Proper operation of activated sludge systems is very important and depends on physical, chemical, and biological parameters of wastewater. In this study, some problems were studied in an existing wastewater treatment plant of a fish-canning factory located in Tehran where thick, brown, stable foam was observed in aeration and clarifier tanks. The effluent of this plant was higher than the standards of the Department of the Environment of Iran, and the pH in aeration tanks was lower than 5. As opposed to other researchers in solving the foaming problem, in this wastewater treatment plant, lime was used instead of organic polymers and other inorganic coagulants. The pH of aeration tanks was adjusted to between 6.5 and 8.5 with an injection of 500 mg/L (47.5 kg/day) lime in the chemical sedimentation tank. It appeared that the solid retention time in this plant was high. Therefore, the rate of return sludge in the aeration tank of the second stage was reduced by about 20%. Foaming was removed in 18 days. During this time, chemical oxygen demand (COD) of effluent went from 500 to 65 mg/L, which indicated 87% reduction. The estimated costs of using lime for removal of foaming were about 0.0029 Euros/day (0.004 dollars/day), which is very low.

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