المعالجة الكيميائية لمياه الشرب

chemical treatment processes
In the production of drinking water, a number of different chemicals may be added to the water. the types and quantities of chemicals can vary widely and will depend on a range of factors including raw water quality, treatment processes employed and treated water quality objectives processes are used Chemical treatment:-
• control algae
• remove turbidity and colour
• remove microorganisms
• remove algal metabolites and synthetic pollutants
• reduce organic matter
• reduce the concentration of iron, manganese and other elements
• reduce pesticides and herbicides
• control taste and odour
• soften
• buffer or modify the pH
• disinfect
• control corrosion in distribution systems
Chemical treatments may also be used for other public health measures, including
• (fl uoridation (to prevent dental caries)
The following sections outline common processes employed in water treatment to achieve objectives THESE
CONTROL OF ALGAE
Algicides are used to reduce toxic or odorous algal blooms in water reservoirs. The chemical commonly
used in the management of algal growth is copper sulfate. Before an algicide is used, the possible
effects on aquatic biota, the accumulation of copper in sediments, the potential impacts on downstream
treatment processes and fi nal treated water quality should be considered
The use of copper as an algicide is controlled in some States. Information on the use of these chemicals
should be obtained from the appropriate State or Territory authority
COAGULATION AND FLOCCULATION
The primary use of coagulant and fl occulant chemicals is in the removal of suspended and colloidal
solids such as clays. Coagulation is particularly important in the treatment of surface waters. Removal
of the solids is achieved by aggregating fi ne suspended matter into larger fl ocs. Coagulant and fl occulant
chemicals will also remove some natural organic matter, colour and microorganisms (e.g. bacteria, viruses
and algae). The size and strength of the fl oc can be controlled and modifi ed, depending on the treatment
process in use, and the fl oc can be removed by sedimentation and fi ltration
ADSORPTION
Adsorption is primarily used to improve water quality through the accumulation of substances at
the interface between two phases, such as a liquid and a solid, due to chemical and physicochemical
interactions. The solid on which adsorption occurs is called the adsorbent. Activated carbon is an excellent adsorbent
Adsorption is commonly used to remove organic contaminants such as herbicides, pesticides, algal
toxins and metabolites; it is also used to remove compounds which may impact on the taste and odour of water
SOFTENING
Softening is undertaken as part of water treatment to remove calcium and magnesium salts, particularly
carbonates and bicarbonates, which cause water hardness. Hard water can cause scale build-up on water
heating elements and can cause problems with the use of soaps and detergents. Softening very hard
waters can also lead to high concentrations of sodium in water. While this may possibly give the water a
salty taste, it is unlikely to present a health concern. Water that is too soft can be corrosive, which may
occur when reverse osmosis is being used for water treatment, in which case it may be necessary to
restore some hardness to prevent corrosion
OXIDATION
Various oxidants may be added to water to oxidise problem compounds. For example, chlorine or
potassium permanganate may be added to control iron and manganese. The oxidised forms of iron
and manganese are readily removed by coagulation, fl occulation and fi ltration. Oxidants may also be
used to oxidise compounds which impact on the taste and odour of water, and organic contaminants
such as pesticides
Ozone, and possibly hydrogen peroxide, may be added to oxidise organic compounds, and thus reduce
the amount of coagulant required. Adding these chemicals also helps to reduce the length of long-chain
organic molecules, which are then more effectively removed by granular activated carbon
DISINFECTION
Disinfection of water is generally used either alone or as the fi nal step in water treatment, after
clarifi cation or fi ltration. Disinfection is widely used to prevent the passage of bacteria, viruses and
some protozoa into the distribution system. Typical chemicals used for disinfection of drinking water
supplies are strong oxidants, such as chlorine (and its derivatives, chlorine dioxide and chloramine),
ozone and hydrogen peroxide
The effi ciency of disinfection depends greatly on the quality of the source or treated water, and can also
be strongly affected by conditions such as chemical contact time, the pH and turbidity of the water, and organic content of the water
The aim of treatment processes used before disinfection should be to produce water with the lowest
possible turbidity and organic content. Excessive particulate matter in the water can protect microorganisms
from the action of disinfection chemicals. Also, excess organic matter and other oxidisable compounds
in water can react with disinfection chemicals intended to inactivate microorganisms and can result in
an increase in the formation of disinfection byproducts (see Section 6.3.2 for general information on
disinfection byproducts, and the fact sheets in Section V for information on specifi c byproducts). Best
practice operation of a conventional water treatment plant should be able to produce treated water with
a turbidity of less than 0.1 nephelometric turbidity units (NTU)
ADJUSTMENT OF PH
Adjustment of pH is important in drinking water treatment processes such as coagulation (particularly
for the removal of natural organic matter), corrosion control and softening
Control of pH is also important for effective disinfection and for minimising the formation of disinfection
byproducts. The effi ciency of certain disinfectants is strongly dependent on pH
ADDITION OF BUFFERING CAPACITY
Soft waters can be subject to pH change as they travel through the distribution system. The rate of
change depends on a number of factors including the water hardness, pipe materials used (e.g. cement
lined pipe), the contact time, temperature. Increasing the buffering capacity of the water can help
control the rate of change of pH through the distribution system
CORROSION INHIBITION
The mechanisms of corrosion in a water distribution system are complex, and involve an interrelated
combination of physical, chemical and biological processes. These depend greatly on the materials used
within the distribution system and the chemical properties of the water, particularly its buffering capacity
Water corrosivity can be minimised by adjustment of pH and increasing calcium carbonate hardness
(resulting in a positive Langelier index). Corrosion can also be reduced by maintaining disinfection
residual throughout the distribution system
Corrosion inhibition chemicals (such as sequestering agents) are used to reduce corrosion of pipes and
household services. They also control the build-up of scale deposits from the dissolved mineral content
of drinking water. This is achieved through the addition of chemicals that form a protective fi lm on the
surface of pipes. While corrosion inhibitors reduce corrosion, limit metal solubility or convert one form
of corrosion to another (e.g. alleviating tuberculation and replacing it with more uniform corrosion))
they do not totally prevent corrosion