As any wastewater professional knows, odor comes with the territory.

Hydrogen sulfide (H2S) is a natural byproduct of the anaerobic decomposition of organic compounds. Because of its low solubility in sewage, the rotten-egg-smelling gas is released into the atmosphere from manholes, curb inlets, and up building vent stacks. Any wastewater professional knows what happens next: complaints.

In 2011, Public Works began a three-year, $14-million street-enhancement project to revitalize downtown Decatur, a central Illinois city of about 76,000 people. Our goal was to finish by the annual Decatur Celebration in August 2015.

The city council rewrote ordinances to encourage sidewalk cafes. We re-routed streets to expand parking, replaced sidewalks and streetlights, updated landscaping, and installed a fiber optic and festival power system.

In many ways, those were the easiest improvements. The biggest challenge was our Broadway Avenue interceptor.

The 6-foot-tall, 4-foot-wide, egg-shaped combined sewer runs diagonally about four blocks east of downtown. Because the sewer follows a natural drainage swale and the downtown area is on higher ground, the interceptor’s flow line is about 25 feet lower than the downtown sewer system. The difference in elevation, combined with the thermal gradient, caused considerable sewer gas to rise into the downtown area.

Cold winter days were especially pungent because industrial plants that process corn and soybeans discharge 130° F effluent into the interceptor. The warm, smelly air would rise to street level via curb inlets and old building vent stacks with cracks and bad joints.

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