Abstract
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•Dual two-stage anaerobic digester increases anaerobic digestion efficiency.•Microwave-oxidative sludge pre-treatment improve sludge hydrolysis and solubilization.•Two-stage anaerobic digester with hybrid feed sludge pre-treatment system.•Superoxide radicals mediate oxidative stress in anaerobic digestion.•The kinetics of anaerobic digestion was also enhanced.
The impacts of hybrid microwave-oxidative (MW-H2O2) feed sludge pretreatment on performance efficiency of conventional single-stage and a novel two-stage semi-continuous anaerobic digestion of mixed waste activated sludge were studied to enhance biogas production and digestate quality. Untreated two-stage anaerobic digestion (thermophilic followed by mesophilic) achieved 76.4 ml/gTCOD methane yield compared to 40.4 ml/gTCOD achieved through conventional mesophilic anaerobic digestion, with an increase in methane percentage. Application of hybrid (MW-H2O2) sludge pretreatment in the two-stage digestion enhanced initial sludge hydrolysis/solubilisation and consequently achieved 143.4 ml/gTCOD methane yield. Also, the highest methane percentage of 71 % was achieved during peak methanogenesis stage in this process. The synergetic effects of hybrid pretreatment were also confirmed by the higher release of extracellular polymeric substances. Oxidative stress exerted by the pretreatment resulted in the accumulation of superoxide radicals in the initial thermophilic phase; followed by increased sludge activity and biomethanation in the later phase of two-stage digestion. Hybrid feed sludge pretreatment in the two-stage system achieved a 73 % volatile solids reduction and more than 90 % reduction of faecal coliform. The various kinetic model parameters were determined by the application of the modified Gompertz model. These results illustrate that a novel semi-continuous two-stage anaerobic digester with hybrid feed sludge pretreatment improved sludge hydrolysis, sludge solubilisation, biogas production, sludge stabilization and reduces sludge retention time, and also achieves “class-A” biosolids by significant pathogen destruction.