| Water shortage is a major condition in China. With the increased level of water pollution and improved demand of drinking water quality for people, the traditional water treatment technology can not satisfy the present requirements. Especially in cold regions of northern China, the winter can be long for 5-6 months. During this period, the water would go into the low temperature and low turbidity state. The existing conventional water treatment processes in many water plants have poor effect, needing to develop efficient and low energy consumption water treatment technology. Considering the operational conditions of northern water plants treatment during the low temperature and low turbidity period, through the comparation and selection of the existing low temperature and low turbidity water treatment processes, we think that enhanced coagulation and micro-flocculation filtration are the most convenient and effective retrofit for the original conventional water treatment. In this paper, enhanced coagulation were studied through coagulant optimization, stirring optimization, coagulant aid, sludge reflux, etc. And based on this,some experiments related to micro-flocculation filtration were studied.In this paper, in the coagulation beaker tests, five common aluminum and ferric coagulant were respectively studied in coagulation tests. According to the results, we believed that polyaluminium chloride (PAC) had the best effect. The formation floc had large size and compact, had high turbidity and organic matter removal. When the PAC dosage was 30 mg/L, the effluent turbidity was approximately 0.2 NTU, the removal rate could be 85%; the effluent CODMn was less than 2 mg/L, the removal rate was about 58%; UV254 removal rate was 67 % and the chroma was less than 5 degrees. By stirring experiment, we found that classification mixing had better results and three-stages mixing had better efficient than two-stages mixing. PAC combined with polyacrylamide (PAM) dosing could increase the ability of floc cohesion and improve its compactness. Sludge reflux method can increase raw water turbidity and improve the coagulation effect. The dosage of PAC could greatly reduced to 5-10 mg/L. And the time of sludge couldn't remain long for 3days.By micro-flocculation bench-scale test, we found that micro-flocculation filtration had a good treatment effect and the coagulant dosage is small. The media type, coagulant dosage and filtration rate had a greatly influence on the treatment effect. The fine quartz sand media (0.5-1.0 mm) and dual-media (upper quartz sand, the lower anthracite) ran better effect. The PAC dosage could be reduced to 12-16 mg/L and was less than the conventional process by about 47-60%.Through the pilot experiment of raw water, we could gain the results that enhanced coagulation-sedimentation-filtration and micro-flocculation filtration made a satisfactory treatment effect. However, due to the characteristics of low temperature and low turbidity water, the effect of sedimentation was poor. Therefore, micro-flocculation filtration not only save chemicals, but also could simplify the process. During the micro-flocculation filtration process, the appropriate dosage of PAC should be 20 mg/L and the filtration rate should be 7 m/h. It's about 90% turbidity removal, the filtrated water turbidity was around 0.2 NTU and the chroma was less than 5 degrees. CODMn removal was about 60% and about 2 mg/L in the filtrated water. |
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