Browsing by Subject "Volatile fatty acids"

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Anaerobic treatment of a biorefinery’s process-wastewater
(2024) Khan, Muhammad Tahir; Lemmer, Andreas
The phrase “one man's trash is another man’s treasure” perfectly embodies the concept of a circular bioeconomy, emphasizing the conversion of waste into valuable resources while embracing a zero-waste approach. In line with this perspective, the primary objective of the current research was to assess the suitability of a biorefinery’s process-wastewater as a potential feedstock for biogas plants by investigating its anaerobic biodegradability and exploring its biogas and methane production potentials. For such a specific purpose, the process-wastewater from a commercial-scale biorefinery specializing in 5-hydroxymethylfurfural (5-HMF) synthesis and refining was utilized. To fulfill the main objective, three independent sub-objectives were formulated. The initial investigation centered on assessing the biochemical methane potential (BMP) of the typical constituents present in process-wastewater, such as furans (5-HMF and furfural), phenols (syringaldehyde, vanillin, and phenol), and weak acids (levulinic and glycolic acid), as well as the full 5-HMF process-wastewater. The BMP assessments for each test substance were conducted separately at different concentrations (2, 4, 6, and 8 gCOD/L) and temperatures (37°C and 53°C) via the Hohenheim batch fermentation test. The model components at 2 gCOD/L, apart from phenol at 53°C, were efficaciously degraded, in most cases to such an extent, that supplementary methane generation was detected i.e. exceeding their maximum theoretical limits. However, increasing the concentrations of the test components in the assays resulted in diminishing methane conversion at both operating temperatures. Eventually, among the tested components, the 5-HMF process-wastewater was evaluated to be one of the most refractory substrates, following phenol, vanillin, and 5-HMF, when tested at its maximum load under mesophilic and thermophilic conditions. The subsequent investigation focused on examining the anaerobic decomposition of the 5-HMF process-wastewater and its main identified constituents, including 5-HMF, furfural, and levulinic acid in continuously operated anaerobic filters (AFs). The test substances were individually injected into the biofilm reactors operating at 43°C in a controlled manner with a randomized experimental design. This study yielded some unusual outcomes i.e., the test substrates exhibited satisfactory degradation, while at other instances, they hampered the process. Introducing butyric acid between the injected components revealed no signs of compromised consortia. The 5-HMF process-wastewater in this investigation emerged as the least favorable substrate for methane conversion. The culmination of the current research involved utilizing the 5-HMF process-wastewater as a sole feedstock for the fixed-bed reactors. Hence, necessary nutrients to support the existing microbial consortia in the AFs were added to the process-wastewater. Given its toxic nature, the substrate dosage was initiated from its reduced concentration of 10 gCOD/L and was gradually increased to 20, 30, 40, and 50 gCOD/L, with corresponding organic loading rates (OLRs) of 2, 4, 6, 8, and 10 gCOD/L.d, respectively, as the trial progressed. Despite meeting the nutrient requirements, the gas yields, in particular methane, were not remarkable. However, a noteworthy finding surfaced: as the gCOD/L of the fed substrate increased, so did the concentrations of the short-chained volatile fatty acids (SCVFAs) in the reactors. This observation led to the conclusion that the low methane yields were at the behest of the accumulation of SCVFAs in the AFs, at both mesophilic and thermophilic temperatures. Ultimately, the subpar performance of the process-wastewater as a substrate is considered to stem from its exceptionally high concentration of the pollutant 5-HMF, which significantly influences its overall characteristics, causing longer lag phases, especially at higher OLRs. This, in turn, triggers the inhibitory behavior, leading to reduced methane yields. Consequently, these factors render the 5-HMF process-wastewater a precarious choice for biogas plants in terms of efficient energy recovery. While AFs are well-suited technology for treating high-strength wastewaters, for the substrate such as 5-HMF process-wastewater, it might be beneficial to increase retention times by decreasing the OLRs. Additionally, reducing the strength via dilution combined with these adjusted process parameters could further enhance its decomposition. Anaerobic digestion (AD), traditionally used for energy recovery from (bio)wastes, has potential beyond biogas production in the bioeconomy. This research showed that the highly recalcitrant 5-HMF process-wastewater can be a viable source for producing SCVFAs through AD. Furthermore, the Muttenz biorefinery could utilize the filtration byproducts to produce levulinic acid, aligning with a cascading biorefinery approach.
Microbial conversion of organic residues into acid rich process liquids and their use in bio-electrochemical systems
(2020) Ravi, Padma Priya; Lemmer, Andreas
In 2016, 2.01 billion tonnes of solid waste were generated worldwide. The volume of waste is expected to grow to 3.40 billion tonnes by 2050. Worldwide, most solid waste is disposed of in landfills or dumps. Due to improper treatment and disposal of solid waste, nearly 1.6 billion tonnes of CO2 equivalents of greenhouse gas emissions were generated worldwide in 2016. This amount is expected to rise to 2.6 billion tonnes of CO2 equivalents per year by 2050. It will therefore become increasingly important in the future not only to treat waste sustainably, but also to use it as an alternative to fossil fuels. Different waste-to-energy concepts are used, particularly for the treatment of OFMSW. As an alternative to the previously dominant biogas production, intensive research is currently being carried out into technologies for the recycling of organic residual materials, including so-called bio-electric systems (BES). In contrast to biogas production, this technology enables the treatment of a wide range of wastes to produce different end products, e.g. electrical energy, hydrogen or methane, can be preferred in BES depending on the selected process parameters. Despite numerous advances in research, considerable additional optimization is still required in order to be able to use the systems in large-scale power generation. In order to use solid organic waste in BES systems, fermentative digestion is required to convert the organic components into dissolved short-chain organic acids (Volatile Fatty Acids (VFA)) and alcohols. In the course of the investigations, the solid waste residues were first digested to acid-rich hydrolysate in a hydrolysis reactor at pH-values of 5.5 and 6.0. However, this hydrolysate also contains particles that are inert to a subsequent degradation step leading to technical process disturbances. These inert particles can be removed by means of a membrane filtration step; a particle-free permeate is produced, which can be fed to the BES reactors. Within the scope of the present work, the basics of the utilization of OFMSW via microbial digestion, membrane filtration and utilization in BES should be investigated. Lab-scale BES reactors were developed and batch tests were carried out. The vegetable waste residues from hydrolysis could be efficiently converted into hydrolysate. At a pH value of 6.0, higher organic acid concentrations were achieved than at pH 5.5. At pH 6.0, based on the added organic dry matter, these were approx. 350 g kg-1 (oDMadded) and at pH 5.5 approx. 215 g kg-1 oDMadded. Likewise, the concentration of chemical oxygen demand (COD) of the hydrolysate at pH 6.0 was 21.85 % higher than at pH 5.5. However, the COD degradation rates in the AF used were insufficient because the inert particles present in the hydrolysate could not be completely microbially degraded. The subsequent integration of ceramic cross-flow membrane filtration into the two-stage system produced a particle-free permeate and significantly the increased microbial degradability. Clear differences could be shown depending on the substrate used (plant waste and grass/maize silage). The filtration step resulted in a significant improvement of the specific methane yield of permeate by 40% (vegetable waste) and 24.5% (grass/maize silage) compared to hydrolysate; proof that inert particles were separated efficiently. Finally, the process liquids hydrolysate and permeate produced by the hydrolysis of maize silage and the subsequent membrane filtration were fed to the anode chamber of two mixed-culture BES reactors. The investigations showed that all organic acids in both process liquids could be completely degraded in the BES. The highest COD (87%) and TOC degradation rates (88%) were achieved with permeate. However, the hydrolysate with added acetic acid yielded the highest current density of 470 µA/cm². Increasing the pH-value of the process liquids from 5.75 to 6.8 also significantly improved the current production and degradation rates. In this batch studies, relatively low Coulomb efficiencies of less than 10% were achieved due to the use of a mixed cultures. The promising results show that at high pH-values (pH 6.0) in hydrolysis organic residues can be efficiently converted into a hydrolysate with high concentrations of organic acids and that the system can be further optimized by coupling membrane filtration. The utilization of the permeate in BES enables, a sustainable production of bioenergy and platform chemicals with permeate enables, depending on the BES reactor configuration. In summary, it was described for the first time that the combination of the fermentative biomass degradation process with filtration via ceramic membranes and the use of permeate in BES systems is possible.
Mono-digestion of 5-Hydroxymethylfurfural process-wastewater in continuously operated anaerobic filters: A cascade utilization approach
(2023) Khan, Muhammad Tahir; Krümpel, Johannes; Wüst, Dominik; Lemmer, Andreas
A proper remedy for the overexploitation of biomass and biobased materials in the bioeconomy is the valorization of biorefineries’ side streams into meaningful products. Hence, in pursuit of a cascade utilization of renewables, a unique biorefinery byproduct was investigated for its biogas potential, specifically methane, in continuously operated anaerobic filters. For this purpose, 5-Hydroxymethylfurfural process-wastewater, after supplementation of necessary nutrients, was diluted down to 10, 20, 30, 40, and 50 gCOD/L concentrations and thereafter tested individually at 43 °C and 55 °C. Maximum methane conversion efficiency at either temperature was observed for test substrates with 10 gCOD/L and 20 gCOD/L concentrations. At 43 °C, the anaerobic filters exhibited their highest biogas yields when supplied with the 30 gCOD/L feedstock. Further exposure of the mesophilic and thermophilic consortia to the ensuing 5-Hydroxymethylfurfural process-wastewater dilutions compromised the stability of the anaerobic process due to the soaring concentrations of short-chained volatile fatty acids. The supplementation of necessary nutrients to unlock the methane potential of the given recalcitrant substrate appears insufficient. Techniques like micro aeration, photolysis, or the use of activated carbon in the fixed bed might have the ability to enhance the biochemical methane conversion of such feedstock; otherwise, the introduction of trace elements alone may be adequate if aiming for platforms (volatile fatty acids) via anaerobic technologies.