COMPARISON OF AN ANAEROBIC FILTER AND AN ANAEROBIC FLUIDIZED-BED REACTOR TREATING PALM OIL MILL EFFLUENT

A laboratory-scale anaerobic filter (AF) and a fluidized-bed reactor ( FBR) were compared for their ability to treat palm oil mill effluent ( POME). The reactors were operated at mesophilic temperature (35 degree s C). Reactor performance was assessed by measuring chemical oxygen de mand (COD), volatile fatty acids (VFA), gas composition and gas produc tion. Sand of 0.3-0.5 mm diameter was used as a biomass support medium in the FBR, and clay rings of 2-5 mm diameter were used in the AF. St art-up of the AF was achieved with 1.5-4.5-day residence times and FBR residence times were maintained at 6 h. After acclimatization, COD re movals higher than 90% were reached in both reactors at 6 h residence time, equivalent to a loading of 20 g COD/litre/day. Ar higher loading s the FBR gave a better performance; even at 40 g COD/litre/day, with 6-h residence times, 78% COD was degraded. The AF could not be operate d above 20 g COD/litre/day without clogging. The AF and FBR performed similar at reactor concentrations up to 1 g COD/litre, while above 2.2 g COD/litre the AF showed a maximum removal rate of 17.0 g COD/litre/ day compared to 31.2 g COD/litre/day for the FBR. These differences we re probably due to diffusion limitations and a less active biomass in the AF. In both reactors the gas production rate and the levels of org anic acids increased in response to a higher feed concentration in les s than one day and this could be explained by substrate limitation. Ot her responses were slower with the microbial culture adapting over per iods of 6-10 days; these were apparently growth related. A doubling in loading always resulted in a large increase in organic acids, especia lly acetic and propionic, as well as increasing the proportion of CO2 in the gas.