Biological Wastewater Treatment

Question 1

Why is biological waste water treatment important for health?

Answer 1

Minimises risk of waterborne disease

Question 2

Why is biological waste water treatment important for ecology?

Answer 2

Minimises the risk to ecological balance

Question 3

Why is biological waste water treatment important aesthetically?

Answer 3

The water’s appearance is maintained

Question 4

Why is biological waste water treatment important economically?

Answer 4

Effective treatment protects from environmental pollution

Question 5

Draw the urban water cycle
There are 3-4 main steps
Details in the answer

Answer 5

Extraction from natural water source

Drinking water treatment

Use in industry/ homes

Sewage treatment

Disturbed for us on crops, sewage biosolid is either recycled or disposed

Crop water goes back into nature

Question 6

What are the main 5 waste water treatment steps?

Answer 6

Screening

Grit removal

Primary sedimentation

Secondary WWT

Secondary sedimentation

Question 7

What does a screen do in WWT?

Answer 7

Removes solid debris which goes to landfill

Question 8

What happens in grit removal?

Answer 8

Grit settles after flow is slowed down in a removal flume.

This grit has hard abrasive grains

Question 9

What happens in primary sedimentation?

Answer 9

Solid fecal matter is removed and we get:

Primary sludge 5% DS (dry solids)

Question 10

After primary sedimentation we get

Answer 10

Settled sewage

Question 11

Settled sewage goes through

Answer 11

Secondary WWT

Question 12

Secondary WWT gives us

Answer 12

Treated WW and new biomass

Question 13

Secondary sedimentation gives us

Answer 13

Secondary sludge 2-3% DS

Question 14

After secondary sedimentation we have

Answer 14

Treated affluent

Which gets returned to the water system

Question 15

What are the two big biological filter processes

Answer 15

Aerobic microbial oxidation
&
Sludge processes

Question 16

Why did sludge processes beat aerobic microbial oxidation?

Answer 16

30-50% less energy needed but a higher footprint

Question 17

What does the circular attached growth biological system entail?

Answer 17

Percolating bio filter
5-10cm aggregate bed
1.5-2m deep overall

Question 18

What is the process of attached growth (4)

Answer 18

Soluble organic and O2 diffuse into biofilm

Extra cellular enzymes hydrolyse organic matter

Direct ingestion by metazoa + Protozoa

Nitrification

Question 19

What are the pros of attached growth?

Answer 19

Low maintenance
Withstands shock loads
Biomass retained, no washout risk
(Overall robust and easy to run)

Question 20

What are the cons of attached growth

Answer 20

Microbial growth blocks filters

Open to air (flies + odour)

Requires pre treatment and primary sedimentation

Question 21

What is suspended growth and what does it involve

Answer 21

Activated sludge process

Uses biomass as a freely suspended floc

Question 22

What are the stages of suspended growth treatment?

Answer 22

Rapid stage

Oxidation stage

Question 23

What does the process of suspended growth involve?

Answer 23

Mixed liquor of biomass and WW in tank

Cell retention 8-12d

Hydraulic retention 8h

High microbial population deliver max rate of decomposition

Question 24

What happens in the rapid stage?

Answer 24

The floc absorbs rapid absorption of solubles:

Capture of particulate solid matter

Rapid reduction of BOD (what is this?)

Question 25

What happens in the oxidation stage?

Answer 25

Respiration increased as organic matter is oxidised

Rapid oxidation of readily degradable organics

Slower oxidation of recalcitrant components

Respiration is <1/10 of original rate at the end of the tank

Question 26

What are the pros of suspended growth?

Answer 26

More control

Reduced odour

Smaller footprint
(Essentially contours all faults of attached growth)

Question 27

What are the cons of suspended growth?

Answer 27

Susceptible to biomass washout and sludge bulking

High energy demand (60% of the whole WWT process)