Sludge

Question 1

Why is sludge important?

Answer 1

Generation is inevitable and it needs to be removed, but can be complex.
Up to 98% water so difficult to transport.
Amount generated will continue to increase as methods for removal decrease.

Question 2

What needs to be considered in a sludge management process?

Answer 2

i) how much is generated
ii) what volume of water will come with the sludge generated
iii) what is the solids recovery rate from the dewatering process.

Question 3

When is sludge commonly produced?

Answer 3

In presedimentation, sedimentation and filtration

Question 4

What is the mass balance equation in words?

Answer 4

Accumulation rate = Input rate - Output rate

Question 5

What does thickening assist in?

Answer 5

i) performance of subsequent treatment
ii) getting rid of water quickly
iii) and equalising flows to subsequent treatment units and minimising unit load.

Question 6

How is thickening usually accomplished?

Answer 6

Gravity settling (gravity thickening) of flotation

Question 7

What does a gravity thickener do?

Answer 7

Slowly stirs sludge in deep circular tanks which induces water release via gravity

Question 8

What are the primary design elements of a gravity thickening tanks?

Answer 8

diameter, depth and required drive torque

Question 9

What are the three parts that the depth of a gravity thickening tank is split into during conceptual design?

Answer 9

i) the free board above the sludge liquid surface
ii) the settling zone where the particulate matter separates from the liquid
iii) and the thickening zone at the bottom of the tank

Question 10

Why sludge conditioning?

Answer 10

Changing sludge properties/structure resulting in easier dewatering. Increases particle size by bringing combining smaller particles into cohesive larger particles that carry less water.

Question 11

What are the two conditioning processes?

Answer 11

Chemical and physical conditioning

Question 12

Why sludge dewatering?

Answer 12

Significant reduction of volume for final disposal (transport, landfill, incineration etc)

Question 13

Briefly describe the mechanical dewatering process (filter press)?

Answer 13

i) sludge is sandwiched between two porous belts and passed over and under various rollers of different diameters
ii) as roller diameter decreases, pressure is exerted on the sludge removing water.

Question 14

What are the various methods of sludge disposal after treatment?

Answer 14

on-site storage, landfill, disposal to WwTW, soil amendment, incorporation into new products e.g. bricks, concrete. Soil conditioner, ocean dumping.