Wastewater Treatment

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 • Protect public health from contamination of water supplies • Reliable and economic operation • Minimum capital cost Aims

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

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 40 - 75% of TSS and about 25 - 40% 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 0.3 - 0.4 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 0.2 - 1 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 0.25 - 0.35 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 > 10 000 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 40 - 75% suspended solids • Some incidental BOD5 reduction 25 - 40% • 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 10 - 15 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 • Inflow to a central stilling box • Radial-horizontal flow • Uses radial scrapers to remove sludge Circular radial flow tank

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

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 .

48. High rate two-stage sludge digester .

49. Anaerobic sludge digester .

50. Aerobic sludge digester .