Design Of Aerobic Granular Sludge Reactors

Introduction
Since several years, conventional wastewater treatment has been dealing with low volumetric loading rates and a high energy consumption (Van Haandel & Van der Lubbe, 2007; Pronk et al., 2017). Especially with the increasing standard of living and an increasing amount of households connected to a sewage system constant improvements are needed (Vlaamse milieumaatschappij, 2019a). The question arises how these challenges can be met in an efficient way. Over the past 20 years, aerobic granular sludge is presented as a promising technology to meet these challenges. Conventional activated sludge flocs, i.e. suspended microorganisms forming bulky aggregates, are converted into compact aerobic granules. This results in 25-75% less land area, 20-50% less energy and up to 7-17% less costs compared to conventional activated sludge plants (Pronk et al, 2017). The conventional use of aerobic granular sludge is in batch systems, but continuous systems are under research as well (Jahn et al., 2019).
The aim of this thesis is to gain further insight in continuous processes with aerobic granular sludge. Given that the current continuous systems are not depreciated, yet cannot meet the demand for higher treatment capacity, continuous aerobic granular sludge systems seem promising. Better settleability of granules could lead to higher biomass concentrations in the existing continuous systems, possibly resulting in a higher treatment capacity. Before researching how to get stable granules in a continuous flow reactor, it is needed to investigate the overall effect of granules on the performance of continuous reactors. In this thesis it is questioned if refurbishment of the current continuous activated sludge plants into continuous aerobic granular sludge plants would be advantageous in terms of treatment capacity and energy consumption, in order to meet the effluent criteria. This was investigated by developing the comparison between continuous systems with activated sludge and with aerobic granular sludge. The comparative study is obtained in different steps. In the literature review, a state-of-the-art on current wastewater treatment with activated sludge and aerobic granular sludge is given. Both the typical aerobic granular sludge implementation in batch systems and perspectives on aerobic granular sludge in continuous systems are discussed and compared based on literature findings. The chapter ‘Materials and methods’ describes the mathematical model based on the Benchmark Simulations Model No. 1 (BSM1) in Matlab-Simulink. A continuous activated sludge system serves as the reference model. Furthermore, this model is adapted to make it representable as a continuous
design with aerobic granular sludge based on two characteristics: better settleability and diffusion limitation.
In the chapter ‘Results and discussion’, the differences between both continuous systems are elucidated to answer the research question. Both the maximal treatment capacity and energy consumption in order to meet the effluent criteria were calculated and compared for both systems. Conclusions are summarized in the chapter ‘General conclusions’ and ‘Recommendations for further research’ are given.