UK legal limit: 10 µg/L. Find out what bromate is, its health effects, and how to check and reduce it in your tap water.
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Bromate is a disinfection by-product formed when ozone — used in advanced water treatment as a powerful disinfectant and oxidant — reacts with naturally occurring bromide in source water. Ozonation is an important treatment step at many UK water treatment works, particularly those treating surface water with high organic content, because ozone destroys a wide range of contaminants including pesticides, taste and odour compounds, and microorganisms that can resist chlorine.
The formation of bromate depends on bromide levels in source water, ozone dose, pH, and temperature. Not all ozone-treated water contains significant bromate — it is only formed when source water contains bromide. The UK legal limit for bromate is 10 µg/L. In practice, the vast majority of UK zones achieve bromate levels far below this limit — typically below 1–2 µg/L.
Bromate is classified by the International Agency for Research on Cancer (IARC) as a Group 2B substance — possibly carcinogenic to humans. This classification is based on animal studies showing kidney tumours at high doses. At the concentrations found in UK tap water (generally well below 5 µg/L), any cancer risk is theoretical and extremely small. The WHO's health-based guideline of 10 µg/L reflects concentrations at which a lifetime excess cancer risk of 1 in 100,000 is calculated — a standard considered acceptable in regulatory risk assessment.
Bromate compliance in the UK is very high. Bromate is one of the less commonly discussed water quality parameters precisely because it is so rarely elevated. Water companies that use ozonation carefully monitor bromate and adjust ozone dose and pH to minimise formation while maintaining disinfection efficacy.
Bromate is more likely to be detected in zones where: ozonation is used at the treatment works, source water contains elevated bromide (some coastal areas, brackish sources), and seasonal temperature increases ozone-bromide reaction rates. Thames Water and some Anglian Water zones have historically reported detectable bromate, though levels have generally remained below the legal limit.
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Bromate is a by-product formed when ozone (used to disinfect water) reacts with naturally occurring bromide in source water. Not all water contains bromate — it depends on the treatment process and source water chemistry.
At UK tap water concentrations (typically well below 5 µg/L), bromate poses a negligible health risk. It is classified as a possible carcinogen based on animal studies at high doses, but the legal limit of 10 µg/L includes a large safety margin.
The UK legal limit for bromate is 10 µg/L, in line with the WHO guideline value.
Zones treated with ozonation where source water contains bromide can have higher bromate. Thames Water and some Anglian Water zones have historically reported detectable levels, though usually well below the legal limit.
Reverse osmosis removes bromate effectively. Activated carbon has limited effectiveness for bromate. Standard jug filters do not remove bromate.
By adjusting ozone dose and pH, and by monitoring bromide levels in source water. If bromate exceeds monitoring thresholds, ozone dose is reduced or other treatment modifications are made.