Sedimentation
Sedimentation: -
These are the units where settleable suspended solids are removed. Sedimentation of water can be plain or aided by coagulants. When the impurities are separated by natural process alone i.e. gravitational force, the sedimentation is called as plain sedimentation, which is usually used as a preliminary process to reduce heavy sediment load prior to subsequent treatment processes such as filtration or coagulation.
Factors Influencing :-
- Size, shape and weight of particles.
- Viscosity and temperature of water.
- Surface overflow rate, surface area.
- Velocity of flow
- Inlet and outlet arrangement.
- Detention period and effective depth of basin.
Settling velocity of particle can be calculated by Stokes Law (for laminar flow regime)
Vs = [g/18µ] [Pp-Pw] dp2
Where, Vs = settling velocity, m/sec.
g = gravitational acceleration. m/sec2
Pp = density of particles, kg/m3.
Pw = density of water, kg/m3
µ = dynamic viscosity of water, kg/m.s
dp = diameter of particle, m
Types of tanks:- Horizontal as well as vertical flow tanks are in use having the shape of circular, square or rectangular. The depth of the tanks vary between 2.5 to 4.0 m.
Detention time:-
For plain settling tanks , 3-4 hrs.
For coagulated water, 2-2.5 hrs.
For vertical flow settling tanks , 1.0-1.5 hrs.
Surface Loading: - For effective settling the overflow rate i.e discharge rate divided by plan area of the tank, shouldn’t exceed the hydraulic subsidence value of the particles required to be removed.
Surface Overflow Rate for Horizontal Flow circular settling tanks = 30-40 Cum/Sqm/day.
Surface Overflow Rate for Vertical Flow settling tanks = 40-50 Cum/Sqm/day.
Weir Loading: - It determines the strength of outlet currents. Normal weir loading is 300 Cum/day/meter length of weir. Well clarified waters can be obtained at the weir loading of even 1500 Cum/d/m. length of weir.
Sludge removal: - sludge can be removed under hydrostatic pressure or by pumping. Hopper slope = 1 : 10 to 1 : 12. The scrapping mechanism is rotated slowly to complete one revolution in about 30-40 min. The power requirement = 0.75 KW/ Sqm of tank area.
Sludge and wash water should be properly disposed without causing any pollution problems in the environment.
Settling tank efficiency: - The hydraulic efficiency of the settling tank is the flow through period which can be expected to approach the critical detention period only in ideal basins. The efficiency is dependent on eddy currents, concentration of particles, and also inlet and outlet arrangements. A well designed tank should be capable of having the volumetric efficiency of 70%.
Tube Settler: - Settling efficiency of basin is primarily depend on surface area and is independent of depth. Sedimentation basin should be constructed as shallow as possible to optimize the removal efficiency. Tube settlers have been developed as an alternate to sedimentation tank. Tube settlers are shallow settling devices consisting of number of small plastic tubes or shallow trays placed at an angle of 30 – 60 o. They are used to enhance the settling characteristics of sedimentation basins. The flow within the basin passes upward through the tubes and exits from the basin above the tubes. The solids that settle in tubes move by gravity in downward direction to the bottom of the basin.
