Operating and performance problems arise when fats and oils are a major fraction of the organic content of the waste water. These problems are frequently associated with the presence of the microorganisms Nocardia spp. and Microthrix parvicella which can cause foaming of activated sludge plants. Bacteria and enzyme additives (B-E-additives) are produced by some companies who claim to prevent the growth of nocardioform actinomycetes by addition of these B-E-additives to the mixed liquor. In order to investigate these claims a research program for the control of sludge foaming was started using a commercial B-E-additive combatting these operational problems. The B-E-additive was investigated in batch experiments in laboratory scale (0.25-40 1) using mixed liquors from different plants (domestic and industrial plants). In a second phase the B-E-additive was applied on a large scale over extended periods in two different activated sludge plants. Plant A had a severe foaming problem due to waste water from a pet-food factory. Plant B treated mainly waste water of domestic origin. A continuous addition of the B-E-additive could not avoid a severe scum formation on the aeration tank after lipid loadings from industry. The addition of the B-E-additive caused a shift in the biocenosis from Nocardia spp. to Microthrix parvicella but did not influence the foam production in the plant. The addition of the B-E-additive in a plant with two identical, parallel-run activated sludge tanks where the product was only applied in one half of the plant, whereas the other unit served for control purpose, did not show significant differences. The results obtained during the investigations did not show any significant advantage in effective grease removal, foam control or aeration efficiency.

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