Dark fermentation is an anaerobic biological process that converts organic substrates into hydrogen, carbon dioxide, and other byproducts. It has gained attention for its dual role in wastewater treatment and biohydrogen production. Unlike conventional treatment methods, dark fermentation does not require aeration, reducing energy consumption while generating a clean energy carrier. The process leverages fermentative bacteria to break down complex organics in wastewater, making it particularly suitable for high-strength industrial effluents and municipal wastewater with high organic loads.
The mechanism involves hydrolytic bacteria decomposing complex organics into simpler compounds like sugars and amino acids. Acidogenic bacteria then convert these intermediates into volatile fatty acids, alcohols, and hydrogen. The absence of methanogens—either through process control or microbial community structuring—ensures hydrogen accumulation rather than methane formation. Operational parameters such as pH, temperature, hydraulic retention time, and substrate composition significantly influence hydrogen yields and treatment efficiency.
One of the key advantages of dark fermentation in wastewater treatment is its ability to handle diverse feedstocks. Food processing wastewater, dairy effluents, and distillery spent wash are among the most studied substrates due to their high carbohydrate content. For example, a study on cheese whey wastewater reported a hydrogen production rate of 1.8 L/L-day alongside a 65% reduction in chemical oxygen demand (COD). Similarly, palm oil mill effluent treated through dark fermentation achieved a hydrogen yield of 0.9 L/g COD removed, demonstrating the process’s potential for industrial applications.
Case studies highlight the scalability of dark fermentation. A pilot-scale reactor treating sugarcane vinasse in Brazil achieved stable hydrogen production for over 90 days, with an average yield of 2.2 mol H2/mol glucose. The system also reduced the organic load by 70%, meeting discharge standards. In another instance, a brewery wastewater treatment plant in Germany integrated dark fermentation as a pretreatment step, cutting energy demand by 30% compared to conventional aerobic systems while producing hydrogen for on-site use.
Despite its benefits, operational challenges persist. Process instability due to microbial community shifts is a common issue. Hydrogen-producing bacteria are sensitive to environmental fluctuations, and their dominance can be easily disrupted by competing organisms like lactic acid bacteria or methanogens. Maintaining a pH between 5.0 and 6.0 is critical, as deviations can lead to metabolic pathway shifts favoring non-hydrogen-producing routes. Temperature control is equally important, with thermophilic conditions (55–60°C) often yielding higher hydrogen outputs than mesophilic ranges (35–40°C) but requiring more energy input.
Substrate variability also poses difficulties. Wastewater composition fluctuates based on source and seasonal factors, affecting microbial activity and hydrogen consistency. Pretreatment methods such as heat shock, acidification, or ultrasonication can enhance biodegradability but add complexity and cost. Nutrient imbalances—particularly the carbon-to-nitrogen ratio—must be carefully managed to avoid microbial inhibition. For instance, excess nitrogen can promote ammonia formation, which is toxic to hydrogen-producing bacteria at high concentrations.
System design further influences performance. Continuous stirred-tank reactors (CSTRs) are widely used due to their simplicity, but they suffer from biomass washout at short retention times. Fixed-bed and granular sludge reactors offer better biomass retention but face clogging and mass transfer limitations. Membrane bioreactors address these issues by retaining high biomass concentrations but incur higher capital and maintenance expenses.
Economic viability remains a hurdle. While dark fermentation reduces wastewater treatment costs through lower energy demand and sludge production, hydrogen yields are often insufficient to justify standalone implementation. Coupling dark fermentation with secondary processes like anaerobic digestion or microbial electrolysis cells can improve overall energy recovery and economics. For example, a two-stage system treating dairy wastewater first produced hydrogen via dark fermentation, followed by methane generation from residual organics, boosting total energy output by 40%.
Regulatory and safety considerations also impact adoption. Hydrogen is flammable, requiring stringent leak prevention and ventilation measures in treatment facilities. Integrating gas collection and purification systems adds to infrastructure costs. Additionally, effluent from dark fermentation may require post-treatment to meet discharge standards, as it typically contains residual organic acids and solvents.
Research continues to address these challenges. Advances in microbial consortia engineering aim to enhance hydrogen-producing bacteria resilience and substrate utilization. Novel reactor configurations, such as biofilm-based systems, show promise in improving stability and yield. Process optimization through real-time monitoring and control algorithms is another active area, leveraging sensors and automation to maintain optimal conditions.
Dark fermentation represents a sustainable approach to wastewater management, aligning with circular economy principles by recovering energy from waste. While technical and economic barriers remain, ongoing innovations and pilot-scale successes underscore its potential. As industries and municipalities seek low-carbon solutions, integrating hydrogen production with wastewater treatment could become a cornerstone of future resource recovery systems. The synergy between pollution abatement and renewable energy generation positions dark fermentation as a compelling option in the transition toward sustainable water and energy infrastructure.