WATER TREATMENT 1323
here to ensure that the product is stripped of butane; oth-
erwise serious explosion hazards exist. It has a low energy
input and has minimized corrosion and scale-forming prop-
erties. However, so far this process has not been examined
on a very large-scale basis.Other Processes There are a number of other processes
which are being considered for desalination, such as solar dis-
tillation, but so far this has been restricted to use on a very
small scale such as survival kits. Ion exchange, such as was
described previously, has some potential here but it is very
much limited down to approximately 3000 mg/liter solids.
A hydrate process has been considered where propane is added
to form a hydrate and react with the water, leaving the salt
behind. Then the propane hydrate is decomposed to recover the
propane and the water; this one is rather difficult to handle.DisinfectionAs mentioned earlier, water has long since been identified
as a means of distributing pathogenic organisms among
society. The purpose therefore of disinfecting water sup-
plies is to prevent the spread of water-borne disease by
destroying pathogenic organisms. Most of the physical
and chemical treatment processes described previously
will remove most of the micro-organisms to some extent.
However, very small numbers of microorganisms which
are viable and pathogenic are all that are required to bring
about disastrous epidemics. Thus, disinfection is consid-
ered to be a necessary final step before treated water is
delivered to a municipal system. This may not be the case
in certain industrial supplies.
A physical process for disinfection was previously
described using ultra-violet irradiation. Other forms of
chemical disinfectant are the halogens such as chlorine, bro-
mine, iodine, and the powerful, unstable oxidant, ozone. In
North America chlorination is the most common of the dis-
infectant processes used, for two reasons. Firstly, it is fairly
simple to handle, can be manufactured inexpensively in bulk
and delivered to the site, can be applied under fairly con-
trolled conditions, and can maintain a measurable residual
in the water supply to indicate safety at all points on a water
distribution system. The first attempt at continuous chlorina-
tion of a public water supply was made in England during
1904, and subsequently in 1908 in Jersey City, New Jersey,
USA. There are certain disadvantages of chlorination, in
that a high residual chlorine will bring about a taste which
is unacceptable to many people; and chlorine furthermore
will react with certain micro-constituents of water, such as
phenols, to bring about substantial odors (chlorophenols)
quite out of proportion to the concentration of the causative
chemicals. The addition of chlorine to water releases a group
of substances, all of which have some disinfecting proper-
ties. The substances so released are:1) hypochlorite ion(OCl);
2) hypochlorous acid (HOCl);3) monochloramine (NH 2 Cl);
4) dichloramine (NHCl 2 );
5) nitrogen trichloride (NCl 3 );
6) organic compounds containing chloride; and
7) chlorine dioxide (ClO 2 ).Hypochlorite ion and hypochlorous acid are known col-
lectively as free available chlorine residuals. The follow-
ing substances are known as chloramines: NH 2 Cl, NHCl 2 ,
NCl 3 , and organic chlorine compounds. The chloramines
are brought about by the reaction of hypochlorous acid with
ammoniaNH 3 HOCl → NH 2 Cl H 2 O
NH 2 Cl HOCl → NHCl 2 H 2 O
NHCl 2 HOCl → NCl 3 H 2 O.The process which brings about the various chloramines
are shown above. Chlorination is applied in a series of dif-
ferent forms as follow:Superchlorination This process represents the addition of
very high concentrations of chlorine which are intended to
oxidize not only the pathogenic and potential pathogenic
microorganisms in the system, but also to oxidize those
organic compounds which might bring about taste and odor.
Following superchlorination, a step involving dechlorina-
tion which involves the addition of sulphur dioxide, sodium
bisulphite, or sodium sulphite or some similar reducing
agent. The bisulphite is frequently used in practice because
it is cheaper and more stable.
If there is any amount of ammonia naturally present
in the water, a strange phenomenon will occur such as
shown above in the graph. On the initial part of the graph,
labelled 1 to 2, the ratio (molar) of chlorine to ammonia
is less than one and the residual chlorine is essentially all
monochloramine. In the next section, between 2 and 3, the
oxidation of ammonia and reduction of chlorine continue
until the complete oxidation reduction occurs at point 3.
At this point once again, all the residual chlorine is in theChlorine ResidualChlorine Dosage123FIGURE 20C023_006_r03.indd 1323C023_006_r03.indd 1323 11/18/2005 1:32:36 PM11/18/2005 1:32:36 PM