Steam explosion versus hydrothermal carbonization: evaluation of applicability for pretreatment of semi-solid waste from beverage industries to facilitate on-site biogas production

Semi-solid wastes from the beverage industry, like agave bagasse or brewery spent grains, were partly turned to the liquid phase by employing hydrothermal carbonization (HTC) or steam explosion (SE). The methane release in anaerobic batch assays of those extracts reached 316–362 mL g⁻¹ of chemical oxygen demand for the SE pretreatment and 162–173 mL g⁻¹ COD for HTC employed as a pretreatment. However, the yield of chemical oxygen demand in the extracts depends on both the pretreatment as well as the substrate. HTC applied to agave bagasse yielded 0.281 g of COD per gram of biomass, which is more than twice the amount from SE. In contrast, SE is more than 3 times effective than HTC using brewery spent grains as substrate with a yield of 0.582 g COD g⁻¹. The combination of both process conversion efficiencies led to an available energy recovery of up to 0.4 for SE of spent grains and up to 0.1 for HTC of agave bagasse in relation to the energy content of substrate. For the latter, the slow and incomplete bioconversion of substrates obtained by HTC must be overcome. This holistic approach to sustainable management of large quantities of agro-industrial wastes turns an on-site environmental challenge into the availability of in-house renewable energy.