS method.[19] Though less accurate than an electrode, these methods can be used to evaluate if a body of water is safe to consume.[19]

[edit] Defluoridation

[edit] History of Defluoridation

The problem of fluorosis is nothing new as fluorosis is naturally occurring in ground water. Fluoride has been called "[...]one of the most widespread endemic health problems associated with natural geochemistry." Indeed, ancient skeletal remains in fluorotic areas exhibt signs of fluorosis.[20] Though an ancient problem, there were little formal attempts to defluoridate water before the 20th century. The problem of fluorosis was not necessarily identified until recent history. Some of the earliest diagnoses of dental fluorosis come from 1888 in Mexico and 1891 in Italy. [2] Still the link between drinking water and fluorosis wasnt established until the 1920s in Colorado, by dentist Dr. Fedrick S. McKay.[21] As many parts of the world where fluorosis is common are relatively isolated, many cultures in fluorotic regions never considered dental fluorosis to be an abnormality until increased communication and travel resulted in changes in perception [7]. Additionally, skeletal fluorosis can often take an extended time to develop visible symptoms, so it is not necessarily obvious for people to link fluorosis to a problem in water quality. [22] It was in the 1930s that several nations first began to more seriously investigate the negative effects of fluoride and how to remove it from drinking water supplies. [16]

[edit] Methods of Defluoridation

As with the treatment of other chemical contaminants, such as arsenic, fluoride cannot be removed by typical water treatment means. Boiling, UV treatment, most methods of filtration, and most chemical treatment options do nothing to remove fluoride concentrations from water. Synthetic ion exchange and precipitation processes, activated alumina filters, and reverse osmosis are typically used to remove fluoride from water in the developed world [23], there is no universally accepted or routinely used defluoridation techniques in the developing world. Thus, defluoridation is a prime example of field in the need for further development of appropriate technologies. Ultimately social, financial, cultural, and environmental factors must all be weighed to determine what solutions should be implemented in a region.

This Activated Alumina filter is conveniently integrated into the home plumbing system
This diagram of the Nalgonda technique alludes to its labor intensive nature

It must be noted that most of the defluoridation methods discussed here are point-of-use (POU) treatment options. These options are preferred for defluoridation in developing world settings. Though many POU treatments are done so to help reduce the risk of recontamination of water, defluoridation techniques, however, are typically POU because it helps to reduce costs. Because only water needed for drinking and cooking (about 25% of total water usage[24]) needs to be treated, municipal level treatment is rare. Still in some places a community scale defluoridation plan can be more appropriate. As with other methods of water treatment, the economics of the situation must be weighed to determine what scale is most appropriate.

[edit] Activated Alumina

Activated alumina is just what it sounds like, Alumina (Al2O3) that has been activated to become adsorbitive. The method of activation is done through dehydration of aluminum hydroxides at temperatures of 300-600oC.[24]

Activated Alumina

An extremely effective technology, activated alumina has been used in defluoridation since 1936 and was first used on a large scale in South Africa in the 1980s. [23] Though it is often used for large scale defluoridation in the developed world[24],activated alumina has long been considered to not be an appropriate technology for use in the developing world because of chemical costs and availability. This technology is recently being reconsidered and is considered economically feasible for many in China, India, and Thailand.[25] This is probably due to higher level of infrastructure established in these countries needed to produce activated alumina. Still, costs prevent activated alumina from taking off in much of the developing world. However, the exploration of lower cost methods of producing activated alumina, as has been developed in one study Vietnam, may result in this technology becoming more accessible to more people.[26]

Activated alumina is used in an adsorption process with very high fluoride removal efficiency (able to treat water with fluoride concentrations from 4-20mg/L).[7] Though it has been used successfully to treat water for a number of contaminants,[4] it has the advantage of having a very high selectivity for fluoride.[27] The theoretical defluoridation capacity of the material increases with lower pHs (as high as 20.4 mg F/g at pH of 3), with any pH less than 9 resulting in a positive charge on the alumina surface that adsorbs the fluoride ions.[27] (It must be noted that the operational capacities observed in the field for activated alumina are often much lower than the theoretical values, being as low as 1 mg/g. [2])

Column Filter Typical for use with Activated Alumina
Fluoride Removal Capacity vs pH for Activated Alumina

Though configurations may vary, activated alumina is typically used in a gravity fed column filter.
[28] Though it depends entirely on the demands put on the filter, activated alumina filter media typically has to be replaced every few moths. Activated alumina has been shown to be successfully regenerated using hydrochloric acid (2%),sulphuric acid (1%), and sodium hydroxide (2%) solutions that create a strong negative charge on the alumina surface, removing the fluoride ions. [27] It should be noted that 5-10% of the media is lost in the regeneration process.[25] The remaining media has been shown to lose 30-40% of its capacity, thus activated alumina can only be used 3-4 times before it must be replaced, though it is common to mix regenerated media with fresh media.[25]

Below is a set of sample design calculations for the construction of an activated alumina column filter

Given Parameters: Unit Column Filter
D Daily personal water demand L/(c x d) 3
N Number of users p 6
OP Operation period months months 3
LT Theoretical sorption capacity g/kg 4
Lo Operational sorption capacity g/kg 1
s Bulk density of medium kg/L 1.2
Fi Raw water fluoride conc. mg/L 5
Ft Treated water average fluoride conc. mg/L 0.2
VRSW/M Volume ratio supernatant water/meduym - 0.2
VRCW/M Volume ratio clean water container medium - 0
Derived Parameters
Q= D x N Daily Water Treatment L/d 18
VT= OP x Q Total volume of water treated in a filter L 1620
FT= VT (Fi-Ft)/1000 Total fluoride removal during a period g 8
M= FT/LO Amount of medium required for renewal kg 8
VM= M/s Volume of medium in the filter 7 35
BV= VT/VM Number of bed volumes treated in a filter period - 250
VSW= VRSW/MxVM Volume capacity of supernatant water L 1.5
VCW= VCW/M/VM Volume capacity of clean water container L 0
VF= VM+VSW+VCW Total Volume of filter L 8
Corresponding dimensions:
Ø Filter diameter (selected as available) cm 12
HF= VF/[pi x (Ø/2)2] Total height of the filter cm 69
[29]



[edit] Bone Char

Bone char is the oldest known technology for water defluoridation, being successfully used since the 1940s.[2] It has been utilized successfully for at least 5 decades and was at one point used heavily in the United States for municipal water defluoridation and sugar refining.[30] Bone char has also been used successfully in the removal of arsenic from water.

Bone Char

Bone char is produced with animal bones that have passed through calcination or pyrolysis processes. Though raw bones have some defluoridation value, it is small and limited by the various organics obstructing the interfaces where chemical reactions with the fluoride take place.[30]

In order to produce bone char, animal bones must first be collected. These bones can be collected from a variety of sources including butchers, restaurants, ranchers, etc. This can create an entirely new market by giving value to what was previously viewed primarily as a waste material.
Bone Collection
Once the bones are collected, they are often washed, rinsed, boiled, or sun dried to remove much of the organics before they are actually charred to be used as filter media. Many different animal bones can work, as the uptake of fluoride hinges upon a reaction with hydroxyapatite (Ca10(PO4)6(OH)2), a mineral found in all bones. One study showed that there is slight variability seen in the capacity of char made from different animals, with cows and pigs yielding char with more capacity than chicken and fish, but the differences are often regarded as minor.[31] There can be significant cultural reasons to select one animal bone type over another. Bone char made from cattle may be a problematic for Hindus, char made from pigs may not be accepted by Muslim and Jewish communities, and char from dogs and hyenas may be rejected in many African communities.[2].

Calcination is the process of subjecting the bones to high temperatures(300-700oC) with atmospheric oxygen present.[30]. This results in a bone char product that is chiefly just hydoxyapatite with the organics burning off as CO2.[32] Hydoxyapatite is the primary mineral component of bones and the mineral in which an ion exchange with fluoride takes place.[30] There is significant variability is the quality of calcined bone char in terms of fluoride uptake.

Pyrolytic bone char is bone char that has been charred in the absence of oxygen. Though potentially more complicated to produced, it has proven to be much more desirable than calcined bone char. Rather than burning the organic materials, pyrolysis results in the conversion of organic materials into a usable activated carbon, which can treat the water beyond fluoride. [30] A well done batch of pyrolytic bone char should result in about 10% activated carbon with the rest chiefly being hydroxyapatite for the fluoride ion exchange process.[30] The quality of pyrolytic bone char is far less variable than that which has been calcined. Additionally, it loses less mass (31% as opposed to the 38% of mass lost in calcination) and capacity through the charring process.[30]

The difference between pyrolytic and calcined bone char is not necessarily black and white. (No pun intended.) Pure pyrolysis results in black bone char will be most likely due to the increased amount of graphite (activated carbon) that is present, while calcination can vary in color form white to dark grey.[30] As implied, generally the closer the charring process is to pure pyrolysis the better. Not only does this result in higher quality bone char in terms of fluoride removal capacity, but in terms of activated carbon present as well.

Example of a furnace designed for the production of bone char.

That being said, true pyrolysis is difficult to achieve on a large scale in the furnaces used in the developing world. Pyrolysis requires far more fuel, and therefore is far more expensive.[33] Thus, there are virtually no large scale furnaces in use designed for the pyrolysis of bone char. Efforts are still typically made in the construction of furnaces to significantly control the inflow of air to these furnaces, but because some atmospheric oxygen is used in the preparation of bone char it is always a calcination process. Generally the ratio of fuel (typically charcoal) to bones is about 8%.[34] There have been numerous studies on what temperatures and charring times produce the best quality bone char, and though the numbers vary most agree that charring temperatures for bones should not be much beyond 500oC because at this point the effectiveness in fluoride removal is compromised.[33]

Physical Properties of Bones Calcined at Different Temperatures

The amount of time the charring is done can vary significantly depending on the quantity of bones being charred and the desired quality. Typically the total charring time is a matter of days, with perhaps a few hours actually at the maximum temperature because it takes a long time to heat up the massive furnaces.

Graphs representing the temperature of two batches of bone charring process as carried out by CDN in Kenya, (Different colors represent different portions of furnace)
Once charred, the bones are then crushed and sieved to achieve desired sizes. As with the temperature there are variations in what is considered to be the optimal grain size for bone char use in defluoridation. Most sources agree that the use of 0.5-4 mm diameter grains can and should be used with both effectiveness and economics in mind. [34]
Table for evaluating bone char filter media based made by defluoridation experts Dahi and Bregnhøj
It has actually been shown that the grain size has no bearing on the dynamic capacity of the bone char, but rather it is the reaction rate that is affected.[35] The reaction rate constant is nearly reciprocal of the grain diameter. That is, smaller diameters result in quicker reactions. [35] Too small of a grain size can result in too much head loss when the char is used as a filter media, so care must be taken to ensure that the grain sizes of the media are not so small as to clog the filter.

As mentioned, the crushed bone char is then used as a filter media. Typically bone char filters are designed as gravity-fed column filters, though there have been a few different configurations implemented.

The primary difference exists between the first two filters and the column filter. The column filters more closely resembles a plug flow reactor, while the other two have more of a mixed flow.[25] Thus, the column filters generally make more efficient use of the filter media. In order to estimate the life of filter media when designing a filter the operational defluoridation capacity of a filter is estimated from the theoretical defluoridation capacity. The theoretical capacity is calculated based on scaling up the amount of fluoride that each grain of bone char can hold. A proposed rule of thumb is that the closer a filter is to being a column filter (and therefore a plug flow reactor) the operational defluoridation capacity is 2/3 the theoretical defluoridation capacity (Typical capacity of 2-6 mg F/g for bone char.[36]) , while the bucket and drum filters have a operational defluoridation capacity closer to 1/3 the theoretical defluoridation capacity. [25] There are a number of design parameters critical in the construction of a bone char filter.

The primary advantages to making a bucket or drum filter all lie in the filter construction. Because the filter unit themselves can be made from a wide variety of materials, it is often far easier to use cheap and readily available containers to make filters, rather than the custom manufacturing of a column filter. [25]

It is recommended that bone char filters be rinsed through a few times before they are actually used as there may be some residual organic materials in the char that may negatively affect the taste of the water. Though this should not be a concern if the charring process was done well. [34] (Pyrolytic bone char yields no odors or unpleasant taste.)[30] Due to variations in the charring process, the actual chemical composition of bone char can vary slightly. Generally its composition is:

57-80% Hydroxyapatite (Ca10(PO4)6(OH)2
6-10% Calcium Carbonate(Ca(CO3))
7-10% Activated Carbon[2]

Though some adsorption of fluoride occurs onto the activated carbon, the primary uptake reaction is believed to be an ion exchange between the hydroxyapatite and fluoride resulting in the formation of fluorapatite.

Ca10(PO4)6(OH)2 + 2F- => Ca10(PO4)6F2 + 2 OH-

This reaction readily occurs due to the fact that fluoride and hydroxide ions have the same charge and radius[4], and hydroxyapatite is more soluble than fluorapatite. [37] Obviously this results in a pH shift in the water to become more alkaline. (This shift is more pronounced with bones that have been poorly calcined than those which were subject to pyrolysis, probably because there is destruction of the apatite structures that result in increased calcium oxide in the water.[30])

This reaction can be reversed to regenerate the saturated bone char media. By adding 1% NaOH to saturated bone char, another ion exchange happens.[36]

Ca10(PO4)6F2 + OH- => Ca10(PO4)6(OH)2 + 2F-

This process results in extremely high pHs (upwards of 13) which are then slowly lowered by rinsing the regenerated bone char with distilled and CO2 enriched water.[38] The fluoride rich sodium hydroxide solution can then be subjected to additions of Ca(OH)2 and CaCl2 which will react with the Fluoride to precipitate out as CaF2, thus allowing the NaOH solution to be reused.[38] It has been shown that this form of regeneration can be successfully used on bone char for several hundred cycles. [36]Generally the regeneration of bone char has only been implemented on large scale or community systems because of the complexities of the process, the close monitoring required, and the cost effectiveness.[2]

Bone char has also been successfully regenerated by recharring depleted bone char. The suspected reaction taking place can be seen below.

Ca10(PO4)6F2 +2OH- + heat => Ca10(PO4)6-2 +2HF+O2
[39]

Though successful, bone char can only be successfully regenerated through this method for a few times as there are significant losses in efficiency after each regeneration.[39]

Like activated alumina, bone char filter media needs to be replaced or regenerated regularly, every few months to few years depending on the construction of the filter and the demand from it.

Bone char has been successfully implemented on some scale in many parts of the world, especially Thailand and Africa, yet there remains to be a successful wide scale implementation of this technology.[2]
Example of a bone char filter in use in East Africa.
Many of the obstacles this technology faces are related to social acceptance. As mentioned many will not consume water treated by the bones of specific animals for religious and cultural reasons. Additionally, when poorly quality bone char can result in treated water having flavor issues, leaving a bad taste in the mouths of those who sampled this technology. When not appropriately managed, the charring process itself is also capable producing horrible odors[22] and can negative associations with cremation in the minds of some users.[22] Still, this technology has the advantages of being simple, relatively cheap, readily available, and potentially able to treat water beyond reducing fluoride when processed properly and regenerated. Having the infrastructure in place to create, maintain, and regenerate the filtered media is necessary to see the widespread adoption of this technology. Examples of an effort to create this infrastructure would be the Catholic Diocese Network (CDN) in Nakuru, Kenya[38] and the fledgling enterprise of Umoja Engineering (affiliated with the Defluoridation Technology Project.)

The CDN in Nakuru, Kenya represents one of the more successful efforts to develop and disseminate bone char technology and defluoridation in general. Approaching the problem of fluorosis from all angles, the CDN has heavily invested in education and awareness initiatives in the region. These efforts include the development of a fluoride education theatre group that are able to educate in a way that is appreciated and understood by many different groups in their communities.[17] The CDN has focused on collaboration with the local communities in bone char and bone char filter production and implementation. As mentioned, they have set up the infrastructure necessary to making bone char available by creating a centralized bone char processing and regeneration facility where filter media can be sold at low prices to the general public while still maintaing quality control over the bone char.[40] The CDN has both community size and household filters successfully in use in the field.[40] That being said, the CDN's efforts in bone char dissemination are not without problems. Because it is based out of one central processing facility and is receiving no income beyond that which cover expenses, the CDN's operational model is unable to grow and is considered to be unsustainable.[40]

Below is a sample calculation for 3 types of bone char filters. It is assumed that the theoretical sorption capacity is 6 mg F/g.

Given Parameters: Unit Drum Type Bucket Type Column Filter
D Daily personal water demand L/(c x d) 3 3 3
N Number of users p 6 6 6
OP Operation period months months 12 3 6
LT Theoretical sorption capacity g/kg 6 6 6
Lo Operational sorption capacity g/kg 2 2 4
s Bulk density of medium kg/L 0.83 0.83 0.83
Fi Raw water fluoride conc. mg/L 10 10 10
Ft Treated water average fluoride conc. mg/L 1 1 1
VRSW/M Volume ratio supernatant water/meduym - 2 2.5 0.2
VRCW/M Volume ratio clean water container medium - 0 3.5 0
Derived Parameters
Q= D x N Daily Water Treatment L/d 18 18 18
VT= OP x Q Total volume of water treated in a filter L 6500 1600 3200
FT= VT (Fi-t)/1000 Total fluoride removal during a period g 60 15 30
M= FT/LO Amount of medium required for renewal kg 30 7 7
VM= M/s Volume of medium in the filter L 35 9 9
BV= VT/VM Number of bed volumes treated in a filter period - 185 185 370
VSW= VRSW/M/VM Volume capacity of supernatant water L 70 22 2
VCW= VCW/M/VM Volume capacity of clean water container L 0 31 0
VF= VM+VSW+VCW Total Volume of filter L 105 62 11
Corresponding dimensions:
Ø Filter diameter (selected as available) cm 42 32 12
HF= VF/[pi x (Ø/2)2] Total height of the filter cm 75 75 92
[25]
[edit] Contact Precipitation
Contact Precipitation Design for Home Use

A more recent development in the usage of

Contact Precipitation Design for Community Use in Ngurdoto by Eli Dahi
bone char in defluoridation is the method of contact precipitation. This method is based on the addition of chemicals to water before running the water through depleted bone char filter media. By adding calcium chloride (CaCl2) and monosodium phosphate (NaH2PO4) to the water the precipitates of fluorapatite and calcium fluoride are formed and removed from the water.[2]

CaCL2 2H2O => Ca2+ + 2Cl-+2H2O
NaH2PO4H2O => PO43- + Na+ +2H+ +H2O
Ca2+ + 2F- => CaF2 (s)
10Ca2+ + 6PO4- + 2F- => Ca10(PO4)6F2

At this point there is not a thorough understanding of how contact precipitation works. It has been noted that the addition of calcium to fluoride rich water generally does not lead to significant precipitation of CaF unless it is in the presence of saturated bone char which catalyses the process. [36] The addition of the calcium and phosphate may also help to coat the surface in a new layer of hydroxyapatite which may be able to add effectiveness to the depleted char.[41] It is also speculated that the addition of calcium to the depleted bone char helps to repair some of the hydroxyapatite structure destroyed in the charring process.[36] Filters to operate for contact precipitation are designed similarly to normal bone char filters except that they have a larger reservoir for the input of raw water so that the chemicals can be added before passing through the depleted char. A effluent reservoir or bed is then added to be able to collect the water without the precipitate. It should be noted in the design and construction of this filter that residence times of 20-30 minutes have been shown to be most effective.[2] Some sketches of potential filter setups can be seen below. This method was developed in East Africa and has been successfully tested and implemented at a village and school level in Arusha, Tanzania successfully.[2] Though not yet widely used on a household level, it is believed to be appropriate for household usage because it is considered to be reliable, have high fluoride removal efficiency, be cheap, and no real risks associated with improper chemical dosage.[25] The primary barrier to the adoption of contact precipitation methods are the efforts required by the users to apply the dosages of the chemicals (though considerably easier than the Nalgonda technique) and the availability of said chemicals.[41]

Sample Calculations to determine Design Criteria associated with Contacte Precipitation. CaCL2 is assumed to contain 27% Calcium and NaH2PO4 65% Phosphate

Given Parameters: Unit Domestic School/Market
D Daily personal water demand L/(c d) 3 0.5
N Number of users p 6 500
Fi Raw water fluoride conc. mg/L 10 10
Ft Treated water average fluoride conc. mg/L 0.4 0.4
E Medium porosity - 0.56 0.56
s Medium Bulk Density kg/L 0.83 0.83
v Filtration velocity m/h 0.5 0.5
tc Contact time (=HcxE/v) h 0.3 0.3
tf Filtration time (=Q/(v x pi x (Ø/2)2))) h 4 4
VRRW/O Volume ration raw water/daily water treated - 1.1 1.2
VRBC/O Volume ration raw water/daily water treated - 0.3 0.3
VRCW/O Volume bone char medium/daily water treated - 1.1 2
MRCC/F Mass ratio calcium chloride/daily fluoride loading - 30 30
MRMSP/F Mass ratio MSP/daily fluoride loading - 15 155
Derived Parameters
Q= D x N Daily water treatment L/d 18 250
FT=QxFi/1000 Total daily fluoride loading (removal) g/d 0.18 2.5
ØBC= 2(Q/tF x v x pi)0.5) Diameter of contact bed cm 11 40
HBC= tC x v/E Height of contact bed medium only cm 27 40
Mcc=FT x WRCC/F Total daily dosage of CC g/d 5 75
MMSP= FT x MRMSP/F Total daily dosage of MSP g/d 3 40
VRW=Q x VRCW/O Volume of raw water bucket/column L 20 300
VBC= Q x VRBC/O Volume of contact bed medium L 2.4 33.5
MBC= VBC x s Mass of contact bed medium kg 2 30
VCW= Q x VRCW/O Volume of clean water bucket/tank L 20 500
Corresponding Parameters
WRW= (VRW)1/3 Width of raw water column cm - 65
LRW= WRW Length of raw water column cm - 65
HRW= VRW/(WRW x LRW) Height of raw water column cm - 70
ØCB= ØBC Diameter of contact bed compartment cm - 40
HCB= HCW Height of contact bed compartment cm - 65
WCW= WRW Width of clean water tank cm - 65
HCW= HCB Height of clean water tank cm - 65
LCW= VCW/(BCW x HCW) Length of clean water tank cm - 120
[25]

[edit] Soils, Clays, and Minerals

Various soil and clay types have long been used for water treatment, and there are records of ancient Egyptians using clay to treat turbid water since ancient times.[2]

A number of soils have been used in defluoridation including: Magnesite, apophyllite, natrolite, stilbite, clinoptilolite, gibbsite, goethite, kaolinite, halloysite, bentonite, vermiculite, zeolite, serpentine, alkaline soil, acidic clay, kaolinitic clay, China clay, aiken soil, Fuller's earth, diatomaceous earth, lateritic clay, and ando soil.[2] All of these soils, most composed chiefly of oxygen, silicon, and aluminum [3], have lattice hydroxyl-groups which are exchanged for fluoride.[2] Typically, these materials are calcined, acid washed, or air dried before use.[2]

Laterite Soil: Clays made from laterite soil can be used in defluoridation

Clays, which are probably some of the more utilized materials listed, are typically calcined at temperatures of 500-700oC with has been shown to be the optimal temperature for calcining to encourage fluoride binding in the material.[3] One study has proposed that though calcining may improve the fluoride uptake abilities of many clays, it is not necessary for fluoride removal. However, calcining is still recommended for the purpose of sterilizing the filter media.[42] One must also be careful as calcining at temperatures of over 800oC have shown to reduce fluoride removal potential or even result in increasing the fluoride content of water.[3] In addition to concerns of sterilization, the usage of soils and clays also requires careful selection of materials, as some soils may actually increase the fluoride content of water when used as a filter media.[43] Though yet to be proven as universally true, there is reason to believe that generally clays and soils from highlands may be more effective in fluoride removal as they often have lower fluoride content. [43].

Schematic of a Clay/Soil/Brick type filter

These defluridation filter media are typically crushed and placed into an upflow filter column or bucket, in order to allow for the settling of suspended solids within the filter bed. [2]

The chief advantage of using clays and soils is that it is locally available and cheap. Though there is significant variability in the fluoride uptake capacity of these materials, they generally have far lower capacities and treat water far too slowly compared to other adsorbtion methods such as bone char or activated alumina.[44] [42]

One exception to this general is the use of Magnesia (MgO) in defluoridation. Magnesia is created by calcining magnesite (MgCO3) and crushing it into small pieces to be used as a filter media.[45] It fosters an ion exchange process and has been shown to have an extremely high removal capacity of over 30 mg F/g.[46].It has the potential to be an especially promising technology in Tanzania where many magnesite mines have been established.[46] Though it has been discussed as a potential solution to defluoridation since the 1930s it has never been successfully implemented. [46] This is largely because the chemical reactions associated with the ion exchange are self-regulating in regards to pH, automatically moving the pH of the solution to its optimal point of 10.5-11.[47]Thus the use of magnesia in defluoridation requires additionally treatment afterwards to lower the pH to an acceptable drinking level.[48]

Clay filters have been used with some success in a number of regions, most notably Sri Lanka.[2]

Below is a sample calculation of the design parameters associated with the construction of a clay filter. Because there is significant variability in constants associated with this filter media, it is assumed that a bucket filter uses clay powder with a capacity of 0.03 mg/g and bulk density of 1.5 kg/L, and a column filter has clay brick grains of 8-16mm, with a capaciry of 0.1mg/g and bulk density of 1.3 kg/L

Given Parameters: Unit Bucket Type Column Type
D Daily personal water demand L/(c d) 3 3
N Number of users p 6 6
OP Operation Period days 1 180
Lo Operational sorption capacity g/kg 0.03 0.1
s Bulk Density of Medium (Powder & Chips) kg/L 1.05 0.086
Fi Raw water fluoride conc/ mg/L 3 3
Ft Treated water average fluoride conc. mg/L 1 1
VRSW/M Volume ratio supernatant water / medium - - 1/5
VRAF/M Volume ratio after-filter water / medium - - 1/2
VRS/Q Volume ratio of sludge / water demand - 1/10 -
VRVS/Q Volume ratio vacant space for mix./ water demand - 1/15 -
Derived Parameters
Q=DxN Daily water treatment L/d 18 18
Vs= QxVRS/Q Volume of Residual Sludge L 2 -
VT= OPxQ+Vs Total volume of water treated in a filter period L 20 3200
FT=VTx(Fi-Ft)/1000 Total fluoride removal during a period g 0.04 6
M=FT/Lo Amount of medium required for removal kg 1.3 65
VM=M/s Volume of the medium in the filter L 0.9 50
BV=VT/VM Number of bed volumes treated in a filter period - - 45
VSW=VMxVRSW/M Volume Capacity of supernatant water L - 15
VAF=VMxVRAF/M Volume of after-filter arrangement L - 40
VVS=QxVRVS/Q Volume capacity of vacant space in bucket L 1.2 0
VB=Q+VS+VVS;VF=VM+VSW+VAF Total Volume of bucket/filter L 40 130
Corresponding dimensions
Ø Filter diameter (selected as available) cm 35 40
H=VB/(pi x (Ø/2)2 or VF/(pi x(Ø/2)2) Total height of the bucket/filter cm 40 100
[25]



[edit] Nalgonda Technique

The Nalgonda technique is a means of fluoride removal that depends on the flocculation, sedimentation, and filtration of fluoride with the addition of aluminum sulfate and lime.[34] This technique was developed by the National Environmental Engineering Research Institute in India in 1975 in response to fluorosis concerns.
A possible home set up for implementing the Nalgonda technique.
[49] Aluminum sulfate (Al2(SO4)318H2O) is added to the water to acts as a flocculent. Though aluminum sulfate is commonly used in general water treatment as a flocculent, the amounts used in defluoridatoin are much higher (150 mg/mgF or 1000mg/L or 20 times normal).[24][50][34] As is typical with flocculation processes, the water must be thoroughly stirred to ensure dispersal of the flocculating agent [2]. Because the reaction results in an excess of H+ ions, Lime (Ca(OH)2) is added to the water during the process to help maintain a neutral pH and hasten the settling of the sediment.[24] The amount of lime added is typically 5% (by mass) of the aluminum sulfate added though some sources say significantly more (20-50% of alum by mass) should be added.[24][2] The chemical processes, though admittedly are not fully understood [50], can be seen below:

Al2(SO4)318H2O => 2Al + 3 SO4 + 18H2O
2AL + 6H2O => 2Al(OH)3 + 6H+
F- + Al(OH)3 => Al-F Complex +undefined product
6Ca(OH)2 + 12H+ => 6Ca2+ + 12H2O

Additionally, some of the fluoride is able to form precipitate with calcium.[51]Ca(OH)2 + 2F- => CaF2 +2OH-

In order to determine the amount of aluminum sulfate that should be added the following equation can be used.[2]

A=((Fr-Ft)xV)/(mxFt1/n)

Fr= The raw water fluoride concentration (mg/l)
Ft= The residual water fluoride concentration (mg/l)
V= The Volume of water treated (l)
m= The sorption capacity constant
n= The sorption intensity constant
A= The amount of Alum to be added (g)
(Under near neutral pH and target concentrations of 1.5 mg/L m=6 and n=1.33)[2]

After waiting for the sediment to settle, filtration is then needed to ensure that none of the created sediment is drank. [2] The actual amounts of aluminum and lime to be added can vary greatly depending on the initial alkalinity, fluoride concentration, and the quality of lime.[50] The amount of aluminum sulfate added must be carefully monitored as both left over aluminum can cause significant health problems including neurological, cardiovascular, and respiratory problems among others and must be kept under 0.2mg/l.[51] Although less serious, left over sulfates must also be monitored as they can cause poor tastes, and must be kept under under 400mg/L which means that no more than 800mg/l of aluminum sulfate should should initially be added to the water.[34]

One advantage of the Nalgonda technique it that is easily scalable and can be used from a household system of buckets to and large scale plant.[34]

Apparatus for using Nalgonda Technique on domestic or community level
Design for a Nalgonda community level plant in Tanzania

As with other defluoridation systems , the Nalgonda technique is often used as a POU treatment, and has been successfully used in many developing nations including, India, China, and Tanzania.[24] People are taught to add the correct amounts of alum and lime based on the fluoride and pH of the raw water being used and to quickly stir in the added chemicals for 1 minute , continue stirring slowly for 5 minutes, and then let it settle for 1 hour.[2]

The Nalgonda Technique has not been embraced on a large scale in the developing world. There are a number of serious drawbacks to the method. Though often considered to be a cheaper defluoridation method available, it is not necessarily cheaper everywhere, particularly where necessary materials are not readily available, such as East Africa.[16] Additionally there are limitations chemically as the Nalgonda Technique has been shown to not be sufficient for the treatment of water with a fluoride concentration greater than 10.0 mg/L. [34] Also the pH is very difficult to regulate with the addition of lime,[50] and sludge created needs to be properly disposed of.[2] Finally, the Nalgonda technique is more time intensive and requires more diligence than other defluoridation options and this is possibly the largest reason why it has yet to be embraced.


[edit] Other Defluoridation Options

[edit] Reverse Osmosis

Reverse osmosis is a technology that has been used more successfully in the developed world than the developing world. This process is achieved by applying high pressure to water against a semipermeable membrane that is capable of rejecting undesired ions from passing through. [51] A variation of this process is known as electrodialysis that relies on DC potential to remove specific ions. [23] In fact, reverse osmosis can be used to remove a variety of undesired quantities from the water depending on the nature of the membrane used.[51] Being a purely physical process, it eliminates many of the problems seen with other defluoridation techniques, like pH balancing and the need for regeneration.[51] Reverse Osmosis has been shown to successfully treat water with fluoride concentrations up to 12 mg/L. [7] Unfortunately, reverse osmosis has not been successfully implemented in the developing world for a number of reasons. The primary being that it is a very costly defluoridation option.[23] Additionally, reverse osmosis requires much electrical power to operate. [23] Also, 20-40% of water is lost in this treatment process, [23] possibly much more. Technological improvements in materials and larger scale operations may someday make this technology affordable to more people in need of defluoridation technology.[51]

[edit] Magnesium Oxide==

Testing of Defluoridating Filter Using MgO-CaO-CaCl2 For Use in Rural Rajasthan, Southern Brazilian Journal of Chemistry, Margandan, Agrawal, R; K; Singh, K; Acharya, R; Sharma, S; Qanungo, Kushal,Vol. 21, No. 21, 2013, p 79-95. http://www.sbjchem.he.com.br/jornal/revista2013.pdf

Field Testing of a Magnesium Oxide-Lime-Calcium Chloride Hydrochloric Acid Based filter, Margandan, Agrawal, R; K; Singh, K; Acharya, R; Sharma, S; Qanungo, Kushal, Ovidius University Annals of Chemistry Volume 24, Number 1, p 43-50, 2013. http://anale-chimie.univ-ovidius.ro/anale-chimie/chemistry/2013-1/pdf/9_karunanithi.pdf

[edit] Synthetic Materials

There are a number of strong base anion exchangers that are able to perform an ion exchange process to remove fluoride from water, typically exchanging for chloride ions.[51] Many of these are not specifically designed for use in removing fluoride, but rather all anions. Thus because they are not specifically designed for removing fluoride, their actually fluoride removal capacities are relatively low.[52] Though able to be regenerated with the use of chloride salts[51], they are not necessarily the most economical and available in areas where they are not produced, and thus, at this point, are not able to really be considered an alternative in the developing world.

[edit] Biological Options

Though relatively unknown, some success has been seen in using biological materials such as strains of fungi, bacteria, and algae.[53] The mechanism of fluoride uptake with these methods are largely not understood.

Additionally, studies have been performed by creating activated carbon from water Hyacinths (Eichhornia crassipes).[51] By charring these plants at 600oC fluoride capacities as high as 4.4 mg/g of carbon were seen.[51]

[edit] Solar distillation

Solar distillation can also be used to remove fluoride from drinking water and F concentration in distilled water will be within the permissible limit of 1.5 mg/L. However, this process mainly weather depended [54].

[edit] Alternatives to Defluoridation

Of course there are a number of ways to help reduce the occurrences of fluorosis other than just treating water. All of the water resource options available should be assessed economically, as it may be more affordable to find and use a different water source.[24] (Often times a water source should just be abandoned rather than treated, especially if it has exceptionally high levels of fluoride.) Rainwater often can provide a fluoride free water source [51] [55], though this isn’t universally agreed upon. Sometimes if multiple water sources are available, the source with lower fluoride concentrations can be used to dilute the fluoride concentration of another.[24]

Additionally, diet can play a significant role in the effects fluorosis has on the body. A diet that is high in protein, vitamin C, calcium can help reduce the effects of fluorosis.[56] That being said, milk consumption should be encouraged.[57] On the other side, general malnourishment generally exacerbates the harmful effects of fluoride[56]and a diet that is high in silicon, a mineral critical in bone mineralization, also has been shown to make the effects of fluorosis worse. [56]


[edit] Evaluation of Defluoridation Treatment Types

There are a number of different defluoridation options, none of which emerge as being more appropriate in all situations. Thus many factors must be taken into account when deciding on what defluoridation option to use. A number of questions should be asked when selecting a technology, including:

Economic Factors
What is affordable to those using this defluoridation method?
Are the proper supply chains in place to ensure that people will always have access to the materials needed for this defluoridation method?
Social Factors
Is this defluoridation methodology simple and easy to use?
Does this technology require any special know-how, skills, or tools?
Are people willing to use this defluoridation method?
Is the water from this treatment method acceptable to drink?
Technical Factors
Does this techology have the fluoride removal capacity necessary for the region's level of fluoride?
Is this technology selective for fluoride/does the water have characteristics that can interfere with the uptake of fluoride?
Does this technology require frequent replacement,recharge, or repair?
Environmental Factors
Does this defluoridation technology have any undesirable by products?
What resources does this technology consume?


An Overall Comparison of Fluoride Removal Techniques for Use in the Developing World (+=PRO, -=CON)[25]

Activated Alumina Bone Char Calcined Clay Contact Precip. Nalgonda
No daily dosage of chemicals (i.e. no daily working load) + + + - -
Dosage designed for Fluoride Conc. independent of unit or plant - - - + +
No risk of false treatment due to breakpoint - - - + +
Removal capacity of medium is independent of Fluoride conc. - - - + -
No regeneration or renewal of medium is required - - - + +
High removal efficiency can be ensured + + -- + -
Easy to construct, even by users + + + + ++
Construction materials are cheap and widely available + + + + ++
Can be sized for one or several families or e.g. a school + + - ++ +
No risk of medium/chemicals unacceptability + - - -/+ +
No risk of deterioration of original water quality -/+ -/+ - + -/+



Additionally, the World Health Organization created the following flowchart to aid in the decision making process[2]:

Fluorideflowchart.jpg

[edit] Sources

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