Hard and Soft Water: Where It Helps, Where It Harms, and Where the Line Is

Soft water is not the same as good water. A boiler needs it, a drinking water line does not always need it, and for a steel pipe excessively soft water is directly harmful. Let's go through the figures and the boundaries, unit by unit.

The Units You Need to Speak

Three hardness measurement systems appear in documents most often. GOST 31865-2012 defines the degree of hardness and the conversion factors:

• 1 °Zh is numerically equal to 1 meq/L and corresponds to 20.04 mg of Ca²⁺ or 12.15 mg of Mg²⁺ per liter
• 1 °Zh = 2.80 °dH, the German degree
• 1 °Zh = 50.04 ppm CaCO₃, the American notation expressed as calcium carbonate
• 1 °Zh = 5.00 °F, the French degree

Keep this conversion at hand. Instructions for European equipment are almost always written in °dH, laboratory analyses in Uzbekistan in meq/L, and English-language studies in mg/L CaCO₃. An error in the factor of 2.8 or 50 changes the entire conclusion.

The Hardness Scale and Where Uzbekistan Sits on It

The World Health Organization, in its background document on hardness, gives the following classification in terms of CaCO₃:

• soft: less than 60 mg/L, that is, less than 1.2 meq/L or less than 3.4 °dH
• moderately hard: 60–120 mg/L, that is, 1.2–2.4 meq/L or 3.4–6.7 °dH
• hard: 120–180 mg/L, that is, 2.4–3.6 meq/L or 6.7–10.1 °dH
• very hard: over 180 mg/L, that is, over 3.6 meq/L or over 10.1 °dH

Uzbekistan's sanitary requirements allow total hardness of drinking water up to 7 meq/L, and up to 10 meq/L where justified. Converted, this is 350–500 mg/L CaCO₃ or 19.6–28 °dH. On the WHO scale this is firmly in the "very hard" category. The standard is not violated, but equipment does not make this distinction.

Field data confirm the picture. A 2024 study of the Chirchik and Akhangaran catchments places virtually all sampled waters in the category of hard freshwaters. Calcium in the samples ranges from 27 to 221 mg/L, magnesium from 3.4 to 46.5 mg/L, and dry residue from 156 to 878 mg/L, increasing downstream.

The conclusion for engineering design is simple. Water in Tashkent is hard. In approximately 85 percent of the city and in all other regions of the republic, the water is hard and of low quality. There are individual districts with acceptable quality, but they are the exception, and even there old trunk mains cause secondary contamination, iron and suspended solids.

Where Hardness Is Actually Harmful

The hardness problem is almost entirely technical, not medical. WHO states explicitly that the data are insufficient to establish a minimum or maximum mineral concentration, and it does not set a guideline value for hardness.

The technical consequences, however, are measurable:

• deposits on the heat-exchange surfaces of boilers, water heaters and heating elements
• fouling of the channels in brazed plate heat exchangers and heated towel rails
• clogging of aerators, shower heads and nozzles, as confirmed in the 2009 Battelle tests
• higher consumption of detergents and limescale marks on plumbing fixtures

The German standard VDI 2035 ties the permissible hardness of make-up water to the output of the heat source. For 50–200 kW it is about 11.2 °dH, for 200–600 kW about 8.4 °dH, and above 600 kW less than 0.3 °dH. The higher the thermal load on the surface, the lower the permissible hardness.

Where Soft Water Is Harmful

Here begins the second half of the topic, which is discussed less often.

The WHO background document on hardness puts it this way: unstabilized soft water has a pronounced tendency to corrode metal surfaces and pipes. The consequence is not abstract: lead and copper pass from pipes and solder into the water.

The review by Kozisek, included in the WHO collection "Nutrients in Drinking Water," describes the mechanism in harsher terms. Demineralized water is aggressive, and distributing it through pipes and storage tanks without correction is not possible. The document cites a case of lead poisoning in three infants, in which lead was leached from brass fittings and soldered seams of tanks, and low-mineral water intensified the process. Concentrations in the samples reached 495–1050 µg/L.

The practical meaning for installation work. If you install a water softener for the whole apartment or the whole house and bring hardness down to zero, you have solved one problem and created another. This applies especially to systems with steel and galvanized sections, brass fittings and soldered joints.

How Much Calcium and Magnesium a Person Needs

The same Kozisek review gives reference values for drinking water:

• calcium: minimum 20 mg/L, optimum 40–80 mg/L
• magnesium: minimum 10 mg/L, optimum 20–30 mg/L
• total hardness: optimum 2–4 mmol/L, that is, 4–8 meq/L or 200–400 mg/L CaCO₃
• recommended upper limit: below 5 mmol/L

Note the contradiction. According to this source, the medical optimum lies in the range of 4–8 meq/L, which on the WHO scale already counts as very hard water. For a boiler, such water is excessively hard. This is not an error in the data, but different criteria for different tasks.

Where the Line Lies

There is no single correct hardness value, because each circuit has its own task:

• heating circuit and boiler make-up: hardness is reduced as required by the manufacturer and by VDI 2035, down to near zero at high capacities, since no one drinks this water
• domestic hot water and process water: softening to values at which scale does not form, usually retaining some residual hardness to protect pipes from corrosion
• drinking water line: zero is not needed here. Calcium and magnesium in drinking water matter, and full demineralization requires subsequent remineralization
• any circuit in an old building: mechanical filtration and iron removal are needed before softening, otherwise the resin will fail ahead of schedule

This leads to the practical approach. A water softener is set not to the minimum but to a target value, and the drinking water line is usually provided with a bypass or blending with the raw water.

What this means for a project in Tashkent

• Analysis first, then the design. You need total hardness, calcium, magnesium, iron, sulfates, dry residue and turbidity. Without these figures, sizing a water softener is impossible.
• Do not bring hardness down to zero across the entire project. Water that is too soft is aggressive to metal, and the drinking water line loses calcium and magnesium to no benefit.
• Separate the circuits. Boiler feed, domestic hot water and drinking water require different degrees of treatment, and a single unit for the whole house rarely solves all three tasks correctly.
• Even in districts where the raw water quality is acceptable, plan for mechanical filtration. Old trunk mains cause secondary contamination regardless of the analysis at the water intake.