OPTIMIZATION OF ANAEROBIC DIGESTION OF SOME SELECTED LIGNOCELLULOSIC MATERIALS FOR BIOGAS PRODUCTION THROUGH CHEMICAL PRETREATMENT METHODS

Abstract

Anaerobic digestion (AD) of five lignocellulosic materials (LMs), rice husk (Oryza sativa) (RH), oil fibre (Elaeisguineensis), corn cob (Zea mays), Sandbox leaves (Hura crepitans) and elephant grass (Pennisetumpurpureum) was investigated at two particle sizes namely 250 and 710 microns and at three dilution ratios of 1:4, 1:6, 1:8 for a retention time of twenty-one days. Based on these parameters, thirty samples were generated and labeled accordingly to reflect their particle sizes and dilution ratios employed. The experiments were performed at ambient average temperature (37±2oC). The first stage of the research was to determine the best substrate out of the five lignocellulosic materials based on biogas production, optimum dilution ratio and particle size, technical digestion time (DT90), biodegradability constant (k-1) and volatile solid reduction. RH with particle size of 250microns anaerobically digested at 1:8 substrate-to-water ratio (i.e. RH218) represented optimum conditions and these conditions were used in the subsequent chemical pretreatments of the feedstock. In the second stage of the experiment, effects of acid (HCl), alkaline (NaOH) and organosolv (EtOH) chemical pretreatments on biogas production were investigated. Three concentrations were used in each of the categories namely 1, 3 and 5%. The samples were designated based on the reagents used, concentrations and reaction temperatures, for example, 1H100, 5H100, 3N100, 3E100 and so on. Solid-state pretreatments were carried out at 100 and 120oC for sixty minutes. Solid-state chemical pretreatment had varied effects on chemical compositions, morphologies and biogas production as confirmed by physico-chemical analyses, Fourier Transform Infra-red, X-ray Diffraction analysis and Scanning Electron Microscope. The best digesters from each categories namely, 5H100, 3N100 and 3E100 were compared with the control. 3E100 had the highest biogas production of 5545 ml followed by 3N100 with 4705ml while 5H100 produced 700ml which was lower than the control. The biogas yields ranked in the order of 3E100>3N100>RH218>5H100. The values were 67.32, 60.89, 32.26 and 9.32 ml/gVS respectively. The methane contents were 46, 1.43, 50.27 and 50.68% for RH, 5H100, 3N100 and 3E100. Both alkaline and organosolv pretreatments improved both biogas production and yield in comparison to the control. Acid pretreatment on the other hand neither improved biogas production nor biogas yield. Biogas productions from 3E100 and 3N100 were 121.80 and 89% higher than the control. The viability of the digestates as biofertilizer was evaluated with the conclusion that they could be used as low grade fertilizers. Overall, from economic point of view 3% ethanol pretreated substrate at 100oC was the most preferred in comparison to the control and other pretreated samples.