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

Wastewater from small communities often has a greater environmental impact than conventional on-site treatment systems can mitigate, yet the flow rate is too low to achieve economies of scale with municipal treatment processes. As a result, the cost of wastewater treatment is often beyond the financial means of the community, in terms of capital costs and annual operational costs. The recirculating gravel filter (RGF) is an attached-growth treatment process for wastewater from small communities. In the RGF process, pre-settled wastewater is recirculated through a gravel filter bed, while a biofilm on the filter media oxidizes the organic matter and ammonia. Effluent from the RGF process has equivalent or lower concentrations of BOD5, TSS and ammonia nitrogen as effluent from other wastewater treatment processes typically employed in small communities. Two small communities in Washington State, USA, have selected the RGF process for wastewater treatment, due to low operational costs, simplicity of equipment, and high effluent quality. For the two communities, the RGF wastewater treatment facilities were estimated to have somewhat lower construction costs and significantly lower annual operational costs than the alternatives evaluated. Low annual operational expenses are important for wastewater system sustainability in small communities.