Volatile fatty acids (VFAs) are key platform chemicals used in a multitude of industries including chemicals, pharmaceuticals, food and agriculture. The current route for VFA production is petrochemical based. VFAs can be biologically produced using organic wastes as substrate, therefore directly contributing to a sustainable economy. This process is commonly known as acidogenic fermentation (AF). This review explores the current research on the development of AF processes optimized for VFA production. Three process steps are considered: feedstock pretreatment, fermentation, and primary product recovery with a focus on in situ recovery. Pretreatment is required for recalcitrant feedstocks, especially lignocellulosic substrates. Different pretreatment techniques for AF application have not been studied in depth. The operational parameters of AF (temperature, pH, hydraulic retention time, substrate concentration, etc.) highly influence microbial activity, VFA yields and product distribution. Optimum conditions are ultimately dependent on substrate composition, however, there is indication that certain operational ranges are beneficial for most feedstocks. VFA recovery and purification are necessary for chemical applications. When recovery is performed in situ, it can help relieve product-induced inhibition and keep alkalinity levels stable enabling further waste degradation. Many techniques have been tested, but none are directly compatible with the fermentation conditions tested. Bio-VFAs have the potential to aid in developing a circular economy, but further development is required. Processes need to be developed with the product market in mind, considering both process integration and systematic process optimization.

Acidogenic fermentation (AF) of wastes provides a biological route to volatile fatty acids (VFA) production as an alternative to fossil hydrocarbons. As VFA are acidic in nature, the pH in AF typically drops, affecting the fermentation performance. The effect of different alkaline substances such as urea, NaOH, or CaCO₃ on AF of organic residual solids (ORS), a food waste-rich fraction of municipal solid waste (MSW), was studied. Enzymes were used as an additive in simultaneous hydrolysis and fermentation (SHF) experiments. Use of urea (66 mg Urea g⁻¹ TS) in SHF resulted in a VFA yield of 0.14 g VFA g⁻¹ VS after 6 days, 177% higher than the control (no urea addition). pH and ammonia concentrations significantly influenced the bacterial population distribution. In SHF using NaOH/CaCO₃, added alkalinity of 11 g CaCO₃ L⁻¹ did not influence VFA yields or pH control with NaOH addition at pH 7 or higher. This study demonstrates that, while increasing pH up to 8.5 using different alkaline substances improves VFA production, higher pH is detrimental to SHF due to enzyme inhibition. Finally, the use of urea as an alkaline source must be considered carefully as the acidic pH naturally reached by AF can inhibit urea hydrolysis.

Due to the climate change and other environmental aspects linked to the use of fossil fuels, renewable energy and clean technologies such as hydrogen/oxygen fuel cells have gained renewed interest in the last decades. However, fuel cell manufacture and management of spent fuel cells could also represent a significant impact on the environment. Several studies to assess the extent of this impact were carried out using life cycle analysis (LCA) [1-6], concluding that the membrane electrode assembly (MEA) is the component that contributes the most to the overall environmental impact, mainly due to the extraction of platinum group metals (PGM), which are the main contributor to the acidification burden of MEA manufacture. These studies also affirm that important reductions in pollution can be achieved if the MEA materials are recycled. The recovery of the PGM can be achieved through thermal, chemical or biological methods [7,8]. All of these methods present benefits and drawbacks. For example, thermal treatments are not an environmentally friendly option when recycling most commonly used MEAs, since the Nafion membrane generates acids in the combustion. On the other hand, although biological recovery of PGM is the most environmentally preferable process, there is lack of studies on the integration of this step with the MEA recycling process. The recovery of PGM or membrane materials from MEAs using chemical methods is well documented. Nonetheless, very few studies seem to propose a method to recover both the catalyst and the membrane at the same time, even though the membrane material highly contributes to the final MEA cost. This work proposes a new recycling method of MEAs from hydrogen-oxygen fuel cells, where all main components like the Nafion membrane, carbon powder and metallic platinum were recovered through chemical methods. [1] M. Pehnt, “Life-cycle analysis of fuel cell system components,” Handb. Fuel Cells – Fundam. Technol. Appl., vol. 4, pp. 1293–1317, 2003. [2] M. Miotti, J. Hofer and C. Bauer, “Integrated environmental and economic assessment of current and future fuel cell vehicles,” Int. J. Life Cycle Assess., pp. 1–17, 2015. [3] A. Simons and C. Bauer, “A life-cycle perspective on automotive fuel cells,” Appl. Energy, vol. 157, pp. 884–896, 2015. [4] S. R. Dhanushkodi, N. Mahinpey, A. Srinivasan and M. Wilson, “Life cycle analysis of fuel cell technology,” J. Environ. Informatics, vol. 11, no. 1, pp. 36–44, 2008. [5] D. A. Notter, K. Kouravelou, T. Karachalios, M. K. Daletou, and N. T. Haberland, “Life cycle assessment of PEM FC applications: electric mobility and μ-CHP,” Energy Environ. Sci., vol. 8, pp. 1969–1985, 2015. [6] D. Garraín and Y. Lechón, “Exploratory environmental impact assessment of the manufacturing and disposal stages of a new PEM fuel cell,” Int. J. Hydrogen Energy, vol. 39, pp. 1769–1774, 2014. [7] H. Dong, J. Zhao, J. Chen, Y. Wu, and B. Li, “Recovery of platinum group metals from spent catalysts: A review,” Int. J. Miner. Process., 2015. [8] M. A. Barakat and M. H. H. Mahmoud, “Recovery of platinum from spent catalyst,” Hydrometallurgy, 2004. Figure 1