Using Wastewater Biogas to Generate Combined Heat and Power

The Sheboygan Wastewater Treatment Plant in Wisconsin is nationally recognized in the water and wastewater treatment sector as a pioneer in energy efficiency. The plant has made great improvements to its efficiency by taking advantage of equipment upgrades.

When a boiler upgrade was planned in late 2005, plant staff took the opportunity to explore ways of using the biogas byproducts to fuel a combined heat and power (CHP) system. To build this system, the plant installed ten 30-kilowatt microturbines, along with heat exchangers and anaerobic digesters, which use heat to break down waste and condition the biogas that emerges. The digester gas fuels two firetube boilers and the CHP microturbine system. Exhaust from the microturbines is diverted to the heat exchangers, converting exhaust heat into usable heat for the anaerobic digesters, completing the cycle. This allows the entire CHP system to recover 877,000 Btu per hour for anaerobic digestion.

Overall, the CHP upgrade reduced the plant’s natural gas consumption by 80 percent (according to the Midwestern Governors Association report Sheboygan Wastewater Treatment Plant [PDF]). The microturbines allow the plant to burn biogas from its anaerobic digesters to produce 2,300 megawatt-hours (MWh) of electricity annually (saving about $78,000 in energy costs per year); the electricity produced is then sold to the city. The CHP system produces 84,000 therms of heat, valued at over $60,000 per year at current natural gas rates.

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