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Wastewater Treatment

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  1. Wastewater Treatment On completion of this module you should be: • Aware of the public health aspects and goals of wastewater treatment • Able to define the design flows to a wastewater treatment plant • Able to describe and discuss 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 • Able to discuss the methods available for nutrient removal Module 8

  2. Wastewater Treatment Public health aspects of wastewater treatment • 3.4 million people, mostly children, die annually from water-related diseases • 2.4 million people lack access to basic sanitation include the poorest in the world • 1.1 billion people lack access to even improved water sources • Access to safe water supply and sanitation is fundamental for better health, poverty alleviation and development (WHO data) Module 8

  3. Typical Characteristics of Wastewater Module 8

  4. Wastewater Treatment Goals • Minimum capital cost • Reliable and economic operation • Protect public health from contamination of water supplies • Removal of floating, suspended and soluble matter Module 8

  5. Wastewater Treatment Goals (cont) • Reduce BOD, COD, pathogenic organisms and nutrient • Efficient collection system for aerobic conditions • Maintain aesthetics of natural water bodies, ecology of water systems Module 8

  6. 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 Module 8

  7. Design Flows Module 8

  8. Primary Treatment • 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 anaerobic • Processes include screening, grit removal and primary settling Module 8

  9. 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 • Velocity through openings about 0.6 - 1 m/s • All screenings to be removed/buried • Location of strong odour from decomposition Module 8

  10. Mechanical bar screen Module 8

  11. Rotating drum screen Module 8

  12. 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 Module 8

  13. Comminutor Module 8

  14. 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 constant velocity and aerated/spiral flow tanks Module 8

  15. 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 Module 8

  16. Constant Velocity Grit Chamber Module 8

  17. Aerated or Spiral Flow Grit Chamber • Flexibility of control; more efficient grit removal and can assist pre-aeration • Suitable for larger population > 10 000 ep • HRT of about 3 min at PWWF Module 8

  18. Aerated or Spiral Flow Grit Chamber Module 8

  19. Vortex Flow Grit Chamber Module 8

  20. Primary Sedimentation • Largely class II settling of flocculent matter and natural coalescence or flocculation occurs • A test column is used to establish settling characteristics and correction factor of 0.65 - 0.85 is applied to overflow rate and 1.25 - 1.5 to detention time values • Per cent removal = hn(Rn + Rn+1)/(2h) • The settled solids are pumped to an anaerobic digestion tank. The effluent (settled sewage) from primary treatment flows to the next stage i.e. secondary treatment Module 8

  21. Primary Sedimentation Per cent removed = dh1(R1 + R2)/(2h5) + dh2(R2 + R3)/(2 h5 ) + ... Module 8

  22. 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 • HRT 1.5 to 3 h; depth 2.5 to 5 m • Even inlet distribution > 3 m/s • Sludge scrapers should not cause re-suspension Module 8

  23. Primary settling % removed vs time Module 8

  24. 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 Module 8

  25. Rectangular horizontal-flow Module 8

  26. 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 Module 8

  27. Up-flow settling tank Module 8

  28. Types of Primary Settling tanks • Radial-horizontal flow • Uses radial scrapers to remove sludge Circular radial flow tank Module 8

  29. Circular Radial Flow Tank Module 8

  30. Circular Radial Flow Tank Module 8

  31. Circular Radial Flow Tank Module 8

  32. Pulteney Bridge and Weir, City of Bath Module 8

  33. Secondary Treatment • Central process is biological in which dissolved organics are utilised by microorganisms • Hence, secondary treatment is often known as biological treatment • The concomitant growth of biomass (cells) and substrate removal must be followed by separation Module 8

  34. Classification of Microorganisms Module 8

  35. Typical microorganisms in activated sludge Module 8

  36. Biological processes • Aerobic condition – presence of free molecular oxygen • Anaerobic condition – devoid of free molecular oxygen • Anoxic – absence of free molecular oxygen but presence of nitrate Module 8

  37. Types of Metabolism Respiratory metabolism • aerobic microorganisms generate energy by enzyme-mediated electron transport from an electron donor to an external electron acceptor eg O2 • anoxic process uses NO3- and SO42- as the electron acceptors Module 8

  38. Types of Metabolism Fermentative metabolism • anaerobic processes that do not involve an external electron acceptor • process is less energy efficient and is characterised by low growth rates and low cell yield • facultative anaerobes can shift from fermentative to aerobic respiratory metabolism depending on the absence or presence of O2 Module 8

  39. Some Concepts of Biological Treatment • Biological growth curve • Food:microorganism ratio ie F/M • Fixed-film (attached) system and suspended growth system Module 8

  40. Biological growth curve • Lag phase • Log-growth or exponential phase • Stationary phase • Log-death or endogenous phase Module 8

  41. Biomass growth and substrate removal curves Module 8

  42. F/M ratio • Food is the substrate i.e. (Q x S) • Microorganisms i.e. (reactor volume x biomass conc.) • F/M is expressed as t-1 • F/M is used as a preliminary design criterion Module 8

  43. Fixed-Film Systems • Land treatment, trickling and rotating biological filters are predominantly aerobic biological processes • Land treatment i.e. broadcasting of sewage is one of the earliest forms of wastewater treatment Module 8

  44. Trickling Filter • Development of a biofilm on an inert surface where macro and microorganisms break down organic matter • Natural sloughing of the biofilm owing to aerobic growth, decay and shear stress at the interface • Filter medium voids promote air circulation and aerobic condition Module 8

  45. Trickling Filter Module 8

  46. Trickling filters at Wetalla Module 8

  47. Interaction of biofilm Module 8

  48. Trickling Filter (cont) • Design for PWWF • Simplicity in construction but little control • Ease of operation but high initial capital cost • Balance of hydraulic and organic loading necessary to prevent clogging of voids Module 8

  49. Trickling Filter (cont) • BOD removal efficiency E = 1/[1 + 0.44(W/VF)] • Speed of distributor is critical • Recirculation ratio 0.5 - 3 • Humus sludge production 0.3 - 0.5 g/g BOD5 removed Module 8

  50. Rotating Biological Contact Unit • A fixed-film aerobic process comprising of large number of discs rotating half submerged in a tank • Wastewater flows through the tank • Development of biofilm on the disc that interacts with the wastewater • The rotating biological contact units are compact with low energy consumption Module 8