“Water hardness” can refer to total hardness, calcium hardness or magnesium hardness. These related results are not interchangeable, and the correct definition depends on the water system and the operating question.
In brief: total hardness is commonly expressed as a calcium-carbonate equivalent and is principally associated with calcium and magnesium in fresh water.
Know which hardness result the process needs
Total hardness commonly represents the combined contribution of calcium and magnesium, expressed as an equivalent concentration of calcium carbonate. A calcium-only result answers a narrower question; magnesium hardness may be calculated or measured separately depending on the method.
Other polyvalent ions can contribute in some waters. State whether the requirement is total hardness, calcium hardness, a specific ion or a method-defined value, and record the reporting basis. Do not assume a conductivity or total-dissolved-solids value can be substituted for hardness.
Connect the sample point to the water-treatment objective
Raw feedwater, softened water, boiler make-up and circulating water represent different process stages. A sample taken after treatment may be appropriate for confirming the output, while an upstream point may be used to characterise the source or investigate loading.
The online water hardness analyser page and boiler water application guide provide relevant starting points. For broader plant monitoring context, see PMA’s online water quality monitoring in power plants article.
Treat reference methods and matrix as part of the specification
The EPA lists Method 130.1 for total hardness by automated EDTA colorimetry. The chosen laboratory procedure, endpoint, calibration and sample preparation define the comparison value. Confirm that the online method and reference method report the same hardness basis before interpreting a difference.
Water with unusual salinity, colour or metal content may behave differently from a standard fresh-water matrix. Supply representative samples and discuss interferences, concentration range, expected trends and the verification procedure before selecting a monitoring configuration.
Use a trend within a defined operating response
Hardness measurements can contribute to a treatment review, but the required action depends on the system. Agree what result prompts a check of softening, make-up quality, blowdown or a confirmatory sample. Avoid using a result outside the validated matrix or range.
For formal compliance, retain the required sampling and reference method. Online monitoring can add frequency to an operational picture, but it should not be described as a replacement for the applicable standard or permit method.
Questions to confirm
- Is the required result total hardness, calcium hardness or magnesium hardness?
- Which equivalent units and reference method will be used?
- Where in the treatment cycle should the sample be taken?
- What salinity, colour or other matrix conditions could affect the method?
- How will the result guide an operator action and be verified?
Review your water-hardness requirement
Tell PMA the water system, hardness definition, units, sample location and reference method so the measurement duty can be assessed.
Source basis
Source basis: US EPA approved methods, including total hardness Method 130.1 and EPA water-quality criteria describing hardness contributions. Related PMA application reading: online water-quality monitoring in power plants.
