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Wastewater Treatment
Aware of the public health aspects and goals of wastewatertreatment
Able to describe the processes involved in primary, secondaryand 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
On completion of this segment you should be:
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Wastewater Treatment Goals
Protect public health from contamination of watersupplies
Reliable and economic operation
Minimum capital cost
Aims
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Wastewater Treatment Goals (cont)
Removal of floating, suspended and soluble matter
Reduce BOD, COD pathogenic organisms and nutrient
Maintain aesthetics of natural water bodies, ecology of water
systems
Outcomes
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Typical Characteristics ofWastewater
Oxygen demand, BOD5 mg/L 200 - 400
Total suspended solids mg/L 200 - 300
Nitrogen mg/L 20 - 30 as NH3
30 - 70 total
Phosphorus, mg/L 8 - 16 total
Total dissolved solids, mg/l 400 - 600
Toxins eg metals, organics
Grease and oil
Total coliform number/mL 105
- 106
Fecal coliform, number/mL 104 - 105
Fecal streptococci number/mL 103 - 104
Enteric virus number/mL 10 - 102
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Treatment Selection
Wastewater treatment comprises primary, secondary andtertiary treatments
The selection of appropriate treatment processes isdependent upon the nature and strength of pollutants, quantityof flow, and discharge licence conditions
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Primary Treatment
Usually the first stage of wastewater treatment compriseslargely 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 andpre-chlorination if the influent is 'stale
Processes include screening, grit removal and primarysettling
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Screens
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
The removal of large objects that may damagepumps or block channels
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Mechanical
bar screen
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Comminutors
These are mechanical cutting screens thatreduce the size of large objects
Shredded matter are returned to the flowstream
A by-pass may be included
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Comminutor
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Grit Chambers
Purpose is to remove inorganic grit/sand 0.2 - 1 mm sizethrough differential settling
Aim is to prevent damage to pumps, blockage of channelsand cementing of sludge in settling tanks
Two types of grit chambers, namely constantly velocity andaerated/spiral flow tanks
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Constant Velocity Grit Chamber
Class I settling - horizontal flow
Uniform velocity at 0.25 - 0.35 m/s
Ideal parabolic shape or approximation
Width:depth ratio 1:1
Length 18 x max. depth
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Constant Velocity Grit Chamber
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Aerated or Spiral Flow Grit Chamber
Flexibility of control; more efficient grit removal and canassist pre-aeration
Air supply or spiral flow controls the amount of silt removed
Suitable for larger population > 10 000 ep
HRT of about 3 min at PWWF
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Aerated or Spiral Flow Grit Chamber
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Vortex Flow Grit Chamber
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Primary Sedimentation
Aim is to remove gross suspended solids (organic matter)
Largely class II settling of flocculent matter and naturalcoalescence or flocculation occurs
Surface skimmers remove floating matter (scum, greaseetc)
The settled solids are pumped to an anaerobic digestiontank. The effluent (settled sewage) from primary treatmentflows to the next stage ie. secondary treatment
Solids separation by gravity
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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
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Primary settling %removed vs time
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Types of Primary Settling tanks
Tanks use less space
Forward velocity 10 - 15 mm/s
Weir loading rate < 300 m3/m.d
Length:width ratio 3:1
Rectangular horizontal-flow
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Rectangular horizontal-flow
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Types of Primary Settling tanks
Square with 60o sludge hopper
No moving parts as sludge is removedhydrostatically
Some possible particle carry over
Up-flow tank
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Up-flow settling tank
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Types of Primary Settling tanks
Inflow to a central stilling box
Radial-horizontal flow
Uses radial scrapers to remove sludge
Circular radial flow tank
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Circular Radial Flow Tank
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Circular
Radial FlowTank
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Circular Radial Flow Tank
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Pulteney Bridgeand Weir, City of
Bath
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Secondary Treatment
Objective is to remove the remaining suspended solidsand also dissolved solids
The process is mainly biological using microorganisms toconvert the dissolved solids to biomass
Two distinct systems are available i.e. fixed film (tricklingfilter) and suspended growth (activated sludge)
The biomass is removed as sludge in final sedimentationtanks (clarifiers)
Removal of dissolved solids through microbial action
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Typical microorganisms in activated sludge
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Fixed-Film Systems
Land treatment, trickling and rotating biological
filters are predominantly aerobic biologicalprocesses
Land treatment ie. broadcasting of sewage, isone of the earliest forms of wastewater
treatment
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Trickling Filter
Comprising an inert structure for growth of biofilmcontaining microorganisms (attached growth)
Microorganisms in biofilm interact with wastewaterand metabolise the organic matter (BOD) intoCO
2
and H2
O
Natural sloughingof the biofilm when it reaches athickness that cannot be sustained
Filter medium voids (40 60%) promote air
circulation and aerobic condition
Solids in the effluent are separated in thesecondary settling (humus) tank
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Interaction of biofilm
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Trickling Filter
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Trickling filters at Wetalla
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A rotating
biologicalcontact unit
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Suspended Growth Systems
Microorganisms are held in suspension as a highconcentration 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 anaerobic suspended growth system. The anaerobic
digester for the break down of waste sludge is anexample of an anaerobic suspended growth system
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Activated Sludge Process
The heart of the process is the reactor where aerationand oxidation of organic compounds occur
Microorganisms are held in suspension by aeration and
stirring
Energy requiring process but has greater control andflexibility
Return activated sludge and sludge wasting maintain the
design biomass concentration (MLVSS)
Final clarifier separates solids from the clear effluent andreturns the settled sludge to the reactor
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Activated sludge process with alternative wasting locations
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Surface aerators
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Final sedimentation tank
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Final clarifier
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Comparison between attached film andsuspended growth systems
Parameter Trickling filter Activated sludge
BOD removal 85 90% > 95%
Lower limit of BOD effluent 15 mg/L < 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 NoneNoise Minimal Moderate
Hydraulic washout No Yes
Plugging Yes No
Drying of media Yes No
Output of sludge moderate High
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Wastewater Disinfection
Some microorganisms (105 107/100 mL) are still present intreated wastewater after secondary treatment
Disinfection is required to reduce pathogenic microorganisms
Chlorine is still the cost-effective disinfection, but requiresminimum contact time and has adverse effects
Other environmental friendly methods include UVL, ozone
disinfection, membrane microfiltration and constructedwetlands
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Sludge Digestion
Sludge from primary and secondary settling tanks (includingwaste 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
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Low rate single-stage sludge digester
.
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High rate two-stage sludge digester
.
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Anaerobic sludge digester
.
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Aerobic sludge digester
.
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Tertiary Treatment
Tertiary maturation ponds an aerobic polishing process withdetention time and further reduction in BOD and TSS (NFR)
Nutrient removal comprising nitrification and denitrification
and phosphorus removal
Microfiltration and reverse osmosis
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Nano-membrane filtration
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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 nitratesand phosphates. They contribute to the eutrophication ofreceiving water
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Nitrogen Removal
Ammonia is first oxidised to nitrites and nitrates through
a process of nitrification by microorganisms
Nitrification uses aerobic autotrophic microorganisms
Dinitrification uses facultative heterotrophicmicroorganisms under anoxic condition where nitrates
are converted to nitrogen gas
Involves two stages of microbial action underdifferent conditions
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Phosphorus Removal
Use of coagulants e.g. lime, aluminium sulfate, ferric
chlorine will precipitate phosphorus
Process is expensive and results in quantities of difficultsludge
Preferred process is through microbial action with uptakeof phosphorus by a select group of microorganisms
Process may be through chemical precipitation orby preferred microbial action
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Biological phosphorus removal
Modified Bardenpho process
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End of Module 18