Ozone Disinfection Byproducts: What They Are and How to Control Them

What Are Ozone Disinfection Byproducts?

Ozone disinfection byproducts are the small set of chemical compounds that can form when ozone gas reacts with substances already present in the water being treated. The direct answer for most plants: ozone forms far fewer and far less harmful byproducts than chlorine. Ozone itself decomposes completely to oxygen within minutes of dosing and leaves no halogenated residue. Its only regulated byproduct of real concern is bromate (BrO3-), which forms when dissolved bromide ions in the source water are oxidised. Ozone can also generate trace aldehydes and ketones (formaldehyde, acetaldehyde) from the partial oxidation of natural organic matter, but these occur at concentrations well below drinking-water guideline values in almost all practical dosing scenarios and are readily removed by downstream biological filtration.

This puts ozone in a fundamentally different byproduct category from chlorine, which reacts with natural organic matter to form trihalomethanes (THMs) and haloacetic acids (HAAs) — compounds classified as possible or probable human carcinogens and tightly regulated under IS 10500:2012. Understanding ozone byproducts is mainly about understanding bromate: where it comes from, when it becomes a real risk, and how a correctly engineered ozone system keeps it below guideline limits.

How Bromate Forms During Ozonation

Bromate forms through a two-step oxidation pathway. Ozone first oxidises bromide (Br-) — a naturally occurring ion in many groundwaters, coastal aquifers, and some surface waters — to hypobromite/hypobromous acid (OBr-/HOBr). If ozone exposure continues, this intermediate is oxidised further to bromate (BrO3-). Because the reaction depends on how much bromide is available and how much ozone contacts it, four variables determine how much bromate actually forms:

Source-water bromide concentration is the starting condition — no bromide, no bromate, regardless of ozone dose. Applied ozone dose and CT (concentration × time) drive the extent of oxidation; higher doses and longer contact times push more bromide through to bromate. pH matters because the bromide-to-bromate pathway accelerates above pH 7.5 and slows measurably as pH drops toward 6.5–7. Ammonia (NH3/NH4+) in the source water competes with bromide for available hypobromite, effectively scavenging the intermediate before it can oxidise further to bromate.

In most Indian inland surface water and groundwater sources, bromide levels are low (typically well under 50 micrograms per litre), and bromate formation at standard disinfection doses stays comfortably under the WHO guideline value of 10 micrograms per litre. The exception is coastal and brackish sources, or groundwater with geological bromide influence, where source-water testing before ozone system design is essential rather than optional.

Ozone Byproducts vs Chlorine Byproducts: A Direct Comparison

The table below sets ozone's byproduct profile against chlorine's, across the dimensions that matter for plant design and regulatory compliance.

• Primary byproduct — Ozone: bromate (BrO3-), only when bromide is present in source water. Chlorine: trihalomethanes (THMs) and haloacetic acids (HAAs), whenever chlorine contacts natural organic matter.
• Formation trigger — Ozone: requires bromide ion in the raw water; absent bromide, essentially no regulated byproduct forms. Chlorine: forms THMs/HAAs from virtually any water containing humic or fulvic organic matter — nearly universal in surface water.
• Regulatory guideline — Ozone (bromate): WHO 10 µg/L; also referenced by BIS/CPCB technical guidance. Chlorine (THMs): IS 10500:2012 limits total THMs to 200 µg/L; HAAs increasingly scrutinised under evolving CPCB reuse norms.
• Health classification — Bromate: IARC Group 2B (possibly carcinogenic to humans), same classification tier as chloroform, the dominant THM. Both warrant engineering control, not alarm.
• Practical control difficulty — Bromate: straightforward — lower pH, manage dose/CT, add ammonia if bromide is high, or use GAC polishing. THMs/HAAs: harder to eliminate entirely because they form from ubiquitous organic matter reacting with the disinfectant itself; the only complete fix is switching the primary oxidant.
• Residual after treatment — Ozone: none; fully decomposes to O2, so no ongoing byproduct accumulation in a closed system. Chlorine: persists for hours to days, continuing to react with residual organics and generate additional THMs/HAAs throughout the distribution network.

Selection Checklist: Sizing an Ozone System to Minimise Byproducts

Byproduct control starts at the design stage, not after commissioning. Work through this checklist before finalising an ozone system specification:

• Test source-water bromide concentration before sizing the generator — this single number determines whether bromate needs active management or is a non-issue at your site.
• Size the ozone dose to the actual treatment objective (disinfection CT or specific COD/colour-removal target), not to an arbitrary safety margin — over-dosing is the single biggest driver of unnecessary bromate formation.
• Specify ORP-based dose control (target 650–750 mV for general disinfection) rather than fixed-rate dosing, so the generator delivers only the ozone the water actually demands as influent quality varies.
• For bromide-rich coastal or brackish sources, budget for pH depression (to 6.5–7) or ammonia dosing ahead of the contact tank as a standard design feature, not a retrofit.
• Where very high doses are unavoidable (advanced oxidation for refractory COD, colour removal), plan for downstream GAC or biological polishing, which adsorbs and biodegrades trace bromate and aldehydes.
• Choose a generator platform with stable, repeatable output — ceramic-electrode DSC-type cells hold dose more precisely across a duty cycle than older tube-style cells, which reduces the dose overshoot that drives bromate formation.

Common Mistakes in Managing Ozone Byproducts

Plant teams new to ozone tend to repeat a small set of avoidable errors. Reviewing them before specification or commissioning is cheaper than fixing them afterward.

• Over-dosing ozone "to be safe": pushing dose well beyond the CT required for the treatment objective increases bromate risk without improving disinfection outcomes — match dose to demand, not to a guess.
• Never testing source-water bromide: a one-time lab test costs little; designing an ozone system blind to bromide concentration is the root cause of most bromate complaints reported in the field.
• Ignoring pH as a control lever: operators focused only on dose and contact time often overlook that dropping pH by even half a unit measurably slows the bromide-to-bromate pathway.
• No downstream monitoring: verifying bromate periodically (lab test or ion chromatography) confirms the system is performing as designed — dissolved-ozone residual readings do not tell you anything about bromate.
• Treating a symptom instead of the source: adding activated carbon to "remove byproducts" without first correcting an over-sized dose or uncontrolled pH addresses the effect while leaving the underlying design fault in place.

Is Ozone-Treated Water Safe Given These Byproducts?

Yes, when the system is dosed and controlled correctly. Bromate at the low concentrations typically seen from properly designed ozone systems on low-bromide Indian source waters sits well within WHO and BIS guidance, and ozone's complete decomposition to oxygen means there is no accumulating chemical residue in the treated water, unlike chlorine's persistent THMs and HAAs. For a full discussion of ozone safety in drinking-water applications, see our guide on whether ozone-treated water is safe to drink. For a side-by-side look at the two disinfectants across cost, contact time, and byproduct profile, read ozone vs chlorine for water treatment.

The practical takeaway for procurement and plant engineers: byproduct risk from ozone is a manageable design parameter, not a reason to default to chlorine. A system sized to the correct dose, with ORP-based control and bromide-aware design for coastal sources, keeps bromate well under guideline limits while delivering ozone's disinfection and oxidation advantages.

Engineering Byproduct Control Into Your Ozone System

Lotus Ozone Tech has designed and manufactured ozone systems in Chennai since 2010, with more than 1,000 installations across drinking water, STP/ETP, aquaculture, and industrial applications, built on 100% in-house components including DSC ceramic-electrode ozone cells engineered for stable, precisely controllable output — the foundation of good dose management and, by extension, good byproduct control. Our ozone technology is designed around ORP-based dosing rather than fixed-rate generation, so the system responds to actual water demand instead of running at a flat rate that risks over-dosing.

If you are evaluating an ozone system for a bromide-sensitive source, or want a dose and contact-time design reviewed for byproduct control, contact our engineering team for a quote. We can assess your source-water chemistry and specify a system sized to your actual treatment objective.