Chlorine dioxide vs chlorine
Chlorine bleach is cheaper and you already have some. There are three jobs it is genuinely worse at, and one of them is the reason most people go looking for chlorine dioxide in the first place.
The short answer
If you are cleaning a hard surface, use bleach. If you are cleaning water — a tank, a well, a system that has stood — chlorine dioxide is the better tool, and the difference is largest exactly where bleach disappoints people: a tank that smells again a fortnight after it was shocked.
By the job
Five situations, and which one to reach for in each.
- A water tank that has stood, or smells
- Chlorine dioxide
- The smell is biofilm, and biofilm is the case bleach handles worst — it is consumed at the surface of the layer instead of getting through it.
- Hard or alkaline water
- Chlorine dioxide
- Bleach weakens as water turns alkaline. Chlorine dioxide carries on across a much wider pH range.
- Water you will actually drink afterwards
- Chlorine dioxide
- No chlorophenol taste, and it does not form trihalomethanes with the organic matter it meets.
- A one-off scrub of a hard surface
- Bleach
- It is cheaper, it is in every shop, and none of the advantages above are in play.
- Keeping stored water drinkable for months
- Neither
- Both are disinfectants and both dissipate. What that job needs is a stabilised preserver.
Why biofilm is the whole argument
A tank that has held water grows a thin living layer on its wall. That layer is what you are smelling, and it is what brings the smell back: shock the tank with bleach and the chlorine is spent oxidising the outside of the film, leaving what is underneath to re-establish itself.
Chlorine dioxide passes into the layer rather than being consumed at its face, which is why it is used on water systems industrially and why it is worth the extra cost on a tank you drink from. How to do an RV or marine tank →
What bleach is still better at
Cost, and being on the shelf already. For wiping down a surface, or any job where the water is not going to be drunk and there is no film to get through, the advantages above do not apply and bleach is the sensible choice. Being able to say that is the point of this page.
And if the job is storage, not cleaning
Disinfecting and preserving are different jobs. Both of the above clean water once and then dissipate; keeping water drinkable for months is what a stabilised preserver is for, and how much to use gives the dose for a tank of any size.
Still working out what chlorine dioxide actually is? Start here →
Questions we are asked
- Is chlorine dioxide better than bleach?
- For water you are going to drink, and for anything with biofilm in it, yes — it gets through the layer, keeps working in alkaline water and leaves no chlorine taste. For wiping down a hard surface, bleach is cheaper and does the job.
- Can I just use bleach in my RV or boat water tank?
- You can, and people do. What it struggles with is the biofilm on the tank wall, which is usually the actual cause of the smell: chlorine is spent at the surface of the film rather than passing into it, so the smell returns. That is the case chlorine dioxide is chosen for.
- Does chlorine dioxide contain chlorine?
- It contains a chlorine atom, which is where the name comes from, but it is not chlorine and it does not behave like it. It stays as ClO₂ in water and disinfects by oxidation instead of by chlorination.
- Why does bleach stop working in hard water?
- Hypochlorite bleach works mostly as hypochlorous acid, and as water turns alkaline that shifts to the much weaker hypochlorite ion. Chlorine dioxide does not depend on that equilibrium.
- Which is more expensive?
- Chlorine dioxide, per use. It cannot be stored at working strength, so it is made where it is used, from two bottles. That is what you are paying for and it is also why it is fresh.
Nothing here is a health claim. Chlorine dioxide is a water treatment chemical, used at the dilutions on the label — including what it is not sold for.