Wastewater Treatment

1
Wastewater Treatment
On completion of this segment you should be

• Aware of the public health aspects and goals of
  wastewater treatment
• Able to describe 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
• Aware of some methods available for nutrient
  removal

2
Wastewater Treatment Goals
Aims

• Protect public health from contamination of water
  supplies
• Reliable and economic operation
• Minimum capital cost

3
Wastewater Treatment Goals (cont)
Outcomes

• Removal of floating, suspended and soluble matter
• Reduce BOD, COD pathogenic organisms and nutrient
• Maintain aesthetics of natural water bodies,
  ecology of water systems

4
Typical Characteristics of Wastewater
5
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

6
Primary Treatment

• Usually 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
  'stale
• Processes include screening, grit removal and
  primary settling

7
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
• Design for PWWF
• All screenings to be removed/buried
• Location of strong odour from decomposition

8
Mechanical bar screen
9
Rotating drum screen
10
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

11
Comminutor
12
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 constantly
  velocity and aerated/spiral flow tanks

13
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

14
Constant Velocity Grit Chamber
15
Aerated or Spiral Flow Grit Chamber

• Flexibility of control more efficient grit
  removal and can assist pre-aeration
• Air supply or spiral flow controls the amount of
  silt removed
• Suitable for larger population gt 10 000 ep
• HRT of about 3 min at PWWF

16
Aerated or Spiral Flow Grit Chamber
17
Vortex Flow Grit Chamber
18
Primary Sedimentation
Solids separation by gravity

• Aim is to remove gross suspended solids (organic
  matter)
• Largely class II settling of flocculent matter
  and natural coalescence or flocculation occurs
• Surface skimmers remove floating matter (scum,
  grease etc)
• The settled solids are pumped to an anaerobic
  digestion tank. The effluent (settled sewage)
  from primary treatment flows to the next stage
  ie. secondary treatment

19
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
• Hydraulic retention time (HRT) 1.5 to 3 h depth
  2.5 to 5 m
• Also act as flow/strength equalisation basins
• Sludge scrapers should not cause re-suspension

20
Primary settling removed vs time
21
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

22
Rectangular horizontal-flow
23
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

24
Up-flow settling tank
25
Types of Primary Settling tanks
Circular radial flow tank

• Inflow to a central stilling box
• Radial-horizontal flow
• Uses radial scrapers to remove sludge

26
Circular Radial Flow Tank
27
Circular Radial Flow Tank
28
Circular Radial Flow Tank
29
Pulteney Bridge and Weir, City of Bath
30
Secondary Treatment
Removal of dissolved solids through microbial
action

• Objective is to remove the remaining suspended
  solids and also dissolved solids
• The process is mainly biological using
  microorganisms to convert the dissolved solids to
  biomass
• Two distinct systems are available i.e. fixed
  film (trickling filter) and suspended growth
  (activated sludge)
• The biomass is removed as sludge in final
  sedimentation tanks (clarifiers)

31
Typical microorganisms in activated sludge
32
Fixed-Film Systems

• Land treatment, trickling and rotating biological
  filters are predominantly aerobic biological
  processes
• Land treatment ie. broadcasting of sewage, is one
  of the earliest forms of wastewater treatment

33
Trickling Filter

• Comprising an inert structure for growth of
  biofilm containing microorganisms (attached
  growth)
• Microorganisms in biofilm interact with
  wastewater and metabolise the organic matter
  (BOD) into CO2 and H2O
• Natural sloughing of the biofilm when it reaches
  a thickness that cannot be sustained
• Filter medium voids (40 60) promote air
  circulation and aerobic condition
• Solids in the effluent are separated in the
  secondary settling (humus) tank

34
Interaction of biofilm
35
Trickling Filter
36
Trickling filters at Wetalla
37
A rotating biological contact unit
38
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

39
Activated Sludge Process

• The heart of the process is the reactor where
  aeration and oxidation of organic compounds occur
• Microorganisms are held in suspension by aeration
  and stirring
• Energy requiring process but has greater control
  and flexibility
• Return activated sludge and sludge wasting
  maintain the design biomass concentration (MLVSS)
• Final clarifier separates solids from the clear
  effluent and returns the settled sludge to the
  reactor

40
Activated sludge process with alternative wasting
locations
41
Surface aerators
42
Final sedimentation tank
43
Final clarifier
44
Comparison between attached film and suspended
growth systems
Parameter Trickling filter Activated sludge
BOD removal 85 90 gt 95
Lower limit of BOD effluent 15 mg/L lt 10 mg/L
Capital cost High Moderate
Operating cost Minimal High
Land requirement High Low
Operator control Limited More
Shock loads Rapid recovery Very slow
Foaming None Often
Odour Yes Minimal
Filter flies Yes None
Noise Minimal Moderate
Hydraulic washout No Yes
Plugging Yes No
Drying of media Yes No
Output of sludge moderate High
45
Wastewater Disinfection

• Some microorganisms (105 107/100 mL) are still
  present in treated wastewater after secondary
  treatment
• Disinfection is required to reduce pathogenic
  microorganisms
• Chlorine is still the cost-effective
  disinfection, but requires minimum contact time
  and has adverse effects
• Other environmental friendly methods include UVL,
  ozone disinfection, membrane microfiltration and
  constructed wetlands

46
Sludge Digestion

• Sludge from primary and secondary settling tanks
  (including waste activated sludge) must be
  treated in digesters
• Sludge is thickened before passing to sludge
  digesters
• Sludge may be treated anaerobically or
  aerobically
• Anaerobic sludge digestion involves 2 sequential
  stages ie. acid formation and methane formation
• Digested sludge is dewatered before disposal

47
Low rate single-stage sludge digester
.
48
High rate two-stage sludge digester
.
49
Anaerobic sludge digester
.
50
Aerobic sludge digester
.
51
Tertiary Treatment

• Tertiary maturation ponds an aerobic polishing
  process with detention time and further reduction
  in BOD and TSS (NFR)
• Nutrient removal comprising nitrification and
  denitrification and phosphorus removal
• Microfiltration and reverse osmosis

52
Nano-membrane filtration
53
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

54
Nitrogen Removal
Involves two stages of microbial action under
different conditions

• Ammonia is first oxidised to nitrites and
  nitrates through a process of nitrification by
  microorganisms
• Nitrification uses aerobic autotrophic
  microorganisms
• Dinitrification uses facultative heterotrophic
  microorganisms under anoxic condition where
  nitrates are converted to nitrogen gas

55
Phosphorus Removal
Process may be through chemical precipitation or
by preferred microbial action

• Use of coagulants e.g. lime, aluminium sulfate,
  ferric chlorine will precipitate phosphorus
• Process is expensive and results in quantities of
  difficult sludge
• Preferred process is through microbial action
  with uptake of phosphorus by a select group of
  microorganisms

56
Biological phosphorus removal
Modified Bardenpho process
57
End of Module 18