Effluent from a paper plant (4500 mg COD/l; 250 mg SO4/l) was treated in 9 litre laboratory scale UASB reactors operated at 25°C and 35°C. COD removals of 85 percent were attained in both reactors at loading rates of 26 kg COD/m3/d. Observations on concentration changes through the reactor from inlet to outlet yielded interesting information in terms of the hypothesis on sludge pellet formation of Sam-Soon et al (1987). Only limited pelletisation is likely with this waste because a high H2 partial pressure zone is not created at the base of the reactor: the influent substrate available for acidogenesis; with its associated H2 production, is limited; rapid-growing sulphate reducing organisms utilise H2 at the reactor inlet.

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