Abstract 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 0.2 m/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 2 h.