Water: the foundation of tea

When brewing tea, warm water absorbs aromatic compounds, polyphenols, caffeine, and amino acids from the tea leaves. The easier this process, the more flavour ends up in the infusion. Minerals in the water influence that process. Calcium and magnesium, the substances that make water "hard", bind to polyphenols and inhibit extraction. The result is a flat cup with a cloudy surface. With green and white tea, where the aroma is often delicate and subtle, this is immediately noticeable. With a robust black tea the effect is smaller, and sometimes even beneficial.

Alongside hardness, pH plays a role. Slightly acidic to neutral water (pH 6 to 7.5) gives the best results. Water with a high pH (alkaline water, the opposite of acidic) accelerates the oxidation of catechins in green tea, leading to bitter infusions. Scientific research confirms that hard water with a high pH significantly reduces the catechin content in green tea.

Beyond hardness and pH, there is chlorine. Almost all Belgian and Dutch water treatment plants add chlorine to keep water safe. But what is good for public health is not always equally good for tea: chlorine has its own flavour that can disturb the delicate aromas of green and white tea. Full evaporation of chlorine requires 24 to 48 hours in an open jug at room temperature, or 15 to 20 minutes in boiling water. A faster and more reliable solution is an activated carbon filter, such as in a BRITA jug or comparable system.

TDS: the key concept

Anyone who delves deeper into water for tea will quickly encounter the term TDS: Total Dissolved Solids, the total amount of dissolved substances, expressed in milligrams per litre (mg/l) or parts per million (ppm). It is a rough measure of how many minerals are present in the water.

The Tea Association of the USA recommends 50 to 150 ppm as ideal for most tea types. For delicate teas such as a fine green or white tea, a TDS of 30 to 80 ppm is better. For a robust black tea, it may go slightly higher, even towards 150 ppm.

A few reference points:

• Belgian tap water: usually contains 200 to 400 mg/l, in parts of Flanders even higher
• Spa Reine: approximately 38 mg/l. With its low mineral content and ideal pH balance, it is an excellent choice for tea lovers. It produces a clean and refreshing flavour, allowing the characteristics of your tea to come through more clearly.
• Montcalm: 32 mg/l, from the French Pyrenees. The low mineral content creates a good balance between the tea leaves and the water.
• Mont Roucous: around 22 mg/l. Known for its exceptional purity, it provides a fine base for brewing tea. The low mineral composition produces a soft and gentle flavour profile.
• Evian: 357 mg/l, too high for delicate tea
• San Pellegrino: approximately 915 mg/l, not suitable for tea

Differences by tea type

Not every tea requires the same water:

• Green and white tea are the most sensitive. Their aroma comes from delicate amino acids and catechins that quickly disappear in hard water. Water below 80 mg/l TDS is recommended here, ideally around 50 mg/l. A Chinese Long Jing or a white tea from Sri Lanka loses its character in tap water.
• Black tea is more tolerant. The entire British tea culture developed on hard English water. A robust black tea, such as an Assam or an English breakfast, softens even in slightly hard water. Up to 150 mg/l is fine here.
• Oolong tea sits between green and black tea. Light, floral oolongs such as a Tieguanyin or a Baozhong require softer water. Heavier, roasted oolongs behave more like black tea.
• Pu'erh calls for experimentation. Some gushu pu'erh teas appear to benefit from water with a degree of mineral structure.

Tap water in Belgium

Belgium generally has medium-hard to hard water, with considerable regional variation. Flanders has water with TDS values that vary significantly by region, reaching 300 to 400 mg/l in some municipalities. Limburg typically has softer tap water. Wallonia is usually softer due to different sources. Brussels, which draws most of its water from Wallonia, ranges from medium-hard to hard depending on the municipality, according to Vivaqua.

Official figures per municipality can be found with your water supplier: in Flanders usually De Watergroep, Pidpa, or FARYS; in Brussels Vivaqua; in Wallonia SWDE.

In closing

Good water does not make mediocre tea great, but bad water makes great tea mediocre. Anyone investing in high-quality loose-leaf tea would do well to consider the water used to brew it. A simple carbon filter solves the biggest problem for most people.

Those who want to make the comparison can brew the same tea with tap water, filtered water, and for example Spa Reine bottled water.