The U.S. Environmental Protection Agency (EPA) is initiating a peer review of draft scientific modeling approaches to inform EPA’s evaluation of...
On the basis of favorable results of an exhaustive test program conducted on site (see "Tests Confirm Alternative Filtration System Technology Meets California Requirements" on page 39), the resultant regulatory approval by California, and in compliance with the National Primary Drinking Water Regulations, the Yucaipa Valley Water District (about 708 miles east of Los Angeles, in the San Bernardino mountains) proceeded with the installation of an alternative technology filtration plant.
Previous experience with the existing equipment at their Oak Glen Treatment Plant, installed in 1973, had failed to conform to Environmental Protection Agency (EPA) and California Health Department regulations. The old plant consisted of a single pressure filter with four individual compartments having a total of 200 sq. ft. of sand and gravel media. The plant was prechlorinated with sodium hypochlorite. Polymers had been used in the last few years, but even then water quality did not seem to improve.
Some of the problems experienced with the old plant included mud balls that formed very quickly because of inconsistent backwash flows; the alarm would sound when influent flows were disturbed causing turbidity above 2.0 NTU; and backwash valves and the waste valve frequently got stuck causing the filter to stay in the backwash mode until the problem could be manually corrected. In addition, the filter was made of steel and corrosion caused the media baffles to break, creating a continuing maintenance problem.
Some alternative technology filtration plants avoid the high cost of conventional water treatment plants for small communities with 25 to 500 connections. It is fortunate that the EPA and now many states (including California) have approved this technology. Otherwise, small communities such as Yucaipa would have faced discontinued service and/or potential fines for non-compliance of the new regulations.
For the moment, the plant is operating at only 450 gpm (0.6 mgd) due to the clearwell size and until a tracer study can be completed for higher rates.
Plant influent is supplied by gravity, primarily from two nearby creeks and a well that is considered to be a surface-water influenced source. Since the plant is capable of producing treated water and backwash with only gravity flow, effluent pumps are eliminated. Elevation, depending on the source in use, provides approximately 20- to 40-psig water to the plant. Two additional creeks provide secondary water sources.
Influent water quality can be described as very good to poor and vary widely due to variations in runoff resulting from such events as thunderstorms.
Effluent flows (also by gravity) to two tanks in series: a 100,000 gallon clearwell that allows the appropriate chlorine disinfection contact time, and then a 42,000 gallon reservoir tank.
The filtration system is a two-stage, fully automatic, Hi-Rate permanent media, modular plant. Figure 3 shows the first-stage, roughing filter consisting of six tanks in parallel, the second-stage polishing filter consisting of four tanks, the unit control panel, and some of the interconnecting piping.
The roughing primary filter tanks contain a fine garnet media in the size range of 50X (0.27-mm); 60/80 (0.15-mm) garnet media is used in the secondary polishing filter. The first-stage filter tanks operate at a flow rate of 5 gpm/sq. ft. of filter area; second-stage tanks operate at a flow rate of 7.5 gpm/sq. ft. of filter area.
A polymer feed pump continuously injects a cationic, polymerized aluminum sulfate into both first and second state filter tanks. This serves to create a pin-point floc on the filter beds that attracts and hold minute particles including Giardia cysts and viruses. Most of this polymer and its attracted impurities remain on or near the filter bed surface, creating the increased pressure differential that triggers filter tank backwash.
The system automatically backwashes both the rough and polishing filters one tank at a time. Backwash is triggered by an increased differential-pressure across any tank media bed, a predetermined time interval or manual activation. Tank backwashing uses clean filtrate from the adjacent tanks.
The entire system is run by a fully automatic, programmable logic controller. Hands-on operation is not required. This control system monitors and records influent and effluent water quality. In accordance with California regulations, the turbidimeter sequentially measures and records effluent turbidity and the turbidity from each of the four second-stage tanks individually on 15-minute intervals. This process eliminates the possibility of a single filter bed breakthrough that could be masked by operation of the remaining tanks.
The Autodialer Alert System provides telephone communication with the filter system and offers the operator notification (Figure 4) for the following conditions.
In addition, an automatic turn-out valve assembly dumps water to waste when it does not meet preset water-quality standards in accordance with applicable federal and state regulations.
The entire filtration system is extremely compact. The six-tank roughing filter measures 19-ft. 3-in. long; the four-tank polishing filter is 12-ft. 9-in. long. Both first and second stage filters are 4-ft. 111¦2 in. high by 5-ft. 81¦4-in. deep.