If you have a swimming pool at your premises or you are planning to have one installed, it is essential for you to be aware of the basics of swimming pool water treatment. The following article emphasizes on the importance of pH and disinfection.
What is pH?
pH is a measure of how acidic or alkaline your pool water is. The pH scale ranges from zero to fourteen; 7 being neutral. The figure below shows an indication of the pH scale.
The pH of swimming pool water shall range from 7.0 to 7.6. It essential that you to test the pH on a regular basis to determine whether the pool water lacks acid (Hydrochloric acid) or alkali (soda ash). It is strongly advisable not to add acid or alkali without having first carried out a pH test. Several user-friendly pH test kits are available on the market.
What to do if the water is too alkaline?
If your pool water is above 8.0, you shall add acid to the pool. The most common acid added to pool water is hydrochloric acid (30 – 33 % concentration). Table below shows the approximate volume of acid that needs to be added at pH 8.0 & pH 8.4 per cubic meter of water (1 cubic meter = 1000 Liters)
| pH | 8.0 | 8.4 |
| Volume of acid to be added per cubic meter (1000 L) | 11 ml | 18 ml |
For example, if your pool volume is 20 m3 (20,000 L,) and your pool pH is 8.4, the amount of acid to be added is 20×18 = 360 ml (0.36 L).
Each time you add acid to your pool, allow the pump to run for at least one hour and repeat the pH test again to verify whether the water pH is within the recommended range.
Your water is too acidic
If the pH of your pool water is below 7.0, you shall add soda ash (sodium carbonate). As a guide it can be considered that 22.5g of soda ash when added to pool water will raise the pH of a 10,000 L pool by 0.1 pH point. For example, if your pool contains 20,000 L (20 cubic meters) and your water pH is 6.0, you shall add roughly 540g of sodium carbonate to reach a pH level of 7.2.
Same as for acid addition, allow the pump to run for at least one hour and repeat the pH test again to verify whether the water pH is within the recommended range.
Calculations as per above example:
Measured pH: 6.0.
Targeted pH: 7.2.
pH difference = 7.2 – 6.0 = 1.2
Above it is mentioned 22.5g of soda ash raises the pH of a 10,000 L swimming pool by 0.1 pH point. Therefore, a difference of 1.2 in pH = 1.2/0.1 = 12 pH points
Hence for a 20,000 pool, quantity of soda ash to be added = (20,000/10,000) x 12 x 22.5 = 540g or 0.54 kg.
Effects of low and high pH values
- Low pH values, below 6.5 cause eyes and skin irritations, make your hairs become brittle and are responsible for the corrosion of mechanical pool equipment (pump, ladders, etc.)
- High pH values, above 8.0 cause skin rashes, inactivates chlorine, and may result in cloudy pool water.
- Low & high pH values damage vinyl liners.
Chlorination of Swimming Pools
The most widely used disinfectant for pool is chlorine (calcium hypochlorite) at 70 % concentration. To keep your pool clean and safe for bathing, make sure the free chlorine concentration in the water is between 1.0 and 3.0 mg/l (analyzed using DPD No.1 pills)
Recommended Chlorine dosage
To get a chlorine concentration of 1.5 mg/l, you shall add approximately 2.2 g chlorine per 1000 L of water. However, it must be taken into consideration that chlorine will first react with organic nitrogen compounds, organic matter and inorganics present in your pool water. So, to be on the safe side add 3 to 3.5g chlorine to every 1000 L. Also, chlorine depletes rapidly on a hot sunny day due to ultraviolet rays. It can take only 2 hours for 90% of chlorine in the water to get depleted in direct sunlight.
As such, for residential pools, it is advisable to add chlorine early in the evening to get best disinfection results. For commercial swimming pools, automatic constant chlorine dosing is strongly recommended.
However, rapid depletion of chlorine can be minimized by the addition of stabilizer (Isocyanuric acid).
Isocyanuric Acid (Pool Water Stabilizer)
You can add stabilizer to your pool to prolong the chlorine residence time in the water. The recommended range varies from 30 mg/l to 60 mg/l – that is you need to add between 30 to 60g per 1000 L of water. It essential to note that Isocyanuric acid does not evaporate like chlorine. The stabilizer is lost only through backwash and splashing (pool water losses). You need to check your stabilizer level each week and add only the correct amount if your concentration fells below 30mg/l. Above 100 mg/l isocyanuric acid in pool water, chlorine becomes too stable and no longer produces free chlorine radicals. Thus inhibiting the action of chlorine.
Note: Some chlorine pills sold on the market already incorporates stabilizer and even algaecide and clarifier. The dosage is often indicated on the package. It is very important to respect the written instructions.
It is a must that you carry out regular pH, free chlorine and isocyanuric acid tests to ensure safe bathing.
Sources:
https://www.lathampool.com/blog/maintain/why-is-my-swimming-pool-ph-level-so-important/
https://www.precisionswimmingpools.com/the-effect-of-sunlight-on-chlorine/
https://www.wikihow.com/Raise-pH-in-Pool
https://www.poolpro.com.au/pool-and-spa-tips/17-chemicals/36-how-do-i-use-hydrochloric-acid
