December 2007, Vol. 19, No.12
Taking Peak Flows by Storm
Problem: Screens caused bypassing during periods of high flow, continued maintenance issues.
Solution: Screening system that combines coarse and fine screens in one unit.
The Mill Creek wastewater treatment plant (WWTP) in Cincinnati is the largest plant in the city. It handles a total of 1,360,000 m³/d (360 mgd) in a combined sewer system and typically encounters high flows and large solid loads during peak rain events.
The Mill Creek plant used four coarse, climber-type screens with openings of 3.8 cm and six fine screens with spacings of 12 mm, which originally were installed for pump protection. During periods of high flow, the plant found that screens — which had a 5-min cleaning cycle — would be blinded, then shut down. This resulted in the flow having to be bypassed, which in turn damaged the pumps these screens were intended to protect. Another problem was the excessive amount of maintenance required for all 10 screens.
The frequency of the problems required significant time and manpower. The 18-m-deep (60-ft-deep) wastewater channels at Mill Creek are in a confined space, which adds a level of difficulty to repairs. When the U.S. Environmental Protection Agency reviewed the plant conditions and asked that these issues be resolved, plant authorities decided to search for a replacement screening system.
Headworks Inc. (Houston) was contacted by plant authorities to find a screening system to suit their specific needs. Having visited installations by the company in both the United States and Austria, the consulting firm CH2M Hill (Englewood, Colo.), along with the Cincinnati Metropolitan Sewer District, decided that installing the Headworks Mahr Bar Screen would greatly benefit the Mill Creek facility.
The bar screen is a front-raked, front-return, chain-driven, multiple-rake system that can operate either continuously or intermittently to remove retained material from the screen field. While in operation, rakes travel in a continuous circuit from the bottom and then engage the bar rack to collect, convey, and deposit the retained material into the discharge chute, located at the top of the machine. This simple positive removal ensures that no solids get washed beyond the screen field.
The sewer district required a mechanically reliable bar screen to provide a high degree of protection for its pump and treatment system. With the fine 9.5-mm openings, the chosen bar screen does not require separate fine screens after pumping, which lowers capital and operations and maintenance costs.
In combined sewer overflow applications, such as Mill Creek, the new bar screen helped out during high flows by automatically increasing the cleaning cycle to every 5 seconds. The system also performs a reversing step to automatically clear obstructions caught in the screen field and then resumes the forward movement and continues to clean the screen field in the regular direction of operation.
The first two Headworks units were installed in 2004, followed in 2006 by a third and fourth screen. The 20-m bar screens replaced the climber-type screens. For 4 years, two of the new bar screens were operating in conjunction with the old fine screens. With the installation in 2006 of the final two Mahr screens, all pre-existing coarse and fine screening equipment was removed.
During the hurricane season of 2005, the Mill Creek WWTP screens had to endure a large quantity of floating materials, which swept into the intake of the plant as a result of flooding in the Cincinnati area. During this time, one screen alone handled 757,000 m³/d. The screens cleared out tree limbs and stumps, large stones, and more. The screen continued to protect the plant from bypass and remained in continuous operation.
For more information, contact Bill Beyer, Cincinnati Metropolitan Sewer District operations supervisor, at (513) 352-4923, or Biju George, assistant director of wastewater, at (513) 244-1367.
Donald Cuthbert of CH2M Hill (Englewood, Colo.) can be reached at (513) 489-0779 or via e-mail at firstname.lastname@example.org. For information on Headworks Inc. (Houston), contact Gerald Seidl, senior vice president of sales and marketing, at (713) 647-6667.