Attempts were made in this study to examine the effectiveness of polymer addition to the aeration tank effluent prior to sludge flotation as practiced in a slaughterhouse wastewater treatment plant. The plant currently uses 10 mg/l of polymer prior to sludge flotation, but alternative, less-expensive, chemicals such as alum could be equally effective. Therefore, experiments were conducted using the Standard Jar test to determine the performance of both alum (Al2SO4 (.) 6H(2)O) and organic polymer. The dosages used, for alum ranged between 0 and 1000 mg/l, whereas polymer dosages varied between 0 and 90 mg/l. The (optimal) removal efficiency for suspended solids in the mixed liquor was obtained at 400 mg/l for alum and 30 mg/l for polymer. It is evident that addition of alum or polymer. results in significant removal of suspended solids reaching up to 99% for alum and 96% for polymer but alum produced a more, compacted sludge. Removal of filterable COD was much lower in both cases since the chemicals used target the colloidal and suspended portion of the COD rather than the soluble (filterable) part of the COD. (C) 2004 Elsevier Ltd. All rights reserved.