|The testing begins with the determination of the optimum dosage of the coagulant. After recording the raw water turbidity, the procedure is as follows:
|The procedure is repeated for the samples taken at each sampling time, i.e., 1, 2, 3, 5, and 10 minutes. For the light scatter turbidimeter a sample of 30 to 35 ml is sufficient for a turbidity reading so that the sample can be taken quickly and the liquid level in the jar is not lowered significantly. Larger samples should not be taken since it would unnecessarily lower the level of the water in the jars.
With four jars, there will be 20 samples for determining turbidities. The turbidimeter should be checked for calibration for the range anticipated. Some instruments are calibrated for over 10 ntu or under. Turbidity readings are then made on each sample and recorded. Samples should be agitated carefully before pouring into the turbidimeter tube. A direct comparison or turbidities for each group of samples, 1, 2, 3, 5, and 10 minutes can be made.
The next variable to be determined will be polymer dose in case the use of a polymer is contemplated. It must be remembered that the charges which the polymers carry determine how they react. The positively charged cationic polymers act like coagulants and are used to supplement the coagulant dose. Sometimes by adding a small dose of cationic polymer during initial mixing, the coagulant dose can be significantly reduced. It may therefore be economical to use a combination of coagulant and polymer. The feasibility of the procedure can be determined by bench scale testing in the laboratory. Along with determining the quantity of polymer, the sequence of addition of coagulant and polymer may be important. It will be necessary therefore to determine which should be added first and which second. Occasionally the polymer alone will form a satisfactory floc.
The non-ionic polymers are used to assist in the agglomeration of floc to assist in settling. In this case, the polymer must be applied after the floc has been formed. If it is applied too early before all the floc has formed, it will be less effective. If applied too late there may not be enough time remaining for optimum floc formation. It is important therefore to carefully test for the best point in the flocculation process to add this polymer. Experience indicates that the best time is about 5 minutes after the beginning of the flocculation cycle.