- Sewage is the waste matter from homes or industries.
- Sewage contains disease-causing bacteria.
- When discharged into rivers or lakes, it may lead to the spread of diseases, eg cholera and typhoid.
- Sewage may also contain nitrates and phosphates which are nutrients for algae and water plants. The condition is known as eutrophication.
Disposal sewage onto Yangtze River
1. Where does the sewage disposed into Yangtze River come from?
URL: https://www.youtube.com/watch?v=gec1CMN6h0c
Sewage Treatment to remove large solid wastes and phosphates. (Video)
1. What chemical is added to turn soluble phosphates into insoluble forms?
2. Which size of solid waste is separated by the solid screens?
3. Solid waste eg plastics is compacted. How is the compacted waste disposed?
4. Coagulants and polymer solution are added to convert the remaining phosphates into flaky substances. This forms another component known as sludge. The sludge is incinerated. What is the ash used for?
URL: http://youtube.com/watch?v=GV-DoisLwm0
Objectives of sewage treatment at water reclamation plants
(i) Removal of organic matter
(ii) Removal of water-soluble nitrates and phosphates
(iii) Elimination of pathogenic microorganisms. (Tertiary treatment)
3 stages of sewage treatment
1. Primary treatment
2. Secondary treatment
3. Tertiary treatment
Stage 1: Primary treatment
The liquid sewage is sent to a large tank known as primary settlement tank.
Liquid flows through the tank very slowly and this allows the large solid particles to settle to the bottom of the tank as primary sludge.
The sludge is removed and feed into an anaerobic digester.
The top liquid from the primary settlement tank flows into an aeration tank.
Stage 2: Secondary treatment
In Singapore, secondary treatment involves the activated sludge process.
Oxygen gas is introduced at the aeration tank for the microorganisms for aerobic respiration.
In the presence of oxygen, microorganisms which are found in the sewage break down and absorb the dissolved organic compounds
Filtration of substance generated from bacterial digestion. Residue containing the disease-causing microorganisms is known as sludge. A portion of the sludge is returned to the aeration tank for reuse.
Excess sludge is sent to the anaerobic digester.
Clean water at the top of the tank is collected and may be discharged as effluent into nearby rivers, streams or lakes. The effluent may be further treated using reverse osmosis technology to produce clean water known as NEWater in Singapore.
Anaerobic digester
The anaerobic digester is a closed tank with no oxygen being supplied.
The anaerobic bacteria decompose the sludge, breaking down the organic matter to produce biogas, mainly methane.
The biogas is used as a fuel to generate electricity for the functioning of the reclamation plant.
The remaining solid material is removed from the tank where it may be used as fertiliser or burnt in an incinerator.
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