Comparison of Pilot Scale Cloth-Media Disk Filters with Full Scale Rapid Sand Filters for Filtration of Secondary Effluent

The cloth-media disk filter has historically been used to filter effluent from conventional activated sludge and sequencing batch reactor systems. In this study, a pilot scale cloth-media disk filter was operated at the Urbana & Champaign Sanitary District's (UCSD's) Northeast Treatment Plant (NETP) in Urbana, Illinois to filter effluent from nitrifying biotowers. The data collected from the pilot scale filter was compared with data from the NETP's existing rapid sand filters.The effluent from both filters was 4.2 mg/l total suspended solids (TSS) or less and 3.0 mg/l 5- day carbonaceous biochemical oxygen demand (CBOD5) of less through the study. This is considerably lower than the plant's permitted monthly average limits of 12 mg/l TSS and 10 mg/l CBOD5. During a peak solids loading test, the solids loading rate peaked at 4.0 lb/day/ft2, which is considerably higher than the 0.25 to 0.60 lb/day/ft2 loading throughout the rest of the study. The effluent quality peaked at 10 mg/l during this peak loading test.At filtration rates from 2.5 gpm/ft2 to 6 gpm/ft2, the cloth-media disk filter removed between 70% and 90% of the TSS from the wastewater. At filtration rates from 1.5 gpm/ft2 to 4 gpm/ft2, the rapid sand filters removed between 60% and 90% of the TSS from the wastewater. In both filters, TSS removal rates are not a function of the filtration rate.At similar filtration rates, the percent of the influent flow returned to the headworks from the cloth-media disk filter is approximately half that returned from the sand filter.Installation of the cloth-media disk filters at the UCSD's Southwest Treatment Plant (SWTP) when the plant is expanded to 10 MGD was recommended for the following reasons: 1) The performance of the cloth-media disk filters for TSS removal is equal to or better than the plant's existing rapid sand filters; 2) The cloth-media disk filters have a smaller footprint and will easily fit inside the plant's existing filter building; 3) The cloth-media disk filters are easy to operate and maintain; 4) The estimated twenty-year present worth of the cloth-media disk filters is 15% less than that of rehabilitating and expanding the existing rapid sand filters.