Analysis of Residential Subsurface Flow Constructed Wetlands Performance in Northern Alabama CONTRIBUTING WRITER Kathleen M. Leonard Ph.D., P.E. ABSTRACT Constructed wetlands are becoming increasingly popular as an alternative onsite wastewater treatment technology in many areas of the U.S. This paper documents a sample collection study and statistical analysis of five such residential constructed wetlands over seasonal variations. Results of the data analysis show that constructed wetlands can reduce organic content and coliform appreciably, an average of 85 percent and 90 percent respectively. The operational parameters obtained from this study are being used by local and state health departments for developing future design guidelines. One of the major causes of nonpoint source water pollution may be poorly operating wastewater treatment procedures, including conventional residential onsite systems. This problem is immense since over 27 million housing units in the U.S. are served by conventional onsite septic systems, and it has been documented that 2.5 million of these systems were malfunctioning (Eddy, 1999). in response to the need for practical treatment alternatives, many communities are investigating constructed wetlands (CWs) for these areas. However, there seems to be a reluctance to design and permit these innovative natural systems due to lack of knowledge of their performance and long-term viability (Cole, 1998). Constructed wetlands are commonly described as either free-water-surface (FWS) or subsurface flow (SF) systems. The FWS systems have visible standing water comparable to shallow lagoons and are usually found in larger, municipal systems. On the other hand, the SF type do not have visible flow, are similar to flow through a porous media (sand or gravel), and are quite similar to other types of attached growth processes (Reed, et al., 1995). Two major design approaches are being promulgated in the U.S. for designing CWs. The first approach was developed by the U.S. Environmental Protection Agency (EPA, 1988) for large municipal wastewater treatment systems and is based on plug-flow kinetics. The Tennessee Valley Authority (TVA) developed the second model for small onsite systems (e.g., less than 76 m 3 /day) with design criteria applicable for temperate climates. A recent article (Sauter and Leonard, 1997) documented differences in these design methods in residential applications. Constructed Wetland Systems in Northern Alabama It is often difficult to site conventional septic tank/leach field systems in Northern Alabama due to the heavy clay soils, limestone geology, and high water tables. This is a significant problem especially for small communities and rural areas that do not have access to sewer lines. Therefore, innovative decentralized and onsite systems are of major interest to health departments and land developers in the region. As a result, several experimental SF constructed wetlands were permitted and installed three to five years ago in suburban neighborhoods near Decatur in North Alabama. To verify performance efficiency, these constructed wetland systems were sampled for a variety of water quality parameters including biochemical oxygen demand (BOD), nitrates, ammonia, total coliform bacterial, turbidity, and solids over a range of seasons (e.g., temperatures). Water samples were taken at the influent to the cells (at hydraulic control structures), at the midpoint of two-cell systems, and from treated effluent at the end of the cell. All of the systems studied had a 1,250-gallon septic tank upstream of the wetland for initial solids and BOD removal. They were all designed for four bedroom homes using
160 140 120 100 80 60 40 20 0 Nov. Jan. March May July Sept. Feb. March June July Jan. Feb. March Date Plot Showing Variability of BOD Concentrations at Site 2 over 18-Month Sampling Study the TVA method for subsurface flow through a pea gravel medium. This design is considered non-discharge since the anterior portion of the final cell is unlined and the effluent infiltrates either through a sand sump or mound system. The sites had a variety of vegetation including sedges, softstem bulrush, canna lilies, and iris. The water surface is not visible and the vegetation is very attractive, giving the appearance of a garden in warmer months. Performance Data As noted previously, the experimental CWs have been in operation a minimum of three years. The systems were originally designed using the TVA method, but actual flows (Q) and organic loadings (BOD) were not verified initially. Based on the data obtained from this study, it was found that the flow rates were much lower than that suggested by the state health department standards (Alabama Department of Public Health). In fact, the average residential flows measured in this study were 73 percent of design criteria. In order to document organic loading and treatment, standard laboratory fice-day BOD tests were performed on the samples and then the data was graphed and analyzed. Samples were taken on a Average monthly basis at each loca- Site BOD in tion. Figure l is a plot (mg/l) showing influent (after sep Site 1 167 tic tank settling) and efflu- Site 2 75 ent concentrations for one site over a range of 18 Site 3 88 months. As illustrated, Site 4 93 there is a high variation in Site 5 82 the incoming BOD con- Average over centration. Since local all sites 101 health departments and engineers had questions about seasonal efficiency of the wetlands, part of this study was to document that the CWs performed over the winter months. The effluent BOD was below the required 30 mg/l except for two sampling periods in January and February of the second year. This could be indicative of clogging or low temperature effects. It should be noted that the March data for both years did not show any problem. Also, samples were only taken at two-month intervals over the first year due to financial constraints. Table 1 is a summary of the above BOD data showing the average values of input and effluent BOD along with average percent removals of the CW. The average removals are about 85 percent; however, as mentioned previously, Site 3 had the lowest removal percentage. The cause of this variability is being investigated. In all cases, the average annual BOD of the effluent was under 22 mg/l, which is acceptable to local health departments for non-discharge treatment within Alabama. The first order model of BOD removal incorporates temperature into the reaction rate coefficient (Tchbanoglous and Schroeder, 1987). One of the significant questions that engi- Average BOD out (mg/l) Percent Removals 5 97% 16 79% 21 76% 10 89% 15 82% 13 85% neers have regarding implementation of CW technology is performance over seasonal variations. Therefore, figure 2 was constructed to illustrate a relationship between water temperature and percent of BOD removal.
... Plot of BOD Reductions Versus Temperature at All Sites Based on this small set of data, the trend-line shows an increase in removal due to temperature, but good reductions occur even during winter months. Nitrogen compounds also are found in significant concentrations in domestic wastewater. In particular, ammonia nitrogen removal is a major concern because of EPA's implementation of more stringent water quality-based ammonia limits (EPA, 1991). The process of mineralization of organic nitrogen to ammonia is the first step in the complicated biological breakdown of nitrogen in natural systems. The process can be aerobic or anaerobic, but occurs much faster under aerobic conditions. Ammonia is biologically converted (nitrification) to nitrate in an aerobic process. (It should be noted that nitrification also is highly temperature and ph dependent.) The final step is the denitrification of the nitrates to nitrite and elemental nitrogen. Since the effluent nitrate concentration is of concern to the health department, both ammonia and nitrate measurements were taken. Figure 3 shows the input and effluent concentrations of ammonia for Site 2 from start-up condition to 24 months into operation. In most cases the nitrification process appears to be the predominant nitrogen reaction as shown by the ammonia reductions. However at startup for Site 2 (first three samples) the output ammonia increased within the wetland cells, indicating that mineralization was the dominant process within the wetland during that time. This could be associated with insufficient nitrifying bacteria, anaerobic conditions, short hydraulic retention times, or excessive organic nitrogen conversion within the cell. However, on average, ammonia nitrogen concentration decreased Date Plot of Ammonia Input and Output Concentration at Site 2
30 25 20 15 10 5 0 0 10 20 30 40 50 60 70 Percent Ammonia Reduction Scatter Plot of Ammonia Reduction Versus Temperature at Site 2 by 35 percent in the CW. Many practicing engineers question the efficiency of the CWs to operate at low temperatures. Figure 4 illustrates the effects of temperature on percent removal of ammonia concentrations at Site 2. As expected from literature (Kadlec & Knight, 1996), mineralization rates are highly temperature dependent. However, the linear trend-line shows (albeit with a very poor fit) of expected percent removal to temperature given as % reduction NH = 0.33 *(Temp) + 21.5 [equation 1] Bacterial reductions of wastewater also are a positive function of CW treatment. Kadlec and Knight (1 996) reported expected average total coliform (TC) bacteria reductions greater than 90 percent and stated that the reductions are a function of influent concentrations. Standard membrane filtration (24-hour incubation period) coupled with plate count methods were performed on the water samples. The units reported are given in colony forming units per 100 ml (cfu/l00 ml). The Northern Alabama data showed high levels of TC reductions at all sites as illustrated in table 2. It was expected that limited bacterial action in colder weather would hamper the natural disinfection process. This is illustrated at Site 2 (January and February) and Site 4 (February) where the reductions are less than 80 percent. However, it also should be noted that the TC removals in July at two sites are also below 80 percent. The cause of this limitation may be the extreme temperatures (>26 C), high levels of natural, in-situ bacteria, or bad data sampling/testing techniques. Reductions at Sites 1 and 3 were very consistent and show no strong temperature dependence. The TC removal data for 12 months is summarized in table 3. Site 1 had the highest average coliform........................................ Coliform Bacteria Removal per Site.
removal efficiency, while Sites 4 and 5 were the lowest. The average for all sites was greater than 90 percent, consistent with the literature. However, although the percent reductions were high, the average effluent concentrations were still above 10,000 cfu/100 ml. Kadlec and Knight reported that bacterial reductions could be represented by an exponential decline equation: C/C o = exp (- k 1/q) [equation 2] Where Co is influent concentration, C is effluent concentration, k 1 is a first-order zero-background rate constant, and q is the hydraulic loading rate (m/day). Rate constants (k 1) calculated using equation 2 and areal flow rates resulted in a range of values from 0.01 to 0.5 for coliform removal of these systems. Percent Turbidity Removal by Site, Season, and Average Although turbidity is a gross measurement, it was of concern to the local health departments since it can be indicative of suspended solids. Turbidity was quantified in a HACH 2100 turbidimeter using the nephalmetric approach. The percent removal measured at each site is listed in table 4 according to season and site. As noted earlier, the influent reading is taken after the septic tank treatment and prior to the CW treatment and thus is not indicative of total turbidity of the raw wastewater. As shown in the figure, Site 4 had negative reductions during the winter months and the header pipes were discovered to be clogged with debris. Thus, the performance concerns were not temperature-related. After cleaning the pipes, the removal rates at Site 4 increased to an average of 80 percent. Conversely, Site 1 had consistently high turbidity reductions. Site 2 had problems during the summer related to low-flow conditions, which could be attributed to the sampling method of using a hand-operated vacuum pump (this site was overdesigned and the second cell was frequently dry). This could have dredged up debris from the bottom of the hydraulic discharge pipes and caused increases in turbtdrty. A statistical analysis of the performance data over all of the sites for a 12month period is presented in table 5. Although this data set is relatively limited (n = 60), the local health department wanted an idea of expected effluent concentrations for future design criteria. The range of values gives an indication of the variability of the data sets at each site and over the seasons. The coefficient of variability (CV) quantifies the standard deviation of each parameter to its mean. From this test it is apparent that the BOD and coliform data are both highly variable. The coefficient of skewness was calculated to determine if the statistical distributions are best approximated by normal or log-normal probability density functions (pdf). The closest to normal distribution (skew = 0) are the TDS, nitrogen ammonia, nitrate, and ph data. This means that although the nitrogen process is very complicated, concentrations of ammonia in the effluent still can be predicted with some accuracy. The others (BOD, coliform and turbidity) are extremely skewed (skew > 1.0 ) and a normal distribution would not be appropriate. Therefore, an analysis of calculating the skewness of the data logarithms was performed to determine if the data could be approximated by a log-normal distribution. Column 6 of table 5 shows the "log skew" of that exercise. Since the log skew of both the coliforms and turbidity are low, log-normal probability density function may be best for predicting the expected values of effluent data. Since the BOD data was not strongly normal or log-normal, a probability density function was constructed using the Weiball plotting position method. This data is shown in figure 5. Next a "best fit" trend-line was constructed using the log-log plot. The resultant equation is given on the chart to be a power function where Performance Data: Effluent Concentrations BOD 5 = 0.36 P 0.93 [equation 3] This equation (or figure 5) can be used to determine the expected effluent concentration at any
1 10 100 Percent of Measured Values Equal to or Less than Value Probability Density Function of Biochemical Oxygen Demand Data. probability (P). For example if the design were to find the effluent concentration that would exceed a probability of 95 percent, the expected BODS would be less than 24 mg/l. REFERENCES CONCLUSIONS The sample collection and data analysis performed at the experimental residential systems was used to evaluate performance and construct statistical models of the operating parameters. This study demonstrated that constructed wetlands are a viable alternative for natural, residential, onsite wastewater treatment. The effluent met BOD requirements for non-discharge systems, although ammonia was an issue on several sampling dates. Specifically, it was determined that average BOD removal rates were greater than 85 percent and total bacteria coliform reductions of 91 percent were realized. The statistical analysis showed that the effluent BOD data was not normally distributed, and a power curve fit was best to calculate the probability of removal rates. The results of this ongoing study will be used to make recommendations for future design guidelines. In particular, new technology to address the ammonia removal problems are currently being tested. Author Guidelines for Juried Article Submissions ACKNOWLEDGMENTS The authors wish to express gratitude to the University of Alabama-Huntsville environmental engineering students who sampled the wetlands and spent hours in the lab analyzing the data. In addition, Mike Roden (Tennessee Valley Resource Conservation and Development Council) was instrumental in allowing access and providing the opportunity for this study.