عدد المساهمات : 3514
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: Chemicals from water treatment and distribution/كيماويات معالجة محطات المياه وشيكات التوزيع الجمعة مارس 02, 2012 12:22 pm|| |
Chemicals from water treatment and
Chemicals from water treatment and distribution reach drinking-water by the most direct
They fall into three broad categories:
• substances resulting from the addition of chemicals used in the treatment process for
coagulation and disinfection — these chemicals are intentionally added and can give rise
to residues or by-products;
• disinfectants that are deliberately added with the intention of maintaining a residual in
distribution, usually to the tap — these chemicals may also give rise to by-products;
• substances that leach from materials used in distribution or plumbing, or that arise from
the corrosion products of pipes.
The WHO Guidelines for Drinking-water Quality (WHO, 2004) cover a significant number
of potential substances from water treatment or distribution , but
only a few of these substances are of practical significance.
It is important that water supply
agencies properly manage any chemicals that they use. In many cases, the best method of
control is through management practices, such as optimization of the treatment process, and
regulation of materials and chemicals that come into contact with drinking-water, rather than
through monitoring and chemical analysis.
This chapter gives guidance on the importance of potential chemicals derived from water
treatment or distribution, from a management perspective.
Chemicals used in treatment
Disinfectants and disinfection by-products
The three chemicals most commonly used as primary disinfectants are chlorine, chlorine
dioxide and ozone. Monochloramine, usually referred to as chloramine, is used as a residual
disinfectant for distribution.
Chlorine is the most widely used primary disinfectant and is also often used to provide
residual disinfection in the distribution system. Monitoring the level of chlorine in drinkingwater
entering a distribution system is normally considered to be a high priority (if it is
possible), because the monitoring is used as an indicator that disinfection has taken place.
Residual concentrations of chlorine of about 0.6 mg/l or more may cause problems of
acceptability for some consumers on the basis of taste. Monitoring free chlorine at different
points in the distribution system is sometimes used to check that there is not an excessive
chlorine demand in distribution that may indicate other problems in the system, such as
ingress of contamination.
Chlorine reacts with naturally occurring organic matter in raw water to form a range of
unwanted by-products. Guideline values have been established for a number of these byproducts.
The compounds most widely considered as representatives of chlorination byproducts
for the purposes of setting standards and monitoring are the trihalomethanes
(THMs) which include chloroform, bromodichloromethane, chlorodibromomethane and
Haloacetic acids (HAAs), such as monochloroacetate, dichloroacetate and
trichloroacetate, can also be formed as the result of reaction of chlorine with organic matter
contained in raw water. Some countries monitor HAAs as well as THMs, but HAAs are much
more difficult and expensive to analyse than THMs.
THMs and HAAs continue to develop within the distribution system; thus, monitoring can be
complex. Optimizing coagulation and filtration is most important in helping to remove the
precursors of these by-products and will, in turn, reduce the formation of THMs, HAAs and
other unwanted by-products.
In order to ensure the microbial safety of drinking-water, disinfection should never be
compromised in trying to meet guidelines for any disinfection by-products.
Chlorine dioxide breaks down to leave the inorganic chemicals chlorite and chlorate. These
are best managed by controlling the dose of chlorine dioxide applied to the water.
can also be found in hypochlorite solution that has been allowed to age. There is no guideline
value for chlorate because of limited data on its toxicology, but this chemical has been shown
to be less toxic than chlorite and is present at lower concentrations.
Controlling chlorite will
generally also adequately control chlorate.
Ozone, used as a primary disinfectant, cannot be monitored in drinking-water, because it
leaves no residual.
Ozonation in the presence of inorganic bromide, which can occur
naturally in raw water, can give rise to low concentrations of bromate.
The analysis of
bromate is difficult and expensive, because a number of other inorganic substances that
interfere with the analysis may be present.
It is considered, therefore, that bromate monitoring
is a low priority, and that management should instead involve controlling the conditions of
Monochloramine, used as a residual disinfectant for distribution, is usually formed from the
reaction of chlorine with ammonia.
Careful control of monochloramine formation in water
treatment is important to avoid the formation of di- and trichloramines, because these can
cause unacceptable tastes and odours.
The formation of nitrite as a consequence of microbial
activity in biofilms in the distribution system is a possibility when monochloramine is used as
a residual disinfectant, particularly if ammonia levels are not sufficiently controlled.
Coagulation and flocculation are important barriers to microbiological contaminants and are
key processes for reducing naturally occurring organic matter and turbidity, which can
seriously affect the efficiency of disinfection.
Chemicals used as coagulants in drinking-water
treatment include aluminium and iron salts, such as aluminium sulfate, polyaluminium
chloride or ferric sulfate.
No health-based guideline values have been set for aluminium and
iron, because neither is considered to be of significance to health when used under normal
circumstances in water treatment.
However, both substances can give rise to problems of
discolouration and deposition of sediment in distribution if present in excessive amounts.
concentrations in drinking-water above which problems are likely to occur are 0.3 mg/l for
iron and 0.2 mg/l for aluminium. This concentration of aluminium should be achievable by
any water treatment works, but a well-run large treatment works should be able to achieve a
routine average residual value of 0.1 mg/l.
The best management strategy for both aluminium and iron when used in treatment is to
ensure that coagulation is optimized to prevent excessive amounts remaining in the drinkingwater.
Sometimes organic polymers, known as coagulant aids, are used to assist with coagulation.
These polymers may contain residual acrylamide or epichlorohydrin monomers. Monitoring
for these chemicals in drinking-water is not normally appropriate, because measurement in
water is very difficult.
Instead, these chemicals are managed by specifying a maximum
amount of residual monomer in the polymer and a maximum concentration of polymer that
can be added to the treatment process.
The WHO Guidelines for Drinking-water Quality
(WHO, 2004) give additional guidance on the approval and control of chemicals and
materials in contact with drinking-water.
Other chemicals and materials used in water treatment
A number of other chemicals may be added in treatment. These include substances such as
sodium hydroxide for adjusting pH and, in certain circumstances, chemicals for fluoridation
In all cases it is appropriate to specify the quality of the chemicals added
so that the final water does not contain unacceptable concentrations of unwanted
Ensuring that chemicals used are of an appropriate quality is generally best
managed by product specification rather than by monitoring drinking-water.
Guidelines for Drinking-water Quality (WHO, 2004) has a section on approval and control of
chemicals and materials for use in contact with drinking-water that provides guidance on
Ion-exchange resins and more advanced treatment processes based on membranes are
increasingly used in drinking-water treatment.
It is possible that chemicals can leach from the
materials used in the manufacture of these systems; therefore, these too should be managed
by appropriate product and materials specifications.
The most widely used metal for pipes and fittings in distribution systems is iron, which may
give rise to corrosion products. These products can cause discoloration at the tap if the
distribution system is not managed correctly.
Monitoring for corrosion products is not
appropriate; instead, it is necessary to manage the problem of corrosion and the accumulation
of corrosion products in distribution.
In some circumstances, iron hand pumps can give rise to
discoloured water if they are corroded by water that is too acid.
In such cases, it may be
appropriate to screen the raw water for low pH and, where a low pH is detected, consider
using alternative materials for the pumps.
The corrosivity of water is a function of many
factors, including pH, low alkalinity, chloride and sulfate ions, sediment and microbial
activity; this topic is covered in more detail in the WHO Guidelines for Drinking-water
Quality (WHO, 2004).
Lead, copper and sometimes zinc may be present in drinking-water, as a consequence of the
use of these metals in pipework in public, commercial and domestic buildings. Monitoring is
complicated by the fact that both occurrence and concentration will vary from building to
building and at different times of the day.
Concentrations will usually be greater the longer
the water is standing in the pipe, so first-draw water will usually have higher levels than
water from a fully flushed system.
Copper and zinc are less likely than lead to occur at levels
of concern, except in very new buildings or where highly corrosive water is supplied;
however, concentrations may be increased in some circumstances when copper piping is used
as a means of earthing the electrical system in a building.
Lead frequently occurs at
concentrations greater than the guideline value in situations where lead pipes and solders are
present. Lead is also a component of brass, bronze and gun-metal, which are used in fittings
in plumbing systems. In some circumstances, fittings made of these metals can be a
significant contributor to the concentrations of lead at the tap.
Monitoring of metals from plumbing is difficult because of variations in concentration with
time and the fact that the levels are frequently property specific.
Where lead pipes are present
in a large number of buildings, the most important requirements are public health surveillance
(to ensure that there is no significant public health problem) and identification of the
buildings that have lead piping.
Consideration of lead in drinking-water should be part of an
overall lead-reduction strategy, because lead exposure from other sources may be more
There are a number of possible approaches to reducing lead levels in drinkingwater,
ranging from targeted replacement of lead pipes to central control of corrosion to
reduce the possibility that lead will dissolve in water.
Lead can also arise if lead solder is used in the installation of copper piping. A control
measure in this case would normally be to avoid the use of lead solders for applications
Polyvinyl chloride (PVC) plastic pipe is also widely used in distribution systems. Lead has
been used as a stabilizer in unplasticised PVC pipe, and may give rise to elevated lead levels
in drinking-water for a time after a new installation. Such pipe is normally of large diameter;
thus, the dilution effect of the water flowing through the pipe will reduce the concentration of
lead and may result in lead concentrations below the guideline value.
There have been cases
where the levels of vinyl chloride monomer remaining in the plastic have been higher than
desirable. However, chemical monitoring of drinking-water is not normally considered to be
appropriate and the most suitable method of management is by product specification, as
indicated above for other materials.
Table 8.1 Suggested risk management strategies for chemicals from water production and
Chlorine Useful indicator for
Critical for good disinfection, monitor posttreatment
to ensure disinfection
Chlorine dioxide Verification, together
chlorite and chlorate
Control through dose optimization
Monochloramine Verification Manage by ensuring correct ammonia dose and
Ozone None Control through dose optimization
Aluminium Verification Above 0.2 mg/l can cause problems of dirty water,
control by treatment optimization
Iron Verification Above 0.3 mg/l can cause problems of dirty water,
control by treatment optimization
Acrylamide None Product specification
Epichlorohydrin None Product specification
Trihalomethanes Verification Control through optimization of coagulation and
filtration to remove precursor substances
for haloacetic acids
Control through optimization of coagulation and
filtration to remove precursor substances
Iron None Corrosion inspection
Lead Part of broader
Important for consideration; inspection and
investigation of pipework in buildings
Copper Part of broader
water quality in
Pipework in buildings not usually a problem unless
very aggressive water
Zinc Part of broader
water quality in
Galvanised pipes in buildings
Vinyl chloride None Product specification of polyvinyl chloride (PVC)