Temporary and permanent hardness respond to polyphosphate treatment in much the same way, because polyphosphate treatment does not remove hardness at all. What changes is the type of deposit that can form when the treatment is under- or over-applied, how quickly heat defeats it, and which competing technologies you should be comparing it against. Carbonate-dominated water usually fails as calcium carbonate scale on hot surfaces. Sulfate- or chloride-dominated water fails differently: as dense, acid-resistant sulfate deposits, or as corrosion that scale-inhibiting chemistry was never intended to control.
So the temporary/permanent split is less about the phosphate chemistry itself and more about risk. It tells you what happens if dosage, contact time, or temperature limits are wrong — and that is what should shape the specification you buy.
What temporary and permanent hardness mean in practice
Hardness is caused by dissolved calcium and magnesium. The traditional classification comes from how those ions behave when the water is heated.
- Temporary hardness (carbonate hardness): calcium and magnesium are paired mainly with bicarbonate. Heating drives off carbon dioxide and converts bicarbonate to carbonate, which precipitates calcium carbonate. This is why kettles, water heaters and heat exchangers fur up. The hardness is “temporary” because boiling can remove it.
- Permanent hardness (non-carbonate hardness): the same metals are paired with sulfate, chloride or nitrate. Boiling does not remove them. Only ion exchange, membrane processes or comparable separation steps reduce their concentration.
- Total hardness is the sum of both. Most real water contains a mixture, and the split can shift seasonally or after blending sources.
A useful field calculation: carbonate hardness is approximately the smaller of alkalinity and total hardness (both expressed as CaCO3); non-carbonate hardness is the remainder. The estimate needs a full water analysis to be meaningful, but it is a quick way to see which type dominates.
Why polyphosphate treatment behaves differently from softening
Polyphosphate media work by releasing small amounts of phosphate into the water as it passes through the bed. The phosphate interferes with the formation and growth of scale crystals — often described as threshold treatment or sequestration — and can form a thin film on metal surfaces. The calcium and magnesium stay in the water.
Three consequences follow directly from that mechanism:
- It is scale inhibition, not softening. Total hardness measured downstream is generally unchanged. If a specification calls for reduced hardness, ion exchange or reverse osmosis is the correct class of technology.
- It is not filtration. Particulate removal, iron and manganese removal, and disinfection are separate functions, even when a phosphate medium is installed inside a filter housing.
- It is not scale removal. Polyphosphate treatment is aimed at preventing new deposit formation. Any loosening of existing scale should be treated as a claim to verify with the specific supplier and product, not as a general property of the category.
Because the mechanism does not depend on the hardness anion, both temporary and permanent hardness fall within its scope. Where the two types diverge is in the operating window and in the cost of failure.
What actually changes between the two hardness types
The table below summarises the practical differences for anyone selecting or troubleshooting a phosphate-based medium.
| Factor | Temporary (carbonate) hardness | Permanent (non-carbonate) hardness | Implication for polyphosphate treatment |
|---|---|---|---|
| Dominant ions | Calcium and magnesium bicarbonates | Calcium and magnesium sulfates, chlorides, nitrates | Same inhibition mechanism applies to both; nothing is removed from solution |
| Behaviour on heating | Bicarbonate decomposes and calcium carbonate precipitates | Salts remain largely soluble, though sulfate scaling is possible under certain concentration and temperature conditions | Heat is the most common failure point for treated carbonate water |
| Typical deposit | Calcium carbonate — porous, readily dissolved by acid | Calcium sulfate and related salts — denser and far less acid-soluble | A failure in sulfate-dominated water costs more to correct and often requires mechanical cleaning |
| Link to pH and alkalinity | Directly tied to alkalinity; pH drifts as CO2 is lost | Often low alkalinity and weaker pH buffering | Polyphosphate dosing does not correct pH; low-alkalinity water may need separate corrosion control |
| Role of chloride | Usually minor | Can dominate the water chemistry | Chloride-driven pitting is a corrosion problem, not a scale problem, and is outside the scope of threshold treatment |
| Common competing solution | Lime softening, dealkalization, acid dosing, ion exchange | Ion exchange softening, membrane processes | Alternatives are often cheaper for carbonate water; for non-carbonate water they may be less practical at small scale |
Three operating conditions that decide whether the treatment holds up
Temperature and residence time
Polyphosphates gradually revert to simpler orthophosphates in water, and that reversion accelerates as temperature rises. Orthophosphate is a much weaker threshold inhibitor than the polyphosphate it came from, so a bed that performs well on a cold mains line may perform poorly on a hot loop or on a long, slow-moving branch. Ask the supplier for the rated temperature range of the specific medium and match it against the hottest point in your system, not the inlet temperature.
pH, alkalinity and carbonate equilibrium
Threshold treatment has a working pH band. Outside it, performance drops and the water itself may become aggressive. Carbonate-hardness water is buffered and tends to raise pH when carbon dioxide is lost, which increases calcium carbonate saturation at exactly the moment scale risk is highest — in heaters and heat exchangers. Very low pH, by contrast, increases corrosion and shortens the useful life of the medium. Neither problem is solved by adding more phosphate.
Competing demand for phosphate
Dissolved iron and manganese, high suspended solids, and oxidising disinfectants all consume or interfere with the treatment. Iron, in particular, can form its own deposits and staining. Free chlorine is worth checking before you commit to a phosphate medium, because residual disinfectant can affect both the medium and the protective film. If these parameters are high, the correct first step may be pre-treatment rather than a larger phosphate dose.
When polyphosphate is the wrong tool
Polyphosphate treatment is a good fit for moderate-temperature, once-through or low-residence systems where the goal is to keep existing dissolved hardness from depositing on surfaces. It is usually the wrong tool when:
- Hardness must actually be reduced — for reverse osmosis feed, low-TDS applications, or boiler feedwater.
- Operating temperatures exceed the range the medium is rated for.
- The system is dominated by chloride, and the real problem is corrosion rather than scale.
- Iron or manganese concentrations are high enough to foul the medium or stain fixtures.
- Local rules restrict phosphate discharge to drains or receiving water, or limit phosphate in the treated water itself.
- What is needed is filtration, disinfection, or removal of an existing hard deposit.
How to evaluate a phosphate-based medium for your water
- Start with a complete analysis — total hardness, calcium, magnesium, alkalinity, pH, sulfate, chloride, TDS, silica, iron, manganese, and any residual disinfectant.
- Split the hardness into carbonate and non-carbonate fractions so you know which failure mode to design against.
- Map the temperature and flow profile — peak temperature, typical and peak flow, daily volume, and how long water sits in the system during low-demand periods.
- Define the objective precisely: prevent new scale, reduce maintenance intervals, protect a specific component, or reduce corrosion. Each objective points to different products and settings.
- Confirm the medium’s own limits — base chemistry, rated temperature, recommended flow range, replacement criteria, and any documentation relevant to your destination market.
- Verify in place — monitor downstream scale on a test surface or a representative heat exchanger, rather than assuming the treatment is working because the medium is still in the housing.
A note on terminology
Siliphos, phosphate crystal, polyphosphate media and “siliphos crystal” are often used loosely, but they are not automatically interchangeable. Siliphos is a trade name associated with a specific slowly dissolving phosphate glass medium, and products sold under similar descriptions may differ in composition, dissolution rate and blending. Do not transfer temperature limits, dosage figures, service life or certification from one product description to another. Confirm the exact product you intend to buy.
Frequently asked questions
Does polyphosphate treatment soften hard water?
No. It inhibits scale formation and keeps hardness minerals dispersed in the water. The calcium and magnesium remain, and standard hardness testing generally still reflects them. If you need lower hardness, use ion exchange or a membrane process.
Does it work on both temporary and permanent hardness?
The inhibition mechanism applies to both, so the temporary/permanent split does not by itself rule polyphosphate treatment in or out. What it changes is the risk profile: carbonate water tends to fail as acid-soluble calcium carbonate, while sulfate-dominated water tends to fail as denser, harder-to-remove deposits. Sulphate-rich water therefore demands a more conservative design and closer verification.
Will polyphosphate treatment remove scale that has already formed?
Do not rely on it. The primary function is preventing new deposition. Some products may gradually loosen existing light deposits, but this depends on the specific medium and the deposit, and should be supported by evidence from the supplier rather than assumed.
Can I install a phosphate medium in a hot water line?
Only within the temperature range the manufacturer rates for that product. Elevated temperature accelerates the reversion of polyphosphate to less effective forms and increases carbonate scaling risk at the same time. If the hot side of your system is the problem, softening or another separation technology is often the more reliable answer.
If a medium is described as food grade, is it approved for drinking water?
Not automatically. Food-grade describes the material’s handling and purity in a manufacturing context. Suitability for potable water depends on the exact product, the dosage and contact conditions, and the regulations of the destination market. Ask for the specific documentation that applies to your application.
Selecting the right medium for your water
Most failures with phosphate-based treatment come from applying a correctly made product to the wrong water or the wrong operating window. If you are comparing options for a specific site, ENVO CHEMICAL can work from your actual conditions rather than from a generic recommendation. When you enquire, include: a recent water analysis (hardness, alkalinity, pH, sulfate, chloride, iron, manganese, TDS), the application and equipment type, housing dimensions and flow rate, peak water temperature, whether the goal is scale prevention or corrosion control, required quantity, and destination market so that the correct documentation can be identified.
