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Wastewater Treatment On completion of this module you should be:

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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 % 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 Module 8

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 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 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 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 > 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 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 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 % 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 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 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
Circular radial flow tank Radial-horizontal flow Uses radial scrapers to remove sludge 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/[ (W/VF)] Speed of distributor is critical Recirculation ratio Humus sludge production 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

51 A rotating biological contact unit
Module 8

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 Module 8

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) Module 8

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 Module 8

55 Activated sludge process with alternative wasting locations
Module 8

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 Module 8

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] Module 8

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 Module 8

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 Module 8

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 Module 8

61 Hydraulic Characteristics of Reactor Tanks
Plug flow system (cont) DO lowest at inlet Lower average MLSS Theoretically more efficient than completely mixed flows Module 8

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 Module 8

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 Module 8

64 In practice non-ideal flow occurs
Module 8

65 Areas of short-circuiting and incomplete mixing
Module 8

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 Module 8

67 Two-film gas-liquid transfer
Module 8

68 Aeration devices Module 8

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 Module 8

70 Aeration (cont) OTRfield = Module 8

71 Dome diffuser Module 8

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 Module 8

73 Surface brushes Module 8

74 Surface aerators Module 8

75 Floating surface aerator
Module 8

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 Module 8

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 Module 8

78 Final sedimentation tank
Module 8

79 Hindered zonal settling
Module 8

80 Final clarifier Module 8

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 Module 8

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 Module 8

83 Analysis of solids flux
Module 8

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) Module 8

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 Module 8

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 Module 8

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 Module 8

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 Module 8

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 Module 8

90 Types of Activated Sludge Systems
Module 8

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 Module 8

92 Continuous extended aeration process
Module 8

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 Module 8

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 Module 8

95 Major advantages of the IDEA process
Cheaper than continuous activated sludge systems Easily modified to remove nutrients Easy operation and minimum attendance Module 8

96 Disadvantages of the IDEA process
High operating energy requirements Sludges are often difficult to settle Not suitable for large flows Module 8

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 Module 8

98 Nano-membrane filtration
Module 8

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 Module 8

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) Module 8

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 Module 8

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 Module 8

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 Module 8

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 Module 8

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 Module 8

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 Module 8

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 Module 8

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 Module 8

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 Module 8

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 Module 8

111 Biological phosphorus removal
Modified Bardenpho process Module 8

112 Biological phosphorus removal
Phosphate transport in and out of bacteria Module 8

113 End of Module 8 Module 8

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