**2. Biological Hydrogen Production**

With the ever-rising energy demand, hydrogen can potentially be an alternative efficient and clean fuel replacement of the conventional ones [28]. An innovative approach to solve the issue of waste generation would be to utilize commercial and residential wastewater for producing electricity [29]. Biohydrogen production can come into action via four processes: (a) bio-photolysis, (b) dark fermentation, (c) photo fermentation, and (d) microbial electrolysis, as described below.
