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3/31/2011
1
Wastewater Characterisation and Treatment
Recommended text books:
Wastewater Engineering – Metcalf and Eddy
Standard Methods for the Examination of Water and Wastewater
Contact: Benoit Guieysse
RC.2.18
Lecture block outline
The big picture: “Understanding the nature of wastewater is essential in the design and operation of collection, treatment, and reuse facilities – and in the engineering management of environmental quality”
We need to know what’s in it before we can decide what to do with it!
Characterisation
Sampling
Bio pollutants
Chemical pollutants
Physical pollutants
Treatment
Disposal
Tertiary
Secondary
Primary
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10. Wastewater Treatment
Example: City of Toronto, Canada
Typical steps in wastewater treatment: from the most cost-efficient to the least!
Preliminary treatment
Primary treatment
Secondary treatment
Tertiary treatment
Sludge treatment
Removal large debris, greaseEqualization
Removal SS (include some COD)
Removal BOD/COD & nutrients
Removal nutrient & pathogens
Sludge digestion, stabilization &
dewateringTypically:Preliminary and Primary treatments are based on physical mechanismsSecondary = biologicalTertiary = specialized
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Common processes for pollutant removalPollutants Common Processes
Debris, large solids Grid removal
Suspended solids (including VSS) Sedimentation
FOG Dissolved Air Flotation
Dissolved solids Coagulation-Flocculation
Organic pollutants* Biological treatment
N Biological nitrification-denitrification
P Chemical precipitation
Priority pollutants** Adsorption
Pathogens** Biological treatment (maturation ponds) and specific treatment (UV irradiation, chlorination)
* A fraction of organic pollutants found as suspended solids will be removed with solids.** A fraction of priority pollutants and pathogen can be indirectly removed with solids.Membrane filtration (from ultra- to nano-filtration and reverse osmosis) are gaining popularity in situations where reuse is necessary or space seriously limited. Membrane bioreactors combine size separation with biological removal.
Waste water↓
Screening↓
Grit removal↓
Flow balancing (optional) ↓
Dissolved air flotation↓
Sedimentation↓
AEROBICSuspended culture
Attached culture
Specialised Processes
Sludge
SLUDGE TREATMENT
DISPOSAL / REUSEDISPOSAL / REUSE
Sludge
PRIMARY
SECONDARY
TERTIARY
SLUDGE THICKENING/ DEWATERING
� Solids � Landfill
ANAEROBICSuspended cultureAnaerobic lagoon
UASBContact processAttached growthAnaerobic filter
PRELIMINARY
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Important definitionsPollutant loading rate = amount of pollutant reaching a tank/process per unit of
time – usually kg/d
Example: A wastewater containing 100 g COD/m3 and 250 g TS/m3 is treated in a
pond. The wastewater flow rate is 300 m3/d. The organic loading rate = 100×300 =
30,000 g COD/d = 30 kg COD/d. Similarly, the solid loading rate = 75 kg TS/d.
Hydraulic Residence Time (HRT) = amount of time a “liquid volume” introduced
into a tank/process will stay inside the tank/process before being removed
Solid Retention Time (SRT) = amount of time a solid would remain in the system.
Pollutant loading rates and the retention times are important design criteria
during WWT.
Preliminary & Primary Treatment
Objectives: Balancing of flows, screening/settling and fat
removal.
Crucial steps that reduce the pollutant load to the rest of the
facility.
Offer best value for $$
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Examples large debris removal
Need for equalization
1 3 6 9 12
Typical daily variation of municipal wastewater: water use peaks during morning and evening are seen with a 1-2 hours delay at the WWT
0 6 12 18 24
Typical yearly variation of municipal wastewater in South France: Winter peaks reflect storms and summer peaks reflect the increase of population (up to 100 times)!
Month
hours
Flow
/or
gani
clo
adin
gFl
ow/
orga
nic
load
ing
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Balancing and storage
Tank or pond used to store wastewater
Reduce impact of changes in wastewater flow-rate and strength
Adjust pH if needed
Storage capacity in case of breakdown
Useful if wastewater is used for irrigation
Fat removal – Dissolved Air Flotation
http://en.wikipedia.org/wiki/File:DAF_Unit.png
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Primary treatment
Physical separation of solids
from the wastewater by
sedimentation
Examples
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Secondary treatment
“All most all wastewaters containing biodegradable constituents can be treated biologically”. You must understand the processes to ensure the proper conditions are produced/controlled effectively.
Main processes for Secondary treatment
Aerobic
suspendedAerobic attached
Anaerobic suspended
Anaerobic attached
Trickling filterRotative disks
Activated SludgeAerobic ponds
UASBAnaerobic pond
+ multitude of hybrid (anaerobic/aerobic or suspended/attached), anoxic (e.g. NO3
- instead of O2) processes in various combinations. Process can also be classified as batch/continuous etc
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Cell
Products: CH4, oil,
NO3-, N2, NO2
-, CO2,
O2, heat, enzymes,
toxins etc
More cells (containing C, N, and P)
Nutrient sources: N, P, H, O etc
Energy source
(Organic and inorganic compounds)
Carbon source (organic compounds)
Electron acceptor: O2,NO3
-, CO2 etc
Main principle of biological removal:Food + organisms = Products + biomass
Aerobic Carbon removalOrganics compounds are used as sources of carbon and energy
C10H19O3N + O2 + N + P → C5H7NO2 + CO2 + H2O + NH4+ ...
Anaerobic carbon removalOrganics → Biomass + CO2 + CH4 + NH4
+
Organic pollutants are converted into methane (energy), CO2 and biomass (sludge)
These reactions summarize far more complex mechanisms!
Symbolize organic matter in WW =
pollutant. In fact, WW is made up of 1000s
of different compounds
Biomass that must also be removed (sludge).
Note the biomass contains N and P and
can be itself considered as organic pollutant
Nutrients
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Anoxic carbon removal
Anoxic treatment often means that oxygen is absent and replaced by nitrate of
sulfate.
Nitrate instead of oxygen:
C10H19O3N + NO3- + nutrients → C5H7NO2 + CO2 + N2 + ...
This process is also known as denitrification (see N-removal)
Sulfate instead of oxygen:
C10H19O3N + SO42- + nutrients → C5H7NO2 + CO2 + H2S + ...
H2S formation results in bad smells
N-Removal
Biological nitrogen removal is based on the conversion of N (organic or
inorganic) into N2 that escapes into the atmosphere. This is a 2-step process:
1. Nitrification (AEROBIC conditions) – conversion of NH4+ to NO3
-
NH4+ + CO2 + O2 → biomass + NO3
-
.
2. Followed by denitrification (ANOXIC) - conversion of NO3- to N2
C10H19O3N + NO3- → biomass + N2 + CO2
As seen above, biomass is made of C, N and P (C5H7NO2). N and P are therefore
removed by assimilation = uptake into cells during population growth. This is
the main mechanism for N and P removal in ponds
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Bio-P removal: relatively new (and unreliable) technology
‘Special’ organisms take up more P than is required for growth
More traditional approach is P removal by precipitation
Phosphorus Removal
Impact of N and P on NZ freshwater:http://www.mfe.govt.nz/environmental-reporting/freshwater/river/nutrients/
The Activated Sludge process
Reactor – micro-organisms are kept in suspension as flocks
Liquid/solids separator – usually a sedimentation tank
Recycle stream – for maintaining adequate biomass conc.
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Trickling Filters
The filter is a non-submerged, fixed film reactor using rock or plastic
packing (almost all new filters are constructed with plastic packing).
The wastewater is evenly distributed over the top of the bed by a rotary
distributor
The mircro-organisms grow on the packing. Treatment occurs as the
wastewater flows over the film.
Anaerobic treatment
Advantages Disadvantages Less energy required (no need for forced aeration)
Longer start-up time
Less biological sludge production May require alkalinity addition
Fewer nutrients required May require polishing
CH4 production – energy source Bio N and P removal is not possible
Elimination of off-gas air pollution (no forced aeration)
Much more sensitive to lower temps
Potential for production of odours
Potential for production of corrosive gases
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UASB (upflow anaerobic sludge blanket)
Rely on ‘granulation’ which enables very high sludge concentrations to accumulate at the base of the reactor.
Liquid and gas flow suspend granules. Baffles retains bacterial granules, separate gas/liquid.
HRT of 0.5 - 1 d
26
Wastewater Ponds (Lagoons)
This is the more “low-tech” of WWT reactors: usually an earthen basin,
which might be covered with an impermeable liner.
Very common for small communities or
as maturation ponds (tertiary treatment)
Normally in series 2/3 ponds
New Zealand: 200
USA: 3,500+
France: 2,500+
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27
Why
pond
s?Disadvantages Advantages
Easy operation
Primary/secondary/tertiary
Low maintenance
Ready equalisation
High SS in effluent
Process modification and control difficult
Temperature effect
Large land area needed
28
Types of ponds
Anaerobic
Facultative aerobic
Maturation (Aerobic pond)
Surface aerated
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29
Anaerobic ponds
2-5m depth, no algae, HRT of 20-50 d
Relatively small with a high organic load 125-300 kg BOD/ha-day.
40-70% BOD removal.
Problem with methane release in the atmosphere (unless the pond is covered and methane is burned).
Treatment mechanisms in a facultative pond(Source: Wastewater Engineering by Metcalf & Eddy, 1991, pg 437)
Facultative ponds
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31
Facultative ponds
Example of design criteria: 10-350 kg BOD/ha day, 1.0-2m depth, 17-200 d HRT
(for temperate-subtropical weather, 10-350 kg BOD/ha day, 1.5-2m depth, 33-100
d HRT)
90% BOD removal
The algae provide oxygen and capture CO2 during photosynthesis. CO2 capture can
cause the pH to increase, which improves pathogen kills, N stripping and P
precipitation
32
Maturation ponds (aerobic lagoons)Look like facultative, 1-1.5m deep
Low organic loading – well oxygenated
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Aerated pondLagoon depth:1 – 3m
earthen basin
typically, mechanical aeration on floats or fixed platforms
Similar as a facultative ponds with the upper layer is aerated with surface
aerators in order to avoid odor formation!
Aeration is often intermittent (during night)
5-25 days retention times
10 g BOD/m3-d
Irrigation
Salinity: related to electrical conductivity (can use TDS as a measure)
Nutrients: provide fertilizer (phosphorus is often bound in the soil but
nitrogen can leach quite readily).
Fats and biological growth can cause the blocking of sprinkler systems; an
issue for using WW for irrigation.
Discharge areas should be rotated (approx every 20 days) to allow organic
and nutrient conversion
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Sludge disposal
‘Sludge’ = wastes from screens, primarily clarifiers and biosolids from bioreactors
Disposal: land application, landfills, incineration
Sludge processing: key process is thickening – helps transportation, digestion, drying and combustion
Sludge can often be digested anaerobically (more common) or aerobically. Digested sludge can be composted for further stabilization (pathogen removal).
ComparisonCriteria Activated
Sludge Plant
Biological
filter
Aerated
Lagoon
Waste
stabilization pond
Plant
Performance
BOD Removal F F G G
Pathogen removal P P G G
SS Removal G G F F
Economic
Factors
Simple & economic
construction
P P F G
Simple operation P F P G
Land Requirement G G F P
Maintenance cost P F P G
Energy Demand P F P G
Sludge Removal
costs
F F F G
G = Good - F = Fair - P = Poor
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General overviewProcess Applications Advantages Disadvantages Cost
Activated
sludge
Low/moderate
conc.
Proven, good control Emissions of volatile compounds
and aerosols, high sludge
production. Aeration costs
+++
Aerated
lagoons /
ponds
Low conc. Simple, low costs Emissions of volatile compounds
and aerosols, sensitive to shock
and climate, land requirment, no
control
+
Trickling
filter
Low conc.,
recalcitrant
organics
Little sludge,
biodiverse
Emissions of volatile compounds,
sensitive to shocks, clogging, odor
+
Anaerobic
process
High-strength Methane production,
low sludge
Sensitive to temperature, higher
capital costs, odor
++
Source: Environmental Biotreatment. CN Mulligan.This is given as an example only, very specific of North America
Conclusions
Most large wastewater treatment systems are based on activated sludge
variations as secondary treatment because this is the best described process.
The trickling filter is used but there are some operational problems (clogging).
Anaerobic technologies are generally recommended for effluents with high
concentrations of biodegradable organic matter. There are therefore very
commonly used for sludge digestion. They remain limited by odor and
instability issues (temperature, toxicity).
Ponds are common for primary and secondary treatment at small scales
(decentralized treatment) or for tertiary treatment as stabilization ponds.
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Example
Case study: treating dairy wastewaters
Characteristic Concentration
Biochemical oxygen demand 90 - 12,400 (mg/l)
Chemical oxygen demand 180 - 23,000 (mg/l)
Suspended solids 7 - 7,200 (mg/l)
Nitrogen 1 – 70 (mg/l)
Fat 0 – 2100 (mg/l)
Phosphorus (as PO4) 4-150 (mg/l)
pH 3 – 13
Temperature 11 – 72 (oC)
Remember wastewater properties can change in with time and location!
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Dairy wastewater
Approx 14.7 billions litres milk produced in NZ each year (2005/06)
Wastewater produced:
0.5 – 2 m3 wastewater per m3 milk received
This accounts for 7 – 29 billion litres of wastewater produced from milk
production only!
Treatment options
Characteristic Concentration
BOD 90 - 12,400 (mg/l)
COD 180 - 23,000 (mg/l)
SS 7 - 7,200 (mg/l)
N 1 – 70 (mg/l)
Fat 0 – 2100 (mg/l)
P (as PO4-) 4 - 150 (mg/l)
pH 3 – 13
Temperature 11 – 72 (oC)
High BOD and COD values + “good” BOD/COD ratio = plenty of biodegradable organic matter = excellent for anaerobic treatment!
Low concentrations of suspended solids = primary settling might not be efficient
A balance tank would be useful (neutralize pH and temperature) and primary settling would help reduce the suspended solids. Fat removal is often necessary