We conducted a pilot-scale study at a research facility in southeastern Connecticut to assess the effects of leachfield aeration on removal of nutrients and pathogens from septic system effluent. Treatments consisted of lysimeters periodically aerated to maintain a headspace O(2) concentration of 0.209 mol mol(-1) (AIR) or vented to an adjacent leachfield trench (LEACH) and were replicated three times. All lysimeters were dosed with effluent from a septic tank for 24 mo at a rate of 12 cm d(-1) and subsequently for 2 mo at 4 cm d(-1). LEACH lysimeters had developed a clogging mat, or biomat, 20 mo before the beginning of our study. The level of aeration in the AIR treatment was held constant regardless of loading rate. No conventional biomat developed in the AIR treatment, whereas a biomat was present in the LEACH lysimeters. The headspace of LEACH lysimeters was considerably depleted in O(2) and enriched in CH(4), CO(2), and H(2)S relative to AIR lysimeters. Drainage water from AIR lysimeters was saturated with O(2) and had significantly lower pH, five-day biological oxygen demand (BOD(5)), and ammonium, and higher levels of nitrate and sulfate than LEACH lysimeters regardless of dosing rate. By contrast, significantly lower levels of total N and fecal coliform bacteria were observed in AIR than in LEACH lysimeters only at the higher dosing rate. No significant differences in total P removal were observed. Our results suggest that aeration may improve the removal of nitrogen, BOD(5), and fecal coliforms in leachfield soil, even in the absence of a biomat.

Aeration improves the capacity of leachfields to decontaminate and reduce the nutrient load of wastewater. To gain a better understanding of the effects of aeration, we examined the faunal and microbial communities of septic system leachfield soil (0-4 and 4-13 cm) using replicated (n = 3) mesocosms that were actively aerated (AIR) or unaerated (LEACH). Protozoa were 40 to 140 times more abundant in AIR than in LEACH soil. No nematodes were found in LEACH soil, whereas AIR soil contained 5 to 14 x 10(3) nematodes (all bacteriovores) kg(-1). Active microbial biomass was four to five times higher in AIR than LEACH soil. Proteobacteria and actinomycetes/sulfate-reducing bacteria constituted a higher proportion of the community in AIR soil, whereas anaerobic Gram-negative bacteria/firmicutes were more prominent in LEACH soil. Ratios of prokaryotic to eukaryotic phospholipid fatty acids (PLFAs) were higher in LEACH soil, as were membrane stress index values, whereas the starvation index was higher in AIR soil. Community-level physiological profiles showed that 29 and 30 different substrates were used for growth by LEACH and AIR soil microorganisms, respectively. The AIR soil had more microorganisms capable of growing on 10 substrates, whereas growth on two substrates was higher in LEACH soil. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis of 16S rRNA gene fragments revealed greater diversity of dominant phylotypes in AIR than LEACH soil, with communities separated by treatment. Aerated leachfield soil had a larger and more diverse faunal and microbial community than unaerated soil, possibly due to differences in the type and availability of electron acceptors.

The depth of soil below the absorption trench of a septic system is considered an important factor in protection of groundwater. We examined the effects of depth on the ability of intermittently aerated sand-filled leachfield mesocosms to renovate domestic wastewater. Mesocosms ͑n =3͒ consisted of lysimeters with a headspace O 2 concentration maintained at 0.21 mol/ mol and containing 7.5, 15, or 30 cm of sand that were dosed with septic tank effluent every 6 h for 328 days ͑12 cm/ d͒. Sand depth had no effect on pH, dissolved O 2 , PO 4 , NH 4 , or BOD 5 levels in percolate water. Nitrate levels in percolate water were higher for 30 cm than for 7.5 and 15 cm during the first 70 d of the experiment, after which no differences were observed. Time-averaged removal rates of N, P, fecal coliform bacteria, and BOD 5 were 22-28, 13-18, 81-92, and 81-99%, respectively, and were unaffected by depth. Wastewater renovation in intermittently aerated leachfield mesocosms appears to take place in a narrow zone ͑Յ7.5 cm͒ below the infiltrative surface, with the medium below contributing little to renovation.

We examined the effects of tetracycline (TET) addition on the function of mesocosms representing aerated and unaerated septic system leachfields. Replicate mesocosms (n = 3) were filled with soil and either vented to a leachfield (LEACH) or aerated intermittently to maintain an O(2) level of approximately 0.21 mol mol(-1) (AIR). All mesocosms were dosed every 6 h for 10 d with 3 cm of domestic wastewater amended with 5 mg TET L(-1). Water quality parameters, headspace gas composition, and soil properties were measured prior to and during the dosing period, and for 42 days after the last antibiotic dose. No significant effect of TET was observed on the pH, level of dissolved O(2) or dissolved organic carbon (DOC) in drainage water from either treatment. In contrast, levels of Fe(2+) and SO(4) in drainage water from LEACH mesocosms decreased in response to TET dosing, with lower levels persisting until Day 52. Persistent increases were observed in the level of NO(3) in drainage water from AIR lysimeters and in NH(4) in LEACH mesocosms in response to TET additions. Removal of total P and DOC were unaffected by TET dosing in either treatment. Nitrogen removal in AIR mesocosms decreased during the TET dosing period, returning to pre-dosing values by Day 52. In contrast, TN removal in LEACH mesocosms increased during TET dosing, returning to pre-dosing values by Day 52. The composition of headspace gases in AIR mesocosms was not affected by tetracycline dosing. TET dosing resulted in significant increases in soil NH(4) concentration in LEACH mesocosms, whereas significant decreases were apparent in AIR mesocosms. Elevated levels of H(2)S and CH(4) in the headspace of LEACH mesocosms coincided with TET dosing and returned to pre-dosing levels when antibiotic dosing ceased. The effects of tetracycline on leachfield mesocosms differed as a function of aeration. Although most effects were transient, with values returning to pre-dosing levels after a 6-week recovery period in both treatments, persistent effects were observed in LEACH mesocosms.

Optimization of N removal in soil-based wastewater treatment systems requires an understanding of the microbial processes involved in N transformations. We examined the fate of NH in intermittently aerated leachfield mesocosms over a 24-h period. Septic tank effluent (STE) was amended with NHCl to help determine N speciation and distribution in drainage water, soil, and headspace gases. Our results show that 5.7% of the N was found in soil, 10.0% in drainage water, and 84.3% in the gas pool. Ammonium accounted for 41.7% of the soil N pool, followed by NO (29.2%), organic N (21.7%), and microbial biomass N (7.5%). In drainage water, NO constituted ∼80% of the N pool, whereas NH was absent from this pool. Nitrous oxide was the dominant form of N in the gas phase 6 h after addition of NH-amended STE to the mesocosms, after which its mass declined exponentially; by contrast, the mass of N was initially low but increased linearly with time to become the dominant form of N after 24 h. Analysis based on the isotopic enrichment of NO and N indicates that nitrification contributed 98.8 and 23.1% of the NO flux after 6 and 24 h, respectively. Our results show that gaseous losses are the main mechanism for NH removal from wastewater in intermittently aerated soil. In addition, nitrification, which is generally not considered a significant pathway for N loss in soil-based wastewater treatment, is an important source process for NO.