oo A HANDBOOK OF CONSTRUCTED WETLANDS a guide to creating wetlands for: AGRICULTURAL WASTEWATER DOMESTIC WASTEWATER COAL MINE DRAINAGE STORMWATER in the Mid-Atlantic Region Volume AGRICULTURAL WASTEWATER ------- ACKNOWLEDGMENTS Many penpl.. contributed to this Handbook. A,i In.«r:u.,u<:y Core Group provido.l the initial impetus for the Handbook, and inter orovid*H guidnnce and technical input during its preparation. The Core Group comprised: Carl DuPoldl. USDA - NRCS. Chester. PA Robert Edwards, Susquuhanna River Basin Commission. Harrisburg. PA Lainonle Garber. Chesapeake Bay Foundation. Harrisburg, PA Barry Isaacs. USDA - NRCS, Harrisburg. PA Jeffrey Lapp. EPA. Philadelphia, PA Timothy Murphy. USDA - NRCS. Harrisburg. PA Glenn Rider. Pennsylvania Department of Environmental Resources, Harrisburg. PA Melanie Sayers. Pennsylvania Department of Agriculture. Harrisburg. PA Fred Suffian. USDA - NRCS, Philadelphia, PA Charles Takita. Susquehanna River Basin Commission. Harrisbura. PA Harold Webster, Penn State University, DuBois. PA. Wet'andS C011tributed fay Providing information and by reviewing and commenting on the Handbook. These Robert Bastian. EPA .Washington. DC William Boyd. USDA - NRCS. Lincoln. NE Robert Brooks, Penn State University. University Park. PA Donald Brown. EPA, Cincinnati. OH Dana Chapman. USDA - NRCS, Auburn. NY Tracy Davenport, USDA • NRCS. Annapolis MD Paul DuBowy, Texas A & M University. College Station. TX Michelle Girts. CH2M HILL. Portland. OR Robert Hedin, Hedin Environmental, Sewickley. PA William Hellier, Pennsylvania Department of Environmental Resources, Hawk Run. PA Robert Kadlec. Wetland Management Services. Chelsea, MI Douglas Kepler, DamariscoUa, Clarion. PA Robert Kleinmann, US Bureau of Mines, Pittsburgh, PA Robert Knight, CH2M HILL, Gainesville. FL Fran Koch, Pennsylvania Department of Environmental Resources, Harrisburg. PA Eric McCleary, Damariscotta, Clarion, PA Gerald Moshiri, Center for Wetlands and Eco-Technology Application, Gulf Breeze, FL John Murtha, Pennsylvania Department of Environmental Resources, Harrisburg, PA Robert Myers. USDA - NRCS, Syracuse, NY Kurt Neumilfer, EPA, Annapolis, MD Richard Reaves, Purdue University, West Lafayette. IN William Sanville, EPA, Cincinnati, OH Dennis Sievers. University of Missouri, Columbia, MO Earl Shaver, Delaware Department of Natural Resources and Environmental Control, Dover, DE Daniel Seibert, USDA - NRCS, Somerset, PA Jeffrey Skousen, West Virginia University, Morgantown, WV Peter Slack. Pennsylvania Department of Environmental Resources, Harrisburg, PA'.... Dennis Verdi, USDA - NRCS, Amherst, MA\ Thomas Walski, Wilkes University, Wilkes- Barre, PA '*", Robert Wengryznek, USDA - NRCS. Orono, ME Alfred Whitehouse, Office of Surface Mining, Pittsburgh, PA Christopher Zabawa, EPA, Washington.^. This document was prepared by Luise Davis for the USDA-Natural Resources Conservation Service and the US Environmental Protection Agency-Region III. in cooperation with the Pennsylvania Department of Environmental Resources. Partial funding has been provided with nonpoint source management program funds under Section 319 of the Federal Clean Water Act. The findings, conclusions, and recommendations contained in the Handbook do not necessarily represent the policy of the USDA - NRCS, EPA - Region HI. the Commonwealth of Pennsylvania, or any other state in the northeastern United States concerning the use of constructed wetlands for the treatment and control of nonpoint sources of pollutants. Each state agency should be consulted to determine specific programs and restrictions in this regard. PS5384RSGZ ------- VOLUME 3 TABLE OF CONTENTS CHAPTER 1. INTRODUCTION • 3 CHAPTER 2. USING CONSTRUCTED WETLANDS IN AGRICULTURE Contaminant Removal Processes '.' Advantages and Limitations of Constructed Wetlands Creating Effective Constructed Wetlands ' 6 Types of Constructed Wetlands ."" Wastewater Characteristics ..'."."...' Water Quality , _ Water Quantity ; "" • Prertreatment ..... Discharge Option , ', '"""" CHAPTERS. PERFORMANCE EXPECTATIONS ] u Introduction ;.... _ ' '"" Biochemical Oxygen demand and Total Suspended Solids n Nitrogen Phosphorus Pathogens _, . •"" " • 14 Toxics CHAPTER 4. SURFACE FLOW WETLANDS ; 17 Wetland Design ; ' Configuration .-. _ Water Depth ' Sizing „ ZZZZZZZZZZZZZZZ 18 Presumptive Method for BOO 18 Field Test Method for BOD '. ZZ.1Z....Z....ZZ... 20 Presumptive Method for Nitrogen . 21 CHAPTER 5. SUBSURFACE FLOW WETLANDS 23 Introduction . ' "• Wetland Design \ _ '""",". 23 Darcy's Law. 23 Media Types .^Z.Z.ZZ 24 Length-to-Width Ratio ",' 24 Bed Slope .......Z.......Z . 25 Sizing ; ZZZZZZZZZZZZZ.25 Biochemical Oxygen Demand ' 25 Total Suspended Solids .ZZZ" 26 Nitrogen _ 26 REFERENCES ' "* ------- LIST OF TABLES Table 1. Removal mechanisms in constructed wetlands Table 2. Advantages and limitations of constructed wetland treatoent'of [[[ 5 domestic wastewater .................................... Table 3. Guidelines for creating constructed wetlands ................. • ........ ............................................... ' ............... 5 Table 4. Summary of agricultural constructed wetland operationaTdata .............................. ' .............................. 6 Table 5. Design summary for surface flow wetlarids ........ [[[ • n Table 6. Design summary for subsurface flow wetlands ............................................... " ..................... ' ............ 17 [[[ 23 LIST OF FIGURES ------- CHAPTER 1 INTRODUCTION This volume focuses on the use of con- structed wetlands to treat agricultural wastewa- ter. It is to be used in combination with Volume 1: General Considerations, which pro- vides information on wetland hydrology, soils and vegetation, and on the design, construction. operation, and maintenance of constructed wetland systems. Constructed wetlands can provide an inex- pensive and easily operated means of removing organic matter, particulates, nutrients, and bacteria from agricultural wastewater. Agricul- tural wastewaters suitable for wetland treatment- include milkhouse wastewaters, runoff from concentrated livestock areas, and effluents from settling tanks and manure treatment lagoons. Interest in using constructed wetlands to treat agricultural wastewaters has been prompted by the success of constructed wet- lands in removing organic matter, particulates, and nutrients from municipal wastewaters. However, the use-of constructed wetlands in agriculture is a fairly recent development and the number of systems that have been installed" is still small. While the data show that properly designed wetlands can be effective in treating agricultural wastewater,.much is not yet under- stood and many of the relationships between design and performance have not been clearly established. The use of constructed wetlands in agriculture will be modified and refined as more systems are installed and monitoring data are gathered over longer periods of time. The guid- ance presented here should be considered as today's "state of the art" and will likely be modi- fied as our understanding of these systems grows. As with other agricultural waste management practices, constructed wetlands are one compo- nent of an overall agricultural waste management system (AWMS). This Handbook discusses the contributions that constructed wetlands can make to an AWMS, the performance that can reasonably be expected, and the factors that are important in the design of effective constructed wetlands. Step- by-step procedures in designing constructed wet- lands are given. This volume incorporates the guidance pre- sented in the Soil Conservation Service (SCS, now the Natural Resources Conservation Service) 'con- structed Wetlands for Agricultural Wastewater Treatment Technical Requirements (1991), which is an important reference for those interested in usin° constructed wetlands in agriculture. ° ------- ------- CHAPTER 2 USING CONSTRUCTED WETLANDS IN AGRICULTURE CONTAMINANT REMOVAL PROCESSES The most important removal mechanisms in agricultural constructed wetlands are physical sedimentation and filtration, and biological assimila- tion, breakdown, and transformation (table I). The suspended solids that remain in the effluent from pretreatment unit are removed in the wetland by sedimentation and filtration. These physical processes also remove a significant portion of other wastewater constituents,such as biochemical oxygen demand (BOD), nutrients, and pathooenc (Brix 1993). Soluble organic compounds are° for the most part, degraded by microbes, especially bacteria, that are attached to the surfaces of pla'nts litter, and the substrate. ADVANTAGES AND LIMITATIONS OF CONSTRUCTED WETLANDS Constructed wetland treatment of agricultural wastewater offers a number of advantages (table 2), Table 1. Removal mechanisms in constructed wetlands (after Brix 1993). Wastewater ConstihiPnf Biochemical oxygen demand Suspended solids Nitrogen Phosphorus Pathogens Removal Microbial degradation (aerobic and anaerobic) Sedimentation (accumulation of organic matter/slud°e sediment surfaces) Sedimentation/filtration on by microbial nitrification and Plant uptake Volatilization of ammonia Soil sorption (adsorption-precipitation reactions with aluminum iron, calcium, and clay minerals in the soil) "«««n. Plant uptake Sedimentation/filtration Natural die-off Table\2. Advantages and limitations of constructed wetland treatment. Advantages • are capable of providing a high level of treatment • can reduce or eliminate odors • are inexpensive to operate • are largely self-maintaining Limita{iong are affected by season and weather, which may reduce treatment reliability are sensitive to high ammonia levels may hold potential for mosquitoes and other insect pests are able to handle variable wastewater loadings * require a continuous supply of water • reduce the amount of area needed for land application require dedicated, single land use • may be more expensive to construct than other treatment options ------- including odor control and simplicity of opera- tion. Constructed wetland treatment is con- strained by a number of limitations, including variability in treatment effectiveness and the sensitivity of wetland plants to high ammonia concentrations. The advantages and limitations of a constructed wetland as an alternative to other treatment options must'be understood and weighed before deciding to install a constructed wetland. CREATING EFFECTIVE CONSTRUCTED WETLANDS Suggestions for creating an effective con- structed wetland are given in table 3. Since the objective of using a constructed wetland is to simplify the handling of wastewater, the system should be made as easy as possible to operate while ensuring reliable treatment. Building a slightly larger system may be more expensive to construct but may be less costly and more reliable to operate than a smaller system. At- tention to several factors will help to ensure successful wetland treatment: • Adequate pretreatment. Pollutant loads in agricultural wastewaters often greatly exceed the ability of a wetland to treat or assimilate them. A wetland can be severely damaged by a wastewater that is too concentrated, for instance, one that contains high levels of ammonia. Pretreatment to lower pollutant loads is essential to avoid overloading the wetland. • Adequate retention time. A wetland treats wastewater through a number of biological (largely microbial). physical, and chemical processes. The water must remain in the wetland long enough for biological and chemical transformations to take place and for Table 3. Guidelines for creating constructed wetlands. Know what you are dealing with Size the wetland generously Wetlands must have water Give the plants a chance Don't overload the wetland Don't kill the wetland Effluent disposal must be addressed Keep an eye on what is happening Get interdisciplinary help Sample the wastewater Know what pretreatment will accomplish Too small a wetland cannot perform well Know the water budget Provide a supplemental source of clean water Allow time for establishment Avoid shock loading Keep ammonia levels to 100 mg/L or less Application rates must not exceed treatment rates - Keep raw milk out of the wetland Keep herbicides out of the wetland Use other recognized practices Monitoring is important Environmental engineer Water quality specialist. Plant materials specialist/biologist/extension agent State agencies VOLUME 3: AnRinn.TttPAi WASTCWATFB ------- sedimentation and deposition to occur. The wetland must be built large enough to pro- vide the necessary retention time. Supplemental water. If a constructed wet- land is to remain healthy, it must remain relatively wet. Wetlands are generally .tolerant of fluctuating flows, but they cannot withstand complete drying. For this reason, either a slow release of wastewater must be assured or a supplemental source of water must be provided. Supplemental water can be used to dilute the wastewater to accept- able levels and also to assure that the wet- land stays wet. Enough water should be supplied to the wetland to maintain a slow flow of water, since stagnant water can lead to problems with odors and mosquitoes. Proper management. Constructed wetlands are "high management, low maintenance" systems. They must be actively managed if they are to perform well. "Management" means watching the wetland for signs of stress or disease and adjusting water levels or input streams accordingly. While wet- lands are low maintenance systems, they are not maintenance-free. For instance, the pretreatment unit must be cleaned periodi- cally to keep excessive solids from entering the wetland, and valves and piping must be checked to detect and correct blockages or leaks. TYPES OF CONSTRUCTED WETLANDS Most wetlands used in agriculture are surface flow (SF) wetlands. The advantages of SF wetlands are that their design and construc- tion are straightforward. Operation and main- tenance are simple. Because the water surface is unconstrained, SF wetlands are able to accept wide variations in flow. SF wetlands can provide excellent removal of 5-day bio- chemical oxygen demand (BOD5) and total suspended solids (TSS). Good removal of ammo- nia and total nitrogen has been achieved at some wetlands. SF wetlands are discussed in Chapter 4. In subsurface flow (SSF) wetlands, the water level must.r.emain below the surface of the sub- strate if the wetland is to perform well. The design and construction of SSF wetlands are therefore more complicated than for SF wetlands and SSF wetlands must be managed and moni- tored much more closely. There have been prob- lems with unintended surface flow and apparent plugging. Because of the hydraulic constraints imposed by the media, SSF wetlands are best suited to relatively uniform flows. Their use in agriculture has thus been limited. SSF wetlands may be appropriate for field drain discharges and research on such systems is being conducted. SSF systems are discussed in Chapter 5. WASTEWATER CHARACTERISTICS To design the wetland correctly, an accurate assessment of contaminant loadings is needed (loading = contaminant concentration x waste- water volume). To calculate loadings, data are needed on the average water quality, the maximum concentrations, and the largest and smallest volumes that may occur. Loadings may vary throughout the year as the volumes of water change in response to climatic factors, such as rainfall and evaporation. Maximum concentra- tions will probably occur in the late summer and fall, when water losses due to evapotranspiration' are greatest. The highest flows can be expected during the wet season, but pollutant concentra- tions may be lower at this time because of dilu- tion. The design should be based on the highest pollutant loadings. WATER QUALITY The characteristics of agricultural wastewater' vary, depending on the specifics of the agricultural operation, and should be determined by laboratory analysis before the wetland is designed. The Soil ------- Conservation Service (SCS. nou- the Natural Resources Conservation Service) recommends the following analyses for agricultural wastewater (SCS 1991): 5-day biochemical oxygen demand (BOD ) total solids (TS) " 3 total Kjeldahl nitrogen (TKN) nitrate nitrogen (NO3-N) ammonia nitrogen (NH3 + NH^-N) total phosphorus (TP). The design of the wetland is usually based on the removal of BOD (usually measured as 5-day biochemical oxygen demand, BODS) or nitrogen (measured as total Kjeldahl nitrogen or nitrate nitrogen). Concentrations of ammonia (NH3 + NH4-N, un-ionized ammonia + the ammonium ion) should be evaluated because of the toxicity of ammonia to \vetland plants. Additional analyses may be needed. For instance, if high salinities could occur, chloride concentrations should be measured to determine the salinities that the vegetation will be exposed to; salinities in the brackish range will suggest that salt-tolerant vegetation should be planted. The likelihood of toxic compounds, and high or low pHs that could affect the biological components of the wetland should be considered. WATER QUANTITY An accurate estimate of the volume of waste- water is needed, including the expected average, maximum, and minimum wastewater flows, An accurate figure for the volume of water to be treated must be determined: too small a wetland will perform poorly; a large wetland may require supplemental water to maintain the wetland during the dry season. The maximum expected flow must be determined. If the maximum ex- pected flow is larger than the capacity of the wetland, a bypass will be required. The minimum expected flow should be calculated to determine the volume of supplemental clean water that may be needed. PRETREATMENT Raw agricultural wastewaters are character- ized by very high concentrations of BODS, nutrients, and total and dissolved solids.3 To avoid overloading \vetland removal capabili- ties, raw wastewaters must be pretreated to lower the concentrations of these contami- nants. Pretreatment is also necessary to lower nutrient and organic loads (particularly ammo- nia) to avoid damaging the wetland vegetation. Pretreatment to lo\ver total organic loading also helps to control mosquito populations (Wieder et al. 1989). Pretreatment can be made through settling tanks and basins, flotation tanks, filters, or mechanical separators, either singly or in combination. The Agricultural Waste Management Field Handbook (SCS 1992) discusses various pretreatment options. In the northeastern United States, percent removals in BODsby settling tanks are gener- ally around 70%. To protect the vegetation, SCS (1991) suggests a target concentration of 100 mg/L ammonia for the effluent from the pretreatment unit. However, Reaves et al. (1995) found that while concentrations of 200 -.300 mg/L ammonia damaged the growth of young cattail shoots in a wetland used to treat swine effluent, mature cattails did not seem to be affected. Reaves et al, (1995) theorize that the ammonia may be present as the non-toxic ammonium ion (NH/) rather than the toxic ammonia ion (NH3). Pretreatment must remove solids. Most settleable, floating, and non-biodegradable solids, such as plastics and grease, must be removed before the water enters the wetland or the wetland will eventually clog. Pretreatment to remove solids also avoids potential clogging of pipelines, gates, and valves. Since bacteria" and viruses adsorb on solids, pretreatment to decrease solids also decreases the numbers of bacteria and viruses (Ives 1988). SCS (1991) suggests 1,5 00 mg/L total solids or less as a target concentration for the effluent from the pretreatment unit. 8 VOLUME 3- Anorr-MtTtiD»t U/» ------- Fats are a particular problem since they float on the surface of the water, causing a scum that blocks gas transport and rapidly depletes dis- solved oxygen. Raw milk can suffocate a wetland and must be kept out of the wetland. An organic filter (a bed of chipped mixed wood bark) is being tested as a means of reducing odors and ammonia concentrations at a veal operation (Murphy et al. 1993). The effluent from the settling tanks passes through the filter by subsurface flow before .entering the wetland. The bed has completely eliminated odors; ammonia removals have been variable. In addition to pretreatment, dilution may be necessary for som.e wastewaters. The wetland system can be designed to recycle the wetland effluent for use in diluting the effluent from the pretreatment unit. DISCHARGE OPTIONS There are several options for the wetland effluent: • storage for later land application • discharge to an infiltration area • recycle through the wetland. Where possible, the effluent should be re- cycled. Recyling the treated effluent from the wetland is an efficient way to dilute influent BOD and suspended solids. Recycling decreases the . potential for odors and may possibly increase dissolved oxygen concentrations, which in turn may enhance nitrification. Recycling is also an efficient way to maintain adequate flows during low-flow periods. The disadvantages of recycling are the in- creased construction costs and increased operation (pumping) costs. Also, recyling may slowly increase salinities as evapotranspiration removes water from the system. ------- ------- CHAPTERS PERFORMANCE EXPECTATIONS INTRODUCTION Data on agricultural systems are limited, both in the number of systems that have been built and in the length of time the systems have been operat- ing. However, a number of constructed wetland systems are being used to treat domestic wastewa- ters, which contain a array of contaminants similar to those in agricultural wastewater. Information from domestic systems is thus useful in assessing the potential of constructed wetlands to .treat agricultural wastewater. BIOCHEMICAL OXYGEN DEMAND AND TOTAL SUSPENDED SOLIDS Wetlands provide a number of mechanisms for removing BOD5 and TSS, and constructed wet- lands are,extremely efficient at removing these contaminants. At seven agricultural systems that have recently begun operating, removals range from about 60% to more than 90% (table 4). In a survey of 324 municipal, industrial, stormwater, and other constructed wetlands, Knight et al. Table 4. Summary of agricultural constructed wetland operational data BODs (mg/L) In Out %. TSS (mg/L) In Out NH3-N (mg/L) In Out %. Dairya Dairy0 Dairy0 Swine1' Swine6 Swine* Chicken manure? Field drain effluent11 37 36 37 1,343 1,688 1,998 354 64 45 45 45 47 45 1,320 1,550 13 65 9 75 11 70 375 68 339 81 397 62 138 61 6 91 28 38 80 80 20 56 24 47 230 83 190 88 137 109 125 700 7,216 1,338 282 105 118 118 118 94 88 1,060 1,300 44 68 56 49 47 62 281 65 151 89 558 42 75 9 •17 10 9 31 36 155 130 73 91 86 92 92 '67 59 85 90 6.5 6.1 7.5 243 208 343 64 54.7 94 94 94 112 112 0.4 93 0.3 95 1.2 84 68 70 110 51 90 SO 29 54 3.5 94 23 76 3 97 2 98 36 68 41 63 0.3 0.3 TN(mg/L) In Out 654 201 56 476 215 57 890 275 36 92 38 57 70 6 91 104 41 61 104 6 94 104 5 95 295 <95 68 320 64 80 23 21 6 TP(mg/L) In Qui 13.5 14.4 14.2 56 75 62 14 25.8 66 66 66 28 27 22 25 0.02 3.3 76 3.8 74 5.7 60 24 58 22 78 23 49 S 66 6.2 76 30 55 23 65 15 77 18 36 16 41 <4 >80 5 80 0.02 0 %: percent removal . a surface flow, milkhouse wash water + runoff + rainfall, Mississippi, 2 years of data (Cooper et al. 1993) b surface flow, bam washwater + yard runoff, Indiana, 1 year of data (arcsine means) (Reaves et al. 1994) c surface flow, milking parlor washwater + yard runoff + rainfall, Oregon, 6 months of data (Skarda et al. 1994) d surface flow, effluent diluted with stormwater pond effluent. Alabama, 1 year of data (Hammer et al. 1993) e surface flow, lagoon effluent, flow rates of 2610 gpd/1094 gpd/540 gpd (lines 1-3). Alabama. 3 months of data (McCaskey et al. 1994) or _ or f surface flow, lagoon effluent, marsh-pond-marsh system, Mississippi, 16 months of data (Cathcart, Hammer. and Triyono 1994). . g subsurface flow, dilute chicken manure, Czechoslovakia, 1 year of data (Vymazal 1993] h subsurface flow, cropland tile drain effluent, Pennsylvania, 4 years of data (Taylor et al. in preparation) ------- r: ^ „!_• s "°! - m r . . :s ™ 1M = •i 4-... _ i .ua J .1 'L 1 : •••r^- .; 1 • — i 4- ' . \" *'•• 1 . i > 1 1 ! i . •• i i : 1 • g ' . " " '" - •" BOD, Mass Loading Rate (kg/ha/d) Figure 1. BOD, mass loading and removal rates in wetland treatment systems (from Knight et al. 1993). (1993) found that BOD, mass removal efficiencies were generally 70% or more at mass loading rates up to 250 Ib/ac/day (280 kg/ha/day), which is considerably higher than the 65 Ib/ac/day recom- mended by SCS (1992) as the maximum BOD, loading rate for agricultural wetlands. A linear regression of the 324 data records used to examine the predictability of BOD, outflow concentration as a function of BOD, inflow concentration and hydraulic loading produced the following (figure 1) (Knight et al. 1993): BODOUT = 0.097*HLR + 0.192*BODIN Rz'= 0.72 where BODOUT = BOD, outflow concentration, mg/L BODIN = BOD, inflow concentration, mg/L HLR = hydraulic loading rate, cm/day. While average annual removal rates were usually high, rates sometimes varied considerably on a monthly or seasonal basis (Knight et al. 1993). In wetlands, BOD is produced within the system by the decomposition of algae and fallen plant litter. As a result, wetland systems do not completely remove BOD and a residual BOD of 5 to 7 mg/L is often present in wetland effluent (EPA 1993). This internal production of BOD decreases removal efficiencies at very low inflow concentrations. TSS is removed primarily by sedimentation and Miration, and removal is enhanced as the density of surfaces within the wetland increases. Cooper et al (1993) found that TSS removals increased as plant litter accumulated. To maximize the removal of BOD5 and TSS, the growth of plants (particularly underground tissues) and the accumulation of litter should be encouraged. Plants and plant litter provide organic carbon and attachment sites for microbial growth, and promote filtration and sedimentation. Because of the impor- tance of microbial processes in removing BOD , adequate residence time must be provided. The SCS (1991) recommends a hydraulic residence time of at least 12 days. NITROGEN In contrast to the simplicity of BOD and TSS removal, the chemistry of nitrogen removal is com- plex (figure 2). Nitrogen occurs in a number of forms, including organic and inorganic compounds, and nitrogen gas. In wetlands, the important forms include nitrogen gas (N2j, nitrate (NO/), nitrite (NO/)', ammonia (NH3), and ammonium (NH/). . In wetlands, the removal of nitrogen involves a series of reactions (Mitsch and Gosselink 1986). Decomposition and mineralization processes .convert a significant part of organic nitrogen to ammonia. Ammonia is oxidized to nitrate by nitrifying bacteria in aerobic zones (nitrification) and nitrates are converted to nitrogen gas by denitrifying bacteria in anoxic zones (denitrification); the gas is released to the atmosphere. The sequence is: mineralization: organic nitrogen -> ammonia aerobic or anaerobic nitrogen reaction nitrification: ammonia nitrogen -> nitrate aerobic reaction nitrogen denitrification: nitrate nitrogen -> nitrogen gas anaerobic reaction, requires a carbon source as food for the bacteria Since nitrification is an aerobic process, rates are controlled by the availability of dissolved oxygen to ------- .--»""" ,„.,„,,„»"— 'callv ^ * and 95% W" °80 to/ac/day. «*a sr^ssiS&sr- f^W^^ M—UMli°ni Residence ^j^. The system ° deep and the v ^ surface to pro °0b\em turbulence of t^ ^ correct thvsp_ ------- ^WaisT* tho^rt co; Tab/e <;• HO ,. -^^fe^"uie'° ,-f' ph°sPnorUs is a h- l * 1 s f°rins- It has ' ^ ------- ££££— TOXICS ------- ------- CHAPTER 4 SURFACE FLOW WETLANDS WETLAND DESIGN Guidelines to designing a SF constructed wetland are given in table 5. The design assumes that the wastewater has been pretreated to reduce. BOP5 by 70%, TSS to less than 1,500 mg/L, and ammonia to less than 100 mg/L. CONFIGURATION The configuration should take advantage of the natural topography of the site to minimize excava- tion and grading costs. The configuration should allow water to move through the wetland by gravity. The wetland should not be placed where excess solids could be washed into the wetland; for instance, sites next to solids;settling pads should be avoided. While treatment wetlands are often designed as rectangles, wetlands can be built as semi-circular or irregular shapes to fit the topography of the site. Using curved shapes also eliminates right-angled corners, which tend to be "dead water" areas. If a shape other than a rect- angle is used, the widest portion should be located at the inlet end to facilitate equal flow distribu- tion. For large wetlands, dividing the, wetland into side-by-side cells should be considered. Dividing a wide wetland into parallel cells lessens the likelihood of preferential flow paths and short- circuiting and promotes the contact of the waste- water with the surfaces in the wetland. It also facilitates maintenance since one set of cells can be taken out of operation temporarily. If the removal of nitrogen and ammonia is-a major objective, including a deeper (2-3 ft) open water pond in the middle of a longer wetland cell should be considered to increase nitrification and denitrification. Pretreatment Configuration Flow Bottom slopes Water depth Vegetation Construction Table 5. Design summary for surface flow wetlands. Reduction of BOD5 by 70% Reduction of solids to <1,5 00 mg/L Reduction of ammonia to <100 mg/L Fit the wetland to the site Divide large wetlands into side-by-side cells 3y gravuy, as much as possible Side-to-side elevations: level Inlet to outlet slopes: almost flat (0.5 -1.0%) 3 - 8 inches, depending on the plants selected 18 inches maximum Complete coverage is more important than the species used Use at least two or three different species Wetland must be sealed to limit infiltration and exfiltration Water table must be below or excluded from the wetland ------- WATER DEPTH The design should plan for 3 to 8 inches c: surface xvater, with a maximum of 18 inches. Deeper water may be advisable in winter to accom- modate the slower reaction rates during cold weather and to guard against freezing. The \vet- land may have to be divided lengthwise into a series of cells to prevent the water in any of the cells from being deeper than desired. Each ceil will then discharge to a downstream cell of the same width. The maximum length of each cell is based on the slope of the bottom of the cell (which should not exceed 0.5 to 1.0%) and the water depth suitable for the wetland vegetation (which is generally 18 inches or less). The number and length of the subdivisions \vill depend on the length of the cells and the slope of the bottom. The bottom of the cells should be flat from side to side to assure an even distribution of water across the cells and to prevent channeling. SIZING Procedures for sizing SF wetlands for the removal of BOD5. TSS, and nitrogen are still preliminary. It has been widely presumed that simple first order chemical reaction rates apply for pollutant removal and that constructed wetlands roughly follow plug flow in their internal hydrol- ogy. However, several recent studies have shown that the movement of water thrtiugh constructed wetlands is considerably more complex than that described by standard flow equations (Kadlec et al. 1993, Kadlec 1994). The flow through a wetland is related to the morphology of the cell, the pattern of vegetation density, and the balance between evapotranspiration and precipitation. Constructed wetlands exhibit mixing characteristics intermedi- ate between plug flow and well-mixed, flows are typically in the transition zone between laminar and turbulent, and hydrologic conditions change continuously with changes in the weather and the seasons. Factors such as obstructions to flow, the development of channeling, recirculation patterns, and the presence of stagnant areas cause further deviations from calculated theoretical flows. Contact times are not often as great as the theoreti- cal residence time calculated from the wetland empty volume and the volumetric flow rate. As a final complicating factor, the chemistry of wetlands is complex, involving interrelated biological reactions and mass transfers. These factors and the lack of good information on factors such as reaction rate constants have probably led to many systems being underdesigned. Agricultural wetland systems are usually sized to remove the necessary amount of BOD5. The SCS (1992) uses two methods to determine the size of a constructed wetland for a specific site: the presumptive method and the field test method. The presumptive method is used when the pretreatment system has not yet been installed and the concentration of BOD, in the pretreatment unit can only be estimated. The presumptive method first calculates a surface area and then determines the resulting hydraulic residence time, adjusting the size as necessary to achieve the required residence times. The field test method uses known, measured BOD5 concentrations in the effluent from the pretreatment unit to calculate hydraulic residence times, from which the required surface area is then calculated. Removal efficiencies for TSS are similar to those for BODS and design for BOD5 removal should achieve similar TSS removal. For wetlands designed to remove ammonia as well as BOD5, the size of the wetland should be based on the removal of the ammonia. Since ammonia removal is a less efficient process than BOD, removal, ammonia removal requires a larger wetland area than does BOD5 removal. PRESUMPTIVE METHOD FOR BOD. This method assumes that a certain amount of BOD, is present in the wastewater and that a certain amount is removed by the pretreatment system. It then uses the remaining BOD, load with ------- a standard wetland areal loading rate to determine the surface area needed for adequate treatment. 1. 2. 3. Determine the production of BOD. per day: Livestock production of BOD5 is as follows: production (Ib) Average ,1000 Ib animal weight Hbl unit (AU1 per day Dairy cows 1300 Beef cattle 750 Swine 200 Poultry - layers 4 Poultry - broilers 2.2 1.6 1.4 2.1 3.7 5.1 4. BODS= BOD5/1000 Ib AU x number of animals x average weight/1000 Ib Add 10% BOD5to account for that in waste hay and feed: Adjusted BOD3= BODsx 1.1 Determine BOD3 remaining after pretreatment. if known. A well-managed settling/flotation tank for milkhouse wastewater will remove 70% of BOP5. Use a removal factor of 0.30 (70% removal): BOD5 remaining in tank effluent = adjusted BOD5x0.30 If using an anaerobic lagoon, use a rate of 40% rate (60% removal): BODS remaining in lagoon effluent = adjusted BOD5x0.40 Determine the water surface area (SA) for the constructed wetland: SA = BOD3 loading / recommended areal BODS loading rate SCS recommends an areal BOD5 loading rate of 65 Ib BOD5 /ac/day. It is known that treatment, and therefore areal loading rate, are affected by climate but no research or field data are available that can be used to quantify the influence of different climate conditions. A standard value is therefore used. Determine the overall dimensions. The optimal length-to-width ratio has not yet been determined. The SCS (1992) recommends a ratio of 3:1 to 4:1. For a length-to-width ratio of3:l: W = width of constructed wetland L= length = 3VV Then SA = 3W x VV = 3W* Determine the hydraulic residence time (t) in days. Data needed are average water depth (D). porosity (P), and daily flow rate (Q). t = SAxD xP/Q (4.1) where: t = hydraulic residence time, days SA = surface area of constructed wetland, ft2 (length x width) D = average water depth in constructed wetland, ft Q = average daily flow rate. ftVday P = porosity, percent as a decimal. The Q value is the average flow in the bed, calculated from flow through the bed plus gains and losses from precipitation and evapo- transpiration. Published values are usually available for precipitation .and evapotranspira- tion for local conditions. Large rainfall and snowmelt volumes can greatly affect Q and must be considered in the design; some con- structed .wetlands have failed because high flows were not factored into the design. The porosity (P) is the ratio of the volume occupied by water to the volume occupied by plants and water combined. The following porosity values have been determined: cattails (Typha spp.) 0.95 (SCS 1992) bulrush 0.86 (Scirpus yalidus] 0.86 woolgrass (S. cyperinus] common reed 0.98 (Phragmites) rushes (Juncus spp.) 0.95 (Watson and Hobson 1989) (Watson and Hobson 1989) {Watson and Hobson 1989) (Watson and Hobson 1989). ------- The volume occupied by underground plant structures (roots and rhizomes) increases over time (Reed 1993) and porosity gradually decreases. Tin; wetland should be designed conservatively to allow for the diminished porositv. The SCS recommends a hydraulic residence lime of at least 12 days since this residence time has been found empirically to provide adequate removal of BOD5. FIELD TEST METHOD FOR BOD. The field test method uses data from samples collected in the pretreatment unit to calculate hydraulic residence times via the following equation: t = 2.7 (In C - In Ce + In F) / 1.1 '™>'. or (4.2) t « (In C. - In Ce + In F) / 65K,. where t = hydraulic residence time, days C( = constructed wetland influent BOD5 concentration, mg/L Ce = desired constructed wetland effluent BODj concentration, mg/L In = natural logarithm F = fraction of BODS that is not removed as settleable solids near the head of the wetland, expressed as a decimal fraction, (soluble BODj/total BOD5) T = water temperature, °C Kj. = temperature-dependent reaction rate constant, days •' The values for C, and F are determined from samples of the supernatant (the liquid above the solid layer) in the pretreatment unit. A composite sample (several samples combined) should be collected within the unit. Ideally, samples should be collected and analyzed during various seasonal conditions. Because BOD. concentrations in these systems can vary widely, and because an adequate safety factor must be assured, the highest sample value should be used to design the system. The temperature of the water (T) is controlled by local climatic conditions. The lowest water temperature under which the wetland will be expected to perform should be used for the design In constructed wetlands in the northern states, if the wetland does not freeze completely, the wetland will continue to function and water temperatures under the ice can be estimated as 40°F (5"C)(Bovd 1991). However, at low tempera- tures, removal rates will be lower and the wetland will have to be larger to accommodate the slower rates. Alternatively, the wastewater can be stored in the pretreatment unit during the cold seasons. In this case, a higher value for water temperature can be used and the wetland made smaller, de- pending on BOD5load. wastewater volume, and local temperatures. The values to be used for F and K_ in design- ing constructed wetlands have not been con- ° firmed. The value often used for F in domestic systems is about 0.52. This should be the lower limit for F unless research determines otherwise. If organic material is adequately removed by pretreatment. the value of F can be increased, but it should not be more than 0.90. For an agricul- tural waste treatment lagoon, the value ofF may- equal 0.90. A value for K,. of 0.0057 (l.lpM)is~ often used. However, experimental data on the values to be used in designing constructed wet- lands have been difficult to obtain because of the . logistic and economic difficulties in experiment- ing with wetlands on a scale large enough to be appropriate. The wetland should be sized gener- ously to accommodate these uncertainties. The hydraulic characteristics of the con- structed wetland should provide the required hydraulic residence time of at least,12 days. The hydraulic design is calculated using the average depth of water and average daily flow rate into the wetland to find an arrangement that results in the required hydraulic residence time: SA=t/(DxP/Q) (4.3) where SA = surface area, ft2 (length x width) t = hydraulic residence time, days D = average water depth in the constructed wetland, ft P = porosity, percent as a decimal Q = average daily flow rate, ftVday. ------- PRESUMPTIVE METHOD FOR NITROGEN The presumptive method can be modified to design wetlands for nitrogen removal. To treat ammonia to concentrations in the wetland effluent of less than 10 mg/L (the usual discharge limit). Hammer (1992) recommends that influent nitrogen not exceed 9 Ib/ac/day as TKN (10 kg/ ha/day). To size the constructed wetland for TKN: 1. Determine the production of ammonia: Ammonia production (Ib) Average 1000 Ib animal weight fib) unit fAU1 per dav Dairy cows 1300 1.6 Beef cattle 750 1.4 Swine 200 2.1 Poultry - layers 4 3.7 Poultry - broilers 2.2 5.1 1. Calculate the concentration of nitrogen (as TKN) per day: Assuming that TKN is 150% of ammonia: mg/L TKN = mg/L ammonia x 1.5 2. Calculate the daily load of TKN: TKN (Ib/day) = mg/L TKN x fWhr of influent x 680 (conversion factor) 3. Determine the surface area needed: Surface area (SA)(ac) = Ib/day TKN + 9 Ib TKN ac/day. ------- ------- CHAPTER 5 SUBSURFACE FLOW WETLANDS INTRODUCTION. The use of SSF wetlands in agriculture has been limited because of the high probability that they will be clogged by water containing more than about 30 mg/L solids. The design information provided in this chapter is a summary of the information in Subsur- face Flow Constructed Wetlands for Wastewater Treatment: A Technology Assessment (Reed 1993). Reed based his recommendations on the perfor- mance of 14 municipal, domestic, hospital, and industrial systems that have provided detailed data and that are thought to be representative of constructed wetland systems in the United States. Many of these systems are in the South and West, and most have been operating for less than five years. Only a limited number of systems\in the Northeast have provided operational data. WETLAND DESIGN Guidelines to designing a SSF constructed wetland are given in table 6. The design assumes that the wastewater has been pretreated to reduce BOD,by 70%, TSS to less than 1,500 mg/L, and ammonia to less than 100 mg/L. DARCY'S LAW The intent of the SSF wetland treatment concept is .to maintain the flow below the surface of the media in the bed. The design of SSF wet- lands has generally been based on Darcy's Law. which describes the flow regime in a porous medium. However, many of the systems designed with Darcy's Law have developed unintended surface flow and may have been under-designed. Pretreatment Configuration Flow Bottom slopes Inlet Outlet Vegetation Construction Tables. Design summary for subsurface flow wetlands. Reduction of BODS by 70% Reduction of solids to <1,5 00 mg/L Reduction of ammonia to <100 mg/L Fit the wetland to the site Divide large wetlands into side-by-side cells By gravity, as much as possible Subsurface flow design based on Darcy's Law Side-to-side elevations: level Inlet to outlet slopes: almost flat (0.5 -1.0%) Surface manifold with adjustable outlets Perforated subsurface manifold connected to adjustable outlet Complete coverage is more important than the species used Use at least two or three different species ; Medium(a) must be clean Wetland must be sealed to limit infiltration and exfiltration Water table must be below or excluded from the wetland ------- Darcy's Law assumes laminar flow, a constant and uniform flow (Q). and lack of short-circuiting. conditions that do not exist in constructed wet- °. lands (see Volume I). Darcy's Law is thought to provide a reasonable approximation of the°hydrau- lic conditions in an SSF bed if small to moderate size gravel (------- lie gradient will help to maintain near-laminar flow in the bed and validate the use of Darcy's Law in the design of the system. Since this approach ensures a relatively wide entry zone, it will also result in low organic loading on the cross-sectional area and thereby lessen concerns over clogging. BED SLOPE The bottom of the cell can be flat or slightly sloping from top to bottom. The top surface of the medium should be level regardless of the slope,of the bottom. A level surface facilitates plant management and minimizes surface flow problems. Once surface flow develops on a downward sloping surface, flow may not penetrate the medium even though the true water level within the medium is well below the surface. SIZING BIOCHEMICAL OXYGEN DEMAND SSF systems are generally designed for BOD5 removal. In SSF systems, the physical removal of BOD5 is believed to occur rapidly through settling and entrapment of particulate matter in the void spaces in the gravel or rock media (Reed 1993). Most of the existing systems in the United States and Europe have been designed as at- tached growth biological reactors using the same equations as those used for SF wetlands (equa- tions 4.1 - 4.3). The plug flow model is pres- ently in general use and seems to provide a general approximation of performance. It is believed that the plug flow rate constant for SSF wetlands is higher than for facultative lagoons or SF wetlands because the surface area available on the media in SSF wetlands is much higher than in the other two cases. This surface area supports the attached growth microorganisms that are believed to provide most of the treat- ment responses in the system. At an apparent organic loading of 98 Ib/ac/day (110 kg/ha/day). the rate constant for the SSF wetland (1.104 d'1) is about an order of magnitude higher than that for facultative lagoons, and about double the value often used for SF wetlands. The "t". or hydraulic residence time factor in equation 4.1 can be defined as: t = nLWd/Q (5,2) where n = porosity (% as a decimal) L = length of bed (ft.'m) W = width of bed (ft, m) d = average water depth (ft, m) Q = average flow rate through bed (ftVday, mVday). The Q value in equation 5.2 is the average flow in the bed [(Q,0 + Q.J/2]'. calculated from flow through the bed plus gains and losses from precipitation and evapotranspiration. This is the same value used in Darcy's Law for hydrau- lic design. The "d" value in the equation is the average depth of liquid in the bed. If- the design hydrau- lic gradient is limited to 10% of the potential available, as recommended above, then the average depth of water in the bed will be equal to 95% of the total depth of the treatment media in the bed. Since the term LVV in equation 5.1 is equal to the surface area of the bed, rearrangement of • terms permits the calculation of the surface area (A ) required to achieve the necessary level of BOD, removal: A, = LxW = Qln(C./C0)/-kTdn (5.3) where Af = bed surface area (ft2) other terms as defined previously. The depth of the media selected will depend on the design intentions for the system. If the vegetation is intended as a major source of oxygen for nitrification in the system, then the depth of ------- the bed should not exceed the potential root penetration depth for the plant species chosen. This will ensure the availability of some oxygen throughout the bed profile but may require man- agement practices which assure root penetration to these depths. The design and sizing of the SSF bed for BODS removal is an iterative process: 1. Determine the media type, vegetation, and depth of bed to be used. 2. By field or laboratory testing, determine the porosity (n) and "effective" hydraulic conduc- tivity (kj of the media to be used. 3. Use equation 5.3 to determine the required surface area of the bed for the desired levels of BOD5 removal. 4. Depending on site topography, select a pre- liminary length-to-width ratio; 0.4:1 up to 3:1 are generally acceptable. 5. Determine bed length (L)'and width (W) for the previously assumed length-to-width ratio, and the results of step 2. 6. Using Darcy's Law (equation 5.1) with the previously recommended limits (ks ------- plants can provie o ^ ^ plants can provide oxy en ^^ Pe. The ^^^^e ?or denitrification. i ould then be ava, able t ri{ication include the substrate. ------- 8 ------- REFERENCES Boyd. W. H. 1991. Adapting constructed wetland technology to treat agricultural wastewater. Paper presented at the Mid-Central Conference of the American Society of Agricultural Engineers, St. Joseph, MO, April 19-20, 199l! -Brix, H. 1993. Wastewater treatment in con- structed wetlands: system design, removal processes, and treatment performance, pp 9-22 in Constructed Wetlands for Water Quality Improvement, G. A. Moshiri (ed.). CRC Press, Boca Raton, FL. Cathcart, T. P.. D. A. Hammer, and S. Triyono. 1994. Performance of a constructed wetland- vegetated strip system used for swine waste treatment, pp 9-22 in Proceedings of the Work- shop on Constructed Wetlands for Animal Waste Treatment, Purdue University, Lafayette, IN. 4-6 April 1994. Conway, T. E., and J. M. Murtha. 1989. The Iselin marsh pond meadow, pp 139-143 in Constructed Wetlands for Wastewater Treatment, D. A. Hammer (ed.). Lewis Publishers, Chelsea, MI. Cooper, C. M., S. Testa, III, and S. S. Knight. 1993. Evaluation of ARS and SCS Constructed Wet- land/Animal Waste Treatment Project at Hernanado, Mississippi. National Sedimentation Laboratory Research Report No. 2. Oxford, MS. - 55 pp. Crumpton, W. G., T. M. Isenhart, and S., W. Fisher. 1993. Fate of non-point source nitrate loads in freshwater wetlands: results from experimental wetland mesocosms. pp 283-291 in Constructed Wetlands for Water Quality Improvement, G. A. Moshiri (ed.). CRC Press, Boca Raton, FL. EPA (Environmental Protection Agency). 1988. Design Manual: Constructed Wetlands and Aquatic Plant Systems for Municipal Wastewater Treatment, EPA/625/1-88/022. Center for Environmental Research, Cincinnati, OH. 83 pp. EPA (Environmental Protection Agency). 1993. Created and Natural Wetlands for Controlling Nonpoint Source Pollution. CRC Press. Boca Raton, FL. 216 pp. Gambrel. R. P., and W. H. Patrick, Jr. 1978. Chemical and microbiological properties of anaerobic soils and sediments, in Plant Life in Anaerobic Environments, D. D.,Hook and R. M. - M. Crawford (eds.). Ann Arbor Sci. Publ., Inc.. Ann Arbor, ML Hammer, D. 1992. Designing constructed wetland systems to treat agricultural nonpoint source pollution. Ecological Engineering 1:49-82. Hammer, D. A., B. P. Pullin, t. A. McCaskey. J. Eason, and V. W. E. Payne. 1993. Treating livestock wastewaters with constructed wet- lands, pp 343-347 in Constructed Wetlands for Water Quality Improvement, G. A. Moshiri (ed.). CRC Press, Boca Raton. FL. Hunt, P. G., A. A. Szogi, F. J. Humenik, J. M. Rice, and K. C. Stone. 1994. Swine wastewater treatment by constructed wetlands in the south- eastern United States, pp 144-154 in Proceed- ings of the Workshop on Constructed Wetlands for Animal Waste Treatment, Purdue University, Lafayette, IN, 4-6 April 1994. Ives, M. 1988. Viral dynamics in artificial wet- lands, pp 48-54 in Proceedings of a Conference on Wetlands for Wastewater Treatment and Resource Enhancement, G. A. Allen and R.A Gearheart (eds.). August 2-4, Humboldt State University. Arcata, CA. Kadlec, R. H. 1994. Detention and mixing in free water wetlands. Ecological Engineering 3:345- 380. Kadlec, R. H., W. Bastiaens, and D. T. Urban. 1993. Hydrological design of free water surface treatment wetlands, pp 77-86 in Constructed Wetlands for Water Quality Improvement, G. A. Moshiri (ed.). CRC Press, Boca Raton, FL. ------- Knight. R. L. R. W. Rible, R. H. Kadlec, and S. Rued. 1993. Wetlands for wastewater treatment: performance database, pp 35-58 in Constructed Wetlands for Water Quality Improvement. G. A. Moshiri (ed.). CRC Press, Boca Raton. FL. Knight. R. L.. R. W. Ruble, R. H. Kadlec, and S. C. Reed. 1994. Wetlands Treatment Database (N'orth American Wetlands for Water Quality Treatment Database). USEPA, Risk Reduction Engineering Laboratory, Cincinnati, OH. The database is available on 3.5" diskette, and requires DOS 3.3 or higher, 640K of memory, and 4MB of free disk space. To order, contact: Don Brown, USEPA (MS-347), Cincinnati. OH 45268: phone (513) 569-7630; fax (513) 569 7677; e-mail: brown.donald@epamail.epa.gov. McCaskey, T.A.. S. N. Britt, T. C. Hannah. J. T. Eason. V.W. E. Payne, and D. A. Hammer. 1994. Treatment of swine lagoon effluent by con- structed wetlands operated at three loading rates, pp 23- 33 in Proceedings of the Workshop on Constructed Wetlands for Animal Waste Treatment, Purdue University, Lafayette. IN, 4-6 April 1994. Mitsch. W. J., and J. G. Gosselink. 1986. Wet- lands. Van Nostrand Reinhold, New York, NY. 539 pp. Murphy. T. J., J. Zaginaylo, III; and F. Geter. 1993. Design and construction of a wetland to treat veal waste. American Society of Agricultural Engineers Meeting, Chicago, IL, 14-17 December 1993. Reaves, R. P., P. J. DuBowy, and B. K. Miller. 1994. Performance of a constructed wetland for dairy waste treatment in Lagrange County, Indiana, pp 43-52 in Proceedings of the Workshop on Con- structed Wetlands for Animal Waste Treatment, Purdue University, Lafayette, IN, 4-6 April 1994. Reaves, R. P., P. J. DuBowy, D. D. Jones, and A. L. Sutton. 1995. First year performance of an experimental constructed wetland for swine waste treatment in Indiana. Proceedings of the Conference of Versatility of Wetlands in the Agricultural Landscape, Tampa, FL, September 1995. Reed. S. C. 1993. Subsurface Flow Constructed Wetlands For Wastewater Treatment: a Technol- ogy Assessment. EPA 832-R-93-001. EPA Office of Wastewater Management, Washington. D. C. Reed. S., and D. S. Brown. 1992. Constructed wetland design - the first generation. Water Environment Research 64(6):776-rai. Richardson, C. J., and C. B. Craft. 1993. Effective phosphorus retention in wetlands: fact or fic- tion? pp 271-282 in Constructed Wetlands for Water Quality Improvement, G. A. Moshiri (ed.). CRC Press, Boca Raton, FL. Skarda, S. M., J. A. Moore, S. F. Niswander, and M. J. Gamroth. 1994. Preliminary results of wetland for treatment of dairy farm wastewater. pp 34-42 in Proceedings of the Workshop on Constructed Wetlands for Animal Waste Treat- ment, Purdue University, Lafayette,.IN, 4-6 April 1994. SCS (Soil Conservation Service). 1991. Con- structed Wetlands for Agricultural Wastewater Treatment Technical Requirements, August 9, 1991.. SCS (Soil Conservation Service). 1992. Agricul- tural Waste Management Field Handbook, April 1992. Taylor, L. E., R. E. Edwards, and C. S. Takita. in preparation. Water Quality and Hydrogeology of Two Small Agricultural Basins in Central Penn- sylvania. Susquehanna River Basin Commis- sion, Harrisburg, PA. Vymazal,]. 1993. Constructed wetlands for , wastewater treatment in Czechoslovakia: state of the art. pp 255-260 in Constructed Wetlands for Water Quality Improvement, G. A. Moshiri (ed.). CRC Press, Boca Raton, FL. Water Pollution Control Federation. 1990. Manual of Practice FD-16, Chapter 9. Alexan- dria, VA. ------- Watson, J. T., and J. A Hobson. 1989. Hydraulic design considerations and control structures for constructed wetlands for wastewater treatment, pp 379-391 in Con- structed Wetlands Tor Wastewater Treatment, D. A. Ham- mer (ed.). Lewis Publishers, Chelsea, MI. Wieder, R K.. G. Tchobanoglous, and R. W. Tuttle. 1989. Preliminary considerations regarding constructed wet- lands for wastewater treatment, pp 297-305 in Con- structed Wetlands for Wastewater Treatment, D. A. Ham- mer (ed.). Lewis Publishers, Chelsea, MI. Wolverton, B. C. 1989. Aquatic plant/microbial filters for treating septic tank effluent, pp 173-178 in Constructed Wetlands for Wastewater Treatment, D. A. Hammer (ed.). . Lewis Publishers, Chelsea, MI. ------- ABBREVIATIONS AND CONVERSION FACTORS MULTIPLY ac, acre cfs, cubic foot per second cfs. cubic fool per second cm, centimeter cm/sec, centimeter per second °F. degree Fahrenheit ft, foot ft2, square foot ft3, cubic foot ft/mi, foot per mile fps, foot per second g/m2/day. gram per square meter per day gal, gallon gal, gallon gpm, gallon per minute ha, hectare inch kg, kilogram kg/ha/day, kilogram per hectare per day kg/m2, kilogram per square meter L, liter L, lite* Ib, pound Ib/ac, pound per acre m, meter m2, square meter m3, cubic meter m3, cubic meter nvVha/day, cubic meter per hectare per day mm, millimeter mi, mile BY 0.4047 448.831 2.8317 x 10'2 0.3937 3.28 X 10"2 5/9(°F-32) 6.305 9.29 x 10'2 2.83 x 10'2 0.1895 18.29 8.92 3.785 3.785 x 10'3 6.308 x 10'2 2.47 2.54 2.205 0.892 0.2 3. 531 x lO'2 0.2642 0.4536 1.121 3.28 10.76 1.31 264.2 106.9 3.94 x 10'2 1.609 TO OBTAIN ha, hectare gpm, gallon per minute m3/s, cubic meter per second inch fps, foot per second °C, degree Celsius m, meter m2- square meter m3. cubic meter m/km, meter per kilometer m/min, meter per minute Ib/ac/day, pound per acre per day L, liter m3, cubic meter L/s, liter per second ac, acre cm, centimeter Ib, pound Ib/ac/day, pound per acre per day Ib/ft2, pound per square foot ft3, cubic foot gal, gallon kg, kilogram kg/ha, kilogram per hectare ft. foot ft2, square foot yd3, cubic yard gallon, gal gallon per day per acre, gpd/ac inch kilometer, km ------- ------- ,?,,P..Q.Qo -------