The hydrodynamic behaviour of a 110 m2 full-scale submerged upflow biofilter, filled with 2.8 m of floating media and used for tertiary nitrification of treated domestic sewage was investigated. Residence time distributions were obtained from lithium chloride tracer experiments. Four trials were performed at 3.7 and 6.5–6.7 mh−1, once without aeration. Spatial differences in the mean residence time of a given cross-section were in the range of 30–40% and can be attributed to the distribution system of the influent at the bottom of the filter. Sampling at the bottom and top of the filter bed media showed that the behaviour of the filter bed itself is close to a dispersed plug flow reactor. The dispersion lengths are in the range 60–100 mm, increase with flow rate and are unaffected by the aeration. From simple model calculations, it was concluded that the heterogeneity observed within the filter bed would have no significant effect on water quality. Heterogeneity due to the distribution of effluent in the lower part of the filter would also have a very limited effect in the case of sudden changes in the effluent substrate concentration.

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