The following FAQ page should provide some insight as to what system to use in the removal of contaminants from wastewater.

The HYDRASEP® Advanced oil water separator can effectively separate any two immiscible liquids having different specific gravities such as oil from water, biodiesel from glycerin, etc. In oil/water separation it is used to recover the oil or and/or to treat, discharge, or re-use the water. The following questions and answers should provide some insight as to what system to use in the removal of contaminants from wastewater.

While the section below focuses on oil and water separation the information is applicable to any two liquids that behave similarly.

An oil water separator is a primary treatment device. It is usually the first device, short of a sediment basin, to receive the wastewater. Therefore, it can be exposed to a flow that will vary in rate and in oil concentration. In most applications the separator will be of the non-mechanical gravity type. Criteria for good design of oil water separators should include: effectiveness, proper inlet design, solids handling capability, enhanced separation, separate oil removal section, and clean water section.

General FAQ

What are the design criteria for oil water separators?

The separator should be able to maintain its effectiveness at separating the liquids even when subjected to sudden variations in flow rate and concentration. It should be designed to handle the anticipated flow rate and contaminant concentrations under normal operation. It should also be capable of accepting flow rate increases up to 125% of rated capacity for a short duration without causing remixing of the oil/water interface. In addition, it should be capable of treating an upset where the quantity of pure oil may be 10% of the volume of the separator.

Proper Inlet Design:
The inlet to the separating section should be configured to uniformly distribute the flow and to prevent disturbing the oil/water interface.

Solids Handling:
Liquid mixtures introduced into the separator may contain entrained settleable solids. The design should allow these solids to settle prior to the introduction of the mixture into the separating section.

Enhanced Separation:
The separating section should be configured to accelerate separation. Enhanced separation may be achieved by reducing turbulence and controlling the velocity profile. It should not only prevent remixing at the oil/water interface and, but also stabilize it. This may be achieved by keeping the water flow parallel to the interface, i.e., never inclining it upwards.

Separate Oil Section:
The separator should be equipped with a separate compartment where oil may be removed without disturbing the interface in the separating section and where the flow within the separator will not disturb the oil removal operation.

Clean Water Section:
A final compartment that is isolated from the other sections should be provided in the separator to receive the clean water and to prevent any possible remixing of the separated components. This section should be designed to control the system hydraulics and regulate effluent discharge.

What is an API separator?

An API (American Petroleum Institute) separator is designed using Stokes' Law to determine the residence time required for particle rise. API recommends that turbulence should be controlled and that the horizontal flow velocity should not exceed 3 ft/s. It recommends a channel depth of 3 to 8 ft and a channel width of 6 to 20 ft with a depth to width ratio of 0.3 to 0.5. API also recommends a minimum channel length of 5 times the width. Based on API's recommendations, the minimum design Reynolds number for flow through the separator is in the range of 400 000. This is 40 times the transition to fully turbulent flow through a tube.

What happens when an oil/water mixture is introduced into an inclined plate separator?

In the inclined plate separator depicted below, the oily water mix enters from the left and flows through the plate bundle. The plates in the bundle are usually set at an angle around 45°. As oil builds up in the separator, it fills the upper passages in the bundle causing higher velocities through the lower plates. As the liquid exits the plates it is directed at the interface. Directing the water at the interface causes remixing of the oil, especially if there is a rag layer, with the exiting water. This remixing becomes more pronounced as the oil layer gets thicker. Dirty water will have fewer passages to go through, increasing its velocity and remixing ability.

When should filter media be used in water treating processes?

Filter media are used in all aspects of liquid and gas treatment to remove low concentrations of particulate matter from the carrying fluid. All filter media are similar, in that particle removal is accomplished by preventing a particle of a given size from passing through a screen, or mesh, of a given opening. The filter opening size is usually defined in micrometers, µm, often referred to as microns, where a micron equals 0.001 mm which converts to an opening of 0.000 039 in. A human hair averages 0.003 inches in diameter, or 76.2 µm. Most filters are sized to remove particulates ranging in size from 40 to 150 µm and are considered as tertiary treatment devices. Rapid clogging occurs if high concentrations of particulate matter are introduced into the filter. Therefore filter media are best used for polishing the stream.

Can recovered or separated oils be automatically or manually pumped from the HYDRASEP®?

Yes. The HYDRASEP® is designed to allow for automatic and/or manual pumping of recovered oils from the unit, without water. The automatic operation uses state of the art programmable controllers with a surface mounted pump.

What is the expected quality of the recovered oils?

Extensive testing of the HYDRASEP® has proven that the increase in moisture content is less than 5% in the recovered oils. In many applications, recovered oils or fuels will remain within ASTM allowable moisture content limits for that particular petroleum product, and can be sold or recovered. HYDRASEP® has separated crude oil from produced water from an ESP equipped well whereas the recovered crude met pipeline quality requirements without further treatment.

Can some of the recovered oils be reused without reprocessing?

It is recommended that recovered oils, such as diesel fuel, be analyzed prior to re-classification. In many instances, however (referring to related codes), recovered oils can be reused, recycled or burned for heating purposes.

Can the HYDRASEP® be efficiently incorporated into other treatment processes?

As previously stated, the HYDRASEP® is designed specifically as an in-line separation device. It is versatile and is adaptable to treatment of any mixtures regardless of their source whether it be storm water runoff, waste from petro-chemical, metal processing, crude oil and mining, food processing, and/or transportation industries. The HYDRASEP® can, in many instances, be incorporated into a process with minimal modifications and using existing piping, pumps, and other resources.

Can the HYDRASEP® be cost effective in large area surface run-off applications?

Yes, HYDRASEP®, Inc. also offers the HYDRAPASS® Bypass Basin for large area surface run-off applications such as parking lots, airport tarmacs, and equipment storage. Facilities that have a low probability of oil spills will gain the economy of design and installation when the collected oils are washed off with the first flush. The separator can be sized to handle the first flush event, and all run-off greater than that may be safely by-passed.

What is meant by maintenance free?

Our definition of maintenance free refers to the fact that the HYDRASEP® has no internal moving parts or filters that must be serviced on a periodic basis. In most applications equipped with a GN series controller, we recommend a monthly inspection of the control unit only, and an annual visual inspection of the separator. This annual inspection requires the removal of three mainway hatches and observation of the liquids within these sections. The annual inspection should take 30 to 60 minutes, depending on the type and location of the installation.


Technical FAQ

What are immiscible liquids?

Immiscible liquids are liquids that are incapable of mixing or attaining homogeneity when combined. An oil/water mix is a good example of two immiscible liquids.(1)

What is a carrying fluid?

In a mixture of immiscible liquids, one liquid is usually trapped in droplet form within the other, which is considered continuous. The continuous liquid is usually predominant by volume and is called the carrying liquid. An oil-in-water mixture is a mixture where the oil is dispersed in water, the water is the carrying medium. The reverse is true for a water-in-oil mixture.(2)

What are the differences between mixtures, emulsions and solutions?

Liquid mixtures consist of two or more liquid components in varying proportions that retain their own properties and are not necessarily uniformly dispersed within the continuous phase.(1)

An emulsion is a stable non-homogeneous liquid mixture consisting of two or more immiscible liquids uniformly dispersed within the carrying phase either through the use of mechanical (blenders) or chemical (emulsifiers) means.(1)

A solution is a homogenous mixture of two or more substances that become chemically bonded to form a stable single liquid.(1)

What is an immiscible liquid separator?

An immiscible liquid separator is a device that causes immiscible liquids to separate.

There are three general categories of separators: decanters, non-mechanical, and mechanical.(2)

Decanter: A decanter is a holding tank in which the liquid is stored until a desired level of separation is reached. It is limited to a batch type operation.

Non-mechanical: A non-mechanical separator is a flow-through device designed to use the flow pattern to its advantage in order to provide separation. It does not require the use of a power source or moving parts. An oil water separator is such a device.

Mechanical: A mechanical separator is a flow-through device, which requires the use of an external power source in order to provide separation. A centrifuge, a clarifier, and a skimmer are such devices.

How many degrees or levels of wastewater treatment are there?

There are three general levels of wastewater treatment: Primary, Secondary, and Tertiary.

Primary: the first level of treatment where coarse suspended solids, mixtures, and some mechanical emulsions are separated from water.

Secondary: the next level of treatment which removes the remaining mechanical emulsions, chemical emulsions, and organic matter in solution using chemical and biochemical processes.

Tertiary: the final level of treatment which removes any impurities remaining in the effluent from the previous primary and/or secondary treatment process. This level usually requires the use of filter media or membrane filters to remove small concentrations of matter, or trace concentrations of metals, salts, or colloids.

The Hydrasep® oil water separator is a primary treatment device.

What defines laminar, turbulent or transitional flow?

Laminar flow occurs when the forced movement of a liquid through a conduit can be described as a telescopic sliding of adjacent concentric layers of liquid without transverse mixing. Each layer tends to maintain its own identity. The resistance to motion is mainly due to molecular interactions within the liquid. In a long straight conduit, the liquid velocity is zero at the wall and gradually increases to its maximum value at the conduit centerline. Laminar flow conditions are usually met when the Reynolds number of the flow (Re = VD/ ) is less than 2,000.(4) The laminar velocity profile within a circular conduit is parabolic.

Turbulent flow occurs when the forced movement of a liquid through a conduit causes erratic mixing. Individual fluid particles no longer follow predictable paths and the resistance to flow is no longer entirely due to the molecular forces within the liquid. The turbulent flow velocity profile is almost flat (in a circular conduit) due to the transverse mixing caused by the random motion of the individual fluid particles. This condition is usually met when the Reynolds number Re = VD/ is greater than 10,000.(4)

Transitional flow is the gray zone in which the flow begins to lose its laminar characteristics to become turbulent due to an increase in velocity. The core becomes turbulent while the external ring remains laminar due to wall effects. Thus, in transition a fluid has characteristics of laminar and of turbulent flow. Making accurate predictions about fluid behavior in the transition zone is difficult. The velocity profile becomes a modified parabola flattened at its center. As the flow velocity increases, the turbulent core grows while the laminar ring shrinks. This condition is usually met when the Reynolds number Re = rVD/m ranges between 2,000 and 10,000.(4)

What are the three basic forces acting on a liquid particle moving with an immiscible liquid?

The three basic forces that act on an immiscible particle suspended in a liquid are:

Weight: the gravitational force acting on the particle. It acts downward.

Buoyancy: the lifting or upward force that a carrying liquid imparts on a floating or submerged body. It is equal to the weight of the displaced volume of the carrying liquid.

Drag: the resistance to motion that the carrying fluid imparts on a particle. It is opposite to the direction of motion of the particle relative to the liquid, and is basically due to frictional effects and the shape of the particle.

For a sphere rising at constant velocity, the buoyant force is dominant and acts upward. The gravity force (weight) acts downward and the drag force acts opposite to the direction of motion. Thus for a rising sphere, Buoyancy = Weight + Drag

What scientific theory is used to determine an oil particle’s rate of rise in a quiescent aqueous medium?

As mentioned in the previous question, a force balance is made for the rising sphere. The buoyancy and weight forces are relatively simple to determine. The drag force, however, varies with the velocity. Under conditions of very slow motion of the sphere, the drag force is inversely proportional to the velocity. Under other conditions, the drag force is a constant.

It is customary to express drag force in terms of a drag coefficient for the sphere, and to express velocity in terms of Reynolds number Re = VD/µ

One familiar theory used to determine the drag force on a submerged oil particle in water is Stokes' Law. Stokes Law can be derived from purely theoretical considerations and applies when the Reynolds number of the moving sphere is less than or equal to 1. (Actually, Stokes' Law relates the drag on a sphere to its velocity as the fluid flows past a stationary sphere.)

When a sphere begins moving through a fluid, it accelerates and eventually reaches a constant velocity, called the terminal velocity. Combining Stokes' Law with the preceding force balance equation, the terminal velocity of a sphere rising in a quiescent liquid can be determined using the following equation(4):


V = The rise velocity of a sphere
g = The acceleration of gravity
µ = The dynamic viscosity of the carrying liquid (water)
w = The density of the carrying liquid (water)
o = The density of the oil sphere
D = The diameter of the oil particle

Because this equation for terminal velocity was derived for a sphere rising in a quiescent (not in motion) liquid, it is more suitable for decanters. In fact, it is only valid when the Reynolds number is less than 1.(5)

Note that there are two Reynolds numbers associated with flow through a separator in which there are oil bubbles suspended in the water (the carrying fluid). One Reynolds number applies to the flow of the mixture as it moves through the separator. The other applies to motion of the oil bubbles as they rise upward through the water. In fact, each oil bubble will, in turn, have a separate Reynolds number. In a well-designed separator, all the oil bubbles will move upward, and droplet coalescence occurs. Applying Stokes' Law to this moving mass of oil, in an effort to model separation, is not possible. Thus, due to the water and oil velocities and motion within the separator, Stokes Law does not apply to the oil bubbles. While Stokes' Law cannot be used to calculate terminal velocity in this case, the overall effect of the parameters (weight, buoyancy and drag) on bubble rise still applies. The only difference is in how the drag force is determined.

Stokes' Law can be applied successfully for determining the terminal velocity of a single sphere moving in another fluid. Stokes' Law can also be used to predict trends such as how terminal velocity is affected by other parameters.

What effects do changes in temperature, viscosity, specific gravity and pressure have on separation?

Viscosity: Viscous effects are basically frictional in nature. They oppose the flow. This can be seen from the equation for terminal velocity, wherein an increase in the viscosity of the carrying liquid will cause a decrease in the terminal velocity of the bubble. The more viscous the carrying fluid is, the slower the rate of separation.

Specific Gravity: From the equation for terminal velocity, we note that as the difference in the specific gravities of the liquids increases, the buoyancy force increases and therefore improves separation. If the specific gravity of the oil approaches that of water, then the buoyancy force approaches zero.

Temperature: The viscosity and specific gravity of a liquid are direct functions of temperature. For example, if the temperature drops from 18°C (65°F) to 10°C (50°F), the specific gravity of the water will increase by 0.12% and its dynamic viscosity will increase by 25%. The rising bubble will experience a net increase in drag and will rise at a slower rate. Therefore, separation is more effective in warm water than it is in cold water.

Pressure: Liquids are generally incompressible; pressure would have very little effect on density and viscosity and therefore would not affect separation.

Are there other factors that may affect separation?

There are several other factors that may affect separation. Some of the most critical factors are listed below:

Bubble Diameter: From Stokes’ Law, the rise velocity is related to the diameter of the bubble. As the bubble increases in size, it rises faster to the surfaces. As it decreases in size, it rises more slowly. Also as it decreases in size, interfacial tension (the force that makes the water surface creep up in a glass) becomes predominant over buoyancy and the small oil bubbles remain trapped in the water.

Turbulence Effects: As defined earlier, turbulence causes erratic mixing. As the flow Reynolds number increases, the mixing intensity increases. The rise of an oil bubble would be undermined by turbulence. Small diameter oil bubbles, subjected to smaller buoyancy forces and slower rise velocities would be very readily remixed.

The pH of the Water: pH ranges from 0 to 14 with 7 being neutral. A low pH indicates the presence of an acid while a high pH indicates the presence of a base. If the pH of the water is high, this indicates that there may be chemicals in the water that would react with oil and cause it to become water-soluble. Oil/water separators are totally ineffective with dissolved oils. Lye and soaps are good examples of such chemicals. Acids may in certain cases enhance separation especially when added to neutralize the high pH chemicals.(3)

Emulsifiers and Cosolvents: The presence of emulsifiers and co-solvents (depending on their concentration) hinders separation. Emulsifiers may have to be chemically neutralized prior to separation.

What happens when wastewater containing oil, grease and grit passes through hydrophobic and/or oleophilic filter media?

Some separators use hydrophobic and/or oleophilic media. These media repel water and attract oil either due to their absorptive characteristics, or electrostatic and surface tension effects. However, these media will not attract oil that is in the bulk of the flow. They will retain the oil particles that impinge upon them or at a distance where microscopic molecular attraction is possible. The media are supposed to allow the oil to rise to the surface, but in many instances, the molecular attraction is so great that the oil ends up saturating the media and re-entrainment occurs.


1. N.I. Sax, R.J. Lewis, Sr., Hawley's Condensed Chemical Dictionary, 11th Edition, Van Nostrand Reinhold, 1987.
2. R.H. Perry, D.W. Green, J.O. Maloney, Perry's Chemical Engineers' Handbook, 6th Edition, McGraw-Hill, 1984.
3. American Petroleum Institute, Monographs on Refinery Environmental Control - Management of Water Discharges, Design and Operation of Oil-Water Separators, API Publication 421, First Edition, February 1990.
4. W.S. Janna, Introduction to Fluid Mechanics, 2nd Edition, PWS Publishers, 1987.
5. R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, John Wiley & Sons, Inc., 1960.