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A numerical scheme is proposed for the simulation of reactive settling insequencing batch reactors (SBRs) in wastewater treatment plants. Reactivesettling is the process of sedimentation of flocculated particles (biomass;activated sludge) consisting of several material components that react withsubstrates dissolved in the fluid. An SBR is operated in cycles of consecutivefill, react, settle, draw and idle stages, which means that the volume in thetank varies and the surface moves with time. The process is modelled by asystem of spatially one-dimensional, nonlinear, strongly degenerate parabolicconvection-diffusion-reaction equations. This system is coupled via conditionsof mass conservation to transport equations on a half line whose origin islocated at a moving boundary and that models the effluent pipe. Afinite-difference scheme is proved to satisfy an invariant-region property (inparticular, it is positivity preserving) if executed in a simple splitting way.Simulations are presented with a modified variant of the established activatedsludge model no.~1 (ASM1).

A moving-boundary model of reactive settling in wastewater treatment. Part 2: Numerical scheme