Preventing Back-Flush

Jan. 23, 2008

Riviera Water District provides potable water for a small community at Clear Lake in Northern California. This natural lake historically produces significant summer algae blooms and has creek sediment runoffs in the winter.

As in many older water treatment plants, a multimedia filtration system is used for potable water production. The media beds are composed of anthracite, sand and gravel and are the primary filters on the influent and raw water side of the plant.

The Assessment

The influent is surface water that contains organic types of debris. California canals and lakes can contain high levels of algae, water weeds, crustaceans and other floating debris, especially during the summer. Additionally, winter storms and the resulting runoff can easily magnify total suspended solid (TSS) and nephelometric turbidity unit (NTU) readings to very high levels.

As a result, these media systems would often back-flush, producing a substantial amount of waste or back-flush water. This cleaning method also took the water treatment plant off line, stopping potable water production. Because of the high NTU and TSS levels, back-flushing the media filters could not be scheduled often enough to meet the community’s water demand. In addition to automatic back-flushing, manual “fire hose flushing” was necessary at 10-hour intervals when NTU levels and water demands were high. Costs significantly increased because additional overtime was needed to produce water at the plant.

The Prescription

In spring and summer 2005, an automatic, self-cleaning Amiad SAF 3000 filter was installed as a prefilter, reducing the load on the multimedia system and allowing the media to extend its flushing intervals. The prefilter would hopefully reduce backwash water, chlorine consumption and flocculants.

Water from the lake is pumped into the prefilter through an inlet flange and passes through a coarse screen to remove large, hard contaminants that cannot pass through the suction scanner nozzles. The dirty water then moves through a multilayer, cylindrical, 316 stainless steel fine screen.

When a 7-psi pressure differential is reached across the screen, the filter begins its cleaning cycle. The remote controller, which monitors the filter at all times, opens the 2-in. exhaust valve and simultaneously starts the electric drive motor.

The suction scanner is a hollow, 316 stainless steel pipe that is connected to the exhaust valve. The opening of this valve connects the inside suction scanner to the outside atmosphere. The nozzles on the suction scanner branch out from this central tube with openings only a few millimeters from the inside of the fine screen. The differential pressure between the water inside the filter body and atmosphere outside the body of the filter creates high suction forces at the openings of the scanner nozzles. This suction force causes water to flow backward through the screen at very high velocities over a small area at each nozzle. This action pulls off the filter cake from the screen and sends it through the hollow suction scanner pipe and out the flush valve.

This “focused back-flushing” cleans less than 1 sq. in. of screen area. The motor drive unit moves the suction scanner linearly across the entire screen area in approximately 20 seconds. This spiral path of the suction scanner is at a fixed speed, allowing the filter cake to remain intact for its removal. There is no interruption of system flow downstream of the filter during a cleaning cycle and exhaust or wastewater required for flushing is typically less than 1% of the total flow.

Independent testing was arranged with the California Department of Health to help monitor NTU levels before and after this automatic filter. TSS levels were sampled and tested at an independent lab. Three interchangeable screens (80, 50 and 25 micron) were also provided for these tests.

The Results

With influent NTU values between 3 and 7 and TSS levels around 35 ppm, the 80-micron screen at 160 gpm flushed approximately once an hour in March 2005. There was a negligible effect on NTUs, though suspended solids were being removed. The 25 micron screen, while effective, was too small (tight) under these water conditions and would flush every two minutes.

From April through peak algae bloom in August, the 50-micron screen was successfully used in the pre-filter. This screen size back-flushed approximately every 20 minutes with influent NTU ranges from 5 to 7. Water samples and NTU tests showed reductions of approximately 71% in TSS and an unexpected 26% decrease in NTUs.

In addition, manual “fire hose cleaning” of the multimedia system was typically performed every 10 hours under high TSS conditions and large water demands. This manual operation was extended to over 27 hours with the installation of the SAF 3000 prefilter.

The Riviera Water District has continued to meet California water requirements and has been very pleased with the prefiltration the filter provides. Prefiltration with the automatic, self-cleaning screen filter significantly reduced turbidity (both TSS and NTUs) of influent water with minimal space requirements, energy consumption and water used in back-flushing. The result was cleaner, more efficient operation of the main media filtration system, lower labor requirements, reduced use of chemicals and lower release of media filter flush water.

About the Author

Matt Aguiar

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