Activated sludge modelling with smoothed particle hydrodynamics

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Michael Meister, Christoph Bergmeister, Daniel Winkler, Wolfgang Rauch

Monday 29 june 2015

14:20 - 14:35h at Central America (level 0)

Themes: (T) Special session, (ST) Smoothed particle hydrodynamics and other meshfree methods

Parallel session: 2H. Special session: Smoothed Particle Hydrodynamics and other meshfree methods

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SPH in wastewater treatment: State-of-the-art wastewater treatment includes the biological clarification of primary cleaned water and the aftertreatment in a secondary clarifier. As depicted in Figure 1 wastewater is not only transferred from the primary to the secondary stage, but activated sludge is also returned from the secondary clarifier to the biological treatment basin to increase the sludge age. Existing biokinetic models like the activated sludge model (ASM) (see e.g. Henze et al. 2000) consider the biological treatment stage as a self-contained basin and therefore neglect the influence of the hydrodynamics on the biokinetic processes. A breakthrough in the modelling of treatment plants may be obtained by applying smoothed particle hydrodynamics (SPH) to wastewater treatment since this method is particularly suitable for modelling the integrated treatment circuit and the coupling between the hydraulics and existing biokinetic models. In the biological treatment the understanding of the air induced mixing and the interfacial oxygen transfer which drive the sequence of biokinetic processes is of key interest, whereas in the secondary clarifier the settling and extraction of sludge particles becomes important (c.f. Figure 1b). Due to its multiphase simulation characteristics SPH is a very promising method for these applications and one drawback of the method, namely the numerical instability at the phase interface in case of high density ratios, is treated by adding weak artificial repulsion forces at the phase interface as suggested by Monaghan (2013). Biokinetic model: The hydraulic boundary conditions in the biological treatment basin are determined with SPH and then passed on to the biokinetic model. As proposed in the ASM model the biological basin is characterized by a continuously stirred tank reactor (CSTR), but we are working on a detailed coupling between SPH and the ASM model to reflect the influence of the local hydrodynamics on the biokinetics. The mplemented biokinetic model, which includes the processes of heterotrophic growth and lysis, hydrolysis, nitrifier growth and nitrifier lysis, describes the evolution of 8 biokinetic components of interest. Exemplarily in Figure 1 the predicted oxygen uptake rate is in good agreement with experimental data.