Rice waste streams as a promising source of biofuels: feedstocks, biotechnologies and future perspectives

Increased environmental concern over climate change due to higher oil usage has made human being to shift to cleaner and greener alternatives. The utilization of abundant agricultural waste streams as renewable feedstock for biofuels production can be a pivotal strategy. Among others, rice is one of the most largely grown crops, and more than 4.8% of the total production goes to waste. Although previous reviews are related to biofuels obtained from some rice waste, most of those are focused on lignocellulosic rice residues with much attention to thermo-chemical processes. The present paper, instead, reviews for the first time the biotechnological approaches to convert all rice wastes, like rice husk, rice straw, broken rice, discolored rice, unripe rice, into liquid (bioethanol, biobutanol, biodiesel), and gaseous (biogas, biohydrogen) biofuels through the use of pure or mixed microbial cultures. The global availability of each rice byproduct has been also investigated and the potential of rice waste as a ‘fuel farm’ has been estimated for bioethanol. The physical, chemical, enzymatic, or microbial pretreatments, which play a key role in making carbon available for hydrolysis and fermentation, are here discussed and evaluated. Despite the great promise of technologies so far developed, further research is still required for their up-scale and industrial commercialization. Moreover, future process integrations will open the landscape to biorefinery schemes where rice waste streams can be processed into biofuels and other added-value products, towards the full exploitation of the feedstocks and the economic and environmental sustainability of the overall process.