Hardness is not the same as TDS
Total dissolved solids describes the overall dissolved ionic content in a broad sense, while hardness is associated mainly with calcium and magnesium. Two waters with similar TDS can have different scaling behaviour.
Why hardness matters before RO
As RO recovers product water, rejected salts become more concentrated in the remaining stream. Scaling risk therefore depends on feed chemistry and operating recovery, which is why pretreatment decisions should not rely on hardness alone.
When softening may be considered
Ion-exchange softening can be useful where hardness control is required, but it should be selected from the application, water analysis and downstream process rather than added automatically.
DPHE's published water-quality parameter table includes hardness as CaCO₃ among its listed parameters. Use the current applicable standard and laboratory result for the intended use.