Effect of pre-aeration and inoculum on the start-up of batch thermophilic anaerobic digestion of municipal solid waste

Abstract

In this study, a short pre-aeration step was investigated as pre-treatment for thermophilic anaerobic digestion of the organic fraction of municipal solid waste (OFMSW). It was found that pre-aeration of 48 hours generated enough biological heat to increase the temperature of bulk OFMSW to 60 o C. This was sufficient selfheating of the bulk OFMSW for the start-up of thermophilic anaerobic digestion without the need for an external heat source. Pre-aeration also reduced excess easily degradable organic compounds in OFMSW, which were the common cause of acidification during the start-up of the batch system. Careful consideration however must be taken to avoid over aeration as this consumes substrate, which would otherwise be available to methanogens to produce biogas. To accelerate methane production and volatile solids destruction, the anaerobic digestion in this study was operated as a wet process with the anaerobic liquid recycled through the OFMSW.Appropriate anaerobic liquid inoculum was found to be particularly beneficial. It provided high buffer capacity as well as suitable microbial inoculum. As a result, acidification during start-up was kept to a minimum. With volatile fatty acids (VFAsacetate in particular) and H 2 accumulation typical of hydrolysis and fermentation of the easily degradable substrates during start-up, inoculum with high numbers of hydrogenotrophic methanogens was critical to not only maximise CH 4 production but also reduce H 2 partial pressure in the system to allow VFAs degradation. In a lab scale bioreactor, the combined pre-aeration and wet thermophilic anaerobic digestion was able to stabilise the OFMSW within a period of only 12 days. The stabilised inert residual material can be used as a soil amendment product.3 of 32

Rapid start-up of thermophilic anaerobic digestion with the turf fraction of MSW as inoculum

“…The methane concentration in biogas was analyzed by Varian Star 3400 gas chromatograph (GC) equipped with a thermal conductivity detector as described by Charles et al (2009) …”

Evaluation of Biodegradability of Waste Before and After Aerobic Treatment

“…The main sources of carbon used by methane microorganisms are cellulose and hemicellulose which are classified as materials difficult to degrade under anaerobic conditions. Aerobic processing of these complex organic substances prior to anaerobic decomposition leads to the degradation of these complex organic components to make them easily available for methane-producing microorganisms [2,10,14,19,15]. Research has shown that 5 weeks aerobic pre-treatment of waste by eliminating the acid phase, accelerates the emergence of stable methanogenesis condition in reactors (high rates of W CH4 ), and that leachate recirculation intensifies degradation of pre-treated waste [11,20,16].…”

“…This requires less investment and operating expenditures in comparison to the anaerobic processes [10]. An important advantage of using aerobic biostabilization for waste prior to its disposal is that it intensifies decomposition of the recalcitrant organic fraction of waste into a form easily assimilable for methanogenic microorganisms involved in anaerobic decomposition of waste in a landfill [2,7,14,15,19]. The length of the aerobic pretreatment of waste influences on the degree of biological stability of waste.…”

Enhancement of waste activated sludge anaerobic digestion by a novel chemical free acid/alkaline pretreatment using electrolysis

“…Details of experimental set up and methane analysis have been fully described in Charles et al (2009).…”