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OCR for page 1
Executive
Summary
DISINFECTION
Chlorination is the most widely used method for disinfecting water
supplies in the United States. It is convenient to use, elective against
most waterborne pathogens, and continues disinfectant activity within
the distribution system. Chlorination is the standard disinfectant against
which others are compared.
However, chlorination can result in the formation of trihalomethanes
(THM's) and other halogenated hydrocarbons. The discovery that some
of these products are carcinogenic for experimental animals has
prompted a reexamination of alternate disinfectants and procedures.
The comparative electiveness of 12 disinfectants or processes tor
inactivating microorganisms (bacteria, viruses, protozoa) were evaluated.
Chlorination, ozonization, and the use of chlorine dioxide come closest
to meeting the criteria established for a drinking water disinfectant.
The ultimate choice among methods will require weighing the
disinfectant efficacy, detailed in this evaluation, against the toxicity of
the products produced by the use of a particular method of disinfection.
CHEMISTRY
The major objective of the review of disinfectant chemistry is the
identification of products that are likely to be formed by the use of
1
OCR for page 2
2 DRINKING WATER AND H"LTH
. .
specific disinfectants. The identification of known and theoretical
products of disinfection, which is attempted herein, is intended to be a
guide to those contaminants that might require removal or toxicological
evaluation.
There is a large and rapidly growing body of scientific literature on the
products of chlorination in drinking water. Comparable information for
other disinfectants is scarce. This lack of data on alternative disinfectants
should not lead to the conclusion that they are free of the difficulties
encountered with chlorine. Quite apart from the question of their efficacy
as alternative disinfectants, there remains the question, "Will the
substitution of a disinfectant for chlorine in water treatment merely
produce a different set of by-products whose ejects on human health
may be as significant, or more so, than those products known to be
produced from chlorine?"
Clearly, each disinfectant chemical that was examined in this survey
produces by-products that may occur in actual water treatment applica-
tions. Of particular concern are the following substances that are either
known to or could result from the use of the various disinfectants.
From chlorine: the trihalomethanes (THM's), trichloroacetone
(CC13COCH3), and other largely uncharacterized chlorinated and
oxidized intermediates that are formed from the complex set of
precursors in natural waters; chloramines; chlorophenols; and the
largely unknown products of dechlorination.
From ozone: epoxides, which theoretically result from unsaturated
substrates such as oleic acid, although none have yet been found in
drinking water; peroxides; and other highly oxidized intermediates such
as glyoxal (OHCCHO) and methylglyoxal (CH3COCHO) from aromatic
precursors.
From bromine and iodine: THM's and other bromine and iodine
analogs of chlorinated species; bromophenols; bromoindoles; bromoani-
soles; plus the halogens themselves, which may remain in drinking water
as residual.
From chlorine dioxide: chlorinated aromatic compounds; chlorate
(C1O3-) and chlorite (C1O2-), which are often present as by-products or
unreacted starting material from production of chloride dioxide; and
chlorine dioxide itself.
This list, incomplete as it is, is compelling in that it shows that the use
of each disinfectant could result in products that should be examined in
more detail.
OCR for page 3
Executive Summary 3
GRANULAR ACTIVATED CARBON
Raw water and disinfected water supplies may contain organic com-
pounds that have been demonstrated to be carcinogenic or otherwise
toxic in experimental animals or in epidemiological studies.
Properly operated granular activated carbon (GAC) systems can
remove or effectively reduce the concentration of many of these harmful
compounds. Less is known about synthetic resins than about GAC, but
it is known that they can be applied to remove certain types of organic
contaminants. This study of GAC provides data on adsorption iso-
therms, percent removal, and competitive equilibria of a wide variety of
organic compounds.
The information available as of this date on the treatment of water
with GAC provides no evidence that harmful health effects are produced
by the process under proper operating conditions. However, there are
incomplete studies on the possible production of such effects with virgin
or regenerated carbon through
or
· reactions that may be catalyzed by the GAC surface;
· reactions of disinfectants with GAC or compounds adsorbed on it:
· reactions mediated by microorganisms that are part of the process;
· by the growth of undesirable microorganisms on GAC.
Studies are also needed on the properties of regenerated activated
carbons and on the adsorption of additional contaminants with potential
health ejects.
The data regarding microorganisms on GAC beds and their metabolic
products, including endotoxin production, are quite limited. The efficacy
of biodegradation is believed to be less than adsorption, although
microbial activity has been shown to remove organic compounds as
measured by group parameters such as total organic carbon, potassium
permanganate demand, chemical oxygen demand, and W absorbance.
No evidence was found for removal of specific organic compounds of
potential harm to health by microbial activity. The evidence for and
against prechlorination and preozonization enhancing biodegradation of
organic compounds is presented.
GAC serves as a catalyst for some reactions in water systems, e.g.,
oxidation, reduction, and polymerization. These reactions can produce
organic and inorganic species that were not present originally. However,
little can be said concerning the degree of their occurrence during water
treatment and their possible impact on public health.
OCR for page 4
4 DRINKING WATER AND H"LTH
Regeneration procedures influence the chemical properties of GAC.
This, in turn, influences adsorption, catalytic properties, and leachable
chemicals. The frequency of GAC regeneration is determined by the
organic compounds in the water and their competitive interactions. The
types and concentrations of organic compounds may vary widely in
different locations and seasons of the year. Competitive interactions are
complex and presently cannot be predicted without data from laboratory
and/or pilot scale tests on the water to be treated.
While there is ample evidence for the effectiveness of GAC in
removing many organics of health concern, more data are needed in the
quantification of any harmful health effects related to the use of GAC.
This need, however, should not prevent the present use of GAC at
locations where analysis of the water supply clearly indicates the
existence of a potential health hazard greater than that which would
result from the use of GAC.
Clarification processes (coagulation, sedimentation, filtrations remove
significant amounts of some organics, especially some types of THM
precursors and relatively insoluble compounds that may be associated
with Articulates. In some cases, the removal of THM precursors by
clarification may be sufficient to eliminate the need for an adsorption
process.
Representative terms from entire chapter:
gac provides
