In this article we will discuss about:- 1. Characteristics of Settleable Solids 2. Types of Settling 3. Analysis of Settling of Discrete Particles-Type 1 Settling 4. Performance of Settling Tanks.
Characteristics of Settleable Solids:
The settleable solids to be removed from sewage in primary and secondary settling tanks after grit removal are mainly organic and flocculent in nature, either dispersed or flocculated. The specific gravity of organic suspended solids may vary from 1.01 to 1.20. The bulk of the finely divided organic solids reaching primary sedimentation tanks are low specific gravity solids incompletely flocculated but are susceptible to flocculation.
Flocculation occurs within primary settling tanks due to eddying motion of the fluid and aggregation of dispersed flocculent solids becomes more complete as the sewage is detained for longer periods (hydraulic residence time) in these tanks. Since the particles are subject to flocculation, such settling tanks cannot be designed on the basis of surface overflow rate alone but will have to take into consideration hydraulic residence time or detention period also. However, rate of flocculation rapidly decreases as detention period is increased beyond certain values.
Types of Settling:
Depending on the concentration of solids and the tendency of particles to interact the following four types of settling may occur:
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1. Type 1 – Discrete settling
2. Type 2 – Flocculent settling
3. Type 3 – Hindered or zone settling
4. Type 4 – Compression settling
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1. Type 1 – Discrete Settling:
In discrete settling, particles settle as individual entities, and there is no significant interaction with neighbouring particles. Discrete particles have little tendency to flocculate or coalesce upon contact with each other and hence they do not change their size, shape or mass during settling. Discrete settling refers to the sedimentation of particles in a suspension of low solids concentration. Grit in sewage behave like discrete particles and hence their settling in grit chambers corresponds to discrete settling.
2. Type 2 – Flocculent Settling:
In flocculent settling, particles flocculate or coalesce during settling. By flocculation or coalescing, the particles increase in mass and thus settle at a faster rate. Flocculent settling refers to the sedimentation of particles in a rather dilute suspension with concentration of solids usually less than 1000 mg/l.
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The degree of flocculation depends on the contact opportunities which in turn are affected by the surface overflow rate, the depth of the basin, the concentration of the particles, the range of particle sizes and the velocity gradient in the system. The removal of organic suspended solids from raw or untreated sewage in primary settling tanks, settling of chemical floes in settling tanks and of bioflocs in the upper portion of secondary settling tanks are the examples of flocculent settling.
3. Type 3 – Hindered or Zone Settling:
When concentration of flocculent particles in in intermediate range, they are close enough together so that inter-particle forces are sufficient to hinder the settling of neighbouring particles resulting in hindered settling. The particles maintain their relative positions with respect to each other and the whole mass of particles settles as a unit or zone.
This type of settling is applicable to concentrated suspensions such as are found in secondary settling tanks used in conjunction with biological treatment units such as trickling filters and activated sludge units. In the hindered settling zone, the concentration of particles increases from top to bottom leading to thickening of sludge.
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Such secondary clarifiers where zone settling occurs are designed on the basis of solids loading or solid flux and checked for surface overflow rate, both of which can be determined by conducting settling column analysis.
4. Type 4 – Compression Settling:
This refers to settling in which the concentration of particles is so high that particles are in physical contact with each other resulting in the formation of a structure with lower layers supporting the weight of upper layers. Consequently further settling occurs due to compression of the whole structure of particles and accompanied by squeezing out of water from the pores between the solid particles.
Compression takes place from the weight of particles which are constantly being added to the structure by sedimentation from the supernatant liquid. Compression settling usually occurs in the lower layers of a deep sludge mass, such as in the bottom of secondary settling tanks following biological treatment by trickling filters and activated sludge process, and in tanks used for thickening of sludge.
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Fig. 12.1 shows the regions of the four types of settling that occur when a concentrated suspension, initially of uniform concentration throughout, is placed in a graduated cylinder. During a sedimentation operation, it is common to have more than one type of settling occurring at a given time, and it is possible to have all four occurring simultaneously. Primary sedimentation and chemical-aided sedimentation involve only Type 1 and Type 2 settlings, and the analysis of each of these two types of settlings are discussed separately.
Analysis of Settling of Discrete Particles-Type 1 Settling:
The settling of discrete, non-flocculating particles can be analyzed by considering the forces causing the settling of particles and those opposing their settling. The settling of a particle is caused by the effective weight of the particle acting in the vertical downward direction and it is opposed by the drag acting in the vertical upward direction.
The effective weight of the particle in this case is its submerged weight which is equal to the actual weight of the particle minus the force of buoyancy. Thus if particle is assumed to be of spherical shape, then the effective weight Ws of the particle is-
Hazen’s equation is applicable for particles of diameter d > 0.1 to 1 mm and Reynolds’ number in the range Re > 1 to 103. In this range the nature of settling of particles is neither laminar nor turbulent and hence it is termed as transitional settling of particles.
Hazen further indicated that for particles having diameter less than or equal to 0.1 mm and Reynolds’ number less than or equal to 1, Stokes’ law is applicable, according to which the value of the drag FD acting on a small spherical particle settling in a viscous fluid neglecting the inertial force is given by-
The kinematic viscosity of water varies with the temperature of water as indicated in Table 12.1.
Since kinematic viscosity depends on temperature, equation 12.4 is also expressed in an alternative form by introducing temperature T in place of kinematic viscosity ʋ as indicated below:
Equation 12.6 is known as Newton’s equation which is applicable for particles of diameter greater than 1 mm and Reynolds’ number in the Range Re > 103 to 104. Thus in the case of turbulent settling for computing the settling velocity of a particle Newton’s equation may be used.
The various equations for settling velocity of discrete particles are summarized in Table 12.2.
The above noted equations, however, give only theoretical value of settling velocity of particles, because the actual rate of settling of particles in sedimentation tanks is affected by various factors such as non-sphericity of the particles, upward displacement of the fluid caused by the settling of particles, convection currents, etc.
As such the actual settling velocity of particles is generally calculated by Hazen’s modified equation as given below:
Relation between Settling Velocity of a Particle and Surface Overflow Rate (or Overflow Rate or Surface Loading):
Consider a rectangular settling tank of length L, width B, and depth H (see Fig. 12.2). It is assumed that the particles are uniformly distributed as the sewage enters the tank at a uniform velocity V.
If Q is the discharge of sewage entering the tank, the velocity V is given by-
Consideration of the assumed criterion for the settling of the particles indicates that all the particles with settling velocity Vs equal to or greater than (Q/BL) will settle down and will be removed.
Now if a smaller particle having settling velocity Vs’ < (Q/BL) enters the tank at point A then it will not settle down in the tank. However, if this smaller particle enters the tank at some other level h as shown in Fig. 12.2, then from geometric consideration-
The ratio (x/x0) therefore represents the removal efficiency of a settling tank for the particles of the same size.
The quantity (Q/BL) which is the discharge per unit of plan area of a settling tank, is known as Surface Overflow Rate (S.O.R), or overflow rate, or surface loading rate. The surface overflow rate represents the hydraulic loading per unit surface area of tank in unit time expressed as m3/d/m2.
For a given discharge Q entering a settling tank, increasing the plan area (B x L) of the tank will reduce the surface overflow rate and hence even those particles which are having lower values of their settling velocities will also settle down and will be removed.
As such an increase in the plan area (i.e., length x width) of a settling tank will increase the settling and removal efficiency of the tank. The surface overflow rate is therefore a significant parameter for the design of a continuous flow type settling tank. In the design of settling tanks surface overflow rates must be checked both at average flow and peak flow.
Equations 12.9 and 12.10 also indicate that for discrete particles and unhindered settling, the settling of particles in a settling tank is solely a function of surface overflow rate and is independent of the depth of the tank. This is, however, not correct in actual practice, because the settling of particles in a settling tank depends on a number of factors and although the surface overflow rate has an important bearing in the design of settling tanks, the depth of the tank is also important both for maintaining the velocity of flow as well as for providing suitable sludge storage space in the tank.
Removal Efficiency of Discrete Suspension:
The removal efficiency of a unisize discrete suspension in a settling tank is given by the ratio of settling velocity of the particles, Vs, and the surface overflow rate as indicated by equation 12.9. Further from equation 12.9 it is evident that the surface overflow rate represents the settling velocity of the slowest settling particles which are 100% removed.
Sewage, however, contains discrete particles of different sizes (or diameters) which will have different settling velocities. Those particles which will have settling velocities equal to or greater than surface overflow rate will be entirely removed, while those which will have settling velocities less than surface overflow rate will be removed in direct proportion of the ratio of their settling velocity Vs’ to the surface overflow rate as indicated by equation 12.11.
Thus if xs is the fraction of particles with a settling velocity Vs’ < the surface overflow rate, then the fraction of particles with settling velocity Vs’ ≥ the surface overflow rate will be equal to (1 – xs) which will be entirely removed. Further the fraction of particles with settling velocity Vs’ < the surface overflow rate, which will be removed is obtained by equation 12.11 as-
For determining the second term in equation 12.12 a curve indicating cumulative distribution of particle settling velocity is drawn by plotting fraction of particles with less than stated settling velocity against the settling velocity as shown in Fig. 12.3.
The integration of the shaded portion of curve then gives the value of the second term. The actual calculations are usually performed by graphical integration by taking finite number of points on the distribution curve, in which case equation 12.12 is approximated as follows-
Analysis of Flocculent Settling – Type 2 Settling:
Particles in relatively dilute solutions sometimes will not act as discrete particles but will coalesce or flocculate during settling. As coalescence or flocculation occurs, the mass of particle increases and it settles faster.
The extent to which flocculation occurs is dependent on the opportunity for contact, which varies with the overflow rate, the depth of the tank or basin, the velocity gradients in the system, the concentration of particles, and the range of particle sizes. To determine the removal efficiency of a flocculent suspension, no adequate mathematical equation exists and settling column analyses are to be performed.
Settling analyses of flocculent suspensions are performed in a column made of plastic tube at least 300 mm in diameter and having height equal to the depth of the proposed tank. The column has ports usually at 0.6 m interval for withdrawal of samples. The flocculent suspension for which the settling characteristics are to be determined is introduced into the column in such a way that a uniform distribution of particle sizes occurs from top to bottom.
The settling is allowed to occur under quiescent conditions and at constant temperature to eliminate convection currents. Samples are withdrawn at various selected time intervals from the ports at different depths and analysed to determine the suspended solids concentrations. The percentage removals of suspended solids are computed at different times and depths and the percentage removal is plotted as a number against time and depth. The iso-percent-removal curves are drawn in a similar manner as contours are drawn from spot levels.
Where R1, R2, R3, R4 and R5 are percent removals and R, is the percent removal at time t and at 100% depth.
The curves can also be used to determine the detention period, depth and surface overflow rate required to obtain a given percentage removal of flocculent particles.
Performance of Settling Tanks:
Primary sedimentation of domestic sewage may be expected to accomplish 45 to 60% removal of Suspended Solids (SS) and 30 to 45% removal of biochemical oxygen demand (BOD), depending on concentration and characteristics of solids in suspension.
Secondary settling tanks, if considered independently, remove a very high percentage of flocculated solids, even more than 99%, particularly following an activated sludge unit where a high mixed liquor suspended solids (MLSS) concentration is maintained in the aeration chamber.
However, the efficiency of the biological treatment process is always defined in terms of the combined efficiency of the biological treatment units and its secondary settling tank with reference to the characteristics of the incoming sewage.