An antiquated 2-inch manual bar screen allowed inorganics smaller than a bed sheet to enter the Bell Road Lift Station in Sun City West, Arizona. Material routinely clogged the four 250 hp horizontal slurry pumps from KSB.

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 cutting out the road around the existing unit, demolishing it, excavating and widening the 3-foot-3-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 to use as an access point, 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.

Their efforts 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. That made access to the impellers 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 for disposal. 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,” Black says. “We immediately banned all operators from entering the vault and hired a contractor with a Vactor truck to clean the screen. His 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. They 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,” Black says. “We lost five mixers in a short period.”

Facelift

Epcor hired Felix Construction Co. for the preparatory work and to install the one-piece 30-foot-tall bar screen. The process took two and a half 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 all the holes and mounting hardware in the vault aligned perfectly to accept the bar screen frame,” says Black. “Then 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 down stroke collects trapped material, and the up stroke delivers it to an auger with internal spray bars that remove organics. Inorganics drop out a shoot into the trash bin, which is emptied weekly. The screen has no moving parts under grade level.

“We immediately had startup problems with the rake,” says Black. “It wasn’t removing all the debris, and it backed high up into the flow channel.” Workers sprayed the screen to help reduce the problem.

Black, along with an Aqualitec mechanical engineer and service technician, planned to add weight to the back of the rake for a better swing and attachment to the bars. He was with the night shift trying to figure out how to install the weight when they noticed the bars weren’t mounted to the bottom of the channel.

“They’d swing away when the rake tried to engage them,” says Black. “Aqualitec engineers called it a free-floating screen because it was only bolted at the top.

That didn’t make sense to us, so we bolted and welded the bottom in place. We haven’t had a problem since.” The system went online in June 2014.

Additional benefits

The result of the modification was immediate. Plant operators noticed a 75 percent reduction in inorganic solids at the headworks. The reduction 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 Fairbanks pumps. They left one larger pump for maintenance activities.

“The ease with which we retrofit the lift station without major demolition and excavation, and the money we saved through decreased maintenance made this unit a cost-effective solution,” Black says.