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1. EN8592 – WASTE WATER
ENGINEERING
UNIT – 5 – SLUDGE TREATMENT AND
DISPOSAL
SESSION – 2 – SLUDGE THICKENING

2. • Wastewater treatment plants commonly use thickening devices to
increase the solids concentration at the end of a particular process step
within the activated sludge process.
• Thickening of sludge increases its solids content and reduces the volume
of free water thereby minimizing the unit load on downstream processes
such as digestion and dewatering.

3. The most commonly used thickening processes include
• Gravity thickening,
• Dissolved air flotation, and
• Rotary drum thickening.
• Centrifuge thickening is also becoming more common. The type of
thickening selected is usually determined by the size of a wastewater
plant, its physical constraints and the downstream process.

4. 1. Gravity Thickening:
• This process involves the concentration of thin sludges to more dense sludge
in special circular tanks designed for this purpose.
• Its use is largely restricted to the watery excess sludge from the activated
sludge process.
• It may also be used to concentrate sludge to primary tanks or a mixture of
primary and excess activated sludge prior to high rate digestion.

5. GRAVITY THICKENER DESIGN
• A gravity thickener is similar to a conventional sedimentation tank in
design, but has a more steep floor slope.
• Tanks range from 6 – 20 m dia & side water depth (SWD) may vary from 3
– 4 m.
• Floor slope is varies from 1:4 to 1:6 depending upon the type of sludge,
time required for thickening & storage volume to absorb peak solids load.
The steeper slope reduces the raking problems by providing more gravity.

6. Gravity Thickening

7. 2. Dissolved Air Floatation
• The objective of flotation-thickening is to attach a minute air bubble to
suspended solids and cause the solids to separate from the water in an
upward direction.
• This is due to the fact that the solid particles have a specific gravity lower
than water when the bubble is attached.

8. • Flotation thickening is most applicable to activated sludges but higher
float concentrations can be achieved by combining primary with activated
sludge.
• Equal or greater concentrations may be achieved by combining sludges in
gravity thickening units

10. 3. Centrifugation
• Centrifugation has been demonstrated to be capable of thickening a variety of
wastewater sludges.
• Centrifuges are a compact, simple, flexible, self-contained unit.
• They have the disadvantages of high capitals, maintenance and power costs
and often a poor, solids-capture efficiency if chemicals are not used for bio
sludges.

11. • Centrifugal thickening is acceleration of sedimentation through the use of
centrifugal force.
• Centrifuges are commonly used for thickening WAS (Waste Activated Sludge).
• Primary sludge is normally not fed to centrifuge as it may contain abrasive
material.
• In addition of being effective in thickening, they have additional advantage of
less space requirement, less odour potential & housekeeping requirement.

12. Centrifugation

13. 4. Rotary Drum Thickener
The rotary drum thickener consists of the following components:
• stainless steel structural frame,
• sludge distribution system,
• filtering drum,
• filtering drum washing system,
• drum drive system,
• full set of covers,
• electric control panel

14. • Water flows out through surface of the drum and sludge is transported by the
spiral screw along the drum and finally out from the end of the drum.
• Sludge is led inside and through the drum over the flow edge.
• The spiral screw transports the sludge along the drum and finally out from the
drum.
• The spiral screw mixes sludge slowly inside the drum so that water can come
out from sludge very effectively.

15. Rotary Drum Thickener