Cassava Waste Pulp Valorization for Biohydrogen Production: A Sustainable Approach to Advancing Circular Economy

In response to the escalating issue of atmospheric emissions from fossil fuels, harnessing biofuel-based renewable energy is considered a highly promising energy option. Biohydrogen, being carbon-free and non-polluting, has gained significant attention. This study investigated the potential of cassava waste pulp (CWP) as a substrate for biohydrogen production through photofermentation, using Rhodopseudomonas palustris as the inoculum. An acid-hydrolyzed substrate with a sugar concentration of 10.2°Brix was produced via 1% hydrochloric acid hydrolysis pretreatment and used in the experiment. The optimum values for the operating variables of hydraulic retention time (HRT) and inoculation volume ratio (IVR) were assessed under controlled conditions, with a constant pH of 8.0, light intensity of 720 lx (120 W), and an ambient room temperature of 25 ± 3 °C. The results showed that the highest biogas yield was achieved with a 60 h HRT and a 30% IVR. The central composite design of the response surface methodology revealed that the model was highly significant, with a p-value of < 0.0001. The result is further supported by its regression coefficient of 0.9605, which indicates a high consistency between the actual and predicted values. Gas chromatography analysis revealed a high biohydrogen gas composition (92.98%), along with a small presence of carbon dioxide (6.01%) and nitrogen (1%). This proves that photofermentation with R. palustris inoculum effectively produces biohydrogen with no traces of carbon monoxide and methane in the gas composition. Approximately 10.2 ± 0.12 L of pure biohydrogen can be optimally produced from one kilogram of CWP at an HRT of 61.86 h and an IVR of 35.25%. These findings highlight the viability of cassava waste pulp as a valuable feedstock for sustainable biohydrogen production, offering a promising solution for reducing reliance on fossil fuels and advancing renewable energy technologies.