When pool or spa water turns cloudy, gives off odours or simply feels less inviting, we often talk about doing a shock treatment. Traditionally, this treatment is done with a strong dose of chlorine. Yet there is a very popular alternative: monopersulfate, also known as active oxygen.

In pools and spas, the term active oxygen very often refers to monopersulfate, an oxidizer used as a chlorine-free shock treatment.

But what exactly is this product, and why can it reduce your chlorine needs? Here is a simple, complete explanation.

What is monopersulfate (active oxygen)?

Monopersulfate (often called active oxygen) is a powerful oxidizer used in pools and spas. Unlike chlorine or bromine, it does not disinfect directly, but it plays an essential role in water quality and clarity.

👉 Its main role is to oxidize contaminants, not to kill bacteria.

Why is it called a “shock treatment”?

A shock treatment is used to quickly restore water to good condition, in particular by:

- reducing organic contaminants
- improving water clarity
- reducing odours
- supporting the effectiveness of your sanitizer

Monopersulfate acts quickly on all of these, which is why it is considered a shock treatment even though it contains no chlorine.

Why “chlorine-free”?

It is called a chlorine-free shock treatment because active oxygen:

- contains no chlorine
- does not raise free chlorine levels
- does not produce chloramines

👉 This often makes it possible to treat the water without the typical chlorine smell, and with a more comfortable experience for many users (as per product recommendations).

Monopersulfate's key role: oxidation

In a pool or spa, the water continually accumulates contaminants, for example:

- sweat
- body oils
- cosmetics
- organic residue
- outdoor pollution (dust, pollen, etc.)

These contaminants consume your sanitizer and can make the water less clear or less pleasant.

Monopersulfate (active oxygen) works by oxidizing them — chemically destroying them before they place demands on your chlorine or bromine.

How does active oxygen help reduce chlorine needs?

When the organic load is high, chlorine is often doing two jobs at once:
1. disinfecting (killing bacteria)
2. oxidizing (burning off contaminants)

The problem: oxidation consumes a lot of chlorine.

By using an active oxygen shock treatment:
- contaminants are oxidized more efficiently
- the chlorine is less “overworked”
- it stays available longer for disinfection

👉 The result: more stable, clearer water, and often less chlorine needed to maintain good comfort.
👉 Important: this is preventive support, not a curative treatment for green water.

Fewer odours and fewer chloramines

A frequent source of that “chlorine” smell is not the chlorine itself, but rather chloramines, which form when chlorine reacts with organic contaminants.

By reducing this organic load, active oxygen can help:

- reduce odours
- improve bathing comfort
- keep the water more pleasant

An ideal ally for spas

Monopersulfate is especially popular in spas because:

- the water is hot
- contaminants build up quickly
- water chemistry changes faster

In this context, a chlorine-free shock can be a very practical way to keep the water clean and comfortable between regular treatments, then maintained with regular follow-up.

Impact on water balance

Another important advantage of monopersulfate is its low impact on water balance compared to some chlorine shock treatments.

- pH very close to neutral
- little effect on alkalinity
- does not increase calcium hardness

👉 This often lets you treat the water without triggering a series of pH or alkalinity corrections.

What monopersulfate does not do

To avoid any confusion:

- ❌ It does not replace a sanitizer
- ❌ It does not maintain residual protection
- ❌ It does not correct unbalanced pH or alkalinity

Monopersulfate (active oxygen) is an excellent tool, but it works as a complement to chlorine or bromine, not a replacement.

When should you use a chlorine-free shock?

An active oxygen shock treatment is generally useful:

- after heavy use (intense bathing)
- when the water starts losing clarity
- to reduce odours
- as preventive maintenance, depending on use

Used regularly, it helps prevent problems rather than fixing them in a hurry.

Conclusion

Monopersulfate, also called active oxygen, is a chlorine-free shock treatment used mainly to oxidize contaminants in the water. By reducing the organic load, it improves clarity, reduces odours and lets your chlorine or bromine work more efficiently.

Used properly, it is a simple, effective way to keep your water cleaner, more stable and more comfortable, especially in spas and heavily used pools.