1. MIC 203: Environmental Microbiology
Sewage Treatment

2. Stages in typical sewage treatment

3. Wastewater treatment processes

4. Primary Sewage treatment
Large floating materials (cans, sticks, plastic packets etc.) are screened out by bar screen.
Grit chamber removes sand, grit, stones, broken glass etc.
Floating debris is shredded & ground

5. Primary Sewage treatment
After that sewage pass through sedimentation tank where colloidal and dissolved organic matters are removed. Skimmers remove floating oil & grease
Sewage solid collecting on the bottom is called primary sludge.
40-60% suspended solid are removed
Primary treatment may remove 70-80% of BOD

6. Primary Sewage treatment (contd.)
The sludge are drawn off from bottom & is subjected to anaerobic digestion before disposal in landfills or agriculture fields
Biological activity is not important.
Some digestion of sludge & dissolved organic matter occur during holding time
The effluent then undergoes secondary treatment

7. Primary Sewage treatment (contd.)

8. Secondary Sewage treatment
Predominantly biological
Removes most organic matter & reduces BOD
Sewage undergoes strong aeration to encourage growth of aerobic bacteria & other microbes that oxidize dissolved organic matter to CO2 & H2O (mineralization).

9. Secondary Sewage treatment (contd.)
Two method are commonly used:
Activated sludge system
Trickling filters
Combination of primary & secondary treatments reduce original BOD of sewage by 80-90%

10. An activated sludge system of secondary sewage treatment

11. An activated sludge system
Popular & common method
Effluent from primary treatment is aerated in a large aeration tank.
Slime bacteria e.g. Zoogloea ramigera grow & form flocs.
Protozoa, small animals, filamentous bacteria and fungi are attached to the flocs.

12. An activated sludge system (contd.)

13. An activated sludge system (contd.)
Holding time is generally 5-10 hrs.
During this time, a part of of organic matter is mineralized and another part is converted into microbial biomass.
The content of the tank is transferred to a settling tank where the flocs settle.

14. An activated sludge system (contd.)
Some floc materials are used as inoculum (hence the name of the process) and the rest is sent to the sludge digester. The clear effluent is disinfected and discharged.

15. An activated sludge system (contd.)
Advantages:
Effective & flexible
Relatively small space is needed
Widely used for treatmnet of domestic & industrial sewages
Reduces the numbers of intestinal pathogens, such as Salmonella, Shigella, E. coli, enteroviruses etc.

16. Trickling filter

17. Trickling filter

18. Trickling filter (condt.)
common method
Wastewater is sprayed over a bed of crusted stone or molded plastic with drains at the bottom.
The bed is designed such a way that air can circulate throughout the bed.
The spray also saturates the sewage with O2

19. Trickling Filter

20. Trickling filter (condt.)
A biofilm consists of aerobic microbes grows on the stone or plastic surface and completely mineralizes organic matter to CO2, NH3, NO3-, SO42- and PO43-. As a result BOD reduces.
Insect mainly fly larvae consume the extra microbial biomass.

21. Trickling filter (condt.)
Advantages:
Simple & inexpensive
Removes 80-85% BOD
Can be used for several times

22. Trickling filter (condt.)
Disadvantages:
Less effective than activated sludge system
Nutrient overload impairs the operation of the filter
Cold weather reduces effectiveness
Make nuisance to nearby residence
Produces aerosols

23. Oxidation pond

24. Oxidation pond
Shallow pond (2-4 ft in depth) to allow algal growth on waste water efluent.
Oxygen is supplied from the air but release of O2 during photosyntensis by alga Chorella provides additional source of O2.

25. Oxidation pond (condt.)
Advantages:
Inexpensive to build & operate
Have a large holding capacity

26. Oxidation pond (condt.)
Disadvantages:
Requires large areas of land & long retention time.
Create odor problem.
Largely restricted to warmer climate.

27. Rotating biological contractor (RBC)

28. Rotating biological contractor (RBC)
Aerobic secondary sewage treatment process.
Also called biodisc system and is an advanced biofilm design.

29. Rotating biological contractor (condt.)
Several closely palced disc are continuously rorated in a trough containing liquid sewage.
Discs are partially submerged in the liquid sewage.
Biofilm develops on the discs.
Continuous rotation keeps biofilm well aerated and in contact with sewage.

30. Rotating biological contractor (condt.)
Advantages:
Require less space
Efficient & stable in operation.
Produce no aerosol.
Disadvantages:
Require higher initial investment

31. Rotating biological contractor (condt.)
Micobial community:
Complex & diversed
Diverse bacterial, protozoan, metazoan populations.
Filamentous microbes: Sphaerotitus, Beggiatoa, Nocardia & Oscillatirio

32. Rotating biological contractor (condt.)
RBC typically has two layers:
An outer layer: whitish layer containing Beggiatoa filaments &
An inner layer: balck layer containing Desulfovibrio. Black color is due to the precipitation of FeS from reaction between Fe and H2S produced by sulfate-reducing Desulfovibrio.

33. Anaerobic sludge digester
An anaerobic sludge digester is designed to encourage the growth of anaerobic bacteria (especially methane-producing bacteria) that degrade these organic solids to soluble substances and gases mostly CH4 (60-70%) and CO2

34. Anaerobic sludge digester

35. Major microbial processes occurring during anaerobic sludge digestion

36. Anaerobic sludge digester
The digester contains high amounts of suspended organic matter. A considerable part of this suspended material is bacterial biomass, viable count can be as high as bacteria/ml.
Anaerobic counts are typically 2-3 orders of magnitude higher than aerobic counts.

37. Anaerobic sludge digester
The operation of anaerobic digester requires close control of several parameters, e.g. :
Retention time
Temperature pH
C:N & C:P
Reactions are usually heated oC for optimal performance.
Control is necessary for pH to stay within with pH 7.0 being optimal.

38. Anaerobic sludge digester
Extremes of pH, influx of heavy metals and toxic materials upset the operation. The reactor must be cleaned and charged with large volume anaerobic sludge or appropriate methagenic bacteria from an operational unit.
Heavy metals-especially cadmium, which can accumulate in plants and copper, which is toxic to plants- are a potential problem with land disposal.

39. Anaerobic sludge digester
Advantages:
CH4 is used as a fuel for heating the digester
Sludge contains soil-conditioning materials e.g. humus and mulch and can be used a fertilizer.

40. Anaerobic sludge digester
Disadvantages:
Expensive to construct.
Require expert maintenance

41. Disinfection & release

42. Septic tank
Short note

43. Septic tank

44. Tertiary sewage treatment
Its importance

45. Tertiary sewage treatment
Process

46. Tertiary sewage treatment
Advantages

47. Thank You