Field evaluation of the performance of engineered on-site wastewater treatment units

A conventional on-site wastewater system (OWS) is traditionally comprised of one or more septic tanks for treatment of raw wastewater followed by percolation through natural soil to achieve purification prior to groundwater recharge. Other types of engineered treatment units (e.g., sand filters, textile media filters) may be implemented where site conditions are not suitable for conventional systems or in sensitive areas, such as those with nitrogen loading concerns. The research presented here is part of a large field study conducted to evaluate the purification performance of OWS employing three different engineered treatment units: a septic tank, a septic tank with a textile filter unit (TFU), and a septic tank with a membrane bioreactor (MBR). The TFU or MBR employ treatment processes that are designed to achieve higher purification compared to a septic tank so that soil treatment can be accomplished at higher hydraulic loading rates and/or with less unsaturated soil depth. This paper describes the installation, operation, and monitoring of the effluent generated from three engineered treatment units operated for a period of 16-28 months. The three treatment units, as expected, achieved different purification efficiencies for organic matter, total suspended solids, nutrients, and bacteria with the relative removal efficiency of MBR>TFU> septic tank. The relative degree of operational complexity, operation and maintenance requirements, energy use, and cost followed the same ranking.