Lecture 16 - waste water and seqage treatment

Question 1

what is the main thing that needs to happen in waste water treatment

Answer 1

• reduce organic content
• reduce solids
• remove pathogenic microbes
  -remove ammonia by converting to nitrate

Question 2

what is BOD

Answer 2

biological oxygen demand
or the amount of O2 consumed by bacteria in the dark in a set time
(used to predict effectiveness of purification process)

Question 3

what is the max BOD figure by law

Answer 3

20mg per L

Question 4

first step of treatment

Answer 4

remove large solids by mesh or filter

Question 5

second step of treatment

Answer 5

separate more solids from liquid via settling tanks
all particles settle to bottom = PRIMARY SLUDGE

Question 6

third step of treatment

Answer 6

liquid goes to get treated via microbes to reduce organic content
either
- percolating filter bed
- activated sludge process

and then settled again = secondary sludge

Question 7

what is percolating filter bed

Answer 7

sewage sprayed onto rocks
rocks have biofilm on surface
1st half = oxidises organic compounds
2nd half = oxides NH4 to NO3 (nitrification)

Question 8

what would happen if waste spray is too much

Answer 8

biofilm gets too thick
cuz sm carbon and nitrogen
so overgrowth
cause blockages

Question 9

how is activated sludge process diff from percolating filter bed

Answer 9

• microbes arent attached to a surface, they grow as flocs on the liquid
• aerated by air diffusers = lots of oxygenation and microbes grow much faster
• 10x faster

Question 10

what is a negative about activated sludge process

Answer 10

higher costs
and because flocks and not biofilms = not resistant to stress, easy to kill microbes

Question 11

what happens to the liquid after secondary settling tank

Answer 11

either gets discharged into water straight away
or
third treatment step of just passing it through sand or grass for extra filtration, then discharged

Question 12

what happens to the primary and secondary sludge

Answer 12

anaerobic digestion

Question 13

what happens in anaerobic digesting process

Answer 13

• sludge incubated in tank at 35celsius
• all aeorbes use up any O2 and it quickly turns into anaerobic environment
• anaerobes metabolise all the organic matter

Question 14

what do anaerobes turn the organic matter into

Answer 14

methane
carbon dioxide
(these 2 typically used as energy for generators, or burned if small amount)
and water

Question 15

what happens to solid content after anaerobic digestion of sludge

Answer 15

filtrated or dried to make sludge cake wtaf is this
can be used a fertiliser methinks

Question 16

what is the metabolic pathway of the organic matter into methane and water

Answer 16

organic matter
=>
organic acids/esters and alcohols
=>
acetic acid + H2 + CO2
=>
CH4 + H2O

Question 17

what type of bact turns organic matter into organic acids and alcohols

Answer 17

fermentative bacteria
(think of alcohol fermentation in anaerobic resp)

Question 18

what turns organic acids and alcohols into acetic acid + H2 + CO2

Answer 18

Acetogenic bacteria

Question 19

what bact turns acetic acid and stuff into methane and water

Answer 19

methanogens

Question 20

main problems with anawrobic digestion

Answer 20

methanogens grow slower than acetogenic bact
= build up of H2 = explosive

Question 21

why is sludge cake not used as fertiliser from cities

Answer 21

high metal content