Attenuation of pollutants in sanitary sewer overflow: comparative evaluation of treatment with fixed media bioreactors.

Five types of fixed media bioreactors (biofilters)--sand, felt (textile), peat, felt/sand, and peat/sand--were used to treat sanitary sewer overflow (SSO). A simulated 6-h peak flow of a 25-yr SSO event contained 40-125 mg/l biochemical oxygen demand (BOD(5)) and was loaded on the bioreactors at a high hydraulic loading rate of 0.2m/h. The sand bioreactors were the most effective in the treatment, reducing BOD(5) by 84+/-9%. The combination media peat/sand and felt/sand showed similar efficiency with peat, higher than felt. After the initial start-up, all the bioreactors reached >90% reduction of total suspended solids. The bioreactors also effectively removed ammonia and total phosphorus concentrations in a 2-h SSO loading, which would occur more often than a 6-h peak flow in a 25-yr SSO event. The effluent concentration of nutrients increased with continued loadings after the first 2h.