Progress in applications of advanced oxidation processes for promotion of biohydrogen production by fermentation processes.

Advanced oxidation processes (AOPs) are powerful methods for treating substrates using radicals that are generated in situ. This study reviewed applications of AOPs in enhancement of biohydrogen production. The AOPs are applied in substrate pretreatment because of their ability to break the complex structure of lignocellulosic biomass for ease of subsequent hydrolysis. The mechanism of solubilization of complex organics resulting in increased biodegradability of substrate during pretreatment has been suggested. Documented studies indicate that up to 98% color removal from organic wastewater is possible by the use of AOPs. Furthermore, a combination of AOPs with biological processes can achieve more than 90% COD removal from biohydrogen production effluent. Sonication, microwave-enhanced AOPs, and electrochemical treatment are the most applied AOPs in enrichment of biohydrogen with up to fivefold increase in biohydrogen yield achieved after electrochemical pre-treatment. The mechanism of enhancement of hydrogen yield in dark fermentation after pretreatment of the substrate and inoculum with AOPs has been proposed. The excess sludge produced during hydrogen fermentation can be pretreated with ozone and ultrasound before biomethanation process. More studies on co-production of biohydrogen and electricity through electrochemical oxidation in fuel cells are necessary. This study proposes the integration of AOPs with conventional processes in biorefinery production approach with aim of improving biohydrogen yields, co-producing it with other biofuels, and reducing the process costs. Future studies should focus on the scale-up of AOPs for commercial applications. Comparative studies on energy requirements for various AOPs applications are lacking and should be carried out. Graphical abstract [ABSTRACT FROM AUTHOR]