L3- Conventional WWTP & DWTP

Question 1

Pre-treatment purpose

Answer 1

Physical separation of large objects (tree, plastics)

i.e. bar scree, grit removal, flow equalisation tank

Question 2

Primary treatment purpose

Answer 2

Physical separation and chemical treatment to remove suspended solids

i.e. clarifier

Question 3

Secondary treatment purpose

Answer 3

Biological treatment to remove organics

i.e. activated sludge, SBR

Question 4

Tertiary treatment purpose

Answer 4

Provide further improvement to effluent quality before discharge (i.e sand filtration).
Physicochemical & biological treatment to remove toxic, pathogens, N, P

Question 5

Sludge treatment purpose

Answer 5

Physical and biological treatment to upgrade sludge.
Includes digestions (anaerobic treatment, composting), thickening and disposal

Question 6

Activated Sludge (secondary treatment) stages

Answer 6

1st- aerobic biological for BOD removal
2nd- sedimentation tank for solids removal

Question 7

Describe m/o concentration graph for activated sludge

Answer 7

Lag phase: induction phase of m/o (VSS) in the environment
Exponential phase: growth, abundance of substrate for m/o
Stationary phase: steady state conditions
Death phase: no subsstrate left, some m/o survive

Question 8

Biomass yield / oxygen requirements equation

Answer 8

m/o molecular mass/glucose molecular mass

O2 requirements: O2 + COD (for new m/o) + COD 9non-degradble substrate)

Question 9

Disinfection

Answer 9

Inactivation of pathogens for safe disposal (UV, chloronation, ozonation)

Question 10

Activated sludge mass balance

Answer 10

General mass balance: Q=QE+QW

In + produced/consumed = out
Solids: QX0 + [X] = QEXE + QWXu
Organics/BOD: QS0 - [S] = QES + QWS

Question 11

What happens to organics and solids concentrations in an activated sludge process?

Answer 11

Activated sludge converts dissolved organics to suspended solids.
Thus, organics decrease and solids increase

Question 12

Wastewater characteristics

Answer 12

High organic load (COD, TOC), high BOD, non-biodegradable, toxic for MO

Question 13

Advantages/disadvantages of WWTP

Answer 13

Adv: cost effective; well established; high efficiency treatment; minimises environmental impacts

Dis: large amounts of sludge produced; incapable to remove non-biodegradable contaminants

Question 14

DWTP Overall Process

Answer 14

Separation: Coagulation-flocculation, sedimentation, (flotation), sand filtration

Disinfection: chlorination

Chemical stabilization: addition of chemicals

Sludge treatment: thickening

Question 15

Coagulation

Answer 15

causes colloidal particles in water to destabilise to be readily separated.
Coagulants added, i.e. hydrated lime

Question 16

Flocculation

Answer 16

To collide individual destabilised colloidal particles to form aggregates/flocs to be removed.
Continuous stirring used.

Question 17

Sedimentation

Answer 17

Aggregates/flocs removed by separation.
Process where aggreagates allow to settle.
Flocs collected at bottom as sludge and removed regularly

Question 18

Flotation

Answer 18

If light flocs not settled, small air bubbles formed which attach to flocs and removed as froth

Question 19

Sand Filtration

Answer 19

Removal of turbidity to low levels:
particles retained in the sand and clean water flows out the bed

Question 20

Disinfection

Answer 20

removal of pathogens from clarified water.
Chlorination usually used, Cl2 gas

Question 21

Chemical stabilisation

Answer 21

Prevent corrosion of pipeline/formation of chemical scale in distribution.
Addition of chemicals

Question 22

Sludge treatment

Answer 22

prevent contamination of ecosystem. Thickened to remove excess water