Effect of Sludge Water Distribution on the Liquid–Solid Separation of a Biological Sludge

Abstract

Water distribution in sludge is worthy of investigation due to its importance in the liquid–solid separation of sludge. Biological sludge contains different physical states of water, including free water, interstitial water, vicinal water, and water of hydration, associated with sludge solids. Conventional dewatering processes can remove only the free water and a part of the interstitial water. Release of interstitial water held inside the cell structure requires the disruption of sludge cells. This study investigates the effect of water distribution on the liquid–solid separation of a biological sludge. Sludge cells were disrupted by alkali treatment, NaCl treatment, heat treatment, and sonication. Water distribution in sludge was determined by differential scanning calorimetry (DSC) and centrifugation. The results of this study indicated that the sludge cell disruption changed the water distribution in sludge and improved its compactibility (as a measure for liquid–solid separation). It released a considerable amount of interstitial water trapped inside the cells. Cell disruption created extra surfaces for water binding and increased the unfreezable water content (mostly vicinal water, water of hydration, and a fraction of interstitial water). Compactibility (liquid–solid separation property) of the sludge in terms of cake solids content was improved to about 2–7.5 folds depending on the disruption method used.