Compliance at a Bargain

Staff innovation helps a Pennsylvania sanitary authority meet strict Chesapeake Bay nutrient requirements at far lower cost than consultants predicted

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If the nitrogen reduction standards for the Chesapeake Bay watershed are not met in the next few years, it won’t be the fault of the Wyoming Valley (Pa.) Sanitary Authority.

 

The agency’s 32 mgd wastewater treatment plant near Wilkes-Barre is using staff-driven solutions to reduce effluent nutrients even more than the Chesapeake Bay strategy regulations mandate. And, contrary to previous predictions, it’s not costing an arm and a leg.

 

“We’re ahead of the game where the new Chesapeake Bay nitrogen and phosphorus caps are concerned,” says director of operations Bernie Biga, a 34-year veteran of the wastewater treatment profession and winner of the 2010 Pennsylvania Water Environment Association Operator Research Award. “In fact, depending on what nitrogen trading arrangements are agreed upon, we’re set up to generate some revenue from our reduction processes.”

 

That’s a big change from the position the plant was in a few years ago, when a consultant predicted that it could cost up to $20 million to meet new nutrient caps. Using probe technology from the Hach Co. among other low-cost modifications and improvements, Biga and his team are achieving nitrogen reductions of nearly 74 percent through the plant and falling more than 40 percent under their Chesapeake Bay cap.

 

Influent brings about 2.1 million pounds of nitrogen to the plant each year, and only 340,000 pounds of it ends up in the receiving stream, the Susquehanna River. Effluent phosphorus is considerably below the cap, as well.

 

Multiple communities

The Wyoming Valley treatment plant is a regional facility, serving industries and residents in metropolitan Wilkes-Barre and 34 other service communities across a 200-square-mile area in northeast Pennsylvania.

 

The original primary plant, now used for combined sewer overflow treatment, dates to 1969. An expansion in 1988 added secondary treatment designed for biological nutrient control, as well as biosolids treatment.

 

Flow originates from a sprawling sewer system containing 60 pumping stations. Where lines remain combined, the stations have diversion chambers to send a portion of the flow into interceptor sewers that lead to the plant. Average flow is around 25 mgd, wet-weather design flow is 50 mgd, and CSO conditions can push flows beyond 100 mgd at times.

 

During normal flow, wastewater passes through fine screens and is lifted by pumps some 35 feet to the headworks, which include grit and scum removal units. The flow moves on to a Parshall flume and then to four activated sludge treatment trains.

 

The plant uses Schreiber low-load counter current extended aeration technology. Air is supplied by rotary lobe blowers to suspended fine-bubble membrane diffusers through single and double rotating bridge structures.

 

All four trains contain two 185-foot-diameter, 16-foot-deep aeration basins with capacity just over 5.5 million gallons; and a 1.44-million-gallon anoxic zone for denitrification. When installed, it was the largest Schreiber installation of its type in the world. After biological treatment, the water enters the final clarifiers and chlorine disinfection before discharge.

 

Better nutrient removal

Pumps move waste biosolids to a Sharples (Alfa Laval) centrifuge for dewatering. A fluid bed furnace incinerates the cake. On-site lagoons receive the incinerator ash as a slurry. The lagoons are cleaned out once a year and the removed ash is landfilled.

 

The Wyoming Valley operation was designed for biological nutrient removal from the start, but the nitrification and denitrification processes have seen their share of difficulties. The original oxygen minimizers, installed to control aeration cycling by turning blowers on and off, did not function as designed. They had inherent problems that led to plant upsets and were abandoned shortly after startup of the secondary upgrade.

 

“In the meantime,” notes Biga, “we were getting good nitrogen removal with high mean cell residence time and high mixed liquor suspended solids concentrations, but nitrification was limited by the capacity of the aeration system because of leaks in the air lines.”

 

At about that time, the $20 million plant upgrade proposal emerged. It included extensive re-piping and new pump stations to accommodate nitrate recycle streams, along with two large denitrification filters and an expensive chemical addition system.

 

Rejecting such a costly fix, Biga and his operations team tried other remedies. An initial attempt to cycle the blowers with timers failed because the rubber gaskets in the air lines deteriorated. Water filled the lines when the air was turned off and then shorted out the collector rings in the center of each rotating bridge when the air was turned on. They couldn’t be repaired because of their position in the system.

 

To remedy the situation, the team abandoned the air lines under the reactors and replaced them with ducts along the floors of the tanks. This eliminated the air leakage, but even then, while Biga’s team was achieving reductions in total effluent nitrogen, the aeration on-off sequence was arbitrary, and they were not getting complete nitrification-denitrification.

 

Probing for a solution

In December 2008, a pilot unit tested ammonia and nitrate probes supplied by Hach. Air supply was based on the range of ammonia and nitrate concentrations detected by the probes. Results were excellent.

 

“The Hach probes are very accurate,” says Biga. The company originally established on-off signals at setpoints between 4 and 2 mg/l of ammonia, but the staff later found the “sweet spot” to be lower.

“On two trains, we’ve set the air to come on at 0.96 mg/l and turn off at 0.72 mg/l,” says Biga. “On the other two trains, the range is between 1.06 and 0.82 mg/l. We calibrate the system once a week, and it’s certainly worth it. The effluent numbers are so good, they’re almost unbelievable. We confirm them by using an outside lab in addition to our own lab.”

 

The numbers now show that the Wyoming Valley plant is reducing total nitrogen dramatically, to about 340,000 pounds annually, versus a total nitrogen cap of about 584,000 pounds per compliance year beginning this coming October.

 

If it sustains the current level of treatment, the plant will have more than 200,000 pounds of total nitrogen credits to trade for cash, assuming new nutrient trading laws for point sources are adopted in the near future.

 

That could make the plant’s nutrient-control efforts even more cost-effective than they are now. So far, Biga reports, costs to limit nutrient discharges have run in the hundreds of thousands of dollars — not the millions. Nonetheless, the plant has prepared a contingency plan in case nutrient reduction becomes more stringent in the years beyond 2011.

 

Those revisions are estimated to cost around $7 million — still far below the $20 million estimated earlier. And they could run even less if delivery rations (DRs) are used to establish nitrogen and phosphorus caps that are actually higher than now anticipated. “In fact,” says Biga, “if that happens, I could see that $7 million drop to a much lower number.”

 

In other words, Wyoming Valley is prepared to meet its nutrient reduction mandates at no more than a third of the originally forecast costs, not counting any income from potential nutrient trading credits.

 

An energy boost

Changes to the plant’s treatment configuration have netted more than nutrient reductions: There’s an energy payoff, too. Biga says the plant is seeing decreases of 30 to 40 percent in energy consumption, worth about $300,000 annually, electrical supervisor Joe Hines estimates.

 

“The Hach probes signal our PLC system (Allen Bradley, a Division of Rockwell Automation), letting us sequence the blowers so they don’t all start at the same time. That reduces load,” Hines says. The plant is also configured so that certain processes can run off emergency power during peak loads that can produce brownout or blackout conditions in the heavily populated area. By shedding load in this way, the plant can actually be reimbursed by the utility.

 

The improvements don’t stop here. The sanitary authority already has an aggressive grease trap inspection program, but Biga’s staff continues to work toward better grease control in the plant headworks.

A plow suspended from a traveling bridge now pushes the grease from one end of the rectangular grease chamber to the other. Then the grease, along with trapped water, is pushed over a beaching plate into dump containers. “It’s an O&M nightmare, needing operator attention and frequent maintenance,” Biga says.

 

The authority is looking at an air conveyance system that uses a series of nozzles to push the grease along the water line into a hopper, where a screw conveyor would dewater the grease and discharge it into dump containers.

 

Other headworks improvements have included replacement of aging enclosed screws with new dry-pit submersible pumps. “We’re also replacing old hydraulic cone valves,” says Chuck Zambito, maintenance supervisor. “We can’t get parts, and they require us to have a pressure vessel inspection.” The units are being phased out and replaced with pressure-controlled duckbill check valves from Red Valve Co. Inc.

 

The staff is also contemplating improvements in solids handling. “Our fluid bed furnace is nearing the end of its useful life,” Biga says. They’re also planning to convert the solids dewatering centrifuge to direct drive to eliminate problematic fluid couplings.

 

Even more changes could be coming. Biga observes that shale deposits in the Wilkes-Barre area have potential to produce enormous quantities of natural gas as the United States seeks local fuel sources. He suggests that plant effluent could be used for hydro-fracturing and then be re-treated for reuse in a zero-discharge process.

 

“We’re always looking to the future, as best we can,” he says.



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