Wastewater Treatment

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

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)

3
Typical Characteristics of Wastewater
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

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

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

7
Design Flows

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

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

10
Mechanical bar screen
11
Rotating drum screen
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

13
Comminutor
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

15
Constant Velocity Grit Chamber

• Class I settling - horizontal flow
• Uniform velocity at 0.25 - 0.35 m/s
• Ideal parabolic shape or approximation
• Widthdepth ratio 11
• Length ? 18 x max. depth

16
Constant Velocity Grit Chamber
17
Aerated or Spiral Flow Grit Chamber

• Flexibility of control more efficient grit
  removal and can assist pre-aeration
• Suitable for larger population gt 10 000 ep
• HRT of about 3 min at PWWF

18
Aerated or Spiral Flow Grit Chamber
19
Vortex Flow Grit Chamber
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 Rn1)/(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
Primary Sedimentation
Per cent removed dh1(R1 R2)/(2h5) dh2(R2
R3)/(2 h5 ) ...
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 gt 3 m/s
• Sludge scrapers should not cause re-suspension

23
Primary settling removed vs time
24
Types of Primary Settling tanks
Rectangular horizontal-flow

• Tanks use less space
• Forward velocity 10 - 15 mm/s
• Weir loading rate lt 300 m3/m.d
• Lengthwidth ratio 31

25
Rectangular horizontal-flow
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

27
Up-flow settling tank
28
Types of Primary Settling tanks
Circular radial flow tank

• Radial-horizontal flow
• Uses radial scrapers to remove sludge

29
Circular Radial Flow Tank
30
Circular Radial Flow Tank
31
Circular Radial Flow Tank
32
Pulteney Bridge and Weir, City of Bath
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

34
Classification of Microorganisms
35
Typical microorganisms in activated sludge
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

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

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

39
Some Concepts of Biological Treatment

• Biological growth curve
• Foodmicroorganism ratio ie F/M
• Fixed-film (attached) system and suspended growth
  system

40
Biological growth curve

• Lag phase
• Log-growth or exponential phase
• Stationary phase
• Log-death or endogenous phase

41
Biomass growth and substrate removal curves
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

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

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

45
Trickling Filter
46
Trickling filters at Wetalla
47
Interaction of biofilm
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

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

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

51
A rotating biological contact unit
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

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)

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

55
Activated sludge process with alternative wasting
locations
56
Some Features of the Activated Sludge Process

• Design for PDWF and F/M ratio
• System is aerobic requires 0.5 - 2 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
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

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 10 - 30 m
  dia. and depth is important 4 - 4.5 m

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

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

61
Hydraulic Characteristics of Reactor Tanks
Plug flow system (cont)

• DO lowest at inlet
• Lower average MLSS
• Theoretically more efficient than completely
  mixed flows

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

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

64
In practice non-ideal flow occurs
65
Areas of short-circuiting and incomplete mixing
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

67
Two-film gas-liquid transfer
68
Aeration devices
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

70
Aeration (cont)
OTRfield
71
Dome diffuser
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 0.5 - 2 mg/L is necessary for aerobic
  condition
• Aeration must balance the oxygen uptake by the
  microorganisms

73
Surface brushes
74
Surface aerators
75
Floating surface aerator
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

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 lt 250 m3/m.d

78
Final sedimentation tank
79
Hindered zonal settling
80
Final clarifier
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. 30 - 40 m3/m2.d for activated
  sludge
• HRT ? 1.5 to 2 h

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

83
Analysis of solids flux
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 50 - 100 mL/g indicate good dense sludge
• SVI gt 150 mL/g are light, poorly compacting
  (bulking sludge)

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

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

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 WD 11 to 2.21 D 3 to 5 m W 6
  to 12 m
• Limited resistance to shock and toxic loads

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

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

90
Types of Activated Sludge Systems
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 gt 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
92
Continuous extended aeration process
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 3500 - 5000 mg/L

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 2.5 h
• Settling 1.0 h
• Effluent decanting 0.5 h

95
Major advantages of the IDEA process

• Cheaper than continuous activated sludge systems
• Easily modified to remove nutrients
• Easy operation and minimum attendance

96
Disadvantages of the IDEA process

• High operating energy requirements
• Sludges are often difficult to settle
• Not suitable for large flows

97
Disinfection
Secondary treatment will remove up to 98 of
microorganisms and 105 - 107/100 mL of coliform
remains

• Chlorine remains the common disinfection agent
• A contact time of 20 - 30 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

98
Nano-membrane filtration
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 CTNTP ? 100256
• Normal plant growth only need CTNTP of 100151

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 (NH4N
60) Nitrite-nitrogen (NO2N) Nitrate-nitrogen
(NO3--N)
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 lt 20 influent nitrogen
• Secondary treatment removes about 30 cumulative
• Limit for ammonium-N in treated effluent lt 2 mg/L

102
Nitrification (cont)

• Nitrification is a 2-stage process by different
  types of aerobic autotrophic bacteria
• NH4 (3/2)O2 NO2- 2H 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

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 gt 0.4
  0.6
• Minimum DO 1.5 mg/L is required

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

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

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

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

108
Phosphorus Removal (cont)

• About 10 of insoluble phosphorus can be removed
  by primary settling
• Conventional biological treatment removes a
  further 15 - 35 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
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 gt Fe3 gt
  Fe2 gt Ca2
• pH range for aluminium and iron salts 5.5 to 7

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

111
Biological phosphorus removal
Modified Bardenpho process
112
Biological phosphorus removal
Phosphate transport in and out of bacteria
113
End of Module 8