Sludge Treatment and Disposal

Question 1

Sludge

Answer 1

Thick, concentrated biologically unstable slurry containing the solids from sewage.

Question 2

Sludge Treatment Process

Answer 2

1. Thickening
2. Anaerobic Digestion
3. Dewatering
4. Incineration
5. Disposal

Question 3

Sludge Thickening

Answer 3

Holding Tanks - The top water can be drawn off as the sludge settles. Liquors may be trapped as the sludge settles so draw off valves at different levels.
Circular Tanks - With gentle stirring encourages entrained water to leave and consolidate - Produces sludge with 9% dry sludge

Question 4

Dissolved air Flotation Process

Answer 4

Gas released is harnessed in the dissolved air flotation process. A saturated solution of air is introduced with a polyelectrolyte forming fire bubbles.

Question 5

Anaerobic Digestion

Answer 5

Natural decomposition occurs in large closed tanks enhanced by heating the sludge to 35 degrees. Efficient mixing enables daily injection of sludge to ensure optimum breakdown conditions. Takes 16 days to become stable.
The sludge is extracted and cooled in a secondary digestion tank.

Question 6

Aerobic Digestion

Answer 6

Uses aerobic microorganisms to stabilise and partly disinfect the sludge. (Not used in the UK)

Question 7

Dewatering

Answer 7

• Drying beds
• Sludge Conditioning
• Filter Presses
• Centrifuges
• Belt Presses

Question 8

Incineration

Answer 8

Produces the smallest volume for disposal because it evaporates the water and oxidises the organic matter into carbon dioxide. Care must be taken with landfill disposal. >30% dry solid input is required.

Question 9

Sludge Disposal

Answer 9

• At sea - occurred until 1998
• Lagooning - temporary method to pump lagoons formed by excavation. Lagoons have concrete stable, valves to withdraw any separated water.
• Disposal to land - subsoil injection, allows fertilisation of land without visual and small nuisance. Raw sludge may be injected into grassland or sprayed onto arable land.