Water hardness is a measure of certain varieties of calcium and magnesium, the most common of which is calcium carbonate.
A hardness measurement is always lower than the TDS (total dissolved solids) as hardness is included in TDS but not all TDS is considered hardness.
The Atla System is perfectly compatible with very hard water.
General guidelines for classification of hardness are: 0 to 60 mg/L (milligrams per liter) as calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard. (1 mg/L = 1 ppm)
Hard water is also measured in grains per gallon. 1 grain per U.S. gallon (gpg) = 17.1 milligrams per liter (mg/L) = 17.1 ppm (parts per million)
Hardness in the US tends to max out at about 250 ppm (there are uncommon exceptions). In other words, hardness is never an issue with the Atla System.
The Atla System is compatible with high TDS water.
The World Health Organisation rates the palatability of TDS in water in this way:
Less than 300mg/L, ppm - Excellent quality
300 - 600 mg/L, ppm - Good
600 - 900 mg/L, ppm - Fair
900 - 1200 mg/L, ppm - Poor
Above 1200 mg/L, ppm - Unacceptable
We recommend the Atla System up to 600 mg/L, ppm TDS, although many customers are happily using the system with even higher TDS levels.