Coagulation efficiency and flocs characteristics of recycling sludge during the treatment of low temperature and micro-polluted water

Abstract: Sludge of drinking water treatment generally characterized as accumulated suspended solids, organic and inorganic matters are vastly produced in the coagulation process. The proper disposal, regeneration or reuse of sludge is becoming a significant environmental issue. Moreover, reused sludge at low temperature periods is an alternative method to enhance traditional coagulation efficiency. The recycling mass of mixed sludge and properties of raw water (such as pH and turbidity) were systematically discussed for optimizing the coagulation efficiency. It was discovered that the appropriate dosage of mixed sludge was 60 mL/L, the effective initial turbidity ranges were below 45.0 NTU, and optimal pH for DOMs and turbidity removal was 6.5~7.0, 8.0 respectively. Furthermore, by comparing the flocs characteristics of flocs with and without recycling sludge, it was found that the flocs structure with sludge was more irregular, whilst average size could grow to 64.7 ?m from 48.1 ?m. Recycling sludge was feasible and successful to enhance pollutants removal and the more irregular flocs structure after recycling might be caused by breakage of reused flocs and incorporation of PAC into larger flocs structure. Applied during the coagulation process, recycling sludge could be significant for the treatment of low temperature and micro-polluted source water.