| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a Geomatrix, LLC, Killingworth, CT 06419
b Laboratory of Soil Ecology and Microbiology, University of Rhode Island, Kingston, RI 02881
* Corresponding author (jam7740u{at}postoffice.uri.edu).
Received for publication December 8, 2003.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Abbreviations: AIR, aerated lysimeters • BOD5, five-day biological oxygen demand • LEACH, lysimeters vented to leachfield
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
The capacity of leachfield soils to enhance water quality can vary substantially with environmental conditions. For instance, fluctuations in depth to the water table and in soil temperature can affect leachfield functioning (Bouma et al., 1975; Cogger and Carlile, 1984; Viraraghavan and Dickenson, 1991). One approach to improving the quality of water coming out of septic systems is to promote conditions that enhance contaminant removal and/or retention in the leachfield. The biogeochemical transformations in leachfield soil are controlled by the type and availability of electron acceptors and donors. For example, sufficiently high levels of oxygen are necessary for microbial oxidation of ammonium to nitrate. Nitrate may then be removed by denitrification, but only if the amount of organic carbon is sufficient to support the activities of denitrifying bacteria. In addition, most of the soil meso- and microfauna thought to be involved in pathogen removal use O2 as an electron acceptor. Levels of O2 may be suboptimal below the biomat due to the combined effects of high rates of microbial activity and low gas diffusion rates through leachfield soil. Enhanced aeration of leachfield soil may thus improve its ability to remove nutrients and inactivate pathogens via abiotic and biological processes that require oxygen. It may also promote anaerobic processes in microsites, such as denitrification, that rely on the products of aerobic processes. Finally, aeration may be expected to enhance wastewater infiltration by reducing or eliminating the biomat (e.g., Erickson and Tyler, 2001).
A patented process for the rejuvenation of leachfields using aeration (Potts, 2000) has been successful in restoring hydraulic function in more than 60 failed onsite wastewater treatment systems in the eastern United States. The effects of this process on water quality leaving the leachfield, however, are not known. We conducted a pilot-scale study to evaluate the effects of aeration levels on the quality of water coming out of leachfields. The effluent from a household septic tank was passed through lysimeters filled with sand to a depth of 30 cm. Silica sand with a high uniformity coefficient was used because it is chemically inert and it represents the shortest retention time and thus the case with the least effluent treatment. Experimental treatments consisted of lysimeters vented to the leachfield, representing the conditions in a conventional system, and lysimeters aerated to maintain an oxygen concentration of 0.20 to 0.21 mol mol–1. We measured the concentration of total N and P, pH, ammonium, nitrate, sulfate, phosphate, reduced iron, BOD5, fecal coliforms, and Escherichia coli in the septic tank effluent coming into the lysimeters and in the drainage water from aerated and leachfield lysimeters. We also determined the composition of the atmosphere in the headspace in both treatments. The effects of aeration were determined at loading rates of 12 cm d–1 (approximately 3 gallons ft–2 d–1) and 4 cm d–1 (approximately 1 gallon ft–2 d–1).
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
A schematic diagram of the experimental setup is shown in Fig. 1. All of the effluent from the septic tank was diverted to a pump station and stored in a high-density polyethylene (HDPE) tank (1325-L [350-gallon] maximum capacity) housed in a climate-controlled (17–19°C) room above the laboratory. The contents of the tank were mixed at regular intervals using a pump. Wastewater from the tank was pumped through a 3.75-cm-diameter (1.5-in) PVC manifold to a series of dosing tanks in the laboratory. Cylindrical, HDPE dosing tanks (30.5-cm [12-in] i.d., 45.7-cm [18-in] height) had a maximum capacity of 38 L (10 gallons) and were dosed every 6 h. Dosing was regulated using electronically actuated valves. Dosing tank overflow was allowed to drain completely until only the desired dose volume was retained.
|
Lysimeters were dosed with wastewater at a rate of 12 cm d–1 (approximately 3.0 gallons ft–2 d–1) for the first 24 mo of the experiment. LEACH lysimeters had developed a clogging mat, or biomat, approximately 20 mo before the beginning of our study, as indicated by periodic visual inspection. On 21 Apr. 2003 the dosing rate was changed to 4 cm d–1 (approximately 1.0 gallon ft–2 d–1) to determine the extent to which the effects of aeration on leachfield water quality were affected by dosing rate. The aeration level in the AIR treatment was kept constant regardless of dosing rate.
Treatments
Treatments consisted of lysimeters with aerated (AIR) or unaerated (LEACH) headspace, with each treatment replicated three times. Aeration was accomplished using a patented process developed by Geomatrix, LLC (Potts, 2000). This process has been used successfully in hydraulic rejuvenation of failed septic system leachfields, but its effects on water quality are not known. Ambient air was pumped at regular intervals into the headspace of AIR lysimeters to maintain an O2 level of 0.20 to 0.21 mol mol–1 using a piston pump. This resulted in a slight (approximately 2.5–6.7 kPa) positive pressure within AIR lysimeters. To mimic the in situ composition of the atmosphere found in the leachfield, the headspace and the gravel bed below the soil of LEACH lysimeters were vented to the septic system leachfield.
Sampling and Analyses
Sampling and Processing
Raw wastewater samples were collected from a valve in the input stream (Fig. 1) and placed in autoclaved, 125-mL polypropylene screw-cap bottles. One to two liters of wastewater were allowed to flow through the valve before sample collection. Water samples from the lysimeters were collected in 3-L Tedlar bags (2-mil thick; SKC, Eighty Four, PA) at ambient temperature (17–19°C). The bag was connected to the lysimeter outlet using Tygon tubing. To ensure that water samples were exposed to an atmosphere with the same composition as that found in the lysimeters, a connection was made from the headspace of each lysimeter to the drainline connected to the sampling bag. Sampling of water from the lysimeters started coincident with a dosing event, and continued for 60 to 90 min. Between 700 and 900 mL of water were collected from each lysimeter on each sampling date.
A portion of the raw wastewater and lysimeter water samples was analyzed immediately for dissolved oxygen and the presence of Fe2+. The remaining sample was kept on ice during transport to the laboratory in Kingston, RI (approximately 1 h). Unfiltered samples were assayed immediately on arrival to the laboratory for BOD5, pH, and total fecal coliforms and E. coli. A portion of the unfiltered sample was frozen for subsequent determination of total N and total P content. The remaining sample was filtered by passing through a glass microfiber filter (GF/F, 25-mm diameter; Whatman, Maidstone, UK). The filtered samples were stored in plastic, screw-cap scintillation vials at 4°C and analyzed for NH+4 and NO–3 within 24 h, for PO3–4 within 48 h, and for SO–24 within 72 h of collection.
Leachfield gases venting into LEACH lysimeters and headspace gases from the AIR and LEACH lysimeters were sampled using a portable soil gas monitor (SoilAir Technology, East Longmeadow, MA). Carbon dioxide, CH4, O2, and H2S were determined using infrared, catalytic bead, galvanic, and electrochemical sensors, respectively. Measurements were made approximately 2 h after dosing of lysimeters with septic tank effluent. Gas samples were drawn at a rate of approximately 0.05663 m3 h–1 (2.0 standard ft3 h–1) for 30 to 60 s and the maximum values detected during that sampling period are reported for all gases except O2, for which the minimum value is reported.
Analyses
Constituent analyses were performed according to methods of the American Public Health Association (1998). Dissolved oxygen was measured using the azide modification of the Winkler titration method. The concentration of Fe2+ in water was determined using EM Quant iron (Fe2+) test strips (EM Industries, Gibbstown, NJ). The pH of water samples was determined using a combination pH electrode and a Model UB-10 pH meter (Denver Instruments, Denver, CO). The concentration of sulfate was measured using the barium chloride turbidimetric method. Nitrate, ammonium, and phosphate concentrations of water samples were determined colorimetrically using an automated nutrient analyzer (Flow Solution IV; Alpkem, College Station, TX). The total N and total P content of water samples was determined using the persulfate digestion method. Samples were digested by autoclaving at 121°C and analyzed colorimetrically for NO–3 and PO3–4 as described above. Fecal coliforms and E. coli were assayed using the standard fecal coliform membrane filtration procedure. The BOD5 was measured on undiluted, unamended samples by manometric respirometry using an OxiTop BOD system (WTW, Fort Myers, FL) at 21 ± 1°C. Volumes were 250 mL for raw and LEACH samples, and 432 mL for AIR samples.
Statistical Analyses
Differences between AIR and LEACH treatments were evaluated using Student's t test at the 95% confidence level (SigmaStat for Windows, Version 2.03; SPSS, 1995). Statistical analyses were performed on untransformed data except for total coliform bacteria, where data were log-transformed.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
|
Dissolved oxygen levels in incoming wastewater ranged from 0 to 0.9 mg O2 L–1 (Fig. 2). Aeration enhanced dissolved oxygen levels, with values ranging from 8.1 to 11.9 mg O2 L–1 in water from AIR treatments. Dissolved oxygen in the LEACH treatment was significantly lower than in the AIR treatment on all sampling dates, with values ranging from 0 to 3.3 mg O2 L–1.
The BOD5 of incoming wastewater ranged from 73 to 198 mg L–1 (Fig. 2). Values of BOD5 in LEACH lysimeters ranged from 38 to 168 mg L–1, with average removal rates of 29.6 and 60.9% at loading rates of 12 and 4 cm d–1, respectively (Table 2). Values of BOD5 in AIR lysimeters were significantly lower than in the LEACH treatment, and were below the detection limit of 1 mg L–1, on all sampling dates, with removal rates higher than 99% (Table 2).
|
|
|
|
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Transformation and Removal of Nitrogen
Aeration had a strong effect on N removal and speciation and it clearly promotes oxidation of ammonium to nitrate. Nitrification results in proton release that, in the poorly buffered quartz sand used in this experiment, results in acidic conditions. The low pH values observed in the AIR lysimeters support this interpretation. The net loss of total N in AIR lysimeters on the first three sampling dates is probably due to denitrification, which is promoted by the presence of high levels of nitrate in the effluent. Conversely, passage of wastewater through LEACH lysimeters only resulted in ammonification of wastewater N, with little or no NO–3 production or net removal of N. The absence of net N losses in LEACH lysimeters suggests that denitrification did not take place in this treatment, probably limited by very low nitrate levels.
Denitrification may occur in anaerobic microsites even when oxygen concentrations in the bulk medium are high (Conrad, 1996; Sexstone et al., 1985). In addition, denitrification can take place under aerobic conditions. For example, Robertson and Kuenen (1991) have shown that in batch culture experiments, Thiosphaera pantotropha, a known denitrifier, can remove O2 and nitrate simultaneously, and the presence of both electron acceptors resulted in more rapid growth on acetate than when either O2 or nitrate was present. Studies of nitrogen removal in buried sand filters (of similar age and under similar loading and temperature conditions to ours) have also suggested that denitrification, rather than microbial immobilization, is responsible for total N losses (e.g., Gold et al., 1992). Measurement of the gaseous products of denitrification (e.g., NO, N2O) should help in elucidating the role this process has in N removal in leachfield soils.
The fact that net N removal in AIR lysimeters was limited to those sampling dates when wastewater was applied at the higher rate (12 cm d–1) suggests that the length of time the soil remains saturated with wastewater has a strong influence on whether N is removed by denitrification in the leachfield. The additional organic C associated with higher wastewater loading may also have enhanced denitrification by providing additional electron donors and/or promoted localized anaerobic conditions. In addition, transient increases in soil water content of a greater magnitude at the higher dosing rate may also have interfered with soil aeration. Denitrification in septic system leachfields is thought to be limited by the availability of organic C in wastewater (Sikora et al., 1976). Plósz et al. (2003) observed a similar effect of organic substrate input on denitrification rates in an anoxic reactor exposed to oxygen, which they attributed to high dissolved oxygen consumption, which in turn supported denitrifying conditions. Gaseous losses of N may also be due to the activities of nitrifying bacteria, which produce NO and N2O under anaerobic or microaerophilic conditions (Groffman, 1991). Such conditions may be established temporally in areas of the lysimeter after dosing with wastewater.
The presence of nitrate in drainage water from LEACH lysimeters, albeit at low levels, indicates that nitrification is taking place in the soil, since there is no detectable nitrate in incoming wastewater. However, the low concentration of nitrate suggests that nitrification is inhibited to some extent under these conditions. Levels of dissolved oxygen in LEACH water were between 0 and 4 mg L–1 (Fig. 2) and O2 was present in the leachfield gases (Table 1), although generally at low concentrations. The high concentration of methane may have inhibited ammonia oxidation. Methane is a competitive inhibitor of ammonia oxidation (Bedard and Knowles, 1989) and was present at concentrations exceeding 20000 x 10–6 mol mol–1 on most sampling dates (Table 1). By contrast, extensive nitrification appeared to take place in AIR lysimeters, as indicated by levels of NO–3 in drainage water that accounted for up to two-thirds of the total N inputs from septic tank effluent (Fig. 4). High levels of dissolved O2 and NH+4 and methane levels that were three orders of magnitude lower than in LEACH lysimeters probably created conditions conducive for nitrification in AIR lysimeters. Our results suggest that the role of methane as an inhibitor of ammonia oxidation in leachfield soils warrants further study.
Removal of Biological Oxygen Demand
Greater BOD5 removal in AIR than in LEACH lysimeters indicates that microbial decomposition of organic carbon is restricted under LEACH conditions. The enhanced availability of O2 as a terminal electron acceptor in AIR lysimeters probably results in a shift toward aerobic respiration, which is more energetically efficient than fermentative and anaerobic respiration pathways (Fuhrmann, 1998). This may explain the absence of biomat development in AIR lysimeters, since a greater proportion of organic C inputs would be oxidized to CO2. Removal of BOD5 in AIR lysimeters was not affected by loading rate, whereas removal in LEACH lysimeters loaded at 4 cm d–1 was double that at 12 cm d–1. The effect of loading rate on BOD5 removal in the LEACH treatment may reflect the limited capacity of the LEACH soil to provide the microbial community with the types and levels of electron acceptors necessary to metabolize organic compounds efficiently.
Sulfate Production
Higher levels of sulfate in AIR than in LEACH lysimeters are indicative of enhanced microbial oxidation of reduced S compounds found in septic tank effluent. For example, a number of species within the genus Thiobacillus derive energy from the oxidation of H2S and S2O2–3 (formed from bacterial reduction of organic sulfur compounds) within the pH values observed in our experiments (Germida et al., 1992). Furthermore, some of these bacteria are also capable of denitrification (Kanter et al., 1998; Robertson and Kuenen, 1991), possibly contributing to the loss of N in AIR lysimeters.
Total Phosphorus and Phosphate
The lack of PO3–4 or total P removal in AIR or LEACH lysimeters is not surprising. The main mechanisms for removal of organic and inorganic phosphate in leachfield soil involve sorption and binding of the PO3–4 to iron and aluminum oxides and oxyhydroxides (Robertson, 2003; Zanini et al., 1998). The sand used in the present study was manufactured from monocrystalline industrial quartz and did not contain appreciable amounts of these oxides. Oxides could have formed on the surface of sand particles originating from reduced forms of iron in incoming water. However, we did not detect Fe2+ in the wastewater input stream. Even if such mineral coatings did form, they did not appear to have an effect on P removal. The increase in total P levels in drainage water from both LEACH and AIR lysimeters relative to raw wastewater was unexpected (Table 2), although it has been reported by others (e.g., Gold et al., 1992). A large number of microorganisms has been shown to accumulate excess phosphate in the form of polyphosphate (Kulaev and Vagabov, 1983). Shifts in redox status and availability of carbon substrates can induce release of phosphate from polyphosphates (Kornberg, 1995). This process has been implicated in the release of phosphate from enhanced biological phosphorus removal processes in wastewater treatment plants (Carucci et al., 1999) and may account for our results.
Removal of Fecal Coliform Bacteria and E. coli
Fecal coliform bacteria removal was similar in both treatments, with a removal rate of 98 to nearly 100%, depending on loading rate. Drainage water from AIR lysimeters had levels of fecal coliform bacteria that were one to four orders of magnitude lower than LEACH lysimeters. Removal of fecal indicator bacteria in soil absorption fields is attributed to a number of factors, including straining, temperature, soil moisture, pH, organic matter, type of bacteria, and antagonistic microflora (Bitton and Harvey, 1992; Hagedorn et al., 1981). The more acidic conditions of AIR lysimeters may have contributed to the greater extent of total coliform removal, as suggested by others (Gold et al., 1992; Reddy et al., 1981; Reneau et al., 1989). We did not observe formation of a biomat in the AIR lysimeters, suggesting it may not be necessary for effective removal of pathogens in well-oxygenated soil absorption fields. Concerns with reduced treatment efficiency in systems that do not develop a biomat (e.g., Postma et al., 1992; Tyler and Converse, 1994) may not be warranted in the present case, since aeration appears to enhance removal of fecal coliform bacteria.
Comparison with Previous Studies
Water quality parameters in LEACH lysimeters appear to differ from previous studies. For example, we observed consistently low dissolved O2 levels in drainage water from LEACH lysimeters (Fig. 2), whereas it is generally thought that the soil beneath leachfields is relatively well oxygenated (USEPA, 2002) as a result of unsaturated flow below the biomat. We also observed that ammonium dominated the inorganic N pool (Fig. 3), whereas a number of studies have found that nitrate is the main form of inorganic N in the soil beneath a leachfield (e.g., Anderson et al., 1994; Bunnell et al., 1999), suggesting that nitrification is an active process in this zone. In addition, significant (30–40%) net loss of total N has been reported in leachfield soils (USEPA, 2002), whereas we observed no net N loss in LEACH lysimeters. There are a number of possible explanations for these discrepancies. The unstructured nature of the sand used in the present study could account for some of the differences. Soils generally exhibit some level of aggregation, not present in the sand used in our lysimeters, that affects the spatial distribution of pore sizes, water, and air, and leads to the formation of anaerobic microsites where denitrification may take place (Sexstone et al., 1985). In addition, determinations of nitrate concentration are often made at depths considerably greater than the 30 cm in this study (e.g., Stewart and Reneau, 1988). In other instances nitrate was measured in ground water receiving the leachfield water (e.g., Arravena et al., 1993). These situations are not comparable with our experimental setup, since they allow for the possibility of contact with oxic soil or water outside the area immediately below the leachfield. Nitrification could be taking place in oxygen-rich areas of the soil profile or in ground water. Nitrate could also have other sources, such as fertilizers or decomposing vegetation. Finally, laboratory experiments simulating leachfield conditions may not take into account the composition of the atmosphere above the leachfield (e.g., Van Cuyk et al., 2001), which our data indicate differs significantly from ambient air.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
ACKNOWLEDGMENTS |
|---|
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Related articles in JEQ:
This article has been cited by other articles:
|
|
 |
|
  L. W. Gill, C. O'Sulleabhain, B. D. R. Misstear, and P. J. Johnston The Treatment Performance of Different Subsoils in Ireland Receiving On-Site Wastewater Effluent J. Environ. Qual., October 24, 2007; 36(6): 1843 - 1855. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. A. Amador, D. A. Potts, M. C. Savin, P. Tomlinson, J. H. Gorres, and E. L. Nicosia Mesocosm-Scale Evaluation of Faunal and Microbial Communities of Aerated and Unaerated Leachfield Soil J. Environ. Qual., May 31, 2006; 35(4): 1160 - 1169. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Vadose Zone Journal | Journal of Plant Registrations | ||||
| Journal of Natural Resources and Life Sciences Education |
Soil Science Society of America Journal |
