Surface pitting on Siliphos balls is usually a normal sign of progressive dissolution, not a defect. The balls slowly give up polyphosphate glass into the water, and their surface roughens and erodes as they do. What makes pitting worth reading is the pattern: a uniformly etched bed, a bed with deep pits on one side, or spheres coated in a hard white crust are telling you three different things about flow, water chemistry, and remaining useful life.
This article is written for the people who actually open the housing — filter and water-treatment equipment manufacturers, distributors and importers handling returns, and service contractors maintaining point-of-entry or small commercial systems. It explains what pitting can and cannot tell you, and when the answer is simply “top up the charge.”
Is surface pitting on Siliphos balls normal?
Yes, in most installations. Siliphos is a slowly dissolving polyphosphate–silicate glass, and it is designed to dissolve. As water passes over the spheres, the outer layer goes into solution. Because the glass is not perfectly homogeneous and because flow hits one surface of a sphere more than another, dissolution is almost never perfectly even. The result is a surface that looks etched, chalky, or covered in small shallow craters.
A simple distinction helps before you investigate anything deeper:
- Progressive surface etching across the whole bed — expected behaviour. The material is working.
- Deep, uneven pits, fractures, crusts, or slime — a symptom worth investigating. Something about the operating conditions or the bed geometry is not what the product was selected for.
How to read the pit pattern
Take a representative sample from the top, middle, and bottom of the bed. Rinse a few balls gently in clean water and look at them under good light. The table below maps the most common surface conditions to their likely causes and the next check to run.
| Surface condition | What it usually indicates | Next check |
|---|---|---|
| Uniform, shallow etching; matte or slightly chalky surface throughout | Normal progressive dissolution | Measure remaining bed depth; top up or replace on schedule |
| Deep irregular pits concentrated on one side, or voids in the bed | Channeling or uneven flow — water is finding preferred paths instead of passing through the whole bed | Check distributor and collector design, bed packing, and whether flow rate is within the range the housing was sized for |
| Cracks, splits, or spheres breaking apart | Mechanical or thermal stress, or dissolution running faster than intended | Verify inlet temperature, flow rate, and water chemistry against the material’s stated operating limits |
| Hard white or grey crust filling the pits | Re-precipitated hardness or silicate deposit sitting on the surface | Review hardness, alkalinity, pH, and any upstream filtration or pretreatment |
| Rust-brown or dark staining inside the pits | Iron, manganese, or corrosion products arriving from upstream | Inspect upstream pipework, pumps, and any iron-bearing components |
| Slimy, dark, or odorous surface | Biological growth, typically where the bed sits stagnant or the system is not sanitised | Review stagnation periods, system hygiene, and whether the vessel is being bypassed for long stretches |
| Fine fragments and dust at the bottom; spheres compacted | Fines migration and bed compaction | Check backwash or collection arrangements and whether the bed has been disturbed during service |
| Spheres soft, mushy, or crumbling between the fingers | Charge has been in service well past its useful life, or the water is chemically aggressive to the medium | Replace the charge and review the operating envelope with the supplier |
None of these observations replace a water test. They narrow down where to look.
What pitting can tell you about remaining service life
Pitting is a rough indicator of how much material has been consumed — it is not a measurement. The reliable measure is bed depth and mass. If the housing holds a known volume of medium and the measured depth has fallen noticeably, that lost volume has gone into the water, which is the intended mechanism.
A practical approach for service contractors:
- Record the initial bed depth and charge weight at commissioning.
- At each service visit, measure the current bed depth with a dipstick or by emptying and weighing a known volume.
- Compare against the replacement point the supplier specifies for that product and application.
- Use the surface condition only to explain why consumption is faster or slower than expected.
A bed that has pitted deeply on one side but still shows full depth elsewhere is not necessarily depleted — it is often a flow-distribution problem that will keep recurring until the housing or the bed packing is corrected.
When pitting points to a real problem
Uneven flow and channeling
If pits cluster along one wall of the vessel or in a vertical band, water is taking a shortcut. That means part of the bed is doing all the work and wearing out early, while the rest is bypassed. Fixing this usually means changing something mechanical — inlet geometry, distributor plate, bed support, or the flow rate the unit is being run at — not simply adding more medium.
Water chemistry outside the intended range
Dissolution rate depends on the water itself. Temperature, pH, alkalinity, hardness, and the presence of oxidising disinfectants all influence how quickly a polyphosphate glass gives up its surface. Oxidisers in particular are commonly reported to accelerate breakdown of polyphosphate media, so a system that has recently changed its disinfection practice is worth re-examining. Because the acceptable ranges differ between products and manufacturers, confirm the figures for the exact grade in use rather than assuming a value from another supplier’s datasheet.
Fouling rather than dissolution
White crusts, rust staining, and biological slime are deposits, not dissolution. They sit on the surface and can slow the release of the active material, so a fouled bed may look “less worn” while actually performing worse. Deposits also signal that something upstream is being carried into the vessel, which is often a separate problem to solve.
What pitting does not tell you
It is worth being clear about the limits, because surface inspection is often asked to answer questions it cannot:
- It does not confirm scale inhibition is working. That requires looking at downstream heat-exchange surfaces, fixtures, or membranes for deposit formation.
- It does not measure residual phosphate. Only a water test can do that.
- It does not diagnose corrosion. Corrosion control depends on water chemistry and pipe material; the medium is one variable among several.
- It does not prove the material is unsuitable. Deep pitting with correct bed depth and correct flow is often just a longer service interval than expected.
Also keep the functions separate when you interpret what you see. Siliphos-type polyphosphate media are used for scale inhibition and, in many formulations, some degree of corrosion inhibition. They are not water softeners — they do not remove hardness ions by ion exchange. They are not scale removers — they do not dissolve existing deposits. And they are not disinfectants. If a system has a hardness problem rather than a scaling problem, no amount of reading the medium’s surface will point you toward a softener, because that is a different treatment step entirely.
Inspection checklist
- Isolate and depressurise the vessel safely; follow the site’s confined-space and chemical-handling procedures.
- Record the bed depth before disturbing anything.
- Sample from top, middle, and bottom — not just the top layer.
- Rinse and photograph representative spheres against a plain background; note the scale.
- Describe the pit pattern: uniform, banded, one-sided, or localised.
- Note any crust, staining, slime, or odour.
- Check the mechanicals: distributor, collector, bed support, and evidence of bypass.
- Confirm the current operating conditions — flow rate, temperature, and any recent change to pretreatment or disinfection.
- Test the treated water for the parameter the system is actually meant to control.
- Decide: top up, replace, or correct the mechanical or chemical cause first.
Frequently asked questions
Does pitting mean the Siliphos balls have failed?
No. Some surface erosion is the intended mechanism. Failure is better judged by remaining bed depth, by whether the treated water still meets its target, and by whether the charge has reached the replacement point the supplier specifies.
Why are some balls pitted and others still smooth?
Flow distribution, position in the bed, and local variation in the glass itself all affect how quickly an individual sphere dissolves. A mixture of smooth and heavily pitted spheres usually points to uneven flow rather than to a batch defect.
Can I just add new balls on top of a pitted charge?
Topping up is common practice, but it depends on the product and the reason for the shortfall. If the pitting is caused by channeling or fouling, adding material on top extends the interval without fixing the cause. Confirm the recommended procedure for the specific product before mixing old and new charges.
Can I tell whether the material is suitable for drinking water from its appearance?
No. Appearance says nothing about compliance. “Food grade” describes a material’s manufacturing category; it does not by itself establish that a finished product is approved for a specific drinking-water application in a specific market. That has to be verified for the exact product, dosage, and destination.
What to send when you request a replacement or a technical review
If the inspection shows the charge needs replacing, or if you want a second opinion on what the pit pattern means, the useful information is specific and short. Tell us the application (domestic point-of-entry, commercial kitchen, cooling loop, or another duty), the equipment type and housing size, the current charge’s product name and any batch or grade reference, the source-water analysis you have, the operating flow rate and temperature, what you observed on the surface — including photos — and the destination market. With those details, ENVO CHEMICAL can confirm which grade and specification fits the duty and what information still needs to be verified for your conditions.
