All you need to understand to be able to look after the chemistry in your pool is that there is a need to have sufficient chlorine and the correct pH. You can adjust you other chemical levels by testing the pool water yourself, getting a pool shop water test or by using a pool service company such as ourselves. That’s it. It really is that easy.
Fill a pool with tap water and let it sit for a while and you will end up with something like this. Years ago, I was called out to look at a pool that needed work. I was taken into an overgrown enclosed area. I had to ask where the pool was. I couldn’t see it. A plant commonly called Singapore Daisy had grown completely over the pool and several things were living underneath it. These are extreme occurrences, but they do demonstrate that you cannot leave a pool to survive on its own. Besides good equipment and plumbing some understanding of pool chemistry is needed. Don’t let that scare you because, like anything else, once understood it is really quite easy.
To understand why particular chemicals are chosen to sanitise a pool we need to briefly look at ORP. Years of research and study by people far smarter than me have concluded the pool water needs to have an ORP of around 750 mV (provided the pool isn’t using cyanuric acid) with a very minimum of 630 mV. There is some debate about the levels. We respond (very quickly) when a monitored level drops below 650 mv. This is a measure of killing power. The results of DPD tests, expressed in ppm, are the common way of testing pool water and are also a measure of killing power. DPD testing is a far easier method of testing and understanding pool chemical levels. The ORP and DPD minimum levels are set so that when an unwanted organism enters pool water it will be killed within a required period of time.
So, ‘What is the DPD Method?‘ It is a method of measuring the chlorine residual in water. The residual may be determined by either titrating or comparing a developed color with color standards. DPD stands for N,N-diethyl-p-phenylenediamine. A set quantity of the DPD is added to a measured volume of pool water and the resultant colour compared to a colour table or scale to get you chlorine read. A DPD test is measured in parts per million. One ppm = 1 mg per liter of water. A more accurate result can be obtained using a photometer.
And, ‘What is ORP?’ It stands for ‘oxidation reduction potential’. ORP is not any easy concept to understand, which is why you don’t hear about it too often. A broad definition of ORP is that, ‘oxidation is the loss of electrons’ while, ‘reduction is the gain of electrons’. One cannot occur without the other. (See what I mean!). It is measured in millivolts. (A scientific explanation of ORP by Ionode, a manufacturer of ORP probes for several makes of the chemical controllers on the market).
NSW Health Department Fact Sheet – Oxidation-reduction potential
An oxidising biocide attacks bacteria, algae and other microorganisms by oxidising the cell structure, disrupting nutrients from passing across the cell wall, causing the organism to die or stop multiplying.
From a pool management point of view, ORP is great because it tells us the oxidising potential of all the oxidising chemicals in the pool regardless of all the other chemical test levels (especially the pH level). To achieve the required positive ORP we need to add an Oxidising Biocide to the water, but one that is strong enough to make an appreciable change to the existing level of ORP.
Examples of strong oxidising biocides used in swimming pools are: –
Areas that concern pool operators
Pool chemical levels are important in three areas of swimming pool operation.
While there are a multitude of chemical reactions occurring is a swimming pool or any other water body (other than pure water) we only need to concern ourselves with those reactions that affect these three areas.The following table has been copied from the “Water quality guidelines for public aquatic facilities – September 2019”.
While we do not agree with some of the parameters in the table, if you abide by those parameters, you will achieve the aims of the three areas above.
While the guidelines are only guidelines and not law, they are a good reference for anyone responsible for the health and well-being of swimming pool patrons. If an operator can show that she/he can demonstrate that they have adhered to the recommendations of the guidelines, then I would think that that would go a long way towards demonstrating a ‘duty of care’. But I’m no lawyer.
Before looking too closely at the sanitation in any pool we need to look at the pool’s hydraulics and filtration. Sanitation levels aren’t much good if those levels are not consistent throughout the pool. The regular use of pool cleaners will go a long way towards improving inefficient pool hydraulics. However, pool cleaners are generally not left in well used pools for very long.
As an example of terrible pool plumbing, the following is what we saw when called out to a private school pool. The equipment room was well set up with nothing lacking in that area, although the chemical controller settings were well different to what they should have been set at (which didn’t say much for the staff). On looking at the 25 m pool there were 5 skimmer boxes down one side, however the pool water returns were directly under the skimmer boxes instead of on the other side of the pool (at the very least). So much for the sanitation of the water in the lane on the opposite side of the pool. The pool was in use (by not many children) and cloudy. That should have set the alarm bells ringing.
Regardless, the important thing here is that while your chemical tests may show your readings to be within the acceptable range, you need to be aware of where those samples are taken from and, you need to be aware of how the water leaves the pool, the filtration system it travels through and where it returns to the pool.
Thought must be given as to where, how and how frequent water samples are taken and the quality and accuracy of your testing equipment. Testing equipment that is completely independent from any chemical supply company must be a first-choice preference. The same goes for any software that is used in conjunction with that testing equipment.
The Australian Pesticides and Veterinary Medicines Authority (APVMA) is the National Registration Authority for agricultural and veterinary chemicals. The APVMA operates the Australian system which evaluates, registers and regulates agricultural and veterinary chemicals. Swimming pool sanitising chemicals come under and, have to be approved by this authority.
The APVMA web page used to have an extensive list of diseases (and there were a lot of them) that could be transmitted in a swimming pool along with the level of risk. If nothing else, it reinforced the need for good sanitation. There seems to be a perception by many domestic pool owners that if the pool isn’t green then it’s alright to swim in it. Not true. We have often been asked if it is alright to let the kids swim in a green pool. Well, I wouldn’t swim in one. A pool will go green due to a lack of sanitiser usually combined with bad balance. A pool can be perfectly clear but not be sufficiently sanitised. You can see algae. You cannot see bacteria, viruses and parasites.
There are many things we cannot prevent getting into a pool, such as sweat, algae spores and medicines. But we can do something about preventing the major bodily waste products getting into a pool. As you might expect, urine and feces are the two biggest contributors to swimming pool sanitation problems. Urine adds nitrogen to the water which reacts with the chlorine compounds to create those smelly chlorine by-products that we do not want in a pool. Feces adds the obvious plus, bacteria, virus and parasites . Cryptosporidium (NSW Health Fact Sheet pdf) and Gardia (NSW Health Fact Sheet pdf) parasites enter a pool via feces. Both of these parasites are very difficult to get rid of, once they enter a pool and we definitely do not want them in a swimming pool. Prevention goes a long way. Anyone with diarrhea should not be allowed to enter a swimming pool. Showering with soap and water before using a swimming pool should be a regulated requirement and it is in some countries. Some Australian Aquatic Centers insist on it, in the interest of public health.
Given that most pools rely upon a chlorine compound as the sanitising agent then, maintaining the required levels of chlorine and pH will be needed to control bacteria and most viruses. It is for this reason that the two most important test results that we need to be concerned about when people are using a pool are: –
1.1 The level of Free Chlorine – is it sufficient? 1.5 to 3 ppm needed for pools (as per the table above)
1.2 The pH of the water – is it at an acceptable level? 7.2 to 7.6.
We don’t call Water, ‘Hydrogen’. We should not call Hypochlorous Acid, “Chlorine”. Hypochlorous Acid it the stuff everyone calls chlorine in a pool. Chlorine is a gas and yes, we are all allergic to that.
Pool sanitiser is Hypochlorous Acid and the quantity of these molecules is dependent on the pH of the pool.
. HOCl ↔ H+ + OCl–
hypochlorous acid hydrogen ion hypochlorite ion
As the pH increases the reaction moves to the right reducing the quantity of hypochlorous acid molecules (HOCl) while increasing the hypochlorite ion (OCl–) concentration.
The ‘chlorine’ we refer to here is termed the “free chlorine”. The products of the above reaction are involved in further reactions mostly with Urea (thanks to urine and sweat) resulting it what is called “combined chlorine”. “Total chlorine” is the total of free chlorine and combined chlorine. We do not want the combined chlorine and it is removed by the addition of more ‘chlorine’ to burn out those compounds. This is what is meant by ‘super chlorination’.
The ideal pH range is 7.2 to 7.6. Most automated systems operate at a pH of 7.4 or 7.5 for several reasons (swimmer comfort being one of them). At a pH of 7.5 the mix is 50% hypochlorous acid and 50% hypochlorite ion. Hypochlorous acid is a 120 times stronger oxidising agent than hypochlorite ion.
I found the following explanation in a report written by Professor Helen Stratton, Melody Christie and Lindsay Hunt from the University of Queensland.
“The hypochlorous acid (HOCl) and the hypochlorite ion (OCl–) binds to lipids, proteins and structures in bacterial cells or viral coats, rendering them oxidised and destroys the organism or viral particle. When a microbal cell comes into contact with a HOCl molecule, the chemical binding and destruction of the cell takes place in seconds. The OCl– ion is slower to oxidise the cellular components of microbes and takes several minutes.”
The Chloride ion comes in many molecular combinations
The main ones we are concerned with are: –
All of these will result in hypochlorous acid (‘chlorine’) being formed when added to water.
The instructions for using these chemicals are written on the labels.
Hypochlorous Acid can also be produced on site by using salt water chlorinators. The chloride ions come from the added Sodium Chloride, Magnesium Chloride or other Chloride Salts.
This is known as liquid chlorine. We believe that this is the best way to add chlorine to a pool because it is: –
The main drawbacks of using liquid chlorine are: –
However, you can have it delivered which will generally remove most of the problems.
This is known as granular chlorine. It is important to only use quality granular chlorine which is identified by having a 70% active ingredient. The molecule has a low chalk result when it breaks down in water. Cheaper brands have a much higher chalk component resulting (which can cause problems) when the molecule breaks down in water.
The advantages are that it is
The disadvantages are that it
This is known as Stabilized Chlorine. This product
The disadvantages are that
This is known as stabilized tablets. This product
The disadvantages are that
Stabilizer is a chemical we add to a pool to extend the life of your chlorine. It reduces the amount of chlorine that you need to use. Since, we generally only run a domestic pool filtration system for 8 hours a day and the chlorine is usually only added while the filtration is running. Stabilizer is added to lock up excess chlorine, which is then released slowly back into the pool as the available chlorine gets used up. Using stabilizer cuts down on the amount of chlorine that is required to be added to the pool. It saves money.
There is a lot of disagreement as to the level of stabilizer that is the optimum for a swimming pool. After a lot of research and talking to working (as against academic) scientists who have a thorough understanding of ORP, we reduced the level that we were recommending down to between 10 and 20 ppm. We have found that the pools look better and the levels of chlorine adequate, no matter when tested.
The required level of chlorine has been set based on the time it takes for the chlorine to kill pathogens. Stabilizer will lengthen that time. That is, it will take longer to kill the pathogen when stabilizer is present than it will without stabilizer. Green pools with high stabilizer levels are almost impossible to clean up. We find it better to empty, refill and re-balance.
Nothing explains the effects of stabilizer on ‘sanitation efficiency’ better than Section 6.5 “Fecal/Vomit/Blood Contamination Response” in the USA CDC Model Aquatic Health Code pdf. Section 18.104.22.168. This section demonstrates that very much higher levels of chlorine are needed to kill Cryptosporidium in a pool which contains stabilizer than is required in a pool which does not contain stabilizer. I rest my case, your Honour.
The pH in a swimming pool and many, many other solutions is measured, along with other things, to tell us what is going on in that solution. As stated above, the pH read, and the pool sanitation level are the two most important measures that we are concerned with during the daily operation of a swimming pool.
The correct pH level is important for
The chart below is displayed to show that what can easily be mistaken as a small change in pH is in fact, quite large. The different pH testing methods that are used for swimming pool water can usually only show a reading between 6.2 and 8.4, or over an even smaller range, so that quite often, we really don’t know what the actual read it is.
As the concentration of hydrogen ions increases in a body of water the pH decreases. As the hydrogen ion concentration decreases the pH increases.
The acceptable range is 7.2 to 7.8. Most operators aim for 7.4 or 7.5.
Daily changes in pH come about mostly due to the addition of sanitising chemicals or caused because of a sanitising method. Buffer is added to a pool to dampen those changes. Stabilizer is added to reduce sanitation demand. The fact that the pH does change is the important issue here. The changes need to be corrected for the range of important reasons as listed above.
Two gases are generated in the cell of a salt water chlorinator. Chlorine gas is produced on one set of plates which reacts quickly with the water to form hypochlorous acid (the stuff everyone calls chlorine). Hydrogen gas is produced on the other set of plates and, while some of it may dissolve in the water, most of it bubbles out of the pool, thus removing the H+ ions (the measure of pH), which increases the pH level. During the summer months when you are running your chlorinator for longer and at high settings the pH will increase more rapidly.
This is most likely the case. We add acid, usually Hydrochloric Acid. Hydrochloric acid quickly breaks down into its ions (which are H+ and Cl–) thus putting hydrogen ions back into the pool resulting in decreasing the pH level. It is recommended you dilute the acid in a bucket by adding the required acid to the water, then pouring it over the returns while the system is running. Run the system for 2 hours before allowing the pool to be used.
This doesn’t happen often, but it does happen, usually because of the incorrect addition of another chemical. We usually add Sodium Carbonate. This puts hydroxide (OH–) into the pool which increases the pH level, by it combining with the hydrogen ions to form water. Adding Buffer (sodium bi-carbonate) to the water will also increase the pH but not as quickly.
Regular testing and adjustment is needed to maintain the correct pH level in a pool. This should be done three times a day in a common property pool or no later than a couple of hours before using the pool. The alternative is to install an acid controller which measures and adds acid as required. With a salt water pool, you can usually expect to add a cup to two cups of acid a week, but this will depend on many things, so testing is needed.
“Alkalinity is the water’s capacity to resist changes in pH that would make the water more acidic.” In a pool we usually keep the alkalinity between 80 and 120 ppm which is done by the controlled addition of sodium bi-carbonate (called buffer). Buffer is used to control pH so that it stays consistent. It stops ‘pH bounce’.
Total Alkalinity requires a less frequent adjustment (than pH and chlorine) depending mainly on the amount of acid that has been required to maintain the required pH since the last alkalinity adjustment. Generally, in a domestic pool it is adjusted monthly. In a chemically controlled pool the adjustment may be required more frequently. The main point here is that the level does not change rapidly but is does change.
Should the alkalinity be accidentally increased to a much higher (than 120 ppm) reading (as well as a higher pH reading) then it will take large additions of hydrochloric acid to bring it (and the pH) back to the required level and that, in itself, indicates that buffer does its job.
All of this is well and good until you are dealing with a Salt Water pool. The hydrogen gas that is produced in the cell (along with chlorine gas) leaves the pool, steadily reducing the hydrogen ion (H+) concentration in the pool. Buffer will not contain the changes in pH under those circumstances.
We have, however observed, after hundreds of pool water tests that the pH can be contained at a lower than otherwise level in a salt water pool if alkalinity is kept at around 80 ppm. For that reason, we generally try to keep alkalinity at the lower end of the recommended range.
All water bodies seek a chemical balance between Total Alkalinity and Calcium Hardness levels at a pH level. Temperature and Total Dissolved Solids also figure in the equation, however, for the simplicity of understanding, we will look at the relationship between Total Alkalinity, Calcium Hardness and the pH level. The following is called the Taylor Watergram which is in the Australian Standard AS 3633- 1989 “Private swimming pools- Water quality”. While the standard is old and is termed ‘withdrawn’ it doesn’t look like any updates are in the system.
A straight line through this table will show the measures for water balance for a body of water. It could be a swamp. For a swimming pool we choose to set the levels that best service a swimming pool, its equipment and its patrons. It’s a compromise. A level that is perfect for one purpose will be totally wrong for another purpose, so a compromise is required.
Professor Langelier was involved in Sanitary Engineering during the early 1900’s. Among his many achievements he published a paper entitled “The Analytical Control of anti-corrosive water treatment” in 1936. In that paper he developed what is known as the ‘Langelier Index’. While his studies related to water sanitation, the same chemistry applies to swimming pools. The Langlier Index has been adopted by the pool industry to give a measure as to whether the pool water is corrosive or scaling. Other, far more sophisticated methods of measuring calcium hardness in waste water are now used.
The index is a result obtained when the pool pH, alkalinity, calcium hardness, total dissolved solids and temperature reads are entered into a formula. If the read is negative, the pool water looks for calcium (corrosive) and, if it is positive, it will drop calcium out of the water (scaling).
Generally, if your pool water tests fall within the acceptable ranges for pH, alkalinity and calcium hardness the result will be less than one either way of zero. If the calcium hardness has been adjusted to an acceptable level, then mostly the calculated result will be positive. This is due mostly to a rise in pH.
You can adjust you pool to a zero Langlier Index read, but the reality is, that for most domestic pools, that figure will change quite quickly. These changes can be better controlled by using a Chemical Controller, but you will still need to do a full test and adjustment on a regular basis.
The measure of Calcium Hardness is the measure of the calcium content in the pool water.
Always have your water tested before adding Hardness Increaser (Calcium Chloride). Ask for Palintest tablet count test (instead of a photometer test) to get an accurate result. Care needs to be taken when adding calcium chloride. If you overdo it, it is difficult to remove as this usually requires dropping water out of the pool.
Calcium Hardness is an important component of Water Balance. Generally, we like to keep it within the range of 180 ppm to 220 ppm. Occasionally we get results that are more than 400 ppm. That isn’t a problem if the other chemical levels are at an acceptable level. The higher the Calcium Hardness the more important it becomes to keep the pH correct. The more apples you have on a tray, the bigger the problem when the tray gets out of balance.
Calcium Hardness can often be dictated by the water supply in an area which could be high in calcium, while in other areas the pool water needs to have chemicals added to achieve a desired level. It is important to know the calcium hardness of your water supply. In Brisbane it sits at around 60 ppm. As luck would have it, for those of you with a sand filter, this level is perfect. When you backwash your filter, you are losing water containing 220-ppm (say) which is balanced by top-up water added (at the 60-ppm rate) to replace the water lost from the back-washing and from evaporation. In other words, the increase in calcium concentration resulting from topping up with town water after evaporation is balanced by the calcium lost during back-washing. Normally (in Brisbane) once the level has been set it will remain unchanged for a considerable length of time.
The optimum alkalinity levels are described above. This is the ‘second part of the alkalinity story’ and the reason we should not neglect maintaining the alkalinity level within the recommended range. We know what pH level we want; we know what calcium hardness level we have so now we can calculate what alkalinity level that is best for the pool we are dealing with, using a Langelier Saturation Index calculator.
The pH read is the one to watch and it needs to be checked daily and more than once throughout the day for a commercial swimming pool or, a commercial type pool, such as an apartment building pool. In a swimming pool which is sanitised using a salt water chlorinator, the pH will increase more rapidly than for pools with other sanitation methods.
On any given day the pH of most swimming pools will increase if acid is not added (with the exception being, pools using stabilized chlorine products). This means that if acid is not added when required, the pH will remain high for long periods of time. Total Alkalinity will most likely stay stable, the pH will most likely end up being well over 8, resulting in the water body becoming out of balance. A body of water does not like to be out of balance and will take measures to change this. If the pH is not controlled, you will have problems.
The water body decides that it has too much calcium and starts to drop the calcium levels. Initially this causes a dull, even cloudy look to the pool water. “The pool has lost its sparkle”. If this condition is allowed to persist then, just like making salt crystals in Grade 8, the calcium crystals start forming on the pool surface. Geologists call two of the calcium carbonate crystal forms “Dog Tooth” and “Nail Spur”. Yes, they are sharp and hard to get rid of if there are lots of them. The calcium drop-out will cause a dulling of the pool finish and can ruin the look of a pool. Other chemicals may also drop out with the calcium and compound the staining. If left untreated (and there goes your leisure time) at a very minimum the pool will need an acid wash at a cost of around $600 or in the worst-case scenario, further down the track, the pool may need resurfacing, starting at around $6,000. Surface damage will occur when calcium crystals start growing between the pool finish and the concrete wall, but this will take years. If you have reached this point, contact us, as we acid wash and resurface pools. Just use the “Request Pool Service” box below and we will contact you.
On the other hand, if the calcium level is too low and has not been corrected, then the body of water will seek out calcium and will proceed to remove it from your tile grout or pool finish. This can happen if the pool isn’t properly balanced when filled for the first time or after a total re-fill. In the long term the pool finish will become damaged, which again, will result in expensive repairs.
Get it right from day one. Many pool problems can be avoided by putting time and effort into the planning of a pool build. If this didn’t happen, it is not the end of the world.
If attending to the above is encroaching too much on your leisure time, then the simplest answer is to install a reliable pH controller. Controlling the pH in your pool is just as important as maintaining the sanitation level and in the long term it is probably more important.
The easier question to start with is “What causes Swimmer Discomfort?” We know that swimmer discomfort is caused by several things.
pH which is too high or too low will cause eye irritation and itchy skin. As mentioned many times above the control of pH is a very important component in swimming pool operation.
Hands up anyone who hasn’t peed in a pool. Bet you thought all that chlorine you could smell would kill any nasties you just contributed to the pool, right? Let’s hope so, especially if you have an infection. Well yes, the germs will be killed if the chlorine levels are correct, but you are still creating a problem. That strong chlorine smell that you are experiencing is most likely thanks to other people that have already peed in the pool. Urine adds urea, which is high in nitrogen. That nitrogen reacts with chlorine compounds that have been produced in the pool and, as a result, form the nasty chlorine by-products, that we do not want in a pool. It’s the high level of chlorine by-products that cause strong chlorine odours which swimmers find objectionable. If you have a strong chlorine smell coming from your pool, then it’s time to super-chlorinate; to burn out those chlorine by-products.
Tests have show the children can drink up to two litres of water during an extended period in a pool. Good idea to tell them that if they pee in the pool they are probably drinking it. Although maybe some kids would think that was funny and pee with glee.
Obviously, we all have a preference to a certain temperature range. Pool Heating.
No doubt salt water on the skin will leave salt when it evaporites and that will contribute to that clammy feeling when it absorbs moisture again, from the air, especially on a humid day. Obviously, this will happen after swimming in a salt water pool, but it will also happen after swimming in a liquid chlorine pool as these pools contain salt manufactured by the chlorine. But, that’s not the full story.
When you think about it, when do most of us swim? Unless you are a kid, it’s usually on a stinking hot day. What are you doing while you swim? Exercise. What does exercise make us do? Sweat.
What happens when you have a shower on a stinking hot day? Within the hour you get that clammy feeling again and you feel like having another shower. That’s what sweat does to you.
I’ve pinched the following from a web page called “health.how stuff works” because I know nothing about sweat except that the production of Urea and Ammonia contribute to the production of unwanted chlorine by-products.
by TOM SCHEVE
Your body is equipped with a very efficient air-conditioning system: 2.6 million sweat glands that cover nearly every inch of your body, and lots of ductwork. You can produce up to 6 pints (3 litres) of sweat an hour, though you probably top out at 2 pints (1 litre) an hour unless you live in a hot environment. This is great if you’re overheated, and not so great if you’re walking through the parking lot to a job interview.
Exercise and heat can cause you to sweat, but so can fear, excitement and anxiety. And, regardless of other factors, some people just plain sweat more than others. (Those who sweat way more than others may have a condition known as hyperhidrosis, a genetic disorder that causes excessive perspiration.)
But what exactly is sweat? And why does it make us smell different?
If you lick the back of your hand after a run, you’ll taste salt. Sodium is one of sweat’s main ingredients, along with chloride and potassium. All three are carried to the surface of the skin by water produced by coils within your sweat glands, and the salt stays on you after the liquid evaporates.
This mix of ingredients is what most of your sweat glands produce. We have two types of sweat glands: eccrine sweat glands that cover most of your body and produce a water-heavy mix, and apocrine sweat glands that produce a thicker, plasma like mix. Apocrine sweat glands are located where there are lots of hair follicles, such as your scalp, in your underarms and around the anus and genital region. Their brew is a little different — the regular recipe plus fatty acids and protein by-products like urea and ammonia.
Interestingly, none of the contents of sweat stink — it’s the bacteria on your skin’s surface that interacts with sweat produced by the apocrine sweat glands to produce the smell.
Additionally, sweat may have an ingredient that comes in handy in the fight against disease. The British Journal of Medicine reports German researchers discovered within sweat the presence of a natural antibiotic they named dermcidin, which may play a role in killing bacteria and viruses upon the skin’s surface [source: Josefson].”
Well, that says it all.