Water filtration and its effects on coffee


Water is probably the most difficult topic in coffee. You need a basic understanding of chemistry to even be able to explain how a filtration system works. It is hugely important that baristas and roasters understand how water impacts their coffee.

For the sake of that, I feel a need to write a few words about a major contribution to water quality - filtration.

Before diving in, I'd like to disclaim how humble my experience in the topic is. My chemistry knowledge is at best around the 8th grade level. I have only worked with filtration systems provided by 3M and BWT. I have only made coffee professionally in London and around. I have not even read Water for Coffee (a few short extracts only).
I will list my references at the end of the post.

As a coffee professional, I pride myself on being able to taste hardness of water through the coffee. I'm aware that I'm able to do that only in certain conditions. And even when I'm convinced I test the water anyway.

I owe that skill to great training and experience of working with badly maintained water treatment equipment.


Water filtration systems


There are many types of water filtration systems. It's important you understand how exactly your system works.

1. Softener - Calcium and Magnesium are exchanged for Potassium or Sodium. That method doesn't change alkalinity (what we like to call carbonate hardness). Thus it's an unsuitable solution for high alkalinity water (like London), coffee will be flat, lacking acidity and more bitter.

2. Decarboniser - Calcium and Magnesium are exchanged for Hydrogen. Change in alkalinity equals the change in hardness (theoretically). This method lowers the pH of the water, increases its CO2 content. I will outline its dangers later, describing Carbonate Cycle.

3. Demineraliser - ion exchanger or Reverse Osmosis. Those remove minerals from water, both alkalinity and hardness are reduced, no additional Carbon Dioxide is produced. The results of measuring RO water and single cartridge filtrations are non-comparable.

The list is not inclusive.

How to test the water

To avoid situations when filtration is impacting the taste of coffee in a negative way, I needed to be on top of water testing.
I've designed a sheet to help with that.



For that, I used a BWT water testing kit, thermometer, conductivity meter and pH meter. Let's describe them:

BWT water testing kit - after researching titration methods I'm not sure what exactly is measured. It states it measures "Total hardness" and "Carbonate hardness". I could not figure out what indicator is used for the former, but the latter is most possibly EDTA.

Thermometer - the temperature will affect other readings, particularly pH and conductivity

Conductivity meter - so-called TDS meter, it's a quite inaccurate tool, but in a particular setting with sampling at the same temperature, it can provide valuable information. Measuring hardness with this on decarbonising filters could trick you. It might serve as such an indicator when using RO.

pH meter - knowing pH and level of carbonate hardness you can derive CO2 content in your water. Why would that be important?

Carbonate cycle

There is a correlation between pH and form of carbon in the water. Specifically, how much carbon is present in the forms of Carbon Dioxide, Bicarbonate and Carbonate.

I understand this is a simplified, partial explanation of the carbon cycle, but we don't need more to understand how water filtration works.

Let me illustrate that:




The amount of Carbon Dioxide in your filtered water matters a lot. When the water containing a significant amount of CO2 enters the espresso basket, it creates additional resistance. It does so in an identical way as a freshly roasted coffee.

You can check how much Carbon Dioxide is present in your water if you know its Carbonate Hardness and pH.




Unfortunately, a very popular method based on Decarboniser Cation Exchanger produces a lot of Carbonic Acid, which leads to excess dissolved Carbon Dioxide in the water. That may explain why you need to degas your coffee for longer than recommended to reach its peak extraction.

Have you ever noticed that cafes with RO degas their coffee for shorter periods while using the same coffee?



Summary and outlook

I'm far from saying RO filtration is superior. For a long time, I was convinced that RO removes too much Magnesium, which is a flavour carrier. I'm not as convinced about it as I used to. I would need to do more research on that.

Another theory I have is that RO based coffee shops would benefit more from decreasing the pressure on their machines.
My reasoning is that CO2 creates additional resistance - so a barista would make the grind size courser to compensate for that and achieve reasonable contact time. But that leads to underextraction.

Now whether that CO2 comes from coffee being very fresh or decarboniser as a water treatment tool it doesn't matter. Increasing the pressure would put more force to counter the resistance of the basket, coffee and CO2. Do you find that very freshly roasted coffee benefits from higher machine pressure?

Understanding your water helps to understand your coffee. For now, this is still a new and unexplored topic. Definitely, it's not common knowledge even among roasters and coffee shop owners, let alone baristas trying to make their coffee taste good.

You can find more information on the hardness of the water on aquarist's forums - that hobby seems to attract more chemists - than coffee ones. There are great things happening thanks to people like Chris Hendon. People are making their own water for coffee. Products like Third Wave Water and Peak Water get a lot of attention. That's great! But don't forget about water in high volume operations.

I encourage everyone in the coffee shop, roastery or on the competition stage to take water seriously and be aware of its impact.



References and links

- Folmer, B. The Craft and Science of Coffee; Elsevier Science, 2016.
- Titrations.info EDTA - determination of water hardness
- Chemistry.bd.psu.edu - Determination of the Hardness of Water
- SimplyDiscus - Relationship between CO2, KH and pH

- Title Photo by Imani on Unsplash

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