In this study, the possibility of using proteins recovered from excess sludge as animal feed was investigated. The proteins were recovered through the processes of sludge disintegration (alkali treatment followed by ultra-sonication), precipitation and drying of the soluble proteins. The compositions and the toxicants of the recovered proteins were analyzed, and the toxicity was assessed by Sprague-Dawley (SD) rat experiments. The results showed that the nutrient compositions were comparable with the commercial protein feeds. Heavy metals were found to be removed after the protein recovery process, and aflatoxin B1, ochratoxin A and Salmonella D groups were not detected. The rat toxicity tests showed that there were no effects on mortality, the incidence of clinical signs, body weight changes, and necropsy findings. The minimum lethal dose (MLD) was higher than 2000 mg/kg. Based on these results, the use of the crude protein recovered from excess sludge as animal feed appears to be technically feasible. (C) 2008 Published by Elsevier Ltd.