Chemical Treatment of Private Swimming Pools

Chlorination in Private Swimming Pools

Chlorine as such, or in its various forms is still the main sterilization chemical used. This paper is limited to its application, a process called chlorination.

The amount of chlorine required to maintain a free chlorine level in the water depends in part on the chlorine demand of the water, ie the pollution brought in by the bathers in the form of albuminoid and nitrogenous compounds, mainly from urine, perspiration, dead skin, hair and mucus. Other factors that increase the chlorine demand are windblown dust, leaves and grass clippings etc.

Bright sunlight is the most important factor in causing the loss of free chlorine from open air pools due to the presence of ultra violet light. This depends on the intensity and duration of the sunlight. During early morning and late afternoon, the effect is not as great as at mid-day. Also the ultra violet light is more intense in summer time or in locations closer to the equator.

By the same token, on cloudy days the ultra violet light is filtered out and the loss of chlorine is negligible without loading. Free chlorine is more reactive with bacteria at a lower pH level. Rise in the temperature of the water also increases the chemical activity of the chlorine and it reacts faster with pollution water.

When the chlorine or chlorine compounds are added to the water it reacts with organic or inorganic mater and at the same time starts to destroy bacteria.

Some of the pollution brought in by the bathers breaks down to form ammonia compounds. The chlorine reacts with these compounds to form chloramines. When all the free chlorine is used up to form chloramines this is called "Marginal chlorination". Combined chlorine in the form of chloramines is bactericidal and is more stable than free chlorine to the action of sunlight. However, its rate of sterilization is much less. It takes about 50 to times as long to kill bacteria than free chlorine, and gives rise to tastes and odour forming compounds that irritate the eyes. Also combined chlorine does not control algae.

Free Residual Chlorination

With certain stabilizing compounds, or continuous dosing equipment, it is found that a free chlorine level can be maintained in the presence of combined chlorine. Maintaining this free chlorine level will continuously destroy the objectionable chloramine compounds and, what is most important, will ensure a safe biological condition.

For every part of ammonia in the water it will take 8 to 10 times the quantity of chlorine to destroy it. The destruction of the chloramines is a slow process and takes two or three hours to complete. As chloramines are being destroyed, fresh ones are being formed by additional pollution introduced by other bathers.

It is therefore only during periods when the pool is not being used, or at night-time, that sufficient chlorine can be added to destroy the chloramines. If sufficient chlorine has been added at night it will be found that the next morning the chlorine in the water will all be in the free form.

Breakpoint Chlorination

To explain breakpoint chlorination we shall take for example a swimming pool, which has been chlorinated, to breakpoint conditions. We shall allow any free chlorine to disappear to zero.

Now we shall let a group of schoolchildren enter the pool for an hour without any chlorination being produced. This will allow a build up of natural pollution.

We shall then have the children leave the pool and whilst nobody is in it, we shall commence to chlorinate. By referring to figure 1, as we add chlorine to the water, we shall notice that we get an increase in the total residual as measured by the ortho-tolidine test, This will continue to rise as we add chlorine till we reach a point A. Now from this point a strange phenomenon occurs: Instead of the total chlorine residual rising with the increase in chlorine dosage, it begins to get less until we reach a point B when the chlorine residual is nearing zero. Further additions of chlorine from this point will show a corresponding increase in total chlorine residual.

The reason for this change in total chlorine residual is that when the children left the water, they left behind a certain amount of pollution in the form of organic compounds and also ammonia compounds.

On part (1) of the curve we have chlorine oxidizing organic matter and being consumed, at the same time it is reacting with ammonia compounds to form monochloramines. When all the ammonia compounds in the water have reacted to form monochloramine, point A of the curve has been reached. From then on, any further additions of chlorine start oxidizing the monochloramine to dichloroamine then to trichloramine and then finally to nitrogen. This means that from point A the total chlorine residual is getting less as the chloramines are oxidized to nitrogen until point B is reached. This is the breakpoint when all the ammonia compound have been destroyed and from then on, any further additions of chlorine will give a corresponding increase in free chlorine residual as shown by point 3 in the curve.

As explained before this breakpoint curve can only be achieved overnight or when there is no loading in the water. However sufficient chlorine should be added so that a certain amount of it is always remains in the free form.