August 2014, Vol. 26, No.8

Problem Solvers

Ohio facility achieves unexpected bonus from aeration systems upgrade


Problem:
A municipal water resource recovery facility (WRRF) experienced high electric bills.
Solution: The WRRF installed high-efficiency bubble diffusers and blowers to save electricity and increase loading capacity.   

High electric bills had been plaguing the Newark Wastewater Treatment Plant (Newark, Ohio) for several years. Located on the east side of the city, the WRRF treats more than 10.5 million m3/yr (2800 million gal/yr) from 16,906 active customers. In 2013, the facility’s operating cost was $3.066 million. As the WRRF’s aeration system approached the end of its design life, the city sought relief from excessive electric bills through significant process upgrades. 

Replacing the 25-year-old equipment would improve process efficiency and reduce operating costs. The WRRF’s single largest operating expense was the electricity powering its large, aging blower motors that aerated the activated sludge. 


Surprising research revelations  

Arcadis Engineering (Highlands Ranch, Colo.) conducted an initial 2-month cost–benefit analysis that confirmed the project was worth pursuing. Next, an engineering study determined primary considerations for new equipment. It showed that the specific oxygen transfer efficiency factor specification was most important.  

The facility uses aerobic bacteria to break down waste in the aeration basins. These bacteria thrive and perform best 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,” said Darin Wise, WRRF 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, research revealed that their panel sleeves had to be made of polyurethane to avoid fouling from the type of content found in the flows. The research also specified that service needed to be fast and easy and the diffusers had to offer a low overall ownership cost. 

About a year after beginning the initial study, three potential vendors were identified and asked to submit bids. “All manufacturers met our specs,” said Bryan Curry, assistant superintendent for the WRRF. “At that point, our decision came down to cost.” The facility chose to install EDI FlexAir® MiniPanel™ diffusers manufactured by Environmental Dynamics International (EDI; Columbia Mo.). 

  

Implementing the plan and making adjustments as needed  

The facility has installed the first set of bubble diffusers in its aeration basins, waste activated sludge tanks, and a post-aeration tank. The first new blower came on-line in early fall 2013.  

Facility operators had to make minor in-house modifications to fully equip the third basin for separate processing. They added an $800 pump to return those treated flows to the main facility stream. They also installed internal mixed liquor recycle 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 denitrification and bacteria conditioning. This regains some air and removes nutrients. They still are tweaking the process for efficiency.  

“It’s not the lowest-hanging fruit,” Wise said, “but it does factor into total efficiency boost and cost savings.” 

  

Calculating the expected and unexpected benefits  

The city received a $67,000 gridSMART grant from power utility AEP Ohio (Columbus, Ohio) to help fund the project. This funding, received after installation based on pre- and post-install metering, only covered the cost of diffusers, which were the only portion able to demonstrate the required 7-year return on investment. 

The new diffusers and blowers not only have improved critical plant process, but also save nearly $14,000 per month in electricity fees. They also create excess capacity that enables more loadingand additional trucked waste streams that have generated new and unexpected revenues. 

It was expected that the majority of electric savings would come from replacing the old 336-kW (450-hp) blowers with new, single-stage turbo blowers, because the new 223-kW (300-hp) blowers require one-third less energy.  

“We installed soft starts, which allow the motors to draw less current over a slightly longer period of time,” Wise said. “They won’t kick a breaker on startup and will start on plant generator power alone. Once those are running, all the other plant equipment is started.”  

Because the blowers run on facility generators, they can perform full wastewater treatment even during extended power outages. “Untreated flows spilling into the waterways during blackouts can be detrimental,” Wise said. “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 inlet vanes and variable diffusers enable the new blowers to be adjusted for amperage draw according to operating needs to avoid breaker trips during hot weather.  

The WRRF did not anticipate just how much more efficient the system would become when new blowers were paired with the new diffusers. Featuring a top-half-only perforation design for optimum oxygen transfer efficiency performance, the 190.5-mm (7.5-in.) diffuser provides a full 245,000 mm2 (380 in.2) of perforated area per single diffuser and 490,000 mm2 (760 in.2) per diffuser assembly. Diffuser geometry supports high-density applications over 65% floor coverage when the highest oxygen transfer efficiency is desired.  

The diffusers’ saddle mount enables relocation or addition of diffusers to match process demands. It also lends itself to different physical configurations depending on layout and available space in the basin. It can provide high-density diffusion in limited spaces, making it suitable for plug-flow type reactors that need to taper the aeration design to match oxygen needs at both ends of the diffusion process system. 

 “Our air requirements dropped significantly — sometimes to a third of what they were,” Wise said. “The MiniPanel diffuser creates more and smaller bubbles, which also rise slower in the tank, to allow more contact time with bacteria. So now the bubble efficiency and our transfer efficiency is better.” 

  

Numbers don’t lie  

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

The third basin now represents profit, and the facility makes some margin on high biochemical oxygen demand/high ammonia flows that require use of the efficient new aeration basin. “If we can treat something in the digesters, we will,” Curry said, “because we scrub our excess gas and sell it to the local utility.” 

Wise is more than pleased with the amount of savings and profit the new blower/diffuser pairing is generating. “We initially had to spend extra on electric to allow for more air use to treat additional loading, but our electric bill still dropped $12,000 to $14,000 a month,” Wise said.Improvements are projected to save $168,000 per year in electrical use. And as of March 31, the facility had taken in an extra $153,000 in loading fees. 

“We were so happy with the savings and effectiveness, we’re now installing a second, identical blower/diffuser combination,” Curry said. “We’ll depend on it as a redundant system.” 

  

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