Application of iron oxide nanoparticles for biogas yield optimization from winery solid waste and sorghum stover

Abstract

Different methods and processes of optimizing the yield of biogas are currently being explored globally for better biomass management and renewable energy security. Winery solid waste is problematic in South Africa due to current disposal method to the environment and the way it is being handled. Similarly, a lot of waste is generated during sorghum harvesting; however, the stover represents a suitable feedstock for anaerobic digestion due to its high carbohydrate and protein content. Anaerobic digestion is one of the renewable energy technologies able to produce biogas from a variety of biomass sources. The addition of iron oxide nanoparticles (ION) has been touted to increase biogas production. Therefore, the aim of this study is to investigate the ability of ION to boost biogas yield via anaerobic digestion process from sorghum stover (SS) and winery solid waste (WSW). Biomethane potential tests were carried out at mesophilic conditions (37°C ± 0.5) in a batch reactor using SS and WSW singly and in combination at 1:1 ratio, in the absence and presence of ION. A 30-day retention time was used for all the tests. Biogas optimization was also carried out. The optimal conditions from three chosen factors viz., solid retention time (SRT), substrate ratio (SS/WSW) and concentration of iron oxide nanoparticles (ION) were investigated for biogas production using response surface methodology (RSM). The effect and relationship between these three factors on the biogas yield were also explored using CCD (central composite design) to determine the anaerobic co-digestion experiment. The upscaling experiment employed the use of optimal values in a 5L batch reactor. The results from the BMP tests for substrates with ION (wION) and without ION (w/oION) showed a cumulative methane yield of 9.5 mLCH4.gVS-1 WSW, 18.5 mLCH4.gVS-1 SS, and 44.6 mLCH4.gVS-1 substrate ratio for w/oION. Similarly, 36.3 mLCH4.gVS-1 WSW, 29.3 mLCH4.gVS-1 SS and 60 mLCH4.gVS-1 WSW+SS were obtained from wION. It was concluded that ION had a significant effect on biogas yield especially with WSW biomass where the increase was tripled. Results from the co-digestion experiment produced more biogas than single digestion. Optimization experiment using optimal conditions of 100 ppm for ION concentration, 80:20 substrate ratio and 25 days SRT produced maximum cumulative biogas yield of 51.9 mLCH4.gVS-1 which is higher than the RSM predicted value of 49.6 mLCH4.gVS-1 by the quadratic model. The RSM model proved successful in the optimization process with a determination coefficient (R2) value of 0.9528. The upscaled experiment using a 5L batch reactor at mesophilic conditions with optimal values resulted in a cumulative biogas production of 522.97 mLCH4.gVS-1 with a methane content of 74%. The results of this study will affect the agro-industry as well as waste management practitioners.