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OCR for page 18
Ill
Chemical Quality of
Water in the
Distribution System
Even if one could eliminate the causes of contamination associated with
pipe breakages, cross-connections, back-siphonages, and other factors in-
herent in water distribution systems, there would still be changes in the
physical, chemical, and biological properties of the water as the result of
either chemical or biological activity.
Chemical activity producing changes in water quality within the
distribution system is associated with corrosion, leaching, deposition, and
reactions involving water treatment chemicals and their residuals. Each of
these topics is discussed separately in this section.
The materials comprising pipes, pumps, storage reservoirs, and other
system components can corrode through contact with water or may leach
constituents in water over time. Solubility and kinetic factors will deter-
mine whether these constituents will deposit (precipitate) onto pipe walls
or whether the materials used in the conveyance system will partially
dissolve or corrode into the water. Chlorine and other treatment chemicals
added at the water treatment plant or in the distribution system itself can
continue to react with organic compounds in the water. Thus, the
chemical content of water at the consumer's tap may be different from
that of water leaving the treatment plant or other source as a result of its
contact with materials in the distribution system and the time available for
reactions to progress.
18
OCR for page 19
Chemical Quality of Water in the Distribution System 19
CHEMICAL WATER QUALITY INDEXES
A number of water quality indexes have been used to predict whether
water will corrode materials used in distribution systems or home plumb-
ing units. In most cases, these indexes are used as a criteria for water
treatment control, but they can also be used as guide to the selection of
materials. Their principal advantage is simplicity, but they are not always
perfect predictors. Long-term tests of materials are more costly to con-
duct, but provide more direct evidence of water quality and its potential to
corrode given materials.
The oldest and most widely used index is the Langelier Index, which is
based on the solubility of calcium carbonate and the potential of the water
to deposit a scale that would protect the pipe. This index has been applied
to both metal and asbestos-cement pipe. A simplified version of the
Langelier Index, called the Aggressiveness index, was developed especi-
ally for asbestos-cement pipe to predict whether the water will either
deposit a protective scale or seek calcium carbonate saturation by dissolv-
ing the pipe's cement. A third index, the Saturation Index, is based on
solubility characteristics of a number of compounds, not just calcium car-
bonate. Its potential application for asbestos-cement pipe is discussed
below.
Langelier Index
The Langelier Index was developed in 1936 in order to investigate sys-
tematically the chemical relationships involved in the corrosion of iron or
galvanized pipe (Langelier, 1936~. It is sometimes referred to as the cal-
cium carbonate saturation index or simply as the Saturation Index. (To
avoid later confusion with the term Saturation Index, which is used for a
number of constituents in addition to calcium carbonate, the term Lange-
lier Index is used herein).
The Langelier Index (LI) can be defined as follows:
LI = pH = pHs,
where
(1)
PHS = saturation pH, the pH at which water of the measured calcium
and alkalinity concentration is in equilibrium with solid calcium
carbonate and
pH = actual or measured pH of the water.
OCR for page 20
20 DRINKING WATER AND HEALTH
In its simplest form, which is applicable between pH 7.0 and pH 9.5, the
equation for calculating pHs is as follows:
pHs = (pK2' -pKst) + pCa2+ + pAlk,
where
(2)
pK2' = negative logarithm of second dissociation constant for car-
bonic acid (H2CO3),
, _ [H+] [CO32
2 [HCO3 - ~
pKs, = negative logarithm of the solubility product of calcium car-
bonate (CaC03),
Ks' = [Ca2+~[CO32- i,
pCa2+ = negative logarithm of the molar concentration of calcium,
and
pAlk = negative logarithm of the equivalents of alkalinity (titrable
base), assuming that [Ark] = tHCO3- i.
The terms K2' and Ks' are dependent upon temperature and ionic
strength, which is a measure of ionic composition of the water. Correc-
tions for temperature and ionic strength are made for each calculation.
The utility of the Langelier Index is that it predicts whether calcium
carbonate will precipitate, dissolve, or be in equilibrium with solid
calcium carbonate. If it precipitates, calcium carbonate can form a pro-
tective scale on pipes including asbestos-cement (A/C) or metal pipe. If
calcium carbonate dissolves in water of a given quality, calcium carbonate
scale, previously deposited at the water-pipe interface, will be removed,
thus exposing the pipe surface to the corrosive effects of the water.
The Langelier Index is interpreted as follows:
When LI > 0, water is supersaturated with respect to solid calcium car-
bonate and will tend to precipitate and form a scale.
When LI = 0, water is at equilibrium.
When LI ~ 0, water is undersaturated with respect to solid calcium car-
bonate and protective calcium carbonate scales on the pipe may dissolve.
Aggressiveness Index
The A/C pipe industry developed the concept of an Aggressiveness Index
for use as a guide in determining whether A/C pipe would be appropriate
OCR for page 21
Chemical Quality of Water in the Distribution System 21
in a given situation. The original purpose of the index was to ensure the
structural integrity of the pipe. More recently, it has been used to predict
whether water quality degradation would occur from pipe dissolution. The
Aggressiveness Index is a simplified form of the Langelier Index and has
some shortcomings, which are noted below.
The Aggressiveness index (AI) is defined as follows:
AI = pH + log (AH),
where
AI = Aggressiveness Index,
A = total alkalinity, mg/liter as calcium carbonate, and
H = calcium hardness, mg/liter as calcium carbonate.
-
The Aggressiveness Index does not incorporate the corrections for
temperature and ionic strength. At a selected temperature (14°C) and
ionic strength (0.01) and by converting to alkalinity and calcium concen-
trations in mg/liter, it can be shown that:
AI = LI + 12.0 (Schock and Buelow, 1980~. (4)
Application of the Aggressiveness Index to determine when A/C pipe
should be used has been incorporated into standards published by the
American Society for Testing and Materials (1976) and the American
Water Works Association (1975b, 1980~. The need for water quality
guidelines is also acknowledged by the A/C Pipe Producers Association
(198()~. The most recent standards apply the Aggressiveness and Langelier
Indexes to relate water quality and the use of A/C pipe (Table III-1~.
These standards recommend that nonaggressive water (AI -12.0) be
used with Type I (nonautoclaved) or Type II (autoclaved) A/C pipe. Type II
pipe is recommended for moderately aggressive water (AI between 10 and
12~. For highly aggressive water, "the serviceability of pipe for such applica-
tions should be established by the purchaser in conjunction with the
manufacturer" (American Water Works Association, 1980~. Recognizing
the relationship between water quality and the use of A/C pipe, the U.S. En-
vironmental Protection Agency ( 1 979a) recently proposed that the Ag-
gressiveness Index should be -12 for water transported through A/C
pipe in order to prevent adverse effects.
Data published by Millette et al. (1979) provide a perspective on the
typical quality of water in the United States as it pertains to the use of A/C
pipe. Through a sampling of representative utilities throughout the
OCR for page 22
22 DRINKING WATER AND HEALTH
TABLE III-1 The Relationship of Water Quality.(Expressed as
Aggressiveness and Langelier Indexes) to Asbestos-Cement
(A/C) Pipea
Aggressiveness to Aggressiveness Langelier
A/C Pipe Index Index
Highly aggressive water < 10.0 < - 2.0
Moderately aggressive water 10.0 to 11.9 - 2.0 to-0.1
Nonaggressive water -12.0 -O
a From American Water Works Association, 1980.
United States, they determined that 52(~o of the water supplies had water
that was at least moderately aggressive (Aggressiveness Index between
10 and 121. Furthermore, 16.5% of the water supplies could be classified
as very aggressive. They concluded that these data suggest that as many as
68.5% of the U.S. water systems carry water that is potentially capable of
corroding A/C Type I pipe and that water supplies with very aggressive
waters ~ ~ 10) may be significantly corrosive to any type of pipe, including
cast iron, galvanized, and other types of pipes.
When using the Aggressiveness Index, one could assume that the me-
chanism for A/C pipe deterioration by aggressive waters is related to
release of calcium from the cement portion of the pipe. If the water is in
fact attacking the pipe, the cement could be dissolving into the water.
This would leave the asbestos fibers unprotected or not encapsulated
within the cement matrix. This would leave the fibers free to be released
into the water. These fibers could be released individually or in bundles.
Hallenbeck et al. (1978) theorized that once fibers are released into the
water, they can be further broken down so that counts of asbestos fiber
from the breakdown products are even higher. Thus, if A/C pipe is used,
there is a potential for consumers to be exposed to significant concentra-
tions of asbestos in some drinking water supplies.
The use of the Aggressiveness Index represents an advance over the
original preconception that A/C pipe is not subject to the effects of water
quality. As recently as a decade ago, Bean (1970) stated that A/C pipe
does not require lining, even with soft water, which could be classified as
aggressive water. Since that time, both manufacturers and pipe producers
have acknowledged that it is not judicious to use A/C pipe with aggressive
water. Thus, the Aggressiveness index has been a means for alerting sup-
pliers and users that A/C pipe cannot be used under all situations and
OCR for page 23
Chemical Quality of Water
in the Distribution System 23
that it is not resistant to corrosion in all cases. It is also simpler to
calculate than the Langelier Index.
Since the Aggressiveness Index (as well as the Langelier Index) is based
on calcium carbonate saturation, it should yield a fairly accurate predic-
tion of "nonaggressiveness" provided by a protective calcium carbonate
coating if water is oversaturated (Schock and Buelow, 1980~. However, if
the water is undersaturated with calcium carbonate, there is no reason to
expect the Aggressiveness Index to predict with accuracy the dissolution of
A/C pipe since calcium carbonate is only a minor constituent of the ce-
ment and calcium silicate is the predominant pipe component. F;urther-
more, the Aggressiveness Index does not account for temperature and
ionic strength as does the Langelier Index. Finally, the Aggressiveness In-
dex fails to account for protective chemical reactions in drinking water.
The Aggressiveness Index has been used for several years by pipe
manufacturers and the water supply industry. Therefore, the majority of
the data on water quality and A/C pipe deterioration contains informa-
tion oil the Aggressiveness Index, calcium, and alkalinity of the water.
In the absence of a better predictor of pipe performance. this index has
been used extensively and is still a simple first approximation for predict
. . ~
ring pipe performance.
Saturation Index
The Saturation Index has been proposed by Schock and Buelow (1980) for
use in predicting performance of A/C pipe under given water quality con-
ditions. In this approach, both the solubility of pipe components and the
possible protective coating of constituents in the water are considered.
The cement matrix of A/C pipe is a complicated combination of more
than 100 compounds and phases. Since electrochemical corrosion is not
an issue, the corrosion of A/C pipe is governed by solubility considera-
tions. Possible dissolution reactions in A/C pipe include:
Ca(OH)2 (s) - Ca+2 + 20H-,
Ca3SiO5(s) + 5H2O - 3Ca+2 + H4SiO4O+ 60H-,
Ca2SiO4(s) + 4H2O - 2Ca+2 + H4SiO4O+ 40H-,
Ca3Al2O6(s) + 6H2O - 3Ca+2 + 2Al +3 + 120H -,
(5)
(6)
(7)
(8)
where s indicates the solid phase.
The first constituent, Ca(OH)2, is lime, and the others are tricalcium
silicate, dicalcium silicate, and tricalcium aluminate. Solubility constants
for pure solids in Reactions 5, 6, and 7 are 10-5 20, 10-8 6, and 10- ~6. For
OCR for page 24
24 DRINKING WATER AND HEALTH
Reaction 8, it is not known. The actual solubility constants in pipe are dif-
ficult to estimate, since solids in pipe are highly substituted. Schock and
Buelow (1980) concluded that these materials are soluble under typical
water quality conditions, but that they dissolve slowly. Pipe dissolution by
Reactions 5 through 8 would increase pH, calcium, and alkalinity of water
in contact with the pipe. The Langelier Index or Aggressiveness Index
would also increase. These phenomena have been observed in several
studies that are described below.
Schock and Buelow (1980) have also used chemical equilibrium calcula-
tions to estimate whether calcium carbonate film would form to protect
pipe. Protection by metal precipitation has also been modeled for iron,
zinc, manganese, and silica since they could form dense solids.
Models were estimated using the aqueous chemical equilibrium com-
puter program called REDEQL.EPAK (Schock and Buelow, 19801. The
thermodynamic state of saturation was quantified by the Saturation Index
(SI), defined as the logarithm of the ratio of the ion activity product (IAP)
to the solubility product constant (Kso). For example, for hydroxyapatite,
the equilibrium reaction is:
Cas(PO4~0H`s' = 5Ca2+ + 3PO4 3- + OH
· (9)
Assuming activity coefficients equal to unity, the Saturation Index (SI)
would be:
[Ca2+~5[PO4 3- ]3~0H
SI = log K
so
(10)
If the solid and solution are in equilibrium, IAP = KSo and SI = 0. If
the solution is supersaturated, the SI is ~ 0, and undersaturation occurs if
SI <0.
The results of SI calculations are shown in Figure III-1. Initial water
quality is 1.0 mg/liter calcium, 24 mg/liter total carbonate, 0.24 mg/liter
magnesium, 0.5 mg/liter zinc, 0 mg/liter iron, 0 mg/liter phosphate, 20
mg/liter sodium, and 11-33 mg/liter chlorine. Based on this model, zinc
hydroxycarbonate [Zns(CO3~2(OH)6] would precipitate if the pH was
higher than 8. None of the other species would precipitate. Schock and
Buelow (1980) suggested that zinc hydroxycarbonate, once precipitated,
could be converted by reactions with silicates in the A/C pipe to a zinc
silicate coating, which is hard and should provide good protection.
This approach to predicting pipe performance by modeling equilibrium
characteristics of a number of protective solids in addition to calcium car-
bonate appears to contribute to the understanding of A/C pipe. Schock
OCR for page 25
Chemical Quality of Water in the Distribution System 25
6
4
X 2
LO
By o
of
o 2
_ _
G
V) -6
-8
ZnS(CO3)2 (OH )6
ZnCO3
- CaCO3 (calcite)
· Zn(OH )2
............ Cal OH )2
10 1 1 1/1 1 1 1.. ~; 1
O-
5 6 7 8 9 10
.
·'
. -
pH
FIGURE III-1 Saturation Index diagram for model system.
and Buelow (1980) have demonstrated the applicability of the Saturation
Index to several model systems. Although it is more difficult to use than
the LangeJier Index, it is expected to produce more accurate predictions.
CORROSION
Uhlig (1971) defined corrosion as "the destructive attack of a metal by
chemical or electrochemical reaction with its environment." He also noted
that the term "rusting" applies to the corrosion of iron or iron-base alloys
to form corrosion products consisting mostly of hydrous ferric oxides.
Therefore, other metals can corrode, but not rust.
A principal concern about corrosion in water distribution systems is the
possibility that its products will have an adverse impact on the health of
consumers exposed to them. Moreover, materials introduced into this
system to mitigate corrosion might themselves provide a source of poten-
tially hazardous chemicals. For example, protective coatings on pipes
could leach such hazardous constituents into the water, or chemicals added
to the water to inhibit corrosion could be toxic. Before discussing all of
these concerns, it is necessary to consider some of the mechanisms of cor-
rosion, its inhibition, and measurement.
OCR for page 26
26 DRINKING WATER AND HEALTH
Although the economic impact of corrosion in water distribution
systems is not of direct concern in this report, it is of some importance
because it provides an incentive for reducing corrosion. Ultimately, this
may have either a positive or negative effect on the generation of corrosion
products to which the consumer is exposed. The reduction of metallic cor-
rosion resulting from economic incentives is likely to benefit human
health; however, corrosion-inhibiting additives or coatings selected
without full awareness of their possibly toxic nature may be counter-
productive.
The Corrosion Process
Corrosion is most often considered to be an electrochemical process. That
is, electrons move through the corroding metal, and separate (but not
necessarily distant) locations at the metal-water interface act as anodes
and cathodes for the oxidation and reduction half cell reactions that oc-
cur. For example, as described by Larson (1971), the corrosion of an iron
surface in contact with water can involve the following reactions:
Anode: Fe - Fe++ + 2e-,
Cathode: 2e- + 2H2O - H2 + 20H-.
This cathodic reaction will generally occur slowly, but a faster alternative
one will occur in the presence of oxygen:
Cathode: 2e- + H2O + ~/2 O2 - 20H-.
For both cathodic reactions, two hydroxide ions will be produced and an
alkaline condition will result near the cathode. However, the ferrous ion
can be further oxidized by oxygen and precipitate ferric hydroxide:
2Fe++ + SH2O + I/2 O2 - 2Fe(OH)3 + 4H+.
This clearly generates acid.
In neutral or near-neutral water, dissolved oxygen is necessary for ap-
preciable corrosion of iron (Uhlig, 19711. The initial high rate of corrosion
will diminish over a period of days as the rust film is formed and acts as a
barrier to oxygen diffusion. The steady-state corrosion rate will be higher
as the relative motion of the water increases with respect to the iron sur-
face. Increased temperatures can also increase iron corrosion when it is
controlled by diffusion of oxygen to the metal surface.
OCR for page 27
Chemical Quality of Water in the Distribution System 27
Because the rates of electrochemical processes are related to the elec-
trochemical potential at the metal-solution interface, processes affecting
potential can hasten or reduce the rate of corrosion. This applies par-
ticularly to "cathodic protection," which is an important approach to cor-
rosion control. This process involves the external application of electric
current, modifying the electrochemical potential at the metal-solution in-
terface, thereby arresting the tendency for metal ions to enter solution. In
some portions of the water systems, such as water tanks, a more easily cor-
roded metal such as magnesium or zinc can be used as a sacrificial anode,
and cathodic protection is achieved without the use of an impressed source
of current.
Two principal types of electrochemical corrosion cells are of concern in
water distribution systems (Larson, 1971; Uhlig, 19711. The first results
from a galvanic cell, which is due to the contact of two different metals.
The rate of the resultant corrosion is increased by greater differences in
electrochemical potential between the two metals, as well as by increased
mineralization of the water. For such a cell, the anodic metal corrodes,
and the cathodic metal is, in effect, protected. Thus, when zinc-coated
(galvanized) steel corrodes, the zinc will generally do so at the expense of
the iron. Galvanic corrosion can be a problem when, for example, copper
is in contact with iron, which will tend to corrode by galvanic action.
The other and often more important corrosion cell is the concentration
cell. This cell involves a single metal, but different portions of the metal
are exposed to different aqueous environments. Such a cell could be
generated by one region of an iron surface exposed to oxygen and another
one nearly protected from oxygen by rust or other surface coatings.
Similarly, differences in pH, metal, or anion concentrations could
generate such a concentration cell. As noted above for the corroding iron
system, the anodic and cathodic reactions generate different corrosion
products, which can enhance the ability of the concentration cell to cause
corrosion.
Corrosion can also be classified with respect to the resulting outward
appearance or altered physical properties of the piping (Uhlig, 19711.
Uniform corrosion takes place at a generally equal rate over the surface.
Pitting refers to a localized attack resulting, in some cases, in marked
depressions. In water containing dissolved oxygen, oxide corrosion prod-
ucts can deposit at the pitting site and form tubercles. Dezincification is a
corrosive reaction on zinc alloys (e.g., brass, which contains copper) in
which the zinc corrodes preferentially and leaves behind a porous residue
of copper and corrosion products. Soft waters high in carbon dioxide con-
tent may be particularly aggressive to brass. Erosion corrosion can result
when the protective (often oxide) film is removed, such as by abrasion oc
OCR for page 28
28 DRINKING WATER AND HEALTH
curring in fast-moving waters. Normally, many metals in contact with
water will form such a protective oxide coating. One example of erosion
corrosion occurs near joints and elbows of copper pipes when water flows
at high velocities.
It is apparent from the above discussion that the corrosion process is
highly complex and is influenced by a large number of factors, including
the nature of the corrodible materials, the physicochemical quality of the
water, and the physical structure and hydrodynamics of the distribution
system.
Biologically Mediated Corrosion
The role of microorganisms in the corrosion of metal pipe in the water
distribution system has been recognized for some time (Hadley, 19481.
Microorganisms may influence corrosion by affecting the rate of cathodic
or anodic activity, producing corrosive end products and metabolites,
creating electrolytic concentration cells on the metal surface, and disrup-
ting or breaking down the protective film (natural or otherwise) at the
metal surface. The microorganisms may be heterotrophic or autotrophic
and may grow under aerobic or anaerobic conditions.
The pipe surface, joints, valves, and gates provide a wide variety of
niches for the growth of many different microorganisms that can alter the
chemical and physical habitat and produce conditions very different from
those observed in the water passing through the pipe. Although water in
the distribution network is generally well aerated, containing several
milligrams of oxygen per liter, microenvironments without oxygen may oc-
cur in the pipe. Concentrations of organic matter promote the growth of
aerobic microorganisms that deplete the oxygen and create anaerobic con-
ditions. Tuberculation, sediments, and pipe joints can yield protected en-
vironments in which neither dissolved oxygen nor disinfectant residuals
can penetrate.
Under anaerobic conditions, low oxidation-reduction potentials occur,
and, in the presence of sulfate, sulfate-reducing bacteria may proliferate.
Desulfovibrio desulfuricans can grow autotrophically under the above
conditions, reduce the sulfate to sulfite, and oxidize the hydrogen. Uhlig
(1971) suggested that an iron surface aids the process by which sulfate-
reducing bacteria function. These anaerobic bacteria generally possess
hydrogenate enzymes that act on hydrogen and require ferrous iron
(Booth and Tiller, 1960~. Since the possible corrosion products of iron
pipe are ferrous iron and hydrogen, the sulfate reducers may provide a
mechanism for the continual removal of corrosion products, thereby in-
fluencing the equilibrium of the corrosion reaction (Lee and O'Connor,
OCR for page 97
Chemical Quality of Water in the Distribution System 97
a small part of the total nitrogen is present as free ammonia. Reactions
with organic nitrogen compounds involve both cleavage of compounds
such as protein and heterocyclic nitrogen-containing compounds and the
formation of N-chloro organic species (Morris, 1967), which may be
analytically mistaken as free chloramines. There are insufficient data to
permit further characterization of these reactions or their effects on water
quality in distribution systems. It is clear from even the earliest data on
hypochlorite ion and aquatic humic reactions that the ultimate concentra-
tion of total trihalomethanes (TTHM) is a function of reaction time,
temperature, and pH, given an initial total aqueous carbon value and the
presence of chlorine as hypochlorite ion. The analytical methodology for
TTHM recognizes these variables and distinguishes between trihalo-
methane values measured at any point in time (instantaneous THM) and
those values for samples held in bottles for longer periods (5-7 days)
(THM formation potential) (Stevens and Symons, 1976~. Recent studies
(Brett and Calverly, 1979; DeMarco, personal communication, 1980) have
verified that THM values actually increase with residence time in distribu-
tion systems as long as both chlorine and organic precursors are available.
It cannot be ascertained whether this phenomenon is due to simple
homogeneous reaction kinetics of the hectic materials and hypochlorite
ion or to more complex heterogeneous reactions controlled by the physical
size and shape of the humic macromolecules. It is also possible that com-
plex homogeneous reactions occur with rate-controlling steps involving
the production of chloroform from several sites in the humic macro-
molecules, the reactivities of which are dependent on partial oxidation by
hypochlorite ion. It is attractive to assume that the THM increase is not
due solely to additional reaction of hypochlorite ion with extraneous
organic precursors in distribution systems, since good correlations have
been observed for municipal systems (Brett and Calverley, 1979) between
treatment plant effluent samples aged in the laboratory and samples
withdrawn from the distribution system after equivalent periods. In these
cases, supported by data on real systems, samples at the consumer tap (3
days system residence) may be approximately twice the THM values leav-
ing the plant (Brett and Calverley, 1979~.
Reaction of hypochlorite ion with extraneous organic material in a
distribution system is probable, although the dominant reaction products
may not be THM's. Organic nitrogen compounds have already been men-
tioned and additional humic input from soil contact should not be
disregarded.
Humic/hypochlorite ion reactions form a variety of other chlorinated
and unchlorinated reaction products in laboratory experiments (Table
III-22. Since the rates of these processes have not been investigated, it is
OCR for page 98
98 DRINKING WATER AND HEALTH
TABLE III-22 Nonvolatile Reaction Products of
Humic Materials and Hypochlorite Ion" h
Ch lorinated
Nonch lorinated
2-Chloropropanoic acid
Dichloroacetic acid
Trichloroacetic acid
I -Chloroprop-2-ene
1,3-dicarboxylic acid
2,3-Dichlorosuccinic acid
Dichloron~aleic acid
Dichlorofumaric acid
Benzoic acid
Hydroxytol uene
Trihydroxybenzene
Hydroxybenzoic acid
Benzene dicarboxylic acid
Benzene tricarboxvlic acid
Benzene tetracarboxylic acid
Benzene pentacarboxylic acid
UFron~ Christian et c`/1980.
h Analytical procedure involved n~ethylation ~ ith diazn~ethane. Therefore. all
acids Here identified as their methyl esters.
not possible to state whether their concentrations might be expected to in-
crease in distribution systems. Indeed, investigators have not even searched
for them in real water distribution systems.
As discussed above, the ubiquity of PAM's in water distribution systems
is well known (Blumer, 1976~. They may enter drinking water via at-
mospheric deposition in open reservoirs or through leaching from lining
materials in distribution systems.
The presence of hypochlorite ion in distribution systems may affect the
qualitative distribution of the PAM's in drinking water. Alben (1980b)
reported that abundant oxygenated and halogenated PAM's were found in
chlorinated coal tar leachate samples, whereas parent PAM's, alkyl- and
nitrogen-substituted PAM's, were predominant in unchlorinated samples.
At chlorination levels of 50 mg/liter, the dominant PAH in leachate
samples was fluorene, whereas phenanthrene dominated unchlorinated
samples. Carlson et al. (1975, 1978) have shown that exposure of PAM's to
aqueous chlorine reduces their concentration and produces material more
lipophilic than the parent hydrocarbon (Table III-23. The relevance of
the reactions and reaction products listed in Table III-23 to real distribu-
tion systems has not been established.
The effect of increased lipophilicity on bioaccumulation factors is
unknown as is the nature of the effect of chlorine substitution on car-
cinogenicity of the compounds. However, it is known that the car-
cinogenicity of chemical compounds is enhanced by the halogen content.
The growth of algae in the reservoirs of distribution systems may result
in the release of significant quantities of metabolic products into the
OCR for page 99
Chemical Quality of Water in the Distribution System 99
water. The excretion of a wide variety of relatively complex organic struc-
tures is apparently common to almost all species of algae and is not con-
fined to stressed cells (Barnesq 19781. Excretion of glycolate by Chlorella
and Chla'~'ydon~o~zas is well documented, and green algae tend to reduce
glycolate excretion in favor of higher molecular weight compounds as the
cultures age. Many other types of compounds have been identified in
cultures of various species (Table III-241.
Decomposition of algal biomass is another source of reactive organic
material. Approximately one-half of the biomass may be converted to
soluble, short-chain fatty acids in the presence of oxygen and bacteria.
The remainder may be converted to refractory humic-like substances.
The reactivity of these materials with hypochlorite ion or with chloramines
is virtually unexplored.
SUMMARY AND CONCLUSIONS
Although one can describe possible reactions between various organic
substrates and different oxidants in distribution systems, hard scientific
data on real distribution systems are extremely limited. Data suggest that
THM concentrations continue to increase in the distribution system as
long as both organic precursors and chlorine are present. It is probable
that the nonvolatile reaction products of humic material and chlorine also
increase in distribution systems, although there are no data for real
systems.
It is attractive to assume that chlorine will react with trace amounts of
other organic substrates in various distribution systems, e.g., PAM's from
TABLE III-23 Chlorination Products of Selected
Polynuclear Aromatic Hydrocarbons
Chlorine PAH,
PAH mg/liter ng/liter Product
Anthracene 2.0 552 Anthraquinone
Phenanthrene 19.3 820 9-Chlorophenanthrene
Fluoranthene 17.7 824 2-Hydroxy-3-chloro
fluoranthene
1-Methylphenanthrene 21 994 1-Methyl-9-chloro
1-Methylnaphthalene 24
Fluorene
531
24 1,166
phenanthrene
1 -Chloro-4-methyl-
naphthalene
2-Chlorofluorene
U From Carlson er al.. 1978.
OCR for page 100
100 DRINKING WATER AND HEALTH
TABLE III-24 Some Extracellular Products of Algae`'
Compound Type Examples
Acid salts Malate, glycerate, lactate, citrate, oxalate,
mesotartrate
Ketoacids a-Ketoglutaric, cx-ketosuccinic, pyruvic,
hydroxypyruvic, cx-ketobutyric, a-ketovaleric
N-Compounds Proteins, peptides, nucleic acids, free amino
acids
Carbohydrates Arabinose, glucose, mannitol, glycerol, com
plex polysaccharides
Lipids C lo fatty acids (from Ochro''`~`c~s dr ''7ic`~)
Enzymes and Acid and alkaline phosphomonoesterase
phosphorus (several species), high-molecular-weight
compounds organophosphorus compounds
Vitamins Ascorbate, pantothenate, nicotinate,
thiamine, biotin
Volatiles Formaldehyde, acetaldehyde, methyl ethyl
ketone, furfuraldehyde, acetone, valeralde
hyde, heptanal, geosmin
Miscellaneous 2,9-Dicetyl-9-azobicyclot4.2.1.1non-2,3-ene
compounds (very fast death factor; from A',ahae''
flosaquae)
a From Barnes. ~ 97~3.
pipe linings or excretion products from algae in open reservoirs. Unfor-
tunately, no existing experimental evidence would permit testing of these
assumptions.
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Representative terms from entire chapter:
drinking water
