Hydrodynamic behaviour of a full-scale submerged biofilter and its possible influence on performances

The hydrodynamic behaviour of a 110 m(2) full-scale submerged upflow biofil ter, filled with 2.8 m of floating media and used for tertiary nitrificatio n of treated domestic sewage was investigated. Residence time distributions were obtained from lithium chloride tracer experiments. Four trials were p erformed at 3.7 and 6.5-6.7 mh(-1), once without aeration. Spatial differen ces in the mean residence time of a given cross-section were in the range o f 30-40% and can be attributed to the distribution system of the influent a t the bottom of the filter. Sampling at the bottom and top of the filter be d media showed that the behaviour of the filter bed itself is close to a di spersed plug flow reactor. The dispersion lengths are in the range 60-100 m m, 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. Heteroge neity due to the distribution of effluent in the lower part of the filter w ould also have a very limited effect in the case of sudden changes in the e ffluent substrate concentration. (C) 1998 IAWQ Published by Elsevier Scienc e Ltd. All rights reserved.