The impact of recycling sludge on water quality in coagulation for treating low-turbidity source water

There is an alternative method for the treatment of low-turbidity source water that reuses coagulation sludge as low-turbidity raw water to enhance conventional coagulation efficiency. This research investigated the impact of the recycling ration by volume of coagulation sludge on enhancing the removal efficiency of specific water quality parameters in a pilot-scale experiment with a capacity of 5 m3/h. This novel test was first continuously run for 150 h (15 recycling runs), and the recycling sludge was cycle used. The results showed a significantly higher removal efficiency of turbidity from the raw water than that of the traditional process, which was probably attributed to the special long-range Van der Waals attractive force, the enhancement of the collision ratio among particles, and the similar “second dosage of coagulant” strategy after the recycling process. Charge neutralization and absorption have mainly enhanced the removal efficiency of organic matter with recycling sludge. To some extent, this enhancement is due to the open and porous structure of the reused sludge, which could absorb the metal ions and further remove them following the sedimentation process. Recycling sludge is a feasible and successful method to enhance pollutant removal and has a significant effect on the treatment of low-turbidity source water.