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Abstract

The Filter Backwash Recycling Rule established regulatory requirements to help ensure that recycle streams do not reduce the microbial quality of the finished water and threaten public health. While treatment of the backwash water is not required by the regulations, the US Environmental Protection Agency (USEPA) has recognized that additional treatment of the backwash water may be desirable to reduce the chance of microbial contamination and ranked sedimentation without polymer addition as the lowest cost treatment alternative followed by sedimentation with polymer addition. To evaluate the benefit of using a medium negative charge, high molecular weight anionic polymer and to investigate the settling properties of backwash water, jar and settling tests were performed using composite backwash water samples from three conventional treatment processes that have high quality surface water supplies. Composite water samples were used to simulate the recommended practice of having an equalization basin upstream of the sedimentation or sedimentation/polymer treatment system. Settling column tests showed that the addition of an anionic polymer increased the removal of turbidity by 0.3 to 0.4 log units for two of the three waters tested while there was no increase in turbidity removal for the other backwash water tested. The most pronounced difference was that the anionic polymer increased the settling rate by a factor of 2 to 5 times. Comparison of full-scale performance to settling column tests showed that a relatively simple settling column test is useful in estimating the overflow rate and effluent turbidity. It was found that coagulant settling was an important particle removal mechanism and that the overflow rate and depth of the tank must be considered together in the design of the sedimentation basin. With this data, a preliminary full-scale system can be designed and a cost/benefit analysis performed to determine if a coagulant should be included in the backwash recycling system. Includes 4 references, tables, figures.