Two field sites with soil limitations (shallow depth to restrictive layers) were used in the evaluation of slow rate spray irrigation as an alternative methodology to conventional on-site wastewater treatment and disposal. Secondary treatment and disinfection of the effluent was performed before application to the soil-plant system at rates of 1.25 and 2.5 cm/wk. After 20 months of effluent application at Site 1 and 12 months of application at Site 2, adequate wastewater renovation was occurring at both sites. Nitrogen removal rates by the soil-plant system were between 85% and 95%, with NO3--N concentrations in shallow groundwaters at both field sites less than 0.8 mg/L. Phosphorus removal rates were at least 96%. Stormwater runoff quality measurements from Site 1 indicated no serious threat to nearby surface waters. Small increases in soil NO3--N and NH4+-N concentrations to the 30-cm depth occurred only at Site 2. There was no evidence of increased soil PO4-P concentrations as a result of wastewater irrigation. The soil moisture content increased slightly at depths measured from 30-150 cm during summer months at both sites. Both sites demonstrated that the renovation capacity of on-site spray irrigation systems in soils with limitations can be very efficient.
Spray irrigation of domestic wastewater has potential as an effective on-site wastewater treatment and disposal method for soils that have limited renovation capacity. Nitrogen removal via denitrification from spray irrigation can fluctuate due to the alternating aerobic and anaerobic conditions caused by irrigation frequency. The objectives of this study were (i)to determine the effect of domestic wastewater application rates and frequencies within a 24-hr. period on nitrous oxide (N2O-N) emissions from soil columns measured by the acetylene block technique, and (ii) to determine the influence of wastewater irrigation on the denitrification capacity of the soil. Treatments used two effluent application rates (1.25 and 2.5 cm/wk) and three daily irrigation frequencies (1, 2, and 3 times/d) and a no-effluent control. Single daily doses of effluent produced low levels of N2O-N emissions directly after effluent application. Gaseous losses of N averaged 5.3 and 26.2% of the applied N at the 1.25 and 2.5 cm/wk effluent loading rates, respectively. The denitrifying capacity of the soils was limited by both N and C. Maintaining effluent in the upper, more microbially active part of the soil column through split applications was important to N removal via denitrification.