Redox Control Bioreactor for Enhanced Nitrogen Removal from Septic Tank Effluent

This paper will report on the evaluation of the Redox Control Bioreactor (RCB) for enhanced removal of nitrogen from septic tank effluent (STE). The RCB is a novel membrane bioreactor that delivers both oxygen and hydrogen gases through juxtaposed hollow fiber (HF) membranes. STE nitrogen is dominated by organic and ammonia nitrogen, and complete nitrogen removal must include both oxidation (nitrification) and reduction (denitrification). In the RCB, nitrifying biofilms are established on O₂-HF, while H₂-HF support autotrophic denitrifying biofilms. RCB prototype studies have demonstrated simultaneous ammonia and nitrate nitrogen removals of 6 and 4 g m^–2 day^–1, respectively. The goal of this project is to develop an RCB that is capable of achieving low effluent concentrations of nitrogen from STE within a controlled treatment module, before discharge to a soil adsorption field or other appropriate system. Advantages of the RCB are the potential to achieve low nitrogen concentrations and low solids production through the use of autotrophic processes. In-reactor nitrogen removal systems also offer a performance monitoring and inspection point to preclude system failure, and low operator attention. The project entails an initial period of bench scale testing followed by field demonstration. In the bench scale testing, the RCB will be preceded by a biochemical oxygen demand (BOD) removal reactor (BRR) receiving only O₂ via HF membranes. BRR pretreatment is considered necessary to reduce the heterotrophic oxygen demand in the RCB, to reduce solids management issues in the RCB and to protect nitrifiers from inhibitory materials. The Year 1 study will evaluate operation and performance of a coupled BRR/RCB system at bench scale. Field validation testing will be performed in Year 2 using actual STE from a residence for sea turtle researchers associated with Hubbs-Sea World Research Institute (HSWRI) at Archie Carr National Wildlife Refuge, Florida.