View report page

74
without a true understanding of the possibilities, and the methods
adopted should not go under the name of ' superchlorination and
dechlorination ' although this designation is often used. There is
a distinct difference between the procedure of chlorination followed
by dechlorination and that of superchlorination, and it is the full
understanding of this difference which has led to the development
of a new chlorination technique. It is only within the last few
years that the importance of pioneer work carried out during the
first world war by Sir Alexander Houston, Director of Water
Examination of the Metropolitan Water Board, has been fully
realised. He demonstrated that high doses of chlorine destroyed
taste and odour, and he advocated that when difficulty arose from
taste created or intensified by chlorine, the solution was ' chlorine
boldness 'not' chlorine timidity,' and suggested that in the
chlorination of water half measures were worse than none. Attention
was focussed on superchlorination again in 1939, and later
by a great many investigators in this form of water treatment; this
eventually led to the development of a new superchlorination
technique which, because of the characteristic' hump ' and ' dip '
in the dosage-residual curve, obtained on most waters during early
investigations, has become widely known under the name of ' breakpoint
' chlorination.
Previously established practice was to chlorinate to a chlorine
residual. It was modified by being called a chloramine residual
when ammonia was applied together with chlorine. Now the
modern practice is to chlorinate to one of two definite specific determinable
chlorine residuals, either combined available chlorine
residual or free available chlorine residual.
Modern chlorination practice, of which break-point process
forms a part, is based on an understanding of these two residuals,
how to determine them and how to obtain and maintain them.
When carrying out break-point chlorination, it is essential
that careful and accurate pH control of the water should be maintained.
pH values are in the nature of a scale of measurement of the
intensity of acidity or alkalinity of the water. The scale being
divided into 14 parts, and from the theoretical reasoning the
absolute neutrality point is 7. Above 7 indicating the alkaline
condition and below 7 the acid condition, increasing in intensity
as the figures differ more and more from 7.