Upflow biological filter: modeling and simulation of filtration

Abstract

One of the most important operating parameters for the operation and the design of biofilters is the headloss due to clogging of the system leading to a semi-continuous operation with filtration cycles and backwashes. Some basic aspects of the filtration operation have been studied. The work consists in understanding and modelling the clogging phenomena due to the suspended solids retention, and validating the simulation results by calibration with experimental and full-scale results. Tracer experiments allowed us to simplify the hydrodynamics within the Biofilters into a plug-flow reactor with axial diffusion. Mass balances for the suspended solids have been therefore written accordingly, including solids retention represented as a mass transfer from the liquid to the solid phase. This affects the porosity of the system, leading on one hand to a modification of the filtration coefficient (responsible for the transfer of SS from liquid to “solid” phase) and on the other hand to an increased clogging measured by the headloss in the system. The Kozeny-Carman equation could successfully be used and a linear relationship between the filter coefficient and the water porosity could be validated. Experimental work has been conducted to calibrate the model and in this paper validations from pilot scale unit to full-scale plant are shown.