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Activated Sludge Aeration System Evaluation

Client: City of Wyoming

Location: Wyoming, Michigan

With the goal of identifying energy-saving improvements, Donohue evaluated the existing activated sludge aeration system at the Wyoming Clean Water Plant (CWP), which has an average design capacity of 22 mgd, and is operated by the City of Wyoming. Three key areas for aeration efficiency improvements were identified: diffuser equipment, blower equipment, and process control upgrades.

Diffusers: The initial data evaluation indicated reduced oxygen transfer efficiency in the aeration system. A field diffuser investigation was performed by Redmon Engineering to identify specific contributions to the reduced oxygen transfer efficiency, so that corrective actions could be determined and considered. The diffuser investigation included in situ off-gas testing in two of the three aeration basins to evaluate oxygen transfer efficiency and laboratory testing of diffusers pulled from service in the aeration basins. The results of the testing indicated that the diffusers have fouled over time, reducing the ability of the diffusers to transfer oxygen into the water.

Three alternatives were evaluated to improve the diffuser performance. Of the three options, full diffuser replacement was estimated to result in the lowest 20-year TPW and is the least labor-intensive option overall, with fewer and less frequent ongoing maintenance requirements.

Donohue then assisted the City with the selection and procurement of new membrane diffusers. After installation was completed, preliminary performance data indicates an overall operational reduction of $130,000/yr. A rebate of $100K was also secured from Consumers Energy. Including the rebate, the overall project payback was less than two years.

Blowers: The main objective of the blower system evaluation was to identify the monetary and non-monetary benefits associated with the complete or partial replacement of the existing multi-stage centrifugal blowers with high-speed direct drive blowers. The 20-year present worth analysis showed that a partial replacement with the high-speed direct drive would have a similar Total Present Worth as operating the less efficient multi-stage blowers. A multi-attribute evaluation matrix was developed to include other non-monetary considerations into the blower evaluation.

Controls: During the evaluation, control upgrades were also identified to improve the aeration system operation efficiency and provide additional energy savings. The modifications included air piping and control valves changes to provide more independent control of airflow along the length of the aeration basin.