Chapter 6: Constructed Wetlands for Wastewater Treatment

CHAPTER 6

CONSTRUCTED WETLANDS FORWASTEWATER TREATMENT

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Chapter 6: Constructed Wetlands for Wastewater Treatment

Coordinating Lead Author s
Yoshitaka Ebie (Japan) and Sirintornthep Towprayoon (Thailand)

Lead Authors
Chart Chiemchaisri (Thailand), Ülo Mander (Estonia) and Sandra Furlan Nogueira (Brazil)Cont ributing Author
Baasansuren Jamsranjav (IPCC TFI TSU)

Review Editors
Christopher Gordon (Ghana) and Detelina Rumyanova Petrova (Bulgaria)

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Chapter 6: Constructed Wetlands for Wastewater Treatment

Contents
6

Constructed wetlands for wastewater treatment .......................................................................................... 5
6.1

Introduction ...................................................................................................................................... 5

6.1.1

Constructed wetlands for wastewater treatment.............................................................................. 5

6.1.2

Relation to 2006 IPCC Guidelines................................................................................................. 8

6.2

Methane emissions from constructed wetlands................................................................................. 10

6.2.1

Methodological issues ................................................................................................................. 10

6.2.1.1

Choice of method ................................................................................................................ 11

6.2.1.2

Choice of emission factors .................................................................................................. 13

6.2.1.3

Choice of activity data......................................................................................................... 13

6.2.2

Time series consistency............................................................................................................... 14

6.2.3

Uncertainties............................................................................................................................... 14

6.2.4

QA/QC, Completeness and Reporting ......................................................................................... 15

6.3

Nitrous oxide emissions from constructed wetlands ........................................................................ 16

6.3.1

Methodological issues ................................................................................................................. 16

6.3.1.1

Choice of method ................................................................................................................ 16

6.3.1.2

Choice of emission factors .................................................................................................. 19

6.3.1.3

Choice of activity data......................................................................................................... 19

6.3.2

Time series consistency............................................................................................................... 20

6.3.3

Uncertainties............................................................................................................................... 20

6.3.4

QA/QC, Completeness and Reporting ......................................................................................... 20

References ...................................................................................................................................................... 22
Annex 6A.1 Estimation of default emission factors for CH4 and N2O in constructed wetlands for wastewater
treatment ........................................................................................................................................................ 25

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Chapter 6: Constructed Wetlands for Wastewater Treatment

Equations
Equation 6.1

CH4 emissions from constructed wetlands ............................................................................ 11

Equation 6.2

CH4 emission factor for constructed wetlands ....................................................................... 13

Equation 6.3

Total organically degradable material in domestic wastewater .............................................. 14

Equation 6.4

Total organically degradable material in industrial wastewater .............................................. 14

Equation 6.5

N2O emissions from constructed wetlands ............................................................................ 16

Equation 6.6

Total nitrogen in domestic wastewater .................................................................................. 19

Equation 6.7

Total nitrogen in industrial wastewater ................................................................................. 19

FiguresFigure 6.1

Classification and configuration of constructed wetlands for wastewater treatment.................. 6

Figure 6.2

Wastewater treatment systems and discharge pathways .......................................................... 9

Figure 6.3

Decision tree for CH4 emissions from constructed wetlands .............................................

Figure 6.4

Decision tree for N2O emission from constructed wetland .................................................... 18

Figure 6A1.1

The relationship between inflow TOC loading and CH4-C emission (left column) and between
inflow TN loading and N2O-N emission (right columns) in SF, HSSF, and VSSF CWs. In allcases, p < 0.05. ................................................................................................................... 26

12

TablesTable 6.1

Selected factors impacting CH4 and N2O emissions in constructed wetlands .......................... 8

Table 6.2

Influent total organic carbon (TOC) and total nitrogen (TN) values, relevant CH4-C and N2ON emissions, and share (%) of CH4-C and N2O-N in the initial loading of TOC and TN inconstructed wetlands ............................................................................................................ 8

Table 6.3

Coverage of wastewater types and greenhouse gas emissions from constructed wetlands .... 10

Table 6.4

Methane Correction Factors by type of constructed wetland ................................................ 13

Table 6.5

Default uncertainty ranges for domestic and industrial wastewater ...................................... 15

Table 6.6

Example of N content in some nitrogen-rich industrial wastewater ...................................... 20

Table 6.7

Nitrous oxide methodology default uncertainties ................................................................. 20

Table 6A1. 1

Average, standard error, median, 2.5% and 97.5% percentile values of CH4-C and N2O-N
emission factors (%) for different types of constructed wetlands.......................................... 25

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Chapter 6: Constructed Wetlands for Wastewater Treatment

6 CONSTRUCTED WETLANDS FORWASTEWATER TREATMENT
6.1

INTRODUCTION

6.1.1

Constructed Wetlands for Wastewater Treatment

Wetland ecosystems can act as sources, sinks, or transformers of nutrients and carbon (C) (Mitsch andGossenlink, 1993). This ability of wetlands has led to a widespread use of natural and constructed wetlands(CWs) for water quality improvement (Brix, 1997).
Constructed wetlands systems are fully human-made wetlands for wastewater treatment, which apply various
technological designs, using natural wetland processes, associated with wetland hydrology, soils, microbes andplants. Thus, CWs are engineered systems that have been designed and constructed to utilize the naturalprocesses involving wetland vegetation, soils, and their associated microbial assemblages to assist in treating
wastewater. Synonymous terms to “constructed” include “man-made”, “engineered” or “artificial” (Vymazal,2007).
“Semi-natural treatment wetlands” (SNTWs) for wastewater treatment are natural wetland systems that havebeen modified for this purpose. Modifications made within these systems are usually based on increasing thevolume of water reserved (i.e. dams) and constructing channels for targeting the influent and effluent. Thesesystems can be found in both freshwater and coastal wetlands. The functioning of SNTWs is similar to those of
surface flow CWs.
This chapter only provides guidance for CWs and SNTWs for wastewater treatment. Decision tree for finding
the appropriate guidance chapter within this supplement or the 2006 IPCC Guidelines for National Greenhouse
Gas Inventories (2006 IPCC Guidelines) is provided as Figure 1.1 in Chapter 1 of this supplement.
It is good practice that reporting of emissions from wastewater treatment be complete, covering all domestic andindustrial wastewater. CW is a wastewater treatment pathway not specifically described in 2006 IPCC
Guidelines. It is good practice that countries apply the guidance in this chapter on “constructed wetlands”, if
emissions from CWs represent a key wastewater treatment pathway. In accordance with Chapter 4 of Volume 1,
those subcategories that together contribute more than 60 percent to a key category should be treated as
significant1. When wastewater treatment is identified as a key category, key pathways are identified in the same
way as significant subcategories. When countries have access to data and information on wastewater treatment
by CWs, it is a good practice to use this guidance to estimate emissions from CWs.
Emissions from CWs and SNTWs must be reported in the waste sector. If freshwater and coastal wetlands are
modified to SNTWs, inventory compilers should check with relevant land-use category in this supplement toavoid double-counting. Constructed wetlands and SNTWs can be used to improve the quality of collected
wastewater including domestic wastewater, industrial wastewater such as wastewater from processing factories
of agricultural products and dairy farms, collected runoff from agricultural lands and leachate from landfills. For
some wastewaters, CWs are the sole treatment; for others, they are one component in a sequence of treatment
processes (US EPA, 1995).
There are various types of CWs used for treatment of wastewater, the following paragraphs highlight the mainclassification of CWs.

TYPE OF CONSTRU CTED WETLANDS FOR WASTEWATER TREATMENT
Constructed wetlands may be categorized according to various design parameters, but the three most importantcriteria are hydrology (water surface flow and subsurface flow), macrophyte growth form (emergent, submerged,
free-floating, and floating leaved plants) and flow path (horizontal and vertical) (see Figure 6.1; Vymazal 2007,2011). Different types of CWs may be combined (which are called hybrid or combined systems) to utilize thespecific advantages of the different systems. For instance, to guarantee more effective removal of ammonia andtotal nitrogen (N), during the 1990s and 2000s an enhanced design approach combined vertical and horizontal
flow CWs to achieve higher treatment efficiency (Vymazal, 2011).
1

An assessment of significance can be based on expert judgment following the protocol described in Annex 2A.1 of Chapter
2, Volume 1 of 2006 IPCC Guidelines (Protocol for Expert Elicitation). Information concerning the percentage of
population connected to wastewater treatment, which may facilitate expert judgment can be obtained from internationalsources (notably UNSTAT or FAO).

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Chapter 6: Constructed Wetlands for Wastewater Treatment

Figure 6.1

Classification and configuration of constructed wetlands for wastewater
treatment
Constructed Wetlands

Surface Flow

Subsurface Flow

Free-Floating plants

Horizontal flow

Floating leaved plants

Submerged plants
Vertical flow
Emergent plants

Hybrid system
Note: Adapted from Vymazal, 2007, 2011. Lower part is original. Most of SNTWs represent surface flow type wetlands.

Co nst ructed Wet lands w it h S urface Flow
Constructed wetlands with surface flow (SF), known as free water surface CWs, contain areas of open water andfloating, submerged, and emergent plants (Kadlec and Wallace 2008). The shallow water depth, low flow
velocity, and presence of plant stalks and litter regulate water flow and, particularly in long, narrow channels(Crites et al. 2005), ensure better water purification. The most common applications for SF CWs are for tertiary
treatment of municipal wastewater and also for stormwater runoff and mine drainage waters (Kadlec and Knight
1996; Kadlec and Wallace 2008). SF CWs are suitable in all climates, including the far north (Mander andJenssen 2003).

Co nst ructed Wet lands w it h S ubs urface Flo w
In horizontal subsurface flow constructed wetlands (HSSF CWs), the wastewater flows from the inlet and flows
slowly through the porous medium under the surface of a bed planted with emergent vegetation to the outlet
where it is collected before leaving via a water level control structure (Vymazal et al., 1998). During passage thewastewater comes into contact with a network of aerobic, anoxic, and anaerobic zones. Most of the bed is
anoxic/anaerobic because of permanent saturation of the beds. The aerobic zones occur around roots andrhizomes that leak oxygen into the substrate (Brix 1987). HSSF CWs are commonly sealed with a liner toprevent seepage and to ensure the controllable outflow. HSSF CWs are commonly used for secondary treatmentof municipal wastewater although many other applications have been reported in the literature (Vymazal andKröpfelova 2008). The oxygen transport capacity in these systems is insufficient to ensure aerobic
decomposition, thus, anaerobic processes play an important role in HSSF CWs (Vymazal and Kröpfelova 2008).
Some HSSF CWs, having the ability to insulate the surface of the bed, are capable of operation under colder
conditions than SF systems (Mander and Jenssen 2003).

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Chapter 6: Constructed Wetlands for Wastewater Treatment

Vertical subsurface flow constructed wetlands (VSSF CWs) comprise a flat bed of graded gravel topped withsand planted with macrophytes. VSSF CWs are fed with large intermittent wastewater flows, which flood thesurface of the bed, then percolate down through the bed and are collected by a drainage network at the bottom.
The bed drains completely which allows air to refill the bed. Thus, VSSF CWs provide greater oxygen transfer
into the bed, producing a nitrified (high NO3-) effluent (Cooper et al., 1996; Cooper 2005). Consequently, VSSF
CWs do not provide suitable conditions for denitrification to complete conversion to gaseous nitrogen forms,
which then escape to the atmosphere.
In recently developed tidal (“fill and drain”) flow systems better contact of wastewater with the microorganisms
growing on the media is guaranteed. This significantly enhances the purification processes (Vymazal 2011).

Hybrid Co ns tructed Wet lands
Various types of CWs can be combined to achieve higher removal efficiency, especially for nitrogen. The design
consists of two stages, several parallel vertical flow (VF) beds followed by 2 or 3 horizontal flow (HF) beds inseries (VSSF-HSSF system). The VSSF wetland is intended to remove organics and suspended solids and topromote nitrification, while in HSSF wetland denitrification and further removal of organics and suspended
solids occur.
Another configuration is a HSSF-VSSF system. A large HSSF bed is placed first to remove organics andsuspended solids and to promote denitrification. An intermittently loaded small VF bed is used for additionalremoval of organics and suspended solids and for nitrification of ammonia into nitrate. To maximize removal of
total N, however, the nitrified effluent from the VF bed must be recycled to a sedimentation tank (Vymazal2011).
VSSF-HSSF and HSSF-VSSF CWs are the most common hybrid systems, but in general, any kind of CWs
could be combined to achieve higher treatment efficiency (Vymazal 2007).

GREENHOUSE GASES EMI SSIONS FROM VARIOUS TYPES OFCONSTRUCTED WETLANDSEmissions of greenhouse gases such as methane (CH4) and nitrous oxide (N2O) are a byproduct of CWs, theimportance of which has been increasing recently. CH4 is produced by methanogenesis whereas N2O is a productof denitrification and/or nitrification of N compounds by microorganisms. Among several environmental factors
controlling the greenhouse gases emissions, the availability of C and nutrients (especially N) which directly
depend on wastewater loading, temperature, hydrological regime (pulsing vs steady-state flow), groundwater
depth, moisture of the filter material (water filled soil pores (WFSP)), and the presence of aerenchyma plants
play a significant role (see Table 6.1).
Soil temperature, oxidation reduction potential and soil moisture (WFSP, depth of the ground water level) are themost significant factors affecting the emissions of CH4 from CWs (Mander et al., 2003; Van der Zaag et al.,2010). Several investigations have shown that a water table deeper than 20 cm from the surface of the wetlands
and/or water-logged soils oxidizes most of the CH4 fluxes (Soosaar et al., 2011; Salm et al., 2012). Fluxes of
N2O, however do not show a clear correlation with soil/air temperature, and significant emissions of N2O fromCWs have been observed in winter (Søvik et al., 2006). Likewise, freezing and thawing cycles enhance N2O
emissions (Yu et al., 2011). The hydrological regime also plays a significant role in greenhouse gases emissions
from CWs. Altor and Mitsch (2008) and Mander et al., (2011) demonstrated that the intermittent loading(pulsing) regime and fluctuating water table in CWs enhance CO2 emissions and significantly decrease CH4
emissions. N2O emissions, in contrast, do not show a clear pattern regarding the pulsing regime.
Table 6.2 shows CH4 and N2O conversion rates derived from the relationship between the initial (input) C and N
loadings and respective CH4 and N2O emissions from the main types of CWs. There is a significant positive
correlation (p < 0.05) between the initial loadings and CH4 and N2O emissions from both SF and VSSF CWs,
whereas no correlation was found for HSSF CWs types. Seemingly, high variability of conditions andcombination of several factors in HSSF CWs may be the reason for that. The limited amount of available data
did not allow derivation of reliable relationships for HSSF CWs. These shares (%) can be used as a base for the
calculation of emission factors for Tier 1 and Tier 2 methodologies. The high emission factor for CH4 from SF
CWs (Table 6.4) is thought to be due to the additional CH4 from sediments accumulated at the bottom of SF
CWs.

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Chapter 6: Constructed Wetlands for Wastewater Treatment

TABLE 6.1
SELECTED FACTORS IMPACTING CH4 AND N2O EMISSIONS IN CONSTRUCTED WETLANDS
Factors/processes

CH4

N2O

Higher water/soil/air temperature

Increase in almost all cases 1-6 withfew exceptions 7

No clear relationship 1-4, 7, 8Higher moisture of soil or filter material(higher value of WFSP)

Clear increase 9, 10

Decrease 9, 10Higher wastewater loading

Increase 1-4, 11, 12

Increase 1, 2, 4, 13

Presence of aerenchymal plants

Increase 14-16
Decrease (depends on conditions) 17

Increase 16, 18
Decrease 16, 19

Pulsing hydrological regime(intermittent loading)

Clear decrease 9, 20

Increase 9, 21, 22
Decrease in some SF CWs 23

Deeper water table (from the surface) in
HSSF CWs

Decrease 9, 10

Increase 9, 10

Source:1

Mander and Jenssen 2003; 2 Mander et al., 2005; 3 Teiter and Mander 2005; 4 Søvik et al., 2006; 5 Kayranli et al., 2010; 6 Van der Zaag etal., 2010; 7 Søvik and Kløve 2007; 8 Fey et al., 1999; 9 Mander et al., 2011; 10 Yang et al., 2013; 11 Tanner et al., 1997; 12 Tai et al., 2002; 13
Hunt et al., 2009; 14 Inamori et al., 2007; 15 Inamori et al., 2008; 16 Wang et al., 2008; 17 Maltais-Landry et al., 2009; 18 Rückauf et al.,
2004; 19 Silvan et al., 2005; 20 Altor and Mitsch 2008; 21 Jia et al., 2011; 22 Van de Riet et al., 2013; 23 Hernandez and Mitsch 2006

TABLE 6.2
INFLUENT TOTAL ORGANIC CARBON (TOC) AND TOTAL NITROGEN (TN) VALUES, RELEVANT CH4-C AND N2O-N
EMISSIONS, AND SHARE (%) OF CH4-C AND N2O-N IN THE INITIAL LOADING OF TOC AND TN IN CONSTRUCTEDWETLANDS

Type of
CW
SF
HSSF
VSSF

Influent TOC*
(mg C m-2 h-1)

CH4-C emission*(mg CH4-C m-2 h-1)

1.04-173.6 (10)0.15-181.0 (10.7) 1-

1-11

11

15.0-2190.2(177) 8, 10-12, 15-20

0.048-17.5 (1.7) 8, 10,17.88-1417.50(317) 6, 8, 10, 12

11, 15-20

0.3-5.4 (1.3) 6, 8, 10, 12

CH4-C/TOC**(%)
42 (20)12 (6.9)
1.17 (0.33)

Influent TN*(mg N m-2 h-1 )N2O-Nemission*(mg N2O-N
m-2 h-1)

0.76-202.8(12) 2, 3, 6-11, 21-23
1.04-295.20(40) 6, 10, 12, 15-17,0.009-0.65(0.03) 2, 6-11, 21-23
0.014-0.89(0.10) 6, 10-12, 15-

24, 25102.5-2105.0(155) 6, 8, 10, 12-14

17, 25

N2O-N/TN**(%)
0.13 (0.02)
0.79 (0.4)

0.033-0.424(0.03) 6, 8, 10, 11,12-14

0.023 (0.005)* Range and standard error (in parentheses)** Average and standard error (in parentheses)
Source: 1 Tanner et al., 1997; 2 Wild et al., 2001; 3 Tai et al., 2002; 4 Johansson et al., 2004; 5 Stadmark and Leonardson 2005; 6 Søvik etal., 2006; 7 Søvik and Kløve 2007; 8 Gui et al., 2007; 9 Ström et al., 2006; 10 Liu et al., 2009; 11 Van der Zaag et al., 2010; 12 Teiter andMander 2005; 13 Inamori et al., 2007; 14 Wang et al., 2008; 15 Mander et al., 2003; 16 Mander et al., 2008, 17 Liikanen et al., 2006; 18
Garcia et al., 2007; 19 Picek et al., 2007; 20 Chiemchaisri et al., 2009; 21 Xue et al., 1999; 22 Johansson et al., 2003; 23 Wu et al., 2009; 24
Inamori et al., 2008; 25 Fey et al., 1999

6.1.2

Relation to 2006 IPCC Guidelines

This chapter is a supplement to Chapter 6 Wastewater Treatment and Discharge of Volume 5 of the 2006 IPCC
Guidelines. The 2006 IPCC Guidelines include a section on estimating CH4 emissions from uncollected
wastewater. This Wetlands Supplement includes guidance on estimation of CH4 and N2O emissions from CWs
and SNTWs. Emission factors for CH4 and N2O emissions from CWs and SNTWs treating industrial wastewater
are the same as those for treating domestic wastewater. CO2 emissions are not included in greenhouse gases
emissions from wastewater treatment as CO2 from wastewater is considered biogenic.

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Chapter 6: Constructed Wetlands for Wastewater Treatment

Figure 6.2

Wastewater treatment systems and discharge pathways
Domestic and Industrial Wastewater

Collected

Uncollected

Untreated

Rivers, Lakes,
Estuaries, Sea

TreatedTreatedStagnant
sewer
Sewered toplant

Aerobic treatment

Treated on site
Domestic: latrine,
septic tankIndustrial: On site
plant

Untreated

Rivers, Lakes,
Estuarine, Sea

To ground

Anaerobic treatment
Constructed wetlands,
Semi-natural treatment
wetlands

SludgeAnaerobic
Digestion

ReactorLand
Disposal

LagoonLandfill orIncineration

Note: This figure was modified from the 2006 IPCC Guidelines. Emissions from boxes with bold frames are accounted for in the 2006 IPCC
Guidelines. This supplement provides emission factors for grey-coloured box: CWs and SNTWs for treatment of collected and uncollected
wastewater.

Coverage o f w as tew ater ty pes and g ases
Chapter 6 of Volume 5 of the 2006 IPCC Guidelines provides guidance on estimating CH4 and N2O emissions
from domestic wastewater using emission factors based on treatment technology. Constructed wetlands in this
supplement are an additional treatment technology. The emission factors provided in this chapter cover CWs and
SNTWs (collected/uncollected and treated; see Figure 6.2).
Methodology is provided for estimation of CH4 and N2O emissions from both domestic and industrial
wastewater (Table 6.3). Indirect N2O emissions from N leaching and runoff from agricultural land are covered in
Chapter 11, Volume 4 of the 2006 IPCC Guidelines. Emissions from processing factories of agricultural
products and dairy farm wastewater, collected runoff from agricultural lands and leachate from landfills are
considered as industrial wastewater. According to Chapter 3 of Volume 5 in the 2006 IPCC Guidelines, allamount of degradable organic carbon (DOC) in solid waste is subjected to estimation of CH4 in landfill site, andcarbon loss with leachate is not considered because of its low percentage. That means that CH4 emissions fromleachate treatment are already covered, and are not included in Section 6.2, while N2O emissions are consideredin Section 6.3 of this supplement. If CH4 emissions from CWs are accounted for, the amount of DOC in theleachate must be subtracted from that in the solid waste to avoid double counting. Because the C in the leachate
is normally presented in terms of COD, conversion rate from COD in the leachate to TOC in the solid waste isrequired to subtract the amount of DOC entering the CW from that in the solid waste. This logic can be appliedin Tier 2 or 3 estimation.

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Chapter 6: Constructed Wetlands for Wastewater Treatment

TABLE 6.3
COVERAGE OF WASTEWATER TYPES AND GREENHOUSE GAS EMISSIONS FROM CONSTRUCTED WETLANDS
Type of wastewater

MethaneNitrous oxide

Domestic wastewater

Included in this supplement (Section
6.2) with provision of methanecorrection factors (MCFs)

Included in this supplement (Section
6.3) with the provision of default
emission factors

Industrial wastewater including
wastewater from processing
factories of agricultural products
and dairy farms *Included in this supplement (Section
6.2) with provision of MCFs

Included in this supplement (Section
6.3) with provision of default emission
factors

Collected runoff fromagricultural lands

Emissions can be calculated using thesame methodology as for industrial
wastewater and are covered in this
supplement (Section 6.2)

Emissions can be calculated using thesame methodology as for industrial
wastewater and are covered in this
supplement (Section 6.3)
Note: Indirect N2O emissions from N
leaching and runoff from agriculturalland are considered in Chapter 11,
Volume 4 of the 2006 IPCC
Guidelines. If agricultural runoff iscollected and treated by CWs or
SNTWs, the amount of N flowing into
CWs or SNTWs must be subtracted to
avoid double counting.

Leachate from landfill

The amount of DOC leached from asolid waste disposal site is notconsidered in the estimation of DOCf.
Generally the amount of DOC lostwith the leachate is less than 1 percent
and can be neglected in thecalculations (Chapter 3, Volume 5,2006 IPCC Guidelines) and is notconsidered in this supplement

Emissions can be calculated using thesame methodology as for industrial
wastewater and are covered in this
supplement (Section 6.3)

*Dairy farm wastewater does not cover the manure itself but comes from other activities in the farm.

6.2

METHANE EMISSIONS FROM CONSTRUCTEDWETLANDS

6.2.1

Methodological issues

Methane emissions are a function of the organic materials loaded into CWs and the emission factor.
Three tiers of methods for estimation of CH4 from CWs are summarised below.
The Tier 1 method applies default values for the emission factor and activity parameters. This method is
considered good practice for countries with limited data.
The Tier 2 method follows the same method as Tier 1 but allows for incorporation of country-specific emission
factor and country-specific activity data. For example, a specific emission factor based on field measurements
can be incorporated under this method.
The Tier 3 method is used by countries with good data and advanced methodologies. A more advanced countryspecific method can be based on treatment system-specific data such as plant species, climate, temperature,seasonal effects and composition of wastewater.
In general anaerobic conditions occur in CWs. However, CH4 generated by CWs is usually not recovered andcombusted in a flare or energy device, and thus CH4 recovery is not considered here.
The amount of vegetation harvested from CWs is generally very small and its impact on total emissions fromCWs is considered insignificant. Moreover, the harvesting is usually not performed on regular basis and thequantity of harvested biomass is typically not recorded so it is not considered in this supplement.

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Chapter 6: Constructed Wetlands for Wastewater Treatment

6.2.1.1

C HOICE OF METHOD

A decision tree for domestic and industrial wastewater is shown in Figure 6.3.
The general equation for estimating CH4 emissions from CWs treating domestic and industrial wastewater isgiven in Equation 6.1.

EQUATION 6.1
CH4 EMISSIONS FROM CONSTRUCTED WETLANDS
∑(

)

∑(

)

Where:
CH4 emissions=

CH4 emissions in inventory year, kg CH4/yr

TOWj

=

total organics in the wastewater entering the CW in inventory year, kg
BOD/yr or kg COD/yr

EFj=emission factor, kg CH4/kg BOD (for domestic wastewater only) or kg
CH4/kg COD (for both domestic and industrial wastewater)
If more than one type of CW is used in an industrial sector this factor would
be a TOWi,j-weighted average.

i=industrial sectorj=type of CW2013 Supplement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories: Wetlands

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Chapter 6: Constructed Wetlands for Wastewater Treatment

Decision tree for CH 4 emissions from constructed wetlands

Figure 6.3
Start

Is CW used for
wastewater treatmentin the country?

No

Use the 2006 IPCC Guidelines forthe estimation of emissions fromother wastewater treatment systemand discharge pathways

No

Collect or estimate activity data on
wastewater treatment in CW

Yes

Are activity data on
wastewater treatment in CW
avaialable?

Yes

Are measurement orsite-specific measurement data
available for CW?

Is a country-specific method
available?

Yes

Yes

Estimate emissions
using site-specificmeasurement data
Box 3: Tier 3NoNo

Are country-specificemission factors available
for CW?

Yes

Estimate emissions using countryspecific emission factor
Box 2: Tier2

No

Is CW a key pathway of
key category?

Yes

No

Estimate country-specific Bo and
MCF

Estimate emissions using default
emission factors
Box 1: Tier 1

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Chapter 6: Constructed Wetlands for Wastewater Treatment

6.2.1.2

C HOICE OF EMISSION FA CTORSThe emission factor for wastewater treatment using CWs is a function of the maximum CH4 producing potential(Bo) and the methane correction factor (MCF).

EQUATION 6.2
CH4 EMISSION FACTOR FOR CONSTRUCTED WETLANDS
Where:
EFj=

emission factor, kg CH4/kg BOD or kg CH4/ kg COD

j=type of CWs

Bo=

maximum CH4 producing capacity, kg CH4/kg BOD or kg CH4/ kg COD

MCFj

=

methane correction factor (fraction), See Table 6.4

Good practice is to use country-specific data for Bo, where available, expressed in terms of kg CH4/kg BOD
removed for domestic wastewater or kg CH4/kg COD removed for industrial wastewater to be consistent withthe activity data. If country-specific data are not available, the following default values can be used.
The 2006 IPCC Guidelines provide default Bo values for domestic and industrial wastewater: 0.6 kg CH4/kg
BOD and 0.25 kg CH4/kg COD, respectively
The MCF indicates the extent to which Bo is realised in each type of CWs. It is an indication of the degree towhich the system is anaerobic. The proposed MCFs for SF, HSSF and VSSF CWs are provided in Table 6.4 andderived from literature-based analysis of CH4 conversion rates. Each MCF in Table 6.4 is calculated from therelation of initial TOC loading to CH4 emission flux derived from references provided in Table 6.2.
TABLE 6.4
METHANE CORRECTION FACTORS (MCF) BY TYPE OF CONSTRUCTED WETLAND (CW)CW typeSurface flow (SF)

MCF
0.4

Range
0.08-0.7

Horizontal subsurface flow (HSSF)

0.1

0.07-0.13

Vertical subsurface flow (VSSF)

0.01

0.004-0.016

These MCF values are based on actual measurement data derived under different operating and environmental
conditions thus factors such as vegetation types and temperature effect have been taken into account. Based on
the reported scientific data, there was insufficient information to differentiate the MCF values by vegetation typeand operating temperature. Nevertheless, these influencing factors can be considered for the estimation using
higher tier approach. There was insufficient actual measurement data of hybrid systems to derive default MCF
values. If the area fractions of SF, VSSF and HSSF for hybrid systems can be determined, the MCF values of thehybrid systems can be estimated as the area-weighted average of the MCFs for SF, VSSF and HSSF. Most
commonly, SNTWs are the SF type (Kadlec and Wallace, 2008), therefore, the default MCF of 0.4 can be used.
If the type of CW cannot be recognized, the MCF for surface flow can be used as a conservative value.
Otherwise country-specific data should be used in higher tier method.

6.2.1.3

C HOICE OF ACTIVITY DA TA

The activity data for this source category is the amount of organic materials (TOW) in the wastewater treated by
the CW. This parameter is a function of the population served by the CW system, and the biochemical oxygen
demand (BOD) generation per person per day. BOD default values for selected countries are provided in the2006 IPCC Guidelines (Table 6.4, Chapter 6 of Volume 5 of the 2006 IPCC Guidelines). In the case of industrial
wastewater, COD loading to the CW system per day (kg COD/day) can be used. Examples of industrial
wastewater data for various industries are provided in Table 6.9, Chapter 6, Volume 5 of the 2006 IPCC
Guidelines.

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If industrial wastewater is released into domestic sewers, it is estimated together with domestic wastewater .
The equations for TOW are:

EQUATION 6.3
TOTAL ORGANICALLY DEGRADABLE MATERIAL IN DOMESTIC WASTEWATEREQUATION 6.4
TOTAL ORGANICALLY DEGRADABLE MATERIAL IN INDUSTRIAL WASTEWATER

Where:
TOWj=

total organics in domestic wastewater treated in the CW in inventory year (kgBOD/year)

TOWi,j=total organics in wastewater from industry i treated in the CW in inventory year (kgCOD/year)i=

industrial sector

Pj=population whose wastewater is treated in the CW. The population should be
subtracted from total the population used in Equation 6.3 in Chapter 6, Volume 5 in
the 2006 IPCC Guidelines to avoid double-counting.

BOD=

per capita BOD generation in inventory year (g BOD/person/day)I=

correction factor for additional industrial BOD discharged into sewers (for collected
the default is 1.25, for uncollected the default is 1.00 as given in the 2006 IPCC
Guidelines)

CODi

=

COD concentration in wastewater from industry i entering the CW in inventory year
(kg COD/m3)

Wi,j

=

daily flow rate of industrial wastewater treated by the CW, m3/day

6.2.2

Time series consistency

The same method and data sets should be used for estimating CH4 emissions from CWs treating wastewater foreach year. The MCF for different treatment systems should not change from year to year, unless such a change isjustifiable and documented. If the share of wastewater treated in different treatment systems changes over thetime period, the reasons for these changes should be documented.
For activity data that are derived from population data, countries must determine the fraction of the population
served by CW systems. If data on the share of wastewater treated are missing for one or more years, the splicing
techniques such as use of surrogate data and extrapolation/interpolation described in Chapter 5, Time Series
Consistency, Volume 1 of the 2006 IPCC Guidelines can be used to estimate emissions. Emissions fromwastewater treated in CWs typically do not fluctuate significantly from year to year.

6.2.3

Uncertainties

Chapter 3 in Volume 1 of the 2006 IPCC Guidelines provides guidance on quantifying uncertainties in practice.
It includes guidance on eliciting and using expert judgments which in combination with empirical data canprovide overall uncertainty estimates. Table 6.5 provides default uncertainty ranges for emission factors andactivity data for domestic and industrial wastewater. The following parameters are believed to be very uncertain:The quantity of wastewater that is treated in CWs or SNTWs.



The fraction of organics that is converted anaerobically to CH4 during wastewater collection. This will
depend on hydraulic retention time and temperature in the wastewater collection pipeline, as well as other6.14

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Chapter 6: Constructed Wetlands for Wastewater Treatment

factors including the presence of anaerobic condition in the wastewater collection pipeline and possiblycomponents that are toxic to anaerobic bacteria in some industrial wastewater.


The amount of industrial TOW from small or medium-scale industries and rural domestic wastewater that isdischarged into CWs in developing countries.



Different plant species applied in CWs that are involved in gas exchange.
TABLE 6.5
DEFAULT UNCERTAINTY RANGES FOR DOMESTIC AND INDUSTRIAL WASTEWATER
Parameter

Uncertainty range*

Emission factor
Maximum CH4 producing capacity (Bo)Methane correction factor (MCF)± 30%
SF: ± 79%
HSSF: ± 31%
VSSF: ± 56%

Activity data
Human population± 5%

BOD per person± 30%

Correction factor for additional industrial BOD
discharged into sewers (I)COD loading from industrial wastewater

For uncollected, the uncertainty is zero %. For collected theuncertainty is ± 20%-55%, +103%* Uncertainty of MCF calculated as 95% confidence interval is shown in Table 1 in the Annex. Uncertainty in the COD
loading from industrial wastewater is calculated based on Table 6.10 in Chapter 6 in Volume 5 of the 2006 IPCC
Guidelines. Others are the same to Table 6.7 in Chapter 6 in Volume 5 of the 2006 IPCC Guidelines.

6.2.4

QA/QC, Completeness and Reporting

It is good practice to conduct quality control (QC) checks and quality assurance (QA) procedures as outlined in
Chapter 6, QA/QC and Verification, Volume 1 of the 2006 IPCC Guidelines. Some fundamental QA/QC
procedures include:
Activity Data
 Make sure that the sum of the wastewater flows for all types of wastewater treatment processes including
CWs equal 100 percent of the wastewater collected/uncollected and treated in the country.


Inventory compilers should compare country-specific data on BOD in domestic wastewater to IPCC default
values. If the inventory compilers use country-specific values they should provide documented justification
why their country-specific values are more appropriate for their national circumstances.

Emission Factors
 For domestic wastewater, inventory compilers can compare country-specific values for Bo with the IPCC
default value (0.25 kg CH4/kg COD or 0.6 kg CH4/kg BOD). As there are no IPCC default values for thefraction of wastewater treated anaerobically, inventory compilers are encouraged to compare values for
MCFs against those from other countries with similar wastewater handling practices.


Inventory compilers should confirm agreement between the units used for organic degradable material in
wastewater (TOW) with the units for Bo. Both parameters should be based on the same units (either BOD or
COD) in order to calculate emissions. This same consideration should be taken into account whencomparing the emissions.



For countries that use country-specific parameters or higher-tier methods, inventory compilers shouldcrosscheck the national estimates with emissions estimated using the IPCC default method and parameters.



For industrial wastewater, inventory compilers should cross-check values for MCFs against those from othernational inventories with similar CW types.

COMPLETENESS
Completeness can be verified on the basis of the degree of utilization of a treatment or discharge system orpathway (T) for all wastewater treatment systems used. The sum of T should equal 100 percent. It is a goodpractice to draw a diagram for the country to consider all potential anaerobic treatment and discharge systems

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Chapter 6: Constructed Wetlands for Wastewater Treatment

and pathways, including collected and uncollected, as well as treated and untreated. CWs and SNTWs are under
treated and collected/uncollected pathway. In general, the amount of vegetation harvested from CWs is verysmall. If vegetation biomass is removed for the purpose of composting, incineration and burning, disposal in
landfills or as fertilizer on agricultural lands, the amount of biomass should be consistent with the data used in
the relevant sectors.
Completeness for estimating emissions from industrial wastewater depends on accurate characterization ofindustrial sectors that produce organic wastewater and the organic loading applied to CW systems. So inventory
compilers should ensure that these sectors are covered. Periodically, inventory compilers should re-survey
industrial sources, particularly if some industries are growing rapidly. This category should only cover industrial
wastewater treated onsite. Emissions from industrial wastewater released into domestic sewer systems should be
addressed and included with domestic wastewater.

REPORTING
Methane emissions from CWs for wastewater treatment are reported in the waste sector under the categories ofdomestic or industrial wastewater. Methane emissions from CWs treating collected runoff from agriculturallands are reported under the category of industrial wastewater.

6.3

NITROUS OXIDE EMISSIONS FROMCONSTRUCTED WETLANDS

6.3.1

Methodological issues

Nitrous oxide (N2O) emissions can occur as direct emissions from wastewater treatment in CWs throughnitrification and denitrification. Emissions are calculated based on the total nitrogen loaded into the CWs andemission factor.
Three tier methods for estimating N2O emissions from this category are summarised below.
The Tier 1 method applies default values for the emission factor and activity parameters. This method is
considered good practice for countries with no country-specific data.
The Tier 2 method follows the same method as Tier 1 but allows for incorporation of country-specific emission
factors and country-specific activity data.
The Tier 3 method is used by countries with good data and advanced methodologies. A more advanced countryspecific method is based on treatment system-specific data such as plant species and composition of wastewater.
The methodology provided assumes typical vegetation harvesting practices. However, to date, the amount ofvegetation harvested from CWs is generally very small and the harvested plant biomass is commonly notrecorded. Therefore, harvesting practice is not considered as an influencing factor in the estimation of emissions.
Emissions from SNTWs treating collected/uncollected wastewater are estimated using the same methodology.
Indirect N2O emission from domestic wastewater treatment effluent that is discharged into aquatic environments
has already been covered in the 2006 IPCC Guidelines.

6.3.1.1

C HOICE OF METHOD

A decision tree for domestic and industrial wastewater is shown in Figure 6.4.
The general equation for estimating N2O emissions from CWs treating domestic or industrial wastewater isshown in Equation 6.5.

EQUATION 6.5
N2O EMISSIONS FROM CONSTRUCTED WETLANDS∑(
Where:
N2O emissions

6.16

=

⁄

)

∑(

⁄

)

N2O emissions in inventory year, kg N2O/yr

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Chapter 6: Constructed Wetlands for Wastewater Treatment

Nj=

total nitrogen in domestic wastewater entering CWs in inventory year, kg
N/yearNi,j=total nitrogen in industrial wastewater entering CW in inventory year, kg
N/yearEFj=

emission factor, kg N2O-N/kg N
If more than one type of CW is used in an industrial sector this factor would
be a Ni,j-weighted average.

i=industrial sectorj=type of CWs

The factor 44/28 is the conversion of kg N2O-N into kg N2O.

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Chapter 6: Constructed Wetlands for Wastewater Treatment

Figure 6.4

Decision tree for N 2 O emission from constructed wetland
Start

Is CW used for
wastewater treatmentin the country?

No

Use the 2006 IPCC Guidelines forthe estimation of emissions fromother wastewater treatment systemand discharge pathways

No

Collect or estimate activity data on
wastewater treatment in CW

Yes

Are activity dataon wastewater treatment in
CW avaialable?

Yes

Are measurement orsite-specific measurement data
available for CW?

Is a country-specific method
available?

Yes

Yes

Estimate emissions
using site-specificmeasurement data
Box 3: Tier 3NoNo

Are country-specificemission factors available
for CW?

Yes

Estimate emissions using countryspecific emission factor
Box 2: Tier2

No

Is CW a key pathway of
key category?

Yes

No

Estimate country-specific emission
factorEstimate emissions using default
emission factors
Box 1: Tier 1

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Chapter 6: Constructed Wetlands for Wastewater Treatment

6.3.1.2

C HOICE OF EMISSION FA CTORSThe default emission factors for N2O emitted from domestic and industrial wastewater treated by CWs are
0.0013 kg N2O-N/kg N for SF, 0.0079 kgN2O-N/kg N for HSSF and 0.00023 kgN2O-N/kg N for VSSF. These
values are based on data provided in the literature (Table 6.2) and are influenced by the extent of nitrificationand denitrification taking place in CWs, coverage of vegetation in CWs and climatic conditions. There wasinsufficient actual measurement data for hybrid systems to derive emission factors. If the area fractions of SF,
VSSF and HSSF for hybrid systems can be determined, the emission factors of the hybrid systems can be
estimated as the area-weighted average of the emission factors for SF, VSSF and HSSF CWs. Good practice isto use country-specific data for emission factor, where available, expressed in term of kg N 2O-N/kg N loaded fordomestic and industrial wastewater to be consistent with the activity data. The amount of N associated with N2Oemissions from CWs must be back calculated and subtracted from the NEFFLUENT (Equation 6.7 in Chapter 6,
Volume 5 of the 2006 IPCC Guidelines).

6.3.1.3

C HOICE OF ACTIVITY DA TA

The activity data for this source category are the amount of nitrogen in the wastewater entering CWs (TN). Thisparameter is a function of the population served by the CW system, annual per capita protein consumption(protein) and a factor for non-consumed nitrogen added to the wastewater for domestic wastewater. In case ofindustrial wastewater, TN loading to the CW system in the inventory year (kg N) can be used directly. The
equations for determining TN for domestic and industrial wastewater are:

EQUATION 6.6
TOTAL NITROGEN IN DOMESTIC WASTEWATEREQUATION 6.7
TOTAL NITROGEN IN INDUSTRIAL WASTEWATER

Where:
Nj=

total nitrogen in domestic wastewater entering the CW in inventory year (kg N/year)Ni=total nitrogen in wastewater from industry i entering the CW in inventory year (kgN/year)i=

industrial sector

Pj=human population whose wastewater entering the CWs

Protein=annual per capita protein consumption, kg/person/yr

FNPR

=

fraction of nitrogen in protein (default is 0.16 kg N/ kg protein as given in the 2006
IPCC Guidelines)

FNON-CON =factor for non-consumed nitrogen added to the wastewater (default is 1.1 for countries
with no garbage disposals, 1.4 for countries with garbage disposals as given in the2006 IPCC Guidelines)

FIND-COM =factor for industrial and commercial co-discharged protein into sewer system (default
is 1.25 as given in 2006 IPCC Guidelines)

TNi=

total nitrogen concentration in wastewater from industry i entering the CWs ininventory year (kg N/m3)

Wi,j

=

flow rate of industrial wastewater entering the CW, m3/yr

Ni is a function of the total N concentration and the flow rate which can be estimated by multiplying theindustrial production P (ton/yr), wastewater generation (m3/ton product) (Table 6.9, Chapter 6, Volume 5 in the2006 IPCC Guidelines) and the N content in Table 6.6 of this supplement.

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Chapter 6: Constructed Wetlands for Wastewater Treatment

TABLE 6.6
EXAMPLE OF N CONTENT IN SOME NITROGEN-RICH INDUSTRIAL WASTEWATER
Wastewater generation W (m3/tonproduct)

N content (kg/m3)

24 (16-32)1

2.40 (0.94-3.86)2

Fish processing industry

5 (2-8)20.60 (0.21-0.98)3

Seasoning source industry

NA

0.60 (0.22-1.00)3

Meat & poultry

13 (8-18)1

0.19 (0.17-0.20)3

Starch production

9 (4-18)1

0.90 (0.80-1.10)4

2.89 (0.46-8.3)20.50 (0.10-0.80)2

15-20% of annual precipitation in wellcompacted landfill site.
25-50% of annual precipitation for not wellcompacted landfill site6.

0.74 (0.01-2.50)5

Industry type
Alcohol refining

Nitrogen fertilizer plant
Landfill leachate

Note: Average value and range (in brackets) are presented
Sources: 1 IPCC 2006; 2Samokhin (1986); 3 Pilot Plant Development and Training Institute (1994); 4 Hulle et.al. (2010); 5 Kjeldsen et al.
(2002); 6 Ehrig (1983)

6.3.2

Time series consistency

The same method and data sets should be used for estimating N2O emissions from CWs for each year. If a
country decides to change the estimation method from the default methodology (Tier 1) to country-specific (Tier2), this change must be made for the entire time series.

6.3.3

Uncertainties

Large uncertainties are associated with the default emission factors for N2O emissions from CWs due to limited
available data (Table 6.7).
TABLE 6.7
NITROUS OXIDE METHODOLOGY DEFAULT UNCERTAINTIESParameter

Default value
0.0013 for SF
0.0079 for HSSF
0.00023 for VSSF

Range
± 90% for SF± 79% for HSSF
± 70% for VSSF

Human population

Country-specific± 10%

Annual per capita protein consumption

Country-specific

± 10%

0.16

0.15-0.17

1.1 for countries with nogarbage disposals,
1.4 for countries withgarbage disposals
Country-specific1.0-1.5

Emission factor (kg N2O-N/kg N)
Activity data

Fraction of nitrogen in protein
Factor for non-consumed nitrogen

TN loading from industrial wastewater-55%, +103%* Uncertainties of emission factors calculated as 95% confidence interval are shown in Table 6A1.1 in Annex. The
uncertainty in TN loading from industrial wastewater is the same to that of COD loading from industrial wastewater(Expert judgement by Authors of this chapter). Others are derived from Tables 6.11 in Chapter 6 in Volume 5 of the 2006
IPCC Guidelines.

6.3.4

QA/QC, Completeness and Reporting

This method makes use of several default parameters. It is recommended to solicit experts’ advice in evaluating
the appropriateness of the proposed default factors. The methodology for estimating emissions is based on N

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Chapter 6: Constructed Wetlands for Wastewater Treatment

associated with domestic and industrial discharges either collected in the collection system and treated in
CWs/SNTWs or uncollected and discharged into CWs/SNTWs. This estimate can be seen as conservative andcovers the entire source associated with domestic and industrial wastewater discharge.

REPORTING
Nitrous oxide emission from CWs for wastewater treatment is reported in waste sector under the categories ofdomestic or industrial wastewater. Nitrous oxide emissions from CWs treating collected runoff from agriculturallands and landfill leachate are reported under the category of industrial wastewater. If agricultural runoff iscollected and treated by CWs or SNTWs, the amount of nitrogen flowing into the CWs/SNTWs must be
subtracted to avoid double counting.

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Chapter 6: Constructed Wetlands for Wastewater Treatment

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Tai, P. D., Li, P. J., Sun, T. H., He, Y. W., Zhou, Q. X., Gong, Z. Q., Mizuochi, M. & Imamori, Y. (2002)
Greenhouse gas emissions from a constructed wetland for municipal sewage treatment. J. Environ. Sci. 14(1):27-33.

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Tanner, C. C., Adams, D. D. & Downes, M. T. (1997) Methane emissions from constructed wetlands treating
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Chapter 6: Constructed Wetlands for Wastewater Treatment

Annex 6A.1 Estimation of default emission factors for CH4 andN2O in constructed wetlands for wastewater treatment
We reviewed about 150 papers published in international peer-reviewed journals indexed by the Thomson
Reuters Web of Knowledge from 1994 to 2013. The terms “free water surface”, “surface flow”, “constructedwetland(s)”, “artificial wetland(s)”, “treatment wetland(s)”, “subsurface flow wetland(s)”, “vertical flow” and“horizontal flow” in combination with the terms “carbon dioxide”, “CO 2”, “methane”, “CH4”, “nitrous oxide”
and “N2O” were searched.
We found a total of 14 publications that provided information on emissions of either CH4, N2O or both gases insurface flow (SF) constructed wetlands (CWs). These publications presented information on 17 different SF CW
systems, whereas for CH4 and N2O, there were 24 and 25 subsystems/measuring events respectively. Six SF
CWs (Nykvarn, Lakeus, Ruka, Skjønhaug, Hässleholm, and Ibaraki) treated domestic wastewater (Johansson etal., 2003, 2004; Søvik et al., 2006, Ström et al., 2006; Gui et al., 2007; Søvik & Kløve, 2007; Liu et al., 2009),
six CWs (mesocosms in Xue et al. (1999), Donaumoos, Genarp, Görarp, Ormastorp, and Hovi) treated waters ofagricultural non-point pollution (Xue et al., 1999; Wild et al., 2001; Stadmark and Leonardson, 2005; Søvik etal., 2006), two systems (Ngatea and Truro) were used for dairy farm wastewater treatment (Tanner et al., 1997;
Van der Zaag et al., 2010), the Kompsasuo CW treated wastewater from a peat extraction area (Søvik et al.,2006), the Jiaonan CW (Tai et al., 2002) purified raw municipal wastewater, and synthetic wastewater is used in
the Jinan laboratory mesocosms (Wu et al., 2009).
For vertical subsurface flow (VSSF) CWs, there were only 4 measurement periods presented for 3 CWs fromwhich CH4 emissions data and ratios could be calculated: Kõo in Estonia (Teiter & Mander 2005; Søvik et al.,2006), Ski in Norway (Søvik et al., 2006), and Miho/Ibaraki, Japan (Gui et al., 2007; Liu et al., 2009). For N2Oemissions, additional laboratory microcosm experiments with different plant species from Ibaraki, Japan(Inamori et al., 2008; Wang et al., 2008) were included.
For CH4 fluxes from horizontal subsurface flow (HSSF) CWs we used data from two system in Estonia treating
domestic wastewater, Kodijärve and Kõo (Mander et al., 2003, 2008; Teiter & Mander, 2005; Søvik et al., 2006),
four CWs treating domestic wastewater in Ski, Norway (Søvik et al., 2006), Barcelona, Spain (Garcia et al.,2007), Miho/Ibaraki, Japan (Gui et al., 2007; Liu et al., 2007) and Slavosovice, Czech Republic (Picek et al.,2007), an HSSF treating wastewater from a peat extraction area in Kompsasuo, Finland (Liikanen et al., 2006),
an HSSF treating landfill leachate in Bangkok, Thailand (Chiemchaisri et al., 2009), and a dairy farm wastewater
treatment HSSF in Truro, Nova Scotia, Canada (Van der Zaag et al., 2010). For N2O emissions from HSSFs, a
CW for dairy farm wastewater treatment in Friedelhausen, Germany (Fey et al., 1998) was also included.
Tanner et al., (1997) presented estimated values for inflow total organic carbon (TOC in), Xue et al., (1999) for
inflow total nitrogen (TNin), and Søvik et al., (2006) for both TOCin and TNin. For most of the systems, TOCin
and TNin values were calculated based on area, hydraulic load and inflow TOC and TN concentration data. For
some systems only biological oxygen demand (BOD) values were usable, and for them the following
approximation based on domestic wastewater data was used: TOC = 0.5 BOD (Garcia et al., 2007). For the
calculations of emission factors, we used data series from one year or at least a vegetation period.
TABLE 6A1. 1
AVERAGE, STANDARD ERROR, MEDIAN, 2.5% AND 97.5% PERCENTILE VALUES OF CH4-C AND N2O-N EMISSION FACTORS(%) FOR DIFFERENT TYPES OF CONSTRUCTED WETLANDS
Type
of
CWs

Emission factor CH4-C/TOC (%)

Emission factor N2O-N/TN (%)

AverageStandard
Error

Median

2.5%

97.5%

AverageStandard
Error

Median

2.5%

97.5%

SF
HSSF
VSSF

42.2
12.0
1.17

20.4
7.56
0.33

18
4.15
1.28

4
0.03
0.38

446
79
1.73

0.13
0.79
0.023

0.024
0.38
0.005

0.11
0.34
0.018

0
0.04
0.001

0.47
3.01
0.096

Table 6A1.1 presents values of emission factors calculated based on literature sources described above.
In Figure 6A1.1, correlations between the inflow TOC loading and CH4-C emissions and between the inflow TN
loading and N2O emissions for SF, HSSF and VSSF CWs are presented.

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Chapter 6: Constructed Wetlands for Wastewater Treatment

Figure 6A1.1

The relationship between inflow TOC loading and CH 4 -C emission (left-handcolumn) and between inflow TN loading and N 2 O-N emission (right-handcolumn) in SF, HSSF, and VSSF CWs. In all cases, p < 0.05.

0.3
N2O-N emission(mg m-2 h-1)

CH4-C emission(mg m-2 h-1)

1000

100
10
1

100

150

0.1

0

200

20

0.5

16

0.4

12
8
4
0
1

HSSF

10

100

1000

0.1
0

0.4

N2O-N emission(mg m-2 h-1)

5

1
0

50

100

150

200

250

300

Inflow TN loading (mg m-2 h-1)

0.5

2

200

0.2

6

3

150

0.3

HSSF

4

100

0

10000

Inflow TOC loading (mg m-2 h-1)

50

Inflow TN loading (mg m-2 h-1)

SF

N2O-N emission(mg m-2 h-1)

CH4-C emission(mg m-2 h-1)

50

Inflow TOC loading (mg m-2 h-1)

SF

CH4-C emission(mg m-2 h-1)

0.15

0
0

0.3

0.2
0.1
0.0

0

6.26

0.2

0.05

0.1

VSSF

0.25

500

1000

Inflow TOC loading (mg m-2 h-1)

1500

0

VSSF

500

1000

1500

2000

Inflow TN loading (mg m-2 h-1)

2013 Supplement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories: Wetlands