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Illinois wastewater treatment plant saves money & space with high-speed blowers
In 2005, the city of Collinsville, Ill., was in the process of planning an expansion of its activated sludge wastewater treatment plant (WWTP) to meet increased flows. The plant was operating close to design capacity and expected to reach critical status during the next five-year permit cycle. Additionally, the plant equipment was nearing the end of its useful life and a complete mechanical replacement evaluation was necessary.
Based on the evaluation of several years’ worth of plant operating data, a comprehensive collection system flow monitoring program and population forecasts, the approved engineer’s report identified the need to expand the plant’s average daily flow from 4.41 million gal per day (mgd) to 6 mgd. The new maximum daily flow design expanded to 10 mgd.
Prior to expansion, the aeration system was designed to deliver 2,058 standard cu ft per minute. The expansion raised the air requirement to 7,783 standard cu ft per minute. Additionally the receiving stream was classified as impaired for dissolved oxygen (DO), which led to a permit condition for the effluent to be discharged with a DO content of 6 mg/L, mandating a post-aeration unit process be added to the expansion plan.
The combination of expansion and post-aeration established new design criteria for air delivery of 5,280 standard cu ft per minute at the wastewater average daily flow rate and a peak delivery capacity of 8,440 standard cu ft per minute at the maximum design flow.
The Decision to Go Turbo
Prior to expansion, fine bubble diffusers supplied oxygen for the WWTP’s activated sludge process. By replacing the diffusers and adding new laterals, this process remained in place after the expansion. The new full coverage design called for 4,584 membrane diffusers in eight aeration basins and 240 membrane diffusers in a single post-aeration basin.
The aeration system was supplied by three centrifugal blowers located in the basement of the administration building. The new requirement demanded either larger replacement blowers or more than the existing three blowers. Analysis of the expected operating points favored a three-blower arrangement for average, peak and installed spare design criteria. Because existing blower space was limited, installing either larger centrifugal blowers or additional conventional blowers could not be accommodated without moving the blowers to a new location.
Upon investigation of alternative types of blowers (multistage centrifugal, positive displacement and high-speed turbo), the turbo-style blowers garnered attention. High-speed turbo-style blowers have become commercially available for the wastewater aeration market. Key features of these blowers include the use of high-speed (up to 40,000 rpm) impellers; advanced bearing designs (airfoil or magnetic); integrated variable-frequency drive controls; and packaged installation. The high-speed turbo installation typically has a smaller footprint than conventional equipment, requires a less robust foundation, and is delivered prewired with controls and power as an integrated package.
The existing multistage centrifugal blowers produced 3,000 standard cu ft per minute at 153-brake hp. One turbo blower is capable of producing 3,900 standard cu ft per minute at 190-brake hp. This allowed for a design point of present operating load to be serviced by one new blower, the future design load (based on growth projections) to be serviced by two blowers and the full plant operating requirement to be met by a three-blower installation.
The overall project offered Collinsville’s WWTP significant benefits as a result of selecting the turbo-style blowers, including:
The combination of these features resulted in a net cost savings for installation of $300,000.
Although the decision to replace the existing blowers was based on service life and expanded operating requirements, a simple payback calculation was performed for informational purposes. Based on the assumptions of the cost of a single blower change-out, a 6.5% energy reduction at $0.08 per kWh, a project can see payback in less than five years. This result opens the possibility of an economic incentive for certain owners to consider blower upgrades even in situations where higher needs do not exist.
This project demonstrated that, in this climate of enhanced energy efficiency awareness, something as seemingly routine as a blower upgrade project may provide the opportunity for an owner to receive outside support for a major capital construction project and realize construction cost savings and long-term operating cost reduction through selection of a high-speed turbo blower.