Size Determination for Sedimentation Basin

Significant aspects in the determination of size of sedimentation basin are the following:

• Surface loading (m³/m²/h)
• Detention time (h)
• Weir loading rate (m³/m/h)

Sedimentation rate of SS depends at the size of sedimentation particles.

        Figure 6: Zones of Sedimentation; (a) Horizontal Flow Clarifier; and (b) Up Flow Clarifier

Inlet Zone

It evenly distributes the flow and suspended solids across the cross-section of the settling zones. It covers approximately 25% of the tank length.

Settling Zone

Where the actual settling of particles takes place.

Sludge Collection Zone of Sludge

Configuration and depth of the sludge zone depends on the method of cleaning and quantity of sludge deposited. Well flocculated solids, 75% settle in the 1/5th of the tank length.

Outlet Zone

Removal of settled water without carrying away any of the flocs. Should be designed to avoid scouring by having either weirs or trough.

Determination of Settling Velocity

Figure 7 shows settling paths of discrete particles in a idealised horizontal flow tank.

Figure 7 : Settling Paths of Discrete Particles in a Horizontal Flow Tank (Idealised)

In Figure 7, v_o is the velocity of the particle falling through the full depth h0 of the settling zone in the detention time t₀. In Figure 7 above v_d is the displacement velocity and v_s is the settling velocity.

Therefore, v₀ = h₀/t₀

t₀ = V/Q = (l×w×h₀)/Q

where,            V = Volume of the settling tank, and

w = Width of the channel.

Therefore, surface loading or overflow velocity can be further written as

v₀  = Q/l × w = Q/A_s

where,            A_s = Surface area.

Therefore, removal of particles from the tank is independent of the depth of the tank. All particles with vs ≥ v0 are removed by way of settling in the tank. Thus, the horizontal velocity component or displacement velocity vd should not be very high else the particles will not settle and get washed away.

For design calculations,

v_o = 0.8 v_s

Particles of vs < v0 can be removed from horizontal flow basins if they are within vertical striking distance of v_s/v_d    × l₀ from the sludge zone.

Settling of dilute suspensions which have little or no tendency to flocculate; Type I Sedimentation (Discrete Particle Settling) is assumed. Considering spherical particles, the terminal velocity is provided as

v_c  = (g (ρs - ρL )/ 18 μ) d ²

where,            v_c = Terminal velocity,

ρs = Density of the particle,

ρL = Density of the liquid,

g = Acceleration due to gravity,

d = Diameter of particle, and

μ = Dynamic viscosity.

Within the design of sedimentation basins, select a particle wi

th terminal velocity v_c and design the basin so that all particles that have settling velocity equal to or greater than v_c will be removed.