Overcoming yard waste recalcitrance through four different liquid hot water pretreatment techniques – Structural evolution, biogas production and energy balance

Highlights: • Yard waste is a valuable renewable energy resource. • Liquid hot water pretreatment is necessary to reduce the recalcitrance of yard waste for anaerobic digestion. • Comparison among four different type of liquid hot water pretreatment technique was investigated. • Microwave pretreatment was found to be the most efficient liquid hot water pretreatment technique. • A positive energy balance of microwave pretreatment step was confirmed. -- Abstract: Liquid hot water (LHW) pretreatment is considered as a key step in reducing biomass recalcitrance and improving its accessibility to microorganisms during anaerobic digestion. In the present study, a comprehensive comparison among hot air oven, hot water bath, autoclave and microwave (MW) pretreated yard waste (YW) was conducted. The effect of treatment temperature and duration for all the four LHW pretreatment techniques were optimized with respect to the organic matter solubilisation. The results showed that all four LHW pretreatment techniques have a positive effect on organic matter solubilisation. The highest organic matter solubilisation was found in case of MW pretreatment for a treatment duration of 100 s at a temperature of 140 °C. Therefore, batch biochemical methane potential (BMP) of untreated and MW pre-treated YW was carried out for this treatment condition. The BMP of MW pretreated YW resulted in an improvement in methane yield to 364.5 ± 11 mL/g VS on 26th day of anaerobic digestion, which was 1.1 times higher than that of untreated (328.9 ± 15 mL/g VS on 45th day of anaerobic digestion). The FTIR and XRD analysis confirmed that LHW pretreatment technique removed amorphous constituents due to the polymerization of polysaccharides present in YW. In addition, SEM analysis confirmed that the LHW pretreatment led irregular and distorted structure of biomass which improved cellulose accessibility to anaerobic bacteria. A positive energy balance confirmed the possible applicability of MW pretreatment in real life projects.