Capabilities of anaerobic granular sludge bed process for the treatment of particle-rich substrates
Doctoral thesis
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2020-01-17Metadata
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Abstract
High-rate anaerobic reactors have qualities that make them more attractive for anaerobic digestion of organic substrates compared to traditional reactors such as Continuous Stirred Tank Reactor (CSTR). These qualities include high organic loading rate (OLR), short hydraulic retention time (HRT) and efficient conversion of organic matter into biogas. However, their use has been largely restricted to organic wastes that are low in suspended solids. Application of high-rate granular sludge bed reactors to particle-rich substrates is limited due to problems that stem from slow particle disintegration and solid accumulation. Since some of the largest renewable biomass resources are particlerich, it is important to find remedies to the slow particle disintegration and solid accumulation so that such resources can be efficiently utilized in high-rate reactors and transformed into a source of renewable energy. In order to accomplish this, a thorough understanding of the interaction of granular sludge bed with particles is essential.
A review of the current state of application of granular sludge beds for anaerobic digestion of particle-rich substrates was carried out in order to establish how and to what extent substrate particles influence granular sludge beds and vice versa. It was found that successful high-rate digestion of particle-rich substrates with solid contents as high as 35% was possible in dry anaerobic digestion processes but in conventional high-rate reactors such as UASB, the TS limit seemed to be around 10%. Pretreatment of substrates has been used to improve the anaerobic digestion process. Economically sustainable methods of pretreatment are, however, limited. Several methods have been tried to improve the disintegration and hydrolysis of solid particulates with varying degree of success. Enzymatic pretreatment and co-digestion are often used. In addition, various reactor modifications have been implemented to deal with the increased solid accumulation associated with particle-rich substrates. Factors that affect disintegration and hydrolysis of particulates were investigated along with the kinetics used to model them. When the solid particulates contain recalcitrant lignocellulosic compounds, it is advantageous to classify them into easily and slowly disintegrating fractions. The success of high-rate reactors depends on the formation and sustenance of granular sludge with high settling characteristics so that they resist being washed out of the reactor. The settling characteristic of granules is crucial and needs to be monitored regularly. In this dissertation, settling velocity and size distribution of granules were studied using image analysis and settling column experiments with the aim to establish a method that uses image data generated using Matlab as a tool to determine the theoretical (calculated) size distribution and settling velocity of granules and compare them with experimental values. Comparable theoretical and experimental mean settling velocity values were obtained. Settling velocities increased with Reynolds number (Re). Significant size differences were found in granules collected at different heights of the lab-scale reactor.
Particle disintegration was studied in batch anaerobic reactors at 35 °C using particlerich substrate from manure supernatant. Two types of samples were applied, one high in suspended particles and another low in suspended particle content. Both feeds were digested with and without cellulase enzyme addition to obtain a better understanding of particle degradation mechanisms, kinetics and stoichiometry. Higher biomethane potential was found in the substrates with high–particle content but with a lower conversion rate. The addition of cellulase increased biomethane production rates in both high– and low–particle content samples enhancing yield by 54% and 40%, respectively and converting 69% and 87% of feed chemical oxygen demand (COD), respectively. Disintegration was modelled by classifying the solid particulates into fast and slow disintegrating fractions resulting in a good fit between experimental and simulated values. Particle disintegration was also studied in continuous reactors at 25–35 °C. Particulate contents ranging from 3.0–9.4 gTSS/L were fed into a 1.3 L lab-scale up-flow anaerobic sludge bed reactor (UASB). Biogas production was monitored while changing the temperature and particle content of the substrates. Biogas production increased with temperature in both high and low particle content substrates, however, the temperature effect was strongest on the high–particle substrate. Both the high– and low–particle samples produced a comparable amount of biogas at 25 °C, suggesting that biogas at this temperature came mainly from the digestion of small particles and soluble components present in similar quantities in both substrates. At 35 °C, the high–particle sample showed significantly higher biogas production than the low–particle sample, which was attributed to increased (temperature-dependent) disintegration of larger solid particulates. Simulation of disintegration was carried out using a similar scheme as in the batch reactors. Classifying the solid particulates into fast and slow disintegrating fractions resulted in comparable results between the simulated and experimental values.
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Article 1: Tassew, F.A., Bergland, W.H., Dinamarca, C. & Bakke, R.: Settling velocity and size distribution measurement of anaerobic granular sludge using microscopic image analysis. Journal of microbiological methods, 159, (2021), 81–90. https://doi.org/10.1016/j.mimet.2019.02.013Article 2: Tassew, F.A., Bergland, W.H., Dinamarca, C., Kommedal, R. & Bakke, R.: Granular sludge bed processes in anaerobic digestion of particle-rich substrates. Energies, 12(15), (2019), 2940. https://doi.org/10.3390/en12152940
Article 3: Tassew, F.A., Bergland, W.H., Dinamarca, C. & Bakke, R.: Effect of Particulate Disintegration on Biomethane Potential of Particle-rich Substrates in Batch Anaerobic Reactor. Applied Sciences, 9(14), (2019), 2880. https://doi.org/10.3390/app9142880
Article 4: Tassew, F.A., Bergland, W.H., Dinamarca, C. & Bakke, R.: Influences of temperature and substrate particle content on granular sludge bed anaerobic digestion. Manuscript submitted to Biomass and Bioenergy. Published in Applied Sciences, 10(1), (2020), 136. https://doi.org/10.3390/app10010136
Conference poster: Tassew, F.A., Bergland, W.H., Dinamarca, C. & Bakke, R.: Image analysis to measure settling characteristics of granular sludge. Manuscript submitted to Biomass and Bioenergy. Presented at IWA Biofilms: Granular Sludge Conference 2018, Delft, Netherlands (2018)
Appendix A: Matlab code for image preprocessing
Appendix B: Matlab code for image processing