title: Conductive heating and microwave hydrolysis under identical heating profiles for advanced anaerobic digestion of municipal sludge.
abstract: Microwave (2.45 GHz, 1200 W) and conventional heating (custom pressure vessel) pretreatments were applied to dewatered municipal waste sludge (18% total solids) using identical heating profiles that span a wide range of temperatures (80-160 °C). Fourteen lab-scale semi-continuous digesters were set up to optimize the energy (methane) output and sludge retention time (SRT) requirements of untreated (control) and thermally pretreated anaerobic digesters operated under mesophilic and thermophilic temperatures. Both pretreatment methods indicated that in the pretreatment range of 80-160 °C, temperature was a statistically significant factor (p-value < 0.05) for increasing solubilization of chemical oxygen demand and biopolymers (proteins, sugars, humic acids) of the waste sludge. However, the type of pretreatment method, i.e. microwave versus conventional heating, had no statistically significant effect (p-value >0.05) on sludge solubilization. With the exception of the control digesters at a 5-d SRT, all control and pretreated digesters achieved steady state at all three SRTs, corresponding to volumetric organic loading rates of 1.74-6.96 g chemical oxygen demand/L/d. At an SRT of 5 d, both mesophilic and thermophilic controls stopped producing biogas after 20 d of operation with total volatile fatty acids concentrations exceeding 1818 mg/L at pH <5.64 for mesophilic and 2853 mg/L at pH <7.02 for thermophilic controls, while the pretreated digesters continued producing biogas. Furthermore, relative (to control) organic removal efficiencies dramatically increased as SRT was shortened from 20 to 10 and then 5 d, indicating that the control digesters were challenged as the organic loading rate was increased. Energy analysis showed that, at an elevated temperature of 160 °C, the amount of methane recovered was not enough to compensate for the energy input. Among the digesters with positive net energy productions, control and pretreated digesters at 80 °C were more favorable at an SRT of 10 d.
authors.affiliation: School of Engineering, University of British Columbia, Okanagan Campus, 1137 Alumni Ave., Kelowna, BC V1V 1V7, Canada.
authors.forename: Seyedeh Neda
authors.initials: SN
authors.lastname: Mehdizadeh
authors.forename: Cigdem
authors.initials: C
authors.lastname: Eskicioglu
authors.forename: Jake
authors.initials: J
authors.lastname: Bobowski
authors.forename: Thomas
authors.initials: T
authors.lastname: Johnson
mesh_terms: D000693:Anaerobiosis; D006361:Heating; D006868:Hydrolysis; D008697:Methane; D008872:Microwaves; D012722:Sewage / Q000032:analysis / Q000382:microbiology; D012995:Solubility; D013696:Temperature; D013997:Time Factors; D014865:Waste Disposal, Fluid / Q000379:methods*; D014874:Water Pollutants, Chemical / Q000737:chemistry
chemical_list: D012722:Sewage; D014874:Water Pollutants, Chemical; D008697:Methane
keywords: Anaerobic digestion; Biosolids pretreatment; Conductive heating; Microwave; Thermal hydrolysis