What's Left?
Throughout the treatment process, solids have been drawn off and separated from the wastewater. DeKalb Sanitary District is a Class I 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.
Biosolid Purification: Primary Anaerobic Digesters
All sludges from all 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.
Biosolid 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.
Where Does It Go?
The distribution of valuable biosolids completes the process of returning the resources contained in the wastewater to the environment. The USEPA strictly regulates the land-application process to ensure that the only environmental impact is the beneficial one of providing a natural substitute for chemical fertilizers.
Recycling Resources
In older plant processes the liquid digested sludge which was 6% solid was hauled to agricultural land to be used as a soil conditioner and nitrogen source. Some sludge was dried in sludge drying beds at the plant until it was about 50% solids and resembled light black soil. It too was hauled to agricultural land to be used as a fertilizer.
With higher population and increasing production, the plant has come to use mechanical de-watering methods to speed the natural process, and the dried biosolids are distributed to farmland by a contractor.
Conservation of Resources
Both methane gas and nonpotable water are recycled by the DeKalb Sanitary District to provide an alternative heating source and non-potable water for landscaping and other processes within the plant.