Sludge Thickening
Primary and secondary sludge thickening is beneficial to the anaerobic digestion process because it reduces biomass volume tank size and heating requirements. Untreated sludge is a suspension of solids in water. One of the primary goals of sludge treatment is the further separation of solids from water to make treatment and disposal more effective and efficient. Sludge thickening to increase the solids content of the sludge is the first step of the sludge treatment process. The Little Ferry WPCF utilizes four gravity thickeners and three gravity belt thickeners to thicken sludge. The gravity thickening process consists of pumping primary and secondary sludge into a gravity thickening tank where the sludge is mixed and agitated gently by a rotating mechanism. The solids tend to settle to the bottom of the tank where they are pumped into the anaerobic digesters. The thickening tank supernatant is pumped to the headworks of the treatment plant for treatment. Sludge is also screened and degritted prior to distribution in gravity thickening tanks to prevent pipe clogs, premature wear, and accumulation of grit in digester tanks.
The gravity belt thickeners are used primarily to thicken secondary sludge that is wasted from the system. Secondary sludge is more difficult to thicken than primary sludge, especially during warm weather when the density of the secondary sludge is reduced. The gravity belt thickeners add thickening capacity during these critical periods.
Sludge thickening whether by gravity or gravity belt thickeners, relies on the addition of polymer to aid the process. Polymers act as flocculating agents, causing the particles of sludge to adhere and form larger particles, or flocs. The flocs are heavier than the smaller particles and settle more readily, thereby improving the efficiency of the sludge thickening process.
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