ANAEROBIC DIGESTION OF SPINELESS CACTI (Opuntia ficus-indica (L.) Mill) BIOMASS IN TANZANIA: THE EFFECTS OF AEROBIC PRE-TREATMENT

Abstract

One of the best options for African countries to meet rural energy needs is to grow on a massive scale in waste lands, care-free crassulacean acid metabolism plants, which can enable bioenergy production without disrupting food supplies and hence sustainable energy supply for the future. Opuntia ficus-indica (L.) Mill is an ideal plant for arid regimes but has barely been studied as a potential bioenergy source. This study analysed Opuntia’s physicochemical parameters which are important for anaerobic digestion and then investigated the effect of aerobic pretreatment on methane yield of Opuntia ficus indica biomass. This effect was investigated in batch bioreactors by varying time from 0 to 72 h. Reducing sugar content and dissolved oxygen levels after pretreatment and methane forming potential through anaerobic digestion was analysed. Reducing sugar content in bioreactors increased with increase in pretreatment time from 12.22 ±0.69 g/l to 59.08 ± 5.35g/l in the control and 72 h pretreated batches respectively. Methane yields after pretreatment were observed to range from 0.286 m3 CH4/kg VS to 0.702 m3 CH4/kg VS at 9 and 72 h of pre-treatment respectively. A 9 h pre-treatment of feedstock prior to anaerobic digestion yielded 123% higher methane when compared to the control. The findings that there was an increase in reducing sugar production and methane yield at 9 h of aerobic pre-treatment suggests that there was increased hydrolysis with pretreatment and subsequently improved methane yield. Hence short pre-treatment period could be an option to increasing solubilisation of Opuntia ficus indica cladodes and promoting methane productivity. Pre-aeration of Opuntia ficus indica, therefore, was shown to be an effective method for enhancing its digestibility and improved methane yield during anaerobic digestion.