Experimental and Numerical Investigation of Backwash Flow in Nutshell Filter

Abstract

Summary The granular nutshell filter is commonly used as deep-bed media filter to remove insoluble hydrocarbons and suspended solids from water. The important advantage of using such a filter is the ability to backwash and regenerate the media. The process of backwash includes fluidization of the media by applying two fluid flows: backwash flow that is upward vs. a jet flow that is downward. A computational fluid dynamics (CFD) simulation of such fluidization was performed for a pilot-scale nutshell filter. The effect of the backwash flow, jet flow, and medium depth in the filter on the fluidization quality during the backwash mode of the filter were investigated. The media fluidization uniformity was studied as the media depth and flow patterns varied. A fair consistency was observed between the experimental values and the results obtained by CFD analysis. It was observed that the longer the media depth, the more uniform the media fluidization will occur. Increasing the media depth resulted in longer fluidization times. Shorter media-depth filters are more sensitive to jet flows. For shorter filters, when the jet flow was reduced, a more uniform fluidization was obtained. The fluidization time (the time for the media to reach the top of the vessel) is strongly related to the jet flow for both shorter- and longer-depth media filters. For long-depth media filters, the effect of increasing the backwash flow and reducing the jet flow on the uniformity of media fluidization vs. the fluidization time was studied. The velocity pattern for the fluid showed vortices close to the floor screen in lower mid-depths of the filter. The CFD analysis for the commercial-scale nutshell filter with a diameter of 12 ft (3.65 m) and height of 8 ft (2.4 m) showed a similar vortex pattern with uniform media fluidization.