Biogas production from thermochemically pretreated sweet potato root waste

This paper presents the results of batch anaerobic digestion (AD) of thermochemically pre-treated sweet potato root waste (SPW). This agricultural waste is available in massive quantities yet it has remained an unexploited resource amid the ever-increasing need for clean energy and waste disposal challenges. Therefore, the waste can be considered for energy production through AD. However, SPW has a complex amylopectin structure that is resistant to digestive enzymes during hydrolysis which could lead to a longer retention time in the digester. In this sense, the effect of thermochemical pre-treatment on biogas production from SPW was investigated by pre-treating the substrate with sodium hydroxide (NaOH) at (0.6 g/L–3.5 g/L), temperature (50 °C–90 °C) and pretreatment time (30–120 min). The central composite design was used to design the number of experiments. SPW was milled to a small size. The physicochemical characteristics of materials were determined using standard methods. The quality of biogas produced in terms of methane content was analysed. The results from the study revealed that thermochemical pre-treatment on SPW improved biogas and methane yields. The pre-treated SPW had superior results to the untreated one. It represented a 33.88% improvement from 28.23 mL/gSPW biogas yield for the untreated SPW to 37.8 mL/gSPW for the treated SPW at optimal conditions. The optimum conditions for biogas production were found at a NaOH concentration of 2.9 g/L, a heating temperature of 82 °C, and a pre-treatment time of 102 min. Methane content in the biogas also improved from 42% to 64% (22% increase). The digester retention time was also reduced from 22 to 16 days. It can therefore be concluded that thermochemical pre-treatment of SPW improves both biogas yield and methane content as well as improves the kinetics of AD.