Adding pool chemicals — carefully!

Macquatics Training
By John McKenny*
Thursday, 08 December, 2016

Adding pool chemicals — carefully!

In this series of topics on basic swimming pool maintenance, the various forms of chlorine were discussed in the first article, then the all-important and often forgotten pH component of the water was the second topic. The third was a brief run-down on pool water testing. With the necessary tests now understood and done correctly, it follows that it’s necessary to have enough knowledge to both confidently and carefully add the necessary chemicals to maintain the water clean and safe for swimming.

Not understanding pool chemicals, and not knowing the effects they have on the pool water when added, is usually the main cause of poor water quality, which — along with algae growth and frustration — leads to some owners filling in their pool to make a garden.

The following concentrations of pool chemicals may differ from some recommended levels in some states, but they are amounts or levels I have found to be adequate after years of trials on commercial and private pools Trust me — it works!

Chlorine concentrations

Chlorine — or, more correctly, ‘free chlorine’ — must be maintained. It’s named free chlorine as this is the amount of chlorine in the pool which is ‘free’ and ‘available’ to attack any contaminants. For outdoor pools, adequate sanitation should result if the free chlorine level is 3–4 mg/L (that’s milligrams per litre, which we used to call ppm or parts per million). I say “should result” as quick, effective and complete sanitation depends a lot on the pool’s design and where the jets and outlet are located. The chlorine level of 3–4 mg/L is for outdoor pools only using the stabiliser cyanuric acid. Cyanurate has an inhibitory effect on the activity of chlorine, so we triple the recommended minimum of 1 mg/L for stabilised outdoor pools.

Indoor pools should have a chlorine level of 2–3 mg/L and must not use cyanuric acid. The reason? Cyanurate protects chlorine from sunlight, and most indoor pools do not have direct sunlight on the pool water. Furthermore, indoor pools quite often have high bather loads, so all of the available chlorine is needed for the quickest possible sanitation and must not be inhibited because of the presence of cyanurate.

If there’s an odour which smells like chlorine in an indoor pool, quite often, raising the chlorine concentration will reduce this ‘chloramine’ odour, as sanitation occurs quicker with the extra chlorine, which assists in reducing the unpleasant smell.

Adding powdered chlorine (calcium hypochlorite) or liquid chlorine (sodium hypochlorite) to water raises the pH, so careful additions of an acid are necessary to maintain the pool water at a pH between 7.3 and 7.5. Regular testing is the only reliable way to know the chlorine and pH levels.

pH — always aim for pH 7.4

Your chlorine will be very active at this level and swimmers will be more comfortable too, as studies have shown that the pH of eye liquid (teardrops of healthy people) is approximately pH 7.4, so with everything else being right, sore eyes won’t be a problem and goggles will be no longer necessary.

If your pH test comes out at, say, pH 7.2, don’t add anything! The eventual addition of chlorine will bring it up slightly, which is exactly what’s needed.

If your pH is showing pH 7.5 or higher, adding an acid will reduce it. Hydrochloric acid (also known as muriatic acid) can be added, but only with great care. It’s essential that before picking up the acid container you have chemical gloves and full face protection, along with a chemical apron and long rubber boots. Purchase all of these and place them all on the acid container, so you have to pick them up before using the acid. Now it’s easy to simply put them on. This could save you from a severe incident, hospitalisation, loss of sight, etc, so always wear the necessary protective equipment!

How much acid to add?

This depends on many factors, like pool volume (which is often a guess), pH test result, pH of the water supply, type of filter, usage and weather.

My recommendation is to run two pH test results of the pool water, then add one litre of liquid acid slowly to the pool when the pump is running and no-one is swimming, leave it running to circulate for an hour, then test again and do another test about 30 minutes after that (or the next day). Now you know how much one litre of hydrochloric acid will lower the pH in the pool, so write it down on your test book for future reference. This has proven to be the best practical way to know how much acid to add to sensibly and carefully reduce the pool’s pH.

Other acids used are sulfuric acid and sodium bisulfate. They both work too but will be dearer than hydrochloric acid. It’s of vital importance that all acids are stored in a locked cabinet, away from children and all other chemicals.

Some large swim centres inject carbon dioxide gas into the water to reduce the pH. The gas mixes with the pool water, forming a weak carbonic acid. It’s a very safe system, but also dearer than using hydrochloric acid.

Acid safety

Use personal protective equipment and never mix an acid with anything! Measure it carefully using a clean, plastic container, then add it slowly to the pool during the circulation of the water. Adding several litres of acid to the pool outlet can cause the filter to ‘release’ everything caught in the filter. For the next few days, you’ll have a very unclean pool containing all the dirt from the filter.

If you’re using powdered or liquid chlorine or salt in the pool, you should never need to add an alkali such as sodium bicarbonate or sodium carbonate. The addition of the chlorine or salt will give you the pH increase. It’s not worth it to add an alkali one day, to raise the alkalinity or the pH, then an acid the next when neither was necessary!

Sodium bicarbonate (aka buffer)

In a previous article I mentioned that, in my opinion, alkalinity levels in pool water are unimportant. Now, some health departments are considering removing this test from their pool guidelines. Progress! A healthy change for pool water — hoorah!

Sodium bicarbonate, the chemical which increases both alkalinity and pH, will soon no longer be needed, as I’ve advocated for years. Less total chemical means less expense and more comfortable water that is clean and safe for swimming.

Note: some people will argue that low alkalinity will cause a thing called ‘pH bounce’, meaning that the water’s pH will vary and not be stable. All the trials I have done disagree with this, but if your pH goes crazy when your alkalinity is very low, then add some bicarb and see if it helps.

Cyanuric acid stabiliser

After trialling, testing, titrating, touching and even tasting cyanurate, then talking with many water chemistry ‘experts’ on the use of this chemical, it can be safely concluded that there is not one sensible reason for not using it at the correct level in any outdoor pool. Trials over many years conclude that, on average, there’s a total chemical saving of 70% when cyanurate is introduced to pools, with no side effects! These savings have averaged out at $9000/million litres over six months, so for large pools of, say, 4 million litres that are open all year, they can save half a million dollars on chemicals in 10 years by using cyanurate. Now that’s worth considering.

Cyanurate must be used sparingly at a concentration in the pool water at between 15 and 20 mg/L. Adding it is simple: if the outdoor pool holds 500,000 L (the approximate volume of some eight-lane 25 m pools) and there is no cyanurate in the water, adding 10 kg of pure cyanurate should give you 20 mg/L. It follows then that a 50,000 L pool will need only 1 kg of pure cyanurate for 20 mg/L of cyanurate.

To add cyanurate, add it from the bag slowly to the pool outlet while the pump is running. It is slow to dissolve, so don’t clean the filters or do a backwash for three days, as it sometimes takes this long to completely dissolve.

This is the easiest way to add cyanurate — no mess, no mixing and minimal handling.

When the test reduces to 15 mg/L, add a small quantity so the test doesn’t go above 20 mg/L. If the test does go over 20 mg/L, wait until it rains or the filters need a clean to help it return to the desired maximum.

If you are using one of the stabilised chlorine compounds (dichlor or trichlor), it’s okay to add this to the pool until the cyanurate level reaches 20 mg/L — then it’s necessary to stop using this chemical and switch to straight chlorine. If you don’t, the cyanurate levels will continue to increase as you add the dichlor or trichlor, and will reach unhealthy, high levels. The only way to reduce it is to slowly replace some of the pool water by backwashing or partial drainage.

So the cyanurate must be tested regularly, but the continuous addition of either of the two stabilised chlorine chemicals will eventually create very unpleasant and unhealthy swimming conditions.

It’s vital that only adequate amounts of necessary chemicals are added to provide clean and healthy water for swimming, and testing is the only way to monitor this.

Calcium hardness

This must be tested and kept above 100 mg/L, with 150 mg/L being a good level. To increase it, add small amounts of calcium chloride to the skimmer box or pool outlet with the pump running. It dissolves quickly, stores well, isn’t expensive and is readily available.

So why is calcium necessary? I use the analogy that calcium is to water as oxygen is to humans. We must have oxygen and pool water must have calcium, and if we don’t supply sufficient calcium, the water will seek it out! It will, over time, extract the grouting from all tile work and, if that isn’t enough, eat away at the actual tile cement until the tiles fall off. During this time, any other cement surfaces will be attacked too, so eventually very expensive repairs are needed. Therefore, you should test calcium monthly and add calcium chloride as necessary.

Other chemicals

Specific problems with trace elements, rust, discolouration, ducks, algae, cloudiness or blondes with green hair (to name a few) require specific chemical treatments. Try to keep all those additives out of your pool if possible. Some aren’t safe.

Any questions on pool chemicals? Please email me on I’m receiving many pool water quality enquiries and happy to 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. Pool+Spa does not necessarily endorse the opinions expressed by our contributors.

Image credit: © Smith

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