Evaluation of On‐Site Waste Disposal System for Nitrogen Reduction

Abstract

One of the enironmental concerns associated with on‐site wastewater disposal is nitrate contamination of groundwater. The purpose of this study was to design and monitor an experimental on‐site watewater disposal system with nitrogen‐reducing capability. Incorporated into the system are two fixed‐film reactors: a nitrification/denitrification (N/D) tank, and a polishing filter. Recirculated denitrified effluent sprayed onto the sand filter portion of the N/D tank is nitrified and the nitrified effluent is then mixed with septic tank effluent in the submerged gravel section for denitrification. The remaining portion of the denitrified effluent enters a polishing filter before final discharge into groundwater. With a recirculation ratio of 3.3, the overall reduction in total nitrogen through the N/D tank was approximately 70%. The N/D component of the system also achieved a reduction in BOD, COD, and suspended solids of 87, 80, and 84%, respectively. With the polishing filter, further enhanced removal of these pollutants was achieved, including a 5‐log reduction of fecal coliform. Using the N/D tank as a pretreatment unit for a soil adsorption system may not only reduce the levels of total nitrogen entering the system but may also extend the life of the system.