Sedimentation and Sedimentation Tanks

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SEDIMENTATION AND SEDIMENTATION TANKS Removal of suspended particles by gravitational settling Sedimentation is accomplished by decreasing the velocity of the water below which the particles will no longer remain in suspension. When the velocity no longer supports the transport of the particles, gravity will remove them from the flow. Sedimentation tanks are designed to reduce the velocity of flow of water to permit to permit suspended solids to settle out of water by gravity

FACTORS AFFECTING SEDIMENTATION Several factors affect the separation of settle able solids from water. Some of the more common types of factors to consider are: PARTICLE SIZE :-The size and type of particles to be removed have a significant effect on the operation of the sedimentation tank. The shape of the particle also affects its settling characteristics. A round particle, for example, will settle much more readily than a particle that has ragged or irregular edges

WATER TEMPERATURE Another factor to consider in the operation of a sedimentation basin is the temperature of the water being treated. When the temperature decreases, the rate of settling becomes slower. CURRENTS Several types of water currents may occur in the sedimentation basin: Density currents caused by the weight of the solids in the tank, the concentration of solids and temperature of the water in the tank. Eddy currents produced by the flow of the water coming into the tank and leaving the tank.

TYPES OF SEDIMENTATION PLAIN SEDIMENTATION - when impurities are separated from fluid by action of natural forces alone CLARIFICATION WHEN CHEMICALS (sedimentation with coagulation) when chemicals are added to hasten aggregation of finely suspended matter CHEMICAL PRECIPITATION when chemicals are added to throw dissolved impurities out of solution than it is called chemical precipitation

TYPE 1 OR DISCRETE SETTLING also known as FREE SETTLING as particles have little tendency to flocculate. It is sedimentation when particles have low concentration particles settles as individual entities Example of Type I settling is the settling of silt, grit etc., from river water before applying to the slow sand filters. TYPE 2 OR FLOCCULANT SETTLING refers to dilute suspensions which flocculate during sedimentation. This results in increase in mass and hence settling rate increase with depth Example of TYPE 2 SETTLING are settling in primary settlers


TYPE 3 OR HINDERED SETTLING refers to settling of suspension of intermediate concentration. Intermediate forces hold particles together and they settle as a unit. In hindered settling, the velocity gradients around each particle are affected by the presence of nearby particles. So the normal drag correlations do not apply Example of HINDERED SETTLING IS settling in secondary clarifiers TYPE 4 OR COMPRESSION SETLING refers to settling of suspension of high concentration. On contact particles form a structure and settling takes place by compression of structure due to weight of particles Settling of SLUDGE is example of compression settling

As seen above, not only does the settling take more time as the slurry becomes more concentrated but, critical sedimentation point occurs at progressively higher interface heights(Ref 4)

SETTLING OF DISCRETE PARTICLES When a discrete particle is placed in a fluid it will accelerate until FRICTIONAL RESISTANCE(FD) also know as DRAG FORCE on the fluid equals the DRIVING FORCE (FI) when (FD) = (FI) particles attain a uniform velocity known as TERMINAL VELOCITY and now settles down with this constant velocity

From Newtons second law mdV/dt = Fg Fb Fd where: m = mass V = velocity of particle Fg = weight of particle Fb = force of buoyancy Fd = drag force

DRIVING FORCE (FI) = Fg - Fb = (Ss- S)gV where S = density of fluid S s = particle density g = acceleration due to gravity Vp = volume of particle

FD depends upon :Dynamic viscosity Mass of fluid Shape and size of fluid DRAG FORCE is given by NEWTONs LAW for frictional drag as FD = CDASVS2/2 FD = DRAG FORCE CD = Drag coefficient A= Projected area of particle

Therefore equation 1 can be rewritten as mdV/dt = (Ss- S)gVp - CDASVS2/2 At steady state terminal velocity will be reached hence dV/dt = 0 Therefore (Ss- S)gVp = CDASVS2/2 Solving for VS we get VS = (4(Ss- S)gd/3CD S) where d = diameter of the spherical particle

CD depends upon Reynolds's Number For R between .5 to 104 CD= 24/R+3/(R)+.34 For high Re >103 to 104 CD = .4 For Re < .5 CD = 24/R

TYPE 2 SETTLING OR FLOCCULATIONParticles that change size, shape and perhaps specific gravity over time due to agglomeration while settling Hence the settling velocity will increase Strokes equation cannot be used because particles are constantly changing shape and size

A suggested equation for the variation of floc density with particle size is Ss S = kd-0.7 Where Ss = floc density S = density of water d = diameter of particle K = coefficient dependent on characteristic of water and chemicals involved The rate of flocculation is proportional to mean velocity gradient in the system , concentration of particle and the particle size


To develop this we have to perform a settling column test in which suspension is kept in long column and allowed to settle under quiescent conditions Samples are withdrawn from this cylinder, at several depths at several time interval and % removal is determined Numerical value of % removal is located at the co ordinates identifying the time and depth of the sample and iso concentration lines connecting points of equal removal are drawn.

TYPES OF SEDIMENTATION TANK Fill and Draw Type Continuous Flow Type Fill and Draw Type In this, sedimentation tank is first filled with incoming water and allowed to rest for a certain period of time and the particles get settled at the bottom Generally a detention period of 24 hours is allowed At the end clear water is taken out through the outlet Cleaning of tank may take another 6 to 12 hours Hence operation takes place in about 36 hours Minimum 3 such units are required for constant supply

Continuous Flow Type velocity of particles is such a parameter which can be easily controlled In this, particles continue to flow with a small velocity and the settable solids settle a the bottom before they reach the outlet They are of two types (a)Horizontal Flow (b)Vertical Flow

HORIZONTAL FLOW SEDIMENTATION TANK Horizontal flow tanks is based on following assumptions 1)Within the settling zone the particles settle in similar manner as they do in a QUIESCENT TANK of equal depth 2)Flow is horizontal and steady and velocity is uniform in all parts of settling zone 3)Concentration of suspended particles of each size is same at all points of vertical cross section at the inlet end 4)Particles are removed when it reaches the bottom of settling zone

The basin is divided into four zones INLET ZONE from where the water enters the tank SETTLING ZONE where the suspended particles get settled under gravity SLUDGE ZONE where the suspended particles get collected after settling OUTLET ZONE from where water and left over suspended particles assemble and get carried away outside the tank

HORIZONTAL DISCHARGE VELOCITY AND TIME OF FLOW Vd = Q/BH to = L/ Vd Where L = length B = breath H = height Q = discharge V d = horizontal discharge velocity to = time of horizontal flow

Consider a particle entering the tank to have a vertical falling speed of vs Time of falling through a distance H will be H/v s For particles to reach the bottom before the water leaves the tank the falling speed should be equal to the time of horizontal flow H/vs = LBH/Q vs = Q/LB Hence it defines the surface overflow rate as flow divided by plan area of the basin Particles with speed greater than Q/A will get settled

to = LBH/Q to is known as detention period If a particle with a falling speed less than vs