Wastewater Treatment Exam Questions
and Revised Answers 100% Correct
What is compression settling? - Answer-Settling in a high concentration where the
particles touch each other and settling can occur only by compression of the
compacting mass. e.g. in the secondary clarifier.
What is the particle settling theory? - Answer-Drag force = gravitational force
What assumption is used to calculate retention time? - Answer-Ideal basin theory.
What is the vertical detention time? - Answer-tv = depth of tank (H)/settling velocity(v0)
What is the horizontal detention time? - Answer-th = volume(LWH)/flow rate (Q)
What are the objectives of a grit chamber? - Answer-1. protect moving mechanical
equipment
2. reduce formation of heavy deposits
3. reduce the frequency of digester cleaning caused by accumulation of grit.
What is an aerated grit chamber? - Answer-Air is blown in the sides of the chamber to
result in a helical liquid flow pattern.
What are the advantages of an aerated grit chamber? - Answer-- work better at variable
flow rates
- higher efficiency (less removal of organics)
- less head
What are the disadvantages of an aerated grit chamber? - Answer-- aeration can
release odours and VOCs
- large power requirements
What are the methods of grit disposal? - Answer-- transport to landfill
- incineration
- lime stabilisation
What type of settling is the primary clarifier? - Answer-Type 2 settling: settling of
flocculated particles in dilute suspension.
What is the selection of primary sedimentation tanks governed by? - Answer-- size of
installation
, - rules and regulations
- local conditions
- experience and judgement of engineer
How many primary sedimentation tanks should be used? - Answer-Two or more. For
maintenance reasons.
Where are rectangular primary sedimentation tanks found? - Answer-Large-scale
treatment plants.
What are the features of a rectangular primary sedimentation tank? - Answer-- high
tolerance to shock overload
- predictable performance
- cost effective due to low construction costs
- lower maintenance
- minimal short circuiting
What is the purpose of the inlet baffles in a primary sedimentation tank? - Answer-
Reduce high initial wastewater velocities and distribute flow over wider areas.
How is the scum moved to the effluent end of a primary sedimentation tank? - Answer-
Flights and then collected in the baffles.
What is the problem with circular sedimentation tanks? - Answer-- harder to construct
- short circuiting
- particle removal problems
What is the process for a standard WWTP? - Answer-1. Raw sewage
2. Screen
3. Grit chamber
4. Equalisation tank
5. Primary settling tank
6. Biological treatment
7. Secondary settling
8. Advanced treatment
What stages are in the primary treatment? - Answer-1. Screen
2. Grit Chamber
3. Equalisation tank
4. Primary settling tank
What stages are in the secondary treatment? - Answer-1. Biological treatment
2. Secondary settling
What are the stages in tertiary treatment? - Answer-1. Advanced treatment
What constituent does primary treatment aim to treat? - Answer-Suspended solids.
and Revised Answers 100% Correct
What is compression settling? - Answer-Settling in a high concentration where the
particles touch each other and settling can occur only by compression of the
compacting mass. e.g. in the secondary clarifier.
What is the particle settling theory? - Answer-Drag force = gravitational force
What assumption is used to calculate retention time? - Answer-Ideal basin theory.
What is the vertical detention time? - Answer-tv = depth of tank (H)/settling velocity(v0)
What is the horizontal detention time? - Answer-th = volume(LWH)/flow rate (Q)
What are the objectives of a grit chamber? - Answer-1. protect moving mechanical
equipment
2. reduce formation of heavy deposits
3. reduce the frequency of digester cleaning caused by accumulation of grit.
What is an aerated grit chamber? - Answer-Air is blown in the sides of the chamber to
result in a helical liquid flow pattern.
What are the advantages of an aerated grit chamber? - Answer-- work better at variable
flow rates
- higher efficiency (less removal of organics)
- less head
What are the disadvantages of an aerated grit chamber? - Answer-- aeration can
release odours and VOCs
- large power requirements
What are the methods of grit disposal? - Answer-- transport to landfill
- incineration
- lime stabilisation
What type of settling is the primary clarifier? - Answer-Type 2 settling: settling of
flocculated particles in dilute suspension.
What is the selection of primary sedimentation tanks governed by? - Answer-- size of
installation
, - rules and regulations
- local conditions
- experience and judgement of engineer
How many primary sedimentation tanks should be used? - Answer-Two or more. For
maintenance reasons.
Where are rectangular primary sedimentation tanks found? - Answer-Large-scale
treatment plants.
What are the features of a rectangular primary sedimentation tank? - Answer-- high
tolerance to shock overload
- predictable performance
- cost effective due to low construction costs
- lower maintenance
- minimal short circuiting
What is the purpose of the inlet baffles in a primary sedimentation tank? - Answer-
Reduce high initial wastewater velocities and distribute flow over wider areas.
How is the scum moved to the effluent end of a primary sedimentation tank? - Answer-
Flights and then collected in the baffles.
What is the problem with circular sedimentation tanks? - Answer-- harder to construct
- short circuiting
- particle removal problems
What is the process for a standard WWTP? - Answer-1. Raw sewage
2. Screen
3. Grit chamber
4. Equalisation tank
5. Primary settling tank
6. Biological treatment
7. Secondary settling
8. Advanced treatment
What stages are in the primary treatment? - Answer-1. Screen
2. Grit Chamber
3. Equalisation tank
4. Primary settling tank
What stages are in the secondary treatment? - Answer-1. Biological treatment
2. Secondary settling
What are the stages in tertiary treatment? - Answer-1. Advanced treatment
What constituent does primary treatment aim to treat? - Answer-Suspended solids.
