Two-Pronged Solution

A combination of technologies helps a Minnesota city comply with tight phosphorus limits and produce reuse-quality water
Two-Pronged Solution

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Wastewater treatment in the City of Mankato, Minn., means more than sending clean effluent back into the local watercourse. It means treating that effluent to reuse standards, mainly so that a nearby electric power plant can use it to cool the turbines.

At the same time, the wastewater treatment plant must meet new water-quality requirements that limit cities along the Minnesota River to 1 mg/l of total phosphorus in effluent by 2015 to help prevent algae blooms and related pollution problems.

Two technologies from Kruger Inc. (a part of Veolia Water Solutions & Technologies) helped the city achieve those aims. A compact clarification system provides phosphorus removal for the plant’s current and future needs, while a disc filter system provides solids removal to meet reuse requirements.

Wastewater facility effluent typically contains less than 0.4 mg/l of total phosphorus, and water from the city’s Water Reclamation Facility meets California Title 22 Standards for Water Reuse.

The project has received a 2006 Governor’s MnGREAT! Award (Minnesota Government Reaching Environmental Achievements Together), and a 2007 Project of the Year Award from the Minnesota chapter of the American Public Works Association, in the category for projects costing more than $10 million.

 

Significant savings

The original Mankato treatment plant was built in the mid 1950s. A major expansion in 1974 added secondary treatment. Three equalization basins were added in 1985, and an upgrade in 2000 added capacity along with phosphorus and ammonia reduction. The Water Reclamation Facility was built in 2006.

The wastewater facility (11.25 mgd design, 22.0 mgd monthly maximum) is owned and operated by Mankato and also serves North Mankato, Eagle Lake, South Bend Township, Skyline Village, Lake Washington District, and the City of Madison Lake. Its effluent flows to the Water Reclamation Facility, whose effluent is pumped underground to serve as cooling tower water for the Mankato Energy Center, a privately owned electric power plant 1.5 miles away.

Wastewater treatment plant superintendent James Bruender reports that the energy center receives water as needed in line with demand for electricity. Monthly average flows range from lows of about 1 mgd to nearly 3 mgd during the peak winter heating and summer air conditioning seasons.

The cooling process evaporates 75 percent of the treated water, and the remainder returns to the wastewater treatment plant, where it is mixed with effluent and discharged to the Minnesota River.

Without the reclamation system, the city would have had to supply water from surface and groundwater sources to accommodate cooling for the energy center. The process saves the city an estimated 680 million gallons of water and $1.5 million in potable water costs per year. Besides preserving its natural water supply and saving money, the system turned a waste into a resource. The water reuse project was the first of its kind in Minnesota.

 

Technologies at work

The city, in partnership with Calpine Corporation, retained the Black & Veatch consulting firm to design the treatment solution. The two-stage treatment process combines the ACTIFLO and Hydrotech Discfilter processes from Kruger.

Wastewater plant effluent first enters the ACTIFLO process for phosphorus removal. This compact clarification system combines coagulation, flocculation, and sedimentation, using microsand as seed for floc formation. The microsand provides surface area that enhances flocculation and acts as a ballast to speed settling.

The water then flows to the Hydrotech Discfilter, which provides additional filtration to meet reuse requirements focused on solids and turbidity reduction. Designed for fine solids removal, the system contains woven cloth filter elements installed on multiple discs and using an inside-out flow pattern.

Water flows by gravity into the filter segments from a center drum. The media mounted on both sides of the partially submerged discs separates the solids out. The filtered water flows through the disc media into the collection tank. Once solids have accumulated on the inside of the media, the discs are cleaned by an automatic counter-current backwash system.

“Our turbidity, averages about 0.8 NTU after the filters,” says

Bruender. “Our permit requires 2.0 NTU, and we typically don’t go over 1.0 NTU.”

 

Easy operation

Both technologies are highly automated and operate through the facilities’ SCADA system. “Maintenance of the ACTIFLO process consists of cleaning the lamellar tubes once a week,” notes plant operator Jason Westphal. “It takes about 15 minutes to drain the system down and hose off the tubes inside and out.”

About once a month, the tertiary filter is cleaned with Super Iron Out rust remover to eliminate iron residue that results from addition of ferric chloride upstream for phosphorus removal.

“The Kruger support and operations people have been very helpful during startup and with troubleshooting,” notes Bruender. “Both systems have worked well since they were installed in 2006.”



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