Throughout the treatment process, solids have been drawn off and separated from the wastewater. DeKalb Sanitary District is a Class One sludge management facility, monitored by the United States Environmental Protection Agency to ensure that biosolids leaving the plant are safe and are put to appropriate uses.
Biosolids Purification: Primary Anaerobic Digesters
All sludges from each of the sedimentation tanks combine with the raw sludges from the primary clarifiers and are pumped to primary digesters for sludge stabilization and dewatering. There they are heated and undergo an anaerobic-mesophylic digestion process. Pathogens and materials that attract insects are removed. Anaerobic bacteria digest the sludges into methane gas and water.
Biosolids Purification: Secondary Anaerobic Digesters
From the primary anaerobic digesters, the sludge is moved to the secondary anaerobic digesters. There the sludge is stabilized further. Its pH rises, methane is produced and water is removed. The resulting biosolids contain nutrients and organic humus.
Disposal of Digested Biosolids
The Biosolids Rehabilitation Project, completed 2012, rehabilitated four very old digesters which are responsible for anaerobically digesting the solids that remain after the treatment process has cleansed the water. A new digester operations building houses equipment to run these digesters. This project was completed by Williams Brothers Construction, Inc. of Peoria, Illinois who was the lowest of nine bidders at the July 31, 2009 bid opening with a bid of $18,640,000.00. Funds for this project came from a federal stimulus loan (American Recovery & Reinvestment Act of 2009) and an IEPA low interest loan.
Although the anaerobic digestion process reduces the bulk of the solids somewhat, the digesters eventually fill up with biosolids in a watery slurry. In the past, the District has paid a contractor to have this liquid removed by truck and taken to farm fields where it is spread as fertilizer. Sometimes, however, fields were not accessible or roads were posted when the digesters needed to be emptied. Because the digesters cannot be allowed to overflow, contractors would be found to bring in portable de-watering equipment on an emergency basis.
Equipment has been installed with which the District can de-water its own biosolids. Being able to do this on site will be more cost efficient, and will allow better control of the sludge digestion process. After dewatering, the biosolids take up much less volume and resemble loam. They are odorless except for an earthy smell. A building on the District’s property South of I-88 and east of South First Street holds biosolids until they can be hauled away by a contractor to be spread on farm fields.