Lecture notes on Environmental Engineering pdf

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LECTURE NOTE COURSE CODE BCE1504 ENVIRONMENTAL ENGINEERING 1 UNDER REVISION Disclaimer This document does not claim any originality and cannot be used as a substitute for prescribed textbooks. The information presented here is merely a collection by the th committee members for their respective teaching assignments for the students of 5 semester B.Tech Civil Engineering of VSSUT, Burla. We would like to acknowledge various sources like freely available materials from internet from which the lecture note was prepared. The ownership of the information lies with the respective authors or institutions. Further, this document is not intended to be used for commercial purpose and the committee members are not accountable for any issues, legal or otherwise, arising out of use of this document. The committee members make no representations or warranties with respect to the accuracy or completeness of the contents of this document and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. 2 UNDER REVISION LECTURE-1 Module-1 Raw Water Source The various sources of water can be classified into two categories: 1. Surface sources, such as a. Ponds and lakes; b. Streams and rivers; c. Storage reservoirs; and d. Oceans, generally not used for water supplies, at present. 2. Sub-surface sources or underground sources, such as a. Springs; b. Infiltration wells ; and c. Wells and Tube-wells. Water Quantity Estimation The quantity of water required for municipal uses for which the water supply scheme has to be designed requires following data: 1. Water consumption rate (Per Capita Demand in litres per day per head) 2. Population to be served. Quantity= Per capita demand x Population Water Consumption Rate It is very difficult to precisely assess the quantity of water demanded by the public, since there are many variable factors affecting water consumption. The various types of water demands, which a city may have, may be broken into following classes: Water Consumption for Various Purposes: Types of Consumption Normal Range Average % (lit/capita/day) 1 Domestic Consumption 65-300 160 35 2 Industrial and Commercial 45-450 135 30 Demand 3 Public Uses including Fire 20-90 45 10 Demand 4 Losses and Waste 45-150 62 25 Fire Fighting Demand: The per capita fire demand is very less on an average basis but the rate at which the water is required is very large. The rate of fire demand is sometimes traeted as a function of population and is worked out from following empirical formulae: Authority Formulae (P in thousand) Q for 1 lakh Population) American Insurance Q (L/min)=4637 √P (1-0.01√P) 41760 1 Association 2 Kuchling's Formula Q (L/min)=3182 √P 31800 4 UNDER REVISION 3 Freeman's Formula Q (L/min)= 1136.5(P/5+10) 35050 Ministry of Urban Q (kilo liters/d)=100 √P for P50000 31623 4 Development Manual Formula Factors affecting per capita demand: a. Size of the city: Per capita demand for big cities is generally large as compared to that for smaller towns as big cities have sewered houses. b. Presence of industries. c. Climatic conditions. d. Habits of people and their economic status. e. Quality of water: If water is aesthetically & medically safe, the consumption will increase as people will not resort to private wells, etc. f. Pressure in the distribution system. g. Efficiency of water works administration: Leaks in water mains and services; and unauthorised use of water can be kept to a minimum by surveys. h. Cost of water. i. Policy of metering and charging method: Water tax is charged in two different ways: on the basis of meter reading and on the basis of certain fixed monthly rate. Fluctuations in Rate of Demand Average Daily Per Capita Demand = Quantity Required in 12 Months/ (365 x Population) If this average demand is supplied at all the times, it will not be sufficient to meet the fluctuations.  Seasonal variation: The demand peaks during summer. Firebreak outs are generally more in summer, increasing demand. So, there is seasonal variation .  Daily variation depends on the activity. People draw out more water on Sundays and Festival days, thus increasing demand on these days.  Hourly variations are very important as they have a wide range. During active household working hours i.e. from six to ten in the morning and four to eight in the evening, the bulk of the daily requirement is taken. During other hours the requirement is negligible. Moreover, if a fire breaks out, a huge quantity of water is required to be supplied during short duration, necessitating the need for a maximum rate of hourly supply. So, an adequate quantity of water must be available to meet the peak demand. To meet all the fluctuations, the supply pipes, service reservoirs and distribution pipes must be properly proportioned. The water is supplied by pumping directly and the pumps and distribution system must be designed to meet the peak demand. The effect of monthly variation influences the design of storage reservoirs and the hourly variations influences the design of pumps and service reservoirs. As the population decreases, the fluctuation rate increases. Maximum daily demand = 1.8 x average daily demand 5 UNDER REVISION Maximum hourly demand of maximum day i.e. Peak demand = 1.5 x average hourly demand = 1.5 x Maximum daily demand/24 = 1.5 x (1.8 x average daily demand)/24 = 2.7 x average daily demand/24 = 2.7 x annual average hourly demand Design Periods & Population Forecast This quantity should be worked out with due provision for the estimated requirements of the future . The future period for which a provision is made in the water supply scheme is known as the design period. Design period is estimated based on the following:  Useful life of the component, considering obsolescence, wear, tear, etc.  Expandability aspect.  Anticipated rate of growth of population, including industrial, commercial developments & migration-immigration.  Available resources.  Performance of the system during initial period. Population Forecasting Methods The various methods adopted for estimating future populations are given below. The particular method to be adopted for a particular case or for a particular city depends largely on the factors discussed in the methods, and the selection is left to the discrection and intelligence of the designer. 1. Arithmetic Increase Method 2. Geometric Increase Method 3. Incremental Increase Method 4. Decreasing Rate of Growth Method 5. Simple Graphical Method 6. Comparative Graphical Method 7. Ratio Method 8. Logistic Curve Method LECTURE-2 Population Forecast by Different Methods Problem: Predict the population for the years 1981, 1991, 1994, and 2001 from the following census figures of a town by different methods. Year 1901 1911 1921 1931 1941 1951 1961 1971 6 UNDER REVISION Population: 60 65 63 72 79 89 97 120 (thousands) Solution: Year Population: Increment per Incremental Percentage Increment per (thousands) Decade Increase Decade 1901 60 - - - 1911 65 +5 - (5+60) x100=+8.33 1921 63 -2 -3 (2+65) x100=-3.07 1931 72 +9 +7 (9+63) x100=+14.28 1941 79 +7 -2 (7+72) x100=+9.72 1951 89 +10 +3 (10+79) x100=+12.66 1961 97 +8 -2 (8+89) x100=8.98 1971 120 +23 +15 (23+97) x100=+23.71 Net values 1 +60 +18 +74.61 Averages - 8.57 3.0 10.66 +=increase; - = decrease Arithmetical Progression Method: P = P + ni n Average increases per decade = i = 8.57 Population for the years, 1981= population 1971 + ni, here n=1 decade = 120 + 8.57 = 128.57 1991= population 1971 + ni, here n=2 decade = 120 + 2 x 8.57 = 137.14 2001= population 1971 + ni, here n=3 decade = 120 + 3 x 8.57 = 145.71 1994= population 1991 + (population 2001 - 1991) x 3/10 = 137.14 + (8.57) x 3/10 = 139.71 Incremental Increase Method: Population for the years, 1981= population 1971 + average increase per decade + average incremental increase = 120 + 8.57 + 3.0 = 131.57 1991= population 1981 + 11.57 = 131.57 + 11.57 = 143.14 2001= population 1991 + 11.57 = 143.14 + 11.57 = 154.71 1994= population 1991 + 11.57 x 3/10 = 143.14 + 3.47 = 146.61 Geometric Progression Method: Average percentage increase per decade = 10.66 n P = P (1+i/100) n n Population for 1981 = Population 1971 x (1+i/100) = 120 x (1+10.66/100), i = 10.66, n = 1 = 120 x 110.66/100 = 132.8 7 UNDER REVISION n Population for 1991 = Population 1971 x (1+i/100) 2 = 120 x (1+10.66/100) , i = 10.66, n = 2 = 120 x 1.2245 = 146.95 n Population for 2001 = Population 1971 x (1+i/100) 3 = 120 x (1+10.66/100) , i = 10.66, n = 3 = 120 x 1.355 = 162.60 Population for 1994 = 146.95 + (15.84 x 3/10) = 151.70 LECTURE-3 Intake Structure The basic function of the intake structure is to help in safely withdrawing water from the source over predetermined pool levels and then to discharge this water into the withdrawal conduit (normally called intake conduit), through which it flows up to water treatment plant. Factors Governing Location of Intake 1. As far as possible, the site should be near the treatment plant so that the cost of conveying water to the city is less. 2. The intake must be located in the purer zone of the source to draw best quality water from the source, thereby reducing load on the treatment plant. 3. The intake must never be located at the downstream or in the vicinity of the point of disposal of wastewater. 4. The site should be such as to permit greater withdrawal of water, if required at a future date. 5. The intake must be located at a place from where it can draw water even during the driest period of the year. 6. The intake site should remain easily accessible during floods and should noy get flooded. Moreover, the flood waters should not be concentrated in the vicinity of the intake. Design Considerations 1. sufficient factor of safety against external forces such as heavy currents, floating materials, submerged bodies, ice pressure, etc. 2. should have sufficient self weight so that it does not float by upthrust of water. Types of Intake Depending on the source of water, the intake works are classified as follows: Pumping A pump is a device, which converts mechanical energy into hydraulic energy. It lifts water from a lower to a higher level and delivers it at high pressure. Pumps are employed in water supply projects at various stages for following purposes: 8 UNDER REVISION 1. To lift raw water from wells. 2. To deliver treated water to the consumer at desired pressure. 3. To supply pressured water for fire hydrants. 4. To boost up pressure in water mains. 5. To fill elevated overhead water tanks. 6. To backwash filters. 7. To pump chemical solutions, needed for water treatment. Classification of Pumps Based on principle of operation, pumps may be classified as follows: 1. Displacement pumps (reciprocating, rotary) 2. Velocity pumps (centrifugal, turbine and jet pumps) 3. Buoyancy pumps (air lift pumps) 4. Impulse pumps (hydraulic rams) Capacity of Pumps Work done by the pump, H.P.=wQH/75 3 3 where, w= specific weight of water kg/m , Q= discharge of pump, m /s; and H= total head gainst which pump has to work. H= H + H + H + (losses due to exit, entrance, bends, valves, and so on) s d f where, H =suction head, H = delivery head, and H = friction loss. s d f Efficiency of pump (E) = wQH/Brake H.P. Total brake horse power required = wQH/E Provide even number of motors say 2,4,with their total capacity being equal to the total BHP and provide half of the motors required as stand-by. Conveyance There are two stages in the transportation of water: 1. Conveyance of water from the source to the treatment plant. 2. Conveyance of treated water from treatment plant to the distribution system. In the first stage water is transported by gravity or by pumping or by the combined action of both, depending upon the relative elevations of the treatment plant and the source of supply. In the second stage water transmission may be either by pumping into an overhead tank and then supplying by gravity or by pumping directly into the water-main for distribution. 9 UNDER REVISION Free Flow System In this system, the surface of water in the conveying section flows freely due to gravity. In such a conduit the hydraulic gradient line coincide with the water surface and is parallel to the bed of the conduit. It is often necessary to construct very long conveying sections, to suit the slope of the existing ground. The sections used for free-flow are: Canals, flumes, grade aqueducts and grade tunnels. Pressure System In pressure conduits, which are closed conduits, the water flows under pressure above the atmospheric pressure. The bed or invert of the conduit in pressure flows is thus independant of the grade of the hydraulic gradient line and can, therefore, follow the natural available ground surface thus requiring lesser length of conduit. The pressure aqueducts may be in the form of closed pipes or closed aqueducts and tunnels called pressure aqueducts or pressure tunnels designed for the pressure likely to come on them. Due to their circular shapes, every pressure conduit is generally termed as a pressure pipe. When a pressure pipe drops beneath a valley, stream, or some other depression, it is called a depressed pipe or an inverted siphon. Depending upon the construction material, the pressure pipes are of following types: Cast iron, steel, R.C.C, hume steel, vitrified clay, asbestos cement, wrought iron, copper, brass and lead, plastic, and glass reinforced plastic pipes. Hydraulic Design The design of water supply conduits depends on the resistance to flow, available pressure or head, and allowable velocities of flow. Generally, Hazen-William's formula for pressure conduits and Manning's formula for freeflow conduits are used. Hazen-William's formula 0.63 0.54 U=0.85 C r S H Manning's formula 1 2/3 1/2 U= / r S n H where, U= velocity, m/s; r = hydraulic radius,m; S= slope, C= Hazen-William's coefficient, and n H = Manning's coefficient. Darcy-Weisbach formula 2 h =(fLU )/(2gd) L The available raw waters must be treated and purified before they can be supplied to the public for their domestic, industrial or any other uses. The extent of treatment required to be given to the particular water depends upon the characteristics and quality of the available water, and also upon the quality requirements for the intended use.. The layout of conventional water treatment plant is as follows: 10 UNDER REVISION Depending upon the magnitude of treatment required, proper unit operations are selected and arranged in the proper sequential order for the purpose of modifying the quality of raw water to meet the desired standards. Indian Standards for drinking water are given in the table below. LECTURE-4 Water Distribution Systems The purpose of distribution system is to deliver water to consumer with appropriate quality, quantity and pressure. Distribution system is used to describe collectively the facilities used to supply water from its source to the point of usage. Requirements of Good Distribution System 1. Water quality should not get deteriorated in the distribution pipes. 2. It should be capable of supplying water at all the intended places with sufficient pressure head. 3. It should be capable of supplying the requisite amount of water during fire fighting. 4. The layout should be such that no consumer would be without water supply, during the repair of any section of the system. 5. All the distribution pipes should be preferably laid one metre away or above the sewer lines. 6. It should be fairly water-tight as to keep losses due to leakage to the minimum. Layouts of Distribution Network The distribution pipes are generally laid below the road pavements, and as such their layouts generally follow the layouts of roads. There are, in general, four different types of pipe networks; any one of which either singly or in combinations, can be used for a particular place. They are: Dead End System Grid Iron System Ring System Radial System Distribution Reservoirs 11 UNDER REVISION Distribution reservoirs, also called service reservoirs, are the storage reservoirs, which store the treated water for supplying water during emergencies (such as during fires, repairs, etc.) and also to help in absorbing the hourly fluctuations in the normal water demand. Functions of Distribution Reservoirs:  to absorb the hourly variations in demand.  to maintain constant pressure in the distribution mains.  water stored can be supplied during emergencies. Location and Height of Distribution Reservoirs:  should be located as close as possible to the center of demand.  water level in the reservoir must be at a sufficient elevation to permit gravity flow at an adequate pressure. Types of Reservoirs 1. Underground reservoirs. 2. Small ground level reservoirs. 3. Large ground level reservoirs. 4. Overhead tanks. Storage Capacity of Distribution Reservoirs The total storage capacity of a distribution reservoir is the summation of: 1. Balancing Storage: The quantity of water required to be stored in the reservoir for equalising or balancing fluctuating demand against constant supply is known as the balancing storage (or equalising or operating storage). The balance storage can be worked out by mass curve method. 2. Breakdown Storage: The breakdown storage or often called emergency storage is the storage preserved in order to tide over the emergencies posed by the failure of pumps, electricity, or any othe mechanism driving the pumps. A value of about 25% of the total storage capacity of reservoirs, or 1.5 to 2 times of the average hourly supply, may be considered as enough provision for accounting this storage. 3. Fire Storage: The third component of the total reservoir storage is the fire storage. This provision takes care of the requirements of water for extinguishing fires. A provision of 1 to 4 per person per day is sufficient to meet the requirement. The total reservoir storage can finally be worked out by adding all the three storages. Pipe Network Analysis Analysis of water distribution system includes determining quantities of flow and head losses in the various pipe lines, and resulting residual pressures. In any pipe network, the following two conditions must be satisfied: 12 UNDER REVISION 1. The algebraic sum of pressure drops around a closed loop must be zero, i.e. there can be no discontinuity in pressure. 2. The flow entering a junction must be equal to the flow leaving that junction; i.e. the law of continuity must be satisfied. Based on these two basic principles, the pipe networks are generally solved by the methods of successive approximation. The widely used method of pipe network analysis is the Hardy-Cross method. Hardy-Cross Method This method consists of assuming a distribution of flow in the network in such a way that the principle of continuity is satisfied at each junction. A correction to these assumed flows is then computed successively for each pipe loop in the network, until the correction is reduced to an acceptable magnitude. If Q is the assumed flow and Q is the actual flow in the pipe, then the correction d is given by a d=Q-Q ; or Q=Q +d a a Now, expressing the head loss (H ) as L x H =K.Q L we have, the head loss in a pipe x =K.(Q +d) a x x-1 =K.Q + x.Q d + .........negligible terms a a x x-1 =K.Q + x.Q d a a Now, around a closed loop, the summation of head losses must be zero. x x-1 SK.Q + x.Q d = 0 a a x x-1 or SK.Q = - SKx Q d a a Since, d is the same for all the pipes of the considered loop, it can be taken out of the summation. x x-1 SK.Q = - d. SKx Q a a x x-1 or d =-SK.Q / Sx.KQ a a Since d is given the same sign (direction) in all pipes of the loop, the denominator of the above equation is taken as the absolute sum of the individual items in the summation. Hence, x x-1 or d =-SK.Q / S l x.KQ l a a 13 UNDER REVISION or d =-SH / x.S lH /Q l L L a where H is the head loss for assumed flow Q . L a The numerator in the above equation is the algebraic sum of the head losses in the various pipes of the closed loop computed with assumed flow. Since the direction and magnitude of flow in these pipes is already assumed, their respective head losses with due regard to sign can be x-1 easily calculated after assuming their diameters. The absolute sum of respective KQ or H /Q a L a is then calculated. Finally the value of d is found out for each loop, and the assumed flows are corrected. Repeated adjustments are made until the desired accuracy is obtained. The value of x in Hardy- Cross method is assumed to be constant (i.e. 1.85 for Hazen-William's formula, and 2 for Darcy-Weisbach formula) LECTURE-5 Flow in Pipes of a Distribution Network by Hardy Cross Method Problem: Calculate the head losses and the corrected flows in the various pipes of a distribution network as shown in figure. The diameters and the lengths of the pipes used are given against each pipe. Compute corrected flows after one corrections. Solution: First of all, the magnitudes as well as the directions of the possible flows in each pipe are assumed keeping in consideration the law of continuity at each junction. The two closed loops, ABCD and CDEF are then analyzed by Hardy Cross method as per tables 1 & 2 respectively, and the corrected flows are computed. 14 UNDER REVISION Table 1 Consider loop ABCD 1.85 Pipe Assumed flow Dia of pipe Length of K Q H = lH /Q l a L L a 1.85 pipe (m) = L K.Q a in in d in 4.87 470 d 4.87 l/sec cumecs m d (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) - AB (+) +0.043 0.30 2.85 500 373 3 X10 +1.12 26 -3 3 43 X10 BC +0.023 0.20 300 1615 +1.52 66 (+) 3.95 9.4 -4 -4 23 X10 X10 CD -0.020 0.20 500 2690 -1.94 97 (-) 20 3.95 7.2 DA -0.035 0.20 300 1615 -3.23 92 -4 -4 X10 X10 (-) 35 - 3.95 2 X10 -4 3 X10 S -2.53 281 1.85 1.85 4.87 H = (Q L)/(0.094 x 100 X d ) L a 1.85 1.85 4.87 or K.Q = (Q L)/(470 X d ) a a 4.87 or K =(L)/(470 X d ) For loop ABCD, we have d =-SH / x.S lH /Q l L L a =(-) -2.53/(1.85 X 281) cumecs =(-) (-2.53 X 1000)/(1.85 X 281) l/s 15 UNDER REVISION =4.86 l/s =5 l/s (say) Hence, corrected flows after first correction are: Pipe AB BC CD DA Corrected flows after first + 48 + 28 - 15 - 30 correction in l/s Table 2 Consider loop DCFE 1.85 Pipe Assumed flow Dia of pipe Length K = L Q H = lH /Q l a L L a 4.87 1.85 of pipe 470 d K.Q a in in d in 4.87 d (m) l/sec cumecs m (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) - DC (+) 20 +0.020 0.20 3.95 500 2690 7.2 X10 +1.94 97 -4 4 X10 CF (+) 28 +0.028 0.15 300 6580 +8.80 314 - 9.7 X10 1.34 5 -3 X10 FE (-) 8 -0.008 0.15 500 10940 -1.47 184 - 9.7 X10 1.34 ED (-) 5 -0.005 0.15 300 6580 -0.37 74 5 -4 X10 - - 9.7 X10 5.6 X10 5 5 S +8.9 669 For loop ABCD, we have d =-SH / x.S lH /Q l L L a =(-) +8.9/(1.85 X 669) cumecs =(-) (+8.9 X 1000)/(1.85 X 669)) l/s = -7.2 l/s Hence, corrected flows after first correction are: Pipe DC CF FE ED Corrected flows after first + + - - 16 UNDER REVISION correction in l/s 12.8 20.8 15.2 12.2 LECTURE-6 Water Quality The raw or treated water is analysed by testing their physical, chemical and bacteriological characteristics: Physical Characteristics: Turbidity Colour Taste and Odour Temperature Chemical Characteristics: pH Acidity Alkalinity Hardness Chlorides Sulphates Iron Solids Nitrates Bacteriological Characteristics: Bacterial examination of water is very important, since it indicates the degree of pollution. Water polluted by sewage contain one or more species of disease producing pathogenic bacteria. Pathogenic organisms cause water borne diseases, and many non pathogenic bacteria such as E.Coli, a member of coliform group, also live in the intestinal tract of human beings. Coliform itself is not a harmful group but it has more resistance to adverse condition than any other group. So, if it is ensured to minimize the number of coliforms, the harmful species will be very less. So, coliform group serves as indicator of contamination of water with sewage and presence of pathogens. The methods to estimate the bacterial quality of water are: Standard Plate Count Test Most Probable Number Membrane Filter Technique Indian Standards for drinking water If no alternative source Parameter Desirable-Tolerable available, limit extended upto 17 UNDER REVISION Physical Turbidity (NTU unit) 10 25 Colour (Hazen scale) 10 50 Taste and Odour Un-objectionable Un-objectionable Chemical pH 7.0-8.5 6.5-9.2 Total Dissolved Solids mg/l 500-1500 3000 Total Hardness mg/l (as CaCO ) 200-300 600 3 Chlorides mg/l (as Cl) 200-250 1000 Sulphates mg/l (as SO ) 150-200 400 4 Fluorides mg/l (as F ) 0.6-1.2 1.5 Nitrates mg/l (as NO ) 45 45 3 Calcium mg/l (as Ca) 75 200 Iron mg/l (as Fe ) 0.1-0.3 1.0 18 UNDER REVISION LECTURE-7 Module II The typical functions of each unit operations are given in the following table: Functions of Water Treatment Units Unit treatment Function (removal) Aeration, chemicals Colour, Odour, Taste use Screening Floating matter Chemical methods Iron, Manganese, etc. Softening Hardness Sedimentation Suspended matter Coagulation Suspended matter, a part of colloidal matter and bacteria Filtration Remaining colloidal dissolved matter, bacteria Pathogenic bacteria, Organic matter and Reducing Disinfection substances The types of treatment required for different sources are given in the following table: Source Treatment required 1. Ground water and spring water fairly free from No treatment or Chlorination contamination 2. Ground water with chemicals, minerals and Aeration, coagulation (if gases necessary), filtration and disinfection 3. Lakes, surface water reservoirs with less Disinfection amount of pollution 4. Other surface waters such as rivers, canals Complete treatment and impounded reservoirs with a considerable amount of pollution Aeration  Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due to algae and related organisms.  Aeration also oxidise iron and manganese, increases dissolved oxygen content in water, removes CO2 and reduces corrosion and removes methane and other flammable gases. 19 UNDER REVISION  Principle of treatment underlines on the fact that volatile gases in water escape into atmosphere from the air-water interface and atmospheric oxygen takes their place in water, provided the water body can expose itself over a vast surface to the atmosphere. This process continues until an equilibrium is reached depending on the partial pressure of each specific gas in the atmosphere. Types of Aerators 1. Gravity aerators 2. Fountain aerators 3. Diffused aerators 4. Mechanical aerators. Gravity Aerators (Cascades): In gravity aerators, water is allowed to fall by gravity such that a large area of water is exposed to atmosphere, sometimes aided by turbulence. Fountain Aerators: These are also known as spray aerators with special nozzles to produce a fine spray. Each nozzle is 2.5 to 4 cm diameter discharging about 18 to 36 l/h. Nozzle spacing should be such that each m3 of water has aerator area of 0.03 to 0.09 m2 for one hour. Injection or Diffused Aerators: It consists of a tank with perforated pipes, tubes or diffuser plates, fixed at the bottom to release fine air bubbles from compressor unit. The tank depth is kept as 3 to 4 m and tank width is within 1.5 times its depth. If depth is more, the diffusers must be placed at 3 to 4 m depth below water surface. Time of aeration is 10 to 30 min and 0.2 to 0.4 litres of air is required for 1 litre of water. Mechanical Aerators: Mixing paddles as in flocculation are used. Paddles may be either submerged or at the surface. Settling Solid liquid separation process in which a suspension is separated into two phases –  Clarified supernatant leaving the top of the sedimentation tank (overflow).  Concentrated sludge leaving the bottom of the sedimentation tank (underflow). Purpose of Settling  To remove coarse dispersed phase.  To remove coagulated and flocculated impurities.  To remove precipitated impurities after chemical treatment.  To settle the sludge (biomass) after activated sludge process / tricking filters. Principle of Settling  Suspended solids present in water having specific gravity greater than that of water tend to settle down by gravity as soon as the turbulence is retarded by offering storage.  Basin in which the flow is retarded is called settling tank.  Theoretical average time for which the water is detained in the settling tank is called the detention period. 20 UNDER REVISION LECTURE-8 Types of Settling Type I: Discrete particle settling - Particles settle individually without interaction with neighboring particles. Type II: Flocculent Particles – Flocculation causes the particles to increase in mass and settle at a faster rate. Type III: Hindered or Zone settling –The mass of particles tends to settle as a unit with individual particles remaining in fixed positions with respect to each other. Type IV: Compression – The concentration of particles is so high that sedimentation can only occur through compaction of the structure. Type I Settling  Size, shape and specific gravity of the particles do not change with time.  Settling velocity remains constant. If a particle is suspended in water, it initially has two forces acting upon it: force of gravity: F =ρ gV g p p Buoyant force quantified by Archimedes as: F =ρgV b p If the density of the particle differs from that of the water, a net force is exerted and the particle is accelaratd in the direction of the force: F =(ρ -ρ)gV net p p This net force becomes the driving force. Once the motion has been initiated, a third force is created due to viscous friction. This force, 2 called the drag force, is quantified by: F =C A ρv /2 d D p C = drag coefficient. D A = projected area of the particle. p Because the drag force acts in the opposite direction to the driving force and increases as the square of the velocity, accelaration occurs at a decreasing rate until a steady velocity is reached at a point where the drag force equals the driving force: 2 (ρ - ρ )gV = C A ρv /2 p p D p For spherical particles, 3 2 V =∏d /6 and A = ∏ d /4 p p 2 Thus, v = 4g(ρ ρ )d p - 3C ρ D Expressions for C change with characteristics of different flow regimes. For laminar, transition, D and turbulent flow, the values of C are: D C = 24 (laminar) D R e C = 24 + 3 +0.34 (transition) D 1/2 R R e e 21 UNDER REVISION

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