Consumption of unsafe water continues to be one of the major causes of the diarrhoeal disease deaths occurring annually, mostly in children. Various processes of the water purification are developed in order to overcome the problem of water pollution.
In the drinking
water treatment, the water has to be collected from their original sources
such as rivers and reservoirs and purifed. After the purification, that water
should be healthy for human consumption and free of harmful microorganisms and
organic and inorganic pollutants.
Drinking water treatment
involves various steps according to the quality of raw water. Water from
different sources and places has different characteristics. Inorganic
compounds such as cadmium, chromium, copper, lead, mercury, arsenic,
etc., natural and synthetic organic compounds and living organisms such
as bacteria, algae, viruses, etc. may be present in water contributing to
turbidity, particles, color, taste, odor, etc.
Common contaminants in
tap water fall into these categories:
• Suspended solid
particles
• Colloids
• Dissolved inorganic
salts
• Dissolved organic
compounds
• Micro-organisms
• Dissolved gasses
The appropriate method of
water purification can be developed according to the composition of the water
to be treated. The water purification is generally carried out by the processes
such as aeration, sedimentation, coagulation, flocculation, filtration,
disinfection etc.
General steps in
purification of drinking water includes Aeration, Sedimentation, Filtration,
Disinfection.
1. Aeration
This is an optional
treatment. Raw water is first collected in large aeraton tank and
the aerated by bubbling compressed air through perforated
pipes. Aeration removes bad odors and volatile organics (e.g.
solvents), carbon dioxide, some taste and odour causing compounds. It also
removes metal such as iron, manganese by precipitating then as
their respective hydroxides.
In groundwater, iron is
usually present as dissolved ferrous compounds. Manganese is usually present as
dissolved manganous compounds. Aeration converts them to insoluble
hydroxides and which is removed by filtration.
Aeration processes
introduce oxygen into water and removes gases and volatile compounds. For
oxygen transfer, aerators are designed so that water flows in a thin film to
achieve efficient aeration.
2.
Sedimentation/Settling
Water
may have large sized organic materials such as leaves, and gravels which have
run off from the soil. Sedimentation is done so that the suspended
particles settle down depending on their size and weight and conditions of the
stored water. Sedimentation can be done in a settling tank.
Aerated
water is placed in settling tank and stored. During storage about 90% of
suspended solids settle down within 24 hrs and the water becomes clear. Certain
heavier toxic chemicals also settle down during storage. Pathogenic
bacteria gradually die and bacterial count decreases by 90% in about 5-7 days
of storage. Sedimentation provides partial reduction of microorganisms in water
but does not sterilize the polluted water. During storage organic matter
present in water is oxidized by microorganisms.
Sedimentation tanks are usually rectangular with inlet
and outlet at opposite ends of the tank. The inlet distributes the incoming
flow as evenly as possible across the tank width and the outlet collect the
clarified water. Sedimentation tanks require cleaning occasionally for good
performance.
-Assisted
sedimentation-
Simple
sedimentation can reduce turbidity and solids in suspension but take
time so the process is enhanced by physical processes such
as flocculation or chemical processes such
as coagulation. These treatments are used to remove very
light suspended solids that do not settle by themselves during storage.
A precipitate, or floc, which entraps these impurities is formed and
settles down with time. Coagulation and flocculation are used to remove
colour, turbidity, algae and other microorganisms. Flocculation is gentle
mechanical agitation which allows suspended particles to form aggregates/flocs and
gradually settle down. The rate of sedimentation is also enhanced by
adding alum, iron, salts, colloid silicates which act as coagulants. The
suspended materials and microorganisms are entrapped by coagulants and settle
down rapidly as flocs. This procedure is called coagulation.
The floc is separated from the treated water by sedimentation and/or
filtration. Sedimentation provides partial reduction of microorganisms in
water due to their settling down on bottom but does not sterilize the polluted
water.
The quantity of
the coagulants to be added are determined by raw water quality. Water
from storage tank is placed in coagulation tank and then some precipitating
agents such as alum (aluminium sulphate), lime, iron salts (ferric
sulphate) etc. are added in water and mixed.
4. Filtration
After
sedimentation the water is further purified by filtration. There are two types
of sand filters which are used in water purification such as slow sand filter
and rapid sand filter:
(i) Slow Sand Filter:
In slow sand filtration plants the rate of filtration of water is slow; hence
the plant requires a considerable area. This plant consists of a concrete floor
containing drainage tubes (for collection of filtered water). The tile is
covered with rock, gravel, coarse sand and then 2 to 1 feet of fine sand. Water
is passed through this plant. Water passes slowly through the filter and
collected by drain pipes at the bottom which later on is pumped into a
reservoir. The capacity of slow sand filter plant is to filter about 5
million of water per acre per day.
If
water is turbid, slow sand filters are clogged soon. Therefore, turbid water,
which is to be filtered, should be clarified first by sedimentation,
thereafter, passed through slow sand filters.
Water
purification is done by straining and action of
microorganisms. In the surface of layers of fine sand, a colloidal material,
consisting of bacteria, algae and protozoa, is attached. This mucilaginous
material called Scmutzdecke or “dirt layer”, closes the pores between
the sand grains and makes filtration more effective. Sand grains have positive
charges and bacterial cell walls have negative charge. Therefore, bacteria
are adsorbed on the surface of sand. Protozoa ingest bacteria.
Due to intense microbial interactions, organic contents of water is
reduced.
Through slow
sand filter plant, the pathogenic microorganisms such as Giardia
and its cysts which are not removed by any other methods can be
filtered from water.
When
filtration efficiency of the plant is reduced, due to deposition of thick
mucilaginous material, the plant is subjected to cleaning. It is cleaned
by forcing cleaned water backward i.e. back washing through the beds of gravels
and sands without disturbing the fine sand.
(ii) Rapid Sand Filter
Similar to slow sand
filter, the rapid sand filter is also constructed. This plant consists of
layers of sand, gravel and rock. The water is allowed to pass through rapid
sand filter plant. This plant depends on physical trapping of fine
particles. The pores of the plants are soon clogged. It is cleaned by
forcing cleaned water backward i.e. back washing through the beds of gravels
and sands without disturbing the fine sand.
About
99% bacteria are removed by this plant. But it does not remove Giardia
lamblia cysts, Cryptospordium oocysts and viruses which are removed through
slow sand filter. Therefore, water collected after filtration needs further
treatment.
Rapid
sand filter plant operates about 50 times faster than slow sand filter plant,
and can deliver about 150 to 200 million gallons of water per acre per
day. It requires less land area, less cost and less maintenance.
4. Disinfection:
Disinfection
is the final step of water purification. Some of the bacteria pass through
filter even after filtration and must be killed before consumption of water.
Therefore, disinfection of public water supply needs to be done.
The filtered water is
finally purified by using disinfectants which kills pathogenic as well as other
microorganism in water. Several disinfection methods are used in
water treatment.
Disinfection
with chlorine is the most widely used method for large water supplies
but is less common in small supplies. Solutions of sodium hypochlorite were
used but in recent years, chlorination of public water supply has become
popular.
Chlorination involves
the release of chlorine gas in water which gets readily mixed up with water.
The amount of chlorine required ie, chlorine demand of a water
body, depends on organic matter and number of microorganisms present in water,
and duration of time to act upon. High concentration of chlorine quickly acts
upon microorganisms and vice-versa.
The
amount of chlorine required for disinfection is called chlorine demand.
Water is chlorinated to contain about 0.1 to 0.2 ppm of residual
chlorine which reaches to this concentration after 20 minutes of its
addition. However, if the concentration of chlorine exceeds its demand,
peculiar odour and tastes are experienced.
The
mechanism of action of chlorine on microorganisms is by the formation
of highly reactive nascent oxygen. After reacting with water, chlorine is
converted into hypochlorous acid which in turn quickly releases nascent oxygen.
The nascent oxygen soon oxidises the cellular components of microorganisms as
well as organic matter.
Chlorination is performed by Breakpoint chlorination method.
Break point is the point till where chlorine is added to water such that the
demand for chlorine is fully met. When chlorine is added first to water,
nascent oxygen liberated oxidises the organic compounds present in the water.
At the break point, free ‘available’ chlorine is detected in
water. This is "residual chlorine". The organic
compounds present are oxidized and the chlorine demand of the water body is
fully satisfied at the break point.
Breakpoint chlorination requires a dose of
around 10 mg/l chlorine and leave a resultant free available chlorine residual
in the range 0.1 to 0.2 ppm. The actual dose depends on water quality
and has to be determined for each water. It is recommended that
the contact time should be at least 30 minutes.
Chlorination can be achieved by using liquefied chlorine gas, sodium
hypochlorite solution or calcium hypochlorite granules. Chlorine gas is very
reactive and highly toxic and must be carefully stored and handled. It is used
for treatment of large public supplies but not recommended for treatment of
small water supplies.
Advantages
· It oxidises
completely organic compounds, ammonia &other reducing compounds
· It removes colour
in water, due to organic matter.
· It destroys
completely all the disease-producing bacteria
· It removes both
odour and taste from water, due to organic matter
· It provides
residual protection against recontamination
· Ease-of-use and
acceptability
· Scalability and
low cost
Disadvantages
· Relatively low
protection against microbial spores, protozoa and viruses.
· Lower disinfection
effectiveness in turbid waters.
· Potential taste
and odor objections.
· Potential
long-term effects of chlorination by-products.
If
action of chlorine prolongs in water containing high amount of organic
matter, chloramines, are formed. Change in odour and taste of water
is due to the formation of chlorophenols. In the presence of high
organic matter, chlorine reacts with it and produces different halomethanes which
are a group of potential carcinogenic compounds.
*Super chlorination is a water treatment process in which the addition of excess amounts of chlorine to a water supply to ensure disinfection within a short contact time. Super chlorination, also known as hyper chlorination, temporarily increases the free chlorine residual in a water distribution system.
Super chlorination is the addition of large doses of chlorine to the water followed by de -chlorination, which is the removal of excess of chlorine after disinfection
This method is applicable to heavily polluted waters whose quality fluctuates greatly.
A high free chlorine residual (i.e. above 5 mg/L) is effective against most bacteria (including Legionella).
Superchlorination should be undertaken :
- While installing a new water infrastructure
- for remediation of affected infrastructure following a detection of Legionella or other microbial hazard of high risk to patients or residents
- based on the complexity of the plumbing infrastructure, in areas where biofilm growth is suspected (e.g. low flow pipe sections), on a scheduled basis (e.g. every six months).
Superchlorination is most commonly used when water has very high bacteria content and generally comes from river sources or where some form of pollution has occurred. It is also an important part of swimming pool maintenance because it keeps chlorine content at the right level to effectively kill off bacteria and other contaminants.
*Shock chlorination is an effective and safe way to remove bacteria from a domestic well and the cold water part of a household water supply system. The chlorine should be present in a concentration that is lethal to bacteria and disinfection should take place long enough to ensure that all bacteria are killed. Shock chlorination is the most widely recommended means of treating bacterial contamination in home water systems such as wells, springs, and cisterns.
Care should be taken to ensure that the dose of chlorine is adequate and that bleach and harmful chemicals are removed from the system before it is used for drinking water supply again.
Shock chlorination is recommended:
- upon completion of a new well or when an unused well is returned to service
· if annual water test results indicate the presence of bacteria
· if a well system is opened for any installation, repair or maintenance
· whenever the well is surrounded by flood waters (standing water around or covering the well casing)
· if well water becomes muddy or cloudy after a rain
Ozone
One
common method of disinfecting wastewater is ozonation (ozone
disinfection). Ozone is an unstable gas that can destroy bacteria and viruses.
It is formed when oxygen molecules (O2) collide with oxygen atoms (O)
to produce ozone (O3).
Ozone
is generated by an electrical discharge through dry air or pure oxygen and is
generated onsite because it decomposes to elemental oxygen in a short amount of
time. After generation, ozone is fed into a contact chamber containing the
wastewater to be disinfected.
· Ozone is more
effective than chlorine in destroying viruses and bacteria.
· The wastewater
needs to be in contact with ozone for just a short time (approximately 10 to 30
minutes).
· Ozone decomposes
rapidly, and therefore, it leaves no harmful residual that would need to be
removed from the wastewater after treatment.
· Ozone is generated
onsite, and thus, there are fewer safety problems associated with shipping and
handling.
However,
· Low dosages may
not effectively inactivate some viruses, spores, and cysts.
· Ozone is very
reactive and corrosive, and extremely irritating/possibly toxic, so need proper
care and equipment.
· Ozonation is not
economical - the cost of treatment is relatively high.
· There is no residual
activity to indicate the efficacy of ozone disinfection.
Ultraviolet
irradiation (UV)
UV
is the preferred method for disinfection of small supplies with small
distribution networks or retention time. Chlorination may be more suitable for
larger operations in which it is necessary to maintain a residual disinfectant
during storage and distribution.
UV
light damages the deoxyribonucleic and ribonucleic acids (DNA and RNA)
and prevents the reproduction of microorganisms. Microorganisms are
thereby inactivated. In general, viruses are most resistant to UV
disinfection compared to protozoan cysts (e.g., Cryptosporidium) and
bacteria.
UV disinfection efficiency is particularly affected by water
quality and flow rate. The water to be disinfected must be of good quality
and particularly low in colour and turbidity.
UV
devices can be scaled to fit any size or type of drinking water treatment need,
from small handheld devices to large systems.
UV
produces far fewer disinfection byproducts compared to other chemical
disinfectants (e.g., chlorine, ozone, chlorine dioxide). UV does not leave any
odour, flavor in the treated water.
The disadvantage of using
UV for disinfection is its inability to provide a residual activity.
Small Scale /household
disinfection
Several types of filters are
used to remove suspended matter from water and reduce turbidity and
microorganisms, or to remove specific inorganic iron, aluminium or manganese
compounds. Devices incorporating activated carbon filtration, reverse
osmosis (RO) or UV disinfection can be used.
Simplest and surest way
of disinfecting water at households is boiling water at 1000C for
5-10 minutes.
After disinfection, water
is pumped into reservoirs/tanks for subsequent domestic distribution. Different
methods of water treatment mentioned so far can be used singly or in
combination depending on the raw water quality and treatment purpose. These ensure
clean and safe water for healthier living.
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