Evaluation of spray irrigation as a methodology for on-site wastewater treatment and disposal on marginal soils

Abstract

Two field sites with soil limitations, supported by column and laboratory studies, were used to evaluate spray irrigation as an alternative methodology for onsite wastewater treatment and disposal. The first site was located in Montgomery Co., VA on a Chilhowie soil (very-fine, mixed, mesic Typic Hapludalf) and the second was located in Alleghany Co., VA on a variant of the Monongahela series (fine-loamy, mixed, mesic Typic Fragiudult). Secondary pretreatment and disinfection of the effluent was performed before application through a slow rate spray irrigation system to the soil-plant system. Effluent application rates were based on the hydraulic and N assimilative capacities of the soil-plant system. Effluent applications of 1.25 and 2.5 cm wk⁻¹ were examined at both sites. After 20 months of effluent application at the Montgomery Co. site and 12 months of application at the Alleghany Co. site, adequate wastewater renovation was occurring in both sites. Water analyses at the 60 cm depth showed no potential groundwater contamination problems. Significant increases in chloride concentrations and EC measurements in the subsurface waters at both sites along with little or no increases in nitrate, ammonium, or phosphorus levels indicated that the effluent was being renovated as it percolated vertically through the soil profile. Runoff water quality measurements from the spray irrigation sites indicated no serious threat to nearby surface waters. Proper filtration of the sheet flow along with the relatively low loading rates, contributed to the good quality of runoff waters in both winter and summer seasons. Plant tissue analysis indicated that plant uptake of N was a large N sink during the growing season. Denitrification column studies examined the effect of different effluent application frequencies. There were no differences between irrigation frequencies in the amount of nitrous oxide accumulated after 24 h except in columns that were suspected to contain pockets of carbon particulates which contributed to high denitrification activity. Single daily effluent applications produced extended periods of low N₂O emissions, while more frequent effluent applications produced short periods (1-3 h) of rapid N₂O emission rates immediately after effluent applications. The denitrification capacity of the soils was shown to be limited by both C and NO₃-N. Results from the denitrification column studies suggested that there is potential for optimizing N loss in spray irrigation systems by maintaining effluent in the microbially active topsoil through proper effluent application frequencies. Both the field and column studies demonstrated that acceptable renovation of surface applied effluent by slow rate spray irrigation systems on sites with soil limitations can be obtained.