A computational model for heterogeneous flow through low headloss biofilter media

Abstract

In an ideal biofilter, air flows parallel to the biofilter axis and at the same velocity through all parts of the bed. In contrast, non-uniform flow causes different parcels of air to experience detention times above or below the average, reducing treatment efficiency. Designers and operators have long been aware that medium nonuniformity, bed compaction, fissuring, and separation from the vessel walls can cause flow channeling and damage biofilter performance. However, as biofilters have been designed for lower headloss and higher flowrates, flow heterogeneity has also arisen, as pressure variation in the headspace affects flow in the medium. In an effort to investigate flow heterogeneity, a two-dimensional steady-state computational fluid dynamics (CFD) model was developed to simulate flow through a typical biofilter. The model assumes incompressible, two-dimensional, Navier-Stokes flow in the spaces above and below the bed and Darcy flow in the porous medium. The modeling effort demonstrated that the optimal design depends on the permeability of the medium, the air flowrate, and biofilter configuration. Several simple options for inlet and outlet location were compared. In very low permeability media, the flow was uniform regardless of the design. With highly permeable media, the optimal design choice depended on the Reynolds number. However, the design with side inlet and same side outlet had the lowest flow heterogeneity of the five designs over a wide range of Reynolds numbers. The model also demonstrated that removal efficiency decreases as flow heterogeneity increases.