Automated Bar Screen Solves Problems At Arizona Lift Station

Screening equipment designed for narrow, deep vaults increases efficiency at an Arizona lift station and treatment plant.
Automated Bar Screen Solves Problems At Arizona Lift Station
Jesse Black (left), senior operator for Epcor Water, meets with staff and Aqualitec engineers at the completed Screentec installation.

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An antiquated 2-inch manual bar screen allowed some objects almost as large as a bed sheet to enter the Bell Road Lift Station in Sun City West, Arizona. Materials routinely clogged the four 250 hp horizontal slurry pumps.

At the headworks 5 miles away, the flow split over two 1/4-inch bar screens. Smaller material slipping through them clogged the in-basin mixers. “We fought this problem for years,” says Jesse Black, senior operator for owner/operator Epcor Water. “The station was built in 1979 and is one of the oldest in the area.”

Replacing the 30-foot-deep bar screen would be expensive. The prep work alone included saw-cutting the road around the existing unit, demolishing it, excavating and widening the 39-inch-wide channel, and pouring concrete.

Then Operations Manager Douglas Griffith saw Aqualitec Corp. demonstrate its Screentec automated bar screen at the 2012 Arizona Water Association conference.

He measured the unit and realized it would slide unaltered into the lift station’s existing flow channel. “Not having to chip concrete or dig dirt made the purchase much more cost-effective,” says Black. “Even when compared with competitive screening systems, this unit was a good option and it solved our ragging problems.”

Best efforts

The 5 mgd (design) lift station has a peak flow of 4 mgd. The pumps, derated to 235 hp for variable-frequency drive, clogged in no particular order. De-ragging them took two workers four or five hours.

Lacking a mechanical joint in nearby pipes as access points, the crew needed a boom truck topside to lift the unbolted spool piece from the front of the pump to expose the impeller. To reduce frequent ragging, they trimmed the impeller vanes, enabling larger solids to slip past.

That reduced clogging to quarterly events. “However, they always seemed to happen around 2 in the morning when the pumps slowed down in response to decreased flows,” says Black.

To avoid ruining the boom truck driver’s sleep, workers used a blowtorch to cut a hatch in each pipe spool. Now access to the impellers was as easy as pulling off the hatch, but sealing it was a different situation. “The men made their own gaskets, but they still leaked a little,” says Black. “In the end, they sealed the hatches using multiple tubes of silicone.”

Additional woes

The old bar screen needed cleaning once or twice a day. Operators would descend a ladder across from the screen, rake off the material and carry it topside. In 2013, the metal grating directly above the flow gave way as a worker stepped off the ladder. “His foot went straight through the flooring, but he wasn’t injured,” says Black. “We immediately banned all operators from entering the vault and hired a contractor with a combination cleaning truck to clean the screen. Those weekly visits became extremely costly.”

Material from the lift station also affected the treatment plant. The headworks split the flow over two 1/4-inch bar screens that caught much of the larger debris, but smaller matter still reached the in-basin mixers. It took two workers two hours to raise a clogged mixer, then de-rag and repair it. “Repeated cloggings overloaded the motors and eventually fried them,” says Black. “We lost five mixers in a short period.”

Giving a facelift

Epcor hired Felix Construction Co. for the preparatory work and to install the one-piece, 30-foot-tall bar screen. That took 2 1/2 months.

“We bypassed the flow from an upstream manhole and pumped it around the screening area into the wet well,” says Black. “Then the contractor removed the metal housing over the top of the old bar screen and the unit.” The concrete in the wet well was showing its age. Another contractor ground down the walls, sandblasted them, then rolled on 98 percent acid-resistant Novolac epoxy resin (Sewer Shield Composite).

Meanwhile, Felix workers poured a concrete structure with guide rails to accommodate the 10-cubic-yard trash bin. The bin hooks to winches that move it back and forth to distribute solid waste evenly as it pours from the enclosed auger. Workers also put a structure over the bin for odor control and built a frame for the bar screen and trash bin control panels.

“The most critical part of the installation was ensuring that all the holes and mounting hardware in the vault aligned perfectly to accept the bar screen frame,” says Black. “It took more than half a day for the crane operator to lower the 1,430-pound unit, but it fit perfectly.”

How it works

After the retrofit, the bypass was disassembled and wastewater flowed through the new 1/2-inch bar screen. A rake on a belt follows two guide rails on each side of the frame. The downstroke collects trapped material and the upstroke delivers it to an auger with internal spray bars that remove organics. Inorganics drop out a chute into the trash bin, which is emptied weekly. The screen has no moving parts under grade level.

After a shakedown period, the results of the modification were evident. Plant operators noticed a 75 percent reduction in inorganic solids at the headworks. That enabled them to take down and clean the basins for the first time in years.

Because the lift station pumps no longer had to pass solids, workers replaced three of them with more economical 135 hp pumps (Fairbanks Nijhuis). They left one larger old pump for maintenance activities. Says Black, “The ease with which we retrofitted the lift station without major demolition and excavation and the money we saved through decreased maintenance made this unit a cost-effective solution.”



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