greywater wetlands 1

8/2/2019 Greywater Wetlands 1

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Design Manual: Greywater BiofiltrationConstructed Wetland System

Dayna Yocum, Bren School of Environmental Science and Management, University ofCalifornia, Santa Barbara

OverviewA greywater biofiltration system is a constructed wetland that removes a significant amount ofpollutants from greywater before it flows into the groundwater, river, or natural wetland. Additionof pathogens, bacteria, and non-biodegradable toxins to the surface water can be avoided withthis biological treatment, to promote a healthier ecosystem and more sanitary conditions. Thesystem 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 includetoilet water, which contains many more pathogens and bacteria. Typically greywater doescontain 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 causepH imbalances, increased oxygen demand and increased turbidity.

Figure 1. A typical subsurface flow greywater wetland system

How it worksFigure 1 is a horizontal diagram of a typical subsurface flow wetland system. Water flows fromthe house or other greywater-producing system into the gravel level of the treatment wetland.The greywater passes through the wetland slowly, and cleaner water exits the system at thesame level as it entered. A hose or pipe lowers the water to the ground, and the water flows tosurface water with gravity, preferably through a vegetated pathway.

Water that is discharged into a greywater wetland biofiltration system will be filtered throughboth 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 thesystem beneath the soil surface, which eliminates the risk for standing pools and mosquitobreeding. The system consists of a thin layer (5cm) of sand topped by a thick layer (45-75cm) ofsmall-medium sized gravel, with a thin layer (5cm) of mulch or rich organic soil on top. Wetlandplants (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 biologicalbreakdown of pollutants and organic materials by microbes (Figure 3). Removal rate varies, butusually the wetland is able to take up a good of the polluting ingredients from greywater. Table 1shows observed BOD removal rates for wetlands around North America, and in India (Critesand Tchobanoglous 1998, Tayade et al 2005). Effluent from a completed system should bemonitored to determine approximate removal rates.

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


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Table 1. BOD Removal rates for Subsurface Flow Wetlands.

LocationVegetation

TypeType of effluent

PretreatmentDetention

TimeBOD,mg/L

Source

Benton,

Kentucky Oxidation Pond 5 65% Watson et al 1989

Mesquite,NV Oxidation pond 3.3 68%

Crites andTchobanoglous1998

Sydney,Australia Secondary 7 86% Bavor et al 1987

Santee, CA Primary 6 88% Gersberg et al 1985

Mumbaicattails andgrasses None N/A 85% Tayade et al 2005

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 amountof Biochemical Oxygen Demand that needs to be reduced. As a general rule, 1 cubic meter ofwetland can process about 135 liters of greywater (Jenkins 2005). To determine a more precisesize for larger systems, Crites and Tchobanoglous propose completing a series of calculationsto determine the size of the wetland, which is explained below (Crites and Tchobanoglous1998). 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 contributingdischarge, and reaction rate. Calculations based on equations presented in Crites and

Tchobanoglous (1998)

Description

Influent intowetland

(m3/day =

1000L/day)

BODlevel ofinfluent

(mg/L)

DesiredBOD ofeffluent

(mg/L)

Days inConstruc

ted

Wetland

Depthof

Medium

(m)

Width(m)

Length (m)

TotalArea(m

2)

Single household system:assumes a contribution of

240L/family/weekfor 1 family,with a conservative reaction rate

of 1.1 and average lowesttemperature of 3C

0.03 33 5 4.62 0.50 0.40 1.99 0.79

Multiple household system:assumes a contribution of

240L/family/week for 5 families,with a conservative reaction rate

of 1.1 and average lowesttemperature of 3C

0.17 33 5 4.62 0.50 0.89 4.45 3.96

Small community system:

assumes a contribution of240L/family/week for 20families, with a conservative

reaction rate of 1.1 and averagelowest temperature of 3C

0.69 33 5 4.62 0.70 1.68 6.73 11.31

Medium community system:assumes a contribution of240L/family/week for 200

families, with a conservativereaction rate of 1.1 and average

lowest temperature of 3

6.86 33 5 4.62 0.70 5.32 21.27113.1

4

Large community system:assumes a contribution of240L/family/week for 400

families, with a conservativereaction rate of 1.1 and average

lowest temperature of 3

13.72 33 5 4.62 0.70 7.52 30.09226.2

8

Small community system:assumes a contribution of240L/family/weekfor 20families, with a semi-

conservative reaction rate of 2.0and average lowest temperature

of 3C

0.69 33 5 2.54 0.50 1.48 5.90 8.71

Medium community system:assumes a contribution of240L/family/week for 200

families, with a semi-conservative reaction rate of 2.0and average lowest temperature

of 3C

6.86 33 5 2.54 0.50 4.67 18.67 87.12

Large community system:assumes a contribution of240L/family/week for 400

families, with a semi-conservative reaction rate of 2.0and average lowest temperature

of 3

13.72 33 5 2.54 0.50 6.60 26.40174.2

3


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To treat small amounts of greywater (a few liters per day), an alternative to a constructedwetland is to simply use the greywater to water non-food-producing houseplants. This can beaccomplished either by connecting a tube or hose to the outlet of the household sink andguiding the stream into the plant pots, or by collecting the water in a bucket and using the waterto pour overtop the plants. Most plants have a high tolerance for greywater. If this option ischosen, 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 thebathroom is used.

Figure 5. A household greywater irrigation system

Applicable SituationsA greywater treatment wetland is appropriate for use wherever a large quantity of greywater isbeing released. For instance, water from a sink, laundry area, shower, or bath can easily bediverted into the wetland system before it enters the river through a pipe or series of pipes thatextend out from the house or clothes-washing facility. The diagrams below show a pipe runningfrom a house (left) and a pipe running from a community clothes-washing station into atreatment wetland (right). The complete design for the ecological clothes-washing station can befound in Appendix A of Design and Implementation of Sustainable Water Resources Programsin San Cristbal de las Casas, Mexico.

Figure 6. A design for a household treatment system (left) and a ecological community clothes-washing 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 bedrained 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 andTchobanoglous 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 constructedwetland (recommended), or can be purchased from a nursery

A containing wall or impermeable layer should surround the entire wetland to preventgreywater from flowing out before it is fully treated. An outflow device will enable water toexit the system after treatment

PurposeA 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/LocationIn 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 forsubsurface flow wetland systems. Water can then flow through the soil, water, and plantmedium 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 candischarge into surface water. Look for a site that already has a similar slope to minimizealteration

Land use and access should be considered. Be sure that it is convenient to walk to thesite and that there is adequate space to perform any necessary maintenance

Be sure that the landowner supports the greywater system on his property, or that theentire community is in support 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

SizingIn 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 thetarget BOD value for water exiting the system. One can try the calculation with depth varyingfrom 55 to 85 centimeters to find an appropriate size. For instance, if there is a restraint on fieldarea available for the constructed wetland, a depth of 85 cm will minimize the footprint of thesystem. These calculations are based on BOD removal, but can be adapted for nitrate removalby modifying the factors in the calculation of the reaction rate constant. Typically, however, thenitrogen levels in greywater are much less than in blackwater, and BOD is the primaryparameter that should be targeted for removal.


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The following approach for calculation constructed wetland cell size has been adapted fromCrites and Tchobanoglous book, Small Decentralized Wastewater Treatment Systems. Anapplication of this calculation can be found in the Ecological Clothes-Washing Station DesignManual in the report from which this document was extracted: Design and Implementation ofSustainable Water Resources Programs in San Cristbal de las Casas, Mexico.

Determine the minimum monthly average ambient temperature, T (C), that the system will workat (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 20C (k20)varies depending on the system. Arange of values have been used in textbooks guiding design of subsurface flow wetlands. Alarger k value indicates faster decomposition of BOD. Crites and Tchobanoglous (1998), a well-establish source, estimate a k20of 1.1 day

-1, while Tchobanoglous and Burton (1991) estimate ak20of 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 blackwater reaction rate due to the more abundant availability of unprocessed organic matter. Thesevalues 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 thisfigure because much of the treatment depends on the activity of the microorganisms in thewetland, which cannot be determined before construction. More research is needed andencouraged to more accurately characterize the reaction rate and ideal design parameters.

[Equation 1] )06.1()20(

20

=

T

r kk

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

[Equation 2]r

o

kCCt )/ln(

=

,

where Co is the BOD concentration of the water entering the system (mg/L = g/m3) and Cis the

desired BOD concentration of the water (mg/L = g/m3) exiting the system, or the goal. Estimatesof 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

usecategor

y

High

Density

Urban

Residential

Agricultural

Open/Pasture

Forest

Wetlands

Water

Barren

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 beexceeded in practice with influent applied up to 5 cm per day. Almost all greywater systems willhave an organic loading rate below this threshold.

[Equation 3] tdC

L worg

))()(( =

Again, Cis 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 systemcan process, but if the medium is too deep, conditions at the bottom become anaerobic andmay result in incomplete removal of the BOD and nutrients. Use the detention time calculatedabove in Equation 2 (t). The effective porosity of the medium,, is defined as the proportion ofthe non-solid volume to the total volume of material, dimensionless, and can be determinedfrom Table 3 according to the size of gravel chosen.

Table 4. Typical values of constructed wetland mediums (Crites and Tchobanoglous 1998). *d10is the diameter of a particle in a weight distribution of particles that is smaller than all but 10% of

the particles

MediumEffective size

d10*, mm

Effectiveporosity

Medium sand 1 0.3

Coarse sand 2 0.32

Gravelly sand 8 0.35

Medium gravel 32 0.4

Coarse gravel 128 0.45

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

[Equation 4] ))((

))((

w

aves

d

tQA

=

where Qave is average daily flow through the wetland (m3/day), tis the detention time calculated

above (day), and dw is the depth of the medium (m). Use the same value for determined forEquation 3. For larger systems, it may be helpful to convert the area to hectares using theconversion of 1 hectare = 10,000m2.

And finally, to calculate the dimensions of the treatment wetland (m), use the following

expression:

[Equation 5]

2/1

=

A

s

R

Aw

,


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where wequals 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 thatthe aspect ratio is between 2:1 and 4:1, but Bounds et al. (1998) found no significant differenceof nutrient and BOD removal in three 25m2 reedbeds treating domestic effluent with aspectratios 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]w

Al s= .

MaterialsGreywater 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 blocksPVC or metal piping (inlet)

PVC or metal piping (outlet)Fine plastic mesh

Impermeable linerValve (to release and retain water)

Sand

GravelMulch

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 andtransporting oxygen deeper than it would naturally travel through water and soil alone. All plantscan use the nutrients and BOD in the wastewater to some extent. However, relatively few plantsthrive 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, manyof which can be found in natural wetlands. Wetland plants found close to the constructedwetland 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 6shows the common wetland plants described below.

Cattails (Typha spp.) are hardy, easy to propagate, and capable of producing a largeannual 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 ConsiderationsThe constructed wetland should be built with the following generalized steps. Many differentmaterials 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% slope0.5% 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 anotherimpermeable material, allowing space to connect the greywater stream to the wetlandcell via the inlet as specified in step 6. Alternatively, the wetland can be constructed intothe ground using an impermeable liner. This has the disadvantage of not being able todrain 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 wetlandsystems, a perforated pipe of a series of pipes can serve this purpose. For largerwetland 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 beequal to the height of the inlet to retain water in the cell. Put a mesh screen over theopening 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 3cm. Apply a 45 to 75 cm layer of gravel. The depth of gravel will vary according to thesize 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 growingshoot) plus some soil should be transplanted. Simply pull out the plant from aneighboring wetland or pond by the base, being careful not to break the root. The rootend should be placed about 5cm below the surface. Above-water stems can be cut toabout 20 cm tall. Cattails should be placed 0.25-1 m apart; reeds and bulrush can beplanted 15 cm apart (Mitch and Gosselink 2000). It is important to eventually acquire adeep consistent root zone by lowering the water level gradually to encourage deeperroot growth.

12. Saturate the soil with water just up to the surface (no farther) and allow it to evaporateslowly. 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 deeperinto the wetland cell. Vegetation should be allowed to become established before

wastewater applications begin, about 2-3 months. If wastewater must be introducedsooner, 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 andallowed to infiltrate into a vegetated area or flow into the surface waters along a plantedor rock-filled pathway. This pathway encourages the slowly-draining effluent to seep intothe ground en route to the surface water, thus potentially gaining the additionaladvantage of groundwater treatment before it flows into the surface water viagroundwater 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 ifingested. It also reduces erosion of the receiving area.

MaintenanceGreywater 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-outletsystem if it is built at the correct height. During initial plant growth, the drain can serve tolower 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 nutrient-rich environment. It is not necessary to harvest the wetland plants, however, if plants arevery wilted even with sufficient water they may suffer from a build up of pollutants andshould 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 flowis lower than usual to prevent further clogging.

Water monitoring: it is recommended that periodic monitoring of nutrient and BOD levelstake 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 sedimentsgetting into the pipes and preventing flow. This can be prevented by installing therecommended screen or trap for large solids at the pipe inlet.

Invasive species: It is important to not introduce wetlands species that are purchased forthe constructed wetlands at nurseries into natural wetlands. Some species are moreaggressive than others and can dominate a natural wetland by killing native species. It isimportant to avoid this in order to maintain natural species diversity.

Overflow during a storm may cause solids that previously settled to re-suspend and bereleased into the surface waters. This should be avoided by leaving the outlet open at alltimes, 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 inthe 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 MatrixMacrophyte 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 SmallFlows Journal 4.1 (1998): 4-14.

"Bulrush Image." Ed. bulrush.jpg: Traders Creek.

Crites, Ronald, and George Tchobanoglous. "Small and Decentralized Wastewater ManagementSystems." Water Resources and Environmental Engineering (1998).

Dallas, Stewart C. "Reedbeds for the Treatment of Greywater as an Application of Ecological Sanitation inRural Costa Rica, Central America." Murdoch University, Western Australia, 2005.

Eifert, W. "Applications of Constructed Wetland Treatment Technology." Proceedings of the Brownfields2002 Conference (2002).

Gersberg, R.M., et al. "Role of Aquatic Plants in Wastewater Treatment by Artificial Wetlands." WaterResearch 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 MunicipalWastewater." The Global Directory of Environmental Technology, ECO Services International,2005.

Torb, Magorzata. "Common Reed Image." Ed. z_0171n.jpg.

Tchobanoglous, George, and Franklin L. Burton, eds. Wastewater Engineering: Treatment, Disposal, andReuse. 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: LewisPublishers, 1989. 319-51.

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