Waste Water Treatment

Question 1

What are the 5 stages of treatment in wastewater?

Summarise what each does

Answer 1

Preliminary
Removes grit and screens large particles

Primary
Removes suspended soils through sedimentation tanks

Secondary
Biological treatment where organic matter is broken down by microorganisms

Tertiary (if required)
Removing remaining BOD, SS, and bacteria to meet standards for a sensitive region to then discharge into a water body

Sludge
Material removed from WW is treated by dewatering, stabilisation and disposal /re-use through landfill etc

Question 2

Why do we need is preliminary treatment?

Answer 2

Remove materials which may cause blockages or damage pumps and pipework at the treatment works

Question 3

What 2 stages make up preliminary treatment?

Answer 3

Screening
Grit Removal

Question 4

Describe screening

Answer 4

Removes large gross materials and protects other treatment processes. Removed solids are incinerated or landfilled.
Large solids are removed initially before finer screens are used.

Question 5

Describe grit removal

Answer 5

Allows for removal of inorganic material (sand, glass etc), preventing damage to downstream equipment, prevents silting and pump damage (very abrasive). Licensed disposal site.

Inorganics settle faster with slow flows required for grit settlement. Designed so grit sinks to bottom whilst sewage continues forward to next process.

Question 6

What is aim of primary treatment?

Answer 6

For settlement of organic solids and the flotation of grease

Question 7

What does primary treatment produce?

Answer 7

Liquid effluent which goes on to be treated by biological filters and the sludge pumped to sludge tanks/

Question 8

What is sedimentation and what does it form?

Answer 8

Particles settling out of suspension under gravity.
Allows remaining solids to settle with settled solids forming a sludge at base of tank to be removed.

Question 9

What do primary sedimentation tanks achieve and what are the 3 types?
What do all require?

Answer 9

Clarify sewage by flotation and settlement.
Upward Flow
Horizontal Flow
Radial Flow
All require slow flows so sludge is not unsettled and resurfaced

Question 10

Describe an upward flow sedimentation tank

Answer 10

Sewage moves upward through the tank causing flocculation to increase particle size. Sludge collects at base.
Difficult to construct and used in small WWTP.

Question 11

Describe an horizontal flow sedimentation tank

Answer 11

Wastewater moves along channel. Floor is sloped towards a sump and sludge is collected by a bridge scraper. Surface scum is collected in a scum box. Wide to avoid turbulence.
Used in large WWTP.

Question 12

Describe an radial flow sedimentation tank

Answer 12

Use circular tanks with skimmer blades to push scum across surface of tank to scum box. Floor slopes to central sump where sludge is gathered by a rotating scraper.
Very common

Question 13

What does secondary treatment do?

Answer 13

Removes colloidal and dissolved material in the wastewater using biological treatment and sedimentation

Question 14

What process does secondary treatment utilise? Explain process

Answer 14

Self-purification
Uses microorganisms to convert soluble and colloidal matter into a dense microbial biomass that can be readily separated by sedimentation to form sludge

Question 15

What are 2 methods that bacteria breaks down OM in secondary treatment? Explain each
What is required for maximum efficiency?

Answer 15

Biological Oxidation
End products remain in solution and can be discharged in final effluent

Biosynthesis
Convert colloidal and soluble OM into particulate biomass which is removed by setllement

Both require excess of O2 so bacteria is broken down at maximum efficiency

Question 16

What are the 2 secondary treatment systems?

Answer 16

Trickling filter
Activated sludge

Question 17

Describe the trickling filter’s process

Answer 17

WW sprinkles through stones with biofilm on stones which break down OM. Voids between stones provides constant ventilation which promotes aerobic conditions. Biofilm grows but then dies becoming detached.
Treated effluent flows by gravity/settlement

Question 18

Describe the activated sludge process

Answer 18

Aeration tank: Suspended MOs break down OM with aerators injecting O2 causing high bacterial reproduction rates and bacteria to aggregate into flocs.
Settlement tank: sludge settles and clarified effluent is discharged
Sludge is recycled into aeration tank since need to ensure there is enough bacteria in the tank.

Question 19

When is tertiary treatment required?

Answer 19

1. WQ standard of effluent is not sufficient
2. Abstraction point is downstream of discharge
3. Outfall recharges groundwater source
4. Low dilution of watercourse

Question 20

What are the main processes of tertiary treatment?

Answer 20

1. Additional settlement
2. Straining through a fine meash
3. Filtration
4. Disinfection

Question 21

Describe the UV disinfection process

Answer 21

Treated effluent passes through a channel under a UV radiation lamp in laminar flow. UV breaks down any remaining micro-organisms.

Question 22

Describe aerated sand filter process

Answer 22

Treated effluent is passed through sand filters which contain beds of assorted sand sizes. Sand layers act as filters to remove any remaining solids. Sand is aerated so to promote nitrifying bacteria to remove ammonia.

Question 23

When would alternative treatment processes be needed?

Answer 23

If STW is in a small, remote location with a limited population or if new legal standards require specific limits or contaminants to be removed

Question 24

What is sludge and when is it produced?

Answer 24

Liquid with a relatively low solids content
Produced at every liquid/solid separation stage

Question 25

How is sludge gathered?

Answer 25

Sludge is pumped to holding tanks where it is stored, settled, and dewatered then transported to a works for treatment. Excess water goes back into works for treatment.

Question 26

What 4 processes are involved in sludge treatment?

Answer 26

Dewatering Anaerobic digestion Liming Drying

Question 27

Describe dewatering

Answer 27

Removes water from sludge to reduce storage and transport volume

Question 28

Describe anaerobic digestion

Answer 28

Sludge is stored at increased temperatures that allow microbial digestion. Pathogens are killed and methane is emitted and captured for electricity

Question 29

Describe liming

Answer 29

Lime is added as a cake or powder to kill pathogens

Question 30

Describe drying

Answer 30

Sludge cake is heated in a drier to drive off remaining water. Pathogens are killed and creates easily recyclable end product

Question 31

What can treated sludge be used for?

Answer 31

Contains organic matter which can be used as a fertiliser. Has energy value which can be recovered by incineration or pyrolysis.

Question 32

Alternative treatment process: Describe Rotating biological contactors

Answer 32

Settled sewage is held in tank which mounted discs rotate through to expose sewage to air. Bacteria grows on disc surface and treats OM bio-chemically

Question 33

Alternative treatment process: Describe Biological aerated filters and submerged aerated filters

Answer 33

Use fixed media which bacteria grows but sewage flows across media while being aerated. Incorporates primary and final settlement stages.

Question 34

Alternative treatment process: Describe membrane bioreactors

Answer 34

Activated sludge process Large sheets of membrane are submerged and activated sludge tank is aerated to provide oxygen for bacteria. Effluent is drawn through membranes with small apertures trapping bacteria within tank.

Question 35

Alternative treatment process: Describe sequential batch reactors

Answer 35

Activated sludge plant but happens in stages. One tank treats sewage Tank is then aerated Sewage then settles Treated effluent is then drawn off

Question 36

Alternative treatment process: Describe reed beds / grass plots

Answer 36

Flows through wetlands at reduced rate. Bacteria on reeds will break down wastes using oxygen dissolved from air in water forming a sludge which can the compost

Question 37

Alternative treatment process: Describe phosphorus removal

Answer 37

For works that discharge to sensitive watercourses. Can chemically or biologically remove by adding salts or using activated sludges with adjusted aeration respectively