The Beaver Dam (Wisconsin) Wastewater Treatment Plant is an extended aeration facility. Its activated sludge process allows the wastewater to spend 24 hours in aeration under average flows.

It takes a lot of air to operate that kind of plant, so saving energy on blowers has a big impact, says Rob Minnema, director of utilities for Beaver Dam, population 16,000, in southeastern Wisconsin. So, when an opportunity arose to upgrade the plant’s blowers at a subsidized price, he jumped on it.

Minnema was faced with overhauling or replacing the 150 hp blowers installed in 1985 at the plant (5.5 mgd design, 4 mgd average). Overhauls would cost about $20,000 apiece for four blowers. The staff had already tried to save electricity by installing a variable-frequency drive on one of the blowers, but it was not very effective in reducing power draw with reduced speeds, given the blowers’ design.

“They were either wide open or off,” Minnema says. “We were looking at spending $80,000 and would still have the same technology.” A grant of nearly $40,000 for new blowers from Focus on Energy, funded by Wisconsin electric utilities, made the decision a lot easier.

Minnema says, “When I went through Focus on Energy, and I did the cost analysis. It was kind of a no-brainer to go to the other type of blower.”

TWO DO THE WORK OF FOUR

Beaver Dam chose a pair of 200 hp ABS Sulzer HST 20 turbocompressors with magnetic bearing technology. “We replaced four blowers with two,” Minnema says. “They are both on VFDs, and they rarely run at 100% of capacity. We basically get the same amount of air out of one blower at 70% speed as from two of the 150 hp blowers.”

Another advantage was low maintenance: “They are essentially zero-maintenance blowers. There is no oil. We just change the filters; that’s about it.” The savings have been substantial. Since the new blowers were installed in 2018, plant electricity costs have decreased by about $4,200 per month. That puts the payback on the investment at about six years.

Although most of the cost reduction was from the blower replacements, there were also some savings from replacing the air header pipes, as there had been some air loss from the pipes connecting the blowers to the aeration chambers.

HIGH BOD

The reason for the plant’s long aeration time is the presence of several food processors in the city. They include a pizza plant, vegetable processor and cream cheese plant. “Our BOD loading is a little bit higher than most cities that have a commercial, residential and industrial mix,” Minnema says. “We get about 50% of our load from industrial customers.”

The wastewater from the cream cheese plant comes in a dedicated force main and is pretreated in a high-rate anaerobic system before being mixed with the rest of the influent. That pretreatment facility was built during a major upgrade of the Beaver Dam plant in 2011.

“They have an extremely high BOD load,” Minnema says. “We have had an agreement with them since back in 2010. We built the pretreatment facility on our site. We have a high-rate anaerobic system that knocks the strength down before it comes into our regular process.”

CAPTURING BIOGAS

The pretreatment facility produces substantial biogas, which is captured and mixed with the gas from the plant’s three digesters and is burned to produce heat and electricity in two 400-kW generators from Caterpillar Inc., Electric Power Division. “We run one of those generators 24/7,” Minnema says. “We produce enough gas to keep a generator going all the time.”

The solids from the digesters go to a 750,000-gallon secondary storage tank, which doubles as a gas storage facility. From there, the solids go to two belt filter presses for dewatering to about 15% solids before land application on farms.

In 2011, when the cogeneration system was set up, it worked out better to send the electricity to the grid. In return, the treatment plant gets a credit worth about $220,000 a year. That credit is likely to decrease significantly after the current contract with Alliant Energy expires in 2021. Still, Minnema expects to continue selling to the grid since using the electricity at the plant would require a substantial investment in infrastructure. 

NET-ZERO ON THE HORIZON?

In the meantime, Minnema is in discussions with a solar developer for an array of solar panels on the plant property. Through a power-purchase arrangement, the developer would build the solar panels and the city would purchase all the power at a fixed price.

The solar array could produce most of the plant’s power, especially since the new blowers have cut demand substantially. “If we went to solar, we’d be close to being energy independent,” Minnema says. “The solar would provide most of our power. We could even be net positive. We could at least be energy neutral, but to be a little bit positive would be better.”