Oil Water Separation

How Does An Oil Water Separator Work?

A standard oil water separator works by gravity separation.  The oil water separator provides a wide area where the rate of wastewater flow is reduced significantly to allow the oil droplets a chance to rise to the surface instead of being carried along by the wastewater.   The flow rate through an oil water separator is a function of the total volume of the separator.   Most oil water separators are designed to hold the water for 10 minutes.  10 minutes is the industry standard for holding time.  For example; if you want to treat a wastewater stream flowing at 7 gallons per minute, holding the water for the standard 10 minutes, the oil water separator tank should be a minimum of 70 gallons.   The math looks like this; 10 minutes holding time X 7 gallons/minute = 70 gallons.

Stokes Law:  In theory the rate of flow through the oil water separators tank must be less that the rate of rise of the oil droplets.  The relationship between flow rate and rate of rise of the oil droplet is described by Stokes Law.    The rate of rise can be calculated if you know the droplet size, specific gravity and the viscosity of the wastewater.    Or you can perform the simple test described below.

Test Your Oily Wastewater.

Test your oily wastewater, as some oily wastewater my take longer than the standard 10 minutes to see any separation.   Here is a simple test.  Collect a sample of your wastewater in a clear container (coke bottles are great for this) set the bottle down so it is not moving and wait 10 minutes.   The amount of oil you see on the top of the water in 10 minutes is representative of what you will be able to separate in a standard designed oil water separator.  Let the test go on for another 20 minutes and make observations of the oil that is rising to the top every five minutes.     After 30 minutes you can stop the test as this is probably all your going to separate in a gravity separator.    Ask yourself, is 10 minutes the optimum time, then a standard separator is right for you.   If 20 minutes was best, then double the size of the standard design.   Its that easy.   If you did not get any separation check the pH.

How does pH effect oil water separation?

pH is a measure of the acidity or alkalinity of water.   It is on a scale of 1 to 14 pH units.    A pH of 7 is considered neutral and less than 7 acidic, greater than 7 alkaline, also called basic.

As a rule of thumb, If the pH is greater than 8.5 any oil in the wastewater will be chemically emulsified.    If the oil is chemically emulsified, gravity separation will not work.    Even with coalescing media in the oil water separator the oil will not come out of its emulsified state.   This can be corrected by lowing the pH with acid to around pH 6.8.

What does the coalescing grid do in an Oil Water Separator?

The coalescing grid is like a honey comb of material that is oleophilic, or oil loving.  The coalescing grid forces the water through a back and forth pattern where the distance to the grid material is never more than about a half inch.   So the oil droplet only needs to rise a half inch and not all the way to the top of the tank.  As the oil droplets adhere to the plastic media they accumulate and begin to work there way to the top where they are skimmed off.

It is not uncommon for a newly installed oil water separator to show no signs of oil being skimmed off the top for days or even weeks.   Once there is enough oil on the grid material you will begin to see free oil that can be separated.

What causes Odor Problems in Oil Water Separators?

Odor problems can occur with any oil water separator.  They are usually the result of bacteria in the water.  The odor causing bacteria are called anaerobic bacteria, meaning bacteria that grow without oxygen.  Also called anoxic.  When anoxic conditions are present these bacteria will produce hydrogen sulfide which smells like rotten eggs.  This is especially noticeable after a period of no flow such as a Monday morning after a weekend of no flow.

We offer our oil water separators with an Ozone Generator for disinfection and odor control.  Ozone is a very powerful disinfectant wan will call most odor causing bacteria.   Ozone only lasts only a short time in the wastewater.  Therefore  there must be flow in the system for the ozone to work in the separation tank.  In other words, system must be running for it to work.

Chlorine tablets from the local swimming pool store can also be used to control odor.  Pick up some chlorine test strips to see if you adding enough.

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