The Pima Country, Arizona, Wastewater Treatment Plant (Figure 1) handles 40 MM gallons a day of raw sewage from the metropolitan Tucson area using an almost self-sufficient power generation system. The plant uses sewer gas produced by anaerobic digestion of solid wastes to fuel six huge Waukesha engines and generators.
[FIGURE 1 OMITTED]
The sludge of removed organics from the wastewater goes through a thickener and into the digester, where it is consumed by anaerobic digestion in about 15 days. The plant's engines are fueled with the byproduct gas produced by bacteria in this digestion stage. The gas is captured, then compressed and chilled to remove most of the moisture from the gas, and to help remove other sedimentation and waste byproducts. It then flows through the plant's main fuel gas system into the engines.
Using recovered gas means not having to purchase sometimes costly and non-renewable natural gas resources. In addition, use of the recovered gas reduces environmental pollution since the gases would otherwise be flared off to the atmosphere.
"This is really a win/win situation because we not only save about US$30,000 a month in reduced natural gas fuel costs but we're also using the sewage effluent for energy rather than blowing it out in the air," said Tom May, Lead Wastewater Treatment Plant Mechanic.
The Waukesha engines used in the Pima plant are "lean burn" dual-fueled (natural gas-digester gas) models, designed to meet EPA emissions standards even with sour gas fuels. The …