Evaluation of a Subsurface-Flow Constructed Wetland for Onsite Wastewater Treatment using NSF Standard 40 Protocol

Transcription

1 Evaluation of a Subsurface-Flow Constructed Wetland for Onsite Wastewater Treatment using NSF Standard 4 Protocol Pablo A. Davila, Joe C. Yelderman Jr., Robert Doyle, Bruce Lesikar, Courtney O Neill, and Samuel Rodriguez* Abstract A subsurface-flow constructed wetland was evaluated (6 February to 31 May 26) using the American National Standards Institute (ANSI)/NSF International- Standard 4 protocol for Class I onsite wastewater treatment at the Baylor Wastewater Research Program (BWRP) site within the Waco Metropolitan Area Regional Sewerage System (WMARSS) treatment plant, Waco, Texas. Raw wastewater from the WMARSS plant was pumped into a two-chambered 1,5 gallon septic tank and flowed by gravity into the 1 feet x 5 feet x 1 foot treatment wetland then returned to the WMARSS plant. Both septic tank and wetland effluent samples were collected and analyzed, according to Standard 4 criteria, for carbonaceous biochemical oxygen demand (CBOD 5 ), total suspended solids (TSS), and ph. In addition, samples were analyzed for nutrients (total nitrogen and total phosphorus). Rainfall and temperature data were also collected for water budget calculations and to aid interpretations. Because this study used the Standard 4 protocol it allows direct comparison of treatment wetlands to other on-site treatment processes evaluated under Standard 4 protocol. The study provides results that may apply to feasibility of constructed wetlands in areas where traditional onsite treatment systems (septic tank and drain field) are not viable alternatives. The study also provides insight toward the appropriateness of Standard 4 for subsurface-flow treatment wetlands. Funding for this study was provided by the Texas On-Site Wastewater Treatment Research Council. *Pablo A. Davila, graduate student, Environmental Studies, Baylor University. Joe C. Yelderman Jr., Professor, Geology, Baylor University. Contact: , Joe_Yelderman@baylor.edu Robert Doyle, Professor, Biology, Baylor University. Bruce Lesikar, Professor, Texas Cooperative Extension, Texas A&M University Courtney O Neill, Extension Assistant, Texas Cooperative Extension, Texas A&M University Samuel Rodriguez, graduate student, Environmental Studies, Baylor University.

2 Introduction In this study a subsurface-flow constructed wetland was evaluated using the ANSI/NSF International Standard 4 protocol for onsite wastewater aerobic treatment units. The importance of this study is that by applying the Standard 4 protocol, a subsurface-flow wetland can be evaluated under a rigorous test schedule and compared directly to other treatment systems evaluated with the same protocol. Wastewater treatment is necessary before dispersal into the environment. Proper wastewater treatment before dispersal prevents deterioration of our water resources and aquatic life. Wastewater treatment can be achieved through a variety of methods. The majority of rural residents use onsite wastewater treatment systems (OWTS) for wastewater treatment. The traditional onsite wastewater treatment system consists of a septic tank and soil absorption field. The septic tank allows for settling of suspended solids and some digestion of organic matter. Septic tank effluent is discharged through a perforated pipe to a soil absorption field where biological processes in the soil further treat the effluent. There is a high demand for onsite wastewater treatment systems. In the United States OWTS serve nearly 22 million households (National Ground Water Association, 25). An advantage of OWTS is their cost effectiveness. Individual houses and small communities can avoid large investments and operating expenses required of a larger-scale municipal treatment system. Another advantage is minimal impact on the environment. The point-source loading of pollutants into receiving waters from centralized plants can impact aquatic life (EPA Onsite Wastewater Treatment Systems Manual, 22), while the dispersed wastewater volumes of OWTS avoid the problem of point-source loading. The problem arises when conventional septic tank pretreatment and gravity distribution drain fields are used in locations where soil absorption fields are not suitable. These include high flooding potential, steep slopes, thin topsoil, high ground water tables or clay soils (Perkins, 1989). Additional pretreatment can be used in these areas to limit the risk of contaminant movement through the soil. Concerns about traditional septic tank appropriateness at certain sites have resulted in the search for alternative OWTS. One alternative is the use of a subsurface-flow constructed wetland in place of the soil absorption field. A subsurface-flow constructed wetland is a system that provides wastewater treatment in filter media that is not directly exposed to the atmosphere but may be slightly influenced by the roots of surface vegetation (EPA Onsite Wastewater Treatment Systems Manual, 2). In a septic tank and subsurface-flow wetland system, the septic tank receives wastewater directly from the household where solids separate from liquid wastes and decompose. The liquid wastes flow into the constructed subsurface-flow wetland for secondary treatment. In a subsurface-flow wetland, wastewater flows through porous media (gravel is most commonly used) as microbial bacteria attached to the media and plant roots decompose organic matter. Figure 1 represents the subsurface-flow wetland used in this study.

3 Fig. 1. Design of Subsurface-Flow Wetland. The width of 1 feet is not shown Source: Wetland profile view, Texas Cooperative Extension, 25. Although subsurface-flow (SSF) constructed wetlands are similar in function to natural wetlands, there are key differences. Natural wetlands are usually free water-surface (FWS) wetlands where water is visible at the surface. In subsurface-flow wetlands water is not exposed at the surface, but maintained within the media. Secondly, water flow in a subsurface-flow wetland treating household wastewater is more consistent than in natural wetlands because volume is controlled by household use. In natural wetlands there is more fluctuation in response to rainfall because precipitation is less consistent and less predictable. A third distinction is the role wetlands serve for fauna. Natural wetlands are inhabited by diverse fauna, but in a subsurface-flow wetland fauna are limited because water is not exposed to the surface Avoiding wastewater exposure to the surface contributes to the lack of odor problems and the decrease of possible health risks from exposure to untreated wastewater. The final difference is the efficiency of treatment per square meter. Subsurface-flow wetlands provide greater surface area to microbial bacteria in the media, which allows for greater treatment efficiency per square meter of wetland surface (Dusel and Pawlewski, 1997; National Small Flows Clearinghouse, 1998; Cole, 1998). These key differences account for several advantages subsurface-flow wetlands have over free watersurface wetlands for wastewater treatment. Studies on subsurface-flow constructed wetlands have verified them as reliable, low-cost, lowenergy processes requiring minimal operational attention (EPA Subsurface Flow Constructed Wetlands for WasteWater Treatment: A Technology Assessment, 1993). Physical, chemical, and biochemical reactions contribute in wastewater treatment to provide a high quality effluent for in-ground disposal (EPA Wastewater Technology Fact Sheet, 2). Studies conducted on constructed wetlands for wastewater treatment show them to be effective for removal of BOD and TSS (EPA Wastewater Technology Fact Sheet, 2). The EPA s Subsurface Flow Constructed Wetlands for Wastewater Treatment-A Technology Assessment (1993) was based on fourteen systems believed to be representative of systems in operation in the United States showed effluent levels below 2 mg/l for BOD and TSS. Table 1 shows the performance of the 14 system averages and parameters tested.

4 Table 1. Summary of Performance for 14 SF Wetland Systems* Constituent Mean Influent mg/l Mean Effluent mg/l BOD 5 28** (5-51)*** 8** (1-15)*** TSS 6 (23-118) 1 (3-23) TKN as N 15 (5-22) 9 (2-18) NH 3 /NH 4 as N 5 (1-1) 5 (2-1) NO 3 as N 9 (1-18) 3 (.1-13) TN 2 (9-48) 9 (7-12) TP 4 (2-6) 2 (.2-3) * Mean detention time 3 d (range 1 to 5 d). ** Mean value. *** Range of values. Source: EPA Wastewater Technology Fact Sheet, 2 Another EPA publication, Constructed Wetlands Treatment of Municipal Wastewaters Manual, provides results of four studies of varied size, location, and hydraulic loading. Performance history for the Mesquite Nevada, June 1992-May 1993, study shows 55% and 77% reduction for BOD and TSS, respectively, as well as average effluent values of 16 mg/l TKN and 6.2 mg/l TP. The report includes monthly effluent characteristics in relation to temperature (Table 2). Removal of BOD and various forms of nitrogen have been shown to be temperature dependent (EPA Wastewater Technology Fact Sheet, 2). Table 2. Monthly effluent characteristics in response to temperature. Month Temp. C BOD mg/l TSS mg/l NH 4 -N mg/l TKN mg/l TP mg/l 1992 Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Source: Constructed Wetlands Treatment of Municipal Wastewaters Manual, 2. Despite studies that show evidence of subsurface-flow constructed wetlands ability to provide a high quality effluent, the question as to how subsurface-flow wetlands perform when applied to the same standards applied to other onsite wastewater treatment systems, such as aerobic systems, remains. The uniqueness of this study is that the same testing regimen used by ANSI/NSF International Standard 4 for onsite aerobic treatment units was applied to a subsurface-flow constructed wetland. ANSI/NSF International is the leading agency in testing onsite wastewater treatment systems. The Standard 4 test, relating to residential wastewater

5 treatment systems, requires six months of performance testing, incorporating stress tests to simulate wash day, working parent, power outage, and vacation conditions. This study provides results on the effectiveness of a constructed subsurface-flow wetland for wastewater treatment during the first 16 weeks of design loading under these rigorous test conditions. The objective was to study a subsurface-flow constructed wetland under the NSF International Standard 4 Class I on-site wastewater treatment system protocol. Three specific objectives of the study were to: a) Evaluate treatment effectiveness under Standard 4 protocol b) Compare treatment effectiveness between septic tank and wetland effluents c) Evaluate appropriateness of Standard 4 Test for constructed wetlands. Hypotheses: 1) The subsurface-flow constructed wetland will produce an effluent that meets or exceeds Standard 4 criteria. 2) The subsurface-flow constructed wetland will significantly reduce total suspended solids (TSS), carbonaceous 5-day biological oxygen demand (CBOD 5 ), total nitrogen, and total phosphorus when comparing septic tank effluent to wetland effluent. 3) Treatment effectiveness will be correlated to precipitation. Effects of rainfall on site will be evaluated. Location The study was conducted at the Baylor Wastewater Research Program site, within the confines of Waco Municipal Area Regional Sewerage System (WMARSS) plant; immediately adjacent to the NSF-Waco certification site. WMARSS is approximately five miles south of downtown Waco, adjacent to the Brazos River. Waco is in McLennan County, midway between the cities of Dallas and Austin on Interstate-35 and generally in the middle of the state of Texas. Materials and Methods The combination of a 1,5 gallon septic tank and subsurface-flow constructed wetland was designed to treat 5 gallons of wastewater per day, the approximate use of a three bedroom, four-person household. Construction As designed by Dr. Bruce Lesikar of Texas Cooperative Extension, wetland dimensions were 5 ft x 1 ft x 1 ft. A 45mil poly-ethylene pond liner and bentonite clay pellets, underneath the liner, were installed to prevent water loss to soil filtration. The wetland was filled with grade 3 concrete rock (washed gravel ¾ to 1½ inches thick). Vegetation includes cattail (Typha latifolia), bulrush (Scirpus cubensis), pickerel weed (Pontederia cordata) and iris (Iris pseudacorus). These were selected based on size, suitability, availability, and seasonal growth pattern. Wastewater dosing was according to Standard 4 protocol; 5 gallons a day during specific time intervals (Table 3). As wastewater was pumped into the septic tank it displaced water in the septic tank that then flowed into the wetland (Note: an effluent filter was installed in the septic

6 tank to minimize potential clogging in the wetland media). There is no gradient in the wetland; water flows as a result of displacement when dosing occurs. Hydraulic residence time varies according to precipitation and evapotranspiration rates, but is approximately two days since the wetland holds approximately 1, gallons. The wetland effluent flows by gravity into a buried pump tank before it returns to the WMARSS plant. The wetland and septic tank used in this study were new and contained water from hydraulic testing before the initial dosing started. Therefore, both the septic tank and the wetland had to mature with microbial communities. The initial dosing started January 24, 26. Collection of the data used in the evaluation started February 6, 26. System Dosing Wastewater dosing to the wetland was accomplished through a digital timer that activates a dosing pump to fill a calibrated 5-gallon bucket and a valve that drains the bucket into the 1,5- gallon septic tank. Table 3 specifies the system dosing schedule. Table 3. Standard 4 design loading specifications. Time Frame Percent of daily hydraulic capacity 6: a.m. 9: a.m. 35% (175 gallons) 11: a.m. 2: p.m. 25% (125 gallons) 5: p.m. 8: p.m. 4% (2 gallons) Total = 5 gallons Source: NSF Residential wastewater treatment systems, 2. Using this schedule there were 1 doses a day at 5 gallons per dose, totaling 5 gallons a day. To monitor doses, a counter and hour meter were installed to verify whether the system received the correct number of full doses. The hour meter records the hours (and hundredths of hours) that power goes to the dosing pump. Sample Collection NSF Standard 4 protocol requires samples be flow-proportional, 24-hour composites. This was accomplished by using timers and peristaltic pumps to extract volumetrically calibrated samples during each dosing cycle. The composite samples (septic and wetland effluent) were collected in refrigerated containers, retrieved each day and analyzed within 48 hours. All programs for the septic sampling pump have at least a five minute delay to allow fresh sample to reach the area near the sample extraction pump. The minimum delay on the wetland timer is ten minutes after dosing starts which is adequate since flow is observed to increase within two to three minutes at the wetland effluent sampling point after dosing starts. Analytical Procedures Standard 4 Protocol requires samples be analyzed for total suspended solids (TSS), carbonaceous 5-day biochemical oxygen demand (CBOD 5 ), and ph. Samples were also analyzed for total nitrogen and total phosphorus.

7 -Carbonaceous 5-day biochemical oxygen demand (CBOD 5 ) and Total Suspended Solids (TSS) Septic and wetland effluent composite samples were collected and analyzed five days a week, Monday through Friday. Samples were sent to an outside lab, AquaTech, for CBOD 5 and TSS analysis. Standard 4 protocol specific requirements for effluent are: 3-day average CBOD 5 shall not exceed 25 mg/l 7-day average CBOD 5 shall not exceed 4 mg/l 3-day average TSS shall not exceed 3 mg/l 7-day average TSS shall not exceed 45 mg/l - ph A YSI 65MDS display and 6QS sonde at both septic and wetland effluent points measured ph every hour and these hourly measurements were averaged for the daily 24-hour composites. -Total nitrogen and total phosphorus Samples were analyzed twice a week by lab technicians at the Baylor University Center for Reservoir and Aquatic Systems Research (CRASR). Laboratory equipment used included Lachat QuickChem 85 Flow Injection Analyzer. -Temperature and ph Values recorded every hour were used to obtain a daily average. For a weekly average, the daily averages from Sunday to Friday were used. When comparing wetland effluent values for TSS and CBOD to Standard 4 requirements a 7-day average had to include a minimum of three data days. A 3-day average required a minimum of 15 data days. To calculate percent removal for CBOD and TSS septic effluent samples were paired with wetland effluent samples discharged two days later. Therefore, there were three data days per week; Monday Septic compared to Wednesday Wetland, Tuesday Septic compared to Thursday Wetland, and Wednesday Septic compared to Friday Wetland. The difference in mg/l was then divided by septic effluent concentrations in mg/l to obtain percent removal. For total nitrogen and total phosphorus a monthly percent removal was calculated because there were only two samples per week for TN and TP. Rainfall events greater than.1 inches (31 additional gallons to wetland) were plotted against CBOD, TSS, and TN. Results All ph values recorded fell within the Standard 4 required range of ph 6 to 9 (Table 4). Weekly wetland effluent values for all parameters are shown in Table 4 and monthly wetland effluent values are shown in Table 5. Figure 2 shows TSS (mg/l) daily values and 7-day averages compared with the Standard 4 requirement. All TSS data points as well as all the 7- day averages are well below the required values for Standard 4. However, the TSS values generally decrease in concentration from February to April. Figure 3 shows TSS daily values and 3-day averages in mg/l compared to the Standard 4 requirement. All the data points and all the 3-day averages are below the required values for Standard 4 and there is a general decrease over time.

8 Table 4. Wetland effluent weekly averages Weeks TSS (mg/l) CBOD (mg/l) ph Temp. ( C) TN (mg/l) TP (mg/l) N/A N/A 3.26 N/A N/A = not available. Table 5. Wetland Effluent monthly averages Feb. Mar. Apr. May CBOD (mg/l) TSS (mg/l) ph Temp. ( C) TN (mg/l) TP (mg/l)

9 TSS (mg/l TSS Daily values 7-day average Standard 4, 7-day avg. 2/6/6 3/6/6 4/3/6 5/1/6 5/29/6 Fig. 2. The TSS daily and 7-day averages compared to Standard 4, 7-day average limit. 35 TSS (mg/l TSS Daily values 3-day average Standard 4, 3-day avg 2/6/6 3/6/6 4/3/6 5/1/6 5/29/6 Fig. 3. The TSS daily and 3-day averages compared to Standard 4, 3-day average limit. Figure 4 compares CBOD (mg/l) daily values to Standard 4 required 7-day average. The 7-day average wetland effluent values for CBOD did not meet the Standard 4 requirement in the early stages of the study but by April the CBOD was well below the required 7-day average. Figure 5 compares CBOD values with the Standard 4 requirement for 3-day averages. Again, the wetland effluent did not meet the Standard in the beginning (February and March), but by April the 3-day average was below the required value.

10 12 1 CBOD Daily values 7-day average Standard 4, 7-day avg. CBOD (mg/l /6/6 3/6/6 4/3/6 5/1/6 5/29/6 Fig. 4. The CBOD daily and 7-day averages compared to Standard 4, 7-day average limit CBOD Daily values 3-day average Standard 4, 3-day avg. CBOD (mg/l /6/6 3/6/6 4/3/6 5/1/6 5/29/6 Fig. 5. The CBOD daily and 3-day averages compared to Standard 4, 3-day average limit. The effectiveness of the wetland was assessed by calculating the percent reduction between the septic tank effluent and the wetland effluent. This was accomplished by using the residence time (2 days) and pairing the two samples that essentially represent the same wastewater. Table 6 shows the monthly averages for percent reduction of TSS, CBOD, TN and TP. The percent reduction for TSS and CBOD in paired data days from February 3 rd to May 31st are shown in Figures 6 and 7.

11 Table 6. Percent reduction between septic and wetland effluent monthly averages. TSS CBOD TN TP Feb Mar Apr May Paired Data Days: February 6th to May 31st % Reduction Fig. 6. TSS percent reduction from septic effluent samples paired with wetland effluent samples two days later. Paired Data Days: February 6th to May 31st % Reduction Fig. 7. CBOD percent reduction from septic effluent samples paired with wetland effluent samples two days later.

12 Rainfall events and daily TSS, CBOD, and TN values were plotted to see if the precipitation had any large effects but none were apparent (Figures 8-1) Rainfall TSS Rainfall (gallons Feb 3-Mar 28-Mar 22-Apr 17-May TSS (mg/l Fig. 8. The TSS values plotted with rainfall events.

13 Rainfall (gallons Rainfall CBOD Feb 3-Mar 28-Mar 22-Apr 17-May CBOD (mg/l Fig. 9. The CBOD values plotted with rainfall events Rainfall (gallons Rainfall Total Nitrogen Total Nitrogen (mg/l 6-Feb 3-Mar 28-Mar 22-Apr 17-May Fig. 1. Rainfall events and TN (mg/l) from February 6 to March 28.

14 Discussion This study indicates the subsurface-flow wetland is effective in reducing TSS, CBOD, TN and TP. For some parameters the wetland was effective immediately while for others it took a while to establish the system. Effluent quality and efficiency of treatment both changed over time for TSS and CBOD but during this time temperature increased (Table 4), biofilm developed and plant growth occurred. All of these factors probably affected the wetland performance but the significance of each has not been determined at this time. Under design loading conditions, the wetland has shown excellent reduction for TSS. Figures 2 and 3 show that since the beginning of the study (start date February 6) there has been no 7-day or 3-day average for TSS above Standard 4 limits. The CBOD values exceeded the Standard 4 limit during the first 2 months but by April the wetland CBOD values for 7-day and 3-day averages were below the Standard 4 requirement. Figures 4 and 5 are probably indicative of the delayed biofilm development. It was not until week 7, March 2, (Table 4) that the wetland effluent CBOD 7-day average approached the Standard 4 7-day average (4 mg/l). However, since that time (March 2) the wetland 7-day average has met the Standard 4 level 8 out of 9 weeks. Since April the 7-day and 3-day averages were below the Standard 4 requirement. One consideration for the delay in the system s effectiveness is near the onset of the study. Precipitation events may have delayed the biofilm development which is critical for CBOD reduction. The reduction of TSS is attributed more to a physical process where as the reduction of CBOD is a biochemical one. When compared with the EPA s Subsurface Flow Constructed Wetlands for Wastewater Treatment-A Technology Assessment, 1993 (Table 1) the wetland performance in this study is superior in TSS effluent levels, but not with regards to CBOD. In this study, the residence time in the wetland averaged two (2) days compared to 3 days in the EPA study. In general, longer residence time equates to better treatment. The EPA Wastewater Technology Fact Sheet (2) mentions BOD removal is temperature dependent. This study has shown a steady improvement in CBOD removal (Table 5). Part is attributed to biofilm development, but this study also began in winter months. As warmer temperatures approach, residence time will increase due to higher evapotranspiration rates, and system performance may continue to improve. There is also a strong correlation between nitrogen and phosphorus removal to water temperature (Ran, et al., 24). Total nitrogen and total phosphorus removal rates also may increase. There were significant rainfall events during the study. The majority of these rainfall events occurred during weekends when no samples were collected. Under Standard 4 protocol, days with greater than 55 gallons through a system, in this case a result of a rainfall cannot be considered valid samples. Plotting rainfall events and all samples- those counted and those not allowed by Standard 4 protocol- indicate that rainfall events do not produce higher or lower values in TSS, CBOD, or total nitrogen (Figures 8, 9 and 1). Figures 8 and 9 do not show any apparent relationship between rainfall events and wetland effluent levels for TSS and CBOD. Rainfall events disrupt the system by adding excess gallons and possibly reducing residence time, but it also may dilute the sample thereby offsetting any major impact.

15 Conclusion The study indicates the combination of a septic tank and subsurface-flow wetland when tested under the Standard 4 protocol: 1) Wetland effluent met the ph requirements for Standard 4 and ranged from 6.5 to 7. ph units during the study 2) The wetland effluent met the Standard 4 requirements for both the 7-day and 3-day average for TSS. 3) The wetland effluent exceeded the Standard 4 requirements for 7-day or 3-day average for CBOD during February and March. 4) The wetland effluent met the Standard 4 requirements for 7-day and 3-day average for CBOD during April and May. Nutrient (phosphorus and nitrogen) removal also occurred, but with less effectiveness. The study showed that the wetland required development time before certain parameters, particularly CBOD, were efficiently reduced. The first phase (weeks 1 6) showed primarily physical treatment but limited biological treatment. The second phase (weeks 7 1) showed the system improving. In the last phase (weeks 11 to present) the wetland demonstrated efficient treatment that met Standard 4 requirements. Standard 4 was designed for aerobic treatment systems which are stand alone enclosed systems isolated from precipitation and insulated from temperature fluctuations. The wetland system is a combination of a septic tank and subsurface-flow constructed wetland. It is exposed to precipitation and its surface location allows temperature to have more effect on the system. The Standard 4 protocol is not appropriate in its current form to properly evaluate a septic tank/wetland system. The Standard 4 protocol, if applied to a wetland system would need to address the septic tank, weather related factors, and perhaps development time in relation to wetland performance. Literature Cited Cole, Stephen The Emergence of Treatment Wetlands. Environmental Science & Technology. Vol. 32, Issue 9: Dusel, Charles E. and Craig W. Pawlewski Constructed Wetlands Offer Flexibility. Wastewater Treatment. National Ground Water Association. Oct Ground Water and On-Site Waste Recycling Groups to Collaborate. < National Sanitation Foundation. < National Small Flows Clearinghouse Constructed Wetlands: A Natural Treatment Alternative. Pipeline. Vol. 9, No. 3: 5-7. NSF International Standard/American National Standard for Wastewater Technology. Residential wastewater treatment systems. NSF/ANSI: July, 2. Perkins, Richard J. Onsite Wastewater Disposal. Chelsea, Michigan: Lewis, Ran, Noemi, Moshe Agami, and Gideon ORon. 24. A pilot study of constructed wetlands using duckweed (Lemna gibba L.) for treatment of domestic primary effluent in Israel. Water Research. 38: U.S. Environmental Protection Agency. Office of Wastewater Management.

16 < eaa f875fbb99!opendocument> U.S. Environmental Protection Agency. National Risk Management Research Laboratory- Office of Research and Development. Constructed Wetlands Treatment of Municipal Wastewaters Manual. Cincinnati: GPO, 2. U.S. Environmental Protection Agency. Onsite Wastewater Treatment Systems Manual. Cincinnati: GPO, 22. U.S. Environmental Protection Agency. Subsurface Flow Constructed Wetlands for WasteWater Treatment: A Technology Assessment. Washington: GPO, U.S. Environmental Protection Agency. Wastewater Technology Fact Sheet. Wetlands: Subsurface Flow. Washington: GPO, 2.