Water: is one of the environmental resource .
Public Water Supply: refers to same clean water for use in home, schools,
hospitals, workplaces, commercial and some industrial activities, street
cleaning, and fine protection. Water
for dainty personal hygiene and sanitary purposes is of paramount importance to
in health and well –being of the society.
Water Quality Standards
Water
contains a variety of chemical, physical and biological substances that are
either dissolved or suspended in it. From the moment it condenses as rain,
water dissolves the chemical components of its surrounding as it falls through
the atmosphere, runs over ground surface, and percolates through the soil.
Water also contends living organisms that react with its physical and chemical
elements. For this reasons, water must often be treated before it is suitable
for use.
Water quality requirement are established in
accordance with the intended use of the water quality is usually judged as the
degree to which water conforms to physical, chemical, and biological standards
set by the user. Water quality standards are also essential in monitoring
treatment processes.
Water
is evaluated for qualify in terms of its physical, chemical, and
microbiological properties.
Physical Characteristics
Tastes, odours, colour and turbidity are controlled in
public water supplies pertly because they make drinking water unpalatable, but also
because of the use of water in beverages, food processing and textiles Tastes
and odours are caused by the presence of volatile chemicals and decomposing
organic matter. Measurement for these are conduced on the basis of the dilution
needed to reduce them to level barely detectable by human observation co lour
in water is caused by minerals such as lion and manganese, organic materials
and co lour water form industries. Testy is done by comparison with a standard
set of concentrations of a chemical that produces a co lour similar to that
found in water. Turbidity, as well as being aesthetically objectionable, is a
health concern because the particles involved could harbor pathogens water with
enough suspended clay particles will be visually turbid. Turbidity measurements
are base on the optical properties of the suspension that cause light to be
scattered or absorbed rather than transmitted in straight lines through the
Samuel.
Chemical Characteristics
Many chemical compounds dissolved in water may be of
natural or industrial origin and may be beneficial or harmful depend on their
composition and concentration. For example, small amounts of lion and manganese
may not just cause co lour, they can also be oxidized to form deposits of
ferric hydroxide and manganese oxide in water mains and industrial equipment
these deposits reduce the capacity of pipes and are expensive to remove.
Hard Water are generally considered to be those water
that require considerable amounts. Of soap to produce a foam or lather and that
also produce scale in not water pipes, heaters, boilers, and other units in
which the temperature of water is increased materially.
Sources of
Water
The quality and quantify of water from surface water
and ground water, the two main sources, are influenced by geography, chi male,
and human activities. Ground water can normally be used with life or no
treatment. Surface water, on the other hand, often needs extensive treatment,
particularly if it is polluted.
Ground Water
Ground water: is water that has percolated down word from the
ground useful through the soil pores. For motions of soil and rock that have
become saturated with water are known as ground water reservoirs, or aquifers.
Soil pore size water viscosity and other factors combine to limit the speed at
which water can move through soil to replenish the well.
Ground water is not as susceptible to pollution as
surface water, but once polluted its restoration, even if possible, is
difficult and long term. This is why municipalities even those located close to
surface water prefer wells for a municipal water supply much less treatment and
there for expense is needed to bring ground water standards. Ground water
quality is difficult to monitor when large number of well are in use.
Surface Water
Surface water is from rivers and levels are important
sources of public water supplies be cause of the light with darnel rates they
can normally sustain. One disadvantage of using surface water is that it is
open to pollution of all kinds. Contaminants are contributed to lakes and
rivers from diverse and intermittent sources, such as industrial and municipal
wastes, run off from urban and agricultural areas, and erosion of soil water
with variable turbidly and a variety of substance that contribute to the taste,
odour, and co lour of the water can necessitate extensive treatment.
Seawater
Seawater
available in almost unlimited quantities, can be converted into fresh water by
a number of processes
·
Desalination is
the general term used for the removal of dissolved salts from water.
Distillation, the oldest desalination techmvue depends
on the evaporation and condensation in water.
·
Energy intensive,
but using solar energy to evaporate water may make it practical in countries
with plentiful sunshine
·
Freezing lowers
the water temperature unit lice crystals free of salt can be separated from the
brine.
·
Electrodialysis involves
forced migration of charged ions through Cation - permeable or anion –permeable
membrane by applying an electric Potential across a cell containing mineralized water.
Reclaimed Waste Water
Reclaimed waste water: is water that has been treated sufficiently
for direct reuse in loop operation may offer only alternative in areas that cannot
obtain enough fresh water. Suspended solids, boded addable organics, and
bacteria can be removed or degraded by normal waste water treatment processes,
but co lour, the inorganic salts or magnesium, sodium, and calcium, synthetic
organics like pesticides, and other taxi substance similar to those by
desalination.
Water Treatment Processes
Water treatment
plant: one of the great achievements of modern technology has been to
drastically reduces the incidence of water borne diseases such as cholera and
typhoid fever these diseases are no longer the great risks to public health
that they once were.
Today’s water treatment plans are designed
to provide water continuously that meets drinking water standards at the top
there are four main consideration involved in accomplishing this:
·
source selection
·
protection of
water quality
·
treatment methods
to be used
·
prevention of re-contamination
Common precaution to prevent ground water and surface
water pollution include.
·
Prohibiting the
discharge of sanitary and storm sewers close to the water reservoir.
·
Installing fences
to prevent pollution from recreational uses of water
·
Restriction on
the application of fertilizers and pesticides in areas that drain to the
reservoir.
Screening, Coagulation /
Flocculation,
Sedimentation,
filtration, and disinfection are the main unit operations involved in the treatment
of surface water.
Water
treatment operation fulfills one or more of three key tasks:
·
Removal of particulate substance such as send and
clay, organic matter, bacterial and algae.
·
Removal of
dissolved substances such as those
causing co lour and hardness.
·
Removal of
destruction of pathogenic bacteria and viruses. The actual selection of
treatment processes depends on the type of water source and the desired water
quality.
In a
former, water flows by gravity through intake structure and pipe, screens
remove larger items, such as fish sticks, and leaves, and low -lift pumps raise
incoming water to the level of the treatment plant. From this point on, water
moves through the plant by gravity. Occasionally raw water with low turbidity
can be plain sedimentation to remove larger par tickles and then filtration to
remove the few prentices that failed to settle out. Usually, however, particles
in the raw water are two small to be removed in a reasonably and simple
filtration alone to remedy this a chemical is added to coagulate / flocculate
the small particles, called Colloids,
into larger ones, which can then be settled out in sedimentation tanks or
removed directly in filters where sedimentation proceeds filtration, filters
can operate for longer periods, or at higher rates before they have to be backwashed.
Filtration of chemically coagulated
/flocculate water with no prior sedimentation (called direct filtration) is
effective for water with low to moderate turbidity and is in fact the practice
in many of the newer water treatment plants. Following filtration and before it
flows into the storage reservoir, the water is disinfected; usually with
chlorine fluoride may also be added because of its ability to retard tooth
decay. Treated water is then pumped by the high-lit pumps into the distribution
system to serve customers and to maintain water levels in storage reservoirs if
required. It is impotent to recognize that water treatment still remains some
what of an at despite many scientific advances in understanding the physical
and chemical principles involved.
Removal of Particulate Matter
The
unit operations employed for the removal of particulate matter for water
include: screening, sedimentation, coagulation, flocculation, and filtration.
 |
Figure 1: schematic of water treatment plant using surface
water source
|
Screening: to remove large solids such as logs, branches, rags,
and small fish is the first stage in the treatment of water. Water intake is
located below the surface of the lake to exclude floating object and minimize
physical damage from lce.
Sedimentation: the oldest and
most widely used form of water and waste water treatment, uses gravity settling
to remove particles from water. It is relatively simple and inexpensive and can be implemented in basin that
are round, square, or rectangular- water containing particulate matter flows
slowly through a sedimentation tank and is thus detained long enough for tank
over a weir at the outlet end.
Coagulation/ flocculation: is a chemical physical predate where by particles too
small for practical removal by clustered together for faster settling.
Coagulation: is a chemical process used to destabilize colloidal particles.
The exact mechanism is not well understood, but the general idea is to add a
chemical that provides positively charged ions to water containing negatively
charged colloids.
Removal of Dissolved Substance: several of the unit operations discussed so far is
partially effective in removing objection able dissolved substances. For
example, co lour in water caused by coagulation flocculation. If these are a
problem, several other unit operations are available:
Aeration: is used to remove excessive amounts of iron and
manganese from ground water. These substance cause taste and co
lour problems, inter fare with laundering, stain plumbing fixtures, and promote
the growth of iron bacteria in water mains.
Softening of water: is a process that removes hardness, caused by the
presence of divalent metallic irons, principally Ca+2 and Mg+2. Hardness, in water is
the result of contact with soil and rock, particularly limestone, in the
presence of CO2.
Activated Carbon: is an extremely adsorbent material used in water
treatment to remove organic contaminants.
Reverse Osmosis (RO), fresh water is forced through a semi permeable
membrane in the direction opposite to the occurring in natural Osmosis. Because
the membrane removes dissolved salts, the main application for Ro has been in
desalination.
Storage
Storage: is necessary in
any municipal water supply system to meet variable demand, to provide fire protection, and for emergency needs.
Types
of Water Storage
·
Equalizing storage: also called operating storage, is used to meet variable water demands
while maintaining adequate pressure on the system.
·
Fire Storage: is calculated by taking the product of fire
flow and fire duration. Fire flow duration times suggested by the national fire
protection Association (NFPA).
·
Emergency storage: of up to five time the maximum daily demand is suggested by the
insurance Advisory Organization, to
provide water during shutdowns for maintenance or repaint to the system.