It was an invitation too good to turn down. Two regulatory bodies wanted to help the Wetumpka Water Works & Sewer Board save energy at its wastewater treatment plant. The resulting aeration changes have saved significant energy.
The Wilako Wastewater Treatment Plant was among those invited to participate in the Energy Management Initiative (EMI) through Region 4 of the U.S. EPA and the Alabama Department of Environmental Management (ADEM) in spring 2014. “They looked at our processes, suggested some changes and gave us some ideas,” says Ronnie Windham, general manager.
The plant (4.5 mgd design, 1.8 mgd average) had been using two aeration basins, each with six 75 hp high-speed vertical turbine aerators operated about six hours apiece per day, and a pair of 40 hp mixers operating 11 hours per day. To save energy needs, the EMI suggested shutting down one basin and operating the other anoxically for about six hours per day.
Chris Bowar, water works superintendent, says one basin had been shut down several years ago when a textile plant closed, reducing plant flow by 2 mgd. That didn’t work because of I&I problems. “This time our chief operator, Don McIlhargey, changed some operations, started tweaking runtimes, watching dissolved oxygen levels, and keeping flows and solids under control.”
McIlhargey put the aerators and mixers in the remaining basin on a rotating schedule, further reducing power demand. “We maximize DO by bringing it up to 1 to 1.5 ppm, and then turn off the aerators and let it get down to 0.5 ppm,” he says. “It hasn’t harmed us at all. Our TSS removal rate is still around 98 or 99 percent. Changing operational procedures has been a big benefit. We needed to do it, and the EMI gave us areas to concentrate on first.”
Russ Barber, plant operator, says the savings will help with a plant that is 20 years old. “It’s showing its age, and we do have some adverse effects of the textile plant because the waste stream they sent here for a number of years was quite corrosive,” he says. “We have a number of improvements to look at and we needed to find all the ways we could to save on operating costs and put money into replacing worn-out equipment.”
No capital cost
The changes reduced power use by 460,000 kWh (24 percent), saving $70,000 from March 2014 through March 2015. “This was about mindset and being motivated to work smarter,” says Bowar. The changes required no new equipment and no purchases.
Bowar doesn’t expect such reductions all the time: “We’ll have savings, but we’re not sure what they will be. We have heating systems to run in winter, and rainfall adds more runtime.”
The aeration changes also reduced effluent nitrogen by 62 percent (12 tons per year) and CO2 emissions by more than 390 tons per year. Effluent remains well below permit levels and is consistent with results achieved before switching to one basin:
- CBOD5 – less than 2 mg/L (permit level 25)
- TSS – 3 mg/L (permit level 30)
- NH4-N – less than 0.04 mg/L (permit level 10)
Demand on operators
The Wilako plant is operated manually, so the new procedures mean operators have to pay closer attention. “Our SCADA has a really nice graphing system, so we’re able to keep good track of our DO,” says McIlhargey. “It’s manual input, but the time it takes out of the day is worth it for saving power.”
There is no plan to automate the process: “I believe our money is better spent replacing worn-out equipment and making bigger upgrades. Automation can be done in the future sometime, but right now we have much bigger priorities.”
Among those is reducing I&I. The plant uses two old treatment lagoons to store influent during storm rain events. “We can make manual adjustments in the collections system and divert some of the excessive flow to the lagoons,” says McIlhargey. “We bleed it back in after the flows drop back down. So far, that has worked very well.” It does require turning up the aerators.
McIlhargey adds that the sewer board is allocating funds annually for I&I upgrades such as replacing sewer lines and lining old brick manholes and clay pipe. With more focus on energy efficiency, lighting upgrades are planned, the digester blowers are being upgraded with variable-frequency drives (VFDs), and aeration basin diffusers will be changed out.
“The ones we’ve had are open diffusers, and we’re going to disc membrane diffusers,” McIlhargey says. “We’re also looking at VFDs for the aerators and mixers. We’re playing with a lot of ideas along the way.”