In the past, it was common to direct the water to be treated through a bed of excelsior or another similar medium, as shown in Figure 7-9, to aid in the coalescing of oil droplets. However, the coalescing medium has a tendency to clog. Many of these devices in oil field service have the medium removed. In such a case they actually act like a vertical skimmer since the oil droplets must flow counter current to the downward flow of the water through the area where the medium was.
Coalescing filters employing sand, anthracite, or a fibrous element to catch the oil droplets and promote coalescence have been used. The filter media are designed for automatic backwash cycles. They are extremely efficient at water cleaning, but clog easily with oil and are difficult to backwash. The backwash fluid must be disposed of, which leads to further complications. Some operators have had success with sand filters in onshore operations where the backwash fluid can be routed to large settling tanks, and where the water has already been treated to 25-75 mg/1 oil.