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.
| Edition : | Vol. - No. |
| File Size : | 1
file |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 12 |
| Published : | 11/15/2004 |
