Biohythane Production from Domestic Wastewater Sludge and Cow Dung Mixture Using Two-Step Anaerobic Fermentation Process

The current study explored bioenergy, particularly biohythane (a combination of biohydrogen (bioH 2 ) and biomethane (bioCH 4 )), production from cow dung and untreated domestic wastewater sludge to valorize the waste into a value-added product. The experimental study consisted of a two-step process: dark fermentation (DF) and anaerobic digestion (AD) with a range of processing conditions varying the temperature and pH (acidic, neutral, and basic). The study maintained thermophilic conditions (55 °C) for bioH 2 production and mesophilic conditions (35 °C) for bioCH 4 production. The highest yields of bioH 2 and bioCH 4 were obtained at a pH of 5.5 (108.04 mL H 2 /g VS) and a pH of 7.5 (768.54 mL CH 4 /g VS), respectively. Microorganisms, such as Lactobacillus brevis and Clostridium butyricum , in the wastewater sludge accelerated the conversion reaction resulting in the highest bioH 2 yield for an acidic environment, while Clostridium and Bacilli enhanced bioCH 4 yield in basic conditions. The maximum cumulative yield of biohythane was obtained under basic pH conditions (pH 7.5) through DF and AD, resulting in 811.12 mL/g VS and a higher volumetric energy density of 3.316 MJ/L as compared to other reaction conditions. The experimental data were modelled using a modified Gompertz’s model at a 95% confidence interval and showed the best-fitting data from experimental and simulation results for biohythane production. The regression coefficient R 2 value was highly significant at 0.995 and 0.992 for bioH 2 and bioCH 4 with the change in pH during biohythane production. Thus, this study presented an effective pathway to utilize untreated domestic wastewater sludge as an inoculum, showcasing the potential of biohythane production and the generation of valuable metabolic end-products across a broad range of pH conditions.