Coffee is mostly water. Roughly 98%. So if the water is off, the cup is off. Simple as that.
We noticed this the practical way. Our filters had run longer than they should, and the cups changed before the logs made it obvious. Flatter profile. Less lift in the finish. Nothing dramatic, just less alive. That is exactly how water issues usually show up, quietly.
What water actually does
Hot water dissolves compounds from ground coffee in sequence. Acids and lighter aromatics come out early. Sugars and body follow. Later on, bitter compounds start to dominate. Water is not neutral in this process. Its mineral profile decides what gets extracted, how quickly, and in what balance.
Two parameters matter most: hardness and alkalinity.
Hardness: calcium and magnesium
Hardness is mainly dissolved calcium and magnesium. Both help extraction, but not in the same way.
Calcium supports structure and body. Too little can make cups feel thin. Too much can make them heavy.
Magnesium tends to pull aromatic compounds more effectively. That is one reason the same coffee can taste more expressive on one water source than another. The Specialty Coffee Association (SCA) target for total hardness is 50 to 175 mg/L as CaCO3, with 75 to 150 mg/L being a practical working range for most bars.
Alkalinity: where brightness gets lost
Alkalinity is mostly bicarbonate. In water treatment, bicarbonate is useful because it buffers pH. In brewing, it can work against you.
Coffee acids are part of what gives clarity and energy in the cup. If alkalinity is too high, bicarbonate neutralizes those acids during extraction. The result is familiar: a cup that is not clearly bitter or sour, just muted.
SCA guidance for alkalinity is 40 to 75 mg/L as CaCO3. Above 100 mg/L, flatness usually becomes consistent. Above 150 mg/L, quality drops hard, regardless of recipe.
TDS: useful, but incomplete
Total dissolved solids (TDS) is the sum of dissolved minerals. SCA target range is 75 to 250 mg/L, with 150 mg/L as a common midpoint.
TDS by itself is not a flavour guarantee. Water can read 150 mg/L and still brew dull coffee if alkalinity is high relative to hardness. We see that pattern often in Dutch tap water.
A practical rule is this: keep hardness clearly above alkalinity. A 3:1 ratio is a strong starting point, for example 150 hardness and 50 alkalinity.
What filtration changes, and what it does not
Activated carbon removes chlorine and some off-flavour compounds. It does little for hardness and alkalinity.
Ion exchange reduces hardness, and with the right cartridge can also lower alkalinity. This is common in under-counter café filtration systems.
Reverse osmosis removes almost everything. Great for scale control, but too empty for coffee unless you remineralize downstream.
Every filter has a rated capacity. Once that capacity is exceeded, performance slips gradually. That is the tricky part. Water does not fail all at once. It drifts.
You can still get a normal TDS reading while the balance has already gone wrong. Then people start changing grind and dose for a problem that is actually in the water.
Track filters by litres processed, not calendar age. A busy bar can exhaust a cartridge in weeks. A quiet site may take months.
How to test your water
A basic TDS meter is a good first step and costs very little. Add a GH/KH test kit if you want a usable view of hardness versus alkalinity. Together, these two tools are enough for most café diagnostics.
If you need more precision, send samples to a lab for full ion analysis. That is worth doing when you are building a new setup or chasing persistent inconsistency.
Our Fortin target
We aim for water around 100 to 150 mg/L TDS, with hardness clearly higher than alkalinity. In practice, about 100 mg/L hardness and 40 to 50 mg/L alkalinity works well for us. Brightness stays intact, body stays present.
Early warning signs are consistent too: espresso feels flatter and V60 loses its clean finish. Once you know that pattern, water becomes one of the most reliable diagnostics on bar.
And unlike many other variables, this one is highly controllable.
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