LIFETECH s.r.o. • Sumavska 15 • 602 00 Brno • Czech Republic
FREE AND COMBINED CHLORINE
In addition to the compounds necessary for disinfection and oxidation of impurities,
substances with adverse effect to human health are present in pool water treated by
gaseous chlorine or by chlorine reagents (hypochlorite, chloroisocyanuric acid). Properly
designed and operated pool technology suppresses the formation of undesirable disinfection
by-products and enables permanent fulfilling of the requirements on pool water quality.
Compounds present in chlorine treated pool water
FREE CHLORINE (disinfection)
Disinfection by-products (DBP)
COMBINED CHLORINE
Other DBP
TOTAL CHLORINE
FREE CHLORINE – monitored parameter
FREE CHLORINE content is a sum of concentrations of hypochlorous acid and its anion,
hypochlorite. Relative distribution of these two species depends on pool water pH. FREE
CHLORINE value is, however, pH-independent.
Disinfection by-products (DBP)
DBP are undesirable compounds with an adverse effect to human health. Some of them are
classified as suspected human carcinogens.
COMBINED CHLORINE – monitored parameter
COMBINED CHLORINE content corresponds to the concentration of chloramines in pool
water. Chloramines are major disinfection by-products in chlorinated pools with
monochloramine as the predominant species at the pool water pH. Their disinfection effect is
substantially lower than it is at FREE CHLORINE compounds. Because chloramines are toxic
and irritating substances, their presence in pool water is undesirable. Chloramines belong
among the main contributors to “chlorine smell” of the pool water.
Other DBP
In addition to the COMBINED CHLORINE compounds, other substances fall into the class of
DBP. These substances are present at low concentrations at usual conditions. They are toxic,
contribute to the “chlorine smell”, and some of them are classified as suspected human
carcinogens.
TOTAL CHLORINE
This parameter is the sum of the free and combined chlorine contents:
TOTAL CHLORINE = FREE CHLORINE + COMBINED CHLORINE
FREE AND COMBINED CHLORINE DETERMINATION
FREE CHLORINE – monitored parameter
FREE CHLORINE content can be monitored by suitable sensor (measuring probe, electrode),
or it can be determined in a water sample by DPD method. At the DPD method, the sample
becomes red-purple and the color intensity is compared with color scale or measured by a
photometer. The results are given in mg/l Cl2 or in ppm (1 mg/l=1 ppm).
TOTAL CHLORINE
TOTAL CHLORINE content is determined by the means introduced above, i.e. continuously
by special sensor or by DPD method in a taken sample. The results are given in mg/l Cl2 or
in ppm (1 mg/l=1 ppm).
COMBINED CHLORINE – monitored parameter
It is determined as the difference between the TOTAL CHLORINE and FREE CHLORINE
values:
COMBINED CHLORINE = TOTAL CHLORINE - FREE CHLORINE
Avoid mistakes in determination by DPD method!
• Reagents for determination of free chlorine and combined chlorine are different.
• It is necessary to keep the recommended time periods for color development (reaction)
and reading of the value.
• The sample cells used for free and total chlorine determinations cannot be interchanged.
• The DPD method can be used only in concentration range given for the test-kit.
• At free chlorine concentrations higher than the maximum value given for the test-kit,
very rapid partial or total discoloration of the sample proceeds after addition of the
reagent for free or total chlorine determination and, therefore, the determined value is
false. In such cases the sample has to be diluted in a suitable proportion by distilled
water or, at least, by water without any oxidants, free and combined chlorine
compounds inclusive.
• The results can be reliable only in the case the sampling was done properly.
• The samples have to be analyzed immediately after sampling.
Determination of free and combined chlorine is described in ISO 7393-2 Standard.
VALUES OF FREE AND COMBINED CHLORINE
World Health Organization in “Guidelines for safe recreational water environments, Vol. 2:
Swimming pools and similar environments”, WHO 2006
Free chlorine:
the limit values 1.0 or 0.5 and 3.0 mg/l, hot tubs up to 5.0 mg/l
Combined chlorine: less than 0.2 mg/l
Germany - DIN 19643-1: 1997-04 Standard
Free chlorine:
the limit values 0.3 and 0.6 mg/l, (hot tubs 0.7 and 1.0 mg/l)
Combined chlorine: the limit value of 0.2 mg/l
USA - National Spa and Pool Institute, recommendation
Free chlorine:
1.0 to 4.0 mg/l
Combined chlorine: none
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ELIMINATION OF ADVERSE EFFECTS OF
COMBINED CHLORINE TO HUMAN HEALTH
The main input of chlorination by-products from pool water into bather’s organism is through
the skin. However, volatile by-products are present also in the air and are inspired by visitors
and pool attendants and take part in the corrosion of building fabric. Irritation of eyes and
mucous membranes, drying out the skin as well as “chlorine smell” are well known
inconveniences. Besides these effects, much more significant potential health risk occurs.
This risk is given by chloramine (combined chlorine) toxicity and by toxicity or even
carcinogenicity of other chlorination by-products.
These are the reasons why the combined chlorine content in the pool water has to be
monitored and why limit values are specified. However, it is difficult to meet these values
even at ordinary care for pool water quality.
In public pools, the best solution is the completion of existing technology by UV reactor,
ozonization or ozone/UV unit, which will decrease the values of combined chlorine to a
minimum. UV radiation or ozonization will support the effect of free chlorine and, therefore,
make possible to keep the free chlorine concentration at the low level limit required by
authorities. Installation of these technological units will find expression not only in the
improvement of water quality but also in water savings and savings of the energy necessary
to heat the water up.
In the case of private and lightly loaded pools, adverse effects to human health can
be completely avoided by chlorine-free treatment of pool water. This solution is based
on a combination of one of the above mentioned technologies (UV, ozone, and ozone/UV)
and application of BlueSpark® preparation, an algaecide with bacteriostatic effects.
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