Case Study: Ohio city's WWTP achieves unexpected bonus from aeration systems

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NEWARK, OHIO, May 2, 2014 -- The city of Newark, Ohio, sought relief from excessive electric bills through significant process upgrades. Its Licking River Wastewater Treatment Plant on the east side of the city completed installation of new, highly-efficient bubble diffusers in its aeration basins, WAS tanks and a post-aeration tank. The new diffusers, along with new blowers have not only improved critical plant process, but save nearly $14,000 per month in electricity fees while creating excess capacity that allows for more plant loading, including trucked waste streams that have generated new and unexpected revenues.

Now in the second phase of its Long Term Control Plan to optimize interactivity between its four plants, Newark is working to provide EPA-compliant, reliable service at reasonable rates. 2013 saw it completing Phase 1 with the installation of a new interceptor line that expresses sanitary sewer flow from the west end of town, through the downtown area's combined system, to the east end treatment plant.

Design work continues on the separation of downtown sewers, in anticipation of renovation work to this area. Among other efforts, a significant element of 2014's plan is completing the upgrade of the east end WWTP aeration system. Priorities for this project are replacement of 25-year-old existing equipment nearing the end of its service life and significantly improving process efficiencies to reduce operating costs. Electricity powering the large, aging motors on blowers that aerate the activated sludge is the plant's single greatest operating expense.

EDI MiniPanel fine bubble diffusers ready to be placed in service at the Licking River Wastewater Treatment Plant. The new, high-efficient diffusers are installed in the plants three aeration basins, WAS tanks and a post-aeration tank.


Surprising Research Revelations

The aeration system upgrade took a couple years to plan. Scott Phipps spearheaded research into available new blower options when he was formerly with consultants Arcadis Engineering, in nearby Columbus, Ohio. Arcadis conducted an initial two-month cost/benefit analysis that confirmed the project was worth pursuing. This was followed by an engineering study to determine primary considerations for new equipment. It showed that the Specific Oxygen Transfer Efficiency (SOTE) factor specification was most important.

The plant uses aerobic bacteria to break down waste in the aeration basins. These bacteria thrive and best perform their function in sufficiently oxygen-rich environments. Blowers force air into the tanks through diffusers that create bubbles. The smaller the bubble, the more bubbles can occupy each unit of flow volume. More bubbles create greater surface area touching the surrounding flows, providing greater aeration efficiency.

"We originally only intended to replace our two aging blowers with new, single-stage, high efficiency units," recalled Darin Wise, the plant's superintendent. "We hadn't really thought it necessary to also replace the diffusers. But the technology has just advanced so much that efficiencies have really improved. Our research showed we could save more money in the long run by combining newer, highly-efficient diffusers with the new blowers. So we decided to do the whole project."

Once diffusers were added to the upgrade, more research revealed that their panel sleeves had to be made of polyurethane to avoid fouling. Other specified requirements included that service had to be fast and easy, and the diffusers had to offer a low overall ownership cost.

Construction crews install the EDI MiniPanel fine bubble diffusers on the aeration grid piping.


Getting to "Yes"

About a year after beginning the initial study, three potential vendors were identified and asked to submit bids. "All manufacturers met our specs," said assistant plant superintendent Bryan Curry. "At that point, our decision came down to cost, and EDI (Environmental Dynamics International -- wastewater.com) had the best price." So EDI FlexAir® MiniPanel™ diffusers were specified for this application. Installation of the first set of new diffusers was completed in late summer 2013, and the first new blower came on line in the early fall.

The city received a $67,130 gridSMART grant from AEP Ohio (aepohio.com), from an application written by Arcadis engineering consultant Derek Dalton, to help fund this energy-saving project. Grant money was awarded after installation, based on pre- and post-install metering. The grant money covered only the cost of diffusers, since a stipulation of the grant was that covered expenditures must show a seven-year return on investment, which only the diffusers could demonstrate.

Benefits -- Expected and Unexpected

EDI MiniPanel diffusers undergo final inspection and testing in the post-aeration basins at the Licking River Wastewater Treatment Plant in Newark Ohio.

It was expected that the majority of electric savings would come from replacing the old 450 h.p. blowers with new, single-stage turbo blowers. That's because these new blowers at 300 h.p., require 33 percent less horsepower. Aeration blowers can draw up to six times their normal running amps upon startup, so the Newark crew installed soft starts, allowing the motors to draw less current over a slightly longer period of time. This means they won't kick a breaker on startup, and will start on plant generator power alone.

One additional benefit of the blowers running on plant generators alone is that they can perform full treatment of wastewater, even during extended power outages. "That’s important," explained Wise, "because untreated flows spilling into the waterways during blackouts can be deadly. With the new setup, I think we're going to be good. As long as we can get fuel deliveries every couple of days, we won't be killing fish in the river during power outages." The new blowers can be adjusted for amperage draw via the inlet vanes and outlets, according to operating needs, avoiding breaker trips during very hot weather.

What wasn't anticipated was just how much more efficient the system would become when these new blowers were paired with the EDI FlexAir MiniPanel diffusers. "Our air requirements have dropped significantly -- sometimes to a third of what they were," said Wise. "Putting air in the water has associated costs, and now the bubble efficiency is better and our transfer efficiency is better. The MiniPanel diffuser creates more and smaller bubbles, which also rise slower in the tank, to allow more contact time with bacteria."

This unexpected boost in efficiency actually allowed 100 percent treatment to be accomplished in just two of three dedicated basins. This freed up the third basin to treat trucked-in industrial waste and septage, generating new disposal fee revenues of $83,000 from Jan. 1, 2014 to Feb. 24, 2014.

Dialing In

The plant crew had to make minor in-house modifications to fully equip this third basin for separate processing. They added an $800 pump to return those treated flows to the main plant stream, but these were one-time costs, far offset by earning potential. They also installed internal mixed liquor recycle (IMLR) pumps to replace the old air-lift ones that were inefficient and used a lot of air.

The new ones have a propeller-type design that returns treated flows at the end of the aeration basin back to the head for de-nitrification and bacteria conditioning. This regains some air and removes nutrients. They're still tweaking the process for efficiency. "It’s not the lowest-hanging fruit," explained Wise, "but it does factor into total efficiency boost and cost savings."

They anticipate a similar ongoing rate of septage truck disposal, but perhaps not as high a premium as they've made all winter. This is because a local industrial facility lost its biological treatment capacity during this unusually cold season, and they took over that treatment. However, it will soon go back online as the weather warms, and Newark Wastewater will lose those flows. They also can't continue to process landfill leachate, due to the UV blocking it causes, further reducing trucked-in loads.

Still, the third basin now represents pure profit, and the plant makes some margin on high BOD/high ammonia flows that require their efficient new aeration basin. "If we can treat something in the digesters, we will," said Curry, "because we scrub our excess gas and sell it to the local utility."

Upgrade work has been going on for about a year and is expected to wind up in June, with a total price tag of $1,875,875. Wise is more than pleased with the amount of savings and profit the new blower/diffuser pairing is generating for his operation. "We initially had to spend extra on electric to allow for more air use to treat additional loading, but our electric bill still dropped significantly -- between $12,000-14,000 a month."

Curry added, "We were so happy with the savings and effectiveness, we're now installing a second, identical blower/diffuser combination. We''l depend on it as a redundant system." These improvements are projected to save $168,000 per year in electrical usage and, as of March 31, the plant has already taken in an extra $153,000 in the first quarter in loading fees.

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