The Stevens Point Water Department decommissioned the combined heat and power system at its wastewater treatment plant and redirected the biogas to drying biosolids.

The result was a significant operating cost reduction. The utility, in Central Wisconsin, received the 2021 Renewable Energy, Energy Efficiency, and Resource Recovery Award from the Central States Water Environment Association.

High maintenance and operating costs convinced utility leaders to abandon the combined heat and power system in 2021. The engine had to be rebuilt three times in the 10 years the system operated. Ultimately, it costs more to run the CHP system than to buy electricity.

“It took eight to nine months to get the money back from a rebuild, and that’s assuming nothing else went wrong,” says Chris Lefebvre, wastewater superintendent.

BIOGAS FOR HEAT

The conventional activated sludge treatment plant (4.55 mgd design, 2.1 mgd average) still produces enough biogas to heat five buildings, heat 2.2 million gallons of mixed material in the anaerobic digesters and heat thermal oil to dry the biosolids.

Digester gas production is boosted by mixing in high-strength food waste trucked in from dairy and potato processing plants and received in a 40,000-gallon tank. A pipeline brings in waste from the Stevens Point Brewery, about two-thirds of a mile away.

The three boilers — one Hurst and two Envirex (Evoqua Water Technologies) — can burn the biogas without significant processing. “We have a skid for moisture removal and then slight pressurization, and then it goes straight to the boilers,” Lefebvre says.

SAVINGS ON SOLIDS

The biosolids are first dewatered on a belt press (BDP Industries), which operates with low horsepower, needs small amounts of polymer, and can be maintained by plant staff. “It makes a huge difference when we don’t have to wait on other people to come in and do maintenance,” Lefebvre says.

The belt press yields material at about 19.5% solids. “We could get a little bit more out of it, but it would cost us more polymer, which is expensive,” Lefebvre says. “We make our biogas on site, so we’re able to evaporate the liquid out rather than press it out.”

The dryer (Andritz) uses a thermal oil heater (Vapor Power) to indirectly heat the biosolids with oil at 320 degrees F to evaporate the water. More than 90% of the fuel for the heater comes from biogas, which can be supplemented with natural gas if needed.

The dryer operates at a low speed by choice: “Our dryer is slightly oversized because of how small we are. And we’re not maxed out on anaerobic digestion capacity. So we’re able to run at a slower rate, at maximum turndown, and use as much biogas as we can to dry.

“We’re trying to run as slow as we can. By feeding it slower, we avoid needing a centrifuge or something similar to get thicker solids.”

The result is a Class A product at more than 90% solids. Previously, the plant yielded about 500 truckloads per year of liquid biosolids for land application. Hauling costs were high. Now the dry biosolids are picked up by a farmer at no charge. “Now we’re well under 100 trucks a year, actually probably under 50, at no cost to us. It’s hundreds of thousands of dollars in operational savings,” Lefebvre says.

The dryer typically runs for about 20 days and then is idle for 10 days, during which maintenance is performed.

SUSTAINABILITY CULTURE

The biogas project is just one of several innovations in sustainability. Since the 1990s the water department has operated a geothermal heating and cooling system using treatment plant effluent. In summer, the effluent is usually at 68-72 degrees F; in winter typically 48-52 degrees F.

In 2018, the utility put a 131 kW solar array on the roof of the garage to power one of the geothermal system heat pumps. On sunny days the panels produce more power than the heat pump can use; the excess is sold to the utility grid,” says Joel Lemke, public utilities director. 

The utility is investigating other ways to use the electricity on site instead of putting it back onto the grid at wholesale rates.

The water department has considered energy use and sustainability with every new project at the treatment plant since a 2002 energy audit by Wisconsin Focus on Energy. Projects have included changing to LED lighting and turning off yard lights when no one is on the property.

“We’ve changed numerous setpoints throughout the wastewater plant to use less energy and in particular less on-peak energy,” Lefebvre says. “We’re trying to do things overnight rather than during the day. Every decision is based on sustainability and reuse, if we can do it.”