In 1986, the Sycamore Creek Wastewater Treatment Plant in Symmes Township in southwest Ohio installed a coarse sand filtering system for the plant's secondary bypasses before disinfection. The eight 12- by 34-foot cells with 10 inches of filter media were rated for a hydraulic capacity of 12 mgd.

As part of a later upgrade, the filters were re-piped to receive secondary effluent and help meet a tough phosphorus limit to be imposed by the Ohio EPA. Even with a well-established preventive maintenance program, plant owners at the Metropolitan Sewer District of Greater Cincinnati began to see increased operation and maintenance costs, along with a decline in effluent quality, as the filters reached end of life.

In September 2009, after a complete filter system failure, city officials and plant personnel saw that fixing the old system would not be cost-effective. The plant staff began researching options for a modern tertiary treatment solution to deliver high performance, ease of operation and maintenance, and competitive life-cycle costs.

The district ultimately chose a Forty-X disc filter system from Siemens Water Technologies. The system was easily retrofitted to the existing filter tanks, saving significant installation cost, and it helped the plant meet its strict new permit limits.

Fitting an upgrade

During the sand filters' last three years of service, there were 155 component and subcomponent failures, resulting in maintenance costs averaging $50,000 per year. The plant maintenance supervisor contacted the filter manufacturer and received a bid of $1.6 million to $2 million to rehabilitate the system.

At the time, the plant was completing a capacity upgrade from 12 mgd to 18 mgd, but improving tertiary treatment was not part of it. Many factors drove the tertiary treatment selection process. First, the system had to accommodate the expansion plans already in place. Second, upper management had to make a business case to the city council and the Hamilton County Board of County Commissioners. Third, the units had to be installed by the summer 2011 NPDES permit season.

Exploring options

Two plant operations managers and two maintenance managers visited three area treatment plants, each using a different tertiary process. They collected data and did comparison studies to evaluate the performance and cost-efficiency of each, then presented the results to district management and city and county officials.

The Forty-X disc filter system finally chosen was easily retrofitted to the existing tanks, saving significant installation cost. The purchase included three units rated at 6 mgd each, plus one backup unit, increasing the plant's tertiary capacity from 12 mgd to 18 mgd. With the new system, the plant could treat more flow with fewer units and with minimal construction. All told, the system installation saved about $1 million as opposed to rehabilitating the old filters.

The engineering firm CH2M HILL designed the project, and PAE Construction and Lake Erie Electric managed the retrofit of the existing tanks and the installation of the new units.

Simple installation

The disc filters are third generation chain-driven units with 10-micron pleated media, inside-out flow, and outside-in backwash. Everything needed to operate the filters is compact and easily accessible for maintenance, making the Sycamore Creek retrofit simple.

Unlike the other proposed systems, the disc filters did not require construction of facilities to house pumps or other equipment. Once the plumbing and electricity for the filters were installed, the units themselves were hoisted into place and ready to run in minimal time.

The filters have proven valuable in meeting the plant's stringent effluent requirements. Although not a key component of the nutrient removal process, the filters aid in solids removal during wet-weather events. In addition to complying with the plant's stringent NPDES permit, lowered effluent turbidity levels help save money through lowered ultraviolet disinfection doses and cleaning frequencies.

Meeting expectations

"We were close to the end of the four phases of our plant upgrade when our sand filter system failed," says Barb Browne, East Section treatment supervisor at Sycamore Creek. "Unfortunately, the replacement of this system was not included in the construction plans.

"Our solids output from the filters was higher than the influent to them. We were facing stricter limits in addition to a phosphorous limit that was to take effect in May 2010. We had no tertiary treatment. The Forty-X disc filter system ended up being the best fit for us because we could retrofit the units into the existing cells with minimal costs."

After a year of service, plant personnel say the system has met expectations in performance, ease of operation and maintenance, and price. In both performance testing and daily operation, it has been critical to helping the plant meet its permit limits and produce high-quality effluent.

About the authors

Jason Grooms (jason.grooms@cincinnati-oh.gov) and Rachel Oscherwitz (rachel.oscherwitz@cincinnati-oh.gov) hold positions as Operator II/III with the Metropolitan Sewer District of Greater Cincinnati.