Performance of Continuous Flow Anaerobic Sludge Digesters after Microwave Pretreatment

Effects of microwave (MW) pretreatment on waste activated sludge (WAS) in mesophilic semicontinuous (SC) anaerobic digesters with acclimatized inoculum at sludge retention times (SRTs) of 5, 10 and 20 d are presented. Preliminary batch digesters were also studied in order determine the optimum MW temperature (T), MW intensity (I), WAS concentration (C) and volume percentage of WAS pretreated (PT) to achieve the highest WAS solubilization [soluble to total chemical oxygen demand (SCOD/TCOD) ratios] and improvements in biogas production. Digesters using conventionally heated (CH) WAS were also run to investigate thermal and athermal effects of MW pretreatment. Preliminary studies indicated that MWirradiation has a potential of damaging activated sludge floc structure and cell membranes and releasing extracellular and intracellular compounds (proteins, sugars and nucleic acid) along with the solubilization of particulate COD. MW irradiated WAS samples resulted in 3.6 ± 0.6 and 3.2 ± 0.1 fold increases in SCOD/TCOD ratios at 1.4 ± 0.02 and 5.4 ± 0.02% total solid (TS, w/w) concentrations, respectively. The MW pretreatment also increased the ultimate bioavailability of sludge components under mesophilic anaerobic digestion with 17 ± 0.0% higher cumulative biogas production (CBP) compared to controls after 34 days of batch digestion. In SC digesters, incremental increases in TS, volatile solids (VS) and TCOD removal efficiency of pretreated digesters compared to controls dramatically increased as SRT was gradually shortened from 20 to 10 to 5 d. WAS pretreated to 96°C by MW and CH achieved 29 and 32% higher TS and 23 and 26% higher VS removal efficiencies compared to controls at SRT of 5 d, while similar reactors at SRT of 20 d had only 16% higher TS and 11 and 12% higher VS removals than those of controls, respectively. Dewaterability of pretreated sludge was also enhanced after batch and SC sludge digestion after MW pretreatment.