A COMPARATIVE STUDY OF CONVENTIONAL COAGULATION AND SLUDGE RECYCLING ENHANCED COAGULATION FOR LOW-TURBIDITY SOURCE WATER: THE EFFECT AND APPLICABILITY

Low coagulation efficiency and high consumption of coagulant are two challenges for low turbidity source water treatment, and sludge recycling enhanced coagulation is generally considered as an effective solution. The effect and applicability of coagulation with sludge recycling was investigated, compared with conventional treatment. In conventional coagulation, the removal efficiency of poly aluminum chloride (PAC) was better than poly aluminum sulfate (PAS) and poly ferric aluminum chloride (PFAC). The economic dosage was 15 mg/L, and the corresponding removal efficiencies of turbidity, CODMn and UV254 were 62.0%, 31.7%, 32.9%, respectively. Recycling sludge could improve the removal efficiency. When 10 mg/L of PAC and sludge with recycle ratio of 6% were added in order, the turbidity, CODMn and UV254 removal efficiencies were 78.8%, 41.6% and 35.8%, respectively. Meanwhile, recycling sludge process increased the system's abilities to resist acid, alkali and turbidity fluctuations. When the initial turbidities were 2NTU and 4NTU, the optimal recycle ratios were both 6%. The floc structures formed from recycling sludge process were tighter and more regular, indicating a better sedimentation property.