Nutshell filters composed of black walnut or pecan granular media are an established produced water treatment technology for tertiary oil removal. Guidelines for the size and operation of nutshell filters have evolved mainly by trial and error, with limited published operating data. This laboratory research program tested a nutshell filter to determine the operating flux limits (flow rate per unit area) that provide suitable oil removal performance. The separation efficiency target was defined as 5 ppmv oil in the outlet stream. The variables tested included medium type (black walnut shell or pecan shell), medium size, filtration flux, water salinity, and oil concentration. The flux limit for common 12/20 media is 12.0 gpm/ft2 for fresh water operation and 13.0 gpm/ft2 for saline water. Decreasing the medium size to 20/30 mesh increased the allowable flux limit but at the expense of a substantial increase in the pressure drop. Oil droplet penetration into the filter bed proceeds via a near plug-flow profile, with the top 18 inches of the bed providing 99% of the oil removal. Full breakthrough is a function of inlet oil concentration, with 20-30 hours of operation expected for a 48-inch-deep bed. Testing for removal of fine sand particles showed a average 5 µm separation size for 12/20 media at 13.5 gpm/ft2 flux.

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