Effects of thermo-chemical pretreatment on biogas production from sweet potato waste

Abstract

Sweet potato (SP) is a tuber crop which is grown in most parts of the world; the crop generates lots of waste throughout its supply chain. Currently the waste has little commercial utilization; the unutilized sweet potato root waste (SPW) is highly perishable and release methane as they decompose posing a serious problem of environmental pollution. Biogas potential of sweet potato has been tested as a mono substrate as well as co-substrate. Other studies have also reported that sweet potato has a complex molecular structure which is resistant to enzymatic digestion; however no studies have reported any pretreatment mechanism to alter the complex structure to ease anaerobic degradation. Consequently, the main objective of this research was to evaluate energy recovery through anaerobic digestion of thermochemically pretreated SPW. The following specific objectives were investigated: physicochemical characterization of SPW; thermochemical pretreatment of sweet potato waste; and determination of pretreatment factors combination which produces optimum biogas. The quality of biogas produced in terms of methane content as well as the resultant digestate as a biofertilizer were also analyzed. SPW was milled and then subjected to thermo-chemical pretreatment where sodium hydroxide of concentration (0.6g/L-3.5g/L), temperature (50 - 90 ) and pre-treatment time (30-120minutes) were varied. The experimental setup was based on central composite design with all the three factors at five levels and biogas yield as a response. The pretreated SPW was anaerobically digested under mesophilic condition over an incubation period of 22 days. The results from the study revealed that, thermochemical pretreatment on SPW improved both biogas yield and methane. The optimum conditions for biogas production were obtained at: NaOH concentration 2.9g/L, heating temperature 82 and treatment time 102 minutes. The pretreated SPW had superior results than the untreated SPW; the untreated SPW cumulatively yielded 28.23 ml/gSPW of biogas, while the thermochemically treated SPW at optimal conditions produced 37.80ml/gSPW, therefore pretreatment improved biogas yield by 33.88%. The untreated SPW produced 42% methane while the thermochemically treated SPW produced 64%, hence pretreatment improved Methane by 22%. SPW in its natural form had a pH value which ranged from 4.8 to 5.0, this was lower than the optimum pH for anaerobic digestion, and hence neutralization step was necessary prior to anaerobic digestion. Carbon nitrogen (C/N) ratio of SPW of 40.86% was obtained in the study was which was higher than the recommended C/N ratio for anaerobic digestion, therefore the use of thermochemically treated SPW as a co- substrate with nitrogen rich substrate could help balance the nutrients. From the analysis of digestate it was observed that, the digestate from the untreated SPW had more macro nutrients (NPK) than the thermochemically treated SPW. It was therefore concluded that, thermochemical pretreatment of SPW improves both biogas yield and methane along with reduction in digester retention time though the quality of digestate as a fertilizer was degraded. Therefore other pretreatment methods on SPW should be tried to obtain a suitable pretreatment method to improve biogas yield without compromising with the quality of fertilizer.