TWO MARK QUESTIONS AND ANWERS
1. What are the important unit operations?
1. Gas transfer: aeration
2. Ion transfer:
a. chemical coagulation
b. chemical precipitation
c. ion exchange
d. adsorption
3. Solute stabilization
a. chlorination
b. liming
c. re-carbonation
d. super-chlorination
2. What are the types of aerators?
i) Gravity aerators
ii) Spray aerators
iii) Diffusers
iv) Mechanical aerators
3. Write the example of chemical precipitation?
i) Flocculation of iron in water, by addition of lime
ii) Precipitation of carbonate hardness by addition of lime.
iii) Precipitation of non-carbonate hardness by addition of soda-ash
4. What are the common forms of solute stabilization?
i) Chlorination: by chlorination, hydrogen sulfide in water is oxidized in to sulfate.
ii)liming : by passing the water through chips of marble ,lime stone or dolomites ,carbon
dioxide in excess of equilibrium requirements is converted in to soluble bicarbonate.
iii) Re-carbonation .by re-carbonation of water softened by excess time treatment, excess
lime is converted in to bicarbonate.
5. What are the purposes of water treatment?
i) To remove color, dissolved gas and murkiness of water.
ii) To remove objectionable tastes and odour.
iii) To remove the disease producing micro0roganisms so that water is safe for drinking
purpose.
iv) To remove hardness.
6. Write about objectives of aeration
i) It removes tastes and odour caused by gases due to organic decomposition.
ii) It increases the dissolved oxygen content of the water.
iii) It removes hydrogen sulphide, and hence odour due to this is also removed.
iv) Due to agitation of water during aeration, bacteria may be killed to some extent.
7. What are the factors affecting the hydraulic performance of fixed –spray pressure
aerators?
i) Orifice and nozzle behavior.
ii) Orifice effects.
iii) Pipe friction associated with multiple take-off.
8. Write on limitations of aeration?
i. it is not efficient method of removal of reduction of tastes and odours caused by
relatively non-volatile substances such as oils of algae.
ii. odour removal is 50 % only when symura was causative organism.
iii. Tastes and odours causes by chemicals due to industrial wastes discharged in to
receiving waters are not satisfactory.
9. What are the types of sedimentation?
Discrete settling
Hindered settling
Zone settling
Compression settling.
10 .what is plain sedimentation?
When the impurities are separate from suspending fluid by action natural forces alone by
gravity and natural aggregation of the settling particles, the operation is called plain
sedimentation.
11. What is chemical precipitation?
When chemicals are added to throw dissolved impurities out of solution, the operation is
called chemical precipitation.
12. What are the assumptions are made in horizontal sedimentation tank?
i. within the settling or sedimentation zone , the particles settle exactly I the same
manner as they do in a quiescent tank of equal depth.
ii. The flow is horizontal and steady and velocity is uniform in all parts of the settling
zone for a time equal to the detention time.
13. What are the elements of sedimentation?
i. detention period and displacement
ii. Overflow rate and surface loading
iii. Basin dimension
iv. in-let and out-let arrangement.
v. sludge removal devices.
14. What are the factors affecting coagulation?
i) Types of coagulant
ii) Quantity or dose of coagulant
iii) Characteristic of water such as
a. type and quantity of suspended matter
b. temperature of water
c. pH of water
iv) Time, violence and method of mixing.
15. Write about common coagulants
1. Alumnium sulphate or alum
2. Chlorinated copperas
3. Ferrous sulphate and lime
4. Magnesium carbonate
5. Polyelectoreds
6. Sodium acuminate.
16. What are the types of filtration?
i) Mechanical straining
ii) Sedimentation
iii) Biological action
iv) Electrolytic action
17 .write about the mechanism of disinfections?
i) Damage to cell wall of micro-organism
ii) Alteration of cell permeability
iii) Changing the colloidal nature of the cell protoplasm
iv) In-activation of critical enzyme systems responsible for metabolic activities.
18. Write notes on break point chlorination.
When chlorine is applied to water, two actions take place one after the other.
i)
it kills bacteria and disinfections is effected
ii)
It oxidizes the organic matter .when chlorine is added to pure water which has no
chlorine demand, a curve, such as line A. From figure, is obtained between the
applied and residual chorine relationship.
The chlorine first performs the functions of removing bacteria .during the disinfections
process ,the amount of residual chlorine will be less in the beginning, during which various
chemicals such as ions of ferrious iron ,sulphides or nitares present in water , will be oxides
,while during stage II ,the combined residual chorine will gradually increase as the demand of
disinfections is satisfied ,till a point C is reached where the ammonia of combined residual
chlorine will be maximum.
19. What are the factors affecting bacterial efficiency of chlorine?
i) Turbidity
ii) Presence of metabolic compounds
iii) Ammonia compounds
iv) pH of water
v) Temperature.
20. What are the advantages of Ozonation?
1. It is more powerful disinfectant over a wide temperature.
2. There is no danger of over treatment since ozone decomposes to oxygen.
3. The oxidation products of ozone are non-poisonous and bio degradable.
16 MARK QUESTIONS AND ANSEWRS
1. Explain about the water treatment process?
Screening
This is adopted to remove all the floating matter from surface waters. It is generally
provide at the intake points.
Aeration
This is adopted to remove objectionable tastes and odour and also to remove the
dissolved gases such as carbon dioxide, hydrogen sulphide etc. the iron and manganese present
in water are also oxidize d to some extent .the process is optional and is not adopted in cases
where water does not contain objectionable taste and odour.
Sedimentation
The purpose of sedimentation is to remove the suspended impurities .with the help of
plain sedimentation, silt, sand etc can be removed. However, with help of sedimentation with
coagulants, very fine suspended particles and some bacteria can be removed.
Filtration
The process of filtration forms very most important stage in the purification of water
.filtration removes very fine suspended impurities and colloidal that may have escaped the
sedimentation tanks. in addition to this, the micro-organisms present in water are largely
removed.
Disinfections
It is carried out to eliminate or reduce to safe minimum limit, the remaining microorganisms, and to prevent the contamination of water during its transit from the treatment plant
to the place of its consumption.
Miscellaneous process
These include water softening, desalination, removal of iron manganese and other harmful
constituents.
2. What are Types of screens?
Coarse screens
Coarse screens or bar screens are intended to intercept only grosser floating material.
They are mostly in the form of bar grill. Then bars are generally of 25 mm size and are spaced at
75 to 100 mm centers. Mostly, bars are kept inclined so that they can be cleaned easily with a
rake. Trash tracks are often included in dams and other intake structures. For the purpose of
cleaning, they are placed on a slope of 3 to 6 vertical to a horizontal.
from figure, the vertical screen is constructed of vertical wires or bars .the strained water enters
the screen box, leaving behind leaves and other debris which drops below as sediment .the
inclined screen can be lifted onto vertical position from cleaning .in most cases, the common
arrangement is to slide a pair of removable screens on to vertical grooves I the walls and bottom
of the inlet channel.
Fine screens
Fine screen are used at surface water intakes, sometimes alone some times following a
bar screen .in order that fine screens does not get clogged up, some device is set up to clean it
continuously .due to this reason, fine screens are usually arranged as end less band or drums of
material perforated with hole of about 6mm diameter .The process is therefore known as
‘automatic straining ‘.
Automatic strainers are self cleaning and they work continuously, eliminating solids from
the liquid flowing through and disposing of these solids, also continuously. In automatic
strainers, appear straining fabric is submerged in water to be strained while the remained of ht
fabric remains above the water.
3. Write briefly about the Types of aerators.
Cascade aerators:
Cascade aerators are the simplest of free fall aerator. Weirs and waterfalls of any kind are
cascade aerators. a simple cascade consists of a series of three or four steps of concrete or metal
.water is allowed to fall through a height of 1 to 3 meters, and due to this it come into close
contact with air .the case cades can be louvered air inlet. The reduction of CO2 is usually in the
range of 50 % to 60%.
Inclined apron aerator with riffle plates:
In this type of aerator, water is allowed to fall along an inclined plane which is usually
studded with riffle plates in hearing bone fashion .the breaking –up to the sheet of water will
cause agitation of water and consequent.
Slat tray aerators.
This is most commonly used .it consist of a closed round or square structure containing a
series of closed structure containing a series of closely stacked superimposed wood- slat trays
.water enters the top of the aerator and is evenly distributed over the topmost tray. the salts in
the trays are staggered so that the films of water raining over the edges of the slat in one tray fall
on the center of he slats in the slats in the tray just below.
Air supplied to the bottom of the aerator with help of a blower, which blows it upward. A
ventilator ids provided at the top, which discharge air and gases to the atmosphere .water is
collected in the collector pan at the bottom, from where it flows to attach basin or reservoir.
Gravel bed aerators.
Cascading through beds of coke, lime –stone or other anthracite is to have more efficient
CO2 removal than other methods .From figure, a typical gravel bed aerator in which water is
applied at the top and trickles down while air is blown upwards. The thickness of gravel bed may
be from 1 to 1.5 m.
Spray aerators
Spray aerators divide the water flow into fine streams and small droplets which come into
intimate contact with air in their trajectory .water is sprinkled in the fine jets through nozzles. It
requires considerable head, but it reduces carbon dioxide by 70 to 90% or more.
Air diffusion
In this method, perforated pipe net work installed at the bottom of the aeration tank, and
compressed air is blown through theses pipes .the air bubbles travel upward through warter, thus
causing aeration. Air diffuser basins have retention period of about 15 minutes and a depth of 3
to 5m.
Alternatively, compressed air may inject into the flows of water in a pipe, or air at
atmosphere pressure may be drawn in the pipe where a constriction, such as the throat of a
venture tube reduces the water pressure below atmospheric. However, aeration under pressure
does not remove CO2.
4. Write the details of plain sedimentation tanks.
Rectangular tanks
In many of these tanks, baffles are provided with the object of preventing shot circuiting.
From figure, a rectangular tank, with out baffles, but with a sludge hopper, and sloping floor.
Such a tank has high settling efficiency .it is provided with mechanical sludge –removal
equipment. The sludge, scrapped by the sludge scrapers is collected in the hopper from where it
is removed through the sludge withdrawal pipe connected ti a sludge pump.
Circular tank with radial or spiral flow:
Though circular tanks are costlier to install, they are convenient for installing mechanical
sludge scrapers. The circular tank may have radial or spiral flow. The path of water in such case
s is generally from a distributor at the center to canting weir at the circumference. Influent enters
through a central pipe and rises up to the baffle box or influent well from where it flows radial
towards the circumference .the outlet channels is provided at the circumference. The racking
arms move slowly to scrap the sludge which is removed through the sludge pipe connected to a
sludge pump.
In case of circular ,a spiral –flow tank, the inlet is provide at the circumference and
directed at an angle between a radius and a tangent .the outlet ,provided in the form of a
submerged weir, is also provided in the form of a submerged weir, is also provide at the outer
circumference ,but is of relatively short length. The effect of the inlet velocity is ti rotate the
water in the tank and to induce a spiral path for the water from inlet to outlet.
Hopper bottom tank with vertical flow:
From the figure, a hopper bottom tank, water enters through the centrally placed inlet
pipe and is deflected towards by the action of deflector box. Water travels vertically downwards.
the sludge settles at the bottom of the hopper, from where it is removed with the help of a sludge
pipe connected to a sludge pump.
5. Explain about of mixing devices.
Centrifugal pump:
In most of cases, centrifugal pump is used to raise the raw water to the settling tank. The
required dose of the chemical therefore can be added to the section line of the pump .when water
fed with the coagulant passes through the impeller of the pump, mixing is complied by the
agitation .however ,after the water comes out ,some gentle agitation is required to get good
results, and to accelerate coagulation and sedimentation.
Compressed air agitation:
In this method the are water fed with coagulant is agitated vigorously by diffusing
compressed air from the bottom of the mixing basins.
Narrow mixing channel with flume
From the figure, a narrow mixing channel with vertical baffles. The coagulant is fed from the
feeding tank. The turbulence caused by vertical baffles mixes the chemical toughly .a flume is
some times provided to develop the hydraulic jump for causing turbulence ,and also to measure
the flow.
Mixing basins with baffle walls:
From figure, two types of mixing basins with baffles. in the horizontal or round the end
type .The water flows horizontally for a short distance makes a complete turn , and continues
back and forth around the ends of the baffles .this causes turbulence ,and hence the mixing
.another type ,known as vertical or “ over and under type’. Mixing basins with baffle walls are
not used now because of high head losses and variations in the velocities.
Mixing basins with mechanical devices
Most of the modern water treatment plants now have mixing basin with mechanical devices.
From the figure, a typical ‘flash mixer ‘in which the raw water and coagulant are agitated
vigorously by a paddle operated by a variable speed motor.
The intensity of mixing is dependent upon the temporal mean velocity gradient. The turbulence
and resultant intensity of mixing is based on the rate of power input to the water. Propeller type
impellers, commonly employed in flash mixers, have revolving speeds ranging from 400 to 1400
rpm.
The usual ratio of impeller diameter to the tank diameter is 0.2 to 0.4..the shaft speed of
propeller is so kept that tangential velocity of greater than 3 m/s, is imparted at the tip of the
blades.
6. Explain the components of slow sand filters.
A slow sand filters filter unit consists of the following parts.
Enclosure tank:
The traditional slow sand filter is an open basin, usually rectangular in size, built below
finished ground level. The water tight basin or tank is constructed either in stone or brick
masonry, with a coatings of water proof material .the floor has a bed slope of 1 in 100 to 1 in 200
towards the central drains. The surface area of the tank varies between 50 square m. to 1000 m 2.
And is found on the basis of filtration rate which varies between 100 to 200 lit. Of the water per
square meter. The depth of the tank from 2.5 m top 4 m.
Filter media
The filter media consist of sand layer, 90 to 110 cm thick .the effective size of sand varies
from 0,2 to 0.35 ,with common value of 0.3. The coefficient of uniformity varies from 2 to 3, the
common value being 2.5 the finer the sand better will be bacterial efficiency, but slower will be
filtration rate.
Fine sand is suitable when pretreatment is poor, high bacteria and turbidity removal
efficiency is desired and saving of wash water is not an important factor. However, if the
pretreatment is good and water to be treated is not highly polluted, coarse sand is suitable. The
sand should be also containing not more than 2% of calcium magnesium computed as
carbonates.
Base material
The filter media is supported on the base material consisting of 30 to 75 cm thick gravel
bed. The gravel base is graded, and laid in layers of 15 cm with topmost layer of the finer size
and bottom most layer of coarse of coarse size.
Under drainage system
The filter media and base material are supported over the under drainage which
eventually collect the filter water and delivers it to clean water with the water reservoir. From the
figure .the plan of slow sand filter with under drainage system consisting of central drain
collecting water from a number of lateral drains .the lateral drains are either earth ware pipes or
perforated pipes of 7.5 to 10 cm dia. They may also be open jointed tiles.
Appurtenances
The various appurtenances that are generally installed for efficiency working are the
devices for (i) measuring loss of head through filter media
(ii) Controlling depth of water above the filter media.
(iii) Maintaining constant rate of flow through the filter.
When a newly constructed or newly washed filter unit is put on operation, the filter head
is only 10 to 15 cm. but as the upper portion of filter sand gets clogged due to impurities of the
water, the filter head increase .in order terminating a constant discharge, the telescopic tube is
adjusted manually.
7. Explain about the forms of chlorination?
Plain chlorination
Plain chlorination is the application of chlorine to plain or raw water supply as it enters
distributions systems. It is also includes the chlorination of raw waters in tanks or reservoirs to
check the growth of weeds, organic matter, algae, and bacteria. It is also removes color and
odour from water. Plain chlorination to untreated water is resorted to when water is relatively
clear, water turbidities less than 20 – 30 ppm. The normal dose between 0.5 to 1 p.p.m
Pre-chlorination
It is the application of the chlorine to water before its treatment –especially before
filtration .sometimes, chlorination is done before the raw water enters the sedimentation tanks.
This helps in reducing in the amount of coagulates required because of eth oxidations of organic
matter. In alkaline water, chlorination may precede aeration. The dose of chlorine residual of 0.1
– 0.5 p.p.m .when it enters the filter plant.
Post chlorination
It is the application of chlorine top water after its treatment. this is the standard form of
chlorination in which chlorine is added to water as leaves the rapid filters and before it enters the
distribution systems .the dose of the chlorine should be adjusted that residual chlorine is about
0.21 – 0.2 p.p.m before water enters the distribution systems .it is useful for protection against
contamination from cross0connections.
Double or multiple chlorination
Double or multiple chlorination refers to the application of chorine at two or more points
in the purifications .Generally, double chlorination is resorted to, in which chlorine is applied
just before water enters the sedimentation tanks and after it leaves the filter plant. These large
amounts of bacteria life and other organic matter.
Break point chlorination
When chlorine is applied to water, two actions take place one after the other.
iii)
it kills bacteria and disinfections is effected
iv)
It oxidizes the organic matter .when chlorine is added to pure water which has no
chlorine demand, a curve, such as line A. From figure, is obtained between the
applied and residual chorine relationship.
The chlorine first performs the functions of removing bacteria .during the disinfections
process ,the amount of residual chlorine will be less in the beginning, during which various
chemicals such as ions of ferrious iron ,sulphides or nitares present in water , will be oxides
,while during stage II ,the combined residual chorine will gradually increase as the demand of
disinfections is satisfied ,till a point C is reached where the ammonia of combined residual
chlorine will be maximum.
This stage of point C is sometimes accompanied by bad taste and odour .Stage II reflects
the forming of combined residuals as the ammonia or amines react with HOCL that has formed
.further increase in the applied chlorine will result increase in the residual chlorine ,indicating the
beginning of eh second action.
During this, action, the relation between applied breaks down choramtes changing them to
nitrogen compounds while the chlorine residual actually drops. At point D, the bad smell and
taste suddenly disappear and the oxidation of organic matter is also complete .the residual
chlorine has it minimum value .further addition of applied chlorine result in increase in the
residual chlorine as represented by line E. the slope of which will be 45 o .point D on the curve
represents break point since further addition of chlorine break through water and appears as
residual chlorine .actually, up to point C. In curve B, chloramines have been recorded has
residual chorine while at point D (break point) true residual free chorine is revolved.
The break point in the chlorination of water may be defined as the point on applied
residual chorine curve at which all, or nearly all, the residuals chorine is free chlorine. Free
chorine residual is part if the total residuals remaining in water (after a specified contact period)
that will react chemically or biologically as hypochlorous acid or hypho chlorite ion.
Super chlorination
Super chlorination is the application of chorine beyond a stage break point .the addition
chlorine sufficient to give a residual chlorine content of 1 to 3 p.p.m. has provide useful to
destroy odours and tastes resulting from chlorine products formed between the
decomposition product from vegetable matter and algae.
Excess chlorine may be added at any points or points of chlorination, though it is usually
applied after flirtation. Super chlorination followed by retention of contact period of 30 -60
min. when residual is discharge by means de-chlorination against.
De-chlorination
It is the process pf excessive chorine from water before distribution to consumer to avoid
chlorine tastes. As stated above, de-chlorination should do in such away in that some residual
chorine remains in water. De-chlorination is achieved either by aeration or by uses of
chemicals such as sodium thio-sulphate, sodium bio-sulphate, activated carbon, KMnO4..
Sodium dioxide gas is applied practically in the same manner, as chlorine, with a contact
period of de-chlorination has not less than 10-15 min. Generally 0.3 to 0.6 p.p.m of sulphur
dioxide may be required by filtering super chlorinated water through beds of granular
activated carbon, excessive chlorine oxidize the carbon to CO2. Due to which odour, taste,
the color is removed.
8. What are the Methods available for water distribution systems?
Gravity systems
In the gravity system, source of water supply is so located with respect to the area of
distribution that water is available with sufficient pressure at various points of the area. No
pumping normally required. However the purification units air located on hill, and sources of
water supply (lake of reservoir) is also located behind hill, pumping may be required to
covey water from the raw water source to the purification work. The purified water then
follows entirely under gravity this is most reliable and economical distribution system.
Combined gravity and pumping system
This is the most common system adopted in most of the case, generally, water
purification work air located almost at the same level as the area of distribution –specifically
when sure of ear water supply is river or a reservoir formed behind a dam. In order to
sufficient distribution pressure, filtered water is pumped into clear water reservoir (C.W.R)
located either on a higher ground or elevated on tower.
The water from elevated from reservoir then flows under gravity as in figure
Pumping systems
In this systems water is pumped directly in to the distribution of system to achieve the
required pressure, as shown in figure. Such as system, is not desirable .generally double
pumping is required first to pump raw water from the source to the treatment work and then
to pump purified water direct in to the distribution mains. The pumps have to be run at
varying speeds according to the variations in the consumption .in case of power failure ,the
entire water distribution systems of the locality is distributed .then systems as requires
constant attendance.