Model Test of Proposed Loading Rates for Onsite Wastewater Treatment Systems

Abstract

State regulatory agencies set standards for onsite wastewater treatment system (OWTS), commonly known as septic systems, based on expected hydraulic performance and nitrogen (N) treatment in soils of differing texture. In a previous study, hydraulic loading rates were proposed for four categories of soils ranging from Group I (sands) to Group IV (clays). Our objective was to use a HYDRUS model to test the hydraulic performance and N treatment for all 12 soil textural classes using 2 yr of weather data. The model simulated water, N, and temperature dynamics in a two-dimensional space surrounding a drain-field trench. It was calibrated in a previous field study on a clay soil and included water content dependent N transformations. In terms of hydraulic performance, we found that the proposed loading rates were adequate except for the sandy clay textural class in which the rate was too high and caused continuous ponding in the trench. Nitrogen treatment varied widely among soils with denitrification losses ranging from 1% in the Group-I sand to 75% in the Group-IV sandy clay, due to water content limits on denitrification. Leaching losses were inversely related to denitrifcation losses, ranging from 97% in the sand to 27% in the sandy clay. Plant uptake and soil storage accounted for 5% or less of the N losses. The shape of the unsaturated hydraulic conductivity curve and water retention curve were important in determining the prevailing pressure head in the soil and the degree to which denitrification was inhibited in the various soil classes.