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Gravity Thickening Tanks and Centrifuge

Primary and secondary sludge thickening is beneficial to the anaerobic digestion process because it reduces biomass volume tank size and heating requirements. The BCUA utilizes four 65’ diameter gravity thickeners, and one centrifuge to thicken sludge prior to digestion. The gravity thickening process consists of pumping both 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 or primary effluent channel 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.

One of two thickening centrifuges is used primarily to thicken wasted secondary sludge. Secondary sludge is more difficult to thicken that primary sludge, especially during warm weather when the density of the secondary sludge is reduced. The centrifuges add thickening capacity during these critical periods.

Sludge thickening whether by gravity or centrifugation, relies on the addition of polymer to aid the process. Polymers act as flocculating agents, causing the particles of sludge to stick together 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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