An Air of Efficiency

A Missouri plant knocks down energy consumption and costs with aeration improvements including variable-frequency drives and SCADA control
An Air of Efficiency

In March 2010, the municipal wastewater treatment plant in Bowling Green, Mo., worked with Alliance Water Resources and equipment vendors to improve the aeration process and drive down energy consumption.

The improvements included replacing coarse-bubble diffusers with fine-bubble diffusers and the addition of variable-frequency drives connected to the existing blower assemblies that provide air supply to the two aerations basins.

After the improvements, the treatment plant (850,000 gpd design flow, 500,000 gpd average) realized a reduction in electric power consumption averaging 40 percent or greater. The municipality benefits through lower monthly electricity bills and reduced quantities of waste activated sludge and biosolids.

The city saved additional money because the local electricity provider later implemented a 10 percent electric rate increase, and because the aeration process improvements reduced power demand and therefore demand charges.

A one-time incentive of $55,220 from the local electricity provider, Ameren UE, helped offset the capital cost of the improvements, reducing the actual project cost from $120,227 to $65,007, a 46 percent savings.

 

Productive partnership

Alliance Water partnered with Environmental Dynamics Inc. (EDI) and Vandevanter Engineering to explore the feasibility and estimate the benefits of installing variable-frequency drives (VFDs), replacing the coarse-bubble diffusers in the two aeration basins and two digester basins with fine-bubble diffusers, and reprogramming the SCADA controls that integrate equipment operations with the attributes of the influent wastewater.

Alliance Water agreed to fund the estimated $120,000 in capital improvements through an enhanced operations contract. Two Altivar 61VFD units (Schneider Electric) now control the rotational speed of the electric motors connected to the three Sutorbilt positive displacement blowers (Gardner Denver), each rated at 75 hp and each providing 900 icfm airflow to the aeration basins.

EDI supplied 96 FlexAir Mini Panel fine-bubble diffusers in each aeration basin, reducing the air volume required and providing airflow over a larger surface area of the basin. Lower air requirements reduced the air-supply demands required from each blower, in turn reducing electric demand from 75 hp per basin to 20 hp per basin.

The control logic ties the blowers to dissolved oxygen probes in the aeration basins. When a basin DO level reaches 2.0 mg/l or less, a SCADA signal calls for the blower to power up. The VFD controls the rotational speed and therefore the air volume delivered to the basin.

For each of the two digesters, EDI supplied and installed 64 FlexAir 63P Magnum fine-bubble diffusers.

 

Sustained compliance

The treatment plant staff took the aeration basins down one at a time for the retrofits, each of which took three days. The first day consisted of emptying the basin and removing the old equipment. On the second day workers installed the new equipment. The third day’s work consisted of pressure testing and commissioning.

The aeration process improvements reduced monthly electricity consumption by an average of 44 percent per month (Graph 1). Before the improvements, from January 2009 through March 2010, average monthly usage was 117,480 kWh. After the improvements, from April 2010 on, the average monthly usage was 65,897 kWh.

The average electric bill (Graph 2) was $6,215 before the improvements and $5,024 after. Therefore, the average monthly reduction in electricity cost was about $1,200, or 19 percent. At the current pace, annual savings of at least $15,000 can be expected, even after the 10 percent electric rate increase.

The aeration process improvements did not adversely affect effluent quality (Table 1). Key parameters remained well below permit limits.

Additional electricity cost savings came from disconnecting two motive pumps as part of the aeration basin work. These pumps (40 hp) used to run continuously but were no longer needed after installation of the new diffusers and equipment.

The plant staff also has observed a reduction in wasting of sludge from the aeration basins from 30,000 gpd to 10,000 gpd, attributed to improved air distribution across the basins, leading to higher microbial activity and increased consumption of organic matter. Less wasting is expected to bring a significant reduction in the volume of biosolids, which are land-applied at about 3 percent solids.

The staff also expects to see less buildup of heavy sludge in the aeration basin bottoms when the basins are taken down for regular maintenance.

The electricity and other savings will continue to reward Bowling Green with dividends long after the estimated three-year payback on the initial investment. Reducing operating costs through an equipment retrofit is a clear illustration of a municipality working to save customers money and is especially valuable during an economic downturn.

 

About the authors

Daniel A. Gummersheimer, P.E., is a division manager with Alliance Water Resources, a provider of contract management and operations services based in Columbia, Mo., and serving Missouri and surrounding states. He can be reached at dgummersheim@alliancewater.com. John Harris is plant operator for Alliance Water at the Bowling Green treatment plant.



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