Sunday, November 4, 2007

UPGRADATION OF WATER TREATMENT PLANT


















CHAPTER 01

SOURCES OF WATER















1. SOURCES OF WATER:

Water in perfect form can never be obtained. When it runs of the surface of earth it carries many impurities, which may be harmful hence treatment of water is necessary before it is consumed.

    1. Surface water: lakes and rivers.

As rain water flows over the surface it picks up or dissolves many things such as

Impurity Effects

  1. Soil Turbidity

  2. Pesticides Chemicals that may be harmful

  3. Hardness Causing agents Clogging of distribution system

    1. Ca and Mg salts

  4. Bacteria and

Other Microorganisms Disease causing organisms



    1. Underground Water:

As water percolates into the soil, soil forms natural filters and filters out most of the suspended impurities. Hence the water found underground is generally pure or clear. However on its way it dissolves certain impurities like:

Impurity Effects

  1. Hardness Causing agents Clogging of distribution system

    1. Ca and Mg salts

  2. Fe and Mn Colour, taste

  3. Salts Taste useless for agriculture

  4. Fluorides Harmful and poisonous

















Sr. No.


CHARACTERISTIC IMPURITY

CAUSE

TREATMENT

1.

Turbidity

Suspended and dissolved solids

Sedimentation and filtration

2.

colour

Coloured soil

Fe, Mn compounds

Sedimentation and aeration

3.

Taste and odor

Salts, organic matter

Dissolved gasses

Sedimentation and aeration

4.

pH

Lime, CO2, Ca & Mg salts

Lime Soda process

5.

Bacteria Agents

Feces

Disinfection

6.

Fluorides

Fluorine

Defoliation





































CHAPTER 02

Necessities of

WATER TREATMENT



  1. NECESSITIES OF WATER TREATMENT



    1. INTRODUCTION:

Water that is absolutely pure is not found is nature. There are many impurities in it with its journey from the vapor to the source. Initially Water vapors condense in the air and as it falls it absorbs dust and gases. On ground surface it takes silt and other organic matter as explained earlier. Surface water will retain these impurities for indefinite period. The part of it that enter the soil will eliminate the suspended solids but adding other impurities.

By treating it we make it potable or attain the required standers for industrial and agricultural uses. The aim of water treatment is to produce water that is hygienically safe, aesthetically attractive and potable in an economical manner. Though the treatment of water would achieve the desire quantity the evaluation of its quality should not be confined to the end of treatment facilities but extended to the point.





    1. POTABLE WATER:

The water which is fit for drinking, cooking and bathing etc. is called as potable water. Pure water is a chemical compound with each of its molecule containing 2 hydrogen atoms and one oxygen atom and nothing else. Pure water can never be available in nature. As the water flows over the surface of soil it picks up or dissolves particles of soil, garbage, sewage pesticides and certain other Human Animal and Industrial Chemical wastes. It also sometimes contains certain decayed organic matter, such plants or dead animal or their parts. These impurities may sometimes make it useful and potable for drinking and other public uses especially for drinking. E.g. certain minerals such as iron, calcium, magnesium, fluorine etc. in small quantities may be useful & good for health of people as human need some amount of these elements in their bodies. But sometimes it may render it harmful and unfit. E.g. sometimes water may contain toxic substances such as arsenic barium cadmium, lead etc. which are harmful or unfit for health. Hence purification of water is necessary.











2.2.1. REQUIREMENTS OF POTABLE WATER

Potable water should be free from bacteria. It should be clear and sparkling. It should have good tastes, colour and odor. It should be cool and soft i.e. it should not corrode pipes in the distribution system. Potable water should in short not contain any objectionable matter.





2.2.2. REQUIRMENTS OF ANALYSIS OF WATER

  1. To determine the quality of water.

  2. To determines the treatment processes to be provided.

  3. To ensure that the treatment of water is properly done during each phase or stage.

  4. To examine whether the treated water is according to standards.





      1. WATER ANALYSIS



The following are the tests which are done during water analysis.

  1. Physical Test

  2. Chemical Test

  3. Bacteriological Test



        1. Physical Test :



    1. Turbidity :

  1. Water is said to be turbid if it contains visible or suspended matter such as clay, silt or any finally divided organic matter or inorganic matter.

  2. The turbidity was measured by using turbidity Rod conventionally with optical observations and is expressed as the amount of suspended matter in ppm or mg/lit.

  3. Now a day it is measured with Jackson turbid meter or Bailey’s turbid meter. The standard unit is that which is produced by 1 ml finally divided silica in 1 liter distilled water.







    1. Colour :

      1. Dissolved organic matter from decaying vegetation and some inorganic matter such as coloured soil etc. may impart colour of the soil. Fe, Mn, Micro organism growth also impart colour of the water.

      2. The standard unit of colour is that produced by 1 ml of platinum cobalt mixed in 1 lit of distilled water.

      3. The colour of water can easily be detected be naked eyes. For precise determination of small colour intensities a Tinometer is used.





  1. Taste and odor:

        1. Taste and odor may be caused by the presence of: Dissolved gases such as H2S, CH4, CO2 combined with organic matter, Mineral substances such as NaCl, iron compounds. Carbonates and Sulphates of elements, micro organisms either dead or alive.

        2. The odor of water changes with temperature & it may be classified as fishy, grassy, earthy, vegetable etc. Odor of water tested by using osmosis.

        3. The extent of taste or odour present in a particular sample of water is measured by a term called ‘odour intensity’; which is related with the ‘Threshold Odour’.

        4. Water to be tested is therefore gradually diluted with odour free water, and the mixture at which the detection of odour by human observation is just lost determined. The number of times the is diluted, represents the ‘The Odour Number’.



  1. Temperature:

  1. Testing the temperature of water has usually no significance because in particle it is not possible to give any treatment to control the temperature of water.



  1. Specific conductivity of water:

  1. The total amount of dissolved salts present in water can be easily estimate by measuring the specific conductivity of water.

  2. The specific conductivity of water is determined by means of a potable ionic water tester and is expressed in micro-mhos per cm at 25 C.











        1. CHEMICAL TEST :



    1. Total Solids and Suspended Solids:

The Total solids in water are due to

        1. Suspended Matter

        2. Dissolved Matter

These are determined separately and then added together. The suspended matter is found by filtering the water through a fine filter. The material retained on a filter is weighed. The filtered water is then evaporated and the residue is weighted. This gives dissolved matter.

    1. pH value of water:

        1. This test is conducted to find the acidity or alkalinity of sample of water. Alkalinity in water caused by bicarbonates or hydroxides sodium, potassium and calcium. Acidity is caused by mineral acids, tree carbon di oxide, sulphate of iron and aluminum.

        2. pH value is defined as the logarithm of the reciprocal of the H+ ion concentration. For pure water pH value is 7

        3. Measurement of pH by:

          1. Electronic Method:

In this method, a potentiometer is used to measure the electrical pressure erected by the pH value directly.

          1. Colourimetric Method:

In this method, chemical reagents or indicators are added to sample of water. The colour produced is compared with standard colour waters kept in sealed tubes of known pH values.



    1. Hardness of Water:

        1. Hardness in water is that characteristic which prevents the formation of sufficient leather or foam, when such hard water is mixed with soap. It is usually caused by the presence of calcium and Magnesium salts present in water, which form scum by reaction with soap.

        2. Hard water is undesirable because it leads to greater consumption of soap, scaling of bodies, corrosion and incrustation of pipes and making food tasteless.

        3. There are two types of Harnesses’.

          1. Temporary Hardness

          2. Permanent Hardness

        4. Temporary Hardness:

It is due to the presence of bicarbonates of calcium and magnesium. It can be removed to some extend by boiling or to full extend by adding lime water.

        1. Permanent Hardness:

It is due the presence of sulphates, chlorides and nitrates of calcium and magnesium. It requires special treatment for removal.

        1. Hardness is tested by EDTA test. For this water is treated against ETDA salt solution using Erichrome Black T as an indicator. In another method hardness is tested by soap solution test. In this standard soap solution is added to water and shaken for five minutes. The difference between the soap solution and leather factor gives the Hardness.





    1. Chloride Content:

        1. Water contains chloride. It may be due to leaching of marine sedimentary deposits, pollution from sea water, brine and domestic wastes etc. In ground water sodium chloride may be present due to soil.

        2. The amount of Sodium chloride present in water can be detected by titrating water with silver nitrate. If chlorides are present then reddish colour will be formed.



    1. Nitrogen Content:

The presence of nitrogen in water is the indication of presence of organic matter and may occur in one or more of the following forms:

        1. Free ammonia: It indicates the very first stage of decomposition of organic matter (thus indicating recent pollution).

        2. Albuminoidal or organic Nitrogen: It indicates the quantity of nitrogen before the decomposition of organic matter has already started.

        3. Nitrites: It indicates the presence of partially decomposed organic matter

        4. Nitrates: It indicates the presence of fully decomposed organic matter in water. (thus indicating old pollution).



    1. Metal and other substances:

        1. Iron and magnesium in concentration greater than 0.3 ppm and 0.5 ppm respectively are undesirable, as they may cause the discoloration of the clothes washed in it. Moreover they may cause reduction in diameter of pipes due to deposition of ferric hydroxide and manganese oxide.

        2. The fluoride concentration of less than 0.8 to 1.0 ppm may be harmful and may carry dental caries due to formation of excessive cavities in teeth. Higher concentration can be estimated by using ‘colour matching method’ by using different indicators. The important metals and indicators used are given below-

          1. Iron – 1, 10 phenanthroline

          2. Manganese – Ammonium persulphate + Nitric acid

          3. Fluoride – zirconium alizarin



    1. Dissolved Gases:

        1. H2S gas if present even in small amounts may give bad taste and odour to the water. Presence of CO2 indicates biological activities. It may impart bad taste and odour to water & may cause corrosion.

        2. Presence of oxygen indicates organic matter and consequently making the water suspicious. The amount of dissolved oxygen is found by exposing the sample of water for 4 hours at 27 C with potassium permanganate of 10% concentration.



        1. Bacterial Test :

          1. Bacteria are the minute single cell organisms possessing no definite nucleus and having no green material to help them manufacture their own food.

          2. Certain micro organisms can be examined under a microscope while others like viruses are much smaller and cannot be detected by observing their reactions to different strains.

          3. Some bacteria are harmless and under certain conditions beneficial to human beings animal and crops called ‘non-pathogenic bacteria’ while certain animals and bacteria are deadly toes of man animals and may enter their tissues, causing water borne diseases called ‘Pathogenic Bacteria’.

          4. The following are the tests carried for bacteria detection:



              1. Total count Test:

In this test sample of water with agar added to it, is placed in an incubator at 20 C for 24 hours or at 37 C for 24 hours. The bacterium in the water grows and forms colonies which can be counted. The total count should not be greater than 100 per CC of water.



              1. E – coli test:

E - Coli is the most abundant caliform. It is further subdivided into Biotype and serotype. Unlike other caliform E-coli is a parasite living only in the human or animal intestine voided in feces, it remains in the environment only for some days. Detection of E-coli in drinking water hence is taken as evidence of recent pollution with human and animal feces. And so unfit for drinking.

E-coli test is done in three stages:

  1. Presumptive test

  2. Confirmed test

  3. Completed test



  1. Presumptive Test:

Sample of water is placed in sterile tubes and lactose is added to them. The tubes are incubated for 24 hours at 37 C. If gas is formed then the test is positive. If no gas is formed, it is again examind at the end of 24 hours. If still no gas is found the test is negative.



  1. Confirmed Test:

A small quantity of the sample of water is taken on plate containing endo-coreosin-methylene blue agar. The plates are kept at 37 C for 24 hours. If colonies are seen, the result is positive A complicated test is then carried out to establish the presence of E-coli group of bacteria.



  1. Completed Test:

The test is carried out by adding sample of water to lactose broth fermentation tubes and agar tubes. The tubes are incubated at 37 C for 24 to 48 hours. If gas is present then the water is not safe for drinking. Further tests are carried out to find out type of bacteria present in water. If the test is negative the water is safe for drinking.





































2.2.4. STANARDS FOR POTABLE WATER:



SR.

NO.

CHARACTERISTICS

ACCEPTANCE

CAUSE FOR REJECTION

1.

Turbidity

1 ppm

10 ppm

2.

Colour

5 Cobalt scale

25 on cobalt scale

3.

Taste

Unobjectionable

Unobjectionable

4.

pH value

7 - 8.5

6.5 - 8.5

5.

Total dissolved solids

500 ppm

600 ppm

6.

Total Hardness as CaCo3,

200 mg/lit

600 mg/lit

7.

Chlorides

200 mg/lit

1000 mg/lit

8.

Sulphates as SO4

200mg/lit

400 mg/ lit

9.

Fluorides as F

1 ppm

1.5ppm

10.

Nitrates

45 ppm

45 ppm

11.

Calcium

75

200

12.

Magnesium

30

150

13.

Iron

0.1

1

14.

Mn

0.005

0.5

15.

Copper

0.05

1.5

16.

Al

0.03

0.2

17.

Alkali

200

600

18.

Residual Chlorine

0.2

1

19.

Zinc

5

50

20.

Rhenolic as phenols

0.001

0.002

21.

Detergents

0.2

1

22.

Mineral Oil

0.01

0.03

Toxic Material

24.

Arsenic

0.01

0.05

25.

Cadmium

0.01

0.01

26.

Chromium

0.05

0.05

27.

Lead

0.05

0.05

28.

Mercury

0.001

0.001





















CHAPTER 03

TREATMENT OF WATER





          1. TREATMENT OF WATER:



    1. Introduction:

Water contains certain impurities as stated earlier. Majorly it contains the following types of impurities:

  1. Physical Impurities: causing bad colour, taste and odour, turbidity as well

  2. Chemical Impurities: causing hardness in water. Thereby causing greater consumption of soap, corrosion of pipes and fittings and chocking effects in them.

  3. Bacteriological Impurities: main cause of water borne diseases. Epidemic diseases cause providing drinking water as a medium of spread for bacteria and other micro-organisms

Hence the objects of treatment of water are:-

        1. To remove unpleasant & objectionable tastes and odour and colour from water.

        2. To remove dissolved gases, suspended and dissolved impurities and harmful minerals.

        3. To kill harmful bacteria.

        4. To make water safe for drinking and domestic purpose.

        5. To eliminate corrosion properties of water which affects and pipes.



For this the following methods for purification of water are adopted

    1. AERATION:

Aeration is the process of bringing the water in contact with air. The water absorbs the oxygen from air during this process. Aeration of water promotes tastes and removes odour by exchange of gases between the water and atmosphere.

Objects of Aeration:

  1. Removal of H2S from raw water.

  2. Removal of CO2 and other volatile substances causing bad taste and odour in water.

  3. It helps in killing Bacteria to a certain extent.

  4. To remove impurities like Iron and manganese.

  5. To add and increase the oxygen content in water for giving freshness to it.







      1. Methods Of Aeration:

  1. Spray nozzle:

In this method water is sprinkled in air through special nozzles which breaks the water into droplets, thus permitting the escape of dissolved gases. CO2 gas is considerably removed (up to 90%) in this method. The nozzle can be operated at the pressure of 10-14 m of water.



  1. Cascade:

In this method water is made to fall through a certain height (1 to 3 m) over a series of steps three to ten with a fall of about 0.15 to 0.3 m in each step. The Structure formed is called a ‘cascade aerator’. The aerator should preferably bee installed in open air. These are efficient in raising oxygen content of water, but not forCO2 removal, which is removed only up to 60-70 %.

  1. Air Diffusion:

In this method perforated pipes are installed at the bottom of the settling tanks and the compressed air blown through them so as to thoroughly mix with water. The compressed air is thus bubbled up from the bottom of the tank. During this process it mixes with water, thereby completing the aeration process.

  1. Trickling Bed:

In this method the water is allowed trickle down the beds of coke, supported over the perforated bottomed trays and arranged vertically in series. Generally three beds are used, the depth of each being about 0.6 m with a clear distance between them about 0.45m. the water is applied from top through the perforated distribution pipes and allowed to trickle down up to the e bottom bed. During this process water gets mixed with water and aeration takes place.



















    1. SEDIMENTATION:

Sedimentation is the separation of water by gravitational setting of suspended particle that are heavier than water. It is one of the most commonly used unit aeration in the flow sheet of conventional water treatment. Sedimentation (settling or clarification) is used to remove readily settling such as sand and silt, coagulated impurities such as colour and turbidity and particulate impurities such as hardness and iron. When suspended solids are removed by natural force alone i.e. by gravitational force with or without natural aggregation the process is called plain sedimentation. Plain sedimentation is a primary process. It is followed by sedimentation aided by coagulants. After detention period of few hours, the light free floating particles are converted into sizable floe that settle and are removed.

The factors that affect the rate of sedimentation are size, shape, density, nature of particles, viscosity and temperature of water, velocity of water and surface overflow rate.



      1. TYPES OF SEDIMENTATION TANKS:



  1. Horizontal Flow tanks:

    1. Circular tank with central feed:

Water enters at the center of the tank to flow readily outward in all directions equally.

    1. Circular tank with peripheral feed:

Water enters the tank from periphery or the rim. It has established that average detention time is greater in peripheral basins leading to better performance.

In case of both of these tanks sludge is pawed to central slump mechanically and is withdrawn mechanically during operation.

    1. Rectangular tank with longitudinal flow:

The tank can be cut from the middle or the sludge can be removed mechanically.

  1. Vertical Flow Tanks:

These normally combine sedimentation with flocculation. Tanks may be circular or rectangular with scraper at the bottom. Flocculation takes place as effluent enters from bottom of tank. The velocity decreases with increase in C/S area of tank. The clarified water is thus withdrawn from central or peripheral weir. These tanks have a few valves. They are very compact.



  1. Clarifloculators:

Clarifloculators are widely used in the country in water treatment. As it combines coagulation and sedimentation. It consists of 2 – 4 flocculating paddles placed equidistantly. The water mixed with chemicals is fed in flocculation compartment fitted with paddles rotating at low speed.

Thus flocculated water passes out from the bottom of the flocculation tank to the clarification. Under quiescent conditioning annular settling zone embedded and suspended particles settle at the bottom and the clear effluent overflows the peripheral launder.



TANK DIMENSIONS:



RECTANGULAR TANK:

Lengths 30 – 100 m

L : B Ratio 3 : 1 to 5 : 1

Narrow tank reduce the effect of cross currents and eddies due to wind action. For deep sections, baffles are preferred to confine the line of action of water.



CIRCULAR TANK:

Diameter : 30 – 60 m

30 m are preferred to reduce wind effects.



SQUARE TANK:

Square tanks are generally smaller with side of 20 m, however for vertical flow. Sides are limited to 10 m to avoid large depth.

1. Depth of Tanks : 2.5 m to 5 m

It depends upon type of sludge handled, storage capacity and cost.

If sludge contains lots of organic matter, it is better to clean the tank at regular intervals.

2. Slopes : is 1% in case of rectangular tank

Up to 8% in circular tanks.



COMMON SURFACE LOADING & DETENTION PERIODS

The removal of particles of varying hydraulic subsidence values is solely a function surface overflow rate called ‘surface loading’



Tank type

Surface loading

m3/m2/d

Detention period

(Hr)

Particles removed


Range

Typical value

Range

value


Plain sedimentation

Upto 6000

15 - 30

0.01 – 15

3 - 4

Sand, silt & clay

Horizontal flow

25 – 75

30 - 40

2 – 8

2 – 2.5

Alum and iron floc

Vertical flow

-

40 – 50

-

1 - 1.5

floculent





INLETS & OUTLETS:

Inlet structures must be:

  1. Uniformly distribute flow and suspended particles over the cross section at right angle to flow within individual tanks and various tanks in parallel

  2. Minimize large scale turbulence.

  3. Initiate longitudinal or radial flow.

WEIR LOADING:

Weir length relative to surface area determines the strength of the outlet current. Normal weir loadings are 300m3/d/m. But when settling tanks are properly designed, well clarified waters can be obtained at weir loadings of even up to 1500m3/d/m.















SLUDGE REMOVAL:

Sludge is normally removed under hydrostatic pressure through pipes. The size of pipe will depend upon flow and quantity of suspended matter.



Mechanized units : 100 – 200 mm dia.

Non-mechanized unit : 200 mm dia.



SLOPE : 1: 10 for manual cleaning

: 1: 12 for scraper cleaning in circular tank.



SCRAPER VELOCITY : 0.3 mm



POWER REQUIRED : 0.75 watts/m2 of tank area.



Manually sludge is removed using high pressure hoses.



      1. PRESEDIMENTATION AND STORAGE:

Turbidity of Raw water from rivers and streams may exhibit wide fluctuations. Removal of large sizes and rapidly settleable and other materials can be accomplished by presedimentation and storage. It has been used for high as well as low turbid waters.

When removal of coarse and rapidly settling silt Is aimed at presedimentation lower detention periods of 0.5 to 3 hrs and higher surface loading of 20-80m3/m2/d is preferred.. The storage basins or reservoirs are designed for large detention periods ranging from a week to over a month.



      1. TUBE SETTLERS:

Settling efficiency of a basin is primarily dependent upon surface area and is independent of depth. Hence small diameter tubes having a large welted perimeter relative to welted area providing laminar conditions and low surface loading are effective.



      1. PLAIN SEDIMENTION:

Most suspended impurities present in water have specific gravity greater than water. As a result if water is left undisturbed long enough most of these impurities will settle leaving the supernatant water clean and clear.

The sedimentation tank of this type will generally have a detention time of 4-6 hours. Hence such tanks are very huge and the process is very slow.







      1. COAGULATION AIDED SEDIMENTATION:

Coagulation describes the effect produced by the addition of a chemical to a colloidal dispersion, resulting in a partial destabilization. This is achieved by addition of appropriate chemical and rapid intense mixing for obtaining uniform dispersion of chemical.

Flocculation is the second stage of formation of settleable particles from the destabilized colloidal sized particles and is achieved by gentle prolonged mixing. The result is that the particles remain suspended even after plain sedimentation; also the rate of settling is much quicker with the detention time being around 20 minutes.



        1. COAGULANT INFLUENCING FACTORS:

  1. Coagulant Dosage:

Though there is some relation in raw water and dosage the exact amount only be determined by trial.

  1. Optimum pH zone:

There is at least one pH zone for any given water in which good flocculation occurs in shortest time for given dosage of coagulant. Coagulation must be carried out within that pH range using acids and alkalis for correction of pH.

  1. Coagulant Aids :

Coagulant aid is chemical, which when used along with main coagulant, improves the accelerates the process of coagulation and flocculation by producing quick formation of dense and rapid settling flocs.

Most commonly used coagulant aids are:

      1. Finely divided clay

      2. Fillers earth

      3. Bentonites

      4. Activated carbon or activated silica

      5. Polyelectrolytes which are polymer containing ionisable units have been used successfully as both coagulant aids and coagulants but should be taken guard against their toxicity.



    1. RAPID MIXING:

Rapid mixing is am operation by which the coagulant is rapidly and uniformly dispersed throughout the volume of water; to create more or less homogenous single or multiple phase system. This helps in formation of micro-flocs and results in proper utilization of coagulant preventing localization of the coagulant. The source of power for rapid mixing to create the desired turbulanve is gravitational, mechanical and pneumonic devices.



      1. GRAVITATIONAL DEVICES:

  1. Hydraulic Jump mixing:

It is achieved by a combination of a chute followed by a channel with or without a sill. The chute creates supercritical flow, the sill defining the location of hydraulic jump, and the gently sloping channel induces the jump. In this loss of head is more than 0.3m, and detention time is brief.

  1. Baffled Channel mixing:

In this method, the channel section is mormally designed for velocity of 0.6m/s. Angle subtended by the baffle in channel is between 40-90degrees with the channel wall. The angle should ensure a minimum velocity of 1.5 m/s while negotiating the baffle.

  1. Other types of Hydraulic Mixing

Sudden drop in hydraulic level of water over a weir can cause turbulence and chemicals can be added at this point with aid of diffusers. In pressure conduits, the chemicals can be added at the throat of century or just upstream of orifice located within the pipe.



      1. MECHANICAL DEVICES:

The usual device is rapid rotation of impellers or blades in water and the other mixing with the aid of a jet or impingement over a plate. Propeller type impellers are used in flash mixers with high revolving speed ranging from 400 to 1400 rpm

In impingement type, water is forced as jet through a nozzle, impinging on a plate where the chemical is added. An auxiliary pump is used to create a jet action. The rapid mixing takes place at the point of impingement where the turbulence occurs.



      1. PNEUMATIC DEVICES:

When air is injected or diffused into water after suitable compression, it normally expands isothermally and the resultant work done by air can be used for necessary agitation. The typical range of velocity gradients and contact times are 3000 to 5000s and 0.5 to 0.4 sec.





    1. SLOW MIXING/ STRIRRING:

Slow mixing is the hydrodynamic process which results in formation of large and readily settleable flocs, by bringing finely divided matter into contact with micro flocs divided matter in to contact with microflocs formed during rapid mixing. These can be subsequently removed in settling tanks and filters.



      1. TYPES OF SLOW MIXERS:



              1. GRAVITATIONAL OR HYDRAULIC TYPE FLOCCULATORS:

  1. Horizontal flow baffled flocculator:

This flocculator consists of several around the end baffles with in between spacing of not less than 0.45m. to permit cleaning. Detention time is 20min. It is suited for small treatment plants.

  1. Vertical flow baffled floculator:

  2. Hydraulic jet action flocculator:

Suitable for small treatment plants. The coagulants are injected in raw water using a special orifice device at bottom of tank. Water is then let into this hoppered tank.

  1. Albama type flocculator:

It has separate chamber in series. Water flows from one chamber to another, entering each adjacent partition at bottom and through outlets facing upwards.

  1. Tangential flocculator

  2. Pipe flocculator



              1. MECHANICAL TYPE FLOCCULATORS:

Paddle flocculators are widely used in practice. In large plants, it is desirable to provide more than one compartment in series to lessen the effect of short circuiting.

The peddles can be driven by electric motors or by turbines rotated by waterfall when sufficient head is available. The direction of head is usually horizontal moving parallel or at right angles to paddle shaft.



              1. PEBBLE BED FLOCCULAOTOR:

It consists of pebbles of size ranging from 1mm to 50mm. Smaller the size of pebbles, better is the efficiency but faster is build up of head loss and vice versa. The depth of flocculation is in between 0.3 to 1m. It requires no moving parts or power.



              1. FLUDISED BED FLOCCULATOR:

In this sand bed is in fluidized form. Even 10% expansion of sand bed is enough to create a required turbulence without clogging media.

Advantages:

  • It has no moving parts

  • It does not require power

  • No head loss is developed across the bed.



              1. PNUMATIC FLOCCULATOR:

In this, air bubbles are allowed to raise through a suspension. This creates velocity gradient necessary for flocculation. It requires an air compressor and the problem of clogging of diffuser is very common.



              1. SURFACE CONTACT FLOCCULATOR:

It was studied in India especially to overcome the inherent problem of choking, which increases the head loss over a period of time, in pebble bed flocculators.



              1. INLINE FLOCCULATOR:

The head loss in this flocculator is less and maintenance cost almost negligible. Only occasional flushing is necessary sine deposition of some flocs takes place.





              1. SLUDGE BLANKET CLARIFIER:

A sludge blanket clarifier includes both flocculation and clarification. Flocculators are generally independent of settling tanks that follow.

























CHAPTER 04

DISINFECTION





04. DISINFECTION

4.1. INTRODUCTION:

The process of killing the bacteria from the water and making it safe for drinking is called disinfection. The chemicals used for killing these bacteria are known as ‘disinfectants’

The filters are unable to remove all the disease causing bacteria; therefore the water which comes out from the filter may contain some bacteria. Before water is supplied to the public it is necessary to kill all the bacteria.

4.2. Minor methods of disinfection:



4.2.1. BOILING OF WATER:

The bacteria in water can be destroyed by boiling it for a long time. It is an effective method but practically for large amount of water it is impossible to boil the water, also boiling changes the taste of water.

4.2.2. Excess Lime treatment:

The excess lime when added to water, raises its pH value making it alkaline. This extreme alkalinity has found to remove the bacterial load by 99.3 to 100% from highly polluted water.

Disadvantages:-

  • Water treated with lime can’t be consumed directly.

  • Excess lime has to be removed by suitable method (recombination hence expensive)

4.2.3. Ozone treatment:

Ozone gas is an unstable allotropic form of oxygen, with each of its molecule containing three oxygen atoms which readily breaks down into normal oxygen and releases nascent oxygen. This is a powerful oxidizing agent and thus removes the bacteria from water.

Disadvantages:-

  • Costly treatment

  • No residual can be maintained because it i.e. highly unstable

  • Qzone is unstable so cannot stored for longer time

  • It is less efficient than chlorine than killing bacteria.

4.2.4. Treatment with iodine and Bromine:

The addition of iodine or bromine to water can help in killing the pathogenic bacteria. These disinfectants are available in the form of pills and thus are very handy.

Disadvantages:-

  • It cannot be used for large scale public supplies

  • It gives certain taste and odour.



4.2.5. Treatment with Ultra violet Rays

UV rays are highly effective in killing all types of bacteria, thus yielding truly sterilized water. These rays are produced by passing electric current through mercury vapor lamps.

Disadvantages:-

  • Costly treatment

  • Unsuitable for treating large public supply systems

  • Ineffective for high turbidity (>15ppm)



4.2.6. Treatment with potassium Permanganate:

This is useful for disinfecting water supplies contaminated with lesser amounts of bacteria. It has also been used as an algaecide and for removing color and iron form water.

Disadvantages:-

  • Nor effective for all bacteria

  • If water treated with passage of time, it produces dark brown precipitate, which is noticeable as a coating on porcelain vessels and difficult to remove without scouring.



4.2.7. Chlorination:

The process of applying chlorine to water is called chlorination. This is the most commonly and widely used method of disinfection for public-water supply.

Advantages:

  • The process is economical and cheap.

  • Harmless to human beings.

  • It is reliable and effective.

  • Residual Cl2 can be maintained in water.



4.2.8. Application of chlorine:

Free chlorine is available in gaseous or liquid for chlorine is stored in cylinders under a pressure of 700 to 100 KN/m2. When required for use the pressure is released and the liquid chlorine gas dissolved in water to make a solution in the chlorinator, which is fed into the water to be treated.

Advantages:

  • It can be stored for longer time without the loss of quality

  • It is very powerful and effective disinfectant

  • Skilled supervision not required



4.2.9. Hypochlorites and Bleaching Powder:

Calcium and sodium hypochlorite may be used for chlorinating small public supplies when such hypochlorite is dissolved in water, it dissociates and to form hypochlorite ions, as

Ca(OCl)2 Ca++ + 2OCl-

Calcium hypochloride calcium ions hypochloride ions

The hypochlorite ions may further combine with the hydrogen ion present in water, so as to form hypochlorous acid.

OCl- + H+ HOCl-

Hypochlorous acid

Another chlorine compound used for disinfection is ‘bleeasching powder or ‘chlorinated lime’ having molecular formula CaOCl2. It contains about 30% of available chlorine and when exposed to air, light and moisture, it rapidly loses its chlorine content. Hence it has to be stored carefully.

The solution is first prepared by mixing it with water in little quantity and then added to water according to requirement.



4.2.10. CHLORINE TABLETS

Chlorine tablets may also sometimes be used to disinfect small quantities of water, but they are costly. These tablets are manufactured in carious strengths and a single tablet of 0.5gm is sufficient to disinfect about 20 liters of water.

4.2.11. CHLORAMINES

As chlorine is not stable in water, it is sometimes mixed with ammonia. These compounds are quiet stable and can remain in water as residuals for a sufficient time. Hence they can provide a greater safeguard against future pollution

Advantages:

  • They don’t give bad taste and odour when left as residuals

  • They are useful when phenols are present in water.



4.3. TYPES OF CHLORINATION:


4.3.1. Plain Chlorination:

In plain chlorination water is fed into the distribution system after giving chlorine treatment only. This help in removing bacteria, organic matter and chlorine from the raw water. The used quantity of chlorine required for plain chlorination is about 0.5 mg/lit or more.


4.3.2. Pre – chlorination:

Pre chlorination is the process of treating water with chlorine before any treatment is given. It helps in improving coagulation and reduces load on filter. The chlorine dose should be about 0.1 to 0.5 mg/lit of residual chlorine comes to the filter plant. The normal doses are as high as 5 – 10 mg/lit


4.3.3. Post chlorination:

The normal standard process of applying at the end when all treatments have been completed is called as post chlorination. It is adopted before water enters into the distribution system. The dosage of Cl2 should be 0.1 to 0.2 mg/lit after a contact period of about 20 minutes.



4.3.4. Double Chlorination:

Both pre and post chlorination are used in double chlorination. Post chlorination is always used but pre chlorination is added when water are highly turbid and contaminated. Sometimes to maintain the residual chlorine content of 0.2 mg/lit at the farthest end of long distribution systems. Post chlorination is done in two stages called double chlorination.



4.3.5. Break Point Chlorination:

Water contains basic two impurities

  • Bacteria

  • Organic Matter

Chlorine is required to destroy both. When Chlorine is applied it kills bacteria first. Any further addition will appear as residual Chlorine increases. After a certain point, the residual chlorine decreases with emition of bad smell and taste. It indicates that chlorine is used for oxidizing organic impurities. Again after sometimes, there is sudden decease in residual chlorine indication that oxidation of impurities is over. The point at which both the demands are satisfied is called as break point/ the process of adding chlorine beyond break point is called break point chlorination.



4.3.6. Super Chlorination:

It indicates the excessive use of chlorine (i.e. 5-15mg/l) to the water. This requires in special cases of when water is colored, plain chlorination produces taste and odour or for highly polluted waters or during epidemics of water borne diseases. The excess chlorine is to be dechlorinated by using d chlorination agents.



4.3.7. Dechlorination:

Removal of chlorine from water is called as dechlorination. In super chlorination residual chlorine up to 0.1 - 0.2 mg/l remains in water, so as the dechlorinaion is carried out. The dechlorinating agents used are SO2, activated carbon, Na2SO3, and ammonia (NH4OH).





























CHAPTER 05

FILTERATION OF WATER



5.0. FILTERATION PROCESS:

5.1. Introduction:

Filtration is a process for separating suspended and colloidal impurities from water passage through a porous medium. Filtration with or without pretreatment has been employed for treatment of water to effectively remove turbidity for e.g. Silt and clay, colour, microorganisms, precipitated hardness from chemically softened waters and precipitated iron and manganese form aerated waters. Removal of turbidity is essential not only form the requirement of aesthetic acceptability but also for efficient disinfection which is difficult in the presence of suspended and colloidal impurities that serve as hideouts for microorganisms.

Filters can be classified according to

  1. Direction of flow

  2. Types of filter media

  3. The method of flow rate control

  4. The filtration rate

Based on filter media and beds filters have been categorized into

    • Granular medium filters

    • Fabric and mat filters and micro strainers

The granular medium filters include single dual media and multimedia (usually tri-media) filters. Sand, coal, crushed coconut, shell, diatomaceous earth and powered or granular activated carbon have been used as filter media but sand filters, have been most widely used as sand is widely available, cheap and effective in removing impurities. The driving force to overcome the frictional resistance encountered by the flowing water can be either the force of gravity or applied pressure force. The filters are accordingly referred to as gravity filters and pressure filters. In fourth category are constant rate and declining or variable rate filters. Lastly dependent upon the flow rates the filters are classified as slow or rapid sand filters.







5.2. SAND FILTERS:

Filter sand is defined in terms of effective size and uniformity coefficient. Effective size is the sieve size in mm that permits 10% by weight to pass. Uniformity in the size is specified by UC which is the ratio between the sieve size that will pass 60% by weight and effective size.



      1. Specifications:

Shape size and quantity of the filter sand shall satisfy the following norms:

  1. Sand shall be hard and resistant quartz of quart rite and free of clay, fine particles, soft grains and dirt of emery description.

  2. Effective size shall be 0.45 to 0.7mm.

  3. Uniformity coefficient shall not be greater than 1.7 nor less than 1.3.

  4. Ignition loss should not exceed 0.7% by weight.

  5. So while fraction in HCL should not exceed 5% by weight

  6. Silica content should not less than 90%.

  7. Specific gravity shall be in the range of 2.55 to 2.66.

  8. wearing loss shall not exceed 30%

      1. Slow sand filters

Slow sand filters can provide a single step treatment for turbidity (<20NTU) when land, labour and filter sand are readily available at low at low cost, chemicals and equipments are difficult to procure and skilled personal to operate and maintain are not available locally

DESCRIPTION

A slow sand filter consists of an open box about 3m deep rectangular or circular in shape and made of concrete or masonry. The box contains a supernatant water layer a bed of filter medium, an under drainage system and a set of control valves and appurtenances and a set of control valves and appurtenances.

The supernatant provides the driving force for the water to flow through the sand bed and to overcome frictional resistance in other parts of the system. It can also provide a storage of several hours to the incoming water before it comes to the surface.

The filter bed consists of natural sand with an effective size of 0.25-0.35mm and uniformity coefficient of 3 to 5. For best efficiency, the thickness of filter bed should not be less than 0.4-0.5m. As a layer of 10-20 mm sand will be removed every time, the filter is cleaned a new filter should be provided with an initial sand depth of about 1m. Resanding will then be necessary once in 2-3 years.

The under drainage system supports the sand bed and provides unobstructed passage for filtered water to leave the underside of the filter. The under drains may be made of enjoined bricks laid to form channels, perforated pipes or porous tiles laid over the drains. Graded gravel to a depth of 0.2, 0.3m is placed on the under drains to prevent the sand from entering the under drains and ensure uniform abstraction of filtered water form entire uniform abstraction of filtered water form entire filter.

A system of control valves facilitates regulation of filter rate and adjustment of water level in the filter at the time of cleaning and backfilling when the filter is put back into operation after cleaning.



        1. SUMMERY GUIDELINES FOR DESIGN OF SLOW SAND FILTERS:-

SR.NO.

DESCRIPTION

DESIDN VALUE

1.

Design period

10 years

2.

Filtration rate

Normal operation

Maximum overload rate



0.1m/hr

0.2m/hr

3.

Number of filter beds

Area : 20 mm2

20-249 mm2

250-649 mm2

650-1200 mm2

1201-2000 mm2



2

3

4

5

6

4.

Depth of supernatant

1 m

5.

Free board

0.2 m

6.

Depth of filter ( sand)

Initial

final



1 m

0.4m

7.

Size of sand

0.2 – 0.3 m

8.

UC

5

9.

Gravel(3 – 4) depth

0.3 m

10.

under drains

0.2 m

11.

Effluent weir Level above sand bed

20 – 30 mm

5.2.3. Rapid sand filters:

Rapid sand filters comprises of bed of sand serving as a single medium granular matrix supported on gravel overlying an underdrain system. The distinctive features of rapid sand filter as compared to slow sand filtration includes pretreatment of raw water to effectively flocculate the colloidal particles, use of higher filtration rates and coarser but more uniform filter media to utilize greater depths of filter media to trap influent solids without excessive head loss and backwashing the filter bed by reversing the flow direction to clean the entire depth of the filter. Pretreatment of the filter influents should be adequate to achieve the efficient removal of colloidal and suspended solids despite fluctuation in raw water quantity.

        1. MECHANISM OF PARTICLE REMOVAL

The removal of the particles within a deep granular medium filter, such as rapid sand filters occurs primarily within the filter bed and is referred to as depth filtration. Several mechanisms either singly or in combinations get to achieve overall removal of suspended solids and colloidal particles in depth filtration. Consequently the removal of particles takes place in two distinct stages; once the particles come closes to the surface, an attachment step is required to strain it on the surface instead of letting it flow down the filter.

        1. RATE OF FILTRATION

Standard rate of filtration is 80-100 lpm/m2. Practice is tending to higher rates of 10 m/hr.

        1. CAPACITY

The capacity of the rapid sand filters should be such that the number of units can take care of the total quantity of water to be filtered and is optimum to keep the filters working without undue loading at anytime. For flexibility of operations minimum of 4 units must be provided, which could be reduced to 2 for smaller plants.

        1. DEPTH OF SAND

Usually the sand layer has depth of 0.6 to 0.75m, but higher rate of filtration, the coarse medium is used, deeper sand beds are suggested. The standing depth of water over filter varies between 1 and 2 m. the free board above the water level should be at least 0.5m so that when air binding problems are encountered, it will facilitate the additional levels of 0.15 to 0.3m of water being provided to overcome trouble.

        1. FILTER BOTTOMS AND STRAINER SYSTEMS:

The under drainage system of filter is intended to collect the filtered water and to distribute the wash water in such a fashion that all portions of bed may perform nearly the same amount of work and when washed, receive nearly same amount of cleaning. Since the rate of wash is several times higher than the rate of filtration, the former is the governing factor in the hydraulic design of filters which are cleaned by backwashing.

The most common type of under drain is the central manifold with laterals either performed on the bottom or having umbrella type strainers on top. Other types such as wheeler bottom- a fake bottom with a strainers spaced for the entire area at regular intervals or a porous plate floor supported on concrete pillars are all satisfactory when properly designed and constructed.

        1. FILTER GRAVEL:

Gravel is placed between sand and the under drainage system to prevent sand from entering the under drain and to aid uniform distribution of wash water. The gravel should accomplish both purpose without being displaced by the rising wash water sizes of gravel vary from 50mm at bottom to 2.5mm at the top with a 0.45m depth. Faster the rate of application of water, larger the gravel size required.

The inherent drawback of rapid sand filter system is the surface clogging tendencies due to unfavorable stratification of sand medium. A rapid sand filter consists of sand bed which becomes stratified after washing.

The size gradation is from fine to coarse with finest sand particles being at the top of the bed. Since majority of impurities are removed and stored in the limited pore space available in top sand layers, it leads to surface clogging with relatively quicker built-up of head loss at higher velocities of filtration leading to the under utilization of the sand bed.

Another drawback of fine to coarse size gradation of filter medium is possibility of poor filtrate quantity resulting from non removal of finer floc particles with escape the lower sand layers also break through the lower layers containing larger size sand medium.



    1. Pressure Filters

Based on the same principle as gravity type rapid sand filters, water is passed through the filter under pressure through a cylindrical tankusually made of steel or cast iron. Whrein the underdrain, sand and gravel are placed. They are compact and can be prefabricated and moved to the site. Economy is possible in certain cases by avoiding double double pumping.

Pretreatment is essential

      1. DISADVANTAGES :

  • The treatment of water under pressure seriously complicates effective mixing and flocculation of water to be filtered.

  • In case of direct supply from pressure filters, it is not possible to provide adequate contact time for chlorine.

  • The water under filtration and the sand bed are out of sight and it is not possible to observe the effectiveness of the back wash to the degree of agitation during the washing process.

  • Because of the inherent shape to he pressure filters it is difficult to provide wash water gutters effectively designed so that material washed from the sand is discharged to waster and not flushed back to other portions of the sand bed.

  • It is difficult to inspect, clean and replace the sand, gravel and under drains of the pressure filters.

  • Because the water is under pressure at the delivery end, on occasions when the pressure on the discharge main is released the entire sand bad might be disturbed violently with disastrous results to the filter effluent.

  • In view of these disadvantages pressure filters are not recommended for community water supplies, particularly for large one. They may be used for industrial needs or swimming pools.






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