Hydro Chemistry and Water Pollution Studies for Water Management in Doon Valley
Chapter: 5
MANAGEMENT AND CONTROL OF WATER
POLLUTION
Water withdrawn directly from rivers, lakes or reservoirs is rarely clean
enough for human consumption. Even water pumped from underground
aquifers often required some degree of treatment to render it potable, which
is suitable for drinking.
The nature and extent of treatment required to prepare potable water from
surface or subsurface sources depends on the quality of raw water (untreated
water). Of course, less treatment is required by better quality water.
Although some groundwater sources have a very high water quality and
need a little or no treatment prior to use, most sources are treated to confirm
to natural drinking water standards.
The primary objective of water purification is to remove harmful
microorganisms or chemicals, thereby, preventing the spread of disease and
protecting public health. It should be crystal clear and should not have any
objectionable colour, taste or odour.
Contaminants in wastewater are removed by physical, chemical and
biological means. Means of treatment in which the application of physical
forces predominate are known as unit operation such as screening, mixing,
flocculation, sedimentation, floatation, elutriation vacuum filtration, heat
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transfer and drying. Means of treatment in which, the removal of
contaminants is brought about by the addition of chemicals or by biological
activity are known as unit programs. For example, precipitation, combustion,
and biological oxidation are examples of unit processes.
Unit operations and processes that have been applied to the treatment of
wastewater are classified as: (1) physical (2) chemical and (3) biological.
These unit operations and processes are used in conjunction with each other
in the treatment of wastewaters. Selection of a given operation, process or
combination there of depend on: (i) use to be made of a treated effluent
(ii)
nature of the waste water (iii) The processes are used in conjunction with
each other in the treatment of waste waters. Selection of a given operation,
process or combination depends on :(1) use to be made of a treated effluent
(2) nature of the waste water (3) the compatibility of the various operations
and processes
(4) the available means for disposing of the ultimate
contaminants and (5) the economical feasibility of the various combinations.
The various unit operations and unit processes employed in the conventional
wastewater treatment are discussed below.
(i) Screens: The first unit operation encountered in wastewater treatment
plants is the filtering of pollution or screening.
Screen is a device with openings generally of uniform size used to retain
coarse solids. The screen ailment may consist of parallel bars, rods or wires,
grating wires, wire mesh or perforated plate, and the opening may be of any
shape generally circular or rectangular slots.
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For example: Cannery and paper Industry:
Fine screens are considered an essential part of the treatment process for
cannery and paper industry waste. Since they provide rapid means of
removing a large portion of suspended matter and thereby reduce the load on
subsequent treatment operations.
(ii) Grit chambers: Most industrial wastes contain at least a small amount
of heavy suspended solids that settle out readily and rapidly when the
turbulence is lessened. These solids are frequently granuled and have
relatively high specific gravity and coarseness. Salt and dirt from vegetable
washing, hide soak-liquor from tanneries and wastes from industrial
laundries are few examples. The heavy solids are simple and are also
worthwhile because it eliminates them from subsequent treatment operations
where they might cause trouble.
(iii) Skimmino tanks: A skimming tank is a chamber so arranged that
floating matter rises and remains on the waste water until removed, while the
liquid flows out continuously through deep outlets or under partitions,
curtain walls, or deep screen boards. This may be accomplished in a separate
tank or combined with primary sedimentation, depending on the process and
nature of the wastewater.
The object of skimming tanks is the separation from the wastewater of the
lighter floating substances. The materials collected on the surface of
skimming tanks, where it can be removed includes oil, grease, soap, pieces
of cork, wood, vegetable debris and fruit skins originating in households and
in industries.
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Oils occur in all liquid effluents from petroleum production and refining,
and in wastes from many other industries including metal finishing, wool
scouring, meat packing and other food and chemical industries.
The efficient removal of floating oils by means of the skimming tanks is an
important part of the treatment of any oily waste; frequently the oils
recovered by waste settling are valuable materials and are returned to the
plant process or specially treated for recovery as by products.
(iv) Removal of coarse particles by sedimentation and filtration
Sedimentation: The impurity in water may be either dissolved or
suspended. The easiest way to remove the suspended material is to let the
force of gravity do the work under quiescent condition. When flow velocity
and turbulence are minimal, particles that are denser than water will be able
to settle to the bottom of a tank. This process is called sedimentation and the
layers of accumulated solids at the bottom of the tank called sludge. The
tank may be called a sedimentation tank, a setting tank or a classifier.
The speed at which suspended particle settle towards the bottom of tank
depends on their size as well as on their density. The larger and heavier
particles will settle faster than smaller or lighter particles.
In a sedimentation tank, there may be up to four different zones or type of
settling that occurs at different depths and exact mathematical analysis of the
process can be quite complicated.
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Hydro Chemistry and Water Pollution Studies for Water Management in Doon Valley
Filtration: Even with the help of chemical coagulation, sedimentation by
gravity is not sufficient to remove all the suspended impurities from water.
About 5% of the suspended solids may still remain as non-settleable floc
particles. These remaining flocs can cause noticeable turbidity and may
shield microorganisms from producing crystal clear potable water. An
additional treatment step following coagulation and sedimentation is
filtration.
Filtration is a physical process. It involves the removal of suspended
particles from the water by passing it through a layer or “Bed” of a porous
granular material such as sand. As the water flows through the filter bed, the
suspended particle becomes trapped within the pores spaces of the filter
material or filter media as it is called. Filtration is very important treatment
process in a surface-water purification plant. In fact many of these facilities
are called “filtration plants” even though filtration is only one step in the
overall treatment sequence.
•
Rapid Filtration: The first filter built for water purification used
very fine sand and as the filter media. Because of the tiny size of the pore
spaces in the fine sand-waters takes a long time to flow through the filter
bed. When the surface becomes clogged with suspended particles, it
becomes necessary to manually scrape the sand surface to clean the filter.
These units called slow sand filters take up a considerable land area because
of slow rate. Slow sand filters are used in several existing treatment plant.
They are effective and relatively inexpensive to operate.
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In modern water treatment plants, the rapid filter has largely replaced the
slow filter. As its name implies, the water flows through filter bed much
faster (about 30 times as fast) than it flows through the slow sand filter. This
naturally makes it necessary to clean the filter much more frequently. But
instead of manual cleaning by scarping of the surface, rapid filters are
cleaned by reversing the direction of flow through the bed.
During filtration, the water flows downward through the bed under the force
of gravity. When the filter is washed, clean water is forced upward
expanding the filter bed slightly and carrying away the accumulated
impurities. This process is called back washing. Cleaning by a backward
operation is a key characteristic of a rapid filter.
Many rapid filters currently in operation use sand as the filter media and are
called rapid sand filters. But the sand grains are larger than the older (and
pore space) slow sand filters. In a rapid sand filter, the effective size of sand
is about 0.5mm.
Because of this small to large gradation of sand grains in the direction of
flow, most of the filtering action takes place in the top layer of the bed. This
results in the inefficient use of the filter. The filter runtime is reduced and
frequent backwash is required.
•
Other types of filters: The gravity-flow rapid filter is the most
common type used for treating public water supplies primarily because it is
the most reliable. But there are other types of filters that are sometimes used
to classify water including pressure filter and the diatomaceous earth filter
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Hydro Chemistry and Water Pollution Studies for Water Management in Doon Valley
pressure fibre. These are more commonly used for filtering water for the
industrial use or in swimming pools. However the pressure filter is less
reliable than the rapid sand or mixed media filter.
(v) Coagulation: Suspended particle cannot be completely removed from
water by plain settling even when they are given very long detention time
and low overflow rates. Some of the very small turbidity causing particles,
called colloids will not settle out of suspension by gravity without some
help.
The size of the particles varies in the range from 0 to 0.1 µm. These particles
do not settle out on standing and cannot be removed by conventional
physical treatment methods.
If one rapidly mixes certain chemicals called coagulants in the water and
then slowly stirs the mixture before allowing sedimentation to occur, they
will settle.
The coagulant chemical however neutralizes the effect of the colloidal
charges. Once neutralized, the colloidal particles can collide and
agglomerate forming larger and heavier particles called flocs, most of the
flocs form during coagulation are settleable and can be removed from the
water in a sedimentation tank. The most common coagulant is considered to
be (NH4)2SO4 referred to as alum. Some times few synthetic organic
chemicals are also added with alum for better flocculation.
The most popular coagulant in waste treatment application is aluminium
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Hydro Chemistry and Water Pollution Studies for Water Management in Doon Valley
sulphate, or alum. It produces a floc of aluminium hydroxide or hydrage, in
the pH range of 6.0 to8.5 Aluminium chloride is less commonly used
because of being strongly corrosive and correspondingly difficult to handle.
Ferrous sulphate is the least expensive of the iron coagulants and is widely
used in treatment. Ferrous salts do not produce good flocs until oxidized to
ferric state, therefore, in the presence of dissolved oxygen at a pH greater
then 7.7 the optimum pH being 9. Ferric sulphate is also an effective
coagulant when used with time at pH 7.0 to 8.5 Sulphuric acid is used as a
common reagent for lowering the pH of a waste and serves as a coagulant
during the process.
A tentative calculation of the optimum dose of a coagulant, depending on
turbidity and colour of the water, can be done using the following empirical
formulas.
(a) For turbid waters
Dc = 3.5
T
(b) For coloured waters
Dc = 4
C
Applications of coagulation: Textile wastes are normally treated with
calcium chloride and alum. Tannery wastes are treated with ferrous sulphate,
carbon dioxide, alum and iron salts. Sulphuric acid is used to bring the pH
within 5.2 to 5.5 and then it is treated with alum for coagulation. Paperboard
wastes can be effectively coagulated with low dosages of alum.
(vi) Precipitation: Some pollutional constituents of industrial wastes can be
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removed from solution by chemical precipitation followed by sedimentation
or filtration. This is distinguished from coagulation in that the pollutional
substance itself enters into chemical reaction with the reagent instead of
being mechanically absorbed or entrained on the reagent floc. Application
of this principle is limited to a few specific chemical pollutants. The
principle field is inorganic or ionic pollutants, with heavy metal ions.
(vii) Ion Exchange: Ion Exchange can be used for the removal of
undesirable anions and cations from an industrial waste. Cations are
exchanged for hydrogen or sodium and anions for hydroxyl ions. In this
process the reversible interchange of ions and a solid with no change in
physical structure of the solid takes place.
Ion exchange as a mean of waste treatment is a new application of traditional
method of water softening. For example chromic acid purificationChromic acid solutions used in chromium plating etc. can be purified by use
of cation exchangers or remove aluminium ions and other cations that
gradually built up in concentration in the solutions during use.
Reduction: In these processes, soluble metallic ion is reduced through an
oxidation-reduction reaction and then precipitated by conversion to an
insoluble metallic hydroxide; the process is applied commercially in the
treatment of plating wastes containing chromium salts.
The reducing agents commonly used are ferrous sulphate, sodium
metabisulphite, sulphur-di-oxide, metallic ion turnings or borings, metallic
sulphide or sulphites etc.
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Hydro Chemistry and Water Pollution Studies for Water Management in Doon Valley
(viii) Oxidation: The unit process of oxidation serves a valuable purpose in
treating almost of all types of industrial wastes; the most abundant oxidizing
agent is atmospheric oxygen. It serves as oxidant in combustion reactions
such as organic sludge incineration. It is less satisfactory as an oxidizer for
substances in dilute solution except when the oxidation is performed
biologically, in bio-oxidation, dissolved oxygen is very important.
In chemical oxidation of industrial wastes, atmospheric O2 or dissolved O2
is occasionally useful; it can be used in the oxidation of ferrous salts in
spent pickle liquor to ferric compounds. Other common oxidizing agents
include chlorine, bleaching powder and related chlorine compounds, ozone,
and some chemical substances such as permanganates, chromates and
nitrates. Electrolytic reduction also has application in electroplating waste
treatment. Air is the cheapest oxidizing agent but mostly used for biological
oxidation. Ozone is a new inclusion after 1950 as oxidizing agent, but very
costly. Electrolytic oxidation is used for the treatment of cyanide waste.
(ix) Disinfection: Coagulating and filtering water through sand purify it
from
suspended
solids
and
partly
decreasing
its
bacteriological
contamination. Complete disinfection is attained by chemical reagents,
which kill the pathogenic micro organisms
There are four different methods of disinfection viz. (i) Chlorination (ii)
Ozone (iii) Ultraviolet radiation (iv) Thermal disinfection
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•
Chlorination: The addition of chlorine or chlorine compound to
water is called chlorination. Chlorination is considered as the single most
important process for preventing the spread of waterborne diseases.
Molecular chlorine (Cl2) is greenish-yellow at ordinary room temperature
and pressure. In gaseous form, it is very toxic and even in low concentration
it is a severe irritant. But when the chlorine is dissolved in low concentration
in clean water, it is not harmful and if it is properly applied, objectionable
tastes and odours due to chlorine and its by product are most noticeable to
the average person.
Although the chlorine is effective in destroying pathogens and preventing
the spread of communicable diseases, there may be an indirect noninfectious health problem caused by chlorination process.
•
Ozone: O3 is a gas at ordinary temperature and pressure and is a very
potent disinfectant. It is unaffected by pH or ammonia content of the water.
It causes no taste or odour problem. It plays no role in the formation of the
harmful toxic compound, but since it is unstable and can’t be stored, so must
be produced at the sites if does not maintained a measurable residual in the
water after the initial contact time.
•
Ultraviolet radiation: Ultraviolet rays can be used to disinfect water
supplies. In this method, there is no chemical handling; no overdoses are
possible, no taste and odour problem. But its high cost of application and
lack of measurable residual makes it a poor competitor of chlorine.
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•
Thermal disinfection:
This is the most common method of
disinfecting water and has been known from time immemorial. It is boiling.
Boiling disinfects small quantities of water. Drinking water is boiled in
public catering establishment, at hospitals, various institutions etc. but the
method is expensive, required large vessels for boiling and fails to kill
spores therefore this method is not used even in minor water supply systems.
(x) Other treatment processes:
Ground water for example does not
ordinarily require clarification since the water is filtered naturally in the
layer of the soil from which it is withdrawn. Disinfection of ground water
supplies required by law for public water supply systems is basically a
precautioning step. Ground water is usually free of bacteria and other micro
organisms. On the other hand because of its contact with soil and rock,
ground water may have high levels of dissolved minerals that must be
removed.
Water Softening: Water that contains dissolved salt or Ca++ and Mg++ is
called hard water. The process of removing these minerals is called water
softening. The two most common methods of softening are the lime soda
method and the ion exchange method by which the ions get precipitated in
the form of CaCO3 and Mg (OH)2.
CO2 is added to precipitate excess of Ca++ and to maintain the pH because
the pH may, rise before, due to treatment with Lime.
•
Aeration:
A physical treatment process in which air is thoroughly
mixed with water is called aeration which can improve the water quality in a
number of ways e.g., taste and odour control and this application is called
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air striping. Aeration also helps in the removal of iron and manganese from
the water.
•
Activated Carbon: Activated carbon derived from coal and wood has
two unique properties. First, it is very porous material and has an extremely
high ration of surface area to weight up to 100 acres of area per pound.
Second the surface of activated carbon attracts and holds many of the
impurities in water, particularly the dissolved organics by the process of
absorption.
•
Corrosion Control: Corrosion or rusting of metals in water supply
system can be a serious problem. Since corrosion involves a transfer of
electrons, control methods are wind at blocking the flow of electrons
between the water and metal that is susceptible to corrosion. One way to do
this is to add complexing agent such as sodium phosphate.
The water is then disturbed through a system of pipes to individual homes
for use. At the home site, water may be further treated by additional filtering
before being used for drinking and cooking, but most people use it directly
from the tap. Some people are suspicious of tap water that runs through
metal pipes to and into their homes and contains chlorine. In recent years,
there has been a growing market for high quality bottled drinking water for
personal consumption.
No doubt all these measures are quite fruitful. But we still need to know
much more about the long-term affects of exposure to low concentrations of
toxins in our drinking water.
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Biological methods of treatment: The purpose of biological treatment is to
remove the suspended solids that did not settle out in the primary tanks as
well as to remove dissolved BOD that is unaffected by physical treatment.
Biological treatment of sewage involves the use of living microscopic
organism. The microbes including bacteria and protozoa consume the
organic pollutants as food. They metabolize the biodegradable organics
converting them into CO2, water and energy for their growth and
reproduction.
The biological sewage treatment system must provide the micro-organism
with a comfortable home in effect the treatment plant allowed the bacteria to
stabilize the organic pollutant in a controlled artificial environment of steel
and concrete rather than in a stream or lake. This helps to protect the
dissolved oxygen balance of the natural aquatic environment. To keep the
microbes happy and productive in their task of water treatment, they must be
provided with enough oxygen, adequate contact with the organic material in
the sewage, suitable temperature and other favourable conditions. The
design and operation of secondary treatment plant is accomplished with
these factors in mind.
The most common biological treatment systems are of two types. This
includes the trickling filter and the activated sludge process. The trickling
filter is a type of fixed growth system. The microbes remain fixed or
attached to the surface, while the wastewater flows over the surface to
provide contact with the organics.
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Activated sludge is characterized as a suspended growth system because the
microbes are thoroughly mixed and suspended in the wastewater rather than
attach to particular surfaces.
•
Trickling Filters: A trickling filter basically consists of a layer or
bed of crushed rock about 2m (6ft.) deep. It is circular in shape and may
have diameter up to 60 meters.
The primary effluent is sprayed over the surface of the crushed stone bed
and trickles downwards through the bed to the under drain system. The
rotary distributed arm with nozzles located along its length is usually used to
spray the sewage.
The under drain system serves to collect and carry away the waste water
from the bottom of the bed as well as to permit air circulation upward
through the stone. As long as topography permits, the sewage flows from the
primary tank to the trickling filter by the force of gravity rather than by
pumping.
As the primary effluent trickles downwards through the bed of stones, a
biological slime of microbes develops on the surface of rock. The
continuous flow of wastewater over this fixed biological growth provides
contacts between the microbes and organics. The microbes in the thin slime
layer absorb organics; thus, remove oxygen-demanding substances from the
wastewater. Air circulation through the void spaces in the bed of stones
provides the needed oxygen for the stabilization of the organics by the
microbes.
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As the micro-organisms grow and multiply the slime layer gets thicken.
Eventually it gets so thick that the flowing wastewater washes it off the
surface of the stones. This is called sloughing since sloughing does occur
periodically; there is a need to provide settling time for the trickling filter
effluent in order to remove the sloughed biological solids. These solids
basically consist of billions of micro -organisms that have absorbed the
dissolved organics into their body.
The trickling filter effluent is collected in the under drained system and then
conveyed to a sedimentation tank, valved secondary clarifier or final
clarifier, as it is sometimes called.
• Recirculation: Recirculation also serves to improve the pollutant
removal efficiency. It allows the microbes to remove organics that
flowed by them during the previous pass through the bed. There are
many recirculation patterns and configurations of trickling filter
plants. One common pattern is called direct recirculation.
• Hydraulic Load: The rate at which the wastewater flows is called the
hydraulic load. Hydraulic load may be expressed in terms of cubic
metres per day square meter of surface area.
• Organic (BOD) Load: The rate at which the organic material is
applied to the trickling filter is called organic or BOD load. It does not
include the BOD added by recirculation. Organic load is expressed in
terms of kilograms of BOD per cubic meter of Bed Volume per day or
kg/m3.
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Activated Sludge Treatment: The activated sludge sewage treatment
system consists of an aeration tank and a secondary settling basin or
clarifier. The wastewater from primary sedimentation enters the aeration
tank. To this some of the sludge from the final sedimentation tank which
contains aerobic (an oxygen nil environment) bacteria that consume organicmaterial is added. Compressed air is injected continuously into the mixture
through porous diffuser located at the bottom of the tank along one side.
The micro organisms in the aeration tank consume the dissolved organics as
food. The microbes absorb aerobically decomposed organics using oxygen
provided by the compressed air. Water, carbon dioxide and other stable
compounds are formed in addition to provide oxygen for the compressed air.
The micro organisms and waste water are thoroughly mixed together when it
rapidly bubbles up to the surface from the diffuser. Sometimes mechanical
propeller like mixture located at the liquid surface is used instead of
compressed air.
The aerobic micro organisms in the tank grow and multiply forming an
active suspension of biological solids called activated sludge. The
combination of activated sludge and wastewater in the aeration tank is called
mixed liquor. In a basic sludge treatment system, a tank detention time is
approximately 6-hours.
After 6-hours, the mixed liquor flows to the secondary tank or clarifier in
which the activated sludge solid settle down by gravity. The supernatant is
discharged. The settled sludge is pumped out from a sludge hopper at the
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Hydro Chemistry and Water Pollution Studies for Water Management in Doon Valley
bottom of the tank. Recycling a portion of the sludge back to float inlet of
the aeration tank is very important.
The settled sludges processes can be maintained continuously by pumping
about 30% of the wastewater flow from the bottom of the clarifier back to
the head of the aeration tank. When mixed with the primary effluent, the
hungry microbes quickly begin to absorb and metabolise the fresh food. It is
not possible to recycle all the sludge. The excess sludge called waste
activated sludge must eventually be treated and disposed off. In fact most of
the sludge from the final sedimentation tank, however, is transported to
sludge digester. There along with the sludge from the primary sedimentation
tank, is treated by anaerobic bacteria, which further degrade the sludge by
microbial digestion. Methane gas is a product of the anaerobic digestion, and
may be used at the plant as a fuel to run equipment or heat and cool
buildings. In some cases, it is burned off.
•
Sludge Settling:
In the activated sludge process, the organic
pollutants are absorbed by the billion of microorganisms in an aeration tank.
These microorganisms essentially are the activated sludge. But without
proper clarification or separation of sludge from the liquid portion of the
mixed liquor, the treatment process will not be effective at all. For this
reason gravity settling in the secondary clarifier is a most important part of
the activated sludge treatment system. If the sludge does not settle fast
enough, some of the clarifier can cause pollution of the receiving body of
water. Under certain conditions, in an activated sludge in sludge sewage
treatment plant, filamentations or bacteria usually of the species Sphaerotilus
natans grow in the aeration tank making the sludge very fluffy and light.
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Sludge, with excessive growth of these filamentous organisms, settles very
softly and a clear supernatant is not formed in the secondary clarifier. Much
of the sludge flows out with the effluent. This condition is called sludge
bulking. The amount of aeration may be a factor. Sludge bulking is
sometimes associated with too much aeration. Occasionally, to solve the
problem, adjustments in the mixed liquor pit are made.
Wastewater from the final sedimentation tank is then disinfected, usually by
chlorination to eliminate disease-causing agents. The treated wastewater is
then discharged into a river, lake or ocean or, in some limited cases used to
irritate farmland. The sludge from the digester is dried and disposed off in a
landfill or may be applied to improve soil.
Adsorption: This treatment of sewage consists in adsorption of the
dissolved substance on the surface of the adsorbent. Static and dynamic
adsorptions are distinguished. Dynamic adsorption occurs on the surface of
the adsorbent when sewage passes the filer packed with the adsorbent, while
adsorption in static conditions consists in adding certain amounts of the
adsorbent to a given amount of water. During static adsorption, the
concentration of the solute decreases to equilibrium, while in dynamic
adsorption process, the concentration of the solute decrease gradually as the
water passes through the adsorbent bed. If the depth of the filtering bed is
sufficiently great, practically all solute can be removed from the solution. If
the adsorbent is a cheap material (peat, saw dust, slags etc.) it can be
discarded together with the adsorbed substance. But if the pollutant and the
adsorbent are of certain value the adsorbent is regenerated by distillation of
the adsorbed material, its extraction with a suitable solvent, or by converting
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the adsorbed substance into a difficultly soluble derivative. It is often
impossible to regenerate the adsorbent completely because part of it reacts
chemically with the adsorbed substance.
Purification of Water from Radioactive Substances
Natural waters can contain radioactive substances of natural and artificial
origin. Natural radioactivity is given to water when it passes rocks rich in
radioactive elements (the isotopes of uranium, radium, thorium, potassium,
and others). Artificial radioactivity of water is due to its contamination with
industrial radioactive wastes, and waste from medical and other institutional
engaged in radioactive research. Natural water can also be contaminated
with radioactive elements by experimental underground nuclear explosions.
Sewage of increased radioactivity, 100 Cu and over is buried in special
containers
or
pumped
into
underground
cavities
which
are
not
communicated with water bodies.
Radioactive substances are accumulated in the plant and animal tissues and
can be transmitted to man by the trophic chain (e.g. through taking fish and
food). Radioactive substances can be concentrated in small organisms which
are used as food by larger animals, beasts of prey, etc., in whom they are
accumulated in dangerous concentration. Radioactivity of some plankton
organisms 1000 times increases the radioactivity of the water. Some freshwater fishes which are one of the most important links in the trophic chain,
are 20-30 thousand times more radioactive than the water where they live.
When the organisms die, they become the source of the secondary
radioactive contamination of water.
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In the USSR, all possible measures are taken to protect the sources of water
supply from radioactive contamination. If however any radioactive waste
gets into water by accident, the use of water from this source is temporarily
suspended.
Water can be decontaminated by two methods:
Water can be kept for a time required for the short-lived isotopes to decay,
and only then delivered to the consumer;
All suspended or dissolved radioactive substances are removed from the
water.
Radioactive substances can be removed from water by distillation, settling,
filtration, coagulation, adsorption (on sand, clay, active carbon, metals, and
other adsorbents), by ion-exchange processes, and also by various
combinations of the mentioned methods.
If radioactive contaminants are suspended in water, they can be removed by
setting. The time of setting of short-lived isotopes is determined by the halflife period of a particular contaminant. Suspended radioactive particles are
absorbed by microorganisms of the biological film on slow-filters. Common
sand filters retain only part of radioactive contaminants since the adsorbing
capacity of quartz sand is low. The percentage of radioactive substances that
can be retained by various filtering materials is as follows: quartz sand, 7280; activated alumina, 94; charcoal, 86; activated carbon, 92; and glauconite,
83.
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Fine dispersions of radioactive substances are removed by coagulation.
Coagulants and their doses are selected experimentally. As a rule, the dose
of the coagulants is slightly higher than required theoretically. For a better
coagulation, the water is alkalyzed; and the concentration of the element in
question is increased by adding the corresponding non radioactive isotope.
This ensures the required inactivation of water. Aluminium sulphate, ferric
sulphate, ferric chloride, phosphates (Na3PO4 and KH2PO4), lime with
activated sodium silicate, poly-electrolytes and other substances can be used
as coagulants.
The coagulation is more effective if water is turbid. To that end, from 1 to 4
g/litre of clay is added to contaminated water and it is alkalyzed to pH 11. In
100 minutes of the contact, the radioactivity of water decreases 90 percent.
Powered iron taken in a dose of 1000 mg/Litre is used for decontamination
of water. The time of contact is 90 minutes.
The products of radioactive decay are adsorbed by aluminium, zinc and
copper. In some countries water is therefore filtered through a bed of metal
chips (0.5-0.8 in deep). This removes about 80-85 percent of radioactivity.
The filter is regenerated by hydrochloric acid.
Some radioactive isotopes ( Ba140, La140 and Cd115)
The maximum allowed concentrations of radioactive substances in water of
open bodies and sources of water supply can be found in the “Protection of
Surface Waters from Contamination with Sewage” (issued by the ministry of
Land Reclamation and Water Management of the USSR, Moscow, 1974).
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Out of the methods discussed above those, which were under practice and
proved to be successful in the treatment of polluted water and hence
management/control of water pollution are as follows:
v Sedimentation
v Filtration
v Coagulation
v Disinfection (Chlorination)
v Water softening
v Aeration
v Corrosion Control
v Physical treatment (Screens-to retain coarse solids)
Protecting water from pesticide contamination and Control of
water pollution
Application of fertilizers and pesticide to the crops for their better yield and
productivity also leads to water pollution. Pesticides include herbicides,
weedicides and insecticides.
Practices should be implemented to reduce the addition of such chemicals in
fields as (a) EPA as linked some pesticides registrations to the incidence of
detection in water. If too many detection incidents are found the product will
lose registration. (b) Pesticides (Herbicides, Weedicides and Insecticides)
should be used in small amounts, which will really make a difference.
There are two basic ways we can protect surface water from herbicide
contamination. They are
1) Reducing the amount applied and
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2) Keeping any product in the field once it is applied.
Following are the practical ways available to reduce the amount applied to a
field. Narrowing rows is a great way to fill the canopy earlier and decrease
the amount of herbicide needed. Tank Mixing is also one of the methods to
reduce the amount of herbicides for particular crop. In this method less
amount of herbicides is mixed with other products. Planned Resprays is a
practical method, which has been applied in the high weed pressure areas.
Management practices were done to reduce soil erosion as it also stop
pesticides from reaching surface waters. Any practice that increases water
infiltration will reduce water runoff, and therefore pesticide runoff. Crop
rotation is also run to reduce the loss of herbicides in water.
Terracing and contouring both manage the slope to reduce runoff velocity
and slope management and hence reduce erosion losses. Point rows in
contoured fields also prove to be beneficial in reducing soil erosion. There is
no easy way to solve water pollution; yet wastes that require treatment must
be collected from various parts of the manufacturing area and transported to
the treatment plant. Waste liquids of different origins should frequently be
kept separate, in order to simplify their subsequent treatment or to prevent
undesired reactions. Under other circumstances mixing may be desirable to
avoid extreme fluctuations or to achieve partial purification by interaction of
different waste materials.
The first step in the pre-treatment to minimize the affects of industrial
wastes on receiving streams and treatment plants is to reduce the volume of
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waste. This may be accomplished by (i) classification of wastes; (ii)
conservation of wastes; (iii) changing production to decrease wastes (iv)
reusing both industrial and municipal effluents for raw water supplies.
i)
Classification of wastes: By classifying wastes so that manufacturing
process waters are separated from cooling waters, the volume of water
requiring treatment may be reduced considerably. Some time it is possible
to classify and separate the process water themselves, so that only the most
polluted ones are treated, and the relatively uncontaminated ones are
discharged without treatment
(ii) Conservation of wastewater: Water conserved is waste saved.
Conservation begins when an industry changes from an open to a closed
system. For example if the white water in a paper mill is recycled, the cost of
water and cost of water treatment can be reduced.
(iii) Changing production to decrease wastes: This is an effective method
of controlling the volume by considering that waste treatment at the source
as an integral part of production. For example, the reduction of sodium
sulphite is used in dyeing.
Sodium cyanide used in plating and other
chemicals used directly with a saving of money. Also balancing the
quantities of acids and alkalis used in a plant often results in a natural waste,
with a saving of chemicals money and time spent in waste treatment.
iv) Reusing both industrial and municipal effluents for raw water
supplies:
Any portion of the final industrial effluent that can be reused will result in
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less waste to treat and dispose of. Similarly, reuse of sewage effluent will
reduce the quantity of pollution discharged by the municipal effluents.
For example in the paper mills, white waters from paper machines are reused
as spray and wash waters.
Waste strength reduction is the second major objective for in industrial
plant. Any effort to find means for reducing the total aced of polluting
matter in industrial wastes will be well retarded by many saved due to the
reduced requirements for waste treatment. The strength of waste may be
reduced by (i) Process changes (ii) Equipment modifications (iii)
Segregation of wastes (iv) Equalization of wastes (v) By-product recovery.
i) Process changes:
There are many examples of industries resolving waste problems through
process changes. Two examples of progressive management are the textile
and metal finishing industries.
For example textile mills use starch and undergo various processes, which
contribute to high BOD loads. If cellulose compounds are used instead of
starch, the BOD loads will be reduced tremendously.
ii) Equipment Modifications:
Quite often slight or even extensive changes can be made in present
equipment to reduce waste. The redesign of large milk cans with smooth
necks which drain out faster and more completely prevented a large volume
of milk waste from entering stream and waste treatment plants.
iii) Segregation of wastes:
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Segregation of wastes reduces the strength and /or other difficulty in treating
the waste from an industrial plant. Segregating a strong waste from less
potent ones will reduce the strength of the main volume, and the small
volume of a strong waste can be handled with methods specific to the
problem it presents. In terms of volume reduction alone, segregation of
cooling waters and storm waters from process wastes will mean a saving of
the final treatment plant.
iv) Equalization of wastes:
Plants, which produce many products using a diversification of processes,
prefer to utilize their wastes. This requires holding of waste for a certain
period of time. Stabilization of pH, BOD, settling solids and heavy metals
are some of the objectives of equalization.
v) By-product Recovery:
This is the utopian aspect of industrial waste treatment, the one phase of the
entire problem, which may lead to economic gain. Obviously, any use of
waste material eliminates at least some of the waste, which eventually must
be disposed of.
Examples:
ii) Paper Mills with the help of multiple effect evaporation recover caustic
soda from cooking liquors. Also multiple use of sulphite liquor wastes.
iii) From the sugar industrial wastes black strip molasses are recovered for
preparation of alcohol. The cellulose residue is used as a fuel and also used
in the manufacture of wail boards.
iv) Slaughter houses recover waste blood, which is used as a hinder in
making laminated wood products, and in the manufacture of glue.
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These are only a few of the many ways in which industry can turn wastes
into valuable products.
Protection of Surface Waters from Pollution with Sewage:
In compliance with the “Rules of Protection of Surface Waters from
pollution with sewage” sewage should be withdrawn from the point where it
is formed. The object of the present Rules is to preclude pollution of water
bodies, such as rivers, springs, large water reservoirs, lakes, ponds, and
artificial canals used for water supply and fish-raising.
The criterion for the pollution of water is the deterioration of its organoleptic
properties and the appearance of substances noxious to man, animals, birds,
fish, and organisms used in the manufacture of fodder, as well as the
increased temperature of water, which changes the conditions for normal
aquatic life.
The conditions of sewage disposal into the water bodies should be agreed
upon with the authorities responsible for the control and protection of
waters, and also with sanitary and epidemiological services responsible for
the protection of aquatic life (fish).
Those who violate the “Rules” should be persecuted by the law or bear
administrative responsibility.
When sewage is discharged into the water bodies, its composition and
amount should be considered, along with proper consideration of the
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industrial and social importance of a given water body, its character and
capacity.
Solid wastes also contribute to a greater percentage in pollution and
therefore its management is mandatory.
The solid wastes like glass containers, crokeries, plastic containers,
polythene and other packing materials, which are used and dumped as
garbage could be disposed to land and not thrown in the open
Solid wastes could also be recovered and reprocessed by recycling. But
before disposal or recovery, the wastage must be collected.
Pollution should be controlled at source i.e. the reduction in waste input
should be done.
Educating people and making them aware of the problem of water pollution
can even control water pollution. Greater public awareness can make a
positive difference. In the early 1990s, when surfers in Britain grew tired of
catching illness from water polluted with sewage, they formed a group
called surfers against sewage to force governments and water companies to
clean up their act.
Certain laws could also be
framed
to control water pollution.
Environmental laws can make it tougher for people to pollute, but to be
readily effective they have to operate across national and international
borders. Most countries have their own water pollution laws. In the United
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States, for e.g., there is the 1972 Water Pollution Control Act and the 1974
Safe Drinking Water Act.
One of the best ways to tackle this problem is by forming a principle called
Polluter Pays Principle. This means that whoever causes pollution should
have to pay to clean it up, one way or another. Polluter pays can operate in
all kinds of ways. It could mean that tanker owners should have to take out
insurance that covers the cost of oil spill clean ups, for e.g. it could also
mean that shoppers should have to pay for their plastic grocery bags to
encourage recycling and minimize waste. Or it could mean that factories that
use rivers must have their water inlet pipes, so if they cause pollution they
themselves are the first people to suffer. Ultimately, the polluter pays
principle is designed to aware the people from polluting by making it less
expensive for them to behave in an environmentally responsible way.
Some of the individual actions, which were practiced to reduce water
pollution are-by using environmentally – friendly detergents, not pouring
oil from drains, reducing pesticides, and so on. Community action could
also be taken by helping out on beach cleans or litter picks to keep our rivers
and seas that little bit cleaner.
Strict actions, if any should also be taken regarding laws that will make
pollution harder and the country less polluted.
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EMULSIFICATION AND DEMULSIFICATION FOR
WATER SEPARATION AND CONTAMINATED GROUND
WATER MANAGEMENT
Oil pollution of water: The greatest damage to water is inflicted by
petroleum and its products. Oil enters water from breakdowns on derricks,
wrecks of oil tankers, accidental spillage, cleaning of fuel tanks by merchant
and war ships, and also from street cleaning.
According to Thor Heyerdahl (1973), more than 100,000 tons of oil is
annually discharged into the Mediterranean Sea, Each square kilometer of
the sea surface near southern Italy is covered with 500 litres of masout. The
Sargasso Sea is also so heavily polluted with masout that investigators could
not take samples of plankton because the cells of their nets got clogged with
masout.
According to UN reports, the annual oil influx into the ocean from tankers
alone is as million tons, while the total amount of oil that enters the ocean is
ten times as great.
Oil and its products endangers the aquatic life in the surface layers and also
the coastal flora and fauna. Heavy petroleum products precipitate to the
bottom or are adsorbed on rock, stone, and sand banks to inhibit the life of
the hydrobionts. One drop of petroleum spreads over a great area to isolate
the water from contact with atmospheric oxygen, while continuous films
inhibit photosynthesis and the formation of oxygen. This inhibits the growth
of plankton, which is the main food source of the hydrobionts inhabiting the
water body.
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All aquatic animals depend, either directly or indirectly, on plankton, which
is the basis of the trophy chain, but plankton can develop only in depths of
water to which the solar radiation penetrates. In trophics the thickness of this
surface layer is 80-100m, and in the northern regions, 15-20m (on sunny
summer days).
Most organisms inhabiting the surface layers are found near the shoreline,
where they obtain the mineral and organic substances required for their vital
processes. But the surface water in contact with the shore is usually
contaminated with oil, which interferes with the normal development of
many hydrobionts.
Oil and grease present in the water can be extracted in petroleum ether,
which is immiscible in water and can be separated by a separatory funnel.
The residue, after evaporation of this petroleum ether will yield the oil and
grease.
200 to 250 mL of sample is taken in a separatory funnel. 10 mL of sulphuric
acid (1+2) and 25 to 50 mL of petroleum ether is added to the sample. The
mixture is then shaken well, and still if suspension prevails, small amount of
ethyl alcohol is added. It is kept for some time to separate the two distinct
layers; the upper one of petroleum ether and lower one of the sample; lower
layer of the sample is discarded through separatory funnel. A pre -weighed
dish or a small beaker is taken which is then run into it, the petroleum ether
from the separatory funnel through a filter paper, which has already been
moistened with fresh petroleum ether. Little more petroleum ether is added
through the wall of filter paper to remove any residual oil and grease on the
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filter paper. The petroleum ether is evaporated on a water bath and the final
weight of the dish or the beaker is taken after cooling in a dessicator.
Petroleum ether should never be heated on a flame.
Calculation
OIL AND GREASE, mg/L = A-B x 1000,000
V
Where A=Final weight of dish in gm
B= Initial weight of dish in gm
V=Volume of sample taken in mL.
Maximum development of groundwater resources for beneficial use involves
planning in terms of an entire groundwater basin. Management objectives
must be selected in order to develop and operate the basin. These involve not
only geologic and hydrologic considerations but also economic, legal,
political and financial aspects. Typically, optimum economic development
of water resources in an area requires an integrated approach that
coordinates the use of both surface water and ground water resources.
The management of a groundwater basin implies a program of development
and utilization of subsurface water for some stated purpose, usually of a
social or economic nature. In general, the desired goal is to obtain the
maximum quantity of water to meet predetermined quality of requirements
at least cost. Because a ground water basin can be visualized as a large
natural underground reservoir, it follows that extraction of water by wells at
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one location influences the quantity of water available at other locations
within the basin.
Groundwater is extracted from the ground just as are other minerals such as
oil, gas, or gold. Water typically carries a special constraint: it is regarded as
a renewable natural resource. In effect, this can only occur if there exists a
balance between water recharged to the basin from surface sources and
water pumped from within the basin by wells.
Development of water supplies from groundwater begins typically with a
few pumping wells scattered over a basin. With times more wells are drilled
and the rate of extraction increases. As wells become more numerous,
development of the basin reaches and exceeds its natural recharge capability.
By regulating inflow to and outflow from the basin, an underground
reservoir can be made to function beneficially and indefinitely just as a
surface water reservoir.
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