Design of recirculating sand filters using a standardized methodology

The recirculating sand filter is a successful wastewater treatment system for producers of small to medium (2 to 4000 m(3)/day or 500 to 1,000,000 gallons/day) wastewater flows. This cost effective, low-maintenance technology provides ideal treatment systems for parks, schools, office complexes, resorts, and even small communities with effluent sewer collection systems. Space requirements are reasonably small, construction costs are moderate, and maintenance and monitoring do not require highly trained personnel. The recirculating sand filter purifies septic tank effluent sufficiently that it can be disposed subsurface in most soils, or, where allowed, disinfected and disposed into surface waters. In the recirculating sand filters investigated by the authors, the quality of effluent is remarkably similar, even though designs, installation costs, and operational costs vary significantly. Incorporation of the most cost-effective methodologies into a standardized RSF design will reduce all those costs without sacrificing efficacy of the treatment. In this paper the authors discuss a standard design technique for a modular recirculating sand filter, developed to simplify the design process and to reduce equipment, installation and operating costs. This design strategy allows original construction to be limited to a size that accommodates existing flows and defers capital costs of additional capacity until it is warranted by increased flows. Variations in the standard design that may be used to meet special performance or site-specific criteria are also discussed.