High temperature microwave pretreatment for enchancement of anaerobic sludge digestion

Abstract

Efficiency of anaerobic digestion (AD) for stabilization of 5 day sludge retention time (SRT) thickened waste activated sludge (TWAS) is limited by hydrolysis of the floc matrix and cell walls of bacteria in the sludge. Heating is an effective pretreatment to enhance the accessibility to intra and/or extra cellular TWAS organics. Microwave (MW) pretreatment was suggested as an alternative method to conventional heating as a result of its lower energy requirements and additional athermal effect from MW irradiation. This study evaluates the effect of high temperature (>100°C) MW irradiation on sludge solubilization and concomitant performance on subsequent mesophilic AD (MAD). Effects of pretreatment conditions (temperature, MW intensity and TWAS concentration) on changes of sludge characteristics as well as AD biodegradability of whole TWAS (soluble + suspended) and the soluble portion only were evaluated. The highest increase in organic solubilization was observed for 175°C with 3.75°C/min MW intensity. sCOD/tCOD ratios were 4.5 +/- 0.8 and 8.8 +/- 0.9 fold greater than untreated TWAS for 6.0 and 11.8% TS concentrations, respectively. Biochemical methane potential (BMP) assays demonstrated acute short term methanogenic inhibition for all MW pretreated samples. However, this was accompanied by a longer exponential phase that enhanced the potential biogas production with increased MW pretreatment temperature once the inhibition was overcome. Additional culture acclimation suppressed the inhibition and for TWAS pretreated to 175°C at 0.83°C/min MW intensity, 31 +/- 6% more biogas vs. the control was produced by the 18th day of the BMP assay. For digestion of soluble portion, pretreatment at 150°C at low MW intensity produced the greatest potential increase in biodegradability resulting in 90% more biogas than that of the control at day 20 of the BMP. Semi-continuous MAD of TWAS with single and dual stage digesters following MW pretreatment at 175°C at two MW intensities were compared to untreated control MAD systems. With MW pretreatment SRT was decreased from 20 to 5 days without significant changes in the effluent COD characteristics. Lower MW intensity resulted in less improvement on COD stabilization and biogas production for the MAD processes. With incorporation of MW pretreatment the single stage digesters performed better than the two stage system in terms of organics stabilization efficiency and biogas production improvement. Dewaterability of digestate improved with MW pretreatment. MAD biodegradability and rates of degradation of soluble organic fractions at different molecular weight (Mw) intervals was assessed. MW intensity was found to influence the soluble Mw profile distributions as well as subsequent MAD performance and pathways assessed by BMP assay. Energy calculations show that MW pretreatment requires approximately 944 MJ/m3 WAS (6% TS). Although MW is an energy intense process, it was found that as long as the excess energy can be used efficiently, MW pretreatment could potentially decrease sludge management costs by up to 58% if treated sludge can be designated Class A which eliminates transportation and landfill tipping costs. At this time, it is recommended that to enhance WAS (5d SRT) solubilization and biogas production a single stage MAD with MW pretreatment at 175°C with 3.75°C/min MW intensity should be used.