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Aeration Blower Upgrades Cut Wastewater Treatment Energy Costs By Nearly 20%

As part of the City of Stratford’s municipal energy planning process, the city-owned wastewater treatment plant was determined to be responsible for more than 20% of the municipality’s total energy consumption and became a priority target for energy efficiency improvements. By utilizing incentives offered through the Save on Energy program, the City of Stratford Wastewater Treatment Plant modernized its aeration blower system, reducing the total treatment and collection costs of wastewater by more than 20% in just one project.
 
BACKGROUND
Built in 1956, the wastewater treatment plant is owned by the City of Stratford and operated by the Ontario Clean Water Agency (OCWA). The plant currently serves 32,000 people and treats an average of 21,700 cubic metres of wastewater per day, which amounts to around 70% of the design capacity. The plant, valued at over $100 million, has sufficient treatment capacity to address the foreseeable population increase, so the focus has shifted to performance instead of expansion. “Our focus is not on capacity increase but improving energy efficiency,” explains George De Groot, Manager of Engineering, Infrastructure and Development Services for the City of Stratford.

The City worked with OCWA and Festival Hydro to perform an energy audit of the facility and prioritize projects that improve the efficiency and operational performance of the plant. “In order to increase efficiency and reliability, the City decided to consider upgrading the aeration system by installing a turbo air blower,” explains De Groot. “The aeration process accounts for 60% of the wastewater treatment plant’s total energy usage, making it an ideal starting point to improve overall energy efficiency.”

OCWA and Festival Hydro aided the plant’s application for Save on Energy incentives. “Festival Hydro was a great help and assisted us by providing metering to quantify the current system baseline, and worked with OCWA to complete the application,” says De Groot.

PROJECT
In order to feed the microorganisms found in the aeration basin of the plant, which break down the organics in wastewater, oxygen is supplied to the aeration basin by large blowers through 1,600 fine bubble diffusers.

The City of Stratford replaced two fixed-speed 200-horsepower centrifugal blowers with a single 350-horsepower turbo air blower, and integrated a variable frequency drive (VFD). The VFD automatically adjusts the flow of air to the aeration basins based on the dissolved oxygen level in the aeration basin. “The old blowers always operated at full output regardless of the oxygen requirements of the process, and often over-aerated,” explains Marcel Misuraca, Water and Wastewater Systems Manager for OCWA. “This caused dissolved oxygen to be 5–10 mg/L, when it should be 2-3 mg/L. Not only were we using more air and energy than required, but the over-aeration also caused problems with shearing of the floc which made the secondary clarifiers less efficient,” he added. The new system uses 30% less energy and has also improved the operational performance of the secondary clarifiers.

While the blower upgrade was not the lowest-cost solution, it did provide energy efficiency with the best long-term financial benefit to the City. “We procure the smartest solution with the lowest total life-cycle cost, including upfront cost, years of service and energy savings,” says De Groot.

The full project took one-and-a-half years to implement, including design, procurement, permits, installation and commissioning. This timeline was possible as no major modifications were required to the Certificate of Approval, and also because the City of Stratford is a Tier One municipality. This reduced the review time by the Ministry of the Environment to just six weeks.

SAVINGS
“The blower upgrade project has saved roughly $68,000 in energy costs over the first nine months of operation,” says De Groot. “This will reduce the $215,000 in electricity costs for aeration by at least 30% annually.”

“Projecting this cost saving over the 20-year life of the project means this blower upgrade will essentially pay the city $1.2 million.” This translates to a return on investment over the life of the equipment of nearly 700%, based on 2% inflation and $68,000 in annual savings.

LOOKING INTO THE FUTURE
The energy efficiency improvements at the plant are just beginning. For other wastewater treatment plants, De Groot highly recommends considering energy efficiency when upgrading infrastructure. “The energy cost savings from one project can be used to help pay for other energy efficiency projects in the future,” De Groot explains.

The original energy audit suggested several other process upgrades that could improve energy efficiency, all of which are also eligible for Save on Energy incentives. Projects include right-sizing pumps, upgrading to high-efficiency motors and installing VFD controls on other systems, including pump stations, influent pumps, deep well lift pumps, screw pumps, sludge pumps and return activated sludge pumps. For these projects, payback periods are expected to range from two to seven years and provide an additional $60,000 in annual energy savings.
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