October 2010, Vol. 22, No.10

Problem Solvers

Two steps to meeting lower phosphorus limits

Problem: Stricter phosphorus discharge requirements.
Solution: Installing a high-rate clarification process and filtration system.

Just 6 years after a major upgrade to its wastewater treatment plant (WWTP), the City of Mankato, Minn., needed to look toward additional improvements. Tighter limits on allowed phosphorus discharges meant the city needed to take action.

It began with a mandate from the state to clean up the Minnesota River. The plan for the cleanup will require cities discharging into the river meet a 1-mg/L total phosphorus limit by 2015. With the total phosphorus discharge from Mankato’s WWTP averaging approximately 1.6 mg/L, it was clear that a change was needed.

But the city also had another motive: preserving local groundwater and surface water supplies. By treating the WWTP effluent to higher quality, the city could provide reclaimed water to the Calpine Corp. (Houston) Mankato Energy Center, which sits approximately 2.4 km (1.5 mi) from the WWTP, and eliminate the need for the energy center to tap local water supplies.

Originally, the plans for the Mankato Energy Center — which were drawn in 2004 and construction fast-tracked — focused on drawing water from the Minnesota River for the cooling towers. But the city decided that to conserve water supplies and save cost, it would install a water reclamation facility (WRF) at the WWTP to provide the power plant with reclaimed water. Utilizing reclaimed water would save on costs for the additional resources needed to take in more groundwater and surface water and treat it to the necessary standards.

 

Finding a solution

Mankato reviewed potential conventional biological and chemical phosphorus removal technologies, as well as high-rate clarification and cloth-media filter systems, as potential solutions to help the WRF meet the new phosphorus limits. The city estimated the capital cost to implement most of these standard technologies would be approximately $30 million. But the city found that combining two systems from Kruger Inc. (Cary, N.C.) offered a more cost-effective solution. By choosing the ACTIFLO® high-rate clarification process, followed by Hydrotech Discfilters, the city saved $10 million in capital costs. The system also offered a small footprint, which helped fit the processes into the existing facility. In addition, such factors as performance guarantees, schedule demands for deliveries, and startup services provided by Kruger fit Mankato’s needs for this project.

Installation

At the WRF, the high-rate clarification process was installed as the first stage of treatment. It utilizes the combination of coagulation, flocculation, and sedimentation for clarification. Microsand provides surface area for floc formation and acts as ballast. Lamella settlers enable the ballasted microsand floc to settle quickly. The settled material is collected and pumped to hydrocyclones, where the microsand is collected and recycled to the system, while the solids are diverted for treatment and disposal, said Mark Stewart, product manager at Kruger.

Water leaving the high-rate clarifiers flows by gravity to the disc filters for polishing to remove suspended solids and phosphorus. The influent enters the filter discs from the center drum. Solids are separated from the water by the filter media mounted on the two sides of the discs, which are partially submerged. The solids are retained within the filter discs while the clean water flows through into the collection tank. This arrangement prevents accumulation of solids in the filter tank and allows operation and maintenance activities in a clean environment, Stewart said.

The filtered effluent provides a suitable source of backwash water, eliminating the need for a separate source or an additional clean-water collection tank for backwashing. During normal operation, the discs remain static until the inlet water rises to a setpoint. Then, the automatic backwash cycle begins. Clean effluent is pumped to the backwash spray header and nozzles, washing solids into the collection trough as the discs rotate. The removed solids flow by gravity out of the unit.

Each disc filter comes as a complete unit, with the drum drive gear-motor and backwash pump mounted on the unit. This, coupled with a straightforward control strategy, is designed to make the entire system easy to install and reduce costs by eliminating additional backwash pumps and automated control valves. The system is designed to be robust and require only minimal maintenance and to have easy access to filter components, Stewart said.

Since installation, the facility has produced effluent with total phosphorus concentrations well below regulatory requirements. Concentrations have averaged between 0.3 and 0.4 mg/L, Stewart said.

In addition, the effluent meets stringent Title 22 reuse quality standards and can supply the energy plant’s cooling tower. Because of this, it is estimated that Mankato will save approximately 2.6 million m3/yr (680 million gal/yr) in water and $1.5 million in potable water costs each year, Stewart said. The Mankato Energy Center estimates that it will save approximately $500,000 each year.