Skimming Oily Wastewater

By M. Thomas Hobson, Managing Director
Abanaki Corporation; Oil Skimmer Division

Don't ignore the basics when weighing disposal options for oily wastewater

As large generators of oily wastewater tighten effluent controls, the U.S. Environmental Protection Agency (EPA) is targeting smaller generators. Some of the firms receiving their attention are smaller manufacturing plants, automotive garages, mobile equipment service shops and truck farms. Many of these firms do not have access to a sanitary sewer system that will accept oily wastewater. Because disposal in storm sewers also is prohibited, they frequently use injection wells, septic system drainfields, dry wells and ground pits to dispose of oily wastewater; one EPA concern is that oily wastewater will find its way into an underground aquifer that is a source of drinking water. Many oily wastes contain organic and inorganic chemicals in concentrations that exceed the primary drinking water standards established by the Safe Drinking Water Act. So the focus of one EPA program is aimed at preventing contamination of roundwater by controlling oily wastewater at the generator's site.

Similar problems are faced by plants that must deal with fugitive oils. These are oils that enter wastewater as a result of leaks, spills and housecleaning operations.

For metalworking plants, there are additional concerns associated with tramp oil in aqueous parts washer fluids, coolants and cutting fluids. For these plants, whether connected to a sanitary sewer or not, recycling may be the most viable option for oily wastewater. In metalworking plants, the primary concern may be reducing purchased fluids and operating costs, which can be achieved by removing tramp oils to extend fluid life. Coincidentally, this also reduces disposal costs.

Disposal options If oily wastewater isn't recycled, it must be disposed of safely. One option is to have it collected and hauled away by a licensed disposal firm. The annual volume of oily wastewater generated by many shops and plants makes hauling too costly. Even firms connected to sanitary or industrial wastewater sewer systems have limitations on the oil content in their effluent. When oil concentration exceeds a certain level, usually 100 ppm or less, the generator can get hit with hefty surcharges by the local government providing wastewater treatment. At some level of oil concentration, the oily water effluent is prohibited from entering the sanitary sewer line.

When recycling is the best option, the most common methods of oil/water separation include decanting tanks, oil skimming, coalescing, membrane separation and various chemical treatments. Any of these methods can be effective. Selection should be based on economic as well as technical considerations.

Although it is a cost effective method of reducing oil contamination, skimming often is overlooked as a primary technique. Frequently, this results from the misperception that skimming is only suitable as a pretreatment ahead of other oil/water separation devices. Certainly, skimming is a retreatment method used to prevent oil overloads in downstream membranes, coalescers and sand bed filters. But it can stand alone as an oil removal method in many applications, reducing oil to only a few parts per million concentration, depending on conditions. In many locales, this is good enough to allow the water to enter a sanitary sewer system without paying connection surcharges.

More exotic methods of oil removal, such as membrane filtration and chemical treatment, are most often required when tight emulsions and other chemicals must be removed. If an emulsion is the water-in-oil type, a skimmer may do the job.