Investigation into the start-up and operation of upflow anaerobic sludge bed reactors

Stott, Rory

Investigation into the start-up and operation of upflow anaerobic sludge bed reactors

Master Thesis

2022

Abstract

High-rate anaerobic biological wastewater treatment using the upflow anaerobic sludge bed (UASB) reactor technology offers the potential to reform wastewater treatment. However, the lack of clarity regarding the mechanisms responsible for self-immobilisation of the microbial consortia involved, known as granulation, presents an obstacle to the wide-spread use of this technology. In this study, two laboratory-scale UASB reactors were commissioned for the purpose of generating datasets for model development. A sucrose-based feed was used for the experiments, which were conducted at 37°C. Deterioration of the sludge granules used as inoculum into undesirable bulking-type sludge resulted in refocusing the study to investigate the granulation process. After consulting the literature on granulation, an experimental investigation into the effect of providing additional hydraulic mixing by recycling reactor effluent on granulation was conducted. It was hypothesised that the additional hydraulic mixing would result in the formation of more settleable granules. However, it was found that inclusion of the additional hydraulic mixing resulted in a less dense sludge bed which contained more visual signs (presence of both more loosely-bound exogenous polymeric substance and long filaments presumed to be Methanosaeta Spp.) of bulking-type sludge. In hindsight it was found that application of too low a sludge loading rate in the experimental investigations was the cause of the granulation issues, but that this was exacerbated by the additional hydraulic mixing. Apart from granulation issues, a low effluent pH of 6.5 was obtained from the reactors during the experimental investigations in spite of a high feed pH of 8.0. It was hypothesised that the production of VFA and consumption of NH3 were the primary causes of the acidity generation. A fixed-conversion model of the digester pH was developed to investigate the conversions of the relevant weak acid and base species present and the effects of these conversions on the digester pH. It was found that the dissolution of CO2 to satisfy the vapour-liquid equilibrium between the headspace CO2 partial pressure and dissolved carbonic acid concentration was predominantly responsible for the decrease in pH across the reactors. It is on the basis of these findings that both hypotheses were refuted.

Keywords

Engineering

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