L6- Sludge Treatment

Question 1

Factors affecting sludge treatment

Answer 1

Source of sludge, characteristics, quantities

Question 2

Sludge composition

Answer 2

45-60% organics (VSS), 25-55% inerts (inorganic salts, SS)

Question 3

Sources of sludge overview

Answer 3

Primary sedimentation: large particles separated easily - might not require thickening

Secondary sedimentation: flocs with water trapped- thickening required to concentrate

Question 4

Sludge thickening purpose

Answer 4

Reduces volume
Makes cheaper and able to transport

Question 5

Volume-mass relationships - define variables

Answer 5

Ws/(Sspw) = Wf/(Sfpw) + Wv/(Sv*pw)

Ws/Wf/Wv = weight of solids, fixed solids, volatile solids
Ss/Sf/Sv = specific gravity of solids, fixed solids and volatile solids
pw = density of water

Applies then to Sludge = solids + water

Question 6

Volume-mass relationships - define variables

Answer 6

Ws/(Sspw) = Wf/(Sfpw) + Wv/(Sv*pw)

Ws/Wf/Wv = weight of solids, fixed solids, volatile solids
Ss/Sf/Sv = specific gravity of solids, fixed solids and volatile solids
pw = density of water

Applies then to Sludge = solids + water

Question 7

Volume of sludge - define variables

Answer 7

V = Ms / (pwSslPs)

V = volume of sludge (m3)
Ms = mass of dry solids (kg)
pw = density of water (kg/m3)
Ssl = specific gravity of sludge
Ps = percent solids (decimal)

Question 8

Handling and processing overview of sludge

Answer 8

Pumping
Preliminary operation
Thickening
Stabilisation
Conditioning
Dewatering
Advanced thermal oxidation/T&S/application to land

Question 9

Primary processing

Answer 9

Grinding (large-> small)
Screening
Degritting
Blending
Storage

Question 10

Thickening purpose & methods

Answer 10

Remove water so volume handled for transport and disposal

Methods: Co-settling/gravity, centrifugal, dissolved air flotation, gravity belt, rotary drum

Question 11

Sludge stabilisation & methods

Answer 11

Reduce pathogens, eliminate odours, inhibit potential putrefaction (decay of organics)

Methods:
Alkaline stabilistation
Anaerobic digestion
Aerboic digestion
Composting

Question 12

Alkaline stabilisation

Answer 12

Addition of alkaline (lime) to maintain high pH to affect disruption of pathogens

Exothermic reaction, but sludge mass increased

Question 13

Anaerobic digestion for sludge stabilisation, Co-digestion and role of pH

Answer 13

No oxygen supply- typically for high BOD

Co-digestion with other organics -> simultaneous digestion of organics to increase biogas production

Role of pH: alkalinity of sludge buffer change of pH

Question 14

Composting for sludge stabilisation

Answer 14

Mixed waste to provide pasteurised product

Question 15

Aerobic digestion for sludge stabilisation

Answer 15

Energy demanding due to oxygen supply needed.
A High BOD in sludge means a high quantity to be treated

Question 16

Aerobic digestion advantages & disadvantages

Answer 16

Adv:

Dis:

Question 17

Sludge Conditioning

Answer 17

Improve dewatering characteristics.
Condition with polymers - dosage and mixing important

Question 18

Sludge Dewatering

Answer 18

Separate water and solid matter in sludge
-> high solids (cake) in final product

Question 19

Sludge End uses

Answer 19

Incineration, fertiliser, landfill (T&S)

Question 20

Aerobic digestion chemical equation

Answer 20

2C5H7NO2 + 11.5O2 -> 10CO2 + N2 + 7H2O

Question 21

Digester tank volume -define variables

Answer 21

V = [Q0(X0+YS0)]/[X(kdPv + (1/tow c))]

V = digester tank volume (m3)
Q0 = influent average flowrate to digester (m3/d)
X0 = influent suspended solids (mg/L)
Y = fraction of influent BOD consisting of raw primary solids
S0 = influent BOD (mg/L)
X = digester suspended solids (mg/L)
Kd = reaction rate constant (1/d)
Pv = volatile fraction of digester suspended solids
tow c = solids retention time (d)