Simultaneous production of biofuel from agricultural wastes and bioremediation of the waste substrates: A review

Pollution from fossil fuel usage coupled with its unsustainability is currently instigating a global drive for affordable and eco-friendly alternatives. A feasible replacement seems to be microbial biofuels. However, the production cost is still high, partly due to the cost of substrates and media. Microalgae, yeasts, moulds and bacteria can grow on cheap and easily available agricultural waste substrates to produce bioethanol, biogas, biobutanol, acetone and/or lipids for biodiesel. Oleaginous microbes, such as several species of Chlorella, Schizochytrium, Rhodotorula, Rhodosporidium, Yarrowia, Aspergillus, Rhodococcus, etc., have been applied in the production of high and choice lipids for biodiesel. High bioethanol, butanol and acetone yields have also been achieved with several agricultural waste substrates either through separate hydrolysis and fermentation or simultaneous saccharification and fermentation. The isolation or transformation of some microbial strains has shown the possibility of using only one microorganism to produce bioethanol from lignocellulose biomass in consolidated bioethanol production. High biogas/biomethane yield has also been recorded from different agricultural wastes, especially in co-digestion systems. Pre-treatment, detoxification, application of microbial co-cultures, co-substrate mixing ratios, organic loading rate, hydraulic retention time among others have been reported to affect the quantity and quality of different biofuels produced from agricultural wastes. Interestingly, these agricultural waste substrates were greatly degraded during fermentation and/or digestion to achieve high level of bioremediation. Therefore, this study presents a holistic review of various agricultural wastes adopted as biofuel substrates, the conditions to attain optimum biofuel productions and cases of simultaneous bioremediation of substrates obtained during biofuel production. The valorization of the biofuel by-products into other essential products to achieve a zero-waste and circular bioeconomy is also properly presented.