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While we in the swimming pool industry are familiar with the term "chlorine demand," we are not the only industry which recognises this situation. In Chemistry of Water Treatment, Faust and Aly quote from Standard Methods a definition of chlorine demand as the quantity of chlorine reduced or converted "to inert or less active forms of chlorine by substances in the water." Faust and Aly go on to add, "Since chlorine is a non-selective oxidant, almost any substance in the water in a reduced valence state will react and consume chlorine."

We see evidence of consumption of large quantities of chlorine in swimming pools. During the 1992 season, Technical Services gathered a great deal of data on chlorine demand, success of various treatment methods and correlation of chlorine demand to ammonia in the water. The 1995 season has begun with indications that chlorine demand is possibly even more widespread than in 1992. In applying knowledge that we have gained in past situations, we have some actions which can help to make dealing with chlorine demand somewhat less difficult.

First and foremost is the use of the Chlorine Demand Test Station. If a dealer does not have this station, or does not use it, determining the quantity of chlorine needed to satisfy the demand for a pool is a hit or miss proposition. By using the Chlorine Demand Test Station, we are able to accurately determine how much shock needs to be added to overcome the various components in the water which cause rapid consumption of the chlorine. Failures encountered when using the Chlorine Demand Station are related to improper test procedures, most often attempting to complete the test before the time designated for reaction has been completed. Second most common error is waiting beyond the designated time before titrating the sample.

Of concern for many dealers are the pools which have a chlorine demand (i.e. cannot maintain a residual following a shock), yet remain clear. While we have not determined the cause of this situation, we have found that it is not always necessary to meet the demand to successfully operate these pools. Some very definitive criteria are involved in deciding not to shock a pool which does not have a chlorine residual:


1. The pool must remain clear. This means no haziness, even in the middle of a hot afternoon with a heavy bather load. Should cloudiness occur, it is time to do a chlorine demand test and shock to meet the demand immediately.
2. There must be adequate sanitiser being added to the pool on a daily basis. This means that the correct number of sticks, tablets or pucks must be dissolving into the pool. This should be checked every 3 to 4 days by visually examining the product to see that it is eroding at the expected rate. If the product is not eroding properly, determine why this is occurring.
3. The pool must be shocked every week, not every other week. This means a weekly addition of 1 bag of Burn Out per 40,000 litres or 1 bag of Burn Out 35 per 30,000 litres.


In our work in 1992, we were successful in the treatment of a clear pool which exhibited chlorine demand 94.4% of the time using the above program. A singular cause for chlorine demand has not been determined. We do know that pools which are closed longer in the Spring (i.e. sit for a week or two of warm weather with no sanitiser) are more prone to chlorine demand. We know that ammonia can be present in rain water and cause a demand. We know that the introduction of fertiliser into the pool will create a demand. We know that the introduction of any compound which can be oxidised by chlorine will result in a demand.

Chlorine demand will continue to require our attention. We can use the chlorine demand test to help properly treat pools experiencing this problem. Careful monitoring and treatment may be successful for a clear pool without having to add a large quantity of shock all at one time.

The above information is supplied by Bio-Lab and represents its best interpretation of available technical information at the time of preparation. The sole purpose is to supply factual information to Bio-Lab customers. It is not to be taken out of context nor used as support for any other claim not made herein.