Residence time distribution determination in a submerged filter with and without aeration using a tracer

Abstract

BACKGROUND: Commercial filter media are commonly regular in shape and the flow patterns can be predicted with simple tools. Using lava stones as filter media, this study analyzes the influence of particle size and aeration on the hydraulic behaviour of a submerged filter prior to biofilm colonization. RESULTS:The filter was packed, separately, with two different sizes of lava stones (4.7 and 9.5 mm) and operated under different hydraulic retention times to establish the causes of deviations from ideal models. Tracer curves were analyzed using two mathematical models (Axial Dispersion and Wolf and Resnick models). The curves show that the tracer appeared in the effluent in a shorter time than expected indicating an effective volume reduction. It was estimated that the dead volume of the filter with aeration was 83% and 22% for the filter without aeration. CONCLUSION:The dead volume is caused, in the case of the filter with aeration, by the combination of two phenomena: the liquid volume displaced by the air bubbles and the turbulence caused by aeration generating preferential channelling along the filter media and creating large zones of stagnant liquid. In the case of the filter without aeration the volume reduction is caused by channels developed by the media irregular shape.