Full Description

The Filter Backwash Recycling Rule established regulatory requirements to help ensurethat recycle streams do not reduce the microbial quality of the finished water and threatenpublic 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 toreduce the chance of microbial contamination and ranked sedimentation without polymeraddition as the lowest cost treatment alternative followed by sedimentation with polymeraddition. To evaluate the benefit of using a medium negative charge, high molecularweight anionic polymer and to investigate the settling properties of backwash water, jarand settling tests were performed using composite backwash water samples from threeconventional treatment processes that have high quality surface water supplies.Composite water samples were used to simulate the recommended practice of having anequalization basin upstream of the sedimentation or sedimentation/polymer treatmentsystem. Settling column tests showed that the addition of an anionic polymer increasedthe removal of turbidity by 0.3 to 0.4 log units for two of the three waters tested whilethere was no increase in turbidity removal for the other backwash water tested. The mostpronounced difference was that the anionic polymer increased the settling rate by a factorof 2 to 5 times. Comparison of full-scale performance to settling column tests showedthat a relatively simple settling column test is useful in estimating the overflow rate andeffluent turbidity. It was found that coagulant settling was an important particle removalmechanism and that the overflow rate and depth of the tank must be considered togetherin the design of the sedimentation basin. With this data, a preliminary full-scale systemcan be designed and a cost/benefit analysis performed to determine if a coagulant shouldbe included in the backwash recycling system. Includes 4 references, tables, figures.

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Edition: Vol. - No. Published: 11/15/2004 Number of Pages: 12File Size: 1 file , 1.2 MB