2b - Water treatment and marine pollution

Question 1

How many sewage treatment works does HK have?

Answer 1

69

Question 2

what are the main steps of water treatment process?

Answer 2

1. Preliminary treatment (screening and degritting)
2. *Primary treatment (Co-setting, primary sedimentation, chemically enhanced primary treatment, flocculation)
3. *secondary biological treatment (aeration, bio treatment)
4. final sedimentation
5. *disinfection processes
6. effluent export

Question 3

what are the steps of sludge treatment?

Answer 3

thickening, digesting, dewatering

Question 4

describe the preliminary step of water treatment

Answer 4

the screening, removal of grit and sedimentation process until total suspended solids (TSS) is ~70% for waterborne particles exceeding 2microns and biochemical oxygen on demand (BOD) ~30%

Question 5

describe the primary step of water treatment.

Answer 5

The screening, removal of grit and a sedimentation process.
Use of a primary clarifier.
Chemically enhanced primary treatment using cationic salts: ferric salt, alum or polyelectrolyte are added to enhance TSS and BOD to 80% and 60% respectively via coagulation into flocs.

Question 6

what is flocculation?

Answer 6

encouraging collision and further aggregation of flocs. can happen spontaneously or with chemical agents ie. polymers

Question 7

how do polymers cause particles to coagulate?

Answer 7

cationic salts neutralise the negative charges of suspended particles causing them to coagulate

Question 8

describe the secondary step of water treatment.

Answer 8

water is stirred and aerated so aerobic bacteria degrade organic pollutants. removes 90% of suspended solids.

Question 9

what are the main end product of the secondary step of water treatment?

Answer 9

carbon doxide and water

Question 10

describe the tertiary step of water treatment.

Answer 10

combination of physical and biological processes, final sedimentation

Question 11

describe the final step (optional) of water treatment

Answer 11

disinfection via UV, chlorine, reverse osmosis.

Question 12

how does chlorine work in treatment of water?

Answer 12

breaks chemical bonds in the molecules of bacteria and viruses

Question 13

why are we limiting the use of chlorine in water treatment?

Answer 13

it’s toxic to aquatic life
it reacts with organic matter creating hazardous compounds (trihalomethane)

Question 14

what are the main steps of drinking water treatment?

Answer 14

1. pretreatment
2. clarification
3. filtration
4. disinfection

Question 15

which 2 sources does Hong kong’s drinking water come from?

Answer 15

rainfall from local catchment and Dongjiang in Guangdong

Question 16

how does HK treat seawater for flushing?

Answer 16

1. screen and remove sizable particles
2. disinfection with sodium hypoclorite (NaClO, bleach)

Question 17

how does the septic system work?

Answer 17

it is a buried, watertight tank.
allows heavy solids to settle to the bottom while grease and lighter solids float.
solids stay in the tank while wastewater is discharged to the drianfield for further treatment.

Question 18

why do we need to treat wastewater?

Answer 18

they give off gasses like hydrogen sulfide (manure gas), carbon dioxide and methane

Question 19

what are the effects of manure gas exposure?

Answer 19

H2S exposure
0-20ppm: distinct rotten egg odour
20-100ppm: severe irritation of eyes and breathing passages
100-250ppm: severe irritation, cough, headaches, nausea, loss of sense of smell
250-500ppm: difficulty breathing, fluid in lungs, vomiting, dizziness, loss of coordination
500-750ppm: stumbling, staggering, collapse of coordination
>750ppm: death within mins due to respiratory paralysis