The solids (sludge) generated from the various processes are further processed to stabilize them before final disposal. Air-diapragm pumps are used to pump sludge from the primary clarifiers into the sludge blend tank. The sludge blend tank holds primary sludge and waster activated sludge (WAS).
The WAS is pumped from the secondary clarifiers into a holding tank. When a sufficient quantity of WAS has been collected, it is thickened in the sludge blend tank. Rotary drum thickeners use polymer to thicken the WAS from 0.5 percent solids to 4.5 percent; which removes some for the water from the sludge and correspondingly reduces the quantity to be handled.
The excess water is then returned to the plant influent flow to be processed again. The sludge blend tank is mixed with a vertical chopper-typle pump to make a more homogeneous sludge blend. Any scum removed from the primary clarifiers is also mixed in the blend tank. Progressing cavity pumps are used to pump the blended sludge to the existing anaerobic digesters for treatment.
Anaerobic digestion is the process used to stabilize and treat the blended sludge. Stabilization reduces pathogens and also reduces the potential for the stabilized sludge (biosolids) to attract “vectors”.
Vectors are organisms such as rodents or insects that can spread disease by transferring pathogens. The IGLSD digestion process includes two primary digesters with fixed concrete covers and two secondary digesters with floating steel gasholder covers. The primary digesters heat the sludge to 95 degrees F to complete the stabilization process. The primary digesters are mixed with internal draft tube mixers mounted on the covers. A by-product on anaerobic digestion is methane, or digester gas. The digester gas is collected and stored under the secondary digester gas holder covers and used in the boilers to provide heat for the digesters.
The biosolids resulting from stabilization are thickened with an existing gravity belt thickener to increase the solids content to approximately 6%. Once ticketed, solids are pumped to a large storage tank on-site. Biosolids are land-applied as fertilizer on local cropland in the spring before planting and in the fall after harvest.