Volatile Fatty Acids: Production and Applications from Organic Residues

Organic waste is often viewed as a disposal problem, but it holds tremendous potential as a resource for producing valuable chemicals. Volatile Fatty Acids (VFAs) represent one of the most promising value-added products that can be generated from organic residues. These short-chain fatty acids serve as essential building blocks for numerous industrial applications, from biodegradable plastics to sustainable energy production-making them critical components in the transition toward a circular economy.

What are volatile fatty acids? 🔗

Volatile Fatty Acids are short-chain organic fatty acids containing two to six carbon atoms, including acetic acid (C2), propionic acid (C3), butyric acid (C4), and valeric acid (C5). These compounds play a fundamental role in organic carbon cycling and are essential intermediates in the breakdown of complex organic matter. Traditionally, VFAs have been manufactured from fossil fuels through petrochemical processes. However, the rising demand-estimated at 18.5 million tons globally in 2020 with 3% annual growth-has driven interest in more sustainable biological production routes.

The biological production of VFAs offers a compelling alternative to fossil fuel-based synthesis. When organic materials such as food waste, agricultural residues, or sewage sludge undergo anaerobic digestion, VFAs are naturally produced as intermediate metabolites. This approach transforms waste management challenges into opportunities for resource recovery, aligning with circular economy principles.

Key VFAs and their production pathways 🔗

The biological production of VFAs occurs through the early stages of anaerobic digestion, a well-established process that converts organic matter into biogas. However, by modifying operating conditions, this process can be redirected to accumulate VFAs rather than allowing their conversion to methane.

The three-stage production process 🔗

VFA production follows a sequential biochemical pathway:

Hydrolysis marks the first phase where complex organic polymers-carbohydrates, proteins, and lipids-are broken down into simpler molecules. Hydrolytic bacteria secrete enzymes that convert these macromolecules into monosaccharides, amino acids, and long-chain fatty acids that can pass through cell membranes. This step is widely recognized as the rate-limiting phase in the overall process, particularly for complex or recalcitrant materials.

Acidogenesis (fermentation) follows hydrolysis, where acidogenic bacteria convert the dissolved organic monomers into VFAs, primarily acetic, propionic, and butyric acids. During this phase, hydrogen gas is also produced as a co-product. The specific composition of VFAs depends on substrate type, microbial community, and operating conditions.

Acetogenesis involves the conversion of longer-chain VFAs and other intermediates into acetic acid, hydrogen, and carbon dioxide through syntrophic associations between acetogenic bacteria and hydrogen-consuming organisms. In conventional biogas production, these products would be further converted to methane by methanogenic archaea. For VFA production, however, this final methanogenic step must be suppressed.

Major VFAs produced 🔗

The primary VFAs generated through anaerobic processes include acetic acid (the most abundant), propionic acid, butyric acid, iso-butyric acid, valeric acid, iso-valeric acid, and caproic acid (C6). The composition varies significantly based on substrate loading and pH conditions-for instance, higher substrate loadings tend to increase the proportion of longer-chain acids like caproic acid.

Factors influencing VFA yield 🔗

Achieving high VFA yields requires careful optimization of several process parameters. Understanding these factors is essential for scaling up production from laboratory to industrial applications.

pH control 🔗

The pH environment significantly influences both VFA yield and composition. Research demonstrates that slightly acidic conditions around pH 6 favor higher VFA production compared to more acidic values of pH 4-5. Maintaining controlled pH throughout the digestion process is particularly important at high substrate loadings, where rapid acid production can cause pH to drop below optimal levels. The production of acetic acid tends to dominate under acidic conditions, while near-neutral pH promotes longer-chain acid formation.

Inoculum-to-substrate ratio 🔗

The ratio of microbial inoculum to organic substrate affects both VFA yield and methane suppression. At higher substrate loadings (lower inoculum-to-substrate ratios), methane production is naturally suppressed due to VFA accumulation inhibiting methanogenic activity. However, these conditions require pH control to prevent excessive acidification that would inhibit the acidogenic bacteria themselves.

Temperature and retention time 🔗

Mesophilic temperatures (around 35-37°C) are commonly used for VFA production, though thermophilic conditions can accelerate hydrolysis rates. Hydraulic retention time must be sufficient for hydrolysis and acidogenesis while preventing significant methane production. Shorter retention times favor VFA accumulation by limiting methanogenic activity.

Methanogen inhibition strategies 🔗

Several approaches can suppress methanogenic activity to maximize VFA accumulation. Chemical inhibitors such as 2-bromoethanesulfonate (BES) effectively inhibit methanogens at low substrate loadings. The presence of small amounts of oxygen can also reduce methanogenic activity without significantly affecting VFA production. Interestingly, at high substrate loadings, the accumulating VFAs themselves inhibit methanogens, eliminating the need for chemical additives.

Inoculum acclimatization 🔗

Adapting the microbial community toward VFA production rather than methane generation dramatically improves yields. Studies show that inoculum acclimatization can increase VFA production by approximately 35-fold compared to non-adapted cultures. This process enriches specific bacterial phyla such as Firmicutes and Bacteroidetes that excel at hydrolysis and acidogenesis.

Diverse industrial applications 🔗

The versatility of VFAs has generated substantial interest across multiple industrial sectors, positioning them as key platform chemicals for a sustainable bioeconomy.

Bioplastics and polyhydroxyalkanoates 🔗

One of the most promising applications for VFAs is as feedstock for polyhydroxyalkanoates (PHAs)-biodegradable polymers that can replace conventional petroleum-based plastics. PHAs are intracellular polyesters synthesized by microorganisms with renewable, biodegradable, and biocompatible properties, making them suitable for applications in food packaging, biomedicine, agriculture, and tissue engineering.

VFAs derived from organic waste fermentation offer a low-cost carbon source for PHA-producing bacteria. The production process typically involves three steps: first, organic matter is converted to VFAs through controlled anaerobic digestion; second, VFAs are fed to selected bacteria with high PHA production capacity; and third, the bacteria polymerize VFAs into PHA through fermentation. Research demonstrates that bacteria like Cupriavidus necator can produce up to 77% of their cell dry weight as PHA when grown on food waste-derived VFAs.

Biological nutrient removal 🔗

VFAs serve as highly effective carbon sources for biological nutrient removal (BNR) in wastewater treatment. They are used as easily degradable and cost-effective substrates for denitrification and biological phosphorus removal processes at wastewater treatment plants.

For nitrogen removal, VFAs provide electron donors for heterotrophic denitrification, where bacteria use nitrate as a terminal electron acceptor, converting it to nitrogen gas. Food waste-derived VFAs have demonstrated effectiveness comparable to or better than synthetic VFAs and methanol as denitrification carbon sources.

For phosphorus removal, VFAs support enhanced biological phosphorus removal (EBPR) by providing carbon sources that polyphosphate-accumulating organisms store as intracellular PHA during anaerobic phases. Using waste-derived VFAs reduces dependence on expensive synthetic carbon sources while improving process sustainability.

Biogas and hydrogen production 🔗

While VFAs are valuable products themselves, they also serve as intermediates in biogas production. Acetic acid can be directly converted to methane by acetoclastic methanogens, while propionic and butyric acids are first oxidized to acetic acid before methanogenic conversion. Optimizing this conversion pathway can enhance overall biogas yields from organic waste.

Hydrogen co-production during acidogenic fermentation provides another valuable energy carrier. Hydrogen and VFAs are formed as intermediate products during the acidogenesis step, and process conditions can be adjusted to maximize either product depending on market demands.

Microbial lipids and biodiesel 🔗

VFAs can serve as carbon sources for oleaginous microorganisms that accumulate lipids suitable for biodiesel production. Both freshwater microalgae and marine thraustochytrids have demonstrated the ability to produce lipids when cultivated on VFAs generated from food waste. These lipids are rich in fatty acids similar to those in vegetable oils, making them suitable feedstocks for biodiesel conversion. This application provides a pathway for converting low-value organic waste into high-value transportation fuels.

Additional industrial applications 🔗

Beyond these major applications, VFAs find use in numerous industrial sectors. In the food industry, they serve as preservatives, acidity regulators, and flavor compounds due to their antimicrobial properties. The pharmaceutical and chemical industries use VFAs as building blocks for synthesizing various compounds. Textile and leather processing also utilizes these organic acids for various treatment processes.

Sustainable waste management integration 🔗

The production of VFAs from organic residues represents a paradigm shift in waste management philosophy-from viewing organic waste as a disposal problem to recognizing it as a valuable resource. Municipal solid waste, food processing residues, agricultural wastes, and sewage sludge all contain organic matter suitable for VFA production.

Organic-rich wastewater from breweries, dairy operations, aquaculture, slaughterhouses, and food processing facilities holds particular promise as feedstock. Converting these waste streams into VFAs addresses multiple challenges simultaneously: reducing treatment costs, creating economic value from waste, and producing sustainable alternatives to fossil-derived chemicals.

The integration of VFA production into existing wastewater treatment infrastructure offers synergistic benefits. Using VFAs produced from waste sludge for biological nutrient removal creates a closed-loop system that reduces external carbon source requirements while improving overall treatment efficiency.

What do you think? As cities worldwide grapple with increasing organic waste generation and the need for sustainable materials, could VFA production become a cornerstone technology linking waste management, renewable energy, and green chemistry? What barriers do you see to wider adoption of these biorefinery approaches?

References

1. https://www.mdpi.com/2311-5637/9/1/13
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC6802927/
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC7571609/
4. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2025.1693596/full
5. https://vb.nweurope.eu/projects/project-search/wow-wider-business-opportunities-for-raw-materials-from-wastewater/news/volatile-fatty-acids-the-substrate-for-bacteria-to-produce-plastic/
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC11532435/
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC8805862/
8. https://www.sciencedirect.com/topics/chemical-engineering/biological-nutrient-removal
9. https://www.sciencedirect.com/science/article/pii/S0960852422013797
10. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.614612/full
11. https://www.sciencedirect.com/science/article/abs/pii/S0960852422007490
12. https://www.sciencedirect.com/science/article/abs/pii/S0043135423004517