Batch anaerobic digesters were used to stabilize microwave (MW)-irradiated waste activated sludge (WAS) at mesophilic temperature. A low temperature range (50-96°C) MW irradiation was applied by a household type MW oven with 2450 MHz frequency. The effects of four different variables
[MW temperature (T) and intensity (I), sludge concentration (C) and percentage of sludge pretreated (PT)] were investigated in a four-stage Nested or Hierarchical type of statistical design containing 54 mesophilic batch reactors, including controls (no pretreatment) and duplicates
by monitoring cumulative biogas production (CBP). A solubilization experiment on microwaved WAS samples resulted in 3.6±0.6 and 3.2±0.1 fold increases in soluble chemical oxygen demand (SCOD)/total chemical oxygen demand (TCOD) ratios at low and high sludge concentrations, respectively.
Solubilization was always slightly higher at 50% than at 100% MW intensities for both high and low sludge concentrations at the same MW temperatures. WAS, microwaved to 96°C, produced the highest amount of biogas with 13±0.7% and 17±0.2% increases over the controls around
18th d at low and high concentrations. Dewateribility of microwaved sludge was also enhanced after anaerobic digestion. A multi-factor analysis of variance (ANOVA) determined only factors of PT, T, and C as significant at the 95.0% confidence level and therefore factor I could be
eliminated from the experimental design.
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