Neutrally buoyant produce labels and other floating bits of plastic were passing through the Dublin San Ramon (Calif.) Services District Recycled Water Treatment Facility and entering the distribution system, plugging meters, irrigation strainers and sprinkler heads.
The facility had continuous-backwash upflow sand filters that kept the plastics suspended, allowing them to pass out with the effluent. Judy Zavadil, district senior civil engineer, could not locate another facility with a similar problem. “We had to find our own cure,” she says.
After evaluating bar and drum screens and step and articulating rake screens, the district considered and decided on a band screen. Many treatment plants, especially those with membrane bioreactors, install band screens at the headworks. Zavadil assumed that if the filters worked in that harsh environment, they certainly should work at the tail end of the treatment process where the water is clean.
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After researching band screens, she put out a request for a proposal with precise specifications. The district ultimately accepted the bid from Headworks for the Eliminator screen. Rather than bid the installation, at a cost nearly equal to that of the equipment, Zavadil entered a contract with Headworks, then assigned it to a contractor installing a bar screen in the treatment plant. “It was a creative move on our part,” she says. “We just gave him a change order.”
The facility became the first of its kind to install a band screen after the wastewater treatment process but ahead of the recycled water process. No one was certain that it would work — but in the end, with a little creativity, it did.
The Eliminator band screen has perforated stainless steel sieves mounted on a self-lubricating roller chain drive. The upper portion of the 5-foot-wide, 15-foot-tall continuous rotating band is fully enclosed, while the lower half sits parallel in the 9-foot-deep influent channel, covered by a grating.
To eliminate carryover, the unit’s center-feed design turns the flow 90 degrees, forcing it out the sides through 2-mm holes in the sieves. The screenings, carried up to the discharge point, are removed by a Dual-Surge spray header system. The resulting slurry — water with little grease balls and plastics — is discharged out an 8-inch pipe.
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Wayne McCauley, senior projects and special projects manager at Headworks, evaluated where Zavadil intended to install the screen. “I had reservations because the flow, around 5 mgd, was very turbulent at that point,” he says. “Turbulence and approach velocity increase impact on the screen and increase headloss. If we exceeded 12 inches of headloss, the water would overflow a bypass around the screen, which eventually had to handle 17 mgd.”
A consultant for the district recommended installing a baffle to direct and calm the flow before it entered the band, but Zavadil decided to forego the baffle and watch how the screen performed. “Because there isn’t much hydraulic data on losses through perforated plates, nobody knew what the exact headloss would be,” she says. “It turned out to be around four inches at current flows, so everything was fine.”
Keeping it simple
Normally, a screw extracts the screenings from the slurry and compresses them into little pellets, while the rinse water falls through the grating and into the channel. However, the area was too small for the screw mechanism.
The district consultant then recommended installing a filter vessel with removable nylon mesh bags. “The contractor plumbed the discharge pipe to a 2-inch inlet on the filter vessel, but sticks and duck feathers from the influent holding basin clogged the plumbing,” says Zavadil.
Operations staff, the contractor, and Zavadil then tried different alternatives to the filter vessel. They drilled holes in the sides and bottom of a 5-gallon pail and inserted a burlap bag. The water didn’t drain fast enough. Zavadil bought window screen material and sewed a bag to it, with identical results. She bought sample filter bags to test, but none of them worked.
While searching for mesh material in a hardware store, Zavadil stumbled upon a mesh wire wastebasket. “We bolted a 90-degree elbow on the discharge pipe so it angled down toward the channel,” she says. “We placed the wastebasket under it on the grate, and we had the perfect solution for less than five dollars. The slurry goes in, the water runs out and through the grate, and the screenings stay behind. We empty the basket once a day depending on flow.”
The band screen provides excellent capture rates with minimal maintenance, enabling the district to deliver particle-free recycled water to its 270 irrigation customers, which include parks, schools, golf courses, businesses, greenbelts and roadways.