2012-08-24 - The City of Beloit in Wisconsin sought improved efficiency and process control for an aeration blower application that would save energy and improve total lifecycle costs. ABB's ACS 2000 medium voltage drive was selected for the job.
Beloit is a mid-sized community in southern Wisconsin, with a waste water treatment facility currently treating an average of 4.6 million gallons a day. The City of Beloit Water Pollution Control Facility (WPCF) uses aeration blowers to supply air to the biological process used to remove dissolved organic pollutants. As the main consumer of electricity in a wastewater treatment plant, usually consuming more than half of the total electric power used at these plants, the aeration system significantly influences operational costs.
The WPCF had an existing blower with an inlet throttling valve, a common solution for blower control. However, variable speed drives offer significant advantages over these traditional methods of flow control. The WPCF was interested in the ACS 2000’s technical specifications and ease of installation as it does not require a transformer to connect to the power supply, minimizing harmonic distortion, eliminating medium voltage power factor correction, and offering a compact and lightweight footprint. In addition, an ABB DriveMonitorTM was selected for monitoring, controlling and remote diagnostics via a wall-mounted PC connected to the telephone line.
Top row: City of Beloit WPCF and a closer look at the aeration basin. Bottom row: The WPCF aeration blower system and the ACS 2000 drive during commissioning.
Fixed vs. variable speed control
Blower performance was evaluated at annual average conditions for the City of Beloit. The power consumption vs. air flow (SCFM) at these conditions was calculated for the uncontrolled blower characteristic curve, the existing inlet throttling control, and variable speed control. The resulting average improvement in energy consumption on the 600 HP blower motor is shown in the chart below; these results are consistent with typical aeration blower system savings using variable speed drives in comparison with mechanical solutions for motor control.
Energy savings with fast payback
The total plant power decreased by over a million kilowatt-hours once the ACS 2000 was installed. At an average composite electric rate of $0.062/kWh the annual savings of $75,000 per year were significantly better than the original projection of $48,000 per year. As power costs continue to rise the economics of the variable speed drive retrofit are improved further. Projected payback estimate is currently at three years.