Browsing by Person "Ravi, Padma Priya"

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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.