Dark fermentation for biohydrogen production involves the breakdown of organic substrates by anaerobic bacteria, yielding hydrogen gas along with byproducts like volatile fatty acids and alcohols. Water plays a critical role in this process, influencing feedstock preparation, microbial activity, and waste management. The water requirements are shaped by feedstock characteristics, process conditions, and the need for efficient effluent handling. Understanding these factors is essential for optimizing water use and ensuring sustainable operations.

Feedstock dilution is a primary water-consuming step in dark fermentation. Many organic substrates, such as agricultural residues or food waste, have high solid content and require significant dilution to achieve optimal consistency for microbial digestion. The typical solid-to-liquid ratio ranges between 5% and 15% total solids, depending on the feedstock type. For example, lignocellulosic materials like corn stover or wheat straw may need more water to reduce viscosity and enhance hydrolysis, whereas liquid-rich waste streams like food processing effluents may require less dilution. Excessive water use can dilute nutrient concentrations, reducing hydrogen yields, while insufficient water may hinder mixing and microbial access to substrates. Balancing these factors is crucial for maintaining process efficiency.

Process water is another major component, serving as a medium for microbial growth and biochemical reactions. The water must meet specific quality standards, as impurities like heavy metals or chlorinated compounds can inhibit bacterial activity. Additionally, pH adjustment often requires water for buffer solutions, typically maintained between 5.5 and 7.0 to favor hydrogen-producing bacteria. Temperature control also demands water, especially in thermophilic systems operating at 50–70°C, where cooling or heating water loops are necessary. The volume of process water depends on reactor design; continuous stirred-tank reactors (CSTRs) generally use more water than batch systems due to constant inflow and outflow requirements.

Effluent management poses significant water challenges, as dark fermentation generates liquid waste containing residual organics, acids, and microbial biomass. This wastewater has high chemical oxygen demand (COD) and may require treatment before discharge or reuse. Anaerobic digestion can further process the effluent to produce methane, reducing its organic load while recovering energy. Alternatively, the effluent may be treated aerobically or through membrane filtration for water recycling. Reusing treated water for feedstock dilution or process needs can significantly reduce freshwater consumption, though care must be taken to avoid accumulation of inhibitory compounds.

Substrate choice directly impacts water use. Agricultural wastes like sugarcane bagasse or rice husks often require pretreatment (e.g., acid or enzymatic hydrolysis) to break down complex polymers, which adds to water demand. In contrast, easily degradable substrates like fruit peels or dairy waste may need less pretreatment but could have higher moisture content, altering the water balance. The carbon-to-nitrogen (C/N) ratio of the feedstock also influences water requirements, as imbalances may necessitate dilution or supplemental nutrients. For instance, nitrogen-rich substrates like poultry litter may require additional water to avoid ammonia toxicity, while carbon-dominant materials like paper waste might need nitrogen supplements dissolved in water.

Opportunities for wastewater reuse are abundant but require careful consideration. Treated effluent can be recycled into the fermentation process, reducing freshwater intake. However, dissolved metabolites like acetic or butyric acid may accumulate over multiple cycles, potentially inhibiting hydrogen production. Periodic monitoring and effluent polishing steps, such as activated carbon adsorption or oxidation, can mitigate this risk. Another approach is integrating dark fermentation with other bioprocesses, such as algal cultivation, where the effluent serves as a nutrient source. This not only conserves water but also creates value-added products like biofuels or fertilizers.

Water consumption in dark fermentation is also influenced by operational parameters. Hydraulic retention time (HRT) affects both water use and hydrogen output; shorter HRTs may increase water turnover but reduce retention of soluble metabolites. Similarly, organic loading rate (OLR) determines the substrate-to-water ratio, with higher OLRs potentially lowering water needs but risking process instability. System configuration plays a role too—two-stage systems separating acidogenesis from hydrogen production may use more water but achieve better control over conditions.

In summary, dark fermentation for biohydrogen production demands careful water management across feedstock preparation, processing, and waste handling. Substrate selection, pretreatment methods, and reactor design all influence water requirements, while effluent reuse offers avenues for conservation. By optimizing these factors, the process can achieve both high hydrogen yields and sustainable water use, aligning with broader goals of resource efficiency in renewable energy systems. Future advancements in water recycling and integrated biorefining could further enhance the viability of dark fermentation as a green hydrogen source.