Ozone Disinfection in Water and Air: Real Strengths, Real Limits, and the Bromate Question

Why Ozone Still Gets Specified

Ozone is a strong oxidant, and its action on microorganisms is a biochemical oxidation reaction rather than a simple chemical kill. Three mechanisms matter, and they run together rather than in sequence. Ozone attacks bacteria and viruses directly, damaging organelles and DNA/RNA so that metabolism is disrupted and the cell dies. It also penetrates cell membrane tissues, acting on outer-membrane lipoproteins and inner lipopolysaccharides, distorting permeability until the cell lyses. And because ozone is a gas, it diffuses through an entire treatment volume, which is the property that removes dead corners rather than merely reducing them.

The outcome is a bacteriolytic method that is thorough, broad-spectrum and residue-free. It works against bacterial vegetative forms and spores, viruses and fungi, and destroys botulinum toxin. It also has a strong killing effect on moulds. Ozone is unstable, which is normally a disadvantage, but here it is the point: it decomposes back to oxygen or single oxygen atoms that recombine into oxygen molecules, so no toxic residue remains. That is why ozone is described as a pollution-free disinfectant and why it needs no post-disinfection re-cleaning.

Where Chlorine and UV Fall Short

Traditional disinfection carries built-in limits. Ultraviolet light cannot act where the light does not reach, and suffers from decay, weak penetration and a short service life. Chemical fumigation leaves residual pollution or odour, demands heavy manual workload, is often incomplete, and can harm the people applying it. Against bacteria and viruses with strong drug resistance, its effect is also not obvious. Ozone has no such blind spot, which is why it has displaced chlorine in a growing share of drinking water treatment. Tests show a clear inactivation effect on almost all bacteria, viruses, fungi, protozoa and oocysts.

The Costs and Complications

None of this makes ozone free of trade-offs. Capital investment is larger and running cost is higher than chlorination. Ozone in water is unstable, so both control and detection require real instrumentation capability. Disinfected water is corrosive to pipework, and because there is no residual ozone in the finished water, a secondary disinfectant is still needed. Ozone also reacts with iron, manganese and organic matter, which can produce micro-flocculation and raise water turbidity.

The Bromate Problem

The complication that decides most design questions is bromate. When ozone oxidises raw water containing bromide ions, bromate is produced. The International Agency for Research on Cancer classifies bromate as a Group 2B potential carcinogen. WHO recommends a maximum bromate content of 25 μg/L in drinking water, and the US EPA drinking water standard sets the maximum allowable bromate concentration at 10 μg/L.

Two pathways generate it: ozone oxidation, and ozone/hydroxyl radical oxidation. Control therefore has two fronts. Formation can be suppressed by lowering pH, adding ammonia, applying the chlorine-ammonia process, or optimising the ozonation conditions. Once bromate exists in the water, physical, chemical and biological methods can be used to remove it. Achieving a balance among ozone dose, pathogen inactivation and bromate yield is a genuine optimisation problem, and the ozone sterilisation mechanism plus bromate control methods are still active research topics rather than settled specifications.

Practical Handling Rules

Ozone irritates the human respiratory mucosa, and the thresholds are well documented. At 0.15 ppm in air it can already be smelled. International standards put 0.5-1 ppm as the point where discomfort such as dry mouth begins, 1-4 ppm as the range that causes coughing, and 4-10 ppm as the range that causes severe coughing. Rooms must therefore be empty during air disinfection, and no one should re-enter until at least 30 minutes afterwards.

Material compatibility is the second rule worth writing into a procedure. Ozone damages many items, and the damage scales with concentration. Copper sheets develop green rust spots. Rubber ages, discolours, loses elasticity and eventually becomes brittle and fractures. Fabrics bleach and fade.

What Ozone Offers in Exchange

The high oxidation-reduction potential that makes ozone awkward to handle is also why it is used for oxidation, decolourisation and deodorisation. Research indicates that ozone dissolved in water can kill or oxidise substances harmful to human health including iron, manganese, chromium, sulfate, phenol, benzene and various oxides, and can decompose organic matter and kill algae. Against conventional sterilisation methods, four properties stand out.

Efficiency comes first: air is the only medium required, no other auxiliary materials or additives are needed, containment is good, sterilisation is thorough, and mould, fishy and foul odours are removed strongly. Cleanliness follows, because ozone is generated from oxygen and surplus oxygen recombines within 30 minutes, leaving no residue, solving the secondary pollution problem that chemical disinfectants create and removing the need for re-cleaning afterwards. Convenience matters in practice: sterilizers are generally installed in clean rooms, air purification systems or sterilisation chambers such as ozone sterilisation cabinets and transfer windows, with timed start and operating times set to the validated concentration and duration. Economy rounds it out, since comparative use and operation across pharmaceutical plants and medical and health institutions shows real economic and social benefit against other methods.

Two operating points are worth building into any procedure. The half-life of ozone in water is about twenty minutes, so it should be prepared fresh and consumed immediately. And because ozone decomposes chemical drugs, medicated water and ozone water must be supplied separately on separate lines. That detail matters in poultry drinking water systems, where ozonated water changes the gut micro-ecological environment, reduces bacteria that compete for host nutrients, raises the activity of secreted amylase and improves feed nutrient utilisation, while also reducing drug resistance and helping prevent intestinal diseases such as pullorum disease in chicks.