Onsite Treatment of Septic Tank Effluent: An Evaluation of Rotating Biological Contactor Capabilities

A rotating biological contactor (RBC) was evaluated with respect to nitrogen removal and practicality of use in tandem with a septic tank at a single family dwelling. The unit was used to treat wastewater generated by five persons and which was discharged into a 1500-gallon septic tank. Principal operational variables evaluated included hydraulic loading rate, disc rotational speed, anoxic stages, and varying recycle of effluent. Two modifications were made to promote nitrogen removal in the system; establishing anoxic conditions in the third stage, and making the first stage anoxic and recycling final effluent. Effluent was recycled at a ratio (recycle: influent) of 3: 1, 2: 1, 1: 1, and zero. The results showed that the RBC was capable of achieving nitrification/denitrification while simultaneously reducing carbonaceous oxygen demand. The optimum treatment efficiency was achieved at a hydraulic loading rate of 1.0 gpd/ft2 and disc rotational speed of 9 rpm conditions. The RBC showed 70%-90% Chemical Oxygen Demand (COD) removal and 60%-70% Total Suspended Solids (TSS) removal efficiency. Even after a four-day power failure, the biofilm was able to regain a stable operation within 24-48 hours and provide a COD removal efficiency in the range of 60%-70%. Total nitrogen removal was maximized (82%) by establishing anoxic conditions in the first stage and eliminating the recycle of effluent. The RBC appears to be particularly attractive for small communities and/or for an individual home because of its high treatment efficiency and low energy demand. Although the system required frequent labor attention to prevent hydraulic operational problems the use of an RBC is recommended where nitrogen contamination of groundwater is problematic.