Technical How-to - swimming pool non chlorine treatment purification specialist

- Organic and Nitrogen Content - . . . Continued

The product of the reaction of hypochlorous acid with ammonia is the formation of a group of compounds known as chloramines. These are unpleasant chemicals causing red eyes and mucosal irritation and have the characteristic "chlorine smell". The odour is misleading since this form of chlorine is not very effective as a disinfectant and long contact times may be required (White, 1972). Hypochlorous acid reacts to form a series of chloramines. They are corrosive gases and often cause damage to buildings which enclose swimming pools (see reactions below).

HOCl + NH3 ---> NH2Cl + H2O (MONOCHLORAMINE)
HOCl + NH2Cl ---> NHCl2 + H2O (DICHLORAMINE)
HOCl + NHCl2 ---> NCl3 + H2O (TRICHLORAMINE)
(White, 1972)

The above reactions are dependent upon pH, ammonia concentration and temperature. Chloramines can be destroyed by superchlorination. Superchlorination is the addition of very high concentrations of chlorine to the water. The pool cannot be used until the concentration of chlorine falls to within acceptable limits for swimming. Nitrogen trichloride (or trichloramine), an extremely objectionable gas, may be formed at this point (White, 1972).

Chlorine is able to form compounds with organic or carbon based matter and can perform addition, substitution and free radical reactions so that a wide range of chlorinated compounds is possible (Harold and Schuetz, 1966). Chloroform, which is toxic and a suspected carcinogen, is often found in chlorinated swimming pool waters, along with a series of chlorine-resistant organic nitrogen compounds (proteins) such as creatinine (White, 1972).

- Ultra-Violet Light and Temperature -

Heat and ultra-violet light must also be considered. At elevated temperatures, chlorine can be destroyed (Yahya et.al., 1989). High temperatures can exist within water heating systems. Ultra-violet light breaks down hypochlorous acid. Swimming pools are particularly prone to loss of sanitizer efficacy due to the action of sunlight. (White and Reinhold, 1972).

Isocyanuric acid is a sparingly soluble cyclic aliphatic compound which acts as a U.V absorber in the water. It is often called "pool conditioner" or "stabilizer" because it is able to prolong the life of chlorine in pool water. It forms a series of chlorinated isocyanurates having various compositions.

These compounds form a chlorine reservoir. Hydrolysis of N-Cl bond causes the release of hypochlorous acid (White, 1972). Studies of outdoor chlorinated pools show that pools without these stabilizers lose about 90 percent of their total chlorine residual on a sunny day in two or three hours. Pools treated with 25 to 50 milligrams per litre of cyanuric acid, however, under the same conditions, lose only 10 to 15 percent of their total chlorine (White, 1972). When "pool conditioner" is used at concentrations up to 100 mg/L, however, the free chlorine residual must be increased from 0.4 mg/L to 1.5 mg/L. Its use above 100 mg/L is not recommended. There is some concern about the toxicity of the dismutated N-base and its accumulation in pool waters.

- Application of Chlorine -

A typical ten metre pool contains about sixty thousand litres of water. The pool is often heated in order to extend the swimming season. A cupful of dry chlorine is routinely added every day. The cupful equates to about 150 g of calcium hypochlorite (55 kg per annum) which contains 65% available chlorine*. When chlorine gas dissolves in water only half of its chlorine content forms hypochlorous acid, the active sanitizer (HOCl). * The term "available chlorine" means available as chlorine gas.

Cl2 + H2O ----> HOCl + HCl
(chlorine gas) (hypochlorous acid) (hydrochloric acid)

Of the original 150 g less than one third (~50 g) can be effective as a disinfectant. The concentration of chlorine added to the pool water is therefore only 0.8 mg/L.

If the pool is in use then this concentration of chlorine is totally inadequate. Children, in particular, are prone to spontaneous urination (micturation), when they are immersed in warm water. Urine is the main source of nitrogen in pool water and contains about 2.5% of urea by weight. Urea is almost 50% nitrogen. One part of ammonia nitrogen uses up to 10 parts of chlorine. Therefore, less than half a litre of urine would be sufficient to destroy all of the free chlorine in the pool and additional nitrogen or organic substances will continue to build up in the pool water; ideal nutrients for algae and bacteria, and then impose an even higher chlorine demand for the next day.

The majority of domestic pool owners operate their pools on the basis of economy first so that the pump and filter are under utilised resulting in less than optimum water turn over. This is followed by a token dose of chlorine on a daily basis followed by a plethora of chemicals such as clarifiers (polyelectrolytes) algicides (quaternary ammonia compounds) pool conditioners (isocynuric acid) and stain removers (organic acids) from the pool shop because "the pool went off".

At this time the pool gets a "hit" of chlorine, usually about five litres of liquid pool chlorine to clean up the algae. The pH goes up because of the highly caustic nature of the hypochlorite solution and the pool becomes cloudy due to precipitation of calcium from the water. Mineral acid must be added to correct the situation but now the total alkalinity has fallen again,..... and so it goes on! The main permanent active sanitiser is not free chlorine but monochloramine. Monochloramine is a very poor bactericide.

A survey of the concentrations of free residual chlorine (hypochlorous acid) in swimming pools was conducted by Melbourne's Waverley City Council. Although operators believed they were dosing chlorine in accordance with health standards, 76% of pools tested were below the recommended levels (Nat. Times, 1980). Some of these pools contained organisms in numbers which were in breach of public health criteria. In a similar survey, Queensland University found that 50% of pools sampled contained coliform bacteria (Nat. Times, 1980).

Such high failure rates imply that the pool operators did not understand the principles of pool chlorination and consequently were unable to maintain their pools in a sanitary condition using chlorine.

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