**7. Conclusions**

Biohythane via two-stage anaerobic fermentation using organic waste could be a promising technology for higher energy recovery and a cleaner transport biofuel than the biogas. The H2 /CH4 ratio of range 0.1–0.25 is suggested for biohythane. A flexible and controllable H2 /CH4 ratio afforded by two-stage fermentation is of great importance in making biohythane. Biohythane can be achieved by two-stage anaerobic fermentation; in the first stage, organic wastes is fermented to H2 , CO2 , VFA, lactic acid and alcohols. Effluents from first stage containing VFA, lactic acid, and alcohols are converted to CH<sup>4</sup> in the second stage by methanogens under a neutral pH range of 7–8 and HRT of 10–15 days. The pH of 5–6 and an HRT of 2–3 days are optimized for first stage that flavor acidogenic bacteria to convert organic wastes to H2 . *Clostridium* sp., *Enterobacter* sp., *Caldicellulosiruptor* sp., *Thermotoga* sp., and *Thermoanaerobacterium* sp., are efficient H<sup>2</sup> producers in the first stage. *Methanosarcina* sp. and *Methanoculleus* sp. played an important role in the second stage CH4 production. The combination of biohydrogen and biomethane production from organic wastes via two-stage anaerobic fermentation could yield a gas with a composition like hythane (10–15% of H<sup>2</sup> , 50–55% of CH4 , and 30–40% of CO2 ) called biohythane. Biohythane could be upgraded to biobased hythane by removing CO2 . The two-stage anaerobic fermentation could increase COD degradation efficiency, increase net energy balance, increase CH<sup>4</sup> production rates as well as high yield and purity of the products. In addition, the two-stage process has advantages of improving negative impacts of inhibitive compounds in feedstock, increased reactor stability with better control of the acid production, higher organic loading rates operation, and significantly reducing the fermentation time.
