The Study Of Residual Aluminum Content In The Outflow Of Densadeg

Densadeg usually uses poly aluminum chloride (PAC) as coagulant. Water is purified efficiently by adding PAM and reflux sludge to enhance the flocculation. The reflux of sludge with aluminum hydroxide as its main component is ne cessary due to the high PAC dosage, giving rise to the exceeding residual aluminum content in the outflow of Densadeg. In addition, another reason for the exceeding aluminum is that the improper operation and management of Densadeg would cause the sludge to float. Such phenomenon is commonly called "Sediment Upturning", and it will make the floc composed of lots of aluminum hydroxide hardly removed. This paper made the research of the main factors of residual aluminum content in the outflow of Densadeg by baker tests, the bench and pilot scale tests in the laboratory. It is concerned that the residual aluminum content in the outflow of Densadeg could meet the requirement of "Hygienic Standard for Drinking Water" (GB 5749-2006) and the operating condition for the lowest residual aluminum was found by this study. The research results are as follows:(1) The study found the iron-based coagulant had a good effect of aluminum remove with the removal rate of more than 40% in the raw water, while the aluminum-based coagulant added the residual aluminum content in the water, which was the main reason for the increase of residual aluminum content in drinking water.(2) With the increase of PAC dosage, the residual aluminum content in the sedimented water decreased then increased. The lowest residual aluminum content was 0.166mg/L with the PAC dosage of 30mg/L. The main reason for the increment of residual aluminum was that when the PAC dosage was higher than 30mg/L, the dissolved aluminum would increase.(3) With the increase of PAM dosage, the residual aluminum content decreased, which means a less than 0.20mg/L dosage of PAM would reduce the turbidity and residual aluminum significantly. Such effect would not be improved with the increase of PAM dosage when it was more than 0.2mg/L. In consideration of the toxicity of acrylamide monomer, the PAM dosage should be controlled at 0.2mg/L as recommended.(4) With the increase of volume of reflux sludge, the residual aluminum content in the sedimented water decreased then increased. The reflux rate of sludge was related to the PAC dosage. The best reflux rate was 1% when the PAC dosage was less than 15mg/L; 5% when the PAC dosage was 30mg/L and 6% when the PAC dosage was 45mg/L.(5) It could be inferred from the bench-scale tests of raw water with different quality that pH of raw water would exert a huge influence on the residual aluminum content in outflow. The lowest residual aluminum content in outflow was 0.162mg/L when the pH was 7.5. The residual aluminum in outflow would increase when pH was higher or lower. The turbidity of raw water had a little effect on the residual aluminum, which was different from the traditional technology. It gradually decreased with the increase of raw water temperature.(6) The results of orthogonal analysis show that the sequence of all the factors which effected the residual aluminum content in outflow of Densadeg was:PAC dosage>PAM dosage > reflux rate of sludge. In consideration of both turbidity and residual aluminum content, it reached its best operating condition with the PAC dosage of 40mg/L, PAM dosage of 0.2mg/L and the reflux rate of 4%. In this case, the turbidity and residual aluminum content in outflow of Densadeg was 0.87NTU and 0.153mg/L respectively, which would meet the requirement of "Drinking Water Sanitary Standard" (GB5749-2006) after filtration follow-up.(7) When the Densadeg was under extreme operating conditions such as a low PAM dosage or a high reflux rate of sludge, the liquid level between sludge and water in sludge concentration zone would raise because of the reduce of compressibility of sludge and concentration time, giving rise to the "Sediment Upturning" and the exceeding residual aluminum content of more than 1.Omg/L in outflow. Such phenomenon should be avoid in the practical operation.