Evaluation of single vs. staged mesophilic anaerobic digestion of kitchen waste with and without microwave pretreatment

• Anaerobic digestion (AD) of kitchen waste (KW) after microwaving was investigated.
• The feasibility of AD of the free liquid separated from liquidized KW was studied.
• MW pretreatment did not enhance the AD of the KW.
• Staging of the AD process had a positive impact on methane yield of KW.
• Pretreatment of KW did not release sufficient organics to the liquid phase for AD.

Effects of single and dual stage (acidogenic–methanogenic) mesophilic anaerobic digestion (AD) of kitchen waste (KW) was evaluated at hydraulic retention times (HRTs) of 20, 15, 12 and 9 d with and without thermal microwave (MW) pretreatment (145 °C). Anaerobic acidification in terms of acid accumulation was superior compared to microaerophilic acidification. Maximum anaerobic acidification of KW was determined to occur with an HRT of 2 d which was then selected for the acidification stage. The dual stage AD system fed with untreated KW produced the maximum biogas and volatile solids (VS) stabilization efficiencies at the shortest HRT of 9 d. Conversely, for free liquid resulting from MW pretreatment of KW the two stage reactor at 20 d HRT produced three fold more methane compared with the untreated free liquid control. However, MW pretreatment and AD of the free liquid fraction only, was not a sustainable treatment option. For KW, staging of the AD process had a greater positive impact on waste stabilization and methane yield compared to single stage reactors or MW pretreatment. KW can be characterized as being a readily biodegradable solid waste; concomitantly it is recommended that digester staging without MW pretreatment be employed to maximize methane yield and production.