Why Is My Coolant Smelly?

Although you might not be able to see bacteria, at some point you’ll likely be able to smell it.

Bad-smelling coolant is very commonplace throughout the metalworking industry, but that doesn’t mean it should be normal. At Excision, we believe that you should pay attention to what your coolant looks and smells like.

The most common reason coolants smell is due to bacterial growth. That means, to understand why your coolant smells, you first need to understand how and why bacteria grows in your coolant.

Throughout this article, you’ll learn what bacteria is (and how it creates the dreaded ‘Monday Morning’ stench), what conditions make the bacteria grow faster (and the smell worse), and finally how to fix the issue.

Smelly coolant isn’t just inconvenient, it is expensive, which means it’s important you get to the bottom of the cause and understand why it’s happened.

Read on to find out.

(Note: we understand that some coolants are smelly from the start, due to sulphur, chlorine of other additives. What we’ll be discussing in this article is related to smell caused by the growth of bacteria.)


A short lesson in bacteria

Bacteria comes from multiple places, such as the water used for diluting and mixing, sludge in the machine sump, parts, tools in contact with the fluid, or the sweat on your hands.

It’s important that you understand there are two different types of bacteria: Aerobic (require oxygen to survive) and Anaerobic (doesn’t need oxygen to grow).

Aerobic Bacteria

This bacteria thrives on oxygen. There isn’t any shortage of oxygen in your coolant system, which means that aerobic bacteria often have many opportunities to thrive. They come into contact with oxygen at the surface of the sump, where the fluid is ejected from the nozzle, and just about anywhere else the fluid is exposed to the air.

Anaerobic Bacteria

Anaerobic bacteria grow much more slowly, in a very low-oxygen environment. They won’t grow until the aerobic bacteria have first attacked the fluids in your tank and removed all the oxygen from it.

It is in this stage that the coolant will develop the rotten-egg stench. As the bacteria feed upon the machine coolant, it produces hydrogen sulfide (the Monday morning odour) which is not pleasant by any standard.

It’s important to remember that you can’t have one without the other. Aerobic bacteria come first, depleting the oxygen, which allows the anaerobic bacteria to grow, creating the smell.

Even if your workshop is as sterile as a medical lab, you’re not ever going to be able to completely prevent your coolant from going smelly. When mixed with water, coolants will always have some level of bacteria in them. There is no way to avoid this. But that doesn’t mean you should give up all hope of controlling it.


What conditions make coolant smell worse?

First up, you need to know what makes bacteria grow faster, and how they create the smell.

Five key factors make bacteria grow faster. The more rapidly the bacteria grow, the faster they’re going to make your coolant reek. And when you consider that one single aerobic bacterium can produce over a billion others in less than 12 hours, things can go from bad to worse quickly.

1. Stagnant Coolant

How many times have you come back on Monday morning to a workshop that reeks of rotten eggs?

Bacteria take advantage of stagnant coolant tanks. This is where a lack of oxygen depletes the aerobic bacteria and allows the ‘smelly’ bacteria to attack. It’s not pleasant, to say the least.

2. Excessive tramp oil

Tramp oil sits in a layer above the coolant sump. This layer restricts the coolant in the sump from having contact with oxygen, allowing anaerobic bacteria to grow.

3. Poor water quality

Mixing coolants with water that is poor in quality, and already has bacteria in it, allows that bacteria to grow easily.

4. High temperatures

Bacteria thrive in warmer conditions, so warmer weather combined with any of the above conditions only exacerbates the growth.

5. Low concentration

Generally, coolants are designed to have enough biocide in them to keep bacteria under control when mixed at a ratio of 5%. This means that when coolants are mixed below this concentration, there isn’t enough biocide, and bacteria can grow rapidly.

Again, you need to remember that you’re not just creating bacteria, you’re allowing it to grow at a faster rate. And given that they grow exponentially, once bacteria get started in a tank it requires some treatment to kill it, or a complete tank clean out.

Your coolant can smell when it’s brand new. Smelly coolant can be caused by the additives that are used in the manufacturing process. The most common additives that cause a pungent smell is Chlorine and Sulphur. Both are used as EP (Extreme Pressure) additives. Also, mineral-based oils can tend to have an engine oil smell – this is normal and due to the base oil being petroleum based.


How can I fix it?

If the smelly coolant is brand new, check the ingredients. It will most likely will have Chlorine or Sulphur as these are cheaper EP additives. Most manufacturers do not state what is in the coolant so the safest option if you're not sure is to find one that says it is free of Chlorine and Sulphur.

Nonetheless, if the smell has come from bacteria growth, you’ll need to follow a process (like the one below) to kill the bacteria.

1. Empty the tank

The most effective way to remove bad-smelling coolant is to empty the entire tank, clean it out and start again. We can almost guarantee this isn’t your favorite job, but it just has to be done in some cases. We wish we had a magic solution, but once coolant has gone off, there is no way to get rid of it other than replace it.

When you’re doing this it’s important to run some system cleaner or shock biocide through the system to make sure that all of the bacteria in the tank is killed before thoroughly cleaning it out. This is because it’s hard to get all coolant out of the hoses, and any remaining coolant with bacteria will infect the new batch before you have even used it (making all of your efforts useless).

You should then clean the tank with fresh water, and add a new emulsion mixed correctly at the right ratio.

We understand that you might not always prefer this method due to the cost involved in emptying a large tank, and the time it takes to get it right – but if the coolant already reeks, it’s likely beyond saving (no amount of added coolant and additives will prevent it going smelly in the future).

2. Check the concentration level

If we’re going to fix the bacteria issue from within the tank, the next step is to check the concentration level of the coolant. If you don’t know how to test the concentration level, you can find out by clicking below.

How do I check my coolant concentration levels?

Once you’ve worked out the concentration, you can determine whether this was a factor in creating the smell. If the concentration is significantly lower than 5%, then this is most likely the case.

If you’re keeping an eye on the concentration level of your coolant, then you’ve got a chance at catching the bacteria before it becomes smelly. You can simply top up the tank at a higher concentration, and therefore bring the entire sump concentration higher. This will increase the amount of biocide in your sump and slow down the bacteria growth.

If you need to work out how much coolant and water to add to your tank, click here: Coolant Concentration Calculator – Excision Pty Ltd

3. Add tank-side biocide

If the bacteria is already rampant, or the concentration is above 5% then adding more coolant into the tank won’t be enough to remove the smell. In this instance, you’ll need to do some additional treatment. You can achieve this by adding tank-side biocide (which is added to the dilution or sump rather than the concentrate).

There are 2 main types of biocides that can be added:

  1. Shock treatment
  2. Maintenance treatment

In some instances, shock treatment will be enough to kill the growing bacteria and remove the smell. This is especially the case when combined with increasing the concentration. This is because the added concentration will include maintenance biocide. In other instances, a maintenance biocide will be needed to keep the bacteria at bay.


How do I prevent it from happening again?

This is the big question. Once the bacteria growth has been slowed down and removed, your last step is to ensure that the conditions that created this bacteria growth are removed, because you don’t want to have to go through this process again.

If we explained that here, it would make this article too big, so we created another blog that explains the 5 steps to prevent bacteria from growing in your coolant.

Related Articles

How Do I Test The Concentration Level In My Coolant?
How Do I Maintain My Coolant Over Summer?

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