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Subsurface flow constructed wetland for greywater
Greywater, also spelled as graywater, grey water or gray water, is all of the effluent water from a household, except that from the toilets, which is sometimes called "blackwater". In some places the effluent from kitchen sinks is not considered greywater as well due to its high BOD. In addition, if greywater is held for too long (>24 hours in temperate climates), it will become blackwater.
The water leaving our homes carries nutrients and value. It may also contain pathogens, and/or harsh chemicals and care should be taken with it. Redirecting the water we use for tasks such as showering allows us to reclaim some of that value to grow plants and recharge the water table.
As the name implies a subsurface flow constructed wetlands means the greywater flows below the media in the wetlands. Generally a subsurface flow wetlands is a "wicking" type system; in the container cell there is generally a base layer of large stone (4"+), a layer of gravel (3/8"-3/4"), a filter fabric, a layer of sand (the wick), topped off by a layer of growing media filled with plants. Over time a biological film develops on the gravel sand filtration and roots of the plants within the system; this in conjunction with the uptake of the plants is what provides treatment of the greywater. Once passed through the system the water generally emerges clear and can be used for irrigation, toilet flushing, or other non-potable uses.
The plumbing system is very dependent on your needs and existing plumbing system. Many possibilities exist, such as just capturing the plumbing from the bathroom sink. Another good option for capturing a lot of your water is dual plumbing to capture the greywater but allow for a valve to run greywater to sewer if needed.
Standards require that any pipe diameter less than or equal to 2” have a slope of 1/4” fall/foot run. And any pipe diameter greater than or equal to 3” have a slope of 1/8” fall/foot run.[verification needed]
The surge tank is some volume to accommodate for high flow, such as times when everyone is showering and washing at once. This is necessary because the back pressure of a subsurface system is relatively high and, therefore, will slow down the flow of water. The piping to the greywater system will provide some surge capacitance, but you risk backing up into the house. You do not want to risk backing up into the house.
Primary treatment of filtering solids and trapping grease is important in maintaining an efficient subsurface flow, constructed wetland for treating greywater. It is possible to combine the filter, grease trap and settling tank into one system. Each part serves a different purpose and has different maintenance needs.
- A filter keeps chunks of organic matter out of the constructed wetland by directly filtering it out.
- These organic chunks can clog the wetland, if allowed to enter. In addition they have very a high BOD.
- Filters often need to be cleaned weekly or as needed. Build the filter so that it is easy to remove.
Old filter (second generation) from the AEF greywater system. The cloth filtered effectively, but the material did lasted less than one year.
Filter (third generation) from the AEF greywater system. The flat metal surface is easy to pick up and clean, but can spill filtered solids.
Filter from the Greywater Ecuador La lagrima purificadora system.
Filter from the Millcreek greywater system. This depressed metal filter is easy to clean and does not spill easily, but can clog quickly.
- A grease trap keeps grease and fat from getting into the constructed wetland, by providing a barrier to floating material.
- Grease and fat can coat surfaces in the constructed wetland and greatly reduce the treatment capacity of the system, if allowed into the wetland.
- The grease trap should be skimmed weekly or as needed. It will start to look nasty quickly.
- A settling tank allows for smaller particles to settle out before entering the wetland, by providing some time of slow water velocity.
- These smaller particles can slowly clog the entrance of the system, if allowed to pass through the pretreatment.
- The settling tank should be inspected weekly and cleaned every few months or as needed.
Grease trap and settling tank from the AEF greywater system. The upside down 5-gallon bucket prevents the grease from passing into the out-to-wetland pipe hidden beneath the bucket. The cut down 55 gallon drum yields some surge protection and retention time for settling.
Grease trap and settling tank from the Greywater Ecuador La lagrima purificadora system. The wood has a open gap at the bottom of the tank preventing grease and fat from passing to the second compartment. The 36 gallon brick and cement basin yields some surge protection and retention time for settling.
Grease trap and settling tank from the Millcreek greywater system. The small plastic cap provides a barrier to the floating grease and fat from passing into the out pipe. The trash can yields some surge protection and retention time for settling.
Subsurface constructed wetland
Greywater is discharged into the subsurface constructed wetland where the water will be filtered by biological processes. The biological processes include the plants in the wetland, the bacteria that live on and around the vegetation and from the decomposition of the settled solids.
The main compartment consists of gravel and sand, with the vegetation growing on the top portion of mulch or rich soil. The greywater flows through the system beneath the soil surface, which eliminates the risk for standing pools and mosquito breeding. Water would flow from the source, into the gravel level of the wetlands, the water passes through the wetland slowly with the water exiting at the same level as it comes in. The effluent is then fed by gravity to the ground where it is used for irrigation, surface water or groundwater supply.
There are many different designs for subsurface constructed wetlands. Some examples are provided below:
- The surface area of the wetland should be sufficient to treat the greywater to the desired water quality level. The level of treatment should be based on the intended final use of the now treated water. See System Sizing for more information and several methods for calculating.
- The pond liner protects the groundwater from contamination and may not be necessary in locations with low percolation, clay soils.
- The baffles can help protect the system from short circuiting, i.e. water finding a quick route from the entrance to the exhaust.
- The gravel makes sure that the system remains subsurface, providing more treatment area (on the surface of the rocks), reducing the attraction to vectors and protecting from accidental contact with humans.
- The plants should be local wetland plants. Such as:
- Brass Button
- Broadleaf Cattail
- Common Mare’s-tail
- Common Rush
- Marsh Pennywort
- Marsh Rosemary
- Umbrella sedge
- Water Parsley
- Water Plantain
To find local wetland plants. Seek out low lying wet areas. In dry climates this may be near a factory, business or residence with a semi-constant effluent stream. In addition, see Emergent plants for constructed wetlands for more.
The outtakes from wetlands can vary depending on the final use. The wetland can feed into a french drain (trench covered with gravel or rock) irrigation system, supply an outlet box, or feed a pond. Select countries and states have rules and regulations pertaining to the use of greywater. It is important to look at the requirements for your area to determine the type of system that will work.
Many variables effect the sizing of a constructed wetland greywater treatment system. For instance, high temperatures make for faster processing, high BOD needs longer retention time, surface use of effluent needs longer retention time, etc. Likewise, there are many ways to size a system. In the opinion of Lonny, sizing a system based upon retention time is misleading, as treatment effectiveness varies with type of system and environment. Instead rules of thumb can be used. Alternatively, BOD and hydraulic loading calculations can be used much as they are to size municipal wastewater treatment marshes. This method can be adapted to small scale greywater.
Rules of Thumb
Some simple rules of thumb can make system sizing a very brief procedure.
- According to Oasis Designs: 1/2 to 1 square foot of subsurface wetland for every gallon of greywater per day
- According to the Humanure Handbook 1 of Constructed Wetland for every 1 gallon of graywater per day.
To make these rules of thumbs match - Humanure handbook must be referring to a maximum two foot depth.
So, for a system for 150 gallons/day of greywater would need a constructed wetland system with a 75 to 150 square foot footprint.
Simple BOD loading method
Steps to determining sizing based upon BOD loading:
- Determine BOD of influent (mg/l convert to lb/gal)
- Reduce that BOD for primary treatment (%)
- Determine greywater daily flow rate (gal/day)
- Use wetland treatment ability (45 lb/acre*day) 
- Calculate the size of your system.
So, for a system with 150 gallons per day of greywater at 100 mg/l BOD and a primary treatment BOD reduction of 40%:
- BOD * reduction percent for primary treatment
- 100 mg/l * .6 = 60 mg/l
- BOD in mg/l converted to lb/gal
- 60 mg/l * 8.35x10-6 (l*lb)/(mg*gal) = 5.01x10-4 lb/gal
- Flow rate * BOD / Treatment
- [150 gal/day * 5.01x10-4 lb/gal] / [45 lb/(acre*day)] = .001669 acre
- Convert acre to square feet
- .001669 acre * 43,560 ft2/acre = 72.7 square feet
The fact that this number is lower than the 75 square feet calculated by the rules of thumb, is probably due to primary treatment that is not included in the Oasis Design figures.
BOD loading (more rigorous) method
The design and sizing of a subsurface constructed wetland is based on reduction of the biochemical oxygen demand (BOD). BOD is a water quality parameter that measures the amount of oxygen required for biochemical activity (e.g., aerobic bacteria) that is present in water to function. High levels of BOD in a system can decrease the available oxygen and be harmful to aquatic life . The average BOD coming into the greywater wetland has been measured as 65 mg/L . The final BOD in a natural system generally ranges from 2 to 7 mg/L .
BOD removal rates can be calculated based on the reaction rate constant for the appropriate temperature using the equation below :
- kr = reaction rate for a specific temperature
- k20 = reaction rate at 20 °C
- T = lowest temperature in wetland system (°C)
The reaction rate constant varies bases on the system and the bacteria present. A larger k value indicates faster decomposition of BOD . For the for the wetland design a k20 value of 1.1 day-1 was used for the reaction rate constant . The area required for the desired BOD reduction can be calculated based on the detention time, which is a log order removal based on the reaction rate constant, and includes the flow, depth and the porosity of media.
- A = Area (m)
- L = Length (m)
- W = Width (m)
- Q = Flow (m3/day)
- Co = Initial concentration (mg/L)
- Cc = Desired effluent concentration (mg/L)
- d = Depth of gravel media (m)
- n = Porosity of media (-)
BOD loading calculator
An excel Residential Subsurface Constructed Wetland Calculator has been designed to size a wetland based on BOD loading, and California weather. The calculator can be used for residence outside California, however information about irrigation water needs will be irrelevant.
To download the calculator click here, the file must be saved and enabled before use.
- Do not use an in sink food disposal (aka garbage disposal)
- Use Garden friendly soaps (no borax, etc.)
Interpretive sign for old CCAT system
- see this doc to find a value
- Metcalf and Eddy, 2003, Wastewater Engineering Treatment and Reuse, McGraw Hill, New York, NY
- Casanova, L.M., V.Little, R.J.Frye, and C.P.Gerba 2001. A survey of the microbial quality of recycled household graywater. Journal of the American Water Resources Association (JAWRA) 37(5)
- Crites, Ronald, and Techobanoglous, George, 1998, "Small and Decentralized Wastewater Management Systems", Water Resources and Environmental Engineering
- EPA, 1993, Subsurface Flow Constructed Wetlands for Wastewater Treatment: A Technology Assessment, Office of Water, 832-R-93-008