| Summary: | The potential of using biogas generated from organic waste as energy source is broadly
recognized and anaerobic digestion has become a major part of modern wastewater treatment
plants (WWTPs). Various pretreatment techniques have been recently developed to increase
the quantity of biogas and reduce digester volume by enhancing the hydrolysis of waste
material fed to the digesters. This study evaluates advanced anaerobic digestion incorporating
thermal pretreatments (microwave at 2.45 GHz and conventional heating) as an alternative
disposal method for the municipal biosolids generated by Kelowna WWTP (BC, Canada)
which are currently composted.
To be able to compare microwave irradiation with conventional heating under identical
conditions (temperature/ heating rates), a custom pressure vessel was built for conventional
heating. Biosolids heated from room temperature up to pretreatment temperatures of 80, 120
and 160oC at heating rate of (7.5oC/min) in the closed vessel microwave unit and the pressure
sealed vessel. Both conventional heating and microwave pretreatments indicated that in a
pretreatment range of 80-160oC, temperature was a statistically significant factor (p<0.05) for
increasing solubilization of chemical oxygen demand (COD) and biopolymers of the
biosolids. Fourteen lab-scale semi-continuous digesters were operated for digestion of the
biosolids to optimize energy (methane) output and sludge retention time (SRT) requirements
of untreated (control) and thermally pretreated digesters.
In general, relative (to control) organic removal efficiencies dramatically increased as SRT
was shortened from 20 to 10 and 5 days, indicating that the control digesters were challenged
as the organic loading was increased. Except the control digesters at the SRT of 5 days, all
control and pretreated digesters achieved steady state at three SRTs, corresponding to
volumetric organic loading rates of 1.74 to 6.96 g COD/L/d. At the SRT of 5 days, the
controls stopped producing biogas after 20 days of operation while the pretreated digesters
continued producing biogas. Energy analysis showed that all digesters had positive net
energy productions except the digesters fed with sludge pretreated at 160oC and operated at
SRT of 20 and 5 days. Digesters operated at 10 days SRT were more favorable, and they
produced net energy of 1.3 - 9.6 (GJ/d/Tonne total solids added).
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