Wk1 - Waste Water Treatment Works

Question 1

What are the 5 basic stages of treatment in order?

Answer 1

• Pre-treatment
• Primary treatment
• Secondary treatment
• Sludge treatment
• Tertiary treatment

Question 2

Briefly describe ‘pre-treatment’ in the waste water treatment method:

Answer 2

• Screens
• Grit settling
• Flow measurement
• Storm overflow weir

Question 3

Briefly describe ‘Primary treatment’ in the waste water treatment method:

Answer 3

• Primary sedimentation
• Storm or balancing tank

Question 4

Briefly describe ‘Secondary treatment’ in the waste water treatment method:

Answer 4

• Biological oxidation

Question 5

Briefly describe ‘Sludge treatment’ in the waste water treatment method:

Answer 5

• Dewatering
• Disposal

Question 6

Briefly describe ‘Tertiary treatment’ in the waste water treatment method:

Answer 6

• Micro strainers
• Sand filters

Question 7

Describe in detail the ‘pre-treatment’ phase of traditional waste water management:

Answer 7

Water arrives from the sewer and undergoes ‘pre-treatment’. This involves ‘screening’, the removal of larger debris and ‘grit removal’, the removal of grit which is easily settled out due to its high specific gravity (high compared to organic particles).

Question 8

What three methods can be used to achieve the settling of grit?

Answer 8

1) Settling tanks

2) Constant velocity channels (velocity is constant irrespective of flow)

3) Aerated grit channels (non-grit particles are stopped from settling by an upwards flow of air bubbles)

Question 9

What happens to grit when it has settled?

Answer 9

It is collected and washed, to remove any organic particles, then it may be tipped or re-used, provided it is inert.

Question 10

Under what conditions does excess flow get diverted to the storm tank?

How is this flow diversion controlled?

Answer 10

When the flows exceed 3x the dry weather flow (DWF)

Flow diversion is controlled by a combination of side weirs & pen-stocks

Question 11

What happens to the flow of sewage water coming from the grit separators when it exceeds 3x DWF?

Where does this excess water usually end up? What if 6x - 10x DWF conditions are present?

Answer 11

Excess flow is diverted to the storm tanks.

The excess flow is stored in the storm tanks until capacity is available in the WWTW.

During bigger storms, 6-10x DWF, the excess flow is spilled via CSOs to water courses.

Question 12

What does WWTW stand for?

Answer 12

Waste water treatment works

Question 13

What does CSOs stand for?

Answer 13

Combined sewer overflows

Question 14

Following screening and grit separation, assuming no overflow requirements, where does the sewage water end up?

Answer 14

In the primary sedimentation tanks

Question 15

What are the three main types of sedimentation tanks?
(C-R)
(R-S)
(U-P)

Answer 15

• Circular tanks (radial flow)
• Rectangular tanks (scraper blades)
• Upwards flow tanks (pressure driven)

Question 16

What is the general principle of a sedimentation tank?

Answer 16

Sewage water is detained in the tank for 6-8 hours at DWF, or 2-2.5 hours at max flow.

During this time, settlement occurs, separating the solids into a ‘sludge’ which is processed separately from the remaining liquid waste.

Question 17

What phase does the liquid sewage go through after passing through the primary sedimentation tank?

Answer 17

Secondary treatment

Question 18

Describe secondary treatment in detail.

Answer 18

After the sludge has been removed in the sedimentation tanks, the ‘liquor’ (remaining liquid component of the sewage) undergoes biological oxidation.

Question 19

What is the basic principle of secondary treatment?

Answer 19

To encourage rapid growth of aerobic bacteria in order to speed up biological oxidation

Question 20

What are the 2 ways of achieving biological oxidation?

Answer 20

• Biological filters (trickling ‘liquor’ through a bed of fragmented material)
• Activated sludge process

Question 21

What happens to the waste water after it has undergone secondary treatment and biological oxidation?

Answer 21

The ‘liquor’ is collected and passed through ‘humus’ tanks, to undergo a secondary settlement process in order to settle out any solids from the biological oxidation process.

Question 22

After the secondary settlement tanks, where does the remaining liquor go?

Answer 22

It moves onto a stage called ‘tertiary treatment’ or ‘disinfection’, whereby the remaining waste is treated to become non-toxic.

Question 23

When is tertiary treatment used?

Answer 23

• Low dilution of final effluent in receiving water course
• Receiving watercourse is used for water supply

Question 24

Provide some examples of tertiary treatment:

Answer 24

• UV treatment
• Micro-strainers
• Rapid gravity sand filters
• Pebble bed clarifiers
• Lagoons (additional sedimentation)

Question 25

How is sludge produced in WWTWs?

Answer 25

Through screening, grit, primary sludge and secondary sludge

Question 26

What is the aim of sludge treatment?

Answer 26

to reduce:
- The amount of organic matter
- The number of disease causing micro-organisms in the solids

Question 27

Describe sludge digestion

Answer 27

Sludge is stored and not aerated, resulting in the sludge becoming septic and undergoing fermentation or ‘digestion’. Sludge gas, methane, is produced. This reduces the solid content of the sludge.

Question 28

Describe filter pressing and incineration in sludge treatment (4)

Answer 28

• Chemicals are added to the sludge to ‘flocculate’ it (aiding mechanical dewatering)
• Sludge is pumped to filter press
• Water is squeezed out through filters
• A solid mass, or ‘sludge cake’ is produced and removed off site

Question 29

What can a ‘sludge cake’ be used for?

Answer 29

Agricultural land spreading

Question 30

Describe centrifuge and incineration in sludge treatment (3)

Answer 30

• Sludge is heated to 40 degrees by waste heat
• A chemical solution is added to the sludge
• Centrifuge is used and produces a sludge cake

Question 31

What is the general principle behind ‘activated sludge’? (5)

Answer 31

• Activated sludge processes provide an alternative to filter beds
• Liquor is circulated around oxidation tanks
• Tanks promote growth of aerobic bacteria (high oxygen levels)
• Activated sludge develops
• New liquor inflow to the tank is ‘seeded’ with the activated sludge

Question 32

What is activated sludge?

Answer 32

Sludge with large numbers of bacteria

Question 33

What are the key differences between traditional wastewater treatment works and a wastewater treatment works that utilises the activated sludge process? (7)

Answer 33

• Activated sludge process is an alternative to filter beds
• Liquor is circulated around large oxidation tanks
• Tanks introduce and maintain high oxygen levels to promote aerobic bacterial growth
• This is managed either mechanically by rotating paddles, or through the injection of air using diffusors
• As the liquid circulates & oxidation takes place, a certain amount of activated sludge develops
• Activated sludge is sludge containing high amounts of bacteria
• The sludge is collected and some of it is mixed in with the new inflow of the tank (i.e. ‘seeding’ the liquor as it enters)

Question 34

Draw the process of a traditional waste water treatment works.

Answer 34

Refer to cue card #1.

Question 35

Draw the process of a waste water treatment works that utilises the activated sludge process.

Answer 35

Refer to cue card #2.