Treatment of Trace Organic Compounds in Onsite Wastewater Systems.

Onsite wastewater systems have historically been designed to treat conventional pollutants and pathogens. However, it is now recognized that onsite systems must be capable to effectively treat contaminants of emerging concern such as endocrine-disrupting organic chemicals. To enable development of design and performance guidance, research was conducted within the Small Flows Program at the Colorado School of Mines. Research encompassed field monitoring of 30 operating systems along with controlled field experiments at the Mines Park Test Site and laboratory tests at CSM. Due to their potential prevalence and adverse effects, a major research effort was focused on consumer product chemicals including sterols, surfactant metabolites, metal-chelating agents, antimicrobials, and stimulants. These compounds were frequently detected in samples of wastewaters obtained from onsite systems at concentrations ranging over three orders of magnitude. Non-residential wastewaters generally had more compounds at higher concentrations than residential wastewaters, likely due to differences in water- and chemical-using activities within the buildings. Within the unit operations present in many onsite systems, processes such as volatilization, sorption, and biotransformation govern the removal of the organic chemicals. Removal of organic contaminants during septic tank treatment was typically low (<50%), with removals attributed to sorption to settling solids and anaerobic biotransformation. Aerobic treatment in biofilters and wetlands enhanced removal for many organic contaminants (>90%) through aerobic degradation processes. To assess the effects of effluent quality and hydraulic loading rate on compound fate during soil treatment, septic tank or textile biofilter effluents were applied to an Ascalon sandy loam soil at 2 or 8 cm/d. Concentrations of organic contaminants in septic tank effluent were reduced by >90% during transport through 240 cm (often within 60 cm) of soil, likely due to sorption and biotransformation. Additional treatment with a textile biofilter reduced effluent concentrations of many compounds, but generally did not effect any changes in soil pore water concentrations.  The soil profile receiving septic tank effluent at a higher design HLR generally had greater removal efficiencies. Several contaminants (e.g., EDTA, NP, NP1EC, and sulfamethoxazole) were measured in soil pore water, indicating the ability of some organic contaminants to reach shallow groundwater.  Under most conditions, however, additional treatment during movement through the subsurface will further decrease concentrations. The findings of this CSM research were integrated with results from other studies to develop design and management guidance concerning the occurrence, treatment, and risk associated with organic compounds in onsite systems. This presentation will highlight the research findings and describe a decision matrix for onsite system design and performance under commonly occurring conditions.