Replacing anoxic zone mixers with much more energy-efficient units is saving more than $22,000 per year for a water pollution control facility in Connecticut.

In pursuing its goal to achieve net zero carbon, the town of Fairfield (population 60,000), wanted to reduce the operating costs of mixers that ran around the clock. The change was part of a capital improvement plan to replace aging assets with modern and more energy-efficient equipment.

“Like all treatment plants, we have been a very big electricity user,” says John Bodie, superintendent of the Fairfield Water Pollution Control Facility. “But when we reviewed our energy usage and developed a full plan, it presented us with an excellent opportunity to adopt green design standards.”

Help available

Numerous treatment facilities are discovering, to their considerable benefit, that help for capital improvements is at hand with grants and incentives from energy providers. In Fairfield, the help came from United Illuminating, a subsidiary of Avangrid, which owns and operates a portfolio of renewable energy generation facilities.

Bodie, who has worked at the Fairfield plant for 30 years, observes: “When we plugged in the numbers, it all added up: smart energy improvements that would give us the greatest return at the lowest cost, as well as big savings for many years to come.

“The help we had toward capital costs from United Illuminating gave us every incentive to go ahead. Our 12 existing mixers were never the most efficient, and we also found them very difficult to work on, mainly because of how heavy they were.”

The mixers at the 8.5 mgd Fairfield facility continuously suspend solids and mix the return activated sludge into the influent stream. The energy input from the old mixers met industry standard recommendations, but the single-point submersible propeller mixers left some dead zones in the denitrification process. They also contributed to scum and foam formation and required high maintenance and frequent rebuilds. 

 Shedding weight

“After looking at several options, we invested in six mixers made by Landia that came highly recommended as the best solution,” notes Bodie. “At 2.4 hp each, compared to the previous 6.5 hp units, we expected a reduction in energy usage. The bonus is that despite being smaller, the new mixers do a better job than the old ones.

“The first six mixers have been installed for just over three years with no issues at all. Previously, when servicing a 6.5 hp mixer weighing in at around 800 pounds, we needed three or sometimes four men to lift it out. Even then, it was very easy for someone to suffer a back strain. Now, with the Landia mixers weighing 200 pounds, it’s easy for just one person. That has also made a big saving on maintenance.”

The new mixers are of a type used for applications such as mixing in aeration basins and moving solids from membrane bioreactors. With their slow revolutions and low floc shear, the mixers are designed for the gentle treatment of activated sludge in continuous applications where sedimentation in anoxic (or anaerobic) tanks must be prevented.

The design of the propeller allows low-speed flow creation with the lowest possible power consumption. Adjustable propeller blades enable energy consumption to be fine-tuned by simply changing the blade angles blades to achieve the desired velocity.

Based on the success of the first six new mixers, six more were installed in 2020, replacing units that also used too much energy and were time-consuming to maintain.

More savings in store

The mixers’ annual electricity cost reduction of more than $22,000 equates to a reduction in energy usage of 182,383 kWh. “The WPCF was constructed in 1950, with upgrades made in 1968, 1972 and 2001,” Bodie says.

“It has always been very well run, but now the capital improvement plan from four years ago can show other treatment facilities what substantial savings can be made in energy. We are pleased with the new mixers, which provide many cost and operational benefits.”

Long-term energy efficiency improvements at the plant also include a 2 MW solar energy system at a former landfill nearby, and a 400 kW fuel cell. A new 1.3 MW natural gas generator was also installed, with interconnecting switchgear. That allows the plant and surrounding town facilities to operate independently in the event of power outages.