Screen Filtration, A Proven Alternative

Federal legislation was passed prohibiting streamside logging. Residents were given a deadline to cease its customary practice. This deadline drove a flurry of streamside clear-cutting activity by local landowners. These once lush, forested stream banks were now bare allowing the movement of colloidal clay into both forks of the Coquille River, the water supply that feeds the town’s water treatment plant.

Gravity flow multimedia filters had been used satisfactorily until deforestation occurred. Now, back flushing could not be scheduled often enough to maintain potable water production needs.

Analysis
The community hired an engineering firm who recommended a 1,200 gpm clarifier. However, the $600,000 installation and operation fees were cost-prohibitive to the community. They elected alternatively to try mechanical screen filtration.

A pilot was conducted to determine the appropriate filtration degree to give ample turbidity reduction while maintaining the assurance that the filters would clean themselves in a dependable fashion. The pilot test concluded that a filtration degree of 100 microns would supply sufficient turbidity removal and offer reliability.

Solution offered
Four fully automatic self-cleaning filters were installed and began operation in September 1999. A fifth filter was added soon after to handle the river’s heavy dirt loads that occur in the spring due to snow melt and heavy rains. The multimedia filters now go three times longer between back flush cycles. Subsequent testing has shown that when the raw river water has turbidities around 230 NTU, the filters reduce turbidity by more than 50% (effluent values of approximately 111 NTU).

As raw turbidity decreases to 20-40 NTU, the percent reduction by the filters generally drops to 20-40% as expected. During the warmer months when most of the total suspended solids (TSS) in the river are the result of algae, the filters remove a majority of this organic matter. Because the screen filters go through a cleaning cycle only as needed, many times they only do so one time a day while at other times, such as during heavy springtime runoff events, the automatic flush cycles could be as close as every five minutes. Since their commissioning in 1999 the filters have performed flawlessly.

Cleaning cycle
The screens are removed from the filter bodies annually and soaked in muriatic acid followed by a high-pressure spray wash. This removes any encrusted material that may have mineralized onto the screen mesh during the year. Other routine maintenance includes checking monthly to make sure the threaded shaft on each filter is well greased and that there are no leaks.

The Myrtle Point installation of five filters includes a single differential pressure switch across the inlet and outlet manifolds of the system. When a differential of 7 psi (0.5 bars) is sensed, the cleaning cycle begins by initiating focused-backflushing in two of the filters simultaneously. After their 20-second cleaning cycle is completed, two more filters flush together. Then the final filter goes through its cycle. This method assured that all filters are cleaned completely during the same cycle and all are doing their share of the work.

Upstream conditions can change. And when they do, downstream municipalities who utilize the river as a source of water must also change.

Myrtle Point found an acceptable alternative while saving over 50% in the process. After five years of continuous operation, the five Amiad SAF-6000 Filters with 100 micron screens have provided the city with pretreated water under widely varied river quality conditions while extending the interval between multimedia flushing by three times.

Not only were the initial purchase and installation costs low but annual operational costs are nearly zero. Screen filtration is a proven alternative to clarifiers for river water pretreatment.