Sand filters as an appropriate treatment facility for small wastewater flows were investigated to determine solute and gas transport under the highly dynamic conditions of intermittent flushing. The effect of biomass accumulation on solute breakthrough was assessed experimentally by tracer studies. With the help of a one-dimensional subsurface water and solute code, solute transport and air diffusion were evaluated mathematically. The code was used to extrapolate to different hydraulic conditions. A second multiphase transport code enabled to include the effects of water and air flow simultaneously. From the calculated results, which were confirmed by gas tracer experiments, oxygen fluxes due to advective and diffusive transport phenomena could be evaluated. The better insight into transport limitations in intermittent filters allows some conclusions for design and operation of these systems.

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