Recirculating gravel filters: high-performance treatment at low cost for two small communities

Implementation of long solids retention time activated sludge process for rural residential community

Most rural communities in the United States are facing increasingly rigorous effluent criteria, especially ammonia, for their wastewater treatment facilities. A new baffled bioreactor (BBR) technology, which employs a preanoxic activated sludge process operated with a long solids retention time (SRT), was installed in a small community in Missouri to address the more stringent effluent limits. In a recent full-year normal operation cycle (2018), the average effluent concentrations of BOD₅ , TSS, and ammonia-nitrogen were 3.2, 2.2, and 0.5 mg/L, respectively, with removal efficiencies of 96%, 85%, and 98%, respectively. All these parameters were significantly better than their respective permit limits. The long SRT afforded an enhanced factor of safety for the process, conferring the ability to nitrify at sustained ambient temperatures as low as -22°C. Long SRT also resulted in significant reductions in waste sludge production, resulting in dramatically reduced operational costs for sludge handling. Ultimately, the long SRT activated sludge process afforded the ability to meet stringent effluent quality standards including ammonia and the numerous unique challenges that are inherent to small flows. PRACTITIONER POINTS: Small community hydraulic and mass loadings are highly variable and difficult to quantify during facility design. A long SRT activated sludge process warrants superior performance and enhanced factor of safety. The long SRT process with preanoxic zones generated no excess sludge during the extended operation period, significantly simplifying plant operation. Long SRT process is well suited to accommodate wastewater variability associated with small communities while maintaining superior treatment quality.