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Abstract
The dry anaerobic digestion process is an innovative waste-recycling method to treat high-solid-content bio-wastes. This can be done without dilution with water by microbial consortia in an oxygen-free environment to recover potential renewable energy and nutrient-rich fertilizer for sustainable solid waste management. It generally takes place at solid concentrations higher than 10% and enables a higher volumetric organic loading rate, minimal material handling, lower energy requirements for heating, limited environmental consequences and energetically effective performance. The long retention time, poor startup performance, incomplete mixing and the accumulation of volatile fatty acids (VFAs) are considered as the main disadvantages for the solid-state fermentation process. In order to develop feasible dry anaerobic digestion processes, it is important to review the optimization techniques and suggest possible areas where improvements could be made. These include reactor configuration, mixing, solid retention time, feedstocks, organic loading rate, inoculation, co-digestion, pretreatment, percolation, additives and environmental conditions within the digester such as temperature, pH, buffering capacity and VFAs concentration. Key words: Solid organic wastes, dry anaerobic digestion process, biogas, optimization.
Highlights
Waste generation is a natural consequence of human life, and is increasing along with population growth, urbanization and industrialization
The results indicate that digested sludge was the best inoculum source and, with an inoculum of 25%, 44% chemical oxygen demand (COD) removal and 43% volatile solid (VS) removal was achieved in the digester over 60 days operating period
The results indicate that the reactor treating the source sorted organic waste and the reactor treating the mixed organic wastes generated some 200 and 60 m3 of biogas per ton of treated waste, respectively while the specific methane production was some 0.40 and 0.13 m3 CH4/kgTVS, respectively
Summary
Waste generation is a natural consequence of human life, and is increasing along with population growth, urbanization and industrialization. Chaudhary (2008) has reported that the dry continuous anaerobic digestion reactor stabilizing source-sorted OFMSW showed stable performance with highest biogas yield (278.4 LCH4/kgVS) and VS reduction of around 59.21% during loading rate 2.5 kg VS/m3.d in thermophilic condition among the three different OLRs of 2.5, 3.3 and 3.9 VS/m3.d for constant retention time of 25 days This happened due to more accumulation of VFAs and decrease in pH during the loading rates 3.3 and 3.9 VS/m3.d. Anaerobic digestion can take place at psychrophilic temperatures below 20°C, but most reactors operate at either mesophilic or thermophilic temperatures with optima at 35 and 55°C respectively (Bhattacharya and Mishra, 2003; Chynoweth et al, 2000; Liu et al, 2006) because the biomass activities and anaerobic treatment capacities have been significantly reduced at lower temperature. Their additional cost must always be balanced against resultant improvements in efficiency
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