Sludge Water Treatment By Enhanced Coagulation In A Water Treatment Plant In Northern China

Sludge wastewater generated in a water treatment plant normarlly accounts for about 3% ~ 8% of the drinking water production. It contains a large number of suspended substance, colloid particles, and metal hydroxide resulted from aluminum salt coagulants, etc. Because of the low temperature in winter in northern China, sludge wastewater tends to be colloidal and challenged for natural sedimentation. It is not unsually that the untreated sludge wastewater are discharged directly into receiving water bodies; not only pollute the surrounding environment, but also a waste of water resources. Therefore, enhanced coagulation technology was applied on sludge wastewater treatment and sludge wastewater reusing was realized.The natural sedimentation performance of sludge wastewater was investigated firstly using static beaker coagulation experiments. The experimental results showed that the concentration of sludge wastewater ranges from 0.1% to 0.2%, the rate of sedimentation is close to zero, thus that natural sedimentation performance is poor. Enhanced coagulation process was used to treat sludge wastewater, coagulation evaluation system was established and process conditions were optimized. According to the results, it is believed that the turbidity of sludge wastewater and solid content, sedimentation rate of sludge wastewater and solid content of sludge have good linear correlation with R2 > 0.95.Therefore, coagulation process can be controlled by monitoring the turbidity of sludge wastewater. The optimal conditions of the coagulation process were determined to be that the coagulant is PAM #2, the dosage is 20 mg/L, the temperature is 15℃, the dosage of Ca O is 300 mg/l, the fast mixing speed is 220 rpm and stirring 1 min, the slow mixing speed is 60 rpm and stirring 10 min.Under the optimum conditions of enhanced coagulation process, the effects of sludge wastewater turbidity and PAM dosage on water quality indexes were invetigated. The results showed that sludge wastewater turbidity impacted organic matter and ammonia nitrogen content while have no obvious effects on turbidity, Al and Fe content. PAM dosage have influence on turbidity, chromaticity, organic matter and ammonia nitrogen content while no obvious effect on Al and Fe content.The enhanced coagulation mechanism of sludge wastewater were discussed preliminary by measuring zeta potential, sludge particle size and floc morphology. Analysis believed that adsorption bridging role is the dominant enhanced coagulation mechanism after dosing flocculants PAM#2. Through enhancing coagulation process, sludge particle size from the original average particle size of 13.19 μm increased to 61.99 μm; particle size and porosity increased obviously. During the growth- crushing – reformmation dynamic balance, floc entered into a state of stable growth and size no longer increased. But the density and strength increased gradually, and the sludge sedimentation and dehydration performance became better, which lead to the desired enhanced coagulation effect.Arccording to water quality standards, sludge wastewater were determined to reuse as industrial cooling water or urban miscellaneous water. Besides, preliminary design and technical and economic analysis of enhancing coagulation process were carried out. The results showed that sludge water coagulation process has a good technical feasibility, which is able to be widely used by water plants in northern China. Reagent cost of coagulation treatment is 0.64 yuan/t, which means reusing sludge water own a good economic and social benefits and environmental benefits under the challenge of water resources shortage in our country.