Return chemical sludge employed in enhancement of phosphate removal from wastewater

In this study, chemical sludge was recycled by using a coagulation process to improve phosphate removal from wastewater. A pilot experiment was conducted to investigate the appropriate ratio of recycled sludge, the appropriate operating conditions, and the mechanism involved in the enhancement of phosphate removal by return chemical sludge. The results showed that, with sludge recirculation, the process of coagulation for phosphate removal was enhanced by up to 12% compared with no sludge recirculation. Those effects were primarily attributed to the removal of relatively fine particles with a size of 100-310 mu m. The sludge settleability was also improved by the addition of chemical sludge, which was mainly attributed to the increase in floc size. The appropriate ratio of recycled sludge was determined to be around 15-20%. Mixing and the aging time of recycled sludge had a significant effect on phosphate removal. Based on these findings, sweep flocculation and/or physical adsorption are expected to play key roles in enhancement of phosphate removal from wastewater.