Pilot Study of Yangtze River Water Treatment in the Winter by Sludge Return

In order to optimize the dosage and improve the coagulation effect when Yangtze river with low temperature and low turbidity was treated by conventional treatment in the winter, pilot study of enhanced conventional treatment was conducted by dosing two coagulant respectively and returning sludge in sedimentation tank to flocculant tank, so the optimal ratio of sludge return and the impacts of sludge return on effluent turbidity in sedimentation tank and the remove effect of COD_Mnwas discussed. The results indicates that the optimal sludge return ratio for Yangtze river in winter is 8%. Under 8% sludge return ratio,the dosage is reduced from 12mg/L (under no sludge return) to 3mg/L~6 mg/L, and dosage is saved 50%~75%. When dosing respectively 3mg/L~6 mg/L composite coagulant and 3mg/L~6 mg/L polymeric aluminum chloride, it can make average effluent turbidity in sedimentation tank decrease to 3.7NTU~2.4 NTU,8.1NTU~4..4 NTU respectively and the average removal rate of turbidity and COD_Mncan reach up to 94.3%~96.3%, 89.9%~94.5%,30.8%~39%,32.5%~38.6% respectively, so the requirement of effluent turbidity in sediment tank is met. Comparing to the treatment effect under no sludge return condition, sludge return can not only improve the treatment effect but also reduce the dosage greatly.