The sludge which is found in the bottom of a sedimentation tank in water treatment plant is primarily composed of water. The solids in the sludge are mainly flocs, excess coagulant, such as alum. Alum sludge has a solids concentration of only about 1 % when automatically removed from the basin, or about 2% if manually removed. Many options exist for disposal of sedimentation sludge such as disposal in streams, lagoons and landfills.

In the past, sludge and backwash water was typically released into streams and other bodies of water. However, this practice is becoming much less common and is now well regulated.

Most of the other alternatives require transporting sludge away from the treatment plant. Sludge is typically dried before it is trucked away since the greater volume of wet sludge makes it much more expensive to transport. This drying process is known as dewatering or thickening. Alum sludge is difficult to thicken, but a variety of devices have been developed to thicken the sludge. Treating the sludge to aid in thickening is known as conditioning the sludge,_Once the sludge has been conditioned, it may be dewatered in a lagoon, drying bed, or one of several other devices. Other processes used to dewater sludge include filter presses, belt filter presses, centrifuges, and vacuum filters. These processes result in sludge with a solids content ranging from 30 to 50 %.

DESIGN CRITERIA

• Surface Loading Rate

The terminal velocity has great significance in the design of settling tanks. It is called design overflow rate or surface loading rate or hydraulic loading (m3 / m2/d). It shall be used for designing the surface area of the sedimentation basin.

• Detention Time

When the surface loading rate is selected for an acceptable settling water quality, the required side water depth is often considered based on detention time, which is another important design parameter. The performance in the actual settling basins is affected by the dead spaces in the basins, eddy currents, wind currents and thermal currents. In the ideal settling basin all of the fluid elements pass through the basin at equal time to the theoretical detention time, t, which is equal to V/Q. Actual basins, however, have most of the fluids passing at a time shorter than the theoretical detention time. Dead spaces and eddy currents have rotational flow and do very little sedimentation since the inflow and outflow from these spaces is very small. As a result, the net volume available for settling is reduced and the mean flow-through time for the fluid element is decreased. Wind and thermal. currents also create flows that pass directly from the inlet to the outlet of the basin, which decreases the mean flow-through time.

If there are dead spaces, the following relationship occurs:
Mean t / Theoretical t < 1

If there are no dead spaces, the relationship is:
Mean t / Theoretical t = 1

If short circuiting is occurring, the time relationship is:
Median t / Mean t < 1

If there is no short circuiting:
Mean t = Median t

The most common use of type I or discrete settling is plain sedimentation basins, which are generally adopted prior to slow sand filter to remove sand, gravel and other discrete particles from raw water sources such as river supply and to reduce the turbidity level to less than 40 NTU. A surface loading rate of 15 to 30 m3/ m2/ d and water depth of 3 to 5 m are used for the design of plain sedimentation basins.

• Weir Loading Rate

Weir loading, also known as weir overflow rate, is the number of cubic metres of water passing over a metre length of weir per day. The standard weir overflow rate is 170 to 600 m3/ day/metre. Longer weirs allow more water to flow out of the sedimentation basin without exceeding the recommended water velocity;

Design criteria for Rectangular Sedimentation Tank

Detention time                                = 2 to 3 hours

Dept of the tank                              = 2.5 to 5.0 m

Velocity of flow through basin= 30 cm/min

Width of the tank                          = 12m

Maximum length of tank           = 30m

Length to breath ratio = 3 to5.1

Surface loading rate:
Granular solids                             = 15 to 30 m3 / day / m2

Floor slope towards sludge hopper = 1% to 4%

Sludge hopper slope                 = 1.2 to 2V : 1H

Scrapper velocity                      = 0.5 to 1.0 cm/ s

Outlet weir loading:
Normal                                           = 300m3 / day / m-length of weir

Velocity of flow in the launder= 0.2     to 0.3 m /s

Head loss                                      = 1.7 times velocity head

Design criteria for Circular Sedimentation Tank

Detention time                           = 1.5 to 3 hours

Dept of the tank                     = 2.5 to 5.0m

Velocity of flow                     <= 30 cm/min

Maximum diameter of tank = 60m (preferable max:30m)

Surface loading rate:
Granular solids                      = 15 to 30m3 / day / m2

Amorphous flow                   = 30 to 40m3 / day / m2

Flocculent materials           = 40 to50m3 / day / m2

Floor slope= 8%

Sludge hopper                        = 1.2 to 2V:1H

Scrapper velocity                 = 0.5 to 1.0 cm/s

Outlet weir loading:
Preferable maximum          = 600m3/day/m-length 0f weir

Normal                                     = 300m3/day/m-length of weir

Velocity of flow                     = 0.2 to 0.3 m/s

Head loss                                 = 1.7 times velocity head