Solving Oil Water Separator Problems

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FAQs and answers from Mohr Separations Research, Inc.

To reveal the answer for each Frequently Asked Question, simply click the question to expand.

How can I be sure that the effluent from the separator will meet the requirements?

The first thing to do is to make very sure you know all of the possible inlet situations – maximum flow rate, minimum temperature and oil types and concentrations that may occur in the inlet. Consider if a spill condition may happen. Make a schematic flow diagram of the system and identify possible pressure drop causing equipment such as pumps and valves. With this information, MSR can estimate the oil droplet sizes that are likely to be present in the inlet water. We use this information in our proprietary computer program to predict operations of the system. We estimate a separator size and configuration and do a simulation calculation to see if the performance will meet the required effluent concentration. We then adjust the size larger or smaller to ensure a proper fit. Because the separation depends on Stokes’s Law and we are careful to ensure (in the separator design) that the conditions of this physical principle are met, the final design system will meet the required effluent. We try to be very conservative in estimating the droplet sizes so that the final design will be conservative and capable of handling variations in flow and other conditions. The physics work and Sir Isaac Newton’s gravity can be trusted to operate correctly and predictably every time. A very convenient property of coalescing plate separators not shared by some types of oil-water separators is that, at flow rates lower than the design flow, they just work better!

Why is it better to direct the flow into a separator by gravity flow than pumped flow?

Anything that decreases the average droplet size in the inlet of a separator will decrease the performance of the separator. The type of equipment that decreases the average droplet size is anything that causes shear in the inlet stream. This includes pumps, especially centrifugal pumps, valves, especially globe and other control valves, small or rough piping, and undue amounts of elbows or other fittings in the inlet pipe.

If I have to pump into a separator, what is the best kind of pump to use?

The best type of pump to use is a positive displacement pump such as a diaphragm pump or screw pump. The worst are centrifugal pumps.

What information do I need to gather for designing an effective, reliable separator?

Numerous factors must be considered in the selection and design of oil-water separation systems. Among these are:

⋅ Flow rate and conditions
⋅ Degree of separation required – effluent quality
⋅ Amount of oil in the water
⋅ Existing equipment
⋅ Emulsification of the oil
⋅ Treated water facilities
⋅ Recovered oil disposal method.

For industrial and some municipal applications, flow rate, amount of oil, flowing temperature, and other conditions affecting separation – such as whether flow is laminar or turbulent – may be easily determined. For storm water applications, however, it may be necessary to estimate water flows. The degree of separation required is usually a matter of statutory or regulatory requirements, but if the water is discharged to a sanitary or industrial treatment plant, it may be negotiable. The amount of oil in the water may be known, especially in industrial applications, but it often will be necessary to estimate the quantity in storm water applications. MSR can provide guidance about quantities to be expected. Existing equipment such as API separators may affect the design of equipment to be used. Often, it is possible to retrofit existing equipment with more sophisticated internals to enhance separation quality. It is necessary to ensure that adequate size piping is provided for downstream treated water removal to avoid flooding the separator and perhaps filling the oil reservoir (if provided) with water. A downstream test point should be provided to allow for effluent testing. Adequate storage facilities for the removed oil should be provided and means for recycling the oil included. Careful records of removed and recycled oil should be kept to allow for preventative maintenance and avoid possible future regulatory problems.


Can I use soaps and detergents and still have my separator work well?

Soaps and detergents (known collective as surfactants, which is a contraction of surface active agent) can cause emulsions that are very difficult to remove. Without careful attention to the amount and type of soaps and detergents used, it is likely that a system will NOT operate satisfactorily if they are used. The degree of emulsification of the oil is difficult to assess, but steps can be taken to discourage the formation of emulsions and encourage the breakup of emulsions that are inadvertently created. It may be necessary to substitute quick-break detergents for conventional detergents that also are emulsion causing. Quick-break detergents are those detergents designed to remove the oil (or grease) from the item to be cleaned and then quickly dissociate again from the oil, leaving the oil as free hydrocarbon droplets in the water. MSR will be glad to make design suggestions and / or send a copy of a study done by the U.S. Army Corps of Engineers on quick-break detergents. Note: The small amount of detergents in detergent motor oils will not cause problems with the operations of oil water separators.

I have dirt and / or grit in my flow stream – what will that do to the system?

Small quantities of dirt and grit will not harm the operations of the separator; they will only cause some eventual plugging that must be removed periodically. If large quantities of solid particles are expected, it is wise to provide a grit removal chamber before the separator. These chambers should be designed according to normal design parameters for grit removal as used in sanitary sewer plant design. MSR will be glad to offer suggestions on this design.

What is the benefit of using separators over absorbents?

The biggest problem with the use of absorbents is that they are quickly used up, and since there no way to determine if they are exhausted without laboratory testing, they are often left in place long after their useful life is done. The used absorbents are sometimes considered hazardous waste and will result in a disposal cost.

What can I do about dissolved hydrocarbons?

In general, hydrocarbons are only marginally soluble with the notable exception of Benzene and some other aromatic hydrocarbons. Even these are removable using coalescing plate separators or other physical means as long as they are either removed so quickly that there is no time for the dissolution to occur or if they are present in concentrations greater than the solubility. Dissolved hydrocarbons are, strictly speaking, not covered by the Clean Water Act since they do not cause a sheen on the water. Many analysis methods will detect dissolved hydrocarbons and some jurisdictions are concerned about them, so it may be necessary in some cases to treat for their removal. Physical methods such as coalescing plate separators will not remove dissolved hydrocarbons and other methods such as biological treatment or absorbents are required.

What can I do with the oil recovered in a separator?

Removing the oil from the separators is not enough to protect the environment, it must also be recycled to ensure that it is disposed of properly. Current U.S. law can hold the owner of the oil-water separator responsible if this oil is not properly disposed of, even if the owner has paid for proper disposal. There are many local firms involved in recycling hydrocarbons. Most will be able to tell you what the local regulations are and usually offer a pick-up service.

What are the ideal inlet conditions for an oil water separator?  I’d like to make it work as well as possible.

Ideal inlet conditions for an oil-water separator are:

⋅ Gravity flow (not pumped) in the inlet piping
⋅ Inlet piping sized for minimum pressure drop
⋅ Inlet piping straight for at least ten pipe diameters upstream of the separator (directly into nozzle)
⋅ Inlet piping containing a minimum of elbows, tees, valves, and other fittings

What analysis method is recommended for determining oil concentration in water?

MSR suggests the use of USEPA analytical Method 1664, which is a spectrophotometric method. Most environmental laboratories can perform this test and it is usually not expensive. It is accepted by most regulatory authorities.