Review on technology of making biofuel from food waste

The disposal of food waste has become an environmental issue of great concern with the increasing consumption of materials. The conversion of food waste into biofuels using biotechnology is a very promising approach. Food waste can be converted into fuels such as bio-methane, bio-hydrogen, bio-ethanol, and bio-diesel by different biotechnologies. Food wastes as raw materials for biofuel preparation are mainly classified into protein, fat, starch, sugar, and cellulose. Carbohydrate-rich wastes such as straw, bagasse, grape and apple pomace, and kitchen garbage can produce biofuels, which can be converted into biofuels through anaerobic digestion, aerobic digestion, and microbial fermentation processes. Co-substrates are increasingly used to increase biofuel production and to overcome the shortcomings of a single substrate. The main raw materials for each biofuel are different. Anaerobic digestion technology can be utilized for the production of biomethane, and the digestion efficiency can be changed by the adjustment of the substrate combination, and reactor configuration. The process of methane production by anaerobic digestion mainly includes hydrolysis, acid production, acetic acid production, and methane production. The main raw materials used for methane preparation are livestock manure (pig manure, chicken manure, cow manure, etc.), straw and kitchen waste. Each raw material has a different methane yield and environmental friendliness. Fermentation technology of Saccharomyces cerevisiae can be used to produce bio-ethanol, which consists mainly of raw material pretreatment, enzymatic digestion (saccharification), fermentation, and ethanol production. Cellulosic raw materials (including hemicellulose) are the most promising raw materials for ethanol production on earth, mainly straw, wood chips, peanut shells, and corn. Coupled biomass dark and photo-fermentation technologies can be utilized to produce bio-hydrogen, which includes degradation, dark fermentation, photo-fermentation, and hydrogen production. Food waste containing lipids, proteins, and complex carbohydrates is a good substrate for the dark fermentation stage. The main raw materials for hydrogen production are kitchen wastewater, sugars (glucose), starch (cassava), and cellulose (water hyacinth and rice straw). Hydrothermal liquefaction, hydrothermal gasification, and hydrothermal carbonization technologies can be utilized to produce bio-oil or bio-char. The raw materials for bio-oil/biochar are mainly Edible vegetable oil, nonedible vegetable oil, animal oil, waste edible oil, coffee residue, peanut shell, brewer's grains, grape pomace, sugarcane peel, straw, and wood chips, etc. The industrialization of biofuel technologies is limited by the lack of relevant research and needs to be extensively and thoroughly evaluated. In this paper, the respective advantages and disadvantages of each biofuel technology are clarified and the effects of culture conditions, pretreatment technologies, and reactors on biofuel production are discussed from a technological perspective. In the future, the technology of producing biofuels from various food wastes needs more research in waste conversion rate, product yield, treatment cycle, and cost effectiveness.