MODIFICATIONS OF WARMF TO ASSESS THE EFFICACY OF ONSITE WASTEWATER SYSTEMS ON PUBLIC HEALTH

To assess the efficacy of onsite wastewater systems (OWS) to protect public health, an existing watershed model was enhanced with the algorithms for a biozone that develops in a soil system receiving septic tank effluent. Biozone is a biologically active soil layer, about 2 cm in thickness, which can further treat the effluent. The model formulations include microorganisms growing on organic matter (BOD) fed by the septic tank effluent. The field capacity of the biozone increases with bacteria biomass, which retains water like a sponge. The porosity of the biozone decreased with the build up of dead bacteria and solid residue. As the gap between field capacity and porosity is reduced over time, the infiltration rate decreases and in some systems this can lead to hydraulic failure of OWS. The model coefficients were calibrated with data collected in laboratory experiments, in which high doses of septic tank effluent were applied to the 60 cm sand columns. The coefficients were adjusted such that the high dose stimulated the rapid growth of bacteria in 30 to 60 days, when the infiltration rate was reduced to a trickle as observed in the experiments.