Evaluation of a slow sand filter in advanced wastewater treatment

Performances of a slow sand filter used in tertiary treatment of domestic wastewater were evaluated by a lab scale test. Influent and effluent COD, turbidity, color and total bacterial counts were analyzed for more than one year. All results showed that, performances of the slow sand filter were nearly the same with its filtration rate 0.3 m/h or 0.1 m/h., but decreased dramatically when its filtration rate was increased to 0.5 m/h from 0.3 m/h. Temperature decrease had a significant negative effect on COD removal but not on turbidity or bacterial removal. The schmutzdecke layer contributed more than half quantities of pollutants removal. Average effluent of COD, turbidity, color and total bacterial counts were 34.9 mg/L, 0.6 NTU, 23.3° and 6200 CFU/ml respectively and corresponding removal rates were 43.9%, 89.5%, 55.6% and 73.5% under filtration rate 0.1 m/h in Summer.