SHC310 – Hydraulics
Water Treatment Works
P1-1
Water Treatment Works
Layout of the presentation
• Introduction
• Main challenges
• Water quality
• Overview of water treatment
processes
• Conventional water treatment
process
• Advanced water treatment process
• Rietvlei water treatment process
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Introduction
• The term “water quality” describes the physical, chemical
and microbiological characteristics of water.
• These properties collectively determine the overall water
quality and the fitness of the water for a specific use.
• Water quality is only meaningful when evaluated in
relation to the use of the water.
• Water that is fit for domestic use (drinking water) must
comply with specific requirements. The most important
requirement is that it must be safe to drink. In addition to
the requirement that water must be safe to drink, water
for domestic use must also be aesthetically pleasing (have
a clean appearance, taste and odour) and it must
furthermore be chemically stable (i.e. it must not cause
corrosion or form deposits in pipes or fixtures such as
geysers).
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Main challenges
The main challenges facing the water industry
today include:
• Deterioration in the quality of many raw water
sources
• Removal of potentially harmful synthetic organic
substances in water sources
• Removal of resistant micro-organisms from water
• Improved training of process controllers for new
processes and process optimisation
• Demands for process integration and flexibility
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Water quality
General aspects of water quality
• Water is a unique substance and one of its unique
characteristics is its capacity to dissolve a variety
of substances.
• As water moves through the hydrological cycle,
comprising of rainfall, runoff, infiltration,
impounding, use and evaporation, it comes into
contact with many different substances that may
be dissolved by the water to a greater or lesser
extent or that may be suspended in the water,
which collectively determine the overall quality of
the water and its fitness for use.
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Water quality
General aspects of water quality
• The types of contaminants or substances of
concern that may occur in water sources vary
over a wide spectrum and include inorganic salts,
micro-organisms, clay particles and organic
material.
• The substances of concern in water can be
categorised in different ways:
– Dissolved or suspended;
– Inorganic or organic;
– Macro or micro substances
– Natural or synthetic substances
– Suspensions of micro-organisms etc.
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Water quality
CHEMICAL WATER QUALITY
• Dissolved organic and inorganic substances
determine the chemical quality of water. These
substances have a wide range of effects on the
chemical properties of the water.
• For example, some of these substances can be
toxic (chromium, arsenic), while other cause the
water to be hard or scale forming (calcium
carbonate), and other chemical compounds may
affect the taste and odour of the water (sodium
chloride, geosmin).
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Water quality
CHEMICAL WATER QUALITY
• A large range of inorganic chemical compounds can be
present in water. These compounds such as sodium
chloride, NaCl and calcium sulphate, CaSO4 dissolve in
water in the form of the respective ions, i.e. Na+, Ca2+, Cl-
and SO42-.
• A very large variety of organic substances can be present
in water. These substances may either be natural
substances such as decaying plant material, algal or
bacterial by-products and carbohydrates, or synthetic
compounds such as pesticides, herbicides and solvents as
well as products formed during water treatment such as
chloroform and other chlorinated products.
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Water quality
CHEMICAL WATER QUALITY
• Organic compounds have carbon as a main element in
their composition and most of them do not go into
solution as ions but go into solution as molecules of the
compound. Organic compounds are determined either
collectively by means of oxidation, as individual
compounds or as groups of compounds.
• An indication of the general organic quality of the water
can be obtained by means of the determination of
cumulative parameters such as:
– Total organic carbon (TOC)
– Dissolved organic carbon (DOC)
– Chemical oxygen demand (COD)
– Biochemical oxygen demand (BOD)
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Water quality
CHEMICAL WATER QUALITY
• There are a number of collective water chemical
parameters specific to water treatment:
• Alkalinity
• Hardness
• Chemical stability
• Free available – and combined chlorine species
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Water quality
PHYSICAL WATER QUALITY
• The physical quality of water is determined by intrinsic
characteristics as well as by dissolved and colloidal
substances in the water.
• Intrinsic physical properties include temperature,
viscosity, and surface tension.
• Other physical properties such as electrical conductivity,
colour, taste and odour are determined by the presence of
dissolved and colloidal substances in the water.
• Some characteristics of water are often indicated as
physical characteristics, while they are in actual fact
chemical in nature, pH being an example.
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Water quality
MICROBIOLOGICAL WATER QUALITY
• The microbiological quality is determined by the type and
numbers of microorganisms present in the water.
• A variety of micro-organisms can be present even in very
good quality domestic waters. Most of these micro-
organisms are harmless but if the water is polluted
pathogens may be present.
• Pathogens are disease-causing micro-organisms such as
those causing cholera, gastro–enteritis, hepatitis, etc.
• It is difficult to determine the presence of all the different
pathogenic organisms and therefore certain indicator
organisms are used to give an indication.
• Indicator organisms are specific types of micro-organisms
that are present in very large numbers in the intestines of
people and warm-blooded animals.
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Water quality
MICROBIOLOGICAL WATER QUALITY
The following are the most commonly used indicator
organisms:
• Total coliforms are a group of closely related bacteria that
are most commonly used as indicator organisms for
drinking water.
• Faecal coliforms and E.coli are subsets of the total
coliform group. Both groups are better indicators of
recent faecal pollution than total coliforms, but they do
not provide a distinction between human and animal
contamination.
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Overview of water treatment processes
• The principal objective of a water treatment
plant is to produce water that is fit for domestic
use at a reasonable cost.
• Many treatment processes (sometimes called unit
processes and unit operations) are linked together
to form a treatment plant in order to produce
water of the desired quality.
• The combination of treatment processes is called
a process train, or a process flow diagram.
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Overview of water treatment processes
The main factors that must be taken into account
when developing a treatment process train include:
• The source water quality (normally referred to as
the raw water quality)
• The seasonal (and other) variations in the raw
water quality
• The required treated water quality
• Regulatory requirements
• Other factors such as plant size (capacity), site
conditions, availability of skilled labour, degree of
automation required, economics and many other
factors.
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Overview of water treatment processes
Unit process Description and application
Trash rack Provided at the intake gate for removal of floating debris
Coarse screen Mechanically cleaned screens provided at the intake gate or in the sump well ahead of
pumps. Remove small solids
Microstrainer Removes algae and plankton from the raw water
Aeration Strips and oxidises taste- and odour-causing volatile organics and gases and oxidises iron
and manganese. Aeration systems include gravity aerator, spray aerator, diffuser and
mechanical aerator
Mixing Provides uniform and rapid distribution of chemical and gases into the water
Pre-oxidation Application of oxidising agents such as chlorine, potassium permanganate and ozone in
raw water and in other treatment units to limit microbiological growth and to oxidise taste,
odour and colour causing compounds as well as iron and manganese compounds.
Coagulation Coagulation is the addition and rapid mixing of coagulant with the water to destabilise
colloidal particles and form small flocs
Flocculation Flocculation causes aggregation of destabilised colloidal particles to form rapid-settling
flocs
Sedimentation Gravity separation of suspended solids or floc produced in treatment processes. It is used
after coagulation and flocculation and chemical precipitation
Sand filtration Removal of flocculated and particulate matter by filtration through granular media
(normally filter sand). Multi media may also be used (sand and anthracite, or sand and
activated carbon, or a third layer may also be incorporated)
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Overview of water treatment processes
Spiral flocculator at the Vereniging works of Rand Water
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Overview of water treatment processes
Sand filter
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Overview of water treatment processes
Unit process Description and application
Slow sand Removal of colloidal matter, micro-organisms and colour by means of slow rate filtration
filtration through a sand bed on which a layer of colloidal matter and micro-organisms is allowed to
form.
Chemical Addition of chemicals in water precipitates dissolved solids with low solubility into
precipitation insoluble form. Removal of hardness, iron and manganese and heavy metals is achieved by
chemical precipitation
Recarbonation Addition of carbon dioxide to reduce pH of water after addition of lime for coagulation or
softening
Activated carbon Removes dissolved organic substances such as taste and odour causing compounds and
adsorption chlorinated compounds. It also removes many metals. It is used as powdered activated
carbon (PAC) at the intake or as a granular activated carbon (GAC) bed after filtration
Disinfection Destroys disease-causing organisms in water. Disinfection is achieved mainly by chlorine,
but ultraviolet radiation and other oxidising chemicals such as ozone and chlorine dioxide
are also used
Ammonia converts free chlorine residual to chloramines. In this form, chlorine is less
Chloramination reactive, lasts longer and has a smaller tendency to combine with organic compounds, thus
limiting taste and odours and THM formation
Fluoridation Addition of sodium fluoride, sodium silicofluoride or hydrofluosilicic acid to produce
water that has optimum fluoride level for prevention of dental caries
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Overview of water treatment processes
Granular activated carbon (GAC)
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Overview of water treatment processes
Chlorine gas leakage
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Overview of water treatment processes
Unit process Description and application
Desalination Involves removal of dissolved salts from the water supply. Desalination may be achieved
by membrane processes, ion exchange and distillation
Reverse osmosis High-quality water permeates very dense membrane under pressure while dissolved solids
(RO) and some organics are prevented from permeating the membrane. RO is also used for
nitrate and arsenic removal
Nanofiltration Less dense membranes (than RO) are used for removal of divalent ions (softening), micro-
(NF) organisms and organics from water under pressure
Ultrafiltration Removal of colloidal material and some micro-organisms from water by membranes under
(UF) pressure
Microfiltration
Removal of all particulate matter and some colloidal matter
(MF)
Ion exchange (IX) The cations and anions in water are selectively removed when water is percolated through
beds containing cation and anion exchange resins. The beds are regenerated when the
exchange capacity of the beds is exhausted. Selective resins are available for hardness,
nitrate and ammonia removal.
Electrodialysis An electrical potential is used to remove cations and anions through ion-selective
(ED/EDR) membranes to produce desalinated water and brine.
Distillation Used mostly for desalination of seawater
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Overview of water treatment processes
Desalination of seawater
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Conventional water treatment process
The term conventional water treatment refers to the
treatment of water from a surface water source by a
series of processes aimed at removing suspended
and colloidal material from the water, disinfecting
the water, and stabilising the water chemically.
Conventional treatment of water for domestic use
involves a number of treatment steps aimed at
achieving the following objectives:
• Removal of suspended and colloidal matter to an
acceptable level by means of coagulation-flocculation,
sedimentation and sand filtration
• Disinfection to produce water that is safe to drink
• Chemical stabilisation of the water to prevent corrosion of
pipelines, attack on concrete pipes and structures etc.
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Conventional water treatment process
The selection of the best combination of processes to treat
water from a particular source depends on a number of
factors:
• the amount of suspended solids;
• the turbidity of the water;
• the nature of the suspended material;
• the chemical properties of the water (alkalinity and pH);
• the volume of water to be treated, and
• the availability of facilities, trained operators and
supervisors.
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Advanced water treatment process
The term advanced treatment processes refers to processes
other than conventional processes.
Advanced processes are membrane processes (reverse
osmosis RO, nanofiltration NF, ultrafiltration UF and
electrodialysis ED), activated carbon adsorption, ozonation,
oxidation processes for iron and manganese removal and
processes for removal of specific substances such as fluoride.
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Rietvlei water treatment process
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Rietvlei water treatment process
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Rietvlei water treatment process
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Rietvlei water treatment process
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Rietvlei water treatment process
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Rietvlei water treatment process
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Rietvlei water treatment process
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Rietvlei water treatment process
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Rietvlei water treatment process
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Rietvlei water treatment process
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Rietvlei water treatment process
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Rietvlei water treatment process
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