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.
Design Criteria
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.
Benefits Realized
The overall project offered Collinsville’s WWTP significant benefits as a result of selecting the turbo-style blowers, including:
- 1. Smaller installed footprint. The overall footprint of the new blowers was accommodated within the existing building space—only the high-speed turbo blowers provided this option. For this project, 1,400 sq ft of building were eliminated.
- 2. Lower energy costs. The expected energy efficiency savings of the high-speed turbo blowers was 6% to 15%. The estimated annual electrical savings is $20,000.
- 3. Construction staging savings. The selection of the high-speed turbo blowers allowed for a significant cost savings from construction sequence. If a new blower facility was to be constructed, a new main feed header would have to be built, the new facility commissioned and a changeover conducted. The installation of the turbo blowers allowed one of the blowers to be installed and connected to the existing header. Only a one-day changeover from the three old blowers to the first new blower was needed. Demolition of the old blowers and installation of the remaining two new blowers occurred in less than a week.
- 4. Higher operation flexibility. The operating characteristics of high-speed turbo blowers include a wide operating range with efficiency maintained through a 50% turndown. This operational flexibility allows dissolved oxygen control strategies that can be optimized for cost reduction.
- 5. Cost recovery through receipt of an energy efficiency grant. The Illinois Department of Commerce and Economic Opportunity Bureau of Energy and Recycling awarded the project a grant of $40,884.48 for projected energy savings.
- 6. Reduced operating noise level. Prior to the high-speed turbo installation, the blower room was posted as hearing protection required. The noise was deafening and conversations could not be heard. After the installation, a normal conversation can be held. The reduction in carry-over noise through the administration building was an unexpected and pleasant benefit.
The combination of these features resulted in a net cost savings for installation of $300,000.
Payback Calculation
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.
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