A polyphosphate filter cartridge should hold enough media to deliver the required phosphate dose for the service interval you have chosen — and no more than the housing can handle without excessive pressure drop. There is no universal gram figure. The correct mass is calculated from three inputs: the target dose in mg/L (ppm), the volume of water the cartridge will treat, and the flow rate the housing and cartridge diameter can support. Everything else — crystal grade, bed depth, housing size, water temperature — adjusts that calculation rather than replacing it.

If you are sizing a cartridge for scale control, the practical answer is: work backwards from the dose and the service life, then check that the cartridge body physically holds that mass with adequate void space for water contact.

Why the answer is a calculation, not a standard number

Polyphosphate media, including siliphos-type crystal products, works by dissolving slowly into the water stream. It is not a filter medium in the strict sense — it does not trap particles, and it does not remove hardness. It releases a small concentration of polyphosphate that interferes with calcium carbonate crystal growth and, in some systems, forms a thin protective film on metal surfaces. Because the mechanism depends on continuous release, the media charge has to match the water volume it will see.

A cartridge that holds too little media exhausts early and stops protecting the system long before the maintenance interval. A cartridge that is overfilled, or packed too tightly for its housing, can create high pressure drop, poor flow distribution, and uneven dissolution, where water channels through part of the bed while the rest of the crystals remain untouched.

Key terms used in this article

• Scale inhibition: slowing or preventing the formation of new mineral scale deposits. This is what polyphosphate media is normally used for.
• Water softening: removing hardness ions (calcium and magnesium) from water, usually by ion exchange. Polyphosphate media does not soften water and does not lower measured hardness.
• Scale removal: dissolving existing deposits. Polyphosphate media is not a descaler for established scale.
• Corrosion inhibition: reducing metal loss in pipework or equipment. Some polyphosphate programs are used for this, but it requires its own dose verification and is not automatic.
• Filtration: physical removal of suspended solids. A polyphosphate cartridge is not a sediment filter unless it is combined with one.

The four inputs you need before sizing a cartridge

1. Required dose (mg/L as phosphate or as product). This must come from the media supplier or from the water-treatment program designer, because it depends on hardness, temperature, pH, and what is being protected.
2. Daily and total water volume (litres). Estimate peak daily consumption and multiply by the number of days between replacements.
3. Flow rate (L/min or m³/h). This sets the minimum cartridge size and the maximum practical bed depth. High flow through a thin bed gives poor contact time.
4. Water conditions. Temperature, pH, hardness, and any pre-treatment affect how fast the crystals dissolve and how much phosphate stays in the water rather than precipitating.

A five-step sizing method

1. Decide the service interval. For example, 1, 3, 6, or 12 months, or a fixed volume such as a set number of cubic metres between changes.
2. Calculate the total water volume the cartridge will treat in that interval (daily volume × days).
3. Convert dose and volume into mass of media: mass (g) = dose (mg/L) × volume (L) ÷ 1000.
4. Add a sizing margin. Release rate is not perfectly linear over the cartridge’s life, and dose typically falls as the bed is consumed, so cartridges are usually replaced before the last crystals dissolve. Choose the margin with your supplier, not by assumption.
5. Check the housing. Confirm the calculated mass fits the cartridge body with enough free volume for water to move around the crystals, and confirm the resulting pressure drop at design flow is acceptable for your system.

Illustration of the arithmetic

The numbers below are only an illustration of the calculation, not a recommendation for any product: if the approved dose is 2 mg/L and the installation uses 500 L per day over a 90-day interval, the treated volume is 45,000 L, and the media mass is 2 × 45,000 ÷ 1000 = 90 g. Substitute your own approved dose and consumption figures. If your dose is different, the mass changes in direct proportion.

How cartridge format limits the answer

Even when the calculation gives a mass, the cartridge has to physically contain it. Different formats impose different limits, and the practical constraint is usually bed geometry rather than raw volume.

Signs the media charge is wrong

• Media exhausted well before the planned change: the cartridge was undersized for the volume treated, the dose was set higher than the release rate supports, or water temperature is higher than assumed.
• Scale reappears quickly despite fresh media: dose or contact time is insufficient, or the problem is not scale inhibition but an existing deposit that needs separate removal.
• Noticeable pressure drop immediately after filling: the bed is overfilled, over-compacted, or the crystals are finer than the housing was designed for.
• Crystals at the top look untouched while the bottom is empty: channelling or uneven flow, often caused by incorrect fill level or a poor flow path.

What polyphosphate media does not do

Buyers frequently confuse categories, which leads to the wrong cartridge size being ordered for the wrong job.

• It does not soften water. Hardness remains in the water; only the tendency to deposit scale is affected.
• It does not remove existing scale from pipes, heat exchangers, or heating elements.
• It does not filter sediment, chlorine, or microorganisms.
• It is not a disinfectant and does not make microbiologically unsafe water safe.
• It is not a general-purpose corrosion inhibitor that can be applied without dose verification.

Polyphosphate additives also raise the phosphate content of the treated water. Many jurisdictions regulate phosphate in drinking water and in wastewater discharge, so discharge and potability rules should be checked for the specific site before installation.

Material and compliance checks before purchasing

• Ask the media supplier for the recommended dose range, temperature range, pH range, and expected release behaviour, stated for the exact product being quoted.
• Confirm the crystal grade and particle size distribution are compatible with the cartridge body and the intended flow rate.
• Do not treat “food grade” as equivalent to “approved for drinking water.” Potable-water suitability depends on the exact product, the application conditions, and the regulations of the destination market, and must be verified separately.
• Request the batch documentation, safety data sheet, and any certification the supplier actually holds — and confirm which of those apply to the grade you are buying.
• For OEM or private-label cartridge production, confirm fill mass tolerance and how it is verified at the filling line.

Frequently asked questions

Can I simply fill a cartridge with as much polyphosphate media as it will hold?

No. A tightly filled cartridge can raise pressure drop, reduce contact time, and cause uneven dissolution. Fill mass should follow the supplier’s recommended range for that cartridge geometry and flow rate.

Does a cartridge with more media make the water softer?

No. Polyphosphate media does not remove calcium or magnesium. Measured hardness will not change; only the behaviour of scale-forming minerals is affected.

How do I know when the media needs replacing?

Use the rated service volume or interval from the media or cartridge supplier, and inspect the cartridge at intervals. Visible depletion of crystals and the return of scaling symptoms are practical indicators, but the rated volume should be the primary guide.

Can the same cartridge serve several points of use?

Only if the total volume and peak flow stay within the cartridge’s rated conditions. Serving multiple outlets usually increases both peak flow and daily volume, which changes the required media mass and may require a larger housing.

Should the polyphosphate dose be measured in the treated water?

Where phosphate level matters — for potability, discharge, or process reasons — periodic measurement of residual phosphate at the outlet is the most direct way to confirm the cartridge is delivering what was intended.

Getting the fill mass right for your cartridge

If you are specifying polyphosphate media or siliphos-type crystal for a cartridge program, the useful starting point is not a grams-per-cartridge figure but your operating data. ENVO CHEMICAL can review the application and discuss the media grade and fill range that fits it, provided the key parameters are known.

When you send an inquiry, include: the intended application (drinking water, appliance protection, industrial circuit, irrigation), source-water hardness, pH and temperature, required flow rate and daily volume, the cartridge or housing type and dimensions, the target dose or the treatment objective, the required certification for your destination market, and the estimated annual quantity. With those details, a media recommendation and fill mass can be discussed on a factual basis rather than a guess.