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Coolant water treatment

Water Preparation for Metalworking Fluids: Hardness, Analysis and Treatment

Water quality is one of the factors that determines the stability of a water-miscible metalworking fluid (MWF). Even a high-quality concentrate may perform inconsistently if the water does not meet the requirements of the specific product.
There is no single type of “ideal water” for every MWF. Before selecting a water-treatment method, it is important to identify the actual issue: hardness, mineral content, iron, suspended solids, microbiological contamination, or a combination of several factors.
For small volumes, a complex water-treatment system is not always justified. In a number of practical situations, excessive hardness can be corrected by chemical treatment. For continuous high-volume consumption or more complex water-quality problems, dedicated treatment equipment may be more appropriate.

Which water parameters should be checked

Parameter	Why it matters for MWF	What an abnormal result may indicate	What to do
Hardness	Primarily related to calcium and magnesium content	Deposits, precipitation or changes in emulsion stability may occur; very soft water may increase foaming with some MWFs	Compare with the requirements of the specific MWF and select treatment if required
Conductivity / total mineral content	Indicates the amount of dissolved ions	High mineral content increases the salt load in the system	If necessary, identify the dissolved salts and the source of mineralisation
Chlorides and other salts	May affect corrosion processes and fluid performance	Higher concentrations may increase the risk of operating problems	Use an extended water analysis where appropriate
pH and alkalinity	May affect the properties of the prepared working fluid	Unusual values should be assessed together with other water parameters	Follow the requirements of the specific product
Iron and mineral impurities	Often relevant to borehole or well water	May contribute to deposits and system contamination	Identify the source and suitable treatment method
Suspended solids	Enter the tank and filtration system	Sediment, tank contamination and additional filter loading	Filtration
Microbiological condition	Microorganisms can enter the MWF with the water	May increase the risk of rapid biological contamination	For unstable sources or where contamination is suspected, perform microbiological testing
Stability over time	Both borehole and mains water can change	An old analysis may no longer represent current water quality	Repeat testing after source changes, supply disruptions or changes in MWF behaviour

Practical water-analysis reference values

The existing Novohim article included a table of practical water reference values. It is retained here as a practical guide, not as a universal specification for every MWF. The requirements of the specific product take priority.

Parameter	Practical reference value
Total hardness	2–8 °dH
Water pH	6.5–8.5
Chlorides	As low as reasonably achievable
Iron	Below 0.5 mg/L
Conductivity	Up to 500–800 µS/cm

Above these hardness reference values, the likelihood of problems may increase for some emulsion-type products. However, the decision to treat the water should be based on the actual water analysis and the requirements of the specific MWF.

How hard water can affect MWF

Water hardness is mainly associated with calcium and magnesium compounds. Depending on the MWF formulation and the composition of the water, excessive hardness may be accompanied by cloudiness, precipitation, deposits, equipment contamination and changes in emulsion stability.
Salt accumulation during operation should also be considered. Part of the water evaporates, while most dissolved mineral substances remain in the system. Subsequent make-up introduces additional salts, so the condition of an operating MWF can gradually differ from that of a freshly prepared fluid. Very soft water is not always better. Extremely soft or demineralised water is not a universal solution. With some water-miscible MWFs, very soft water may increase the tendency to foam. Water treatment should therefore address a specific problem rather than simply reducing hardness to the lowest possible level.

Mains water: why it should also be monitored

A centralised water supply does not by itself guarantee that the technological characteristics of the water will remain constant.
Backup sources may be introduced or supply schemes may change to maintain water delivery. Seasonal conditions and precipitation can also affect the source water.
A previously satisfactory analysis should therefore not be treated as valid indefinitely. If an MWF that was operating normally suddenly begins to foam, form deposits or otherwise behave differently, the water should be checked again, particularly after supply interruptions, accidents, repairs or changes in the water source.
Compliance with drinking-water requirements and suitability for a particular MWF are different questions.

Can borehole or well water be used

Yes, but only after its actual characteristics have been evaluated. In practice, borehole or well water often requires more careful preparation.
Its composition can change significantly during the year depending on precipitation, aquifer recharge, groundwater level, withdrawal rate, the condition of the well and other hydrogeological factors. A single analysis should therefore not be assumed to represent the water throughout the entire operating period.
For borehole or well water, particular attention should be paid to hardness, mineral content and conductivity, iron and other mineral components, suspended solids, stability over time and microbiological condition.
Microbiological testing is not always included in routine industrial water checks. However, bacteria and fungi can enter the working MWF directly with the water. If the source is unstable or the fluid begins to suffer rapid biological contamination, the microbiological condition of the water should not be ignored.

Water from open sources

River, lake and other open-source water may contain not only dissolved salts but also suspended solids, organic matter and microbiological contamination. It should not be used to prepare MWF without prior assessment and, where required, appropriate treatment.

Soda ash for hardness correction

For small and medium water volumes, soda ash — sodium carbonate (Na₂CO₃) — can be a practical method for correcting excessive hardness.
During treatment, sodium carbonate helps convert some hardness-forming compounds into sparingly soluble forms. The resulting precipitate must be taken into account and should not be transferred into the prepared MWF.
Practical treatment rates used for this purpose are:

Soda ash dosage	Amount per 100 kg of water
0,5%	0.5 kg
1,0%	1.0 kg

The choice between 0.5% and 1.0% should take the initial water characteristics and the treatment result into account. Where necessary, the treated water is allowed to settle and the clarified portion is used without transferring the precipitate into the MWF system.
The economic advantage can be particularly significant for small volumes. For 100 kg of water, a 1% dosage requires only 1 kg of soda ash. In such a case, purchasing, installing and maintaining a dedicated reverse-osmosis system solely to prepare a small amount of water may not be economically justified.
Soda ash is not a substitute for every water-treatment method. It does not automatically solve high total mineral content, undesirable dissolved salts, suspended solids or microbiological contamination.

Other chemicals used in water treatment

In addition to soda ash, water-treatment processes may use sodium hexametaphosphate, trisodium phosphate, sodium silicate and complexing agents.
These chemicals are retained here as possible water-conditioning options, but a universal dosage should not be specified for them. The required amount depends on the composition of the source water, the selected treatment process and the required result.
Before chemical treatment is selected, determine which water parameter actually needs to be corrected.

Which method should be selected: chemical treatment, softening or reverse osmosis

Method	Typical purpose	Complexity / cost	Practical consideration
Soda ash, 0.5–1%	Correction of excessive hardness, particularly for smaller volumes	Low	Low chemical consumption; the resulting precipitate must be considered and separated
Sodium hexametaphosphate	Chemical conditioning of selected water properties	Depends on the process	Dosage is determined from the water analysis and treatment process
Trisodium phosphate	Chemical treatment in selected process schemes	Depends on the process	Dosage and application require separate calculation
Mechanical filtration	Sand, rust and suspended solids	Low–medium	Does not remove dissolved salts or reduce hardness
Ion-exchange softening	Regular requirement to reduce hardness	Medium	Requires equipment and maintenance
Reverse osmosis	High mineral content and complex dissolved-salt problems	High	Requires equipment, maintenance and consumables; the resulting water may be too soft for some MWFs
Demineralisation	Major reduction in dissolved mineral content	High	Not automatically the best option for every MWF
Blending water streams	Adjustment where water of different quality is available	Medium	Final water parameters must be controlled

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The economics should be calculated for the actual volume required. If a plant only needs to prepare around 100 kg of water periodically and the main problem is hardness, chemical treatment may be far more rational than installing a dedicated reverse-osmosis unit. For continuous high-volume use or complex water-composition problems, the opposite may be true.

When should the water be analysed again

• the plant uses its own borehole or well;
• the water source has changed;
• there have been accidents, repairs or prolonged supply interruptions;
• water characteristics change noticeably with the seasons;
• the MWF unexpectedly begins to foam or form deposits;
• corrosion problems recur;
• a freshly prepared MWF changes condition rapidly;
• the same product performs differently in different areas of the plant;
• the plant is planning to change to another MWF.

For an unstable water source, monitoring over time is important. One satisfactory analysis does not guarantee that the water will have the same characteristics several months later.

How to prepare the MWF after water treatment

After the water has been prepared, follow the mixing procedure specified for the particular product. For typical water-miscible MWFs, the concentrate is added to the water while mixing, rather than adding water to the concentrate.
After preparation, check the working concentration. A refractometer is convenient for routine control, using the correction factor specified for the particular MWF.
[Internal link: Refractometer for MWF]

When it may be better to select another MWF

If the plant water is difficult or unstable, it is not always rational to force it toward an assumed universal standard. In some cases, it is simpler to select an MWF that is better suited to the actual water conditions.
Product selection should consider the machining operation, workpiece material, lubrication requirements, circulation system and available water characteristics.
[Internal link: article on selecting an MWF]

Common mistakes

• assuming mains water never changes and using the same old analysis for years;
• using borehole or well water on the basis of one old analysis;
• evaluating water only by hardness;
• assuming the softest possible water is best for every MWF;
• installing expensive treatment equipment before identifying the actual cause of the problem;
• adding treatment chemicals without understanding the source-water composition;
• allowing precipitate formed during chemical treatment to enter the MWF tank;
• blaming the concentrate alone without checking the water and the condition of the system.

FAQ

What water is best for an MWF?
Water whose characteristics meet the requirements of the specific product and remain sufficiently stable over time. There is no universal water specification for every MWF.
Can ordinary mains water be used?
In many cases, yes. However, after a source change, supply interruption or sudden change in MWF behaviour, the water should be checked again.
Can borehole or well water be used?
Yes, after analysis and treatment where required. Seasonal changes in composition and microbiological condition are particularly important for such sources.
How much soda ash should be added?
Practical water-treatment rates of 0.5% and 1.0% are used, corresponding to 0.5 or 1 kg of soda ash per 100 kg of water. The choice depends on the initial water and the treatment result.
Is reverse osmosis always necessary?
No. For small volumes where hardness is the main issue, chemical correction can be much simpler and less expensive. Reverse osmosis may be justified for high mineral content or continuous high-volume demand.
Can sodium hexametaphosphate or trisodium phosphate be used?
These chemicals are used in water treatment, but their dosage should be determined for the specific water and treatment process.
Why can the operating MWF become more mineralised over time?
Water evaporates while much of the dissolved mineral content remains in the system. Additional salts enter with make-up fluid, so mineral content can gradually increase.

Conclusion

Water preparation for MWF should begin not with purchasing equipment or adding a chemical, but with understanding the actual water-quality problem.
For small volumes and excessive hardness, soda ash at 0.5–1.0% can be a practical and economical option. More complex water conditions may require other chemicals, filtration, ion-exchange softening, reverse osmosis or combined treatment.
The water source itself should not be assumed to remain constant. Borehole water is subject to seasonal variation, while centralised water may change when sources or supply conditions change. If MWF behaviour changes, rechecking the water is one of the first practical steps.