Opportunities for Resource Recovery After Hydrothermal Pretreatment of Biodegradable Municipal Solid Waste: A Mini-review

Abstract

The disposal of biodegradable fraction of municipal solid waste (MSW) is a major problem worldwide. Composting and anaerobic digestion are suggested as potential treatment methods for such a waste. Due to long processing time, odor emissions and susceptibility to toxic metals, these processes have received limited success at majority of places. Therefore, there is enhanced interest in hydrothermal carbonization (HTC) process for the pretreatment of biomass waste. HTC is conducted at 180-260°C in the presence of moisture and auto-generated pressure for few minutes to hours to form a carbon-rich solid mass (also known as hydrochar) and wastewater with high organics concentration. Hydrochar has high heating value (>20 MJ/kg) and can be used as co-fuel. Alternatively, it can be utilized as soil conditioner. The wastewater can be used for biogas recovery, carbohydrate recovery or bioethanol formation via fermentation. Around 90% carbon recovery is possible in solid and liquid fractions after HTC pretreatment. Establishment of decentralized-scale plants for high moisture wastes (>70%) such as, kitchen waste, garden trimmings, sewage sludge, institutional wet waste, and food processing industrial waste can help in recovery of energy from waste at source. However, optimization of the reaction conditions for different wastes and analysis of wastewater characteristics to assess its recovery potential are yet to be performed. In this mini-review, the recent literature on HTC of MSW, possible reactions during HTC and final products are discussed and gaps in the existing information are highlighted.