This paper studied the effect of ferric chloride on waste sludge digestion, dewatering and sedimentation under the optimized doses in co-precipitation phosphorus removal process. The experimental results showed that the concentration of mixed liquid suspended solid (MLSS) was 2436 mg center dot L-1 and 2385 mg center dot L-1 in co-precipitation phosphorus removal process (CPR) and biological phosphorous removal process (BPR), respectively. The sludge reduction ratio for each process was 22.6% and 24.6% in aerobic digestion, and 27.6% and 29.9% in anaerobic digestion, respectively. Due to the addition of chemical to the end of aeration tank, the sludge content of CPR was slightly higher than that of BPR, but the sludge reduction rate for both processes had no distinct difference. The sludge volume index (SVI) and sludge specific resistance of BPR were 126 ml center dot g(-1) and 11.7x10(12) m center dot kg(-1), respectively, while those of CPR were only 98 ml center dot g(-1) and 7.1x10(12) m center dot kg(-1), indicating that CPR chemical could improve sludge settling and dewatering.