Cyanurate: stabilising outdoor pools

Macquatics Training
By John McKenny*
Monday, 06 February, 2017

Cyanurate: stabilising outdoor pools

In this article, the fourth in a series of non-technical articles, John McKenny* discusses cyanurate: what it does, why it’s important and how best to use it. Keep your costs down and your pool clean and safe for swimming by following ‘Good water matters’ in each publication.

Cyanurate is probably the most misunderstood and overused chemical in the aquatics industry. Let’s go over the basics to clear up any misconceptions.

Cyanurate stabilisation

Definition: cyanurate stabilisation of swimming pool water is a process of effectively and safely reducing the destruction of chlorine in the water, caused by the action of ultraviolet (UV) rays from the sun.

Many trials over several years have concluded that on a clear day, the chlorine in a pool not containing cyanurate will be 90% destroyed by sunlight (UV rays) in five hours. Even on days of total cloud, chlorine in these unstabilised pools will weaken as the UV rays penetrate through the cloud, and after five hours, the chlorine concentration will still reduce by 50%.

It is surprising to learn further that it is mainly the weather and not the bather load which contributes to increased chlorine usage, but pools have to be open and closed on hot days to determine this. Testing has shown that on full cloud days with high pool bather loads, chlorine usage increased only slightly — and far less than on clear, hot days with low bather loads.

It can be concluded that without cyanurate, chlorine has to be added to outdoor pools several times each day to maintain chlorine levels and keep the water safe to swim in. Most commercial/council-type swim centres have ‘auto dosing’ systems, feeding both chlorine and acid into the system on demand, 24/7.

The chemical

Cyanurate or cyanuric acid is a pure chemical that is slightly acidic and is easy to store and handle.

It should be added to every outdoor pool or water feature, but at the correct level.

It has been suggested that for some water play parks where the water feature has a sloping zero depth floor, the majority of water is away from sunlight and underground in a balance tank, and therefore the use of cyanurate may not be beneficial.

How to add cyanurate to a pool

It’s slow to dissolve, so the easiest way to add it is to pour small quantities of the dry chemical into the pool outlet drain or skimmer box while the pump is running. It can take up to three days to completely dissolve in colder water, so don’t backwash or clean the filter for three days after its addition, as the cyanurate will be slowly dissolving in the filter and backwashing will wash some of it away.

Amount to add

If the total pool (+ balance tank) volume is known, it’s easy to add the correct amount of cyanurate without overdosing it. If the pool doesn’t have any cyanurate in it, for every 1,000,000 L of pool water, add 20 kg of the pure cyanurate. (This is equivalent to adding 2 kg for every 100,000 L of pool water or 1 kg for 50,000 L.) Pure cyanurate/cyanuric acid doesn’t contain chlorine and is the best chemical to use to obtain the correct concentration in the water.

Don’t overdose! The amounts above will give a cyanurate concentration very close to 20 mg/L and this level should never be exceeded.

Remember: cyanurate is probably the most misunderstood and overused chemical in the aquatics industry.

The ideal cyanurate level in a pool for safe swimming is 15–20 mg/L!

Adding any more than these recommended levels has no appreciable benefit and will not create the savings discussed below. It will be a waste of chemical and therefore a waste of money, with bathers swimming in unnecessarily high concentrations of cyanurate. This chemical will be wasted as it’s lost during dilution through backwashing etc, so have the level tested regularly and maintain this level.

Why a maximum of 20 mg/L is best

To explain it further, having the cyanurate at a maximum of 20 mg/L will provide approximately 96% chlorine protection from the sunlight. Increasing the cyanurate level to 40 mg/L (as many people will say is best) will increase the chlorine protection to approximately 98%. But the cyanurate concentration in the pool has been doubled for a 2% advantage! The extra cyanuric acid now in the pool can cause many problems to the water quality and for swimmers. Understanding this chemical is important before adding it unnecessarily and at elevated levels.

Effects on chlorine

Cyanurate attaches itself partially to the chlorine molecule in the water, meaning the chlorine will be not as free to go out and attack and destroy any contaminants. Because of this, it’s necessary to triple the free chlorine concentration to 3–4 mg/L and always maintain this level. With cyanurate now in the pool water, it becomes much easier to maintain the chlorine level, as this stabiliser continuously prevents the sunlight from quickly destroying the chlorine.

Stabilised chlorines

Known as ‘dichlor’ and ‘trichlor’, these chemicals contain both chlorine and cyanurate.

If water testing is not carried out often enough, they cause water quality problems. The reason for this is that the chlorine in these pool chemicals is used up as it breaks down the contaminants, but the cyanurate portion doesn’t break down, instead accumulating slowly, often leading to the water becoming unsafe for swimming.

This can be easily avoided by using a pure chlorine chemical and pure cyanurate/cyanuric acid as separate chemicals, having the water tested regularly and keeping the cyanurate maximum at 20 mg/L.

Using up your stabilised chlorine

It’s quite okay to use up these stabilised chlorine compounds until the cyanurate level reaches 20 mg/L, then it’s necessary to cease using them and switch to pure chlorine so the cyanurate doesn’t keep increasing. Once the cyanurate level drops to 15 mg/L, it’s okay to return to using the stabilised chlorine until the cyanurate level again reaches 20 mg/L.

The savings using cyanurate

My own research has shown that there can be savings of at least $12,000 per million litres of pool water annually in an outdoor pool. Smaller pools of, for instance, 100,000 litres can save about $1000 annually on total chemicals.

Trials I’ve done show that cyanurate introduction to outdoor pool water will give total chemical savings of at least 60%. That’s 60% of all chemicals — not just the chlorine amount — so there’ll be similar savings with any acid and alkali used.

Less total chemical in the water means lower levels of total dissolved chemical or total dissolved solids (TDS), resulting in cleaner, clearer water with the added advantage of less money spent on pool chemicals.


Run the cyanurate in every outdoor pool using chlorine or salt at 15–20 mg/L, the chlorine at 3–4 mg/L and the pH between pH 7.35 and 7.45. There are lots of good reasons for doing so and remember, there’s a cheap and simple-to-use test kit available for these tests.

Let me know your thoughts, concerns or problems. Contact me on or go to Happy to read your comments or assist.

*John McKenny has managed and leased swim centres for more than 30 years. He commenced TAFE teaching in swim centre operations and management 25 years ago and continues today. John is the author of The Complete Swimming Pool Handbook and The Leisure Pool and Spa Handbook.

Editor’s note: As with all content published, readers are advised to make their own enquiries and ensure that they adhere to the relevant regulations in their region. The opinions expressed in this article are the opinions of the author(s) and not necessarily those of Pool+Spa.

Image credit: ©

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