GRIT HANDLING SYSTEM IMPROVEMENTS: A CASE STUDY AT DECATUR, ILLINOIS

The Sanitary District of Decatur owns and operates a 41-mgd (125 peak mgd) wastewater treatment plant. The service area includes a collection system that is partially combined. The wastewater treatment plant includes a headworks facility with catenary mechanical bar screens and Dorr-Oliver “Detritor” grit removal equipment. The headworks area of the plant has been chronically plagued with reliability problems. The District's goal for this project was to improve overall system reliability.Plugging of the “catenary” screens has been a recurring problem during the fall months, and clogging of the grit pumping systems frequently occurs after significant rainfall events. In addition, the District has experienced maintenance and reliability problems with the headworks equipment. The improvements for this project focused on the grit removal equipment.A detailed listing of the facility improvements implemented as part of this project is as follows:Increase “Detritor” rake tip speed from 19 fpm to 38 fpm.Two new grit washer-classifiers.Four new variable speed grit pumps.A new shaft-less screw conveyor for transporting the clean grit into a roll-off container.All new grit slurry piping was added. The grit piping's interior surfaces were custom-coated with abrasion-resistant Kanigen® coating used to help withstand the deteriorating effects of pumping abrasive grit from the wastewater.A new flow control vane gate on the headworks' influent channels systems, for uniformly directing the influent grit load to the facility's two grit chambers.A new screenings handling unit that grinds, washes, and compacts the screenings.A new masonry grit loading building. The building features included low-maintenance, interior walls that constructed of structural glazed tile that do not require painting.Additions to the Intellution-based SCADA system to merge the new grit and screenings equipment controls into the existing system.The improvements noted above have dramatically reduced system downtime and increased reliability.