Removal of hydrocarbons from urban stormwater runoff by gravity separation

Abstract

Three rain events were sampled from a storm drain at Manassas Mall in Manassas, Virginia. The urban runoff samples obtained were placed into lab-scale Plexiglas~ settling columns to monitor removal of total hydrocarbons (THC) by extended quiescent settling. Samples were collected from the columns at specific depths and times over the 48-hour settling period. The samples were analyzed for total hydrocarbon content on a Horiba oil content analyzer. Hydrocarbon values were averaged at each column depth in order to construct average THC concentration and average THC percent removal profiles over settling time. Maximum average THC removals were 77.8%,32.5%, and 73.6%, respectively, for Storms #1, 2, and 3 after 48 hours of quiescent settling. These average removals corresponded to depths of -2 feet, -1 foot, and -3 feet in the 5-foot tall columns. According to traditional sedimentation theory, pollutants settle out of water to the bottom of the container of interest. This did not appear to be solely the case in this hydrocarbon sedimentation study. It appeared that sedimentation was not the sole removal mechanism at work. Some fractions of oil and grease seemed to reorganize into low-density sub-groups and float to upper regions of the column. Also, the majority of THe removal occurred within the first 18 hours of settling for two of the three storms sampled.