Population increase and industrialization has resulted in high energy demand and consumptions, and presently, fossil fuels are the major source of staple energy, supplying 80% of the entire consumption. This has contributed immensely to the greenhouse gas emission and leading to global warming, and as a result of this, there is a tremendous urgency to investigate and improve fresh and renewable energy sources worldwide. One of such renewable energy sources is biogas that is generated by anaerobic fermentation that uses different wastes such as agricultural residues, animal manure, and other organic wastes. During anaerobic digestion, hydrolysis of substrates is regarded as the most crucial stage in the process of biogas generation. However, this process is not always efficient because of the domineering stableness of substrates to enzymatic or bacteria assaults, but substrates’ pretreatment before biogas production will enhance biogas production. The principal objective of pretreatments is to ease the accessibility of the enzymes to the lignin, cellulose, and hemicellulose which leads to degradation of the substrates. Hence, the use of pretreatment for catalysis of lignocellulose substrates is beneficial for the production of cost-efficient and eco-friendly process. In this review, we discussed different pretreatment technologies of hydrolysis and their restrictions. The review has shown that different pretreatments have varying effects on lignin, cellulose, and hemicellulose degradation and biogas yield of different substrate and the choice of pretreatment technique will devolve on the intending final products of the process.

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Conventional production of biodiesel employs the use of alkaline catalysts because they are cheaper and less corrosive, and they use minimal energy when compared to other acidic catalysts. Heterogeneous catalysts have also shown significant effects on biodiesel production with its ease of recovery and reusability. Three reaction variables-temperature, time, and molar ratio of alcohol to oil-were optimized for biodiesel production from milk bush oil using snail shell as a catalyst. The catalyst was prepared by calcinating waste giant African land snail in an electric oven for 3.5 hours at 900°C. Also, a control transesterification experiment was carried out using potassium hydroxide (KOH) as a catalyst. Catalyst concentrations of 3.0 wt % of calcined snail shell (CSS) and KOH were used for the transesterification of the oil. A response surface analysis of biodiesel production using CSS as a catalyst showed that all reaction variables were significant. Biodiesel yield of 81% was recorded experimentally as the highest yield when temperature, reaction time, and alcohol-to-oil ratio were 65°C, 2 hours, and 9:1, respectively. An average yield of 94.33% was obtained at these same reaction conditions when KOH was used as a catalyst.

Air pollution is a precursor to many health issues such as difficulty breathing, asthma, lung and heart diseases, and cancer. This study presents a concise view of biodiesel combustion in mitigating pollutant emissions which are generated by the combustion of fossil fuels, thereby eliminating the negative effects on human health and the environment. Gaseous pollutants such as carbon monoxide, unburned hydrocarbons, nitrogen oxides, particulate matter, and carbon dioxide are found to be major exhaust emissions from vehicles running on fossil fuels. Excessive exposure to these pollutants was found to be a precursor to reductions in life expectancy via health complications in humans. Greenhouse gas emissions from the transport sector were found to be 24% of total annual emissions, 74.5% of which came from the combustion of fossil fuel in road vehicles. Biodiesel combustion in vehicular engines is established to be a control technology in reducing gaseous pollutants toward building a sustainable and healthy human–environment scenario. The emissions reduction index from the United States National Biodiesel Board showed that the combustion of biodiesel wholly as a transportation fuel decreased total hydrocarbons, polycyclic aromatic hydrocarbons, carbon, and sulfur emissions by 67%, 80%, 48%, and 100%, respectively. Evaluation of emission results from topical literature strongly suggests that the use of biodiesel is effective in the reduction in pollutants, which is beneficial to human and environmental sustainability.