Biohydrogen from Fruit and Vegetable Industry Wastes

Abstract

Hydrogen gas produced during the microbial fermentation of organic materials is termed biohydrogen. It possesses higher energy in comparison to fossil fuels which makes it a potential source of biofuel or energy, and hence has been the topic of study recently. Biohydrogen can be obtained from fruit and vegetable waste by the process of dark fermentation which occurs in the absence of oxygen and light. Almost 50% of the food wasted in the world consists of fruit and vegetable residue obtained from household or kitchen waste, restaurants, wholesale and retail markets, municipal solid waste, fruit juice vendors, fruit processing industries, etc. It has proved to be an ideal substrate for biohydrogen production by dark fermentation on account of its high carbohydrate content, easy availability, sustainability, low cost, biodegradability, moisture content, high volatile solids, presence of micronutrients, hydrolyzable organic components, etc. Besides, the fruit and vegetable waste dumping in landfills or incineration causes odor, toxic gas emission, water pollution, and other environmental concerns, and its use for biohydrogen production can reduce the issue of costly solid waste management. Fruit and vegetable wastes acting as substrates are basically lignocellulosic that prevents the activity of microbial enzymes upon them. Different pretreatments including grinding, acid/alkali treatment, heat treatment, etc. help to hydrolyze the substrate for better biohydrogen yield. Another concern for biohydrogen production is the formation of organic acids like lactic acid, propionic acid, acetic acid, and butyric acid during the fermentation process which can be overcome by co-digestion of fruit and vegetable waste with other substrates that helps to regulate the volatile organic acid concentration, besides the addition of alkali. On the other hand, biohydrogen production is increased on using a mixed inoculum derived from an indigenous consortium, but it contains hydrogen-consuming bacteria like methanogens and lactic acid bacteria, which reduces the yield. This problem is solved by using a heat-shock treatment on the inoculum before being placed into the reactor. Hydrogen-producing bacteria can form spores and survive the harsh environment, while the hydrogen consumers are killed in the process. Biohydrogen yield is dependent on a number of factors including pH, temperature, fermentation time, inoculum concentration, pretreatment methods, percolation frequency, substrate composition, and trace metal elements. Further research to improve the biohydrogen yield and economic viability of the production process and the application in the industrial scale will ensure superior results.