Two separate chlorine-free routes
There are two common chlorine-free sanitising systems for pools and spas: the PHMB (biguanide) route and the bromine route. They are not interchangeable and must not be mixed.
PHMB route: PHMB sanitiser (20% polyhexanide, CAS 32289-58-0) provides a residual biguanide that kills bacteria by disrupting cell membranes. Weekly oxidation uses hydrogen peroxide (typically 30–35%), not chlorine or MPS.
Bromine route: BCDMH bromine tablets (CAS 16079-88-2) slowly release both bromine and chlorine; the bromide bank — built up with sodium bromide (NaBr, CAS 7647-15-6) — converts any oxidant addition back into free hypobromous acid, the real sanitiser. MPS or a small chlorine shock is used to reactivate the bank rather than for direct sanitation.
Building the bromine bank: sodium bromide
On start-up or after a drain, add 30 g/m³ of sodium bromide to establish the bromide reserve. Dissolve in a bucket of pool water first, then distribute around the returns. Without a bromide bank, your BCDMH tablets are wasting half their chemistry on chlorine oxidation that vanishes quickly in UV.
Top up the bank at 10–15 g/m³ after a large water change, heavy bather load, or a season-opening shock. A PHMB system does not use NaBr at all — the two routes are separate.
PHMB: establish the residual, then maintain it
For a fresh fill, dose 50 ml/m³ of 20% PHMB solution with the pump running to reach 30–50 ppm. Use a PHMB-specific test strip — chlorine strips do not read biguanide. Weekly maintenance is typically 5–10 ml/m³. Oxidise weekly with a hydrogen peroxide dose per the oxidiser label.
PHMB degrades in the presence of any chlorine oxidiser. If a well-meaning pool shop adds chlorine to your PHMB pool, the residual is gone. Re-establish from scratch — this means a PHMB test, a fresh dose, and a peroxide oxidiser pass.
Compatibility: what not to stack
PHMB + chlorine (any form) → deactivated PHMB. PHMB + MPS shock → incompatible; MPS is an oxidiser that degrades the biguanide. Bromine tablets (BCDMH) + PHMB → incompatible for the same reason. Do not put all three in the same store cupboard without clear labelling.
Bromine and chlorine are compatible in principle (BCDMH releases both) but test separately. BAC and DDAC pool algaecides (quats) can be used in a bromine or chlorine pool at the label dose — check the particular product's compatibility note before adding.
FAQ
- Is PHMB the same as polyhexanide?
- Yes — PHMB (polyhexamethylene biguanide, CAS 32289-58-0) and polyhexanide are the same polymer sold under both names. The pool-trade brand Baquacil uses the PHMB name; wound-care and lens products sometimes say polyhexanide.
- Can I use bromine tablets with a PHMB pool?
- No. BCDMH bromine tablets release chlorine as well as bromine; the chlorine fraction deactivates PHMB. A PHMB pool uses a hydrogen-peroxide oxidiser for weekly oxidation, not bromine tablets. If you want bromine sanitising, run the NaBr + BCDMH tablet system — not PHMB.
- Which is cheaper long-term, chlorine or PHMB?
- Chlorine is generally cheaper per season. PHMB concentrate costs more per litre and requires a dedicated hydrogen-peroxide oxidiser. The appeal is no chloramine formation, no strong bleach odour, and gentler water for skin-sensitive swimmers — not cost saving.
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Pool & spa
Pool Sanitiser & Algicide
Baquacil
- Retail pack
- £39.991 litre bottle
- Raw active
- PHMB solution (polyhexanide 20%, pool grade)32289-58-0
Baquacil sanitiser is 20% PHMB (polyhexanide, CAS 32289-58-0) in water. PHMB is a broad-spectrum polymeric biguanide used in contact lens solution, wound irrigation and industrial biocides. The pool trade charges a heavy premium per litre; the same molecule is available as a 20% pool-grade solution from aquatics distributors at roughly half the retail price per litre.
Pool & spa
Bromine Activator / Bromide Bank Salt
Lonza
- Retail pack
- £19.992 kg tub
- Raw active
- Sodium bromide technical grade (NaBr)7647-15-6
Bromine activator is pure sodium bromide (NaBr, CAS 7647-15-6). The pool and spa trade sells it in branded 2 kg tubs; lab and photo suppliers sell the same technical-grade sodium bromide in 5 kg bags. There is no magic activation chemistry in the tub — you pay for the label and the small pack.
Pool & spa
Bromine Tablets
Clearwater
- Retail pack
- £24.001 kg tub
- Raw active
- BCDMH bromine tablets 20 g16079-88-2
Spa bromine tablets are BCDMH. The 1 kg tub is the same 20 g pucks sold in 5 kg tubs to holiday-park plant rooms. You still need a floater and a test kit.
Pool & spa
Non-Chlorine Shock / Spa Oxidiser
Clearwater
- Retail pack
- £18.991 kg tub
- Raw active
- Potassium monopersulphate triple salt (MPS)70693-62-8
‘Non-chlorine shock’ is Oxone-type MPS — the same triple salt used in bulk oxidisers. A 1 kg spa tub is a markup on a 5 kg bucket. It oxidises organics; it is not a sanitiser on its own.
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