A Wisconsin plant goes greener with biological phosphorus removal, higher-efficiency equipment, biogas-to-energy and other upgrades.
To serve a growing population, the Eau Claire Wastewater Treatment Plant needed improvements.
A 2007 facility plan for the plant in northwestern Wisconsin pointed toward adding an activated sludge process with biological phosphorus removal (bio-P) capability to meet increasing demand and satisfy new regulations.
A higher level of treatment normally means using more energy. But Jeff Pippenger, utilities administrator, says Eau Claire has offset that increase by using more biogas and heat recovery; adding high-efficiency motors, blowers, mixers and variable frequency drives; upgrading heating and lighting; and reducing chemical usage in treatment.
The facility plan included a number of changes to improve energy and water efficiency at the 1980s-era 11.5 mgd (design) plant. Serving 75,000 people in Eau Claire and Altoona, the plant has an average flow of 5.1 mgd.
“Our rotating biological contactors weren’t doing a very good job on the biological oxygen demand (BOD),” Pippenger says. “We were seeing our loading coming up because the city has increased in population, and we have more industrial companies coming in. Our secondary treatment process wasn’t keeping up with it.”
The $40 million project began in spring 2013, and most of the improvements were online by spring 2015, immediately improving treatment efficiency.
Two old 240 kW engine/generators were replaced with two 275 kW models (Gen-Tech). The additional generating capacity will save the plant about $300,000 a year at current electricity rates.
The plant’s heating needs are being met partly by heat recovery from the engines (about 20 percent of demand) and by the addition of boilers for digester heating. “We added two 5,000 MMBtu/hr biogas boilers from Hurst to help heat the sludge before it enters the digesters,” says Pippenger. “So we’re using the gas produced in the digesters to heat the digesters.” The plant has three primary digesters and one secondary unit.
Put online in early 2014, the boilers burned 9 million cubic feet of biogas in the first 22 months, saving the plant about $40,000 over natural gas fuel.
Other equipment upgrades included the replacement of 86 motors 1/4 hp to 350 hp with high-efficiency models from Marathon, Baldor-Reliance and Toshiba. Forty Eaton VFDs were installed throughout the facility. To help further reduce energy demand, the plant cut back on areas that are heated in winter and added LED lights, zone lighting controls and motion sensors in offices and equipment rooms.
To reduce the amount of potable water used, the wastewater treatment plant is reclaiming about 740 gpm of effluent and using it for purposes including:
- Cleaning and washing a variety of equipment, including the gravity belt thickener, process tanks, washer/compactor screens, grit classifier, lime slurry preparation tanks, scum troughs and sludge thickener hopper
- Digester foam suppression in the biogas handling vessel
- Elutriation for thickening primary solids in the gravity thickener
- Odor control in the engineered media system in the headworks
- General site cleaning and landscape irrigation
To stay below its 1 mg/L permit limit on phosphorus, the facility used to chemically treat its effluent. That is no longer necessary. “It used to be our highest chemical cost,” says Pippenger.
“We spent hundreds of thousands of dollars a year on ferric chloride to make sure we maintained our limit.”
Fewer chemicals does mean more energy expended, but the biological process was designed for efficiency. It is automatically controlled through oxygen, ammonia and pH probes and inline orthophosphate analysis. “Phosphorus removal happens in the selector zone of each aeration basin,” says Pippenger. “That’s an anoxic zone, so we have low-energy mixers. The six aeration tanks have high-efficiency blowers.”
One result of the new treatment system is more solids, since more waste material is being captured. The plant’s biosolids are spread on area farms. “Last year we land-applied 7.75 million gallons of biosolids in the spring and fall,” says Pippenger. About a dozen farms receive material at no charge, accounting for 3,000 acres.
The biosolids are injected with a plant-owned TerraGator vehicle (AGCO). “We’re fortunate that we have quite a few farmers close by,” Pippenger says. “We don’t have to drive an hour to get to the field. They’re typically within 10 minutes.”
In 2016, Pippenger is busy upgrading drinking water facilities as the city continues to improve its infrastructure. “We have water and sewer mains that date back to the 1880s,” he says. “Those are the things that keep you up at night.”