OPTIMUM DOSE OF COAGULANT FOR NATURAL SURFACE WATERSA. KHAJABABU AND S.K. GUPTA
Conventionally in a water treatment plant fine suspended matter is made to agglomerate into larger size flocs by adding chemicals that neutralize the inherent negative charges on colloidal impurities that repel them from each other and interfere with their settling mechanism. It is important that the exact dosage of the chemical (coagulant) is determined prior to addition, because if the dosage is low then the negative charge on all the particles shall not be neutralized, and if the dosage becomes more than the particles acquire net positive charge resulting in their repulsion from each other and non-settlement. In water treatment plant, optimum dose of coagulant is generally evaluated by jar test method. Limitation of this test is that it is time consuming, expensive, and the accuracy of results depends upon experience of the plant operator. Coagulant doses are often approximated, resulting in poor quality effluent from secondary settling tanks. In this study water samples were collected locally from different sources e.g. lake, river, village pond and a canal. A review of the commonly available coagulants was made and Alum was identified as the potential coagulant for this study. Optimum alum dose tests were carried out on all samples and the results have been analyzed and presented in graphical format to give some idea to plant operator on the approximate values of optimum alum dosages for various sources of water in the region. It is hoped that this study shall reduce the time, effort and costs involved in finding the optimum alum dosages for these surface waters, because the operator can lay his/her hands on rough value as a starting point and then carry out the experimentation to refine his/her results. The study aims to reduce time, cost and effort in operation and maintenance of water treatment plants where turbidity values of the incoming water fluctuate dynamically with time, and offers a sustainable outcome to this issue.
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