Enhancement of biogas production from waste activated sludge after microwave pretreatment

Elagroudy, S.1, El-Gohary, F.2, Ghobrial, F.1 and Warith, M.3, 1Ain Shams University, Egypt, 2Water Pollution Research Department, National Research Centre, Egypt, 3Ryerson University, Canada



The impact of MW irradiation of WAS on biogas production has been investigated. For this purpose, WAS from a nearby activated sludge treatment plant and domestic MW equipment were used. The effect of varying the MW temp (T), power intensity (P), contact time (t), and sludge concentration (C) on sludge hydrolysis and biogas production have been studied. Available data indicated that MW irradiation destroys sludge flocs. This can be identified by the the increase of the soluble COD values in the microwaved WAS samples. The experimental results revealed 1.25±0.1 and 2.71±0.2 fold higher CODS/CODT values at 0.64% TS and 1.2% TS sludge concentrations at a pretreatment temperature of 94 0C, respectively. Soluble COD was always higher at 50% than at 100% MW power intensity for both sludge concentrations at the same pretreatment temperatures. This can be attributed to longer exposure time at low MW intensity.

WAS microwaved at a temperature of 950C and power intensity of 100% produced the greatest improvement in cumulative biogas production with 116% increase as compared to the control samples after digestion for 43 days and at sludge concentration of 1.2% TSS (w/w). Sludge samples microwaved at 50% power intensity and temperature of 520C, were hydrolyzed more rapidly than other samples and therefore concluded higher . Anaerobic digestion was slower as MW power intensity and temperature increased. The values of CODS of digester effluent were the lowest at the highest MW power intensity (100%), for both MW temperatures (T = 49±3, 95±1 0C).

Key words

Anaerobic sludge digestion; Biochemical methane potential test; Microwave irradiation; pretreatment; Hydrolysis coefficient


The minimization of sludge volume produced by wastewater treatment plants (WWTPs) has become increasingly important because of the rising energy and operating costs as well as issues related to sludge disposal on land.

Mesophilic AD of waste activated sludge (WAS) is an economic and environmentally friendly  approach for sludge volume reduction and at the same time produces a biogas which is considered a renewable source of energy (OTV 1997). Moreover, according to literature (Suh and Roussaux 2002), anaerobic digestion combined with agricultural land application is the most environmentally friendly process thanks to few emissions and low energy consumption.

Anaerobic digestion process of particulate materials is achieved through four major stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. For WAS degradation, the rate-limiting step is the hydrolysis (Tiehm et al. 2001). This is because; to use particulates as a substrate, bacteria release extracellular enzymes that break down and solubilize organic particulate matter (Madigan et al. 2005). To improve digestion efficiency, the most logical approach is to disrupt the microbial cells in the sludge. The disintegration of sludge has therefore been introduced to solubilize and convert slowly biodegradable, particulate organic materials to readily biodegradable low-molecular- weight compounds.

Disintegration of sludge may be performed through several pretreatment techniques: mechanical, thermal, chemical or biological. The aim of these treatments is to solubilize and/or to reduce the   concentration of refractory organic compounds, make them more amenable to biodegradation (Weemaes et al. 2000). Final quantity of residual sludge and time of digestion can thus be reduced and biogas production can be increased (Goel et al. 2003). The use of microwave (MW) irradiations in WAS pretreatment has been investigated during the last 15 years. In the electromagnetic spectrum, MW irradiation occurs at wavelengths of 1 cm – 1 m at corresponding frequencies between 30 GHz and 300 MHz respectively (Vollmer 2004).  The application of MW for environmental engineering projects is based on the property of the MW, i.e. rapid and selective heating.

Ratios of CODS to CODT of WAS increased from 0.08 to 0.18 after microwaving samples at 70οC (Hong 2002). Park et al. (2004) investigated the effect of MW irradiation on municipal wastewater sludge for anaerobic digestion. They found that CODS/CODT values were 0.19 and 0.21 when secondary sludge was subjected to MW heating at 91 and 100oC, respectively. Eskicioglu et al. 2007 used thickened WAS (TWAS) sludge heated by MW up to 96 0C in a batch anaerobic digestion test. Results showed that microwaved TWAS, had 3.6 and 3.2 fold increase in CODS/CODT ratio at concentrations of 1.4 and 3% TS, respectively, with similar improvement in VS destruction compared to controls. It was found that a 17% increase in biogas volume over untreated sludge. Park e al. 2004 reported that MW treated sludge could produce 79% higher methane production than untreated sludge. Park et al. (2009) quantified the effect of microwave pretreatment on the degree of solubilization of waste-activated sludge. He found that the power, temperature, and total solid concentration significantly affected the degree of sludge solubilization.

The present study will address the effect of microwave irradiation on the physico-chemical characteristics of sludge in order to determine the optimum MW operational conditions. MW operational parameters tested include contact time (t), Temperature (T), intensity of MW irradiation (I), and solids content of the sludge (C). The impact of MW irradiation on biogas volume produced after anaerobic digestion has been investigated at different temperatures and MW intensities.

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