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The ability of microorganisms to use chlorate (C103) as an electron acceptor for respiration under anaerobic conditions was studied in batch and continuous tests. Complex microbial communities were cultivated anaerobically in defined media containing chlorate, all essential minerals, and acetate as the sole energy and carbon source. It was shown that chlorate was reduced to chloride, while acetate was oxidized to carbon dioxide and water and used as the carbon source for synthesis of new biomass. A biomass yield of 1.9 to 3.8 g of volatile suspended solids per equivalent of available electrons was obtained, showing that anaerobic growth with chlorate as an electron acceptor gives a high energy yield. This indicates that microbial reduction of chlorate to chloride in anaerobic systems is coupled with electron transport phgsphorylation. Chlorate, C103-, is a highly oxidized compound and should therefore be suitable as an electron acceptor in anaerobic respiration. However, since chlorate, which does not occur naturally, was introduced into the environment only by the activities of humans, time has been short, from

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Anaerobic biological removal of chlorate was studied on a laboratory and pilot plant scale. Continuous laboratory tests in an anaerobic fixed-film process showed that chlorate can be removed completely from kraft bleach effluent at such a short hydraulic retention time as 0.6 h. The efficiency of biological chlorate removal was confirmed under practical conditions in a 20 m3 pilot plant, operating at a Swedish craft mill. Four chlorate reducing bacterial strains were isolated and characterized. All four isolates were gram-negative, catalase- and oxidase-positive, motile rods. None of the four isolates could ferment glucose, while they could all grow aerobically and with nitrate as electron acceptor.

A combination of the suspended carrier biofilm process and the activated sludge process (biofilm-activated sludge--BAS) has been shown to be very successful for the treatment of different types of pulp and paper mill effluents. The robust biofilm pre-treatment in combination with activated sludge results in a stable, compact and highly efficient process. Recent findings have shown that nutrient limited operation of the biofilm process greatly improves the sludge characteristics in the following activated sludge stage, while minimising sludge production and effluent discharge of nutrients. The nutrient limited BAS process was implemented at full scale at the Södra Cell Värö kraft mill and taken into operation in July 2002. After start-up and optimisation over about 5 months, the process meets all effluent discharge limits. The removal of COD is close to 70% and the removal of EDTA greater than 90%. Typical effluent concentrations of suspended solids and nutrients during stable operations have been 20-30 mg/L TSS, 0.3-0.5 mg/L phosphorus and 3-5 mg/L nitrogen. The sludge production was 0.09 kgSS/kg COD removed and the sludge volume index was 50-100 mL/g.

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