(390f) Pretreatment of Primary Sludge Prior to Anaerobic Digestion

Anaerobic digestion, which is the most widely used sludge stabilization process worldwide, utilizes anaerobic bacteria to break down about 40-50% of the organic matter of the sludge to form valuable biogas (methane) and to reduce pathogen content of the sludge prior to disposal. Anaerobic degradation of particulate material and macromolecules is considered to follow a sequence of four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In the case of sewage sludge digestion, the biological hydrolysis has been identified as the rate-limiting step (Eastman and Ferguson, 1981; Shimizu et al., 1993), causing large residence times in the fermenters. Pretreatment of sewage sludge by mechanical, chemical, or thermal disintegration methods can improve the subsequent anaerobic digestion (Chiu et al., 1997; Dohaxnyos et al., 1997; Hiraoka et al., 1984; Mueller et al., 1998). However, there is a lack of information as to how different degrees of sludge disintegration can impact the sludge parameters and subsequent changes in the digestion process.

Ultrasonication, an advanced oxidation process is a well-known method for the break-up of microbial cells to extract intracellular material (Harrison, 1991). When the ultrasound wave (>20 kHz) propagates in a medium such as sludge, it generates a repeating pattern of compressions and rarefactions in the medium. The rarefactions are regions of low pressure (excessively large negative pressure) in which liquid or slurry is torn apart. Micro-bubbles or cavitation bubbles are formed in the rarefaction regions. As the wave fronts propagate, micro-bubbles oscillate under the influence of positive pressure, thereby growing to an unstable size before they violently collapse. The collapsing of the bubbles often results in localized temperatures up to 5000 K and pressures up to 180 MPa (Suslick, 1990; Flint and Suslick, 1991). The sudden and violent collapse of huge numbers of micro-bubbles generates powerful hydro-mechanical shear forces in the bulk liquid surrounding the bubbles (Kuttruff, 1991). The collapsing bubbles disrupt adjacent bacterial cells by extreme shear forces, rupturing the cell wall and membranes. The localized high temperature and pressure could also assist in sludge disintegration. At high temperatures, lipids in the cytoplasmic membrane are decomposed, resulting in ruptures within the cell membrane, through which intracellular materials are released in to the aqueous phase (Wang et al., 2005).

The objective of this study was to determine the effect of ultrasonication on various parameters of the sludge and examine the effect of different degrees of sludge disintegration on the anaerobic sludge stabilization process, mainly the extent of gas production.