When an energy audit recommended variable-frequency drives for blowers, team members at the New Ulm (Minn.) Wastewater Treatment Plant weren’t so sure that was the best way to reduce energy use.

“I don’t think the energy audit took into account the cost of VFDs for motors that size,” says Dan O’Connor, wastewater treatment supervisor. The plant had three 900 hp blowers, only one of them used at any time. They were located in a basement, making it difficult to install VFDs in the space available.

In addition, O’Connor thought it might be a waste of money to spend $250,000 on blowers more than 30 years old. When they were installed, the plant was designed to treat 6.7 mgd, but the average flow had declined to about 2 mgd — the blowers were feeding more air than necessary for treatment.

“We decided to remove one of the old blowers and replace it with two 300 hp high-efficiency turbo-blowers from K-Turbo [since purchased by Aerzen],” says O’Connor. They came with VFDs and could interface with the plant SCADA system. One of the new blowers can handle most of the plant’s loading, and the second can be activated during times of high demand. The old blowers now serve as backup and are exercised once a year to keep them functional.

Air-handling upgrades

With new blowers going in, the city also decided to upgrade the plant’s air distribution system. “We have four square aeration tanks with a center island,” O’Connor says. “Each had 12 headers installed three feet off the tank floor. We basically had aeration throughout the middle of the tanks, but we didn’t have very good dispersion.”

The aeration tanks now have full floor coverage with aerators installed 18 inches off the floor. The system, from Environmental Dynamics International, has a rubber membrane with 2,500 fine-bubble diffusers. “It gives a complete mix and better transfer of air and we don’t have dead spots in the corners like we did before,” O’Connor says.

One concern is that the tanks have to be drained to replace heads or clean diffusers. “Time will tell how easy that is,” O’Connor says. “We put in anaerobic-anoxic tanks in 2008 to treat our phosphorous, and they’re a bit temperamental. You can’t just drain an 800,000-gallon tank and think it’s not going to affect other parts of the plant.”

The air distribution improvements went into operation in summer 2011, before the blowers were installed in December. That enabled the plant team to measure the improvement. “It increased our dissolved oxygen by about 2 ppm, so that made our plant more efficient,” says O’Connor.

The work also included dissolved oxygen probes (Endress+ Hauser) to control the blowers through the SCADA system. The plant can now maintain oxygen levels to avoid over-aerating the tanks in summer. The tanks remain over-aerated in winter, but the blowers run at low speed, so there are still savings on electricity.

Projections come through

An engineering assessment that estimated annual electrical savings of $100,000 proved accurate: Running the blowers for 11 months in 2012 saved almost $92,000. The savings could have been higher if not for some operational issues shortly after startup.

New Ulm Public Utilities spent $1.3 million on the project: $150,000 for electrical work and connection to the SCADA system, $488,000 for the aeration improvements, and $390,000 for the new blowers (including removal of the old blower). Another $100,000 went to electrical engineering work to match the reduced electrical demand to the service coming from Heartland Consumer Power District.

Better biosolids

Another improvement to the facility was a microbe management program that improved treatment and saved energy. The need for improvement was identified in 2006 when the plant’s biosolids exceeded the limit for nickel and could not be land-applied.

The source was a grease and tallow company that recycled materials for animal feed and was using a hydrogenated oil that contained a nickel compound as a preservative. O’Connor worked with the company to keep the nickel out of the sewer system, and the contaminated biosolids were slowly blended with new material, which then could be land-applied. The process took almost a year.

The incident led to discussion about improving microbe management. “We realized we would rather pump 10,000 gallons a day of 4 percent solids versus 15,000 gallons of 2 percent,” he says. “When we were able to do that effectively, we were able to change the way we treated it in our autothermal thermophilic aerobic digester (ATAD).”

Adding air to the ATAD boosted microbial activity, which increased the temperature. “It wipes out almost all of the pathogens and gives us about 40 to 50 percent volatile solids reduction,” O’Connor says. That allowed the plant to operate two digesters instead of three. “We were able to take 13 motors out of operation, ranging in size from 2 to 12 hp,” O’Connor says. “That saves roughly $1,500 a month in electrical costs.”

Reduced energy costs and a smaller carbon footprint have helped the New Ulm plant protect the environment even beyond its role of treating wastewater.