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Module 81 Wastewater Treatment  Aware of the public health aspects and goals of wastewater treatment  Able to define the design flows to a wastewater.

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Presentation on theme: "Module 81 Wastewater Treatment  Aware of the public health aspects and goals of wastewater treatment  Able to define the design flows to a wastewater."— Presentation transcript:

1 Module 81 Wastewater Treatment  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 On completion of this module you should be:

2 Module 82 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) Public health aspects of wastewater treatment

3 Module 83 Typical Characteristics of Wastewater

4 Module 84 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

5 Module 85 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

6 Module 86 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

7 Module 87 Design Flows

8 Module 88 Primary Treatment  The first stage of wastewater treatment comprises largely physical processes.  A well-designed primary treatment should remove about % of TSS and about % BOD 5  A possible pre-treatment is the injection of air, O 2, H 2 O 2 and pre-chlorination if the influent is anaerobic  Processes include screening, grit removal and primary settling

9 Module 89 Screens  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 The removal of large objects that may damage pumps or block channels

10 Module 810 Mechanical bar screen

11 Module 811 Rotating drum screen

12 Module 812 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

13 Module 813 Comminutor

14 Module 814 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

15 Module 815 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

16 Module 816 Constant Velocity Grit Chamber

17 Module 817 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

18 Module 818 Aerated or Spiral Flow Grit Chamber

19 Module 819 Vortex Flow Grit Chamber

20 Module 820 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 =  h n (R n + R n+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

21 Module 821 Primary Sedimentation Per cent removed =  h 1 (R 1 + R 2 )/(2h 5 ) +  h 2 (R 2 + R 3 )/(2 h 5 ) +...

22 Module 822 Some Features of Primary Settling  Design to accept 2 to 3 x ADWF  Removal of % suspended solids  Some incidental BOD 5 reduction %  Hydraulic loading Q/A  30 m 3 /m 2.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

23 Module 823 Primary settling % removed vs time

24 Module 824 Types of Primary Settling tanks  Tanks use less space  Forward velocity mm/s  Weir loading rate < 300 m 3 /m.d  Length:width ratio 3:1 Rectangular horizontal-flow

25 Module 825 Rectangular horizontal-flow

26 Module 826 Types of Primary Settling tanks  Square with 60 o sludge hopper  No moving parts as sludge is removed hydrostatically  Some possible particle carry over Up-flow tank

27 Module 827 Up-flow settling tank

28 Module 828 Types of Primary Settling tanks  Radial-horizontal flow  Uses radial scrapers to remove sludge Circular radial flow tank

29 Module 829 Circular Radial Flow Tank

30 Module 830 Circular Radial Flow Tank

31 Module 831 Circular Radial Flow Tank

32 Module 832 Pulteney Bridge and Weir, City of Bath

33 Module 833 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

34 Module 834 Classification of Microorganisms

35 Module 835 Typical microorganisms in activated sludge

36 Module 836 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

37 Module 837 Types of Metabolism  aerobic microorganisms generate energy by enzyme-mediated electron transport from an electron donor to an external electron acceptor eg O 2  anoxic process uses NO 3 - and SO 4 2- as the electron acceptors Respiratory metabolism

38 Module 838 Types of 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 O 2 Fermentative metabolism

39 Module 839 Some Concepts of Biological Treatment  Biological growth curve  Food:microorganism ratio ie F/M  Fixed-film (attached) system and suspended growth system

40 Module 840 Biological growth curve  Lag phase  Log-growth or exponential phase  Stationary phase  Log-death or endogenous phase

41 Module 841 Biomass growth and substrate removal curves

42 Module 842 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

43 Module 843 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

44 Module 844 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

45 Module 845 Trickling Filter

46 Module 846 Trickling filters at Wetalla

47 Module 847 Interaction of biofilm

48 Module 848 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

49 Module 849 Trickling Filter (cont)  BOD removal efficiency E = 1/[  (W/VF)]  Speed of distributor is critical  Recirculation ratio  Humus sludge production g/g BOD 5 removed

50 Module 850 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

51 Module 851 A rotating biological contact unit

52 Module 852 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 CO 2, H 2 O 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

53 Module 853 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)

54 Module 854 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

55 Module 855 Activated sludge process with alternative wasting locations

56 Module 856 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

57 Module 857 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/[Q w X w + (Q - Q w )X e ]

58 Module 858 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

59 Module 859 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

60 Module 860 Hydraulic Characteristics of Reactor Tanks  Each element has the same residence time  Long and narrow in dimension  No longitudinal mixing  BOD highest at inlet Plug flow system

61 Module 861 Hydraulic Characteristics of Reactor Tanks  DO lowest at inlet  Lower average MLSS  Theoretically more efficient than completely mixed flows Plug flow system (cont)

62 Module 862 Hydraulic Characteristics of Reactor Tanks  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 Completely-mixed system

63 Module 863 Hydraulic Characteristics of Reactor Tanks  Higher MLSS  Substrate concentration in tank and effluent are equal  Better resistance to shock hydraulic and pollutant loads  Better resistance to toxic loads Completely-mixed system (cont)

64 Module 864 In practice non- ideal flow occurs

65 Module 865 Areas of short-circuiting and incomplete mixing

66 Module 866 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

67 Module 867 Two-film gas-liquid transfer

68 Module 868 Aeration devices

69 Module 869 Aeration (cont)  Aim of the aerator - to increase O 2 transfer from liquid film to the bulk liquid at a rate sufficient to meet the O 2 demands of metabolism  A major energy consuming process  K L a is the overall oxygen mass transfer coefficient. It is a function of the equipment, tank geometry and wastewater characteristics  Oxygen transfer rate, OTR = K L a C 20 V kg O 2 /h

70 Module 870 Aeration (cont) OTR field =

71 Module 871 Dome diffuser

72 Module 872 Aeration (cont)  Function of O 2 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

73 Module 873 Surface brushes

74 Module 874 Surface aerators

75 Module 875 Floating surface aerator

76 Module 876 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

77 Module 877 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 m 3 /m.d

78 Module 878 Final sedimentation tank

79 Module 879 Hindered zonal settling

80 Module 880 Final clarifier

81 Module 881 Final Sedimentation Tank (cont)  Involves 2 important functions  Clarification –Hydraulic loading must not exceed the settling velocity of the slowest settling particle –v s = Q/A ie m 3 /m 2.d for activated sludge –HRT  1.5 to 2 h

82 Module 882 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  G L = Q(1 + R)X/(1000A) kg/m 2.d  Measuring the settleability of sludge, SVI

83 Module 883 Analysis of solids flux

84 Module 884 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)

85 Module 885 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

86 Module 886 Return Activated Sludge (RAS)  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 Rate of return, R = 100/[10 6 /(X.SVI) - 1]

87 Module 887 Types of Activated Sludge Systems  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 Conventional activated sludge

88 Module 888 Types of Activated Sludge Systems  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 O 2 demand Continuous extended aeration process

89 Module 889 Types of Activated Sludge Systems  Exhibits completely mixed; hence more stable to fluctuations in flow and loading (organic); requires less stringent recycle  Examples are continuous oxidation ditches eg. carousels Continuous extended aeration process (cont)

90 Module 890 Types of Activated Sludge Systems

91 Module 891 A comparison of Activated Sludge Systems ConventionalExtended aeration Large flowsSmall flows Plug flowCompletely mixed HRT 4 – 8 hHRT 18 – 36 h F/M 0.2 – 0.4F/M 0.04 – 0.15 Sludge age 5 – 15 dSludge age > 15 d MLSS 1500 – 3000 mg/LMLSS 3000 – 6000 mg/L BOD removed 80 –90%BOD removed 85 – 95% R 0.25 – 0.5R 0.75 – 1.5

92 Module 892 Continuous extended aeration process

93 Module 893  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 Intermittent Decanting Extended Aeration (IDEA)

94 Module 894  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 2.5 h  Settling 1.0 h  Effluent decanting0.5 h Pasveer Oxidation Ditch

95 Module 895  Cheaper than continuous activated sludge systems  Easily modified to remove nutrients  Easy operation and minimum attendance Major advantages of the IDEA process

96 Module 896  High operating energy requirements  Sludges are often difficult to settle  Not suitable for large flows Disadvantages of the IDEA process

97 Module 897 Disinfection  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 Secondary treatment will remove up to 98% of microorganisms and /100 mL of coliform remains

98 Module 898 Nano-membrane filtration

99 Module 899 Nutrient Removal  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 The major components of nutrients in wastewater are nitrates and phosphates. They contribute to the eutrophication of receiving water

100 Module 8100 Nitrification In the nitrogen cycle, organic and ammonium nitrogen are converted first to nitrite and then to nitrate Sources: Organic nitrogen (40%) Ammonium-nitrogen (NH 4 + –N 60%) Nitrite-nitrogen (NO 2 — N) Nitrate-nitrogen (NO 3 - -N )

101 Module 8101 Nitrification (cont)  Ammonia in wastewater is toxic to fish; it has a high O 2 demand; it increases Cl 2 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

102 Module 8102 Nitrification (cont)  Nitrification is a 2-stage process by different types of aerobic autotrophic bacteria  NH (3/2) O 2 = NO H + + H 2 O  NO (1/2) O 2 = NO 3 -  Nitrifying bacteria are sensitive to toxic substances; grow more slowly (high  c ); optimum temp o C and decreases with low temp

103 Module 8103 Nitrification (cont)  Operating pH 6.5 – 8  Nitrification reduces alkalinity (7.1 g of alkalinity as CaCO 3 is exhausted by 1 g NH 4 + -N) nitrified)  Nitrification is adversely affected by F/M > 0.4 – 0.6  Minimum DO 1.5 mg/L is required

104 Module 8104 Denitrification  Denitrification is a type of respiration carried out by facultative heterotrophs; a process known as anoxic as NO 3 - is the terminal electron acceptor  Organic-C + NO 3 - = CO 2 + H 2 O + OH - + N 2 + energy  Alkalinity is increase but by about half the amount removed by nitrification Conversion of nitrates (derived from nitrification) to nitrogen gas

105 Module 8105 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

106 Module 8106 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

107 Module 8107 Phosphorus Removal  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 Sources are from domestic wastewater, trade and agricultural wastes; usually present in 3 forms

108 Module 8108 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

109 Module 8109 Phosphorus Removal (cont)  Lime, Ca 2+  Aluminium sulfate, Al 3+  Ferrous sulfate (pickle liquor), Fe 2+  Ferric chloride / ferric sulfate, Fe 3+  Relative cost of coagulants, Al 3+ > Fe 3+ > Fe 2+ > Ca 2+  pH range for aluminium and iron salts 5.5 to 7 Using chemical precipitation

110 Module 8110 Phosphorus Removal (cont)  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 A more efficient process is biological phosphorus removal

111 Module 8111 Biological phosphorus removal Modified Bardenpho process

112 Module 8112 Biological phosphorus removal Phosphate transport in and out of bacteria

113 Module 8113 End of Module 8


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