Effectiveness of chitosan as natural coagulant aid in removal of turbidity and bacteria from turbid waters

摘要

There has been considerable interest in the development of natural coagulants such as chitosan. By using natural coagulants, considerable savings in chemicals and sludge handling cost may be achieved. Chitosan, a natural linear biopolyaminosaccharide, is obtained by alkaline deacetylation of chitin. Present study is aimed to examine the effects of aluminium sulfate (alum) as coagulant in conjunction with chitosan as coagulant aid on removal of turbidity and bacteria from turbid waters. These tests were carried out using artificial water and kaoline as model suspensions to represent the wide range of natural turbid waters. A conventional jar test apparatus was employed for the tests. After determining of optimum mixing intensity and duration, alum suspensions were added to the samples and after one minute, the desired doses of natural chitosan were added. In optimum condition, residual Al+3 in treated water was less than 0.2 mg/l and meets the EPA guidelines. Turbidity removal efficiency was 74.3- 98.2% by chitosan at a pH 7.0-7.5 for all turbidities. In addition, chitosan significantly reduced the required dosage of primary coagulant 50-87.5%. Bacteria reduction of 2-4 log units (99 - 99.99%) was obtained within the first 1 to 2 h of treatment. Overall results indicate that E.coli was removed better than S. faecalis. The main effects of coagulation by chitosan on bacteria are enmeshment and stack on the microbial cell surface. We demonstrated that optimal design method is an efficient approach for optimization of coagulation-flocculation process and appropriate for raw water treatment.