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
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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)
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3. Typical Characteristics of Wastewater
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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
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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
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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
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7. Design Flows
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8. Primary Treatment
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 anaerobic
Processes include screening, grit removal and primary settling
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9. Screens
The removal of large objects that may damage pumps or block channels
Fixed or mechanical
Velocity in channels about m/s
Velocity through openings about m/s
All screenings to be removed/buried
Location of strong odour from decomposition
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10. Mechanical bar screen
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11. Rotating drum screen
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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
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13. Comminutor
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14. 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 constant velocity and aerated/spiral flow tanks
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15. 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
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16. Constant Velocity Grit Chamber
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17. Aerated or Spiral Flow Grit Chamber
Flexibility of control; more efficient grit removal and can assist pre-aeration
Suitable for larger population > ep
HRT of about 3 min at PWWF
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18. Aerated or Spiral Flow Grit Chamber
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19. Vortex Flow Grit Chamber
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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 is applied to overflow rate and 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
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21. Primary Sedimentation
Per cent removed = dh1(R1 + R2)/(2h5) + dh2(R2 + R3)/(2 h5 ) + ...
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22. 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
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
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23. Primary settling % removed vs time
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24. 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
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25. Rectangular horizontal-flow
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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
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27. Up-flow settling tank
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28. Types of Primary Settling tanks
Circular radial flow tank
Radial-horizontal flow
Uses radial scrapers to remove sludge
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29. Circular Radial Flow Tank
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30. Circular Radial Flow Tank
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31. Circular Radial Flow Tank
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32. Pulteney Bridge and Weir, City of Bath
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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
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34. Classification of Microorganisms
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35. Typical microorganisms in activated sludge
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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
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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
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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
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39. Some Concepts of Biological Treatment
Biological growth curve
Food:microorganism ratio ie F/M
Fixed-film (attached) system and suspended growth system
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40. Biological growth curve
Lag phase
Log-growth or exponential phase
Stationary phase
Log-death or endogenous phase
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41. Biomass growth and substrate removal curves
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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
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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
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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
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45. Trickling Filter
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46. Trickling filters at Wetalla
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47. Interaction of biofilm
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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
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49. Trickling Filter (cont)
BOD removal efficiency E = 1/[ (W/VF)]
Speed of distributor is critical
Recirculation ratio
Humus sludge production g/g BOD5 removed
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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
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51. A rotating biological contact unit
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52. 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
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53. Activated Sludge Process
Influent (or settled sewage from primary treatment) enters the reactor (aerator tank) where contaminants are biodegraded by selected microorganisms
Reaction processes lead to the reduction of contaminants and increase of biomass (cells)
In the activated sludge process the biomass is often referred as the mixed liquor volatile suspended solids (MLVSS)
MLSS is the mixed liquor suspended solids (MLVSS  0.8 MLSS)
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54. Activated Sludge Process (cont)
The biomass is separated in a final sedimentation tank (clarifier) as settled sludge and recirculated as return activated sludge (RAS) to the reactor
The clarified effluent is often of a standard that may be discharged into receiving waters
The RAS increases the MLVSS concentration in the reactor
To maintain a designed MLVSS (at steady state) some biomass must be wasted
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55. Activated sludge process with alternative wasting locations
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56. Some Features of the Activated Sludge Process
Design for PDWF and F/M ratio
System is aerobic; requires mg/L DO using diffused air, surface aerators, turbines
Microorganisms are mainly aerobic & facultative heterotrophs and some autotrophs for nitrification.
Microorganisms are kept in suspension by mixing
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57. Some Features of the Activated Sludge Process (cont)
Sludge recycle (RAS) is an essential part of the process.
Owing to recycle the HRT is not the same as the solids retention time (SRT) or sludge age
Sludge age is controlled by wasting the correct mass of sludge daily
c = X V/[Qw Xw + (Q - Qw)Xe]
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58. Some Features of the Activated Sludge Process (cont)
Mixed liquor suspended solids (MLSS) is a mixture of microorganisms and particulate matter
MLSS serves as a quantitative measure of activated sludge concentration
Final clarifiers separate the MLSS from the treated wastewater using class III and IV for settling and thickening sludge
Clarifier tanks are usually circular m dia. and depth is important m
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59. Some Features of the Activated Sludge Process (cont)
Mixing regimes in reactor tanks may be plug flow or completely mixed system
Several variations of activated sludge processes are possible
These range from the conventional systems with high F/M to extended aeration plants with low F/M
Better effluent quality from activated sludge plants compared with trickling filters
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60. Hydraulic Characteristics of Reactor Tanks
Plug flow system
Each element has the same residence time
Long and narrow in dimension
No longitudinal mixing
BOD highest at inlet
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61. Hydraulic Characteristics of Reactor Tanks
Plug flow system (cont)
DO lowest at inlet
Lower average MLSS
Theoretically more efficient than completely mixed flows
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62. Hydraulic Characteristics of Reactor Tanks
Completely-mixed system
Each element may not have the same HRT
Continuous and thorough mixing
Rectangular tanks, typically 6 -7 m width x 3-5 m depth
Uniform MLSS and BOD
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63. Hydraulic Characteristics of Reactor Tanks
Completely-mixed system (cont)
Higher MLSS
Substrate concentration in tank and effluent are equal
Better resistance to shock hydraulic and pollutant loads
Better resistance to toxic loads
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64. In practice non-ideal flow occurs
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65. Areas of short-circuiting and incomplete mixing
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66. Aeration
Two-film theory - a physical mass transport across gas film and liquid film
For the transfer of gas molecules from the gas phase to the liquid phase, slightly soluble gases encounter the primary resistance from the liquid film
Very soluble gases encounter the primary resistance to transfer from the gaseous film
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67. Two-film gas-liquid transfer
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68. Aeration devices
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69. Aeration (cont)
Aim of the aerator - to increase O2 transfer from liquid film to the bulk liquid at a rate sufficient to meet the O2 demands of metabolism
A major energy consuming process
KLa is the overall oxygen mass transfer coefficient. It is a function of the equipment, tank geometry and wastewater characteristics
Oxygen transfer rate, OTR = KLa C20 V kg O2/h
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70. Aeration (cont)
OTRfield =
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71. Dome diffuser
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72. Aeration (cont)
Function of O2 in activated sludge is a two stage process
Aeration provides the DO (electron acceptor) for aerobic metabolism
DO of mg/L is necessary for aerobic condition
Aeration must balance the oxygen uptake by the microorganisms
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73. Surface brushes
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74. Surface aerators
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75. Floating surface aerator
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76. Separation of the treated wastewater from the solids
Occurs after the biological or transformation process
A physical process of settling generally in a separate tank
In some processes this removal of solids can also occur in the same tank but separated in time
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77. Final Sedimentation Tank
A physical separation process to settle the solids (microorganisms, particulate) from the clarified effluent
Thicken sludge is returned to the reactor tank
Design for 3 x ADWF or PWWF
Class III and IV settlings; depth is relevant
Weir overflow rate < 250 m3/m.d
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78. Final sedimentation tank
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79. Hindered zonal settling
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80. Final clarifier
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81. Final Sedimentation Tank (cont)
Involves 2 important functions
Clarification
Hydraulic loading must not exceed the settling velocity of the slowest settling particle
vs = Q/A ie m3/m2.d for activated sludge
HRT  1.5 to 2 h
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82. Final Sedimentation Tank (cont)
Thickening capacity is based on the Solids Flux theory
A concept of maximum quantity of solids that can be handled by a settling tank at a given underflow removal rate without affecting performance. It involves the solids loading rate
GL = Q(1 + R)X/(1000A) kg/m2.d
Measuring the settleability of sludge, SVI
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83. Analysis of solids flux
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84. Sludge Volume Index (SVI)
A criterion for measuring the settleability of sludge
It is related to the recycling of activated sludge
SVI is defined as the settled volume of sludge (mL/L) in 30 minutes per unit MLSS (mg/L)
SVI of mL/g indicate good dense sludge
SVI > 150 mL/g are light, poorly compacting (bulking sludge)
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85. Factors affecting SVI
Sewage composition; relationship between zoogloeal and filamentous growth are dependent on industrial wastes, carbohydrates etc
Degree of longitudinal mixing in reactor tank; plug flow is less prone to bulking
Anoxic conditions and nitrifying systems result in low SVI
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86. Return Activated Sludge (RAS)
Rate of return, R = 100/[106 /(X.SVI) - 1]
Represents the underflow of the final clarifier to the reactor tank
An essential feature of the activated sludge system to maintain the desired MLSS
Rate of return activated sludge varies from 20 to 150% of ADWF
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87. Types of Activated Sludge Systems
Conventional activated sludge
Operates at F/M ratios of 0.2 to 0.5
Design to remove BOD and may also nitrify
Plug flow, limited longitudinal mixing, spiral flow along tank through diffusers
Reactor tanks are long, narrow up to 150 m length; W:D = 1:1 to 2.2:1; D = 3 to 5 m; W = 6 to 12 m
Limited resistance to shock and toxic loads
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88. Types of Activated Sludge Systems
Continuous extended aeration process
Systems operate with low organic loadings (F/M); high c and high HRT
Process minimises sludge handling, consequently have no primary sedimentation tanks
Increased endogenous respiration results in less sludge, but increase O2 demand
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89. Types of Activated Sludge Systems
Continuous extended aeration process (cont)
Exhibits completely mixed; hence more stable to fluctuations in flow and loading (organic); requires less stringent recycle
Examples are continuous oxidation ditches eg. carousels
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90. Types of Activated Sludge Systems
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91. A comparison of Activated Sludge Systems
Conventional
Extended aeration
Large flows
Small flows
Plug flow
Completely mixed
HRT 4 – 8 h
HRT 18 – 36 h
F/M 0.2 – 0.4
F/M 0.04 – 0.15
Sludge age 5 – 15 d
Sludge age > 15 d
MLSS 1500 – 3000 mg/L
MLSS 3000 – 6000 mg/L
BOD removed 80 –90%
BOD removed 85 – 95%
R 0.25 – 0.5
R 0.75 – 1.5
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92. Continuous extended aeration process
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93. Intermittent Decanting Extended Aeration (IDEA)
Biological oxidation and final clarification occur in the same tank: functions are only separated in time
Primary treatment is not necessary
Treated and clarified water is decanted intermittently but raw sewage is fed continuously
Sludge is wasted during the aeration cycle to maintain a constant MLSS of mg/L
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94. Pasveer Oxidation Ditch
An example of the intermittent decanting extended aeration process serving 500 to 2000 ep
4 hours operating cycle for normal operation; 3 phases per cycle controlled by an automatic timing device
Aeration h
Settling h
Effluent decanting 0.5 h
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95. Major advantages of the IDEA process
Cheaper than continuous activated sludge systems
Easily modified to remove nutrients
Easy operation and minimum attendance
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96. Disadvantages of the IDEA process
High operating energy requirements
Sludges are often difficult to settle
Not suitable for large flows
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97. Disinfection
Secondary treatment will remove up to 98% of microorganisms and /100 mL of coliform remains
Chlorine remains the common disinfection agent
A contact time of minutes is required
Much debate continues on the use of chlorine
Other environmentally friendly methods are preferred such as:
UVL, ozone, membrane filtration, artificial wetlands
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98. Nano-membrane filtration
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99. 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
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100. Nitrification
In the nitrogen cycle, organic and ammonium nitrogen are converted first to nitrite and then to nitrate
Sources:
Organic nitrogen (40%)
Ammonium-nitrogen (NH4+–N 60%)
Nitrite-nitrogen (NO2—N)
Nitrate-nitrogen (NO3--N)
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101. Nitrification (cont)
Ammonia in wastewater is toxic to fish; it has a high O2 demand; it increases Cl2 demand during disinfection
Primary treatment removes < 20% influent nitrogen
Secondary treatment removes about 30% cumulative
Limit for ammonium-N in treated effluent < 2 mg/L
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102. Nitrification (cont) NO2- + (1/2)O2 = NO3-
Nitrification is a 2-stage process by different types of aerobic autotrophic bacteria
NH4+ + (3/2)O2 = NO H+ + H2O
NO2- + (1/2)O2 = NO3-
Nitrifying bacteria are sensitive to toxic substances; grow more slowly (high c); optimum temp 28-39oC and decreases with low temp
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103. Nitrification (cont) Operating pH 6.5 – 8
Nitrification reduces alkalinity (7.1 g of alkalinity as CaCO3 is exhausted by 1 g NH4+-N) nitrified)
Nitrification is adversely affected by F/M > 0.4 – 0.6
Minimum DO 1.5 mg/L is required
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104. Denitrification
Conversion of nitrates (derived from nitrification) to nitrogen gas
Denitrification is a type of respiration carried out by facultative heterotrophs; a process known as anoxic as NO3- is the terminal electron acceptor
Organic-C + NO3- = CO2 + H2O + OH- + N2 + energy
Alkalinity is increase but by about half the amount removed by nitrification
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105. Denitrification (cont)
DO inhibits denitrification
A carbon source must be available (external or recycled endogenous carbon)
Some BOD is removed but more slowly than aerobic respiration
Denitrification can be induced in the anoxic part of fixed growth systems by making the filter bed deeper (2.5 – 3 m) but use of activated sludge is the normal process
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106. Denitrification (cont)
In conventional activated sludge, the anoxic zone within the reactor tank may be 30 – 40% of volume and precedes the aerobic zone
In carousel systems, the establishment of sequential aerobic zones coupled with long HRT and high c promote endogenous denitrification
Denitrification can be achieved in separate reactors using suitable organic source
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107. Phosphorus Removal
Sources are from domestic wastewater, trade and agricultural wastes; usually present in 3 forms
Orthophosphate (removed by chemical/or biological processes)
Polyphosphate
Organic phosphorus
Polyphosphate and organic phosphorus are less easily removed until transformed to orthophosphate after secondary treatment
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108. Phosphorus Removal (cont)
About 10% of insoluble phosphorus can be removed by primary settling
Conventional biological treatment removes a further % by assimilation during biomass growth, but a well designed BNR (biological nutrient removal) plant can remove up to 95% of P
Almost all soluble phosphorus can be removed by chemical precipitation
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109. Phosphorus Removal (cont)
Using chemical precipitation
Lime, Ca2+
Aluminium sulfate, Al3+
Ferrous sulfate (pickle liquor), Fe2+
Ferric chloride / ferric sulfate, Fe3+
Relative cost of coagulants, Al3+ > Fe3+ > Fe2+ > Ca2+
pH range for aluminium and iron salts 5.5 to 7
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110. Phosphorus Removal (cont)
A more efficient process is biological phosphorus removal
One example is the modified University of Cape Town model (UCT) for biological nutrient removal
Denitrifying plants can be modified by a fermentation zone at the head of the aeration tank
Selective growth of bacteria (acinetobacter) absorbs the phosphorus
Daily wasting of activated sludge removes the stored phosphorus
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111. Biological phosphorus removal
Modified Bardenpho process
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112. Biological phosphorus removal
Phosphate transport in and out of bacteria
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113. End of Module 8
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