Denitrification in onsite wastewater treatment and disposal systems

The effects of effluent type, effluent loading rate, dosing interval, and temperature on denitrification in onsite wastewater treatment and disposal systems (OSWTDSs) were evaluated in this study. The variables were soil horizon, effluent type, effluent loading rate, dosing interval, and temperature. Surface and subsurface soil cores were collected from a Groseclose silt loam soil (clayey, mixed, mesic Typic Hapludult) and subjected to the following treatments: aerobic and anaerobic effluent, loading rates of 0.5, 1.0, and 1.5 times the Virginia Department of Health (VDH)-recommended levels, 24-hour and 48-hour dosing rates, and summer and winter temperatures. The effects of the treatments on denitrification were evaluated based on analyses of leachate from the cores, soil chemical analyses, and microcosm studies to estimate actual denitrification activity. From the study, a model was developed that predicted the mean nitrous oxide (N20) production for each combination of the experimental treatments. The results of the study and the model indicate that denitrification can be enhanced in OSWTDSs by the application of anaerobic effluent at the VDH-recommended effluent loading rate to surface soil horizons using a 48-hour dosing interval.

A field study was conducted on a Lowell silt loam soil (fine, mixed, mesic Typic Hapludalf). Denitrification was measured at this site using acetylene blocking, and the results compared to those predicted by the denitrification model developed from the laboratory data. The field measurements of denitrification based on N20 concentration in the soil atmosphere were almost three orders of magnitude higher than that predicted by the model.