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Why Coolant Becomes Unusable

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Why Coolant Becomes Unusable

Dirty machine coolant is expensive and dangerous

10% annual loss of machine value

Dirty machine coolant is most expensive for the damage it does to machines.   Dirty machine coolant can cause a loss of as much as 10% of the cost of the grinder.  With a $50,000 grinder this is $5,000 a year.  Refer back to the Machine Coolant Index for ways to keep Machine Coolant Clean.

Dirty machine coolant eats rubber, plastic, Plexiglas, insulation and human flesh. 

Machine coolant never really wears out.  Machine coolants and cutting oils become unusable when they become excessively contaminated with bacteria, tramp oil, metal fines, or wastes. They can break down from use but, if water based, are most commonly broken down by bacteria growing in the system.

When a system develops a drastic change in pH, detergency, lubricity, or odor, a bacteria check should be made. 

Bacteria find their way into fluids through a variety of ways. For instance, there are bacteria in the water used for diluting and mixing.  Bacteria from an operator's hands, sweat, and saliva also get into machine coolants.  Bacteria thrive on wet parts and lurk in the air. 

Surprisingly, there are people who find it easy to confuse machine coolant sumps and holding tanks with toilets or food disposal units. Poor housekeeping practices add huge numbers of microorganisms to the system.  Bacteria also live in the sludge that settles in machine sumps and machine coolant flumes. In short, bacteria are everywhere.

Bacteria & Fungi

Bacteria chemically alter machine coolants, destroying lubricants and corrosion inhibitors, while releasing corrosive acids and salts into the fluid.  Microbial action directly affects the machine coolant resulting in the splitting of emulsions, decreased pH, increased corrosion, degradation of the ingredients in the machine coolant and a loss of lubricating ability within the machine coolant.  Odors may develop including hydrogen sulfide as a product of the bacteria’s metabolism.  Bacteria may expose workers to pathogens and contribute to respiratory irritation and skin irritation, like dermatitis.  Work piece quality decreases, resulting in increased surface blemishes, decreased tool life, and increased time to treat for bacteria and repair equipment. The bacteria may also cause increased foaming and separation in the system and cause clogged lines, filters, and valves. 

The presence of wastes, tramp oils and metal fines can stimulate the growth of bacteria. The life span of most water based machine coolants can be extended two or twelve fold with proper management techniques.

 Bacteria and other microbial organisms thrive in the environment created by the impurities in the machine coolant.  Tramp oil, lubricants, hydraulic oils leaked by machinery, minerals in the water, mineral oils, fatty acids, emulsifiers, corrosion inhibitors, other additives, discarded food and waxes are food for microorganisms.

Microbial contamination is a major cause of fluid spoilage.  All water‑miscible fluids are susceptible to microbial deterioration that can significantly reduce fluid life.  Bacteria prefer water‑miscible fluids, because they need water to grow.  Without moisture, most bacteria cannot reproduce and multiply once introduced to a new environment.

Bacteria populations can double as frequently as every 30 minutes.  The greater the bacterial growth rate in a fluid, the faster the fluid becomes rancid.  As bacteria multiply, they produce acids, which lower the pH of the fluid, causing increased corrosion and reduced lubricity.  Bacteria may also darken the fluid significantly, resulting in stained parts.


Most bacteria, which cause fluid to become rancid, are aerobic.  Aerobic bacteria need oxygen for metabolism.  In the typical machine coolant system, there is no shortage of oxygen.  As machine coolants cycle through, they come into contact with air at the surface of sumps and holding tanks.  Fluids pick up oxygen as they are ejected from the nozzle and just about anywhere else machine coolant is exposed to the air.

In the absence of oxygen, they will continue to survive, but grow very slowly until oxygen is reintroduced.  Once they can "breathe" again, facultative bacteria will resume reproduction.  Aerobic bacteria reproduce by dividing in two approximately every 20 to 30 minutes.  A single well‑fed bacterium can produce billions of others in less than half a day. 


Anaerobic bacteria grow in oxygen‑poor environments.  They grow in systems that are inactive for long periods of time. Inactivity allows tramp oil to rise to the top of the sump, creating an effective barrier between the metalworking fluid and atmospheric oxygen.  Consequently, the amount of oxygen present in the fluid decreases, aerobic bacteria die, and anaerobic bacteria begin to flourish.  They feed upon the machine coolant and produce hydrogen sulfide, which produces the rotten‑egg odor, which causes Monday morning odor.

This type of bacteria grows much more slowly than the aerobic, dividing in two approximately every four hours.  Anaerobic bacteria will usually not grow until aerobic bacteria have first attacked the fluid and the oxygen is depleted.  Some components of machine coolant‑emulsifier systems are naturally toxic to anaerobic bacteria; that's why adding fresh machine coolant makeup to stinky machine coolant will "freshen up" the mix but only for a short time.


Besides bacterial growth, there is another whole group of microorganisms that can infect machine coolants.  Collectively, they are referred to as fungus, which, in reality, is a combination of molds and yeasts.

Fungi degrade metalworking fluids by depleting rust inhibitors.  Fungi also cause musty or mildew‑like odors and form slimy, rubber‑like masses on machine system components that may eventually plug fluid lines. 

While bacteria are essentially animal‑like and grow rapidly as discrete cells or particles within the fluid, fungi are plant like, growing in layers on surfaces (e.g., inside pipes).  Bacteria and fungus compete for the same food sources, and because of bacteria's animal‑like nature, they can generally out compete fungus.  As a consequence, if a machine coolant system has a moderate, stable (neither rapidly increasing nor decreasing) bacterial population, the bacteria will generally control fungal growth. 

Conversely, if a machine coolant system has a consistently low bacterial population and the fluid will allow fungal growth (soluble oils tend to support bacterial growth and chemical [synthetic] fluids tend to support fungal growth), the fluid will likely develop a significant fungal population eventually.

Fungus can be even more of a problem than bacteria because fungal accumulations can interfere with machine functioning and can plug machine coolant filters and piping.  While there are a number of effective microbiocides that can be added to machine coolants to control bacteria, there are very few effective fungicides for use in machine coolants. 

Bacteria Control

The more rapidly bacteria grow, the faster they alter the fluid. That makes controlling the rate of bacterial growth extremely important.  If the rate of growth is limited, the harmful effects are reduced.  Biological growth is controlled through a combination of practices. These include water quality control, proper maintenance of fluid concentration and pH, routine equipment maintenance, biocide treatment and aeration.

How Machine Coolants Get Contaminated

1.  Water used for mixing and diluting.

2.  Sludge in the machine sump or machine coolant flumes.

3.  Parts, tools, and other objects in contact with the fluid.

4.  Operators' hands, sweat, and saliva.

5.  Extraneous contaminants such as food scraps, human waste, etc. 

Factors that promote the growth of bacteria include

1. Oxygen and organic nutrients.

2. Low pH (salts from water) and low temperature.

3. Lubricant carry through.

4. Poor hygiene throughout the plant.

5. Hydraulic leaks, tramp oil, and greases.

6. Chemical additives.

The top 10 ways to minimize bacterial attack.

1.  Select high quality, bioresistant fluids.

2.  Incorporate germicides judiciously.

3.  Practice good housekeeping.

4.  Remove tramp oil contamination.

5.  Remove fines and other particulate matter (the better the filtration, the lower the bacterial growth).

6.  Use pure, mineral free water.

7.  Clean machines and sumps thoroughly and regularly.

8.  Remove sludge from sumps and flumes.

9.  Use machine coolant friendly cleaners on tools and workspace areas

10. Train machine operators on machine coolant maintenance. 


The best way to effectively control bacterial growth is good housekeeping. 

The best method for controlling biological growth is through routine cleaning of machines, machine coolant lines, and sumps/reservoirs.  Machines, exhaust blowers, and hydraulic seals should also be maintained to prevent oil leaks from contaminating the fluid. 

Train your employees to keep trash, solvents, tramp oil and other foreign material out of the machine coolant system. 

It does little good to put fresh machine coolant into a dirty machine if you're only going to suck out the old before putting in the new. This practice provides fresh food for the bacteria that are in the swarf and sludge that remain in the machine sump, on machine surfaces, and in the machine coolant circulating systems. 

Use good water

The minerals present in most water can destroy desirable machine coolant properties and form deposits.  In addition, Fungi feed on dissolved minerals in water. Controlling the mineral content of the water used for metalworking fluids can control fungi growth.  Consider using deionized water when mixing machine coolant formulations.

 Fluid selection

One of the most important methods of keeping bacterial growth in check is to be sure that the cutting fluid manufacturer you select uses high purity materials for its products.  Bacteria have very specific appetites and some materials are much better food for them than others. 

Fluid manufacturers that understand the biology of microorganisms select raw materials that bacteria do not find appetizing. Machine coolants formulated with materials bacteria find undesirable are less vulnerable to bacterial attack. 

Clean machine coolant in process

Screen out and recycle metal fines.  Remove tramp oils and metal chips. 

Accumulations of chips and fines in a sump promote bacterial and fungal growth. These particulates increase the surface area available for microbial attachment, and biocides cannot effectively reach the fluid trapped in these chips and fines. Particulates in the bottom of a sump become septic or rancid if not periodically removed.

Even if the majority of the fluid is free of bacteria, the sludge in the bottom will continue to harbor bacteria and create a septic condition. This can dissolve metals, possibly increasing the toxicity of the fluid to a level at which disposal through a local wastewater treatment plant is no longer permitted.  Laboratory analysis will reveal whether the toxicity of the fluid makes it a hazardous waste. 

Some bacteria prefer oil as a food source, so they tend to grow rapidly in those machines that leak substantial amounts of lubricating and hydraulic oils.  Consequently, everything should be done to reduce tramp oil contamination.  Contaminant oils and greases that do make it into machine coolant should be skimmed off the surface or centrifuged out of the fluid.

Monitor your sumps

Monitor machine coolant quality.  Know when you must add make up or additives.  Know when bacterial growth is about to get out of hand.  One way to do this is by keeping records of when and how much additives or make up machine coolant are added.  Signs of bacterial growth should also be recorded. 

Many machine coolant concentrates contain biocides and pH buffers.  Therefore, preventing fluid from becoming overly diluted helps control microorganisms.

Ideally, the pH for water miscible metalworking fluids should be kept in the limited range of 8.6 to 9.0.  This slightly alkaline range optimizes the cleaning ability of the fluid while preventing corrosion, minimizes the potential for dermatitis and controls biological growth.  If the pH drops below 8.5, the fluid loses efficiency, can attack ferrous metals (rusting), and biological activity will significantly increase.  A pH greater than 9.0 may cause dermatitis and corrosion of nonferrous metals.

Regular monitoring of a fluid's pH is a simple means of anticipating problems.  Fluid pH should be measured and recorded daily after the machine is placed in operation.  Steady pH readings give an indication of consistent fluid quality.  Swings in pH outside the acceptable range indicate a need for machine cleaning, concentration adjustment or the addition of biocide. Each action taken to adjust the pH to the desired operating range should be documented in the machine logbook and evaluated for effectiveness.  Any rapid change in pH should be investigated and action should be taken to prevent damage to the fluid. 

Although fluid pH usually remains constant because of buffers contained in the concentrate, it can change after initial mixing due to water evaporation.  Improper control of microbial growth will also alter fluid pH.  Byproducts of microorganisms produce offensive odors and lower fluid pH.  As the fluid becomes rancid or septic, it becomes more acidic.  Sudden downshifts in pH usually indicate increased biological activity or a sudden change in concentration due to contamination.  If machine coolant concentration and pH both jump downward, the sump has been contaminated.  If machine coolant concentration remains fairly constant while pH decreases, biological activity has probably increased significantly. 

Weekly or biweekly monitoring is typically recommended for detection of microbial contamination, especially during the early stages of developing a fluid management program.  With experience, machine shops may determine that a less frequent monitoring schedule is suitable for their operation.

Biocides & other means

Germicides can be helpful in preventing or retarding bacterial degradation.  However, very few germicides are truly effective in machine coolant systems, and they must be used with great care.  Moreover, those biocides that have proven to be most effective often cannot be used in all types of machine coolants.

When machine coolants require the addition of biocides, it's important to carefully follow the supplier's recommendations.  Do not add too little or too much machine coolant.   Too little can actually stimulate microbial growth, while too much can cause operator skin irritations.  In short, never add any chemicals to your machine coolants without specific instructions from your machine coolant supplier.

The addition of biocides inhibits biological degradation of the fluid by controlling bacteria and fungi.  Relying strictly on biocides for microbial control is discouraged since these chemicals are expensive and can create hazards for the operator's skin.

Generally, biocides should be used sparingly in as low a concentration as possible.  Due to the variety of bacteria that may be present in a fluid, use of only one biocide may control certain bacterial species while allowing others to proliferate. Random use of various types of biocides may prove to be more effective.

Biocide treatment patterns play an important role in controlling microbial growth.  During one study on biocide treatment patterns, fouled fluids were treated with a commercial biocide at various concentrations and frequencies while microorganism populations were monitored.  For all biocide application rates tested, the efficiency of antimicrobial control was found to vary widely with treatment pattern.  Less frequent doses with higher concentrations of biocide were found to be much more effective than low level, frequent doses.

Use pasteurization or non-chlorinated biocides to reduce the growth of bacteria.  Be aware that pasteurization cannot prevent rapid regrowth of bacteria while biocides provide continuous protection.

Some businesses have found that using an aerator and diffuser rod purchased at an aquarium shop can reduce bacterial growth.  The aerator is used when machines are off line for an extended period such as a weekend.

Aeration can be used in conjunction with biocide additives to control anaerobic microbial growth in systems during periods of inactivity.  Aeration oxygenates the fluid, producing an atmosphere hostile to the odor producing anaerobic bacteria.  While the machine is running or sitting idle, a small pump can bubble air into the sump, either continuously or periodically, to agitate stagnant areas within the sump.

Clean sumps

Sanitize sumps and machinery on a regular basis.  Remove all old machine coolant and sludges.  Flush out comers.  It's no use putting good machine coolant into bacteria laden equipment.  If necessary, retrofit sumps to make cleaning easier by lining with sheet metal or epoxy and creating rounded corners.

If a machine is thoroughly cleaned with a proper cleaner, thoroughly rinsed, and then filled with clean, fresh fluid, the fluid will last four to six times longer than it would in a machine that is simply "sucked out" and recharged with fresh machine coolant.

Central Systems

Central systems generally involve quantities of fluid three to five times the amount used for an individual machine.  As a result, if the bacteria in a central system grow at the usual rate, the proportionately smaller addition of fresh makeup will give the bacteria a greater period of time in which to break down the machine coolant than would be the case with individual machines.

Further compounding the problem, bacteria tend to settle to the bottom of tanks.  In any system where the fine metal particles and other silt settle, so do the bacteria.  This combination allows the anaerobic bacteria to achieve full growth potential, since they have plenty of food and are far away from the oxygen at the fluid surface.

Consequently, it is usually more difficult to control bacterial growth in large central systems than it is in individual machine sumps.  However, proper cleaning of such systems with each machine coolant change and good filtration to prevent sludge accumulation can help keep bacterial growth under control.