Fate and Transport of Trace Organic Chemicals In Soil Treatment Units Associated with Onsite Wastewater Treatment Systems.

In the United States one quarter of the population currently utilizes onsite wastewater treatment, and also commonly groundwater wells for drinking water.  Thus, it is imperative to understand the limitations and capabilities of these systems not only with respect to traditional wastewater contaminants, but also trace organic chemicals (TOrCs).  In the most basic form, onsite wastewater systems collect wastewater into an anaerobic tank for primary digestion followed by dispersal to a soil treatment unit (STU) for further treatment utilizing sorption, filtration, and biotransformation processes.  A series of bench-scale column experiments were performed to evaluate the efficacy of TOrC attenuation in STUs as a function of hydraulic loading rate.  Soil columns (30 cm length X 8 cm in diameter), used to represent STUs, were operated at five loading rates (30, 12, 8, 4, 1 cm/day).  Each loading rate was tested on three side-by-side columns, and an abiotic column was operated to evaluate sorptive losses.  Columns were initially dosed with raw wastewater to introduce an appropriate microbial community after which they were fed with synthetic wastewater and TOrC spike to provide a consistent influent water quality.  Column effluent had reduction of dissolved organic carbon concentrations by >85% with nearly complete oxidation of ammonia to nitrate, comparable to performance of field STUs.  Attenuation of TOrC was largely compound specific.  Some TOrCs were effectively attenuated at all loading rates tested (i.e., acetaminophen and caffeine) while others were not attenuated at any loading rate tested (i.e., carbamazepine and primidone).  However there were a subset of TOrCs that exhibited a correlation between attenuation and hydraulic loading rate (i.e. cimetidine).  Results suggest STUs are effective in removing many TOrC, and design and operation can be optimized for maximum TOrC attenuation.