Two very important but different areas in swimming pool operation depend on the correct chemical levels,
Luckily, both areas need chemicals which are common to both.
Errors with chemical levels will lead to expensive fixes in the short term in the case of sanitation and very expensive fixes in the long term in the case of water balance.
RECOMMENDED CHEMICAL LEVELS – QUEENSLAND HEALTH
The following table lists the chemical levels recommended in the Queensland Health “Water quality guidelines for public aquatic facilities”, December 2019.
Anyone managing a swimming pool should have a copy of the local authority’s pool operational guidelines. Guidelines are not law, but I would think that they are a very good place to start if you want to satisfy duty of care or compliance requirements.
THE ESSENTIALS OF THE DIFFERENT PARAMETERS
Free Chlorine (FC)
Includes Cl2(g) (Chlorine gas), HOCl(aq) (Hypochlorous Acid) and OCl-(aq) (Hypochlorite Ion).
Total Chlorine (TC)
Total Chlorine = Free Chlorine + Combined Chlorine
Combined Chlorine (CC)
Combined Chlorine = Total Chlorine – Free Chlorine.
Combined Chlorine is the result of the reaction of Free Chlorine with (mostly) ammonia which is added to the pool via sweat and urine. These compounds are called chloramines.
Combined chlorine is that strong chlorine stink that you get in some pools, especially indoor pools that are not designed with good circulation or are badly managed.
Combined Chlorine is removed by super chlorinating which burns out those chemical compounds we do not want.
Combined Chlorine can cause health problems, especially for pool attendants who spend more than just a couple of hours in an indoor pool that does not manage the elimination of chloramines.
Outdoor pools generally have good air circulation which removes these highly volatile (evaporite easily) chemicals. This doesn’t exclude the pool manager from having a management process in place to control the problem. That’s if you don’t want your patrons going home stinking of chlorine like we did as kids or was that a result of combined chlorine forming on our wet towels and clothes? Well now, there’s a thought. In any event we do not want combined chlorine in the pool.
For those of you with a scientific bent there are further reactions that can take place in a pool that can result in all sorts of combined chlorine compounds and that alone emphasises the importance a of regular super chlorination.
Saltwater pools have an ongoing super chlorination which occurs when the water passes through the cell or cells providing the turnover is high enough.
pH
Select either 7.4, 7.5 or 7.6 as your set point. The pH of our eyes is 7.45 as a guide. If you are operating a chemical controller using ORP as your chlorine addition controller, the 7.6 set point will result in a higher DPD test result for the same ORP than a set point of 7.4, because the controller will need to add more chlorine to maintain an ORP at that higher pH set point.
Total Alkalinity
This is the tricky one. Get this one wrong and it will cost you more money than necessary especially if you have a pH controller.
Alkalinity plays a big part in maintaining the correct pH in a swimming pool.
Making the effort to get a thorough understanding of the relationship between alkalinity and pH will give you far greater control over your pool and its running costs. A quality chemical controller is a tremendous asset when you are trying to achieve this.
Keeping your pH accurate and maintaining your alkalinity within a tight range prevents calcium drop out which adds to the turbidity of your pool water.
Turbidity prevention is not just chemical. Filtration system efficiency and site service procedures (as in, are they correct and being followed) will also affect turbidity.
Cyanuric Acid (CNA)
Calcium Hardness
While calcium hardness has nothing to do with sanitation it does play an important part in water balance.
When you have decided on your pH and worked out what alkalinity level is giving you the best economic results you then need to establish what calcium hardness level you need to be within the acceptable Langelier Index range of between minus 0.5 and plus 0.5.
The easiest way of doing this is to use one on the online aps that work this out for you.
You will find that if you are working with an alkalinity of around 80ppm then you will need a much higher level of Calcium Hardness in the pool. While this will generally be a one-off addition of calcium chloride you will need to take care in maintaining the correct pH.
Should the pH get too high there will be more calcium in the pool ready to respond to dropping out of the water. In other words, it will go visibly cloudy quicker.
Salt
The salt level required will be designated by the manufacturer of the chlorinator. You should have a copy of their operating manual.
Using 20kg bags, the formula is,
Number of bags required = Volume/20,000 x number of 1,000 ppm required.
So, to increase the salt level in a 320,000 litre pool by 1,300 ppm, then
Number of bags required = 320,000/20,000 x 1.3 = 20.8 bags, say 21 bags.
Phosphates
If you find you are going through more chlorine than normal, then there is a good chance that you have a build-up of phosphates. Kids (including the big ones) are pretty good at adding these to the pools.
Use a phosphate treatment process to remove them as quickly as possible. I find that LoChlor Starver M does an excellent job.
The level needs to be close to zero.
