Recirculating gravel filters (RGF) can convert septic tank effluent into an oxidized, nitrified effluent without generating waste biological sludge. An RGF serving a small development has been monitored weekly for the last 5 years. The design loading rate on the filter is 120 mm/d at a design flow of 53,000 L/d. The average flow for the last 12 months (1998) was 33,000 L/d with an actual average hydraulic loading rate on the filter of 74 mm/d. The effluent contains less than 5 mg/L BOD, 3.5 mg/L TSS and a median ammonia concentration of less than 0.77 mg/L. Because the biological system does not produce waste sludge, it is sustainable as well as robust.