Mixed-Media Filtration of Oily Waste Waters

Abstract

The development of mixed-media filtration and its application to oily wastes, including oilfield-produced water, refinery waste water, and other waters containing oil, is reviewed. Several case histories are presented and data show that simple filtration through mixed-media filters can remove essentially all visible oil. Introduction Filtration through granular materials to achieve removal of suspended matter and associated materials such as oil is an old concept. It is, however, receiving new attention in many areas in the petroleum industry where there is a concern for improving the quality of injection waters in secondary recovery operations and reducing the biochemical oxygen demand (BOD), oil, and suspended solids levels in refinery waste waters. There are many granular filter systems available. The purpose of this paper is to discuss mixed-media filtration - how it was developed, where it has been used, and its application to the petroleum industry. A mixed-media filter uses three or more materials, each of different size and density, that intermix, not stratify, to form a filter grading from coarse at the top to fine at the bottom in the direction of flow. Typically, anthracite coal, silica sand, and garnet sand are used. The coal is the lightest density and largest in size, with the garnet highest in density and finest in size. The effective size of the media particles decreases in the direction of flow. The purpose is obvious - to increase the storage capacity and to increase filter efficiency with depth. The term "mixed media" has been wrongly used to describe a filter of two materials, such as coal and sand, that generally stratify into two separate and distinct layers. A review of the evolution of filter media design will provide a better insight into the differences between provide a better insight into the differences between media systems. The slow sand filter was developed in the early 1800's and employed sand with a size of 0.2 mm. Operating rates of 0.05 to 0.15 gal/min per square foot of filter area were used, and applied turbidities were very low. A large surface area was required and cleaning was accomplished by periodically scraping off the top layer of sand. In the early 1900's, the rapid sand filter was introduced, together with chemical coagulation and periodic backwashing of the filter. Effective size of periodic backwashing of the filter. Effective size of the rapid sand filter media might vary from 0.35 to 1.0 mm; typical size is 0.5 mm. This type of filter has demonstrated the ability to remove effectively applied suspended solids of 5 to 10 ppm at flow rates of 2 to 3 gal/min per square foot of bed area. Fig. 1 describes a typical rapid sand filter. Note that hydraulic grading of the sand occurs in the backwash cycle, with the very finest sand forming the top of the bed. Filtration occurs in the top few inches. Once a suspended solid passes the top layer, larger and larger voids occur, and the chance of passing through the filter is greatly increased. A filter with a coarse upper layer over the sand layer - a dual-media filter - was introduced to overcome this shortcoming. "Capped" filters - that is, filters with 4 to 6 in. of coal over sand - have been used for many years, operating at 2 to 3 gal/min per square foot of filter area. JPT P. 157