Greywater Wetlands 1 2d2e6b

Design Manual: Greywater Biofiltration Constructed Wetland System Dayna Yocum, Bren School of Environmental Science and Management, University of California, Santa Barbara Overview A greywater biofiltration system is a constructed wetland that removes a significant amount of pollutants from greywater before it flows into the groundwater, river, or natural wetland. Addition of pathogens, bacteria, and non-biodegradable toxins to the surface water can be avoided with this biological treatment, to promote a healthier ecosystem and more sanitary conditions. The system can be built for a single household or a group of households, typically at a low cost. Greywater is the wastewater produced from sinks, baths, or clothes-washing; it does not include toilet water, which contains many more pathogens and bacteria. Typically greywater does contain nitrate, phosphate, soaps, salt, bacteria, bleach, foam, food particles, organic matter, suspended solids, perfumes and dye. Additions of grey water to surface water bodies can cause pH imbalances, increased oxygen demand and increased turbidity.

Figure 1. A typical subsurface flow greywater wetland system

How it works Figure 1 is a horizontal diagram of a typical subsurface flow wetland system. Water flows from the house or other greywater-producing system into the gravel level of the treatment wetland. The greywater es through the wetland slowly, and cleaner water exits the system at the same level as it entered. A hose or pipe lowers the water to the ground, and the water flows to surface water with gravity, preferably through a vegetated pathway. Water that is discharged into a greywater wetland biofiltration system will be filtered through both mechanical and biological processes by both the plants in the system and the microbes that live around the plant roots. In subsurface flow wetlands, the greywater flows through the system beneath the soil surface, which eliminates the risk for standing pools and mosquito breeding. The system consists of a thin layer (5cm) of sand topped by a thick layer (45-75cm) of small-medium sized gravel, with a thin layer (5cm) of mulch or rich organic soil on top. Wetland plants (cattails, reeds, etc) are planted in the topsoil and roots grow into the gravel substrate. Figure 2 shows a cross-section of a wetland cell.

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Figure 2. Greywater wetland cell cross-section

Greywater enters the wetland by gravity and is first filtered by mechanical processes - the suspended solids settle into the substrate as the water moves through the soil and plants. Wetland plants transfer oxygen to the submerged root zone, which allows for the biological breakdown of pollutants and organic materials by microbes (Figure 3). Removal rate varies, but usually the wetland is able to take up a good of the polluting ingredients from greywater. Table 1 shows observed BOD removal rates for wetlands around North America, and in India (Crites and Tchobanoglous 1998, Tayade et al 2005). Effluent from a completed system should be monitored to determine approximate removal rates.

Figure 3. Contaminant Removal Mechanisms in a Constructed Wetland (Eifert 2002)

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Table 1. BOD Removal rates for Subsurface Flow Wetlands. Location

Vegetation Type

Benton, Kentucky Mesquite, NV Sydney, Australia Santee, CA Mumbai

Type of effluent Pretreatment

Detention Time

BOD, mg/L

Oxidation Pond

5

65%

Oxidation pond

3.3

68%

Watson et al 1989 Crites and Tchobanoglous 1998

7 6

86% 88%

Bavor et al 1987 Gersberg et al 1985

N/A

85%

Tayade et al 2005

Secondary Primary cattails and grasses

None

Source

Figure 4. In a subsurface flow wetland, water is below ground and flows through soil or gravel. Roots penetrate into the gravel medium (University of Florida IFAS Extension)

The size of a constructed wetland depends on the amount of effluent entering it and the amount of Biochemical Oxygen Demand that needs to be reduced. As a general rule, 1 cubic meter of wetland can process about 135 liters of greywater (Jenkins 2005). To determine a more precise size for larger systems, Crites and Tchobanoglous propose completing a series of calculations to determine the size of the wetland, which is explained below (Crites and Tchobanoglous 1998). The size matrix in Table 2 gives a general idea of size ranges when considering different amounts of discharge levels.

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Table 2. Size Matrix for Greywater Wetlands, varying depth of medium, size of contributing discharge, and reaction rate. Calculations based on equations presented in Crites and Tchobanoglous (1998) Description

Influent into wetland 3 (m /day = 1000L/day)

BOD level of influent (mg/L)

Desired BOD of effluent (mg/L)

Days in Construc ted Wetland

Depth of Medium (m)

Width (m)

Lengt h (m)

Total Area 2 (m )

Single household system: assumes a contribution of 240L/family/week for 1 family, with a conservative reaction rate of 1.1 and average lowest temperature of 3°C Multiple household system: assumes a contribution of 240L/family/week for 5 families, with a conservative reaction rate of 1.1 and average lowest temperature of 3°C Small community system: assumes a contribution of 240L/family/week for 20 families, with a conservative reaction rate of 1.1 and average lowest temperature of 3°C

0.03

33

5

4.62

0.50

0.40

1.99

0.79

0.17

33

5

4.62

0.50

0.89

4.45

3.96

0.69

33

5

4.62

0.70

1.68

6.73

11.31

Medium community system: assumes a contribution of 240L/family/week for 200 families, with a conservative reaction rate of 1.1 and average lowest temperature of 3°

6.86

33

5

4.62

0.70

5.32

21.27

113.1 4

13.72

33

5

4.62

0.70

7.52

30.09

226.2 8

0.69

33

5

2.54

0.50

1.48

5.90

8.71

6.86

33

5

2.54

0.50

4.67

18.67

87.12

13.72

33

5

2.54

0.50

6.60

26.40

174.2 3

Large community system: assumes a contribution of 240L/family/week for 400 families, with a conservative reaction rate of 1.1 and average lowest temperature of 3° Small community system: assumes a contribution of 240L/family/week for 20 families, with a semiconservative reaction rate of 2.0 and average lowest temperature of 3°C Medium community system: assumes a contribution of 240L/family/week for 200 families, with a semiconservative reaction rate of 2.0 and average lowest temperature of 3°C Large community system: assumes a contribution of 240L/family/week for 400 families, with a semiconservative reaction rate of 2.0 and average lowest temperature of 3°

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To treat small amounts of greywater (a few liters per day), an alternative to a constructed wetland is to simply use the greywater to water non-food-producing houseplants. This can be accomplished either by connecting a tube or hose to the outlet of the household sink and guiding the stream into the plant pots, or by collecting the water in a bucket and using the water to pour overtop the plants. Most plants have a high tolerance for greywater. If this option is chosen, it should not be used to water food-producing plants because there may be traces of fecal coliform in the greywater if water from diaper-washing or hand washing after using the bathroom is used.

Figure 5. A household greywater irrigation system

Applicable Situations A greywater treatment wetland is appropriate for use wherever a large quantity of greywater is being released. For instance, water from a sink, laundry area, shower, or bath can easily be diverted into the wetland system before it enters the river through a pipe or series of pipes that extend out from the house or clothes-washing facility. The diagrams below show a pipe running from a house (left) and a pipe running from a community clothes-washing station into a treatment wetland (right). The complete design for the ecological clothes-washing station can be found in Appendix A of “Design and Implementation of Sustainable Water Resources Programs in San Cristóbal de las Casas, Mexico.”

Figure 6. A design for a household treatment system (left) and a ecological community clotheswashing facility treatment system (right)

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Criteria to consider before electing a constructed wetland as a greywater treatment facility: • Water must be available year-round to keep the plants and bacteria alive • Large flows (caused by torrential rainfall) can overwhelm the system, and it must be drained following a large rain event until water is below the soil surface • Greywater should flow naturally via gravity into the wetland or plants • Water should remain in the system for an average of 2-10 (Jenkins 2005; Crites and Tchobanoglous 1998) days to allow treatment by plants • Greywater should not be allowed to pool • Plants from a local natural wetland can be transplanted for use in the constructed wetland (recommended), or can be purchased from a nursery • A containing wall or impermeable layer should surround the entire wetland to prevent greywater from flowing out before it is fully treated. An outflow device will enable water to exit the system after treatment Purpose A greywater filtration system can serve the following functions: • Provide a sanitary way to dispose of household greywater • Prevent bad odors from pooling stagnant greywater • Prevent eutrophication (nutrient overload) of surface waters • Prevent the contamination of groundwater and surface water Siting/Location In order to decide on a location for the greywater treatment system, consider the following: • A greywater filtration system should be directly on the receiving end of an effluent flow • Full sun is ideal for a wetland filtration system • A downhill slope of about 0.5% (Crites and Tchobanoglous 1998) is recommended for subsurface flow wetland systems. Water can then flow through the soil, water, and plant medium by gravity; after it has traveled the full length of the wetland, it can be released into an open field for infiltration, or if the load has been sufficiently reduced it can discharge into surface water. Look for a site that already has a similar slope to minimize alteration • Land use and access should be considered. Be sure that it is convenient to walk to the site and that there is adequate space to perform any necessary maintenance • Be sure that the landowner s the greywater system on his property, or that the entire community is in of a system built in a public area • Do not construct the greywater treatment system in a pre-existing wetland • Discharge permits may still be required to return the treated water to a water body Sizing In order to determine the size of a large biological filtration system, one must first know the minimum air temperature of the proposed site (°C), amount of BOD currently produced, and the target BOD value for water exiting the system. One can try the calculation with depth varying from 55 to 85 centimeters to find an appropriate size. For instance, if there is a restraint on field area available for the constructed wetland, a depth of 85 cm will minimize the footprint of the system. These calculations are based on BOD removal, but can be adapted for nitrate removal by modifying the factors in the calculation of the reaction rate constant. Typically, however, the nitrogen levels in greywater are much less than in blackwater, and BOD is the primary parameter that should be targeted for removal.

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The following approach for calculation constructed wetland cell size has been adapted from Crites’ and Tchobanoglous’ book, Small Decentralized Wastewater Treatment Systems. An application of this calculation can be found in the Ecological Clothes-Washing Station Design Manual in the report from which this document was extracted: “Design and Implementation of Sustainable Water Resources Programs in San Cristóbal de las Casas, Mexico.” Determine the minimum monthly average ambient temperature, T (°C), that the system will work at (e.g. average for January). Calculate the reaction rate constant, kT (day-1) for BOD at the appropriate temperature using the following equation. The reaction rate constant at 20°C (k20) varies depending on the system. A range of values have been used in textbooks guiding design of subsurface flow wetlands. A larger k value indicates faster decomposition of BOD. Crites and Tchobanoglous (1998), a wellestablish source, estimate a k20 of 1.1 day-1, while Tchobanoglous and Burton (1991) estimate a k20 of 1.35 day-1 for black water treatment wetlands. A study in Sweden (Olsen et al 1967) demonstrated that the reaction rate for greywater wetlands was 4.5 times higher than the black water reaction rate due to the more abundant availability of unprocessed organic matter. These values are based on the performance of the wetland, and cannot be accurately obtained until the system is built and monitored. It is recommended to use a conservative (low) value for this figure because much of the treatment depends on the activity of the microorganisms in the wetland, which cannot be determined before construction. More research is needed and encouraged to more accurately characterize the reaction rate and ideal design parameters.

k r = k 20 (1.06(T −20) )

[Equation 1]

Calculate the detention time t (day), the time the water should remain in the system in order to reach desired BOD level with the equation

t=

[Equation 2]

− ln(C / C o ) , kr

where Co is the BOD concentration of the water entering the system (mg/L = g/m3) and C is the desired BOD concentration of the water (mg/L = g/m3) exiting the system, or the goal. Estimates of typical BOD values of runoff water are shown in Table 2. A reasonable goal is from 3-7 mg/L, as a treatment wetland can decrease levels of BOD, but cannot eliminate it. Table 3. Estimated BOD mean concentration for non point source loading from various land uses (Benaman 1996) Land High Barre use Densit Residentia Agricultura Open/Pastur Fores Wetland Wate categor y l l e t s r n y Urban BOD (mg/L)

9

15

4

6

6

6

0

13

Check the organic loading rate, Lorg (g BOD/m2-day), using the equation below. This number will indicate the mass of BOD per area per day that the system is expected to receive. As a general

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rule, the organic loading rate should not exceed 11.2 g BOD/m2-day. This threshold will not be exceeded in practice with influent applied up to 5 cm per day. Almost all greywater systems will have an organic loading rate below this threshold.

[Equation 3]

Lorg =

(C )(d w )(η ) t

Again, C is the BOD (mg/L = g/m3) of the influent water, dw (m) is the depth of the medium, which typically can be from 0.4 m to 0.85 m. The deeper the medium the more load the system can process, but if the medium is too deep, conditions at the bottom become anaerobic and may result in incomplete removal of the BOD and nutrients. Use the detention time calculated above in Equation 2 (t). The effective porosity of the medium,η, is defined as the proportion of the non-solid volume to the total volume of material, dimensionless, and can be determined from Table 3 according to the size of gravel chosen. Table 4. Typical values of constructed wetland mediums (Crites and Tchobanoglous 1998). *d10 is the diameter of a particle in a weight distribution of particles that is smaller than all but 10% of the particles Effective Effective size Medium porosity d10*, mm η Medium sand 1 0.3 0.32 Coarse sand 2 0.35 Gravelly sand 8 0.4 Medium gravel 32 Coarse gravel 128 0.45

Determine the necessary field area for the subsurface flow bed (m2),

[Equation 4]

As =

(Qave )(t ) (η )(d w )

where Qave is average daily flow through the wetland (m3/day), t is the detention time calculated above (day), and dw is the depth of the medium (m). Use the same value for η determined for Equation 3. For larger systems, it may be helpful to convert the area to hectares using the conversion of 1 hectare = 10,000m2. And finally, to calculate the dimensions of the treatment wetland (m), use the following expression: 1/ 2

[Equation 5]

A  w =  s   RA 

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,

where w equals width (m), As is the area of the wetland (m2), and RA is the aspect ratio, as length/width. For subsurface flow wetlands, Crites and Tchobanoglous (1998) recommend that the aspect ratio is between 2:1 and 4:1, but Bounds et al. (1998) found no significant difference of nutrient and BOD removal in three 25m2 reedbeds treating domestic effluent with aspect ratios ranging from 4:1, 10:1, and 30:1 over a two year period (qtd. in Dallas 2005). The length, l, of the constructed wetland (m) can be calculated by the expression:

[Equation 6]

l=

As w

.

Materials Greywater wetlands can be built above ground or below ground within a cell of concrete blocks. The size of the cell will affect the cost of the system. Materials Cement Concrete blocks PVC or metal piping (inlet) PVC or metal piping (outlet) Fine plastic mesh Impermeable liner Valve (to release and retain water) Sand Gravel Mulch Vegetation (transplants from a nearby wetland)

Vegetation All types of plants act on the pollutants in the same way - by penetrating the soil and gravel and transporting oxygen deeper than it would naturally travel through water and soil alone. All plants can use the nutrients and BOD in the wastewater to some extent. However, relatively few plants thrive in the high-nutrient, high-BOD waters of treatment wetlands (Mitch and Gosselink 2000). There are a few plants that are most frequently used for greywater biofiltration wetlands, many of which can be found in natural wetlands. Wetland plants found close to the constructed wetland area are the most beneficial because they are already accustomed to the local climate. If these plants cannot be found locally, any wetland plants that grow well can be used. Figure 6 shows the common wetland plants described below. • Cattails (Typha spp.) are hardy, easy to propagate, and capable of producing a large annual biomass. Typically they remove large amounts of nitrate and phosphate. • Bulrushes (Schoenoplectus spp., Scirpus spp.) grow in clumps and grow well in water 5 cm to 3 m deep. This aggressive plant achieves a high pollutant removal. • Reed Grasses (Phragmites australis) are tall plants with deep roots, enabling more oxygen to reach the root zone than the above two plants

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Figure 7. (Left to Right) Cattails, Bulrushes, and Common Reed Grass

Construction Steps and Design Considerations The constructed wetland should be built with the following generalized steps. Many different materials can be used to construct the treatment wetland, so local designs can vary. 1. Identify a location for the wetland at the end of the greywater stream. 2. Calculate the size of the constructed wetland you will build (see above). 3. Grade the land so the water will flow downhill at a 0.5% slope (Figure 7).

0.5% slope

Figure 8. The slope of the wetland flood should be about 0.5%

4. Construct the wetland cell above ground with cement blocks and cement, or another impermeable material, allowing space to connect the greywater stream to the wetland cell via the inlet as specified in step 6. Alternatively, the wetland can be constructed into the ground using an impermeable liner. This has the disadvantage of not being able to drain the cell. Either way, the cell must be impermeable, as cracks or holes in the liner may contaminate the groundwater. 5. Integrate a drain valve into the bottom of the downgradient side of the cell. This valve will serve to lower the water level to encourage deeper plant root growth. 6. Incorporate the greywater inlet. Greywater should be distributed evenly into the inlet area to promote even infiltration around the mouth of the wetland. For smaller wetland systems, a perforated pipe of a series of pipes can serve this purpose. For larger wetland systems, gated pipes, slotted pipes or troughs with V-notch weirs can distribute

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the water along a wide inlet. Put a mesh screen over the opening to deter clogging ( Figure 9).

Figure 9. Illustration of Mesh Filters on the Outlet

7. Add the outlet valve at the downgradient end of the cell. The outlet height should be equal to the height of the inlet to retain water in the cell. Put a mesh screen over the opening to deter clogging. 8. Apply a layer of sand 5 cm thick to the bottom of the cell. 9. Place gravel on top of it. Gravel size close to the inlet should be about 5cm; this reduces the risk of clogging as larger pieces of the suspended solids settle in this area. Throughout the rest of the system, gravel should be of a uniform size between 0.5 and 3 cm. Apply a 45 to 75 cm layer of gravel. The depth of gravel will vary according to the size calculations. 10. Cover the gravel with 5 cm of mulch or rich top soil. 11. Transplant wetland plants from a local natural wetland (recommended) or nursery. When using plants from another wetland, the entire plant (leaves, stem, roots, and growing shoot) plus some soil should be transplanted. Simply pull out the plant from a neighboring wetland or pond by the base, being careful not to break the root. The root end should be placed about 5cm below the surface. Above-water stems can be cut to about 20 cm tall. Cattails should be placed 0.25-1 m apart; reeds and bulrush can be planted 15 cm apart (Mitch and Gosselink 2000). It is important to eventually acquire a deep consistent root zone by lowering the water level gradually to encourage deeper root growth. 12. Saturate the soil with water just up to the surface (no farther) and allow it to evaporate slowly. Keep the soil moist during the entire propagation period. As the plants grow, allow the water level to decrease gradually, encouraging the plants roots to grow deeper into the wetland cell. Vegetation should be allowed to become established before wastewater applications begin, about 2-3 months. If wastewater must be introduced sooner, those plants that die due to shock can be replaced. 13. Effluent from the wetland cell should be lowered to the ground with a hose or tube and allowed to infiltrate into a vegetated area or flow into the surface waters along a planted or rock-filled pathway. This pathway encourages the slowly-draining effluent to seep into the ground en route to the surface water, thus potentially gaining the additional advantage of groundwater treatment before it flows into the surface water via groundwater flow. Since the effluent water is not potable, it is important that the water

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does not splash as it hits the ground, as residual pollutants could cause sicknesses if ingested. It also reduces erosion of the receiving area. Maintenance Greywater wetlands require limited maintenance. • Water depth adjustment to encourage plant root growth: The water level should be kept below the mulch layer at all times. This will be naturally regulated by the inlet-outlet system if it is built at the correct height. During initial plant growth, the drain can serve to lower the water level to encourage deeper plant root penetration into the gravel medium. Eventually the plant roots should extend to the bottom of the media. • Vegetation: Greywater is not toxic to plants, so the vegetation will thrive in this nutrientrich environment. It is not necessary to harvest the wetland plants, however, if plants are very wilted even with sufficient water they may suffer from a build up of pollutants and should be replaced. If many plants wilt, they should be replaced with other plants. • Periodic cleaning: The mesh on the inlet and outlet should be cleaned out when the flow is lower than usual to prevent further clogging. • Water monitoring: it is recommended that periodic monitoring of nutrient and BOD levels take place to estimate removal and identify potential problems. A local laboratory or research institution may be able to aid in organizing a monitoring program. Potential Limitations • Clogging: Some constructed wetlands are susceptible to clogging due to sediments getting into the pipes and preventing flow. This can be prevented by installing the recommended screen or trap for large solids at the pipe inlet. • Invasive species: It is important to not introduce wetlands species that are purchased for the constructed wetlands at nurseries into natural wetlands. Some species are more aggressive than others and can dominate a natural wetland by killing native species. It is important to avoid this in order to maintain natural species diversity. • Overflow during a storm may cause solids that previously settled to re-suspend and be released into the surface waters. This should be avoided by leaving the outlet open at all times, allowing the wetland to maintain its water level naturally.

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References Benaman, Jennifer, Neal Armstrong, and David Maidment. Modeling of Dissolved Oxygen in the Houston Ship Channel Using Wasp5 and Geographic Information Systems. Austin, Texas: Center for Research in Water Resources, The University of Texas at Austin, 1996. "Cattail Image." Ed. cattail.gif: Purdue University Agricultural and Biological Engineering. Bavor, H.F, D.J. Roser, and S.A. McKersie. "Nutrient Removal Using Shallow Lagood-Solid Matrix Macrophyte Systems." Aquatic Plants for Water Treatment and Resource Recovery. Eds. K.R. Reddy and W.H. Smith. Orlando, FL: Magnolia Publishing, 1987. 228-36. Bounds, H.C., et al. "Effects of Length-Width Ratio and Stress on Rock-Plant Filter Operation." The Small Flows Journal 4.1 (1998): 4-14. "Bulrush Image." Ed. bulrush.jpg: Traders Creek. Crites, Ronald, and George Tchobanoglous. "Small and Decentralized Wastewater Management Systems." Water Resources and Environmental Engineering (1998). Dallas, Stewart C. "Reedbeds for the Treatment of Greywater as an Application of Ecological Sanitation in Rural Costa Rica, Central America." Murdoch University, Western Australia, 2005. Eifert, W. "Applications of Constructed Wetland Treatment Technology." Proceedings of the Brownfields 2002 Conference (2002). Gersberg, R.M., et al. "Role of Aquatic Plants in Wastewater Treatment by Artificial Wetlands." Water Research 20 (1985): 363-67. Jenkins, Joseph. Humanure Handbook. Chelsea Green Publishing, 2005. Mitsch, William J., and James G. Gosselink. Wetlands. 3rd Edition ed. New York: John Wiley and Sons, Inc, 2000. Olson, E. et al. "Residential Wastewater." The Swedish National Institute for Building Research, 1967. Tayade, Sandeep T., et al. "Feasibility Study of Constructed Wetland for Treatment of Municipal Wastewater." The Global Directory of Environmental Technology, ECO Services International, 2005. Torbé, Małgorzata. "Common Reed Image." Ed. z_0171n.jpg. Tchobanoglous, George, and Franklin L. Burton, eds. Wastewater Engineering: Treatment, Disposal, and Reuse. McGraw-Hill, Inc., 1991. University of Florida IFAS Extension . "Subsurface Flow Wetland Diagram." Ed. 296682164.jpg: University of Florida IFAS Extension. Watson, J.T., et al. "Performance Expectations and Loading Rates for Constructed Wetlands." Constructed Wetlands for Wastewater Treatment. Ed. D.A. Hammer. Chelsea, MI: Lewis Publishers, 1989. 319-51.

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