by Tom Colcombe

Water and oil condensate exist wherever compressed air is used. Federal and local legislation requires that oil condensate be separated from water condensate and disposed of properly before water condensate is discharged to a public sewer system.

Separation devices, called oil-water separators, are very effective in removing such oil from compressed air condensate.

Oil-free? Maybe not
The use of a non-lubricated or "oil-free" air compressor does not guarantee the absence of oil in the compressed air stream. This is because atmospheric air introduced into the compressor intake contains hydrocarbon vapors.

For example, a typical light industrial area can have 0.5 parts per million hydrocarbon contamination.

When compressing atmospheric air, some hydrocarbon vapors form oil aerosols. You must remove these aerosols and properly dispose of them.

The amount of oil present in a compressed air stream is typically expressed as a concentration in parts per million (ppm). This refers to the mass of oil (in grams), compared to 1 million grams of air (29,000 standard cubic feet).

An easier way to understand the amount of oil entering a compressed air system over time is to convert ppm into volume (gallons).

How much oil is present in a compressed air stream? As shown in the chart below, even a small concentration of oil in compressed air results in many gallons of oil per year entering the environment.

The Federal Water Pollution Control Act, or Clean Water Act, regulates the pollution discharged into the nation’s waters. Under the act, the Federal EPA is responsible for establishing national effluent regulations for specific industrial categories, including a subcategory called "oily wastes."

The federal regulations control the concentration of oil (in oil-laden water) directly dischargeable into the environment.

Local regulations regarding the minimum acceptable oil content in effluent vary from treatment facility to facility within a municipality. These regulations control the concentration of oil directly dischargeable in a sewer system. The typical local regulation matches the federal version, which states no more than 15 ppm of oil can be discarded, with no visible standing oil sheen.

Before discarding treated water condensate into a sewer system, contact your local wastewater treatment facility to verify your area’s minimum acceptable oil concentration discharge.

Three solutions
There are three main devices that remove oil from compressed air condensate. These are electrically heated, membrane and gravity type oil-water separators.

Which system is best for you? The oil-water separator you choose depends on the type of oil-water condensate formed in your system. Condensate containing stable oil-water emulsions requires different treatment from condensate containing oils that won’t emulsify. An emulsion is characterized as liquid droplets dispersed throughout another liquid (for example, particles of butterfat dispersed in homogenized milk).

A typical emulsion in compressed air condensate looks like very thin mayonnaise.

Electrically heated oil-water separators heat the condensate mixture so water evaporates and escapes as steam. The remaining oil drains from the unit and is properly discarded. This type of separation technology is ideally suited to condensate that forms a very stable oil-water emulsion.

Membrane oil-water separators are effective in treating stable oil-water emulsion condensate. These devices capitalize on the difference in size between oil and water molecules found in the condensate mixture.

In this type, oil-water condensate is fed into a settling chamber. Oil molecules float to the surface, where they are drained and collected for discard. A pump forces the condensate through the capillaries of a membrane to separate the oil from the water. Water molecules are small enough to fit through the capillaries, but the larger oil molecules can’t pass. The oil returns to the settling chamber for removal.

The pump assures flow velocities through the membrane are high enough to prevent the oil from blocking capillary openings.

Gravity type separators are effective when treating condensate containing oils that won’t form stable emulsions. These devices take advantage of the difference in density between oil and water molecules. Gravity pulls the denser, heavier water molecules to the bottom of a separation tank. The less dense, lighter oil molecules float to the surface.

The oil is skimmed off the surface, while the water is further treated to remove any suspended oil droplets.

In these systems, oil-water condensate enters an air expansion chamber with an activated carbon filter to eliminate oil vapors from the exhaust air. The condensate then enters the settling and flotation chamber, where the majority of separation occurs.

The water condensate passes through clarification filters and activated carbon filters. This removes the remaining oil droplets from the water stream.

The process purifies the water condensate down to 10 ppm of oil. This level of purity complies with federal regulations and most municipality regulations. The purified water can go directly into the local sewer system.

Ensure compliance
Per federal and local regulations, oil condensate must be separated from water condensate and properly disposed. Since local regulations vary from municipality to municipality, verify the regulations pertaining to your specific installation to ensure compliance.

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