Journal of Nutrition & Food Sciences
Open Access

Transforming Waste into Energy Using Dark Fermentation of Hydrothermal Carbonization Effluents

Isabelle Dupont^{* *}Correspondence: Isabelle Dupont, Department of Clinical and Public Health Nutrition, Université de Montpellier, Montpellier, France, Email:

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Description

The growing urgency to develop sustainable and circular waste management strategies has brought renewed attention to the valorization of food waste, particularly through thermochemical and biological conversion technologies. Among these, Hydrothermal Carbonization (HTC) has emerged as a potential approach for converting wet biomass, such as food waste, into valuable solid carbonaceous materials (hydrochar). However, HTC also generates a significant fraction of process water an aqueous phase rich in dissolved organic carbon, nutrients and inhibitory compounds which is often overlooked or underutilized. In recent years, researchers have begun to explore the potential of valorizing this HTC process water through Dark Fermentation (DF), an anaerobic biological process capable of producing hydrogen-rich biogas. This coupling of thermochemical and biochemical methods represents a novel integration within the bioeconomy, yet it also brings with it several scientific and practical challenges that deserve closer scrutiny.

The core rationale behind this valorization strategy is rooted in sustainability and resource efficiency. HTC process water typically contains a cocktail of volatile fatty acids, sugars, phenolic compounds and nitrogen-containing species, many of which are suitable substrates for microbial conversion. Rather than disposing of this aqueous stream which may pose environmental hazards due to its chemical oxygen demand channeling it into DF provides a pathway to extract further energy value in the form of biohydrogen, a clean fuel with broad industrial applications. This integration can potentially transform HTC from a stand-alone process into part of a comprehensive biorefinery concept, aligning with principles of circular economy and zero-waste paradigms.

From a microbial and biochemical standpoint, the feasibility of using HTC process water as a feedstock for DF is promising, though complex. Studies have shown that, under optimized conditions, selected microbial consortia can tolerate the moderately acidic pH and inhibitory compounds present in the process water, converting available organics into hydrogen, carbon dioxide and short-chain fatty acids. Pre-treatment strategies such as dilution, pH neutralization, or adsorption of inhibitory phenolics are often necessary to improve the fermentability of the aqueous stream. However, these add additional operational complexity and cost, which may hinder scalability.

It is worth noting that the hydrogen yields from DF using HTC process water are often lower than those obtained from synthetic substrates or easily degradable organic wastes. This limitation is partially due to the recalcitrant nature of some dissolved organics and the presence of toxic intermediates generated during hydrothermal reactions, such as furans and aromatic compounds. Addressing these challenges requires an interdisciplinary approach, involving microbial ecology, process engineering and system integration. For example, enriching strong microbial communities capable of degrading complex aromatic compounds or employing two-stage fermentation systems might significantly enhance overall conversion efficiency.

Beyond the technical feasibility, the economic and environmental implications of this integrated approach must also be evaluated. Valorizing HTC process water through DF adds value to a waste stream that would otherwise require costly treatment, thus improving the overall energy and economic balance of the HTC process. Moreover, the co-production of hydrogen can contribute to the decentralization of energy systems, particularly in urban or industrial settings where food waste is abundant. Lifecycle assessments of such integrated systems, though still limited in number, suggest that combining HTC and DF can reduce greenhouse gas emissions, provided that the system is designed and managed with efficiency in mind.

In conclusion, the valorization of HTC process water through dark fermentation is a compelling example of how we can rethink waste streams as resources rather than liabilities. This approach not only improves the energy recovery from food waste but also exemplifies the synergies possible when thermochemical and biological processes are integrated. However, realizing the full potential of this strategy will require concerted efforts in research, process optimization and policy support. If these challenges can be addressed, this hybrid system could play a meaningful role in sustainable waste management and renewable energy production in the years to come.