Shocking your pool is one of the most important procedures in swimming pool maintenance but we often get a lot of customers asking why it’s so important? So in this blog post I will explain.
The main reason why you need to shock treat your pool is to prevent algae growing in the water. Algae are microscopic plant life that grows very quickly and rapidly in sunny and warm conditions and if they are not killed you will soon be faced with a horrible green mess instead of a nice sparklingly clear pool! The truth is nearly all outdoor pool owners in the UK will have encountered this problem in the past. No matter how good your filtration system is or how vigilant you are with your chlorine levels algae spores will still find a way into your pool water. Commonly this is via dirt from rain or bathers.
Maintaining a free chlorine level between 1 and 3 parts per million (ppm) will kill the majority of algae spores that enter the water but over time the remaining algae spores will build up and develop into algae blooms (green mess). This is especially true in sunny and warm conditions as mentioned earlier. This is where shocking your pool becomes imperative. Once a fortnight, or once a week in the summer, you should use Calcium Hypochlorite Granules (Chlorine Shock) to super-chlorinate the pool and increase the chlorine level up towards 10ppm. This rapid increase in chlorine will obliterate any algae spores that are in the water and it will ensure you maintain a sparklingly clear pool. Remember to let your chlorine level drop back into the recommended range before re-entering the pool.
Just a quick note on why you need to use Calcium Hypochlorite Granules (Chlorine Shock). Calcium Hypochlorite is an unstabilised form of granular chlorine that is most commonly used as a shock treatment in swimming pools. It has 70% available chlorine which makes it stronger than other chlorine based chemicals such as Sodium Hypochlorite (14-15%) and Stabilised Chlorine Granules (56%). Another major advantage of using calcium hypochlorite as a shock treatment is that it doesn’t contain any stabiliser. There can be occasions where you need to add a large shock dose in order to kill algae. If you used stabilised chlorine like the aforementioned Stabilised Chlorine Granules, then it would still kill the algae but it would also increase the stabiliser level in the water. Over time the stabiliser level will build up and eventually it will get too high and you will have to reduce it by draining a proportion of pool water and replacing with fresh water.
Another reason why shocking your pool is important is to reduce the combined chlorine level. This is more applicable to public pools that experience a high bather load but it is important for domestic pool owners to be aware of it as well. When you add chlorine to the pool water it produces hypochlorous acid (HOCl), which is commonly referred to as free chlorine. As noted earlier the free chlorine is responsible for killing bacteria, viruses and algae spores in the pool water but even if you maintain a suitable free chlorine level (1-3ppm) it is inevitable that some pollution will still enter the water. This could be in the form of dirt and algae spores from the rain or perspiration, oils and cosmetics from bathers.
When the pollution enters the water a chemical reaction will take place with the free chlorine. This reaction produces combined chlorine which consists of the chloramines called mono-chloramine, dichloramine and nitrogen trichloride. The mono-chloramine is responsible for the distinct chlorine swimming pool smell and the nitrogen trichloride is very irritating to bathers eyes and noses. Although combined chlorine contains a lot of chlorine (hence the smell) it is all locked away therefore it does not kill bacteria, viruses and algae. So in summary you do not want combined chlorine in your pool.
There are ways in which you could minimise the combined chlorine level without using any chemicals. For example you could use a pool cover when the pool isn’t in use and you could insist that bathers showered before using the pool. Sounds straightforward but the first proposal can be expensive and we all know the latter just won’t happen. Therefore it is important to reduce the combined chlorine level by regularly shocking your pool with an oxidising agent such as Calcium Hypochlorite (Chlorine Shock) or Oxy Shock (Potassium Mono-Peroxysulphate). The addition of either of these chemicals will remove all pollution and reduce the combined chlorine level. In normal circumstances once a fortnight should be adequate but if you experience a high bather load, sustained period of hot weather or a substantial rainstorm then you may need to shock your pool once a week.
Finally although I have talked about combined chlorine you don’t need to worry too much about testing for it. Providing you shock your pool regularly a domestic family pool should not encounter any of the problems caused by combined chlorine. However if you are interested in testing for combined chlorine you can revisit a previous blog.