Filtration of municipal sewage by ring shaped floating plastic net media

Characteristics of floating media filtration were examined to develop a process for primary treatment of municipal sewage during dry weather and high-rate treatment of combined sewer overflow during stormy weather. The media used for filtration were ring-shaped polypropylene net (2.2 cm in diameter, 2.5 cm in length and 6 mm in mesh size), which characteristically has a high void ratio (90%) and a low specific gravity (0.93). Experiments with bench scale and pilot scale filters showed that the removal rates of pollutants were 80–90% for suspended solids and 44% for BOD5 under an operating condition of 1000 m/day of flow velocity and 2–3 mg/l of cationic polyelectrolyte addition. Mass capture reached 7–10 kg/m3 with a low head loss, less than 0.2 m, with a media depth of 2 m. High removal rate of suspended solids and low head loss should be principal advantages of the process.