Why Some Oil/Water Separators Need Secondary Coalescers

Posted on Tue, May 01, 2012 @ 12:35 PM

The Secondary Coalescer is a Warning Signal: 

This treatment option is most often seen in the below ground cylindrical “simple tank” oil/water separator world. These packs are largely used in the storm water, petroleum marketing, and transportation industries. Below ground tank systems do have corrugated plastic plate coalescers, then they are followed up by mesh adsorption packs consisting of polypropylene mono filament fibers. In theory, the coalescer in the oil/water separator is the primary separation device and these mesh packs are supposed to allow the water to flow through while any of the small oil droplets coalesce into larger droplets and rise to the top. In most cases the plate coalescers are designed at too shallow an angle to get efficient oil separation, so the manufacturers add adsorption packs to “improve” removal efficiency and meet effluent standards.

Secondary CoalescerPolishing Coalescer

View Mercer lead designer Dave Goding discuss this type of crutch practice by some oil/water separator manufacturers:

Relying on polishing mesh packs to “make up for” the shortcomings of an irregularly shaped plastic coalescer is not sound treatment. The design philosophy goes much like this: “Since we cannot meet 60 micron removal requirements with our oil/water separator at design at maximum flows, we will simply insert this high surface area mesh in the back and cover ourselves.”

Here is an important question to consider: If this were an effective design method to treat oily wastewater, why wouldn’t every oil water separator manufacturer simply fill the entire tank with this mesh and leave out the primary coalescer?

This is the worst fouling treatment in the marketplace. Originally created for treatment in mist eliminators, and a decent alternative for flock matter, this is by no means an effective alternative. Once these clog very quickly, the flow actually builds pressure back and makes these cartridges sag and rip very easily. They are not industrial duty and therefore present constant replacement costs over the life of the unit.

A Crutch for an Under Performing Coalescer--When insufficient coalescer design was recognized, especially in the below ground cylindrical market, manufacturers had to come up with a way to promise stated removal values. These “polishing” packs readily plug at their entrance points, and actually serve as a flow obstruction within the treatment.

Clogged oil/water separator meshFouled mesh pack

Tags: below ground oil/water separators, mesh packs, polishing coalescers, secondary coalescers