Settling and Floatation – Part 1. and Flotation Settling, . Gravity separation is a physical water and wastewater treatment processes in which suspended and floating solids are removed from water by the force of gravity.
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Gravity separation is a physical water and wastewater treatment processes in which suspended and floating solids are removed from water by the force of gravity.
Suspended and floating solids are either heavier than water or lighter than water and accordingly there are two gravity separation processes, sedimentation and flotation. Some fine solid particles with densities heavier than water get attached with air bubbles and are also removed by flotation.
Settling or sedimentation is a unit operation in which solids are drawn toward a source of attraction.
In gravitational settling, solids are drawn toward gravity; in centrifugal settling, solids are drawn toward the sides of cyclones as a result of the centrifugal field; and in electric-field settling, as in electrostatic precipitators, solids are drawn to charge plates.
Flotation is a unit operation in which solids are made to float to the surface on account of their adhering to minute bubbles of gases (air) that rises to the surface.
On account of the solids adhering to the rising bubbles, they are separated out from the water. This chapter discusses these three types of unit operations as applied to the physical treatment of water and wastewater.
Generally, two types of sedimentation basins (sometimes called also tanks, or clarifiers) are used:
Rectangular settling, basins or clarifiers, as they are also called, are basins that are rectangular in plans and cross sections. In plan, the length may vary from two to four times the width.
The length may also vary from ten to 20 times the depth. The depth of the basin may vary from 2 to 6 M. The influent is introduced at one end and allowed to flow through the length of the clarifier toward the other end.
Sedimentation or settling is a process in which water is collected in basins and given proper detention time during which suspended solids present in water is allowed to settle.
Sedimentation is a process with low cost and low energy requirements, however, proper basin design is very important for proper operation and better efficiency.
In specifying a water and wastewater sedimentation tank size, the major features to be considered are:
- tank cross sectional area,
- tank depth,
- and type of cleaning mechanism used.
In specifying a design basis for water and wastewater sedimentation tanks; three conditions are commonly considered:
- solid handling capacity (ton/day),
- overflow rate (gpm/ft2),
Additional design data required to ascertain mechanical construction, specific gravity of solids, size distribution of solids, underflow construction, operating temperature, and geographical location. Typical dimensions of sedimentation tanks are given in Table 1 bellow.
Depth, m 3‑5 3.5
Length, m 15‑90 25‑40
Width, m 3‑24 6‑10
Diameter, m 4‑60 12‑45
Depth, m 3‑5 4.5
Bottom Slope, mm/m 60‑160 80
Water and wastewater sedimentation tanks are mostly
rectangular in shape (See Figures bellow).
The ratio between width to length in rectangular sedimentation tanks is ranging between 1:4 to 1:6 (see Table in slide #5).
Suspended particles in water and wastewater have been categorized into three general classes:
1 ‑ Discrete particles الحبيبات المنفردة: Particles that will not readily flocculate, independent, settling rate is independent of concentration, and flow rate is critical (see Figure bellow‑class 1). Examples of discrete particles are sand, gravel washing, and silt.
2 ‑ Flocculent particles الحبيبات المتلبدة: Particles with relatively low concentration, possible aggloromation, and their settling is highly affected by detention time and flow rate(see Figure bellow‑class 2).
3 ‑ Hindered particles الحبيبات عالية التركيز أوالمضغوطة: Particles with high suspended concentrations (as in sludge thickening), their settling is affected by mixing and the duration of detention time (see Figure bellow‑class 3).
Types of sedimentation are dictated by
types of solids to be removed from water, therefore, for the three types of particles in aqueous suspensions , three separate mechanisms and theory of estimating settling velocities and removal rates
better understanding of the process let us first define the different settling properties of.
For discrete particles in aqueous suspensions and which have density greater than water, it will accelerate downward under the force of gravity until the resistance of the liquid equals the effective weight of the particle. According to its weight, shape, and specific weight or density, discrete particle is affected by the gravity, drag, and buoyancy force (see Figure bellow).
Settling velocity for a discrete particle is approximately constant and its magnitude depend on shape, size, and density of the article, and density and viscosity of the liquid. The force balance on the particle;
FG = FD + FB
νs= settling velocity
ρs = density of particle (kg/m3)
ρ = density of fluid (kg/m3)
g = gravitational constant (m/s2)
d = particle diameter (m)
μ = dynamic viscosity (Pa·s)
Force balance (zero acceleration)
We haven’t yet assumed a shape
Assume a _______
Laminar flow R < 1
Transitional flow 1 < R < 104
Fully turbulent flow R > 104
Increase d (stick particles together)
Increase g (centrifuge)
Increase density difference
(dissolved air flotation)
Decrease viscosity (increase temperature)
Q * C
Design Criteria for Sedimentation Tanks
No scour of settled particles
Slow moving particle collection system
Q/As must be small (to capture small particles)
This will be one of the ways you can improve the performance of your water filtration plant.
Example (3)For the water quality data given in the following table, determine the overall removal efficiency and the change in fractional removal ?
A settling basin is designed to have a surface overflowrate of 32.6 m/d. Determine the overall removal efficiency for a suspension with particle size distribution given bellow:
dp, mm 0.15 0.12 0.10 0.08 0.06 0.04 0.02 0.01
Fraction 5 10 35 60 80 90 95 100
Also given the characteristics of water and solid particles:
Density, kg/l 1.350 0.999
Kin. Viscosity ‑ 1.027 x 10‑3
Temperature, oC ‑ 20oC
Settling velocity of a solid particle is 0.0044 m/s in water at 15 oC. Compute the overflowrate in gpd/ft2. What is the minimum detention time in hours to settle out this flocc if the depth of the sedimentation tank is 15 feet ?
A rectangular sedimentation tank is to be designed for a flow of 1 mgd using a 6:1 length/width ratio, an overflowrate of 0.00077 fps, and a detention time of 3 hours. What are the dimensions of the basin ?
A 120 feet in diameter 15 feet deep cylindrical sedimentation tank has an influent flow of 10 mgd. Compute the overflowrate and detention time? Is the estimated overflowrate and detention time are within the acceptable range? With influent suspended solids concentration of 1500 mg/l. estimate the solid loading rate?
A sedimentation tank 25 m in diameter and 4.5 deep treating 15 mgd of surface water. With suspended solids of 1500 mg/l. Estimate the detention time, the overflow rate and the solid loading rate?