1. Wastewater Treatment On completion of this segment you should be:
Aware of the public health aspects and goals of wastewater treatment
Able to describe the processes involved in primary, secondary and tertiary treatment
Able to compare the differences between the fixed-film and suspended growth systems in biological treatment
Aware of some methods available for nutrient removal

2. Wastewater Treatment Goals
Aims
Protect public health from contamination of water supplies
Reliable and economic operation
Minimum capital cost

3. Wastewater Treatment Goals (cont)
Outcomes
Removal of floating, suspended and soluble matter
Reduce BOD, COD pathogenic organisms and nutrient
Maintain aesthetics of natural water bodies, ecology of water systems

4. Typical Characteristics of Wastewater

5. Treatment Selection
Wastewater treatment comprises primary, secondary and tertiary treatments
The selection of appropriate treatment processes is dependent upon the nature and strength of pollutants, quantity of flow, and discharge licence conditions

6. Primary Treatment
Usually the first stage of wastewater treatment comprises largely physical processes.
A well-designed primary treatment should remove about % of TSS and about % BOD5
A possible pre-treatment is the injection of air, O2, H2O2 and pre-chlorination if the influent is 'stale’
Processes include screening, grit removal and primary settling

7. Screens
The removal of large objects that may damage pumps or block channels
Fixed or mechanical
Velocity in channels about m/s
Design for PWWF
All screenings to be removed/buried
Location of strong odour from decomposition

8. Mechanical bar screen

9. Rotating drum screen

10. Comminutors
These are mechanical cutting screens that reduce the size of large objects
Shredded matter are returned to the flow stream
A by-pass may be included

11. Comminutor

12. Grit Chambers
Purpose is to remove inorganic grit/sand mm size through differential settling
Aim is to prevent damage to pumps, blockage of channels and cementing of sludge in settling tanks
Two types of grit chambers, namely constantly velocity and aerated/spiral flow tanks

13. Constant Velocity Grit Chamber
Class I settling - horizontal flow
Uniform velocity at m/s
Ideal parabolic shape or approximation
Width:depth ratio 1:1
Length  18 x max. depth

14. Constant Velocity Grit Chamber

15. Aerated or Spiral Flow Grit Chamber
Flexibility of control; more efficient grit removal and can assist pre-aeration
Air supply or spiral flow controls the amount of silt removed
Suitable for larger population > ep
HRT of about 3 min at PWWF

16. Aerated or Spiral Flow Grit Chamber

17. Vortex Flow Grit Chamber

18. Primary Sedimentation
Solids separation by gravity
Aim is to remove gross suspended solids (organic matter)
Largely class II settling of flocculent matter and natural coalescence or flocculation occurs
Surface skimmers remove floating matter (scum, grease etc)
The settled solids are pumped to an anaerobic digestion tank. The effluent (settled sewage) from primary treatment flows to the next stage ie. secondary treatment

19. Some Features of Primary Settling
Design to accept 2 to 3 x ADWF
Removal of % suspended solids
Some incidental BOD5 reduction %
Hydraulic loading Q/A  30 m3/m2.d
Hydraulic retention time (HRT) 1.5 to 3 h; depth 2.5 to 5 m
Also act as flow/strength equalisation basins
Sludge scrapers should not cause re-suspension

20. Primary settling % removed vs time

21. Types of Primary Settling tanks
Rectangular horizontal-flow
Tanks use less space
Forward velocity mm/s
Weir loading rate < 300 m3/m.d
Length:width ratio 3:1

22. Rectangular horizontal-flow

23. Types of Primary Settling tanks
Up-flow tank
Square with 60o sludge hopper
No moving parts as sludge is removed hydrostatically
Some possible particle carry over

24. Up-flow settling tank

25. Types of Primary Settling tanks
Circular radial flow tank
Inflow to a central stilling box
Radial-horizontal flow
Uses radial scrapers to remove sludge

26. Circular Radial Flow Tank

27. Circular Radial Flow Tank

28. Circular Radial Flow Tank

29. Pulteney Bridge and Weir, City of Bath

30. Secondary Treatment
Removal of dissolved solids through microbial action
Objective is to remove the remaining suspended solids and also dissolved solids
The process is mainly biological using microorganisms to convert the dissolved solids to biomass
Two distinct systems are available i.e. fixed film (trickling filter) and suspended growth (activated sludge)
The biomass is removed as sludge in final sedimentation tanks (clarifiers)

31. Typical microorganisms in activated sludge

32. Fixed-Film Systems
Land treatment, trickling and rotating biological filters are predominantly aerobic biological processes
Land treatment ie. broadcasting of sewage, is one of the earliest forms of wastewater treatment

33. Trickling Filter
Comprising an inert structure for growth of biofilm containing microorganisms (attached growth)
Microorganisms in biofilm interact with wastewater and metabolise the organic matter (BOD) into CO2 and H2O
Natural sloughing of the biofilm when it reaches a thickness that cannot be sustained
Filter medium voids (40 – 60%) promote air circulation and aerobic condition
Solids in the effluent are separated in the secondary settling (humus) tank

34. Interaction of biofilm

35. Trickling Filter

36. Trickling filters at Wetalla

37. A rotating biological contact unit

38. Suspended Growth Systems
Microorganisms are held in suspension as a high concentration flocculent, bulky matter through agitation, stirring
The microorganisms interact with influent wastewater and biodegrade organic matter into CO2, H2O and by- products, releasing energy for growth of new cells
The activated sludge process is an example of an aerobic suspended growth system. The anaerobic digester for the break down of waste sludge is an example of an anaerobic suspended growth system

39. Activated Sludge Process
The heart of the process is the reactor where aeration and oxidation of organic compounds occur
Microorganisms are held in suspension by aeration and stirring
Energy requiring process but has greater control and flexibility
Return activated sludge and sludge wasting maintain the design biomass concentration (MLVSS)
Final clarifier separates solids from the clear effluent and returns the settled sludge to the reactor

40. Activated sludge process with alternative wasting locations

41. Surface aerators

42. Final sedimentation tank

43. Final clarifier

44. Comparison between attached film and suspended growth systems
Parameter
Trickling filter
Activated sludge
BOD removal
85 – 90%
> 95%
Lower limit of BOD effluent
15 mg/L
< 10 mg/L
Capital cost
High
Moderate
Operating cost
Minimal
Land requirement
Low
Operator control
Limited
More
Shock loads
Rapid recovery
Very slow
Foaming
None
Often
Odour
Yes
Filter flies
Noise
Hydraulic washout No
Plugging
Drying of media
Output of sludge
moderate

45. Wastewater Disinfection
Some microorganisms (105 – 107/100 mL) are still present in treated wastewater after secondary treatment
Disinfection is required to reduce pathogenic microorganisms
Chlorine is still the cost-effective disinfection, but requires minimum contact time and has adverse effects
Other environmental friendly methods include UVL, ozone disinfection, membrane microfiltration and constructed wetlands

46. Sludge Digestion
Sludge from primary and secondary settling tanks (including waste activated sludge) must be treated in digesters
Sludge is thickened before passing to sludge digesters
Sludge may be treated anaerobically or aerobically
Anaerobic sludge digestion involves 2 sequential stages ie. acid formation and methane formation
Digested sludge is dewatered before disposal

47. Low rate single-stage sludge digester.
Low rate single-stage sludge digester

48. High rate two-stage sludge digester.
High rate two-stage sludge digester

49. Anaerobic sludge digester.
Anaerobic sludge digester

50. Aerobic sludge digester.
Aerobic sludge digester

51. Tertiary Treatment
Tertiary maturation ponds – an aerobic polishing process with detention time and further reduction in BOD and TSS (NFR)
Nutrient removal comprising nitrification and denitrification and phosphorus removal
Microfiltration and reverse osmosis

52. Nano-membrane filtration

53. Nutrient Removal
The major components of nutrients in wastewater are nitrates and phosphates. They contribute to the eutrophication of receiving water
Total nitrogen may be about 35 mg/L and total phosphorus 8 mg/L after secondary treatment
Raw sewage composition of C:TN:TP  100:25:6
Normal plant growth only need C:TN:TP of 100:15:1

54. Nitrogen Removal
Involves two stages of microbial action under different conditions
Ammonia is first oxidised to nitrites and nitrates through a process of nitrification by microorganisms
Nitrification uses aerobic autotrophic microorganisms
Dinitrification uses facultative heterotrophic microorganisms under anoxic condition where nitrates are converted to nitrogen gas

55. Phosphorus Removal
Process may be through chemical precipitation or by preferred microbial action
Use of coagulants e.g. lime, aluminium sulfate, ferric chlorine will precipitate phosphorus
Process is expensive and results in quantities of difficult sludge
Preferred process is through microbial action with uptake of phosphorus by a select group of microorganisms

56. Biological phosphorus removal
Modified Bardenpho process

57. End of Module 18