Chemical Treatment of Private Swimming Pools

Disinfection Chemicals Used for Sterilizing Pools

The following disinfection chemicals are the ones commonly used in practice.

1. Chlorine gas
2. Sodium Hypochlorite,111/2 % available chlorine
3. Tropical Bleaching Powder, 33 % available chlorine
4. Calcium Hypochlorite, 70 % available chlorine
5. Chlorinated cyanurates.

 

1. When chlorine gas is used the end product of its reaction with organic matter, or by the action of ultra violet light (sunlight), is hydrochloric acid. The continued addition would lower the pH and would eventually turn the water acid (pH below 7.0). This would cause the formation of the higher chloramines (nitrogen trichloride) that cause eye irritation and the so called smell of chlorine.
2. It is therefore necessary to add 11/2 lb. of soda ash, or 2.4 lb. of sodium bicarbonate, for every 1 lb. of chlorine gas added to the water in order to maintain a constant pH.
3. Sodium Hypochlorite, 111/2% available chlorine, is not a stable product and should be stored in a cool area out of direct sunlight (ultra violet light decomposes the available chlorine). It contains free caustic soda as a stabiliser. The continued addition would increase the pH above the desired range because of this caustic soda. For every gallon of sodium hypochlorite added it may require up to 8 oz. of muriatic acid to keep a constant pH.
4. Tropical Bleaching Powder contains free lime as a stabiliser. This should not be added direct to a pool. A stock solution should be made up and the lime allowed to settle out and the clear solution decanted off. This, however, will contain soluble lime, which will require neutralising as the pH of the water rises.
5. Calcium Hypochlorite, 70 % available chlorine. Although this product has little insoluble matter and can be added direct to the water, it also contains free alkali and for every 1 lb. added it will require about 21/2 - 3 oz. of muriatic acid to neutralise it.
6. Chlorinated Cyanurates. These are a group of products now being used, comprising -
   

• sodium dichloroisocyanurate
• potassium dichloroisocyanurate
• trichloroisocyanurate
• dichloroisocyanurate
• cyanuric acid

The sodium dichloroisocyanurate containing 60% available chlorine is the one commonly used. This product is highly soluble (24%). Has an almost neutral pH, therefore will not alter the pH of the water, contains no calcium or scale forming compounds and is compatible with other chemicals.

The available chlorine is readily released in the water to react with bactericidal and organic matter and after the chlorine is used up, cyanuric acid is left behind. When the cyanuric acid level builds up to 25 - 30 ppm it acts as a screening agent and filters out the ultra violet light from the sun's rays. Therefore, after this stage, chlorine is not dissipated by the action of sunlight but only on organic matter.

It should be noted that cyanuric acid also retards the activity of chlorine in killing bacteria in the same way as ammonia does, although to a very lesser degree. This is shown in the following table giving the relationship of cyanuric acid concentration to killing time for bacteria at a constant free chlorine level of 0.25 ppm., and constant pH and temperature.

Cyanuric Acid (ppm)	Time for 100% Kill
0	1 minute
25	3 minutes
50	15 minutes
100	27 Minutes

This is one reason why the United States Public Health Service has recommended that because of this retarding action of cyanuric acid the limit should not exceed 100 ppm.

NOTE: - With a normal daily dosage of sodium dichloroisocyanurate a figure of 200 ppm of cyanuric acid could be built up in a swimming pool using diatomaceous earth filters within a season.

However, to offset this, it is possible to maintain a higher free chlorine level without loss to sunlight (killing rate is proportional to concentration of free chlorine). This is shown in the following table where the cyanuric acid is kept at 25 ppm with constant temperature and pH, the rate of bacterial kill is given for different free chlorine levels.

Chlorine Concentration (ppm)	Time for 100% Kill	Ratio Concentration
0.25	2 minutes	0.28
0.5	3 minutes	0.42
1.0	7 minutes	1.0

The obvious conclusion is therefore to change to another form of chlorine dosage after the cyanuric acid level is built up to 25 ppm.

Cyanuric Acid

In order to get maximum screening action at the start, cyanuric acid as such can be added to the water as a conditioning agent and then the chlorinated cyanurates can then be added as a daily dose or any other form of chlorine.

To obtain the recommended level of 30 ppm, 3 lbs. of cyanuric acid should be added for each 10,000 gallon capacity in the pool.

However, as cyanuric acid has only a low solubility (3 lbs. per 100 gallons at 77oF), it should be mixed into a slurry (2 - 3 lbs. per bucket of water) and broadcast evenly over the surface of the pool, preferably at night with the circulation running all the time to ensure that the chemical has dissolved by the morning.

The slurry should not be added directly to the inlet of the filter as the powder may "hang up" on the filter material and be lost during backwash.

Disinfectant Dosages

Dosages vary widely, much more being required under hot conditions or heavy loadings. The following table is for average conditions for a pool of 10,000 gallon capacity.

Chemical	Form as supplied	Dose	Frequency
Sodium Hypochlorite	Liquid	2 pints	Daily
Bleaching Powder or Chlorosene	Powder	1lb	Daily
Calcium Hypochlorite 70% available chlorine	Powder or tablets.	5 - 8 oz.	Daily
Sodium Dichloroisocyanurate	Powder	8 oz.	Once weekly

NOTE: - The above small doses for sodium dichloroisocyanurate apply only after the cyanuric acid content of the pool has reached 25 - 30 ppm.

To reach 25 - 30 ppm of cyanuric acid dose 4 oz. of sodium dichloroisocyanurate each day for 20 days.

After the cyanuric acid content has been built up to 30 ppm. then 8 oz. of sodium dichloroisocyanurate can be added weekly or sodium hypochlorite, bleaching powder or calcium hypochlorite at approximately 1/4 of the above rates.

When a pool has been stabilized with cyanuric acid it is essential that super chlorination be carried out every week or fortnight by adding 2 - 3 times the normal daily dosage. This will oxidize or "burn out" any accumulation of nitrogenous compounds and reduce the chloramine content of the water.

It will be found that after rain that free chlorine content of the water will disappear rapidly due to the impurities washed out of the air. Therefore irrespective of the normal dose given, more chlorine will be required to maintain a free chlorine level immediately after rain has ceased.

Free Chlorine Levels

With non-stabilized water, a free chlorine level of 0.2 - 0.5 ppm during the day is satisfactory.

With water stabilized with cyanuric acid the following levels are required:

- 25 - 60 ppm cyanuric acid requires 1.0 ppm. free chlorine

- 60 - 100 ppm cyanuric acid requires 1.5 ppm. free chlorine

It is not recommended that the cyanuric acid level should exceed 100 ppm.