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UBC Theses and Dissertations

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UBC Theses and Dissertations

Assessment of microbial cell viability in municipal sludge after microwave and ultrasound and subsequent impacts on anaerobic digestion Cella, Monica Angela

Abstract

Anaerobic digestion (AD) is an effective method of waste sludge stabilization with lower costs and energy footprints; recently, researchers have begun to optimize the enhancement of AD by pretreating sludge prior to AD. In this study, two sludge pretreatment methods, ultrasonication (US) and microwave irradiation (MW) were compared to determine the relative amounts of microbial inactivation within pretreated secondary sludge, and how this cellular destruction translates to AD processes. The intensities chosen for MWpretreatment were based on previous studies using sludge from the wastewater treatment plant in Kelowna, BC, Canada. The intensity range chosen for MW possessed specific energy inputs of 2.17, 2.62, 4.89, and 6.48 kJ/g total solids (TS). For comparison purposes, US intensities were calculated to be 2.37, 4.74, 6.73, 23.09, and 27.71 kJ/g TS. Using a novel approach, the extent of cellular destruction caused at these intensities was measured using microbial viability fluorophore assays: 1) a molecular assay to measure live and dead cells and 2) a fluorescein diacetate assay to measure relative metabolic activity of sludge micro-organisms. From the results of the viability assays, it was determined that MW had the greatest effect on cells, having several times greater cell death and inactivation than both US-pretreated and non-pretreated sludge, even at lower specific energy values. Additionally, a MW and US intensity with similar specific energy inputs (2.62 kJ/gTS and 2.37 kJ/gTS, respectively) were applied to feed sludge of bench-scale digesters to compare effects on AD over three consecutive sludge retention times (SRTs), at 20 d, 14 d, and 7 d. The MW-fed digester had the highest overall methane production (248.2 L CH₄/kg volatile solids), greatest pathogen removal (73.4% and 69.8% less fecal coliforms over control during the 14 d and 7 d SRTs, respectively), and greatest solids removal (44.2% TS reduction). Interestingly, fecal coliform concentrations in the digester fed USpretreated sludge increased over the control for both the 14 d (31%) and 7 d (39.6%) SRT. All digesters possessed positive net energy production over the three chosen SRTs.

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Attribution-NonCommercial-NoDerivs 2.5 Canada