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Transcript of 1 Wastewater Treatment Aware of the public health aspects and goals of wastewater treatment Able to...
1
Wastewater Treatment
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
On completion of this segment you should be:
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Wastewater Treatment Goals
Protect public health from contamination of water supplies
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 of Wastewater
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 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
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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
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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 damage pumps or block channels
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Mechanical bar screen
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Rotating drum screen
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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
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Comminutor
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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
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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 can assist 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 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
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 removed hydrostatically
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 Flow
Tank
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Circular Radial Flow Tank
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Pulteney Bridge and Weir, City of
Bath
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Secondary Treatment
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)
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 biological processes
Land treatment ie. broadcasting of sewage, is one of the earliest forms of wastewater treatment
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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
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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 biological
contact unit
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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
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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
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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 and suspended 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 None
Noise 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 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
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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
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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 with detention 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 nitrates and phosphates. They contribute to the eutrophication of receiving 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 heterotrophic microorganisms under anoxic condition where nitrates are converted to nitrogen gas
Involves two stages of microbial action under different 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 difficult sludge
Preferred process is through microbial action with uptake of phosphorus by a select group of microorganisms
Process may be through chemical precipitation or by preferred microbial action
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Biological phosphorus removal
Modified Bardenpho process
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End of Module 18