Selection and Design of Oil Water Separators

By Peter Smyth
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A typical drainage system consists of the trench drain, a sand and grit interceptor and an oil/water separator.  In many cases, the oil/water separator is designed with an integral sand interceptor compartment to permit sand and gravel to settle out before the wastewater enters the oil/water separation chamber.

An oil/water separator is typically an underground wastewater treatment tank filled with water.  The oil/water separator intercept and collects free floating oil, grease and settleable solids directly from the facility’s wastewater drainage system and prevents their discharge into the environment.  Internal baffles and high performance inclined parallel-corrugated plate and oleophilic (oil loving) coalescers optimize the separation process.  Corrugated plates are used to enhance the performance of oil/water separators.

A property designed plate coalescer will:

  • Enhance the separation of oil from water through added horizontal surface area and by creating less turbulent flow.
  • Amplify the difference in densities by providing longer path for the fluid to travel.
  • Minimize the distance an oil droplet needs to travel to find another oil droplet.
  • Promote coalescence; as the oil droplets collect they coalesce with other droplets and the larger droplets rise more rapidly to the water’s surface.

The separated oily waste accumulates within the separator until removal while effluent is usually discharged by gravity.  Oil/water separators can also be installed above ground.  These separators work in conjunction with the trench drains and catch/collect basins.  Flow to the separator is usually pumped using positive displacement pumps.  Like the below ground units, these separators are equipped with advanced coalescers that accelerate the separation process and greatly reduce the level of oil and oily coated solids discharged into municipal sanitary or storm sewer systems.

The engineer should know exactly where the oil/water separator effluent will be going and design for the fire, sewer, or environmental regulations governing that particular system.  Oil/water separator must be performance rated and capable of producing effluent discharges equal to or better than the controls established by the specific regulatory agency.  National design standards, such as the American Petroleum Institute standards, establish the mathematical formulas, derived from Stoke’s Law, required substantiating the coalescer’s oil removal efficiencies.

Most importantly, independently certified factory and field test reports must be demanded prior to accepting a manufacturer’s claims and equipment.  Currently code enforcement officials consider Underwriter’s Laboratories, Inc. UL-SU2215 certification as being the preeminent consensus standard for oil/water separator design, construction and performance.