LU12 aquatic microbiology and sewage treatment

Question 1

5 steps water Treatment

in a typical municipal water purification

Answer 1

1. raw water is held in reservoir to allow particulates to settle.
   2a. water is mixed with flocculant.
   2b. as aggregates of floc settle out, they carry colloidal particles out of suspension.
2. water undergoes filtration.
3. water is disinfected by chlorination, ozone treatment, exposure to uv light.
4. water is stored before used by consumers

raw water reservoir –> pumping station –> flocclant, mixing tank –> flocculation tank –> filtration, particulate filter –> storage tank –> consumer

Question 2

Sewage Treatment – Primary Treatment

removal of, removes % bod

Answer 2

• removal of solid matter (sludge)
• Septic tanks can be used in rural areas to provide primary treatment of sewage [they require a large leaching field for the effluent]
• Biological activity is not very important in primary treatment

Primary treatment removes approximately 25-35 % of the biochemical oxygen demand (BOD) of the sewage.
- BOD is a measure of the biologically degradable organic matter in water.
- determined by measuring the amount of oxygen bacteria require to degrade the organic matter.

Question 3

Sewage Treatment – Tertiary Treatment

Answer 3

• uses physical filtration and chemical precipitation to remove all the BOD, nitrogen, and phosphorus from water.
• provides drinkable water, whereas secondary treatment provides water usable only for irrigation.

Question 3

Sewage Treatment – Secondary Treatment

Answer 3

• biological degradation of organic matter in
  sewage after primary treatment.
• activated sludge and trickling filters - methods of secondary treatment.
• small communities can use oxidation ponds [these require a large area in which to build an artificial lake]
• during secondary treatment, microorganisms degrade the organic matter aerobically.
• Secondary treatment removes up to 95% of the BOD.

Question 3

After the secondary treatment, the effluent may undergo two processes:

Sewage Treatment – Disinfection and Digestion

Answer 3

1. Disinfection and Release.
2. Sludge Digestion.

Question 4

1. Disinfection and Release
2. Sludge Digestion

Answer 4

1. Disinfection and Release.
   - Treated sewage is disinfected before discharge onto land or into water.
2. Sludge Digestion.
   - Sludge is placed in an anaerobic sludge digester.
   - In the anaerobic sludge digestion chamber, bacteria degrade organic matter & produce simpler organic compounds such as biogas (i.e.
   methane – CH4 and CO2) as the final products.
   CO2 + 4H2 –> CH4 + 2H2O
   CH3COOH –> CH4 + CO2

• The methane produced in the digester is used to heat the digester and
  to operate other equipment.
• Excess sludge is periodically removed from the digester, dried, and
  disposed of (as landfill or as soil conditioner) or incinerated