Volatile Fatty Acid Production from Organic Waste with the Emphasis on Membrane-Based Recovery

Anhydrous volatile fatty acid extraction through omniphobic membranes by hydrophobic deep eutectic solvents: Mechanistic understanding and future perspective

Volatile fatty acids (VFAs) derived from arrested anaerobic digestion (AD) can be recovered as a valuable commodity for value-added synthesis. However, separating VFAs from digestate with complex constituents and a high-water content is an energy-prohibitive process. This study developed an innovative technology to overcome this barrier by integrating deep eutectic solvents (DESs) with an omniphobic membrane into a membrane contactor for efficient extraction of anhydrous VFAs with low energy consumption. A kinetic model was developed to elucidate the mechanistic differences between this novel omniphobic membrane-enabled DES extraction and the previous hydrophobic membrane-enabled NaOH extraction. Experimental results and mechanistic modeling suggested that VFA extraction by the DES is a reversible adsorption process facilitating subsequent VFA separation via anhydrous distillation. High vapor pressure of shorter-chain VFAs and low Nernst distribution coefficients of longer-chain VFAs contributed to DES-driven extraction, which could enable continuous and in-situ recovery and conversion of VFAs from AD streams.

Integrated Biorefinery for a Next-Generation Methanization Process Focusing on Volatile Fatty Acid Valorization: A Critical Review

This review addresses the critical issue of a rapidly increasing worldwide waste stream and the need for sustainable management. The paper proposes an integrated transformation toward a next-generation methanization process, which leads not only to treating waste but also to converting it into higher value compounds and greener energy. Although the current and commonly used anaerobic digestion process is useful for biogas production, it presents limitations of resource exploitation and some negative environmental impacts. Focusing on the acidogenic stage in waste stream processing, the paper discusses the recent strategies to enhance the recovery of volatile fatty acids (VFAs). These acids serve as precursors for synthesizing a variety of biochemicals and biofuels, offering higher value products than solely energy recovery and soil fertilizers. Additionally, the importance of recycling the fermentation residues back into the biorefinery process is highlighted. This recycling not only generates additional VFAs but also contributes to generating clean energy, thereby enhancing the overall sustainability and efficiency of the waste management system. Moreover, the review discusses the necessity to integrate life cycle assessment (LCA) and techno-economic analysis (TEA) to evaluate the environmental impacts, sustainability, and processing costs of the proposed biorefinery.

Elevated caproic acid production from one-stage anaerobic fermentation of organic waste and its selective recovery by electro-membrane process

A constructed microbial consortia-based strategy to enhance caproic acid production from one-stage mixed-fermentation of glucose was developed, which incubated with acidogens (Clostridium sensu stricto 1, 11 dominated) and chain elongators (including Clostridium sensu stricto 12, Sporanaerobacter, and Caproiciproducens) acclimated from anaerobic sludge. Significant product upgrading toward caproic acid (8.31 g‧L-1) and improved substrate degradation was achieved, which can be greatly attributed to the lactic acid platform. Whereas, a small amount of caproic acid was observed in the control incubating with acidogens, with an average concentration of 2.09 g‧L-1. The strategy accelerated the shape and cooperation of the specific microbial community dominated by Clostridium sensu stricto and Caproiciproducens, which thereby contributed to caproic acid production via the fatty acid biosynthesis pathway. Moreover, the tailored electrodialysis with bipolar membrane enabled progressive up-concentration and acidification, allowing selective separation of caproic acid as an immiscible product with a purity of 82.58 % from the mixture.

Evaluation of volatile fatty acids and ammonia recovery approach from landfill leachate using pilot-scale mechanical vapor recompression

Treatment of landfill leachate is still a current problem due to the high treatment costs in addition to the difficulty of meeting the discharge criteria. However, there is a more important issue that should be underlined; it is also valuable compounds that leachate contains. Conventional methods used for treatment of leachate such as membrane filtration, advanced oxidation processes, biological processes and their combinations have largely focused on treatment. However, the recovery of ammonia and volatile organic acids (VFA) in leachate is a promising approach both to overcome high treatment costs and to sustainably manage leachate. In this study, leachate treatment potential was investigated by mechanical vapor recompression (MVR) process, which offers an operational opportunity to recover high value-added products from leachate while providing an effective treatment for wastewater. Optimum operating conditions for the pilot-scale MVR process have been determined by laboratory-scale studies. VFAs were recovered as organic acid salts from the pilot-scale MVR distillate, while ammonia recovery was accomplished as ammonium sulfate from a highly contaminated concentrate stream. VFA and ammonia recovery rates were 89% and 99%, respectively. The treatment cost of leachate with MVR process was calculated according to the data obtained in pilot scale MVR studies considering the operating cost, chemical cost and economical contribution of value-added products. The results showed that the integrated MVR-crystallization process, all treatment costs are covered, with a net gain of 3.8 USD/m3. Consequently, MVR integrated crystallization process offers an economical and sustainable solution for the treatment of leachate by recovering valuable products.

Two-stage biohydrogen and methane production from sugarcane-based sugar and ethanol industrial wastes: A comprehensive review

The transition to renewable energy sources is crucial to ensure a sustainable future. Although the sugar and ethanol industries benefit from this transition, there are untapped opportunities to utilize the waste generated from the sugar and ethanol process chains through two-stage anaerobic digestion (TSAD). This review comprehensively discusses the utilization of various sugarcane-based industrial wastes by TSAD for sequential biohydrogen and methane production. Factors influencing TSAD process performance, including pH, temperature, hydraulic retention time, volatile fatty acids and alkalinity, nutrient imbalance, microbial population, and inhibitors, were discussed in detail. The potential of TSAD to reduce emissions of greenhouse gases is demonstrated. Recent findings, implications, and promising future research related to TSAD, including the integration of meta-omics approaches, gene manipulation and bioaugmentation, and application of artificial intelligence, are highlighted. The review can serve as important literature for the implementation, improvement, and advancements in TSAD research.

Magnetite enhancing sludge anaerobic fermentation to improve wastewater biological nitrogen removal: Pilot-scale verification

Alkaline anaerobic fermentation for acids production has been considered as an effective method to recover resources from waste activated sludge, and magnetite could improve the quality of fermentation liquid. Here we have constructed a pilot-scale sludge alkaline anaerobic fermentation process enhanced by magnetite to produce short chain fatty acids (SCFAs), and used them as external carbon sources to improve the biological nitrogen removal of municipal sewage. Results showed that the addition of magnetite could significantly increase the production of SCFAs. The average concentration of SCFAs in fermentation liquid reached 3718.6 ± 101.5 mg COD/L and the average concentration of acetic acid reached 2368.8 ± 132.1 mg COD/L. The fermentation liquid enhanced by magnetite were used in the mainstream A²O process, and the TN removal efficiency increased from 48.0% ± 5.4%-62.2% ± 6.6%. The main reason is that the fermentation liquid is conducive to the succession of microbial community in the denitrification process, increasing the abundance of denitrification functional bacteria and realizing the enhancement of denitrification process. Besides, magnetite can promote the activity of enzyme to enhance biological nitrogen removal. Finally, the economic analysis showed that magnetite enhancing sludge anaerobic fermentation was economically and technically feasible to promote biological nitrogen removal of municipal sewage.

Reactive extraction of lactic and acetic acids from leached bed reactor leachate and process optimization by response surface methodology

The present work focused on extracting lactic and acetic acids from the leachate collected from leached bed reactor (LBR) during acidogenesis of food waste using the reactive extraction (RE) process. A wide range of diluents was screened either alone by physical extraction (PE) or in combination with extractants using RE to extract acids from the VFA mix. Aliquat 336-Butyl acetate/MIBK extractants in RE demonstrated higher distribution coefficients (k) and extraction yield (E %) than PE. The response surface methodology (RSM) was used to optimize the extraction of lactic and acetic acids from the synthetic acid mix, using three variables (extractant concentrations, solute/acid concentration and time). Consequently, these three variables were optimized for LBR leachate. The RE was promising, and extraction efficiencies of 65% (lactate), 75% (acetate), 86.2% (propionate) and almost 100% for butyrate and medium-chain fatty acids (MCFA) were achieved after 16 h of extraction. The RSM optimization predicted a maximum E % of 59.60% and 34.67% for lactate and acetate in 5.5 and 1.17 min, respectively. In the leachate experiment, an increase in E% and k was observed with increasing extractant concentration and lactate and acetate concentrations over time. Using a 1M reactive extractant mix and 1.25 and 12 g/L of solute concentrations, the maximum E % of acetate and lactate were 38.66% and 61.8% in 10 min. The results could contribute to developing a rapid in-situ product recovery system integrated with food waste acidogenesis for lactate and acetate recovery, contributing to the bio-economy.

Polyhydroxyalkanoates production from ethanol- and lactate-rich fermentate of confectionary industry effluents

Polyhydroxyalkanoate (PHA) production has been the focus of considerable research to increase productivities and reduce production costs. In this study, a fermented confectionary industry wastewater was used as feedstock for mixed microbial culture PHA production. The feedstock was dominated by lactate and ethanol (60-90 % of all soluble fermentation products). The culture selection reactor was inoculated with municipal activated sludge and was operated at an organic loading rate (OLR) of 100 Cmmol·L^-1·d^-1, achieving a robust PHA-accumulating enrichment, which produced up to 52.6 ± 0.4 wt% of PHA in accumulation assays. An OLR increase in the culture selection stage to 150 Cmmol·L^-1·d^-1 led to a PHA content of 59.1 ± 0.6, a yield of 0.93 ± 0.01 Cmol-PHA·Cmol-S^-1 and a productivity of 0.93 ± 0.01 g-PHA L^-1·h^-1. A correlation analysis of the impact of ethanol concentrations from 3.19 to 20.3 Cmmol·L^-1 in the reactor showed that ethanol inhibited PHA production rate and yield and the consumption of other carbon sources available. Microbial community analysis revealed the increase of Amaricoccus genus during the bioreactor operation time, a known PHA accumulator. The produced polymer was poly(3-hydroxybutyrate) with an average molecular weight of 4.3 × 10⁵ Da and a polydispersity index of 1.88.

Current Status and Prospects of Valorizing Organic Waste via Arrested Anaerobic Digestion: Production and Separation of Volatile Fatty Acids

Volatile fatty acids (VFA) are intermediary degradation products during anaerobic digestion (AD) that are subsequently converted to methanogenic substrates, such as hydrogen (H2), carbon dioxide (CO2), and acetic acid (CH3COOH). The final step of AD is the conversion of these methanogenic substrates into biogas, a mixture of methane (CH4) and CO2. In arrested AD (AAD), the methanogenic step is suppressed to inhibit VFA conversion to biogas, making VFA the main product of AAD, with CO2 and H2. VFA recovered from the AAD fermentation can be further converted to sustainable biofuels and bioproducts. Although this concept is known, commercialization of the AAD concept has been hindered by low VFA titers and productivity and lack of cost-effective separation methods for recovering VFA. This article reviews the different techniques used to rewire AD to AAD and the current state of the art of VFA production with AAD, emphasizing recent developments made for increasing the production and separation of VFA from complex organic materials. Finally, this paper discusses VFA production by AAD could play a pivotal role in producing sustainable jet fuels from agricultural biomass and wet organic waste materials.

Volatile Fatty Acids (VFA) Production and Recovery from Chicken Manure Using a High-Solid Anaerobic Membrane Bioreactor (AnMBR)

Acidogenic fermentation of chicken manure (CM) for production and recovery of volatile fatty acids (VFA) is an interesting biological waste-to-value approach compared to benchmark organic waste management strategies. Considering the wide range of high value applications of VFA, a semi-continuous immersed anaerobic membrane bioreactor (AnMBR) was applied to boost VFA productivity and yield, while reducing downstream processing stages assisting the recovery of VFA. In this regard, the effect of parameters such as pH and organic loading rates (OLR) on the overall bioconversion and filtration performance was investigated. Thermal-shocked CM was applied both as inoculum and substrate. A very high VFA yield (0.90 g-VFA/g-VS) was obtained in the treatment with no pH control (~8.2) at an OLR of 2 g-VS/(L·d), presenting 24% higher yield compared to that of the controlled pH. Batch assays further demonstrated the enhanced hydrolysis and acidogenesis activities at weak alkaline conditions. A long-term (78 days) fermentation and filtration was successfully performed, where stable membrane filtration performance was experienced for about 50 days under high-solid (suspended solid of 37-45 g/L) and high flux (20 L/(m²·h)) conditions. Results suggest that AnMBR of CM is a feasible and promising process for VFA production and recovery.

Acidogenic fermentation of food waste in a leachate bed reactor (LBR) at high volumetric organic Loading: Effect of granular activated carbon (GAC) and sequential enrichment of inoculum

This study investigated the impact of different granular activated carbon (GAC) loadings and inoculum enrichment on acidogenic fermentation of food waste in a leachate bed reactor (LBR) operated at a high volumetric organic loading of 49 g VS/L_reactor. LBR with a high GAC loading of 0.51 g GAC/g VS_{food waste} achieved hydrolysis yield of 620 g SCOD/kg VS_added, significantly (P ≤ 0.05) higher to that obtained for LBRs with low or no GAC loading. A high GAC loading also resulted in a higher acidification yield of 507 g COD_SCFA/kg VS_added. Butyrate dominated the short-chain fatty acid (SCFA) composition by constituting 57-60 % of total SCFA at high GAC loadings, while the composition of acetate (38-40 %) and butyrate (36-38 %) were similar at lower GAC loadings. Inoculum enrichment further improved the hydrolysis and acidogenesis yields by 10-22 % resulting in the final hydrolysis yield of 683 g SCOD/kg VS_added and acidification yield of 617 g COD_SCFA/kg VS_added.

Recovery of volatile fatty acids using forward osmosis: Influence of solution chemistry, temperature, and membrane orientation

This study investigates the suitability of forward osmosis (FO) for recovery of volatile fatty acids (VFAs) from anaerobic digesters (ADs) and identifies the conditions favorable for commercially viable maximum recovery of VFAs. The recovery efficiency of VFAs is evaluated using a polyamide (PA)-based thin-film composite (TFC) membrane. The pH (3, 5, 7, and 9), temperature (20 °C and 40 °C), and membrane orientation (active-layer [AL]-facing FS and AL facingDS) were changed, and water flux, reverse salt flux (RSF), rejection rate, and concentration factor (CF) were evaluated for five VFAs. The water flux and RSF were higher at a higher pH, temperature and in AL-DS mode. A low rejection rate of 23-36% and a CF of 0.20-1.90 were observed at a pH below the pKa due to the solubility of molecular VFAs, while rejection rates was 80-97% and concentration increase by 1 to 4.8-fold at a pH above the pKa values were achieved due to deprotonation of VFAs and changes in membrane surface charges. With an equal increase in temperature of FS and DS from 20 to 40 °C, the rejection rate decreased by almost 20%. While with a transmembrane temperature change, a decrease in rejection rate of 20% was observed compared with baseline experiments due to decreases in viscosity and high diffusivity. In AL-DS mode, VFAs were rejected at a rate of almost 20% lower than that in AL-FS mode due to internal concentration polarization and membrane properties. These findings provide useful information on the factors that can influence optimal recovery rates of VFAs.

Improved Spectrophotometric Method for Determination of High-Range Volatile Fatty Acids in Mixed Acid Fermentation of Organic Residues

Volatile fatty acids (VFAs) are the important intermediates indicating the stability and performance of fermentation process. This study developed the spectrophotometric method for determining high-range VFA concentration in mixed-acid fermentation samples. The performance was compared with the gas chromatography (GC) technique. The calibration curves of the modified method showed linearity over a wide and high concentration range of 250–5000 mg/L for individual C2–C6 VFAs in both linear and branched chains. In order to evaluate the modified method for VFA determination in complex fermentation matrices, fermentation samples produced from acidogenic fermentation of plant materials were spiked with acetic (500–1500 mg/L) and butyric acids (1000 mg/L). The accuracy and precision of the modified method for VFA determination were in the range of 94.68–106.50% and 2.35–9.26%, respectively, comparable to the GC method (94.42–99.13% and 0.17–1.93%). The developed method was applicable to measuring all C2–C6 compounds and VFA concentrations in the fermentation samples and had an acceptable accuracy and precision. The proposed method is analytically reliable and offers significant advantages in the rapid determination of VFAs in mixed acid fermentation of organic residues.

A comprehensive review on current status and future perspectives of microbial volatile fatty acids production as platform chemicals

Volatile fatty acids (VFA), the secondary metabolite of microbial fermentation, are used in a wide range of industries for production of commercially valuable chemicals. In this review, the fermentative production of VFAs by both pure as well mixed microbial cultures is highlighted along with the strategies for enhancing the VFA production through innovations in existing approaches. Role of conventionally applied tools for the optimization of operational parameters such as pH, temperature, retention time, organic loading rate, and headspace pressure has been discussed. Furthermore, a comparative assessment of above strategies on VFA production has been done with alternate developments such as co-fermentation, substrate pre-treatment, and in situ removal from fermented broth. The review also highlights the applications of different bioreactor geometries in the optimum production of VFAs and how metagenomic tools could provide a detailed insight into the microbial communities and their functional attributes that could be subjected to metabolic engineering for the efficient production of VFAs.

Establishing Mixotrophic Growth of Cupriavidus necator H16 on CO2 and Volatile Fatty Acids

The facultative chemolithoautotroph Cupriavidus necator H16 is able to grow aerobically either with organic substrates or H2 and CO2 s and it can accumulate large amounts of (up to 90%) poly (3-hydroxybutyrate), a polyhydroxyalkanoate (PHA) biopolymer. The ability of this organism to co-utilize volatile fatty acids (VFAs) and CO2 as sources of carbon under mixotrophic growth conditions was investigated and PHA production was monitored. PHA accumulation was assessed under aerobic conditions, with either individual VFAs or in mixtures, under three different conditions—with CO2 as additional carbon source, without CO2 and with CO2 and H2 as additional sources of carbon and energy. VFAs utilisation rates were slower in the presence of CO2. PHA production was significantly higher when cultures were grown mixotrophically and with H2 as an additional energy source compared to heterotrophic or mixotrophic growth conditions, without H2. Furthermore, a two-step VFA feeding regime was found to be the most effective method for PHA accumulation. It was used for PHA production mixotrophically using CO2, H2 and VFA mixture derived from an anaerobic digestor (AD). The data obtained demonstrated that process parameters need to be carefully monitored to avoid VFA toxicity and low product accumulation.

Recovery Techniques Enabling Circular Chemistry from Wastewater

In an era where it becomes less and less accepted to just send waste to landfills and release wastewater into the environment without treatment, numerous initiatives are pursued to facilitate chemical production from waste. This includes microbial conversions of waste in digesters, and with this type of approach, a variety of chemicals can be produced. Typical for digestion systems is that the products are present only in (very) dilute amounts. For such productions to be technically and economically interesting to pursue, it is of key importance that effective product recovery strategies are being developed. In this review, we focus on the recovery of biologically produced carboxylic acids, including volatile fatty acids (VFAs), medium-chain carboxylic acids (MCCAs), long-chain dicarboxylic acids (LCDAs) being directly produced by microorganisms, and indirectly produced unsaturated short-chain acids (USCA), as well as polymers. Key recovery techniques for carboxylic acids in solution include liquid-liquid extraction, adsorption, and membrane separations. The route toward USCA is discussed, including their production by thermal treatment of intracellular polyhydroxyalkanoates (PHA) polymers and the downstream separations. Polymers included in this review are extracellular polymeric substances (EPS). Strategies for fractionation of the different fractions of EPS are discussed, aiming at the valorization of both polysaccharides and proteins. It is concluded that several separation strategies have the potential to further develop the wastewater valorization chains.

Advances in downstream processes and applications of biological carboxylic acids derived from organic wastes

Recovering carboxylic acids derived from organic wastes from fermentation broth is challenging. To provide a reference for future study and industrial application, this review summarized recent advances in recovery technologies of carboxylic acids including precipitation, extraction, adsorption, membrane-based processes, etc. Meanwhile, applications of recovered carboxylic acids are summarized as well to help choose suitable downstream processes according to purity requirement. Integrated processes are required to remove the impurities from the complicated fermentation broth, at the cost of loss and expense. Compared with chemical processes, biological synthesis is better options due to low requirements for the substrates. Generally, the use of toxic agents, consumption of acid/alkaline and membrane fouling hamper the sustainability and scale-up of the downstream processes. Future research on novel solvents and materials will facilitate the sustainable recovery and reduce the cost of the downstream processes.

Kinetic Study of the Anaerobic Digestion of Recycled Paper Mill Effluent (RPME) by Using a Novel Modified Anaerobic Hybrid Baffled (MAHB) Reactor

The process kinetics of an anaerobic digestion process for treating recycled paper mill effluent (RPME) was investigated. A laboratory-scale modified anaerobic hybrid baffled reactor (MAHB) was operated at hydraulic retention times of 1, 3, 5, and 7 days, and the results were analyzed for the kinetic models. A kinetic study was conducted by examining the phase kinetics of the anaerobic digestion process, which were divided into three main stages: hydrolysis kinetics, acetogenesis kinetics, and methane production kinetics. The study demonstrated that hydrolysis was the rate-limiting step. The applied Monod and Contois kinetic models showed satisfactory prediction with μmax values of 1.476 and 0.6796 L day−1, respectively.

A Glimpse of the World of Volatile Fatty Acids Production and Application: A review

Sustainable provision of chemicals and materials is undoubtedly a defining factor in guaranteeing economic, environmental, and social stability of future societies. Among the most sought-after chemical building blocks are volatile fatty acids (VFAs). VFAs such as acetic, propionic, and butyric acids have numerous industrial applications supporting from food and pharmaceuticals industries to wastewater treatment. The fact that VFAs can be produced synthetically from petrochemical derivatives and also through biological routes, for example, anaerobic digestion of organic mixed waste highlights their provision flexibility and sustainability. In this regard, this review presents a detailed overview of the applications associated with petrochemically and biologically generated VFAs, individually or in mixture, in industrial and laboratory scale, conventional and novel applications.

An Overview of Recent Advancements in Microbial Polyhydroxyalkanoates (PHA) Production from Dark Fermentation Acidogenic Effluents: A Path to an Integrated Bio-Refinery

Global energy consumption has been increasing in tandem with economic growth motivating researchers to focus on renewable energy sources. Dark fermentative hydrogen synthesis utilizing various biomass resources is a promising, less costly, and less energy-intensive bioprocess relative to other biohydrogen production routes. The generated acidogenic dark fermentative effluent [e.g., volatile fatty acids (VFAs)] has potential as a reliable and sustainable carbon substrate for polyhydroxyalkanoate (PHA) synthesis. PHA, an important alternative to petrochemical based polymers has attracted interest recently, owing to its biodegradability and biocompatibility. This review illustrates methods for the conversion of acidogenic effluents (VFAs), such as acetate, butyrate, propionate, lactate, valerate, and mixtures of VFAs, into the value-added compound PHA. In addition, the review provides a comprehensive update on research progress of VFAs to PHA conversion and related enhancement techniques including optimization of operational parameters, fermentation strategies, and genetic engineering approaches. Finally, potential bottlenecks and future directions for the conversion of VFAs to PHA are outlined. This review offers insights to researchers on an integrated biorefinery route for sustainable and cost-effective bioplastics production.