West Wastewater Treatment Plant Expansion and CSO Storage Basin

As part of Evansville’s Integrated Overflow Control Plan, the Evansville Water and Sewer Utility (EWSU) retained Greeley and Hansen to design the required increase to the West Wastewater Treatment Plant’s peak flow treatment capacity from its permit capacity of 30.6 million gallons per day (MGD) to 45 MGD, and addition of 6.1 MG to upstream storage to reduce combined sewer overflow (CSO) discharges to the Ohio River.  The project was also designed to meet a new effluent Total Phosphorus limit of 1 mg/l.  

 The project has been recognized throughout the industry for a cost-effective solution that utilized a dual benefit screening structure, improved operations of existing facilities, utilized a new technology for phosphorus removal in the biological aerated filters and re-purposed the existing main process building to a state-of-the-art operations building. 

While the CSO Long Term Control Plan initially located the 6.1 MGD storage basin in the collection system, the project team analyzed locating the basin at the plant to provide additional benefits.  These benefits included improved access to plant staff, no disturbance to existing public space, reduced stress on the collection system and a shared screening structure.  The shared screening structure not only reduced the number of structures being constructed, but also limited the number of locations for solids removal, leading to capital and future operations and maintenance savings of approximately $20M.   

Additionally, Greeley and Hansen reviewed two alternatives to increase the secondary capacity of the plant.  These included increasing the capacity of the existing conventional activated sludge system and expansion of the existing secondary BAF system.  The existing conventional activated sludge system was already operating outside of normal design standards during wet weather events leading to higher than desired effluent solids levels. A biological aerated filter is a non-traditional secondary treatment system that completed both steps of a conventional activated sludge system (biological and settling) in one facility.  While more popular in Europe, there are only a handful installed in the United States.  They have a small footprint compared to the conventional activated sludge system.  The design team determined the best approach was to double the capacity of the existing BAF system to provide the required wet weather capacity increase, reduce the load to the existing conventional secondary system to improve operations and provide capacity for industrial growth.

EWSU desired to meet a new phosphorus removal limit through chemical addition and settling, but the BAF treatment technology had never utilized chemical addition for phosphorus removal. Chemical trials were completed and a full-scale pilot test of two different chemicals was successfully completed to evaluate the impact. EWSU has the first BAF with chemical phosphorus removal in the United States. 

Project Highlights: 

• Recognized with a National Merit Award by American Council of Engineers and an Honor Award by ACEC Indiana 
• Planned reduction of CSOs by 150 MG in a typical year 
• Locating the CSO basin at the plant saved project construction and operation and maintenance costs. 
• Increasing the capacity of the existing BAF to meet the new wet weather capacity, but also provided benefits such as improving the operations of the final clarifiers and providing dry weather capacity for industrial growth. 
• Retrofitting the existing chlorine contact tank to reduce the capital cost associated with the new UV disinfection system.  Conversion to UV disinfection eliminated dangerous gaseous chlorine cylinders on site.