High loading rates and high conversion rates can be obtained in biomothanation of industrial wastewaters by accumulating active biomass in anaerobic reactors. A particular technique of active biomass accumulation, using cell immobilisation on inert solid support particles fluidised in an upflow reactor, is presented. The results obtained in a lab-scale reactor of 0.5 1 total volume, using spent liquor from citric acid fermentation, are quite encouraging. Volumetric loads up to 42 000 mg COD × 1−1 reactor × d−1 with more than 70 % conversion are reached under steady state conditions and volumetric load increases of 10 % per day with stable conversion are feasible during transition steps. It is also shown that loading and conversion rates are drastically higher in immobilised fluidised cell systems than in conventional completely mixed systems, with similar food to microorganism ratios and gas production rates per active biomass unit in the reactor. It is concluded that these performances are directly related to the accumulation of active biomass, including methanogenic bacteria, in immobilised fluidised cells reactors. A more qualitative approach is used to compare the immobilised fluidised cell technique with other active biomass accumulation techniques.

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