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72
and is therefore unsatisfactory. Chlorination after filtration in
this country has so far been done by three methods :
(1) The addition of bleaching powder in solution;
(2) The addition of liquids known as hypochlorite solution
containing usually from 10 to 14 per cent, of free
chlorine;
(3) By the administration of chlorine gas in solution.
With the great variation in ' bathing loads ' and the consequent
large variations in pollution, the ' chlorine demand ' of the
water renders the problem of efficient chlorination without giving
rise to complaints, by no means a simple one. Proper chlorination
therefore, involves accurate dosage which must be capable of easy
and rapid variation to meet the large variations in the incoming
pollution, and so maintaining the correct amount of free chlorine
in the water, after its ' chlorine demand ' has been satisfied. This
in turn involves the provision of an appliance that is sufficiently
precise to give an estimation of the free chlorine, accurate between
the limits required, yet simple enough to be operated by a member
of the Baths staff, who is not necessarily an expert chemist.
The Ministry of Health recommended that the free chlorine
actually present in the water of the pool (not merely in the filtrate)
should amount to not less than 0.2 and not more than 0.5 parts per
million of water. The maximum of 0.5 parts per million of free
chlorine being a reasonable and sufficient margin of safety to
obviate complaints of smell of chlorine or smarting eyes.
A further development in sterilisation of swimming bath water
was the use of ammonia in conjunction with chlorine, forming a
compound known as chloramine. The use of chloramine in connection
with swimming bath sterilisation compared more favourably
than the use of chlorine alone, owing to the prolonged effective
action which is characteristic of such compounds. The reason is
that none of the chloramine is used up in the oxidation of the
organic matter, whilst a considerable proportion of added chlorine
is invariably destroyed in this manner. The addition of ammonia
seems to increase not only the efficacy of the chlorine, but the length
of time during which it is able to produce its disinfecting action;
also the method is less liable to produce complaint than the simple
administration of chlorine. As in the case of chlorine, the
ammonia is administered in gaseous form by a special apparatus