Transcript Course 2 Unit 5 Introduction to Constructed Wetlands

Course 2 Unit 5 Introduction to Constructed Wetlands
Part A – Overview and types of constructed wetlands
Part B – Basic principles of wastewater treatment in constructed wetlands
Part C – Design of constructed wetlands
Part D – Operation and maintenance of constructed wetlands; examples for greywater treatment
Lecturer: Dr Diederik Rousseau
for info on Diederik see next slide
[email protected]
About Diederik Rousseau
Dr. Diederik Rousseau is now Programme Coordinator & Lecturer
Environmental Sciences at University College West Flanders.
Before that, he was lecturer in Environmental Engineering in the
Department of Environmental Resources at UNESCO-IHE Delft
from October 2005 – end 2011. He holds both an MSc (1999)
and PhD (2005) degree from Ghent University in Belgium in
Applied Biological Sciences - Environmental Technology.
His teaching activities mainly focused on bio-monitoring based on
plankton and macro-invertebrate communities, and on natural
systems for wastewater treatment. Research was oriented on
model-based evaluation of the performance of constructed
wetlands.
Course 2 Unit 5
Note: You will see a number of different names used for the same
system (this can be confusing)
C2U5 Part A
OVERVIEW AND TYPES OF
CONSTRUCTED WETLANDS
Classification overview: soil filters
unplanted filters
(without plants)
vertical flow
horizontal flow
Also called: subsurface biofilters,
percolation beds, infiltration beds or
intermittent sand filters
planted filters
(with plants)
vertical flow
horizontal flow
= constructed wetlands
Prerequisites for being able to use
constructed wetlands
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Wastewater not too toxic for bacteria and plants
Sufficient incident light to allow photosynthesis
Temperature not too low*
Adequate quantities of nutrients to support growth
Detention time long enough
Organic loading not too high (expressed as g
BOD/m2/day)
 Enough space, because it is a low-rate system
* But they also work in cold-climate countries such as Sweden and Norway
Course 2 Unit 5
Applications – types of wastewater
sewage
domestic
combined / conventional
municipal
separate: greywater
dairy
ecosan
cattle
agricultural
swine
poultry
mine drainage
industrial
food processing
coal, metal
winery, abattoir, fish, potato, vegetable, meat,
cheese, sugar , milk productions
heavy industry
runoff
urban
highway
airport
field crop
nursery
greenhouse
Applications of constructed wetlands in
the ecosan context
 Constructed wetlands can be used for:
– Greywater treatment
– Faecal sludge treatment (less common)
– Post-treatment after anaerobic treatment of blackwater
General strong points and disadvantages
of constructed wetlands
Strong points:
Disadvantages/challenges:
 Site location flexibility
(compared to natural wetlands)
 Mosquitoes (in Free Water Surface
Systems)
 Simple operation and
maintenance
 Can be integrated attractively
into landscaping
 Start-up problems
 Space requirement
 Variable performance possible
 Designs still largely empirical (to date)
Classification of constructed wetlands
Based on water flow characteristics
 (free water) surface flow (abbreviated as FWS or SF)
 subsurface flow (abbreviated as SSF)
Based on plant species characteristics
 floating plants (e.g. Lemna, Nymphaea)
 submerged plants (e.g. Elodea)
 emergent plants (e.g. Phragmites, Papyrus)
Course 2 Unit 5
Different groups of macrophytes
Helophytes
Pleustophytes
Hydrophytes
Pleustophytes
(Source: Vymazal et al., 1998)
Types of plants used
Cattail
Reed
Rush
Bulrush
Sedge
And many others
Cattail
Selection of macrophyte species
 Most often used are emergent plants (reeds, rushes,
sedges) and floating plants (water hyacinth, duckweed)
 Recommended to use local, indigenous species and not to
import exotic, possibly invasive species
 Easy lab-scale growth tests can be performed to check
whether or not the plants can survive and grow in the given
wastewater
 Plants should have high biomass production, an extensive
root system and should be able to withstand shock loads
and short dry periods
CONSTRUCTED WETLANDS
Free floating plants
Floating leaved
plants
Type 1
Surface flow
(FWS)
Upflow
Downflow
Emergent
plants
Submerged plants
Sub-surface flow
(SSF)
Vertical flow
(VSSF)
Type 3
Any combination of the above systems is called a “hybrid” system
Horizontal flow
(HSSF)
Type 2
Type 4
Table 1: Rule of thumb area requirements
for different wetlands
Type of CW
Design area requirement
(m2/PE)
Type 1: FWS
5-10
Type 2: HSSF
3-5
Type 3: VSSF
2-3
Type 4: Hybrids (horizontal – vertical, or H-V)
2.5 – 3
(but note: better nitrogen removal
(denitrification) than VSSF)
Important:
1 PE = 1 Person Equivalent = 60 gBOD/cap/d
= 120 L/cap/d (in the Netherlands)
Valid for mixed domestic
wastewater → lower values
for greywater!
Course 2 Unit 5
Example: simple calculation for area
requirement
(see Part C for more details)
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Design for 50 people
Use Type 2 wetland (HSSF)
Take conservative figure of 5 m2/PE
Therefore, an area of 5 x 50 = 250 m2 is needed to
treat the wastewater of these 50 people in a HSSF
constructed wetland
Type 1: FWS (Free water surface flow)
Emergent plants
Slotted pipe for
wastewater
distribution
Effluent outlet
Slope 1%
Rhizome network
Soil, sand
or gravel
Water flows on top of soil medium, water depth < 50 cm deep.
Mostly planted with sedges, reeds, rushes.
This is a very land intensive system (5-10 m2 per PE).
Watertight membrane
FWS - Example of typical lay-out (slide 1 of 3)
diversion weir
pre-settlement
Diversion weir
• normal flow goes to CW
• storm flow bypassed
Photo: Deinze, Belgium
FWS - Example of typical lay-out (slide 2 of 3)
diversion weir
pre-settlement
Presettlement pond, removes large part of
suspended solids, enables easy access for
desludging
Photo: Deurle, Belgium
Course 2 Unit 5
FWS - Example of typical lay-out (slide 3 of 3)
diversion weir
pre-settlement
Wetland basins, serpentine shape (high
length-to-width ratio) promotes plug flow and
avoids dead zones.
Photo: Kruishoutem, Belgium
Type 2: HSSF or vegetated submerged beds
(horizontal subsurface flow)
Emergent plants
Slotted pipe for
wastewater
distribution
Effluent
outlet
Slope 1%
Rhizome network
Soil, sand
or gravel
Watertight membrane
Water flows inside a layer of sand, gravel or soil (60-80cm).
Most often emergent plants like cattails or reeds are used. Amount of land
reduced (3-5 m2 per PE) compared to FWS CW.
HSSF - Example of typical lay-out (slide 1 of 3)
septic tank
gravel, sand or soil bed planted with
emergent macrophytes
Adequate pretreatment extremely important to avoid clogging (reduction
of pore volume by accumulation of solids).
HSSF - Example of typical lay-out (slide 2 of 3)
The inlet zone is filled with small
rocks or coarser gravel. Together
with multiple vertical riser pipes,
this ensures that the wastewater
is distributed equally over the
entire width and depth of the
wetland.
Photo: Zemst, Belgium
Course 2 Unit 5
HSSF - Example of typical lay-out (slide 3 of 3)
Inlet zone (small rocks)
Gravel bed (fine gravel)
Outlet zone (small rocks)
Type 3: VSSF or “infiltration beds” (vertical
subsurface flow)
Water is pumped on the surface and then drains down through the filter layer which
consists of coarse sand or fine gravel. Amount of land is minimal (2-3 m2 per PE).
VF - Example of single-household system
(slide 1 of 3; VF = vertical flow)
Construction of
impermeable basins.
With parallel basins, one
bed is loaded, the other one
is resting. During the resting
period, accumulated
organic material can be
degraded and oxygen can
penetrate down the filter.
Photos provided by Peter Vandersnickt (at his house in Stabroek, Belgium)
VF - Example of single-household system
(slide 2 of 3)
Basins have been filled
with sand.
On top you can see the
influent distribution
system (tubes with
openings at regular
distances to ensure
equal distribution of
water over entire
surface).
Course 2 Unit 5
VF - Example of single-household system
(slide 3 of 3)
Basins have been planted with young
reed plants.
Top layer of gravel to cover the
influent distribution system.
Garden has been replanted.
Type 4: Hybrid systems
Anoxic → denitrification
Aerobic → nitrification
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Two-step constructed wetland consisting of a HSSF and VSSF flow bed.
Different conditions in both wetlands trigger different removal pathways → hybrid systems are
usually more efficient (particularly for nitrogen removal)
The recycle flow is to improve denitrification in the horizontal (anoxic) first compartment; aerobic
conditions prevail in the vertical compartment
C2U5 Part B
BASIC PRINCIPLES OF
WASTEWATER TREATMENT
IN CONSTRUCTED WETLANDS
29
Compartments in wetlands
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Sediment / gravel bed
Root zone / pore water
Litter / detritus
Water
Air
Plants
Roots
Bacteria growing in biofilms
→ Treatment is the result of complex interactions
between all these compartments
Wastewater treatment mechanisms (slide 1 of 4)
 BOD removal
 particulate BOD by settling and filtration, then converted
to soluble BOD by hydrolysis
 soluble BOD due to degradation by attached microbial
growth (biofilms on stems, roots, gravel particles etc)
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(Note: possible greenhouse gas emissions due to anaerobic processes; negligible
however when compared to other sources)
 Suspended solids removal
 removal occurs within few meters near inlet by settling
and filtration
* See appendix at the end for further information on BOD
Wastewater treatment mechanisms (slide 2 of 4)
 Nitrogen removal
 nitrification/denitrification in biofilms
 plant uptake
 volatilization as ammonia (at pH > 8.5)
 Phosphorus removal
 plant uptake
 retention in the soil (adsorption)
 precipitation with Ca, Al and Fe
Course 2 Unit 5
Wastewater treatment mechanisms (slide 3 of 4)
 Pathogen removal
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predation by protozoa
sedimentation and/or filtration
absorption
die-off from unfavorable environmental conditions (UV
light, pH and temperatures)
 Heavy metal removal
 precipitation and adsorption
 plant uptake
 Trace organics removal
 adsorption by the organic matter and clay particles
Wastewater treatment mechanisms (slide 4 of 4)
 Redox conditions
 FWS generally aerobic in the upper water layers, anaerobic
in the sediment
 HSSF mostly anaerobic (greenhouse gas emissions!)
 VF mostly aerobic due to intermittent loading (pores can refill
with air in between two loads)
 Aerobic patches around roots due to oxygen release
 Upper layers of biofilms can be aerobic whereas deeper
layers can be anoxic/anaerobic
 In most CWs there is thus a mosaic of sites with different
redox conditions which trigger different removal processes
(this is one of the major differences and advantages
compared to other technologies!)
Role of aquatic plants in free water surface
(FWS) constructed wetlands
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Nutrient uptake
Heavy metal accumulation in plant tissue
(Note: usually not a problem with domestic wastewater or grey water)
Habitat for wildlife
Aesthetics
Stems = mechanical filter + attachment of biofilm
Limitation of algal growth by providing shadow
Reduce water current velocity → increases settling
Role of aquatic plants in sub-surface flow
(SSF) constructed wetlands
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Nutrient uptake
Heavy metal accumulation in plant tissue
(Note: usually not a problem with domestic wastewater or grey water)
Habitat for wildlife
Aesthetics
Root system = mechanical filter + attachment of
biofilm
 Root system maintains hydraulic conductivity
 Oxygen transfer (active and passive) → plants are
transporting oxygen to their root zone to allow the
roots to survive in anaerobic conditions. Part of this
oxygen is available for microbial processes.
C2U5 Part C
DESIGN OF CONSTRUCTED
WETLANDS
37
Basic design question: how much area needed?
Wind
Land
Sun
Seeds
Soils
Plants
Microbes
Natural systems
(rule of thumb 2-10 m2/PE)
Since wetlands are low-rate systems which are completely depending
on solar energy, they need a much larger surface area than conventional
systems with electrical energy input (e.g. activated sludge plants).
Table 2: Design criteria for different types of
constructed wetlands
Design parameter
Data is for which wastewater type
Detention time (days)
FWS
(free water
surface)
HSSF
(horizontal subsurface flow)
Mixed domestic wastewater
VSSF (vertical
sub-surface flow)
greywater
5 - 14
2-7
N/A
8
7.5
4-6
Water or substrate depth (m)
0.1 – 0.5
0.1 – 1.0
N/A
Hydraulic loading rate (mm/d)
7 - 60
2 - 30
40 - 80
Area requirement (ha/m³/day)
0.002 – 0.014
0.001 – 0.007
N/A
2:1 to 10:1
0.25:1 to 5:1
N/A
Required
Not required
Not required
3-5
3-5
N/A
Max. BOD loading rate (g/m2/day)
Aspect ratio – length/width
Mosquito control
Harvest frequency (years)
Source: Wood (1995) for FWS and SSF; Ridderstolpe (2004) for VSSF
Course 2 Unit 5
Conversions of different units
 BOD load:
– 1 kg/ha/d = 0.1 g/m2/d
– (because 1 hectare = 10,000 m2)
 Hydraulic load:
– 1 L/m2/d = 1 mm/d
– (because 1 L = 10-3 m3, and 1 m = 1000 mm)
Further design criteria for horizontal flow
constructed wetlands
Source: Morel and Diener (2006), p. 33 - Same type of information is available for the other types
of soil filters / constructed wetlands in that publication
Example photos of horizontal flow constructed
wetlands for greywater treatment
Source: Morel and Diener (2006), p. 34
Example calculation based on BOD loading
rate
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You need to know the design flowrate (Q) and BOD concentration (measure existing
greywater or make estimate), e.g. 0.01 ML/d and 200 mg/L BOD (see also Course 1 Unit 2
for greywater characteristics)
Now calculate the BOD load:
LBOD = Q · CBOD = 0.01 ML/d · 200 mg/L = 2 kg/d = 2000 g/d
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Pick a design value from Table 2, say for VSSF constructed wetland: qdesign = 6 gBOD/ m2 /d
Then area required is:
A = LBOD / qdesign = 2000 g/d / 6 g/ m2 /d = 333 m2
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So the required area for this design is 333 m2
Course 2 Unit 5
Table 3: Comparison of actual design
parameters and costs for 107 constructed
wetlands in Flanders, Belgium
Parameter
FWS
VSSF
HSSF
Combined
Design size (PE)
1 – 2000
4 – 2000
152 and 350
5 - 750
Area (m2/per PE)
7
3.8
5.9 and 3.7
5
Values from Table 1 (m2/per PE)
5 - 10
2-3
3-5
2.5 - 3
Investment cost (€/PE)
392
507
1636 and 879
919
Source: Rousseau et al. (2004)
Note: all these wetlands treat mixed domestic wastewater
The flowrates are in most cases not measured, so there is no information on the hydraulic or BOD
load to these wetlands.
Table 4: Comparison of actual performance for 107
constructed wetlands in Flanders, Belgium (based
on measured average concentrations)
Parameter
FWS
VSSF
HSSF
Combined
VSSF
greywater
COD removal (%)
61
94
72
91
90 – 99
(BOD)
SS removal (%)
75
98
86
94
90-99
TN removal (%)
31
52
33
65
30
TP removal (%)
26
70
48
52
30 – 95
Source: Rousseau et al. (2004)
Note: all these wetlands in Flanders treat mixed domestic wastewater
The column on the right in green is for greywater treatment in vertical sub-surface flow
wetlands (Ridderstolpe, 2004)
Comments on previous slide regarding
performance
 Comparisons of systems in Flanders regarding removal performance:
– FWS: worst overall performance
– VSSF: best performance for COD, SS and TP
– Combined/hybrid: best performance for TN
 But the data gives no indication on how much area is required to
achieve this performance – the key parameter to compare would be
load of BOD removed per m2 and d (in g/ m2 /d)
– But to determine this value, one would have to know the flow rate
– Unfortunately, the flow rate is rarely measured at small constructed
wetlands
Rules of thumb for greywater treatment
in VSSF CW
 Water should percolate through the soil in an unsaturated
flow
 Design life: 25-30 years
 Removal efficiencies:
– 90-99% for SS and BOD
– 30-95% for phosphorus
– 30% for nitrogen
 Greywater may need pre-treatment before wetland to remove
suspended solids and excessive amounts of fat
Source: Ridderstolpe (2004)
Rules of thumb for mixed domestic
wastewater treatment in VSSF CW
 Water should percolate through the soil in an unsaturated flow
(hydraulic loading rates of up to 800 mm/day have been achieved
without clogging)
 Intermittent loading needed (generally 2-4 times/ day)
 Effluent quality (BOD removal + nitrification of domestic wastewater):
– < 10 mg/L BOD
– < 10 mg/L TSS
– < 2 mg/L NH4-N
 Design: 2-3 m2 / PE
 Note: wastewater may need pre-treatment before wetland to remove
suspended solids and excessive amounts of fat
Source: Cooper (2004)
Course 2 Unit 5
Other considerations: water balance
 Influent wastewater flow
 - effluent wastewater flow
 + precipitation
 - evapo-transpiration
 = change in water volume over time
 Because of their large surface areas, constructed
wetlands are very sensitive to precipitation and
evapotranspiration!
Other considerations: slopes and liners
Bottom slope:
 0.5%or less for FWS systems
 2%or less for HSSF systems
Liners (clay or plastic):
 when groundwater contamination or water
conservation is a concern (depends on local soil
characteristics)
Other considerations: filling medium
 The selection of the filling medium of a subsurface flow
constructed wetland is based on:
 hydraulic conductivity
→ high enough to allow easy water flow
 local availability
→ reduced transport costs
* Less important for greywater
 phosphate sorption capacity*
→ the more P-binding sites available (depending on Fe, Al and Ca
content), the longer and the more P can be adsorbed
Sand has better P-sorption capacity but lower hydraulic conductivity
than gravel → higher clogging risk
Design summary
 No significant difference between design for conventional
wastewater and greywater
– If design is based on BOD load then check BOD concentration of
greywater in question
– If design is based on a per person pollutant load then remember that
the per person load is lower for a greywater treatment system since
faeces and urine are treated separately
 CWs for greywater are in general slightly smaller than CWs
for domestic wastewater (for same number of people)
C2U5 Part D
O&M OF CONSTRUCTED
WETLANDS AND EXAMPLES FOR GREYWATER
TREATMENT
53
Basic maintenance
 ‘natural’, low-tech systems  require low but still adequate
maintenance
 Vymazal et al. (1998) recommends checking larger systems
(> 500 PE) on a daily basis, including:
– pretreatment units
– inlet structures
– outlet structures
 If maintenance is ignored:
– uneven flow distribution
– local overloading
– deterioration of treatment efficiency in the long term
Lack of maintenance (or wrong design): consequences
(example for a vertical sub-surface flow CW)
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This wetland is overloaded (no pre-treatment); sludge is accumulating very quickly on the surface
(normally sludge should only accumulate at a much slower rate and be removed about once in 10
years)
Excessive sludge accumulation threatens to block the influent distribution system and the pore
spaces
Should plants from a constructed wetland
be harvested or not?
 Advantages of harvesting:
– net nutrient export from the system
– prevention of thick layers of dead plant material with stagnant water in
FWS which are ideal pest breeding places
 Advantages of not harvesting:
–
–
–
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creation of an isolating layer of dead plant material
provision of a detritus layer that can adsorb trace metals
provision of a carbon source for denitrification
no alteration of the ecological functioning of wetlands
 Recommendation: harvest every 2-3 years (in winter when
applicable)
The mosquito problem
(applies only to FWS CWs)
The problem: mosquitos can bread in free water flow
constructed wetlands
There is less mosquito breeding if the biodiversity and
complexity of food web is high
Solutions:
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No above-ground flow (use SSF wetlands instead)
Pretreatment to reduce organic loading rate
Temporary drying of the beds will eradicate larvae
Lower water depths  higher stream velocity
Insecticides  not sustainable, very expensive
Open, unplanted water areas support growth of predators
(fish, invertebrates)
Biological control agents can be added: Bacillus
thuringiensis, Gambusia
Gambusia fish
Course 2 Unit 5
Ex 1: Culemborg, NL (slide 1 of 3)
Vertical flow constructed wetland for greywater treatment at EVA Lanxmeer
(ecological residential area in Culemborg, The Netherlands).
Ex 1 (cont’d): Schematic (slide 2 of 3)
Distribution
pipes
Pump
for
greywater
Reed plants
Sand filter
Gravel + shells
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Lining of basin
Drainage
floor
Sampling
point
Treats domestic wastewater from houses, offices and schools
Pre-settling tank before constructed wetland
Vertical flow over reed beds (helophytes) and bed of sand and gravel
Micro-organisms in the sand bed and on the roots of the plants degrade the wastewater
Discharge of treated effluent to surface water
Not much performance data available for this wetland
Ex 1 (cont’d) – Performance data (slide 3 of 3)
Pollutant concentrations in effluent samples taken at Culemborg on 27 January
2006 (winter conditions)
 Effluent discharged in surface water
pH
Chloride (mg Cl/L)
COD (mg/L)
BOD5 (mg/L)
KJN (mg N/L)
NH4 (mg N/L)
TP (mg P/L)
Effluent filter 1
7.8
38
13
<1
0.6
< 0.05
0.95
Effluent filter 2
7.7
36
11
<1
0.5
< 0.05
1.3
Source of this data: provided by Sander Booms, Municipality Culemborg
Ex 2 – Kathmandu, Nepal: Vertical flow constructed
wetland for single household (slide 1 of 2)
A system for one household of 7 people
Source: Morel and Diener (2006), p. 66
Course 2 Unit 5
Ex 2 – Example Kathmandu, Nepal: Performance data
(slide 2 of 2)
Source: Morel and Diener (2006), p. 67
Research outlook for CWs in general
 The real challenge is to open up the ‘black box’ that
constructed wetlands still are:
 Process identification (rising evidence that for instance
ANAMMOX* might take place)
 use of microbial marker techniques etc
 Process quantification
 e.g. root oxygen release, sulphate reduction
 Process modelling
– Comprehensive mathematical models that represent the complex network of
interacting processes
– Can be used to optimize design and operation
* ANAMMOX = Anaerobic ammonia oxidation
References

Cooper P. (2005). The performance of vertical flow constructed wetland systems with special reference
to the significance of oxygen transfer and hydraulic loading rates. Proceedings IWA Conference on
Constructed wetlands, Avignon, France.

Morel A. and Diener S. (2006) Greywater Management in Low and Middle-Income Countries, Review of
different treatment systems for households or neighbourhoods. Swiss Federal Institute of Aquatic
Science and Technology (Eawag). Dübendorf, Switzerland. Download from www.sandec.ch (also placed
under Extra Reading)

Ridderstolpe, P. (2004) Introduction to greywater management, Stockholm Environment Institute,
Sweden, Report 2004-4. Download from www.ecosanres.org (also placed under Extra Reading)

Rousseau D.P.L., P.A. Vanrolleghem and N. De Pauw (2004). Constructed wetlands in Flanders: a
performance analysis. Ecological Engineering, 23(3), 151-163. Also placed under Extra Reading

Vymazal, J., H. Brix, P.F. Cooper, B. Green and R. Haberl (Eds) (1998). Constructed wetlands for
wastewater treatment in Europe. Backhuys Publishers, Leiden, 366 p.

Wood, A. (1995). Constructed wetlands in water pollution control: fundamentals to their understanding.
Water Science and Technology, 32(3), 21-29.
Further reading if you are really interested
in constructed wetlands
 Kadlec, R.H. and R.L. Knight (1996). Treatment
wetlands. CRC Press, Boca Raton, FL, USA. – A
new edition will come out in 2007 or 2008
Useful websites on constructed wetlands
in general (not just for ecosan)
 US EPA: Constructed wetlands for wastewater treatment and
wildlife habitat www.epa.gov/owow/wetlands/construc/
 Constructed Wetlands Association www.constructedwetland.org/
Course 2 Unit 5
Appendix: information about BOD and COD
Biochemical oxygen demand (BOD)
The BOD test is a bioassay in which the rate (and extent) of the
aerobic degradation of organic matter is assessed in terms of the
amount of oxygen consumed in its degradation.
Unit is mgO2/L, or in short mg/L
 It is used to determine:
 The organic strength of the wastewater
 The approximate amount of oxygen required to
biologically stabilize the organics in wastewater
 The size (capacity) of wastewater treatment
facilities (based on BOD load in kg/d)
 Efficiency of some treatment processes (e.g.
% BOD removal)
 Compliance with wastewater discharge permits
(effluent BOD concentration)
Principles of BOD test
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Simplified reaction:
Microorganisms + Organic matter + O2 
More microorganisms + CO2+ H2O + Residual organic matter
To ensure the presence of excess dissolved oxygen (DO) throughout
the test, a DO depletion of no more than 70% is allowed for a valid test.
Water at 20C (the standard temperature of the BOD test) contains only
about 9 mg/L DO. Therefore, samples for BOD measurement must
usually be diluted.
Oxygen is supplied by saturating the sample with air.
Microorganisms are supplied by seeding the dilution water with an
appropriate inoculum (usually sewage, treated sewage or, in some
cases, microorganisms acclimatised to the particular substrate of
interest).
BOD5 test in practice
To the BOD bottle add:
– Diluted sample (usually diluted
50 times or more for
wastewater!)
– Innoculum (bacteria)
– Oxygen to saturate to ~9 mg/L
Measure how much oxygen is left
after 5 days (must still be > 2.5
mg/L else dilute more)
The less oxygen is left, the higher
the BOD, therefore the more
organic pollution
The BOD test is time
consuming and has a low
precision
Chemical oxygen demand (COD)
Expressed as amount of oxygen required for chemical oxidation
of organic matter by a strong oxidant (permanganate or
dichromate) in acid solution.
Unit is mgO2/L, or in short mg/L
Consumption of permanganate or dichromate is converted into an
equivalent oxygen demand (amount of oxygen which will be consumed
if the oxidation had taken place by using oxygen)
COD test in practice
To perform a test, users have to add a wastewater sample to a cuvette (with
reagent) and leave it in a heater for 2 hours. At the end of this period the
intensity of colour in the solution is directly related to the COD value in the
sample, and can be measured with a spectrophotometer.
Cuvettes with reagent
Spectrophotometer
Relationship of COD to BOD
 The COD concentration is always higher than the
BOD concentration for a given sample because:
– many organic substances which are difficult to oxidise
biologically can be oxidised chemically
– inorganic substrates that are oxidised by the dichromate
increase the apparent organic content (COD value) of the
sample
– certain organic substances may be toxic to the microorganisms
used in the BOD test