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In short, this analysis determines the most cost-effective option among different competing alternatives to purchase, own, operate, maintain and, finally, dispose of Oil Water Separators, when each is equally appropriate to be implemented on technical grounds. Upfront savings are often made up on the backend by higher costs to maintain: higher costs of parts or long leadtimes mean increased outages, inherent designs that require more manpower to maintain due to complexity and power useage and waste are all factors in why your initial savings can end up costing you in the long run. Read more to ensure you've covered all your bases.

When evaluating various manufacturers’ oil water separator offerings, there are some very important considerations to make in regard to total life-cycle costs of the equipment. Like most industrial wastewater equipment, an oil water separator is not a set-it-and-forget-it system. The initial purchase price of the separator is only the “tip of the iceberg” when it comes to understanding the overall cost during its useful lifespan. Procurement, installation, and maintenance costs are the three main areas that need to be carefully analyzed to help ensure the various manufacturers’ equipment offerings are being fairly compared.

Mercer International provides an informational document which serves as a review of both the initial and long-term costs that should be considered when comparing oil/water separator designs, and explains how investing in a Compliance Master™ oil/water separator saves clients substantial operation and maintenance capital. Like anything else in this world . . .you usually get what you pay for…or less! Mercer takes great pride in being a leader in oil water separation design. Superior design produces superior results. Superior results make the choice for a premium product easily justifiable to the end user once they clearly identify the long-term benefits and cost savings obtained by a more thoughtfully-designed system. The Mercer advantage offers an end user a better solution with higher performance and substantial operational cost savings over the life of the equipment.

Demonstrating this long-term savings, while protecting the end user from environmental citations and fines, is the focus of this document.

Click here to read more about the True Cost of Ownership.

Vertical Tube Coalescers (VTC)

This Vertical Tube Coalescer (“VTC”) technology is somewhat of a hybrid between a traditional coalescer and a secondary mesh pack. This design does not utilize “stoke’s law” (or the rise rate of oil) as its primary means to remove oil from the horizontal flow of wastewater. Rather, as the oil droplets drift by the polypropylene diamond-mesh tubes they are attracted to the tubes by adsorption (very much the same principal as the secondary mesh pack).

In theory the VTC coalescer should collect droplets of oil and agglomerate them (by coalescence) into larger droplets. The larger droplets are then to break free from the polypropylene matrix and move their way to the surface along the diamond-shaped coalescer. Solids are to simply fall down and out the vertical tubes.

These tubes are heat welded together and equipped with a handle to insert and remove the media from a tank. The idea is to tightly pack the entire volume of the tank with these bundles, and remove them for cleaning when necessary.

Below: a brief exerpt from a recent keynote presentation where lead design engineer Dave Goding was asked to compare the mainstream coalescing media on the market for use in oil water separators.

In Reality

The VTC coalescer’s diamond-mesh pattern that creates a fixed coalescer matrix is too “tight”. The openings for wastewater to flow are between 1/8” to 1/4” – which is way too small for typical industrial wastewater applications. The coalescer gets easily blinded by leaves, plastic(s), and solids that are 1/8” and larger. The fixed, tight matrix does attract oil well. However, at the same time the finer solids drift through the pack, they come into contact with the oil and simply cling to the oil particles—creating an oily sludge deep within the coalescer pack.

In practice, solids do not drift down and out of the pack’s tubes. They simply get caught in the coalescer as solids get caught in any filter. Once plugged up, the packs are almost impossible to clean completely. These packs need replacement more often than most other types of coalescers, and become an ongoing and expensive replacement part.

This is a disposable model, as there is no way to clean fully into the center of each bundle. Higher flow rates can have upwards of 20 of these tube packs that need to be pulled out and cleaned/replaced on a monthly basis.

You get what you pay for, and most times much less. Below is an excerpt from a virtual trade show where lead design engineer Dave Goding discusses the fouling problems that are guaranteed to occur when using honeycomb/egg-carton plastic coalescer designs:

Once inexpensive corrugated plastic media took over as the “standard” in oil/water separator design, it became a race by manufactures to produce separators with low price as their selling feature. Now the oil/water separator industry was selling their equipment on price instead of features & benefits. Once price became the main selling function, many long-standing internal components and sound design standards began to be cheapened or eliminated. By designing down the coalescer, the inlet and outlet baffles, solids handling capacities, etc., manufactures began sacrificing functionality and efficiency. This drastically reduced the end user’s ability to stay in compliance.

Manufactures tightly pack these plastic bundles in separator units, selling promises of higher surface area and (theoretically) better oil removal capabilities.

The “nooks and crannies” within these coalescers are the problem with this design. The various nooks, crannies, ridges and flat spots cause solids to lodge in and on the plates, building up and fouling the packs prematurely. Further, there is no clear path for solids particles to make their way out of the coalescer. The settling solids have nowhere to go and “dead end” within the coalescer causing even more rapid fouling.

To further complicate the problem with this design, these style coalescers are nearly impossible to clean. Because of their design, high-pressure washers can only penetrate from 6”-12” into the packs before the velocity is dissipated.

The cleaning becomes very labor intensive. Many times the packs cannot be cleaned all the way through, therefore the end user is forced to throw away these packs after a few cleanings and replace them with new ones. In certain more high-solids applications or applications with “sticky” oils and solids, this could entail a full coalescer replacement every time maintenance is performed.

Why Oil/Water Separator Manufactures Went The Way Of Plastic Corrugated Coalescing Media Decades Ago

Once those treating oily wastewater realized they no longer needed to take on the large treatment footprint of installing API oil/water separation pits or tanks, they were greeted by several new companies entering the oil/water separation marketplace. They also were faced with the difficulty of weighing one manufacturer's removal promises against another.

In response to this, the American Petroleum Institute (API) released a new set of sizing criteria in Feb of 1990 that dealt with the emergence of "plate pack style" or "Coalescing" oil water separators. This document, API Publication 421, went about laying out several calculations that would assist end users in determining how much surface area was needed to achieve an acceptable removal rate at a given flow. 

At that time, the growing fad within the Oil/Water separator industry was to make the coalescing plates corrugated to give the bundles more structural integrity (much in the way a corrugated cardboard box is able to maintain its shape). Originally this allowed a once flat 1/4" piece of steel to go to an 1/8" thickness or less and save on material costs. The manufacturing process went through an even more drastic cost reduction when these sinusoidal shaped plates began to be fabricated out of different types of plastic. The reason for the corrugation was basically cost reductions, as manufactures all went to battle against each other claiming higher removal rates at bargain pricing. 

The standard practice became to position these progressively slanted/tilted S shaped plates in an upflow or downflow position, so gravity could help in one of the two important aspects of the treatment process--downflow for solids and upflow for oil. The problem with the sinusoidal design, or the CPI (corrugated plate interceptor), was that solids would all concentrate in the  trough areas where they would quickly accumulate. The trade off for producing the required surface areas to keep up with higher removal mandates was a design that fouled very quickly.  

The unequal distribution of solids loading combined with oils or solids having to work against a given flow, gave way to a new set of problems that would become commonplace in oil/water separation. The frustration with the new coalescing variety of oil/water separators back then, is still a problem today.  Oil/water separators that present themselves as low cost options have serious design flaws that foul prematurely and compromise efficiency. 

See Why Mercer Uses A Cross Flow Designed Oil/Water Separator With A Flat Plate 

In January of this year lead process designer Dave Goding had the opportunity to review several of the mainstream coalescing technologies during a keynote at Globalspec's water quality and reclamation virtual conference. Below is an excerpt from his portion that covered CPI oil/water separators.

The Origins

Simple gravity separation is where the marketplace began to deal with the removal of oil from water as well as spill containment.  Many older large holding tanks are commonly referred to as API oil/water separators, referring to the American Petroleum Institute who created the design parameters for certain flow rates. The American Petroleum Institute provides design criteria for oil/water separators. A design method is provided in the API Manual on Disposal of Refinery Wastes, Chapters 5 and 6- Oil-Water Separator Process Design and Construction Details (API publication 1630, 1979).  

The API oil/water separator is a simple gravity separation device designed by using Stokes Law to define the rise velocity of oil droplets based on their density and size. The design of the separator is based on the specific gravity difference between the oil and the wastewater because that difference is usually smaller than the specific gravity difference between the suspended solids and water. Based on that design criterion, most of the suspended solids will settle to the bottom of the separator as a sediment layer, the oil will rise to top of the separator, and the wastewater will be the middle layer between the oil on top and the solids on the bottom.

Advantages of the API separator are simplicity of design, low day-to-day maintenance, and resistance to plugging with solids. 

(Below is an excerpt from a presentation Mercer gave at an online Wastewater conference)

Drawbacks:

API oil/water separators are designed to remove oil droplets down to 150 micron, which is not nearly enough removal necessary to stay compliant today.  Most API oil/water separators are not going to do any better than 50 parts per million as opposed to most state & federal regulations that today require below 20 ppm.

Most industries have abandoned building these large gravity settling vessels because they are simply too costly to justify. Most are built of concrete and with the high cost of labor and the need to reinforce these tanks with steel, new construction costs rise quickly. Plus API units have an excessively large footprint and are far too much of a real estate burden to consider this as a viable option.

Those who have API oil/water separators acknowledge they are easy to operate and maintain on a day-to-day basis, but when it is time to clean out the tank, the cost to pump out the sludge and clean this large space is just a nightmare. Some units have a chain and scrapper mechanism. This helps move the solids to one side to be pumped out, but the automation piece for these are usually as much as the construction of the pit itself—and require significant maintenance as well.

Oil + Solids + Time = SLUDGE 

It would be so much easier if we could simply calculate a flow rate, run some viscosity and specific gravity numbers on the types of oils present in a certain wastewater, and easily come up with a suitable product.  But the fact remains that real life applications are not at all like the perfect scenarios laid out on the 

The most important consideration is the affect the solids present will have on the functionality of the oil water separator. Calculations made that kick out ideal surface area numbers and rise rates are only the first half of the evaluation process. The amount of solids present is the most important consideration; and even more important is the way a potential oil/water separator design is positioned to handle this solids loading.  Even the smallest amounts of solids can present enormous maintenance problems, reduced efficiencies, and lead to drastically more expensive lifecycle costs.

Mercer International has dubbed its Compliance Master™ Oil Water Separators "High Performance" because of the coalescing separation design improvements that were made to process out the solids (as opposed to simply push them through the pack). The outright removal of the solids is the most important way to ensure the resulting effluent is not just compliant...but compliant for a prolonged period of time.

Just because your less expensive purchase based on the sharpened pencil looks like this:

Doesn't mean you don't have to consider what you are going to do in 10 days when it looks like this: