If polyphosphate consumption has doubled while the water meter shows roughly the same volume, there are only three realistic explanations: the meter is not seeing all the water that passes through the crystal bed, the media is dissolving faster per litre than it used to, or the consumption figure itself is wrong. Work through them in that order — measurement first, media last — and the cause usually becomes clear quickly.

That order matters because the instinct is to blame the crystal. In most systems it is the water balance or the record-keeping that has changed, not the product.

What “consumption” actually measures in a crystal dosing system

In a crystal-type dosing unit, polyphosphate crystals — often supplied under names such as siliphos crystal or phosphate crystal — sit in a contact vessel. Water flows through the bed, dissolves a small amount of the material, and carries the dissolved polyphosphate downstream. The intent is flow-proportional dosing: more water through the bed, more crystal consumed.

Consumption is therefore driven by several inputs at once:

• the volume of water that physically passes through the bed;
• water temperature and chemistry, which affect how quickly the crystal dissolves;
• flow velocity and the contact pattern inside the vessel;
• the solubility and particle characteristics of the specific crystal in use.

If consumption doubles, at least one of those inputs has changed. The job is to find out which one, using evidence rather than assumption.

Check the easiest explanation first: the consumption record

Before touching pipework, verify that consumption genuinely doubled. Refill logs fail in predictable ways:

• Unit confusion. Sacks, kilograms and pounds appearing in the same log, or a supplier changing the pack weight.
• Double counting. Two operators logging the same refill, or a top-up recorded as a full charge.
• Moisture uptake. Opened material absorbing humidity, so a nominal 10 kg charge no longer weighs 10 kg unless it is weighed rather than counted.
• Weighing versus estimating. “The vessel looked low” is not a consumption measurement.
• A recent recharge. A freshly charged, loosely packed bed can release more per litre than an aged, compacted bed, so a replacement can shift the baseline even when nothing else changed.
• A different grade or supplier. Crystal size, bulk density and solubility vary between products, and not every product sold under a name like siliphos behaves identically.

If the numbers survive this review, the cause is physical.

Water that passes through the media but never through the meter

This is the most common real-world answer to the title question. The meter and the dosing vessel are often not measuring the same stream. Look for these paths:

• Under-registration at low flow. Mechanical meters have a starting-flow threshold. A continuous small draw — a passing toilet valve, a leaking float valve, a slow process feed — can pass through the meter without moving the register, while still flowing through the crystal bed and consuming media around the clock.
• An unmetered branch. Irrigation, washdown, a second building, a temporary connection or a fire line that is not captured by the same meter as the treated supply.
• Recirculation or backflow. A check valve that no longer seals, or a recirculation loop, can send the same water through the bed repeatedly while the meter counts it once.
• Backwash or flush water. Any periodic flush through or around the vessel adds to consumption without adding to the metered total.
• A bypass left partly open. A maintenance bypass that was never fully closed can put additional flow through the vessel.

A simple night-flow test is one of the fastest ways to expose this: record the meter reading and the vessel level before and after a period when the site should be using no water. Any overnight crystal loss with no metered flow points directly at an unmetered or unregistered path.

Meter and monitoring faults

Meters themselves drift. Consider:

• A meter installed upstream of a tee, so it only sees part of the flow.
• A fouled or scaled impeller or turbine, causing under-reading at low and mid flows.
• A meter sized far larger than the actual flow range, which harms accuracy at the low end.
• A pulse output or K-factor that was rescaled in the BMS or SCADA, so the displayed volume no longer matches the physical meter.
• A bypass around the meter installed during past maintenance and never removed.

Cross-check the meter against a second measurement method — a temporary clamp-on flow meter, a tank-fill test, or a comparison with utility billing — before concluding that the media is at fault.

When the media really is dissolving faster

If the water balance and the meter both check out, the dissolution rate itself has shifted. Relevant factors include:

• Water temperature. Warmer water generally dissolves polyphosphate crystal faster. A new mixing valve, a seasonal change, or a relocated supply point can change the temperature at the vessel even when the volume is unchanged.
• Source-water chemistry. A switch between sources, blending of two supplies, or a change in pH and alkalinity can change how quickly the crystal goes into solution.
• Flow velocity and contact time. A resized pump, a throttled valve, or a partly blocked outlet alters how long water stays in contact with the bed.
• Product change. A different crystal grade, particle size distribution, or a substitute product will not dissolve at the same rate as the previous material.
• Vessel condition. A compacted or channelled bed changes the effective contact surface, sometimes increasing local dissolution and sometimes reducing dosing performance even though consumption looks normal.

Use the outlet residual as the deciding clue

Measurement of the treated-water phosphate residual is what separates “the water volume changed” from “the media changed”. Compare the current residual against your own historical baseline for the same point in the system, taken under comparable flow and temperature conditions. Do not rely on a single spot sample.

A practical diagnostic sequence

1. Weigh or reconcile the stock and the refill log rather than counting sacks.
2. Read the meter at two fixed times and compare against a normal-load day from the previous season.
3. Run a night-flow check to find unmetered or unregistered draw.
4. Measure phosphate residual at the outlet and at least one downstream point.
5. Log the water temperature at the vessel inlet for a full day.
6. Confirm whether the source water or blending ratio has changed.
7. Inspect the vessel: bed level, compaction, channelling, and the condition of any bypass.
8. Verify meter accuracy with an independent measurement.

Frequently asked questions

Can temperature alone double polyphosphate consumption?

Temperature strongly affects dissolution rate, but a doubling is a large shift. Temperature is more likely to explain a gradual seasonal rise than a sudden step change. Check it alongside volume and residual data rather than in isolation.

Does higher flow rate increase crystal consumption?

Total consumption rises with the volume of water passing through the bed, because more water contacts the crystal. Dissolution per litre can also change with velocity and contact time, and the size of that effect depends on the vessel design.

Does polyphosphate crystal soften water?

No. It does not remove hardness ions the way an ion-exchange softener does. Products in this family are generally positioned as scale inhibitors and sequestrants — they work against new scale formation and can help condition the water film in a distribution system. They are not the same as a softener, a filter, a scale remover or a disinfectant, and the functions should not be treated as interchangeable when specifying a system.

Are all siliphos-type crystals interchangeable?

No. Siliphos is a trade name, and products sold under similar names can differ in composition, particle size, bulk density and dissolution behaviour. Substituting one for another can change consumption and dosing performance, so a change of material should be treated as a system change, not a like-for-like swap.

What to record before changing anything

Good records turn a mystery into a trend. Capture water volume, inlet temperature, source-water analysis, outlet phosphate residual, vessel type and media quantity, the date of the last recharge, and the product grade used. If you do change the media, keep the old and new grades side by side in the log so future discrepancies can be traced.

If the system supplies drinking water, confirm that the exact product and grade are suitable for potable contact under the conditions and in the destination market. “Food grade” on a datasheet is not, by itself, a verification of drinking-water suitability.

When the clues point to the media specification

If your checks show that the water balance and meter are sound and the residual data points to the crystal itself, the next step is comparing the current material against the specification the system was designed for. ENVO CHEMICAL can help review that comparison when you supply the relevant details.

When you get in touch, include: the application (potable supply, industrial process, cooling or other), source-water analysis, water temperature at the vessel, normal and peak flow rates, vessel or dosing equipment type, metered volume versus actual treated volume, target phosphate residual, current product grade and quantity used, required specification, order quantity, and destination market. With those details, a meaningful technical answer is possible — without them, any recommendation would be guesswork.