Clarifiers-No, Filtration-Yes: A Case Study

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Courtesy of Amiad Filtration Systems Ltd.

Problem

Twenty miles southeast of Coos Bay, Oregon situated at the conjunction of the North and South Forks of the Coquille River lays the town of Myrtle Point. Just five years ago this community of 2670 residents was facing a large potable water problem with an even larger price tag. The town feeds its water treatment plant from the North Fork Coquille River. Prior to the 1990s the water treatment plant (WTP) could take water from the river with turbidities as high as 400 NTU and still provide clean safe water to its customers in only 8 hours of operation per day. Coagulating chemicals were added to the raw water which was sent to a flash-flocculation basin and then to a sedimentation basin. The sedimentation basin was equipped with 45° tube settlers to reduce residence time and minimize the footprint.

Gravity flow multimedia filters finished the polishing process followed by chlorination. Potable water was then sent to a three million gallon storage tank before distribution. All was well until the early 1990s when various environmental groups forced the Federal Government to passed laws prohibiting streamside logging. Such logging practices were to cease on a certain date in 1992. This legislation caused a flurry of streamside clear-cutting activity by landowners during the period from the law’s announcement to its commencement date. Once-forested stream banks now lay bare allowing the movement of fine colloidal clays into both forks of the Coquille River. The existing flocculation and sedimentation basins at the WTP were overloaded causing some colloidal clays to migrate to the multimedia filters. Unlike the basins which just allow the excess material to pass on through, the multimedia filters stopped the clays but were quickly blinded in the process. Back flushing the filters could not be scheduled often enough to maintain the required potable water production to meet community demand.

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