Can you smell chlorine? Then it's probably a badly managed swimming pool.

Posted by on Monday, May 23, 2016 Under: Disinfection
combined chlorine

When the disinfectant gets into the pool water, the free chlorine contained within in immediately gets to work and starts combining with pollution. Once chlorine combines it hangs around in the pool water and is no longer effective as a disinfectant and is now actually more of a pollutant itself. It needs to be removed from the pool by a combination of dilution and filtration.

Combined chlorine is measured by calculating the difference between the total chlorine and the free chlorine.
Free Chlorine (DPD1) + Combined Chlorine (total minus free) = Total Chlorine (DPD3)

Much of the chemical pollution in swimming pools is in the firm of ammonia, which is a decomposition by-product of urea (which comes from sweat and urine etc. from bathers). This ammonia reacts with chlorine to form what are known as ‘chloramines’. These are measured as combined chlorine by subtracting the free chlorine reading from the total chlorine reading.

Combined chlorine levels should be kept as low as possible, and certainly no more than 50% of the free chlorine level.

There are four main categories of chloramines to be aware of:

•    Monochloramine 
•    Dichloramine
•    Trichloramines
•    Organic Chloramines  

Monochloramine is one of the chloramines which contribute to the level of combined chlorine in the water. It is produced when chlorine reacts with ammonia. In simple terms, the reaction is:

Chlorine + Ammonia > Monochloramine

Monochloramine isn't really that much of a problem, in fact, it acts as a disinfectant itself, although it is nowhere near as effective as free chlorine. Things don't stop there though and further chemical reactions will take place to produce dichloramine and trichloramine (these are the chloramines that are the cause of problems and a pool plant operator needs to know how to get rid of them and minimise their production in the first place).

Dichloramine is one of the chloramines that contribute to combined chlorine levels in the pool water. It is the second stage of the chemical reaction that takes place between chlorine and ammonia. It is produced when chlorine (or, to be more specific; hypochlorous acid) reacts with monochloramine (which are produced during the first stage of the reaction):

Hypochlorous Acid + Monochloramine > Dichloramine

You don't want high levels of dichloramine in your pool as it can go on to form further chemical reaction by-products such as trichloramine. It's an unstable chemical though and as long as your pH is at the correct level it will break down fairly easily. At this point, your combined chlorine readings will reduce because there are no chloramines left to react with. This is known as 'breakpoint chlorination'.

Typically, in practice, you won’t know how much monochloramine and dichloramine you have in your pool as normal testing procedures don't distinguish between the two. As long as you keep combined chlorine levels under control (i.e., less than half of the free chlorine), you won’t really need to know, but if you start having difficulties keeping combined chlorine levels low enough, you may need to carry out a DPD2 test in order to find out which of the chloramines is contributing most to the combined chlorine levels.

Breakpoint Chlorination
In basic terms, as far as pool plant operators are concerned, breakpoint chlorination describes the point at which there is twice the amount of free chlorine than combined chlorine.

Imagine a swimming pool that has high levels of pollution. If you were to introduce some much-needed chlorine into the pool, it would quickly end up as combined chlorine as it literally combines with bacteria etc. kills it and then becomes virtually useless as a disinfectant. In fact, combined chlorine could now be classed as pollution and it is combined chlorine that makes people’s eyes sting and causes irritation to nasal passages etc. Combined chlorine is something we want as little of as possible, as close to zero as we can possibly get it and certainly no more than 1.00mg/l.

As all the chlorine we have introduced has now become combined chlorine, we need to add some more. We always need to have some chlorine available (or, free) in order to quickly neutralize microbiological contamination. This is referred to as 'free chlorine'. We need the free chlorine levels to be at least double the combined chlorine levels. Free Chlorine is measured with the DPD 1 test, Total Chlorine is measured with the DPD 3 test, and Combined Chlorine is the difference between the two.

As you continue to add free chlorine into the pool, and as that free chlorine continues to convert into combined chlorine, the combined chlorine reading (the difference between the free and the total chlorine readings) will increase. Eventually though, if everything is operating properly, the chemical reactions that were discussed earlier will continue and progress even further:

Monochloramine + Dichloramine > Hydrochloric Acid + Nitrogen

The above reaction is dichloramine decomposing, and as long as this reaction continues, there will come a point where there is no more dichloramine left for the monochloramine to react with. At this point, the combined readings will start to fall, rather than continue to rise. This is referred to as breakpoint chlorination, which can be more easily achieved by adopting the following good practices:

  • Get people into the habit of taking a shower before swimming so that there is less pollution available for the chlorine to combine with;
  • Dilute the swimming pool water with enough fresh water (30 litres per bather, per day);
  • Ensure enough chlorine is being dosed into the swimming pool (an automated system is always recommended for any type of commercial facility);
  • Make sure that the pool turnover time is fast enough for your type of pool;
  • Make sure you’re not overloading your pool.
  • Keep the pH within the recommended parameters.

If you follow these steps, you should have no problem achieving breakpoint chlorination. However, if these good practices are not followed, things can start to go wrong. Instead of the dichloramine decomposing away, it starts to react with the hypochlorous acid in the pool and forms hydrochloric acid and trichloramine. The reaction looks like this:

Dichloramine + Hypochlorous Acid > Hydrochloric Acid + Trichloramine (Nitrogen Trichloride)

Trichloramine (and to a lesser extent dichloramine) are the chemicals that can cause the strong chlorine smell in badly-run swimming pools. It can also cause irritation to the mucous membranes by forming hydrochloric acid on them when they react with water. They can also trigger asthma attacks in people already suffering from asthma, but are not thought to actually cause the condition.

Organic chloramines are formed by the reaction between chlorine and organic nitrogen compounds. These are introduced into the pool at the same time as ammonia from bather urine/sweat etc. The key difference for the pool operator with organic chloramines is that they are stable and will not break down by the addition of more chlorine. In fact, the opposite is true; the levels will increase with the addition of more chlorine. Therefore, the levels must be kept under control by a process of dilution at the appropriate rate.

In : Disinfection 

Tags: "combined chlorine"