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CHAPTER 6

CONSTRUCTED WETLANDS FORWASTEWATER TREATMENT

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Coordinating Lead Authors

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Yoshitaka Ebie (Japan), Sirintornthep Towprayoon (Thailand)

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Lead Authors

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Chart Chiemchaisri (Thailand), Ülo Mander (Estonia) and Sandra Furlan Nogueira (Brazil)

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Contributing Author

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Baasansuren Jamsranjav (TFI TSU)

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Review Editors

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Christopher Gordon (Ghana), Detelina Rumyanova Petrova (Bulgaria)

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Contents

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Constructed wetlands for wastewater treatment .............................................................................................. 4

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6.1

Introduction ............................................................................................................................................ 4

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6.1.1

Constructed wetlands for wastewater treatment ................................................................................. 4

21

6.1.2

Relation to 2006 IPCC Guidelines ..................................................................................................... 7

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6.2

23

Methane emissions from constructed wetlands ...................................................................................... 9

6.2.1

Methodological issues ........................................................................................................................ 924

6.2.1.1

Choice of method ..................................................................................................................... 1025

6.2.1.2

Choice of emission factors ....................................................................................................... 1226

6.2.1.3

Choice of activity data ............................................................................................................. 12

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6.2.2

Time series consistency.................................................................................................................... 13

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6.2.3

Uncertainties .................................................................................................................................... 13

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6.2.4

QA/QC, Completeness and Reporting ............................................................................................. 14

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6.3

Nitrous oxide emissions from constructed wetlands ........................................................................... 15

6.3.1

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Methodological issues ...................................................................................................................... 15

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6.3.1.1

Choice of method ..................................................................................................................... 1533

6.3.1.2

Choice of emission factors ....................................................................................................... 1834

6.3.1.3

Choice of activity data ............................................................................................................. 18

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6.3.2

Time series consistency.................................................................................................................... 19

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6.3.3

Uncertainties .................................................................................................................................... 19

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6.3.4

QA/QC, Completeness and Reporting ............................................................................................. 19

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References ............................................................................................................................................................. 21

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Annex 6A.1 Estimation of default emission factors for CH4 and N2O in constructed wetlands for wastewater
treatment ............................................................................................................................................................... 24

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Equations43

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Equation 6.1

CH4 emissions from constructed wetlands................................................................................ 1045

Equation 6.2

CH4 emission factor for constructed wetlands .......................................................................... 1246

Equation 6.3

Total organically degradable material in domestic wastewater ................................................ 13

47

Equation 6.4

Total organically degradable material in industrial wastewater ................................................ 1348

Equation 6.5

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

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Equation 6.6

Total nitrogen in domestic wastewater ..................................................................................... 18

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Equation 6.7

Total nitrogen in industrial wastewater ..................................................................................... 18

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Figures52

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Figure 6.1

Classification and configuration of constructed wetlands for wastewater treatment .................. 5

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Figure 6.2

Wastewater treatment systems and discharge pathways............................................................. 855

Figure 6.3

Decision tree for CH4 emissions from constructed wetlands................................................. 1156

Figure 6.4

Decision tree for N2O emission from constructed wetland ...................................................... 17

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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. ........................................................................................................................ 25

Tables60

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Table 6.1

Selected factors impacting CH4 and N2O emissions in constructed wetlands ........................... 7

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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 ................................................................................................................. 7

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Table 6.3

Coverage of wastewater types and greenhouse gas emissions from constructed wetlands ...... 966

Table 6.4

Methane Correction Factors by type of constructed wetland .................................................. 1267

Table 6.5

Default uncertainty ranges for domestic and industrial wastewater ........................................ 1468

Table 6.6

Example of N content in some nitrogen-rich industrial wastewater........................................ 1969

Table 6.7

Nitrous oxide methodology default uncertainties.................................................................... 1970
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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 ........................................... 24

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6 CONSTRUCTED WETLANDS FORWASTEWATER TREATMENT

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6.1

INTRODUCTION

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6.1.1

Constructed Wetlands for Wastewater Treatment

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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).

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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).

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"Semi-natural treatment wetlands" (SNTWs) for wastewater treatment are natural wetland systems that havebeen modified for this purpose. The modifications made within these systems usually are 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 that of
surface flow CWs.

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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.

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Emissions from CWs and SNTWs must be reported in 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.

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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 farm, collected runoff from agricultural land and leachate from landfill. For some wastewaters, CWs
are the sole treatment; for others, they are one component in a sequence of treatment processes (US EPA, 1995).

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There are various types of CWs used for treatment of wastewater, and the following paragraphs highlight themain classification of CWs.

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TYPE OF CONSTRUCTED WETLANDS FOR WASTEWATER TREATMENT

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Constructed wetlands may be categorized according to the various design parameters, but the three mostimportant criteria 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 the specific advantages of the different systems. For instance, to guarantee more effective removal ofammonia and total nitrogen (N), during the 1990s and 2000s an enhanced design approach combined vertical andhorizontal flow CWs to achieve higher treatment efficiency (Vymazal, 2011).

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Figure 6.1

Classification and configuration of constructed wetlands for wastewater
treatment

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

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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 the plant stalks and litter regulate water flow and, especially in long, narrow channels(Crites et al. 2005), ensure better water purification. The most common application for SF CWs is 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).

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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 the 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 due to permanent saturation of the beds. The aerobic zones occur around roots and rhizomesthat leak oxygen into the substrate (Brix 1987). HSSF CWs are commonly sealed with a liner to prevent seepage
and to ensure the controllable outflow. HSSF CWs are commonly used for secondary treatment of municipal
wastewater but many other applications have been reported in the literature (Vymazal and Kröpfelova 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 theability to insulate the surface of the bed, are capable of operation under colder conditions than SF systems(Mander and Jenssen 2003).

Constructed Wetlands with Surface Flow

Constructed Wetlands with Subsurface Flow

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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.

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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).

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Hybrid Constructed Wetlands

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Another configuration is a HSSF-VSSF system. The 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 the sedimentation tank (Vymazal2011).

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The 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 effect (Vymazal 2007).

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GREENHOUSE GASES EMISSIONS FROM VARIOUS TYPES OFCONSTRUCTED WETLANDS

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Emissions of greenhouse gases such as methane (CH4) and nitrous oxide (N2O) are a byproduct of CWs, theimportance of which has been increasing recently. Methane is produced in methanogenesis whereas N2O is a
product of denitrification and/or nitrification of N compounds by microorganisms. Among several environmental
factors controlling the greenhouse gases emissions, 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 filter material (water filled soil pores (WFSP)), and presence of aerenchyma plants play asignificant role (see Table 6.1).

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Soil temperature, oxidation reduction potential and the soil moisture (WFSP, depth of ground water level) are themost significant factors affecting emissions of CH4 from CWs (Mander et al., 2003; Van der Zaag et al., 2010).
Several investigations show that a water table deeper than 20 cm from the surface of wetlands and/or waterlogged soils oxidizes most CH4 fluxes (Soosaar et al., 2011; Salm et al., 2012). Fluxes of N2O, however do notshow a clear correlation with soil/air temperature, and significant emissions of N2O from CWs have been
observed in winter (Søvik et al., 2006). Likewise, freezing and thawing cycles enhance N2O emissions (Yu et al.,2011). 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 andfluctuating water table in CWs enhance CO2 emissions and significantly decrease CH4 emissions. N2O emissions,in contrast, do not show a clear pattern regarding pulsing regime.

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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 types. Seemingly, high variability of conditions and combination ofseveral factors in HSSF CWs may be the reason for that. The limited number of available data did not allow
derivation of reliable relationships for HSSF CWs. These shares (%) can be used as a base for the calculation ofemission factors for Tier 1 and Tier 2 methodologies. The high emission factor for CH4 in SF CWs (Table 6.4)
is thought to be due to the additional CH4 from sediments accumulated at the bottom of SF CWs.

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.

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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 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

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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 CONSTRUCTED WETLANDS

Type of
CW
SF
HSSF
VSSF

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

CH4-C emission*(mg CH4-C m-2 h1)

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

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

0.048-17.5 (1.7) 8, 10,

1-11

17.88-1417.50(317) 6, 8, 10, 12

11

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 h1)

N2O-N
emission*(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, 250.033-0.424(0.03) 6, 8, 10, 11,
12-14

N2O-N/TN**(%)
0.13 (0.02)
0.79 (0.4)
0.023 (0.005)* Range and standard error (in bracket)** Average and standard error (in bracket)
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 et al.,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 and Mander
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

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6.1.2

Relation to 2006 IPCC Guidelines

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This chapter is a supplement to Chapter 6 Wastewater Treatment and Discharge of the Volume 5 of the 2006
IPCC Guidelines. The 2006 IPCC Guidelines include a section to estimate CH4 emissions from uncollected
wastewater. The section is expanded in this supplement to cover CH4 emissions from CWs and SNTWs. This
Wetlands Supplement includes guidance on estimation of N2O emissions from CWs and SNTWs. Emission
factors of CH4 and N2O emissions from CWs and SNTWs treating industrial wastewater are the same as thosetreating 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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Figure 6.2

Wastewater treatment systems and discharge pathways

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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 gray-colored box: CWs and SNTWs for treatment of collected wastewater.

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Coverage of wastewater types and gases

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The methodology is provided for estimation of CH4 and N2O emissions from both domestic and industrial
wastewater (Table 6.3). The indirect N2O emissions from N leaching and runoff from agricultural land arecovered in Chapter 11, Volume 4 of the 2006 IPCC Guidelines. Emissions from processing factories ofagricultural products and dairy farm wastewater, collected runoff from agricultural land and leachate fromlandfill are considered as industrial wastewater. According to Chapter 3 of the Volume 5 in the 2006 IPCC
Guidelines, all amount of degradable organic carbon (DOC) in solid waste is subjected to estimation of CH4 in
landfill site, and carbon loss with leachate is not considered because of its low percentage. That means that CH4
emissions from leachate treatment are already covered, and are not included in Section 6.2, while N2O emissions
are considered in Section 6.3 of this supplement. If CH4 emission from CWs is accounted, the amount of DOC inleachate must be subtracted from that in solid waste to avoid double counting. Because C in leachate is normallyindicated in terms of COD, conversion rate from COD in leachate to TOC in solid waste is required in order tosubtract the amount of DOC entering CWs from that in solid waste. This logic can be applied in Tier 2 or 3
estimation.

Chapter 6 of the Volume 5 of the 2006 IPCC Guidelines provides guidance on estimation of CH4 and N2Oemissions from domestic wastewater with emission factors based on treatment technology. Constructed wetlandsin this supplement are an additional treatment technology. The emission factors provided in this chapter coverCWs and SNTWs (collected and treated; see Figure 6.2).

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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
provision of default
emission factors

Industrial wastewater including
wastewater from processing
factories of agricultural products
and dairy farm *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 from agricultural
land

Emissions can be calculated using same
methodology as industrial wastewaterand are covered in this supplement(Section 6.2)

Emissions can be calculated usingsame methodology as 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 flows into
CWs or SNTWs must be subtracted to
avoid double counting.

Leachate from landfill

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

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

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

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6.2

METHANE EMISSIONS FROM CONSTRUCTEDWETLANDS

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6.2.1

Methodological issues

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Methane emissions are a function of the organic materials loaded into CWs and an emission factor.

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Three tiers of methods for estimation of CH4 from CWs are summarized below.

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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.

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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.

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The Tier 3 method is used by countries with good data and advanced methodologies. A more advanced countryspecific method could be based on treatment system-specific data such as plant species and composition ofwastewater.

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In general anaerobic conditions occur in CWs. However, CH4 generated by CWs is not usually recovered andcombusted in a flare or energy device, and so CH4 recovery is not considered here.

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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 commonly not recorded so it is not considered in this supplement.

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6.2.1.1

C HOICE OF METHOD

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A decision tree for domestic or industrial wastewater is shown in Figure 6.3.

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The general equation to estimate CH4 emissions from CWs treating domestic or industrial wastewater is given in
Equation 6.1.

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EQUATION 6.1
CH4 EMISSIONS FROM CONSTRUCTED WETLANDS
∙

,

∙

,

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Where:

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CH4 emissions=

CH4 emissions in inventory year, kg CH4/yr

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TOWj

=

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

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EFj

=

emission factor, kg CH4/kg BOD (for domestic wastewater only) or kg
CH4/kg COD (for both domestic and industrial wastewater)

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If more than one type of CW is used in an industrial sector this factor would
need to be a TOWi,j-weighted average.

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i

=

industrial sector

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j

=

type of CW

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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 or
measured 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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6.2.1.2

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The emission factor for wastewater treatment using CWs is a function of maximum CH4 producing potential (Bo)and the methane correction factor (MCF).

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EQUATION 6.2
CH4 EMISSION FACTOR FOR CONSTRUCTED WETLANDS∙

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C HOICE OF EMISSION FACTORS

Where:

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EFj

=

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

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j

=

type of CWs

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Bo

=

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

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MCFj

=

methane correction factor (fraction), See Table 6.4

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275
276
277
278
279
280
281
282
283
284

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.
The MCF indicates the extent to which Bo is realized 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 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 BY TYPE OF CONSTRUCTED WETLAND
CW type

MCF

Surface flow (SF)

0.4

Horizontal subsurface flow (HSSF)

0.1

Vertical subsurface flow (VSSF)

0.01

285
286
287
288
289
290
291
292
293
294
295

These MCF values are derived based on actual measurement data and thus the operating and environmental
conditions such as vegetation types and temperature effect have been taken into account. Based on the reported
scientific data, there was insufficient information to distinguish the MCF values by vegetation types andoperating temperatures. 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 the hybrid
systems can be estimated as the area-weighted average of the MCFs for SF, VSSF and HSSF. Most commonly,
SNTWs are surface flow type (Kadlec and Wallace, 2008), therefore, the default MCF of 0.4 can be used. If thetype of CW cannot be recognized, the MCF of surface flow can be used in order to be conservative. Otherwise
country-specific data should be used in higher tier method.

296

6.2.1.3

297
298
299
300
301
302
303

The activity data for this source category is the amount of organic materials (TOW) in the wastewater treated by
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 from various industries are provided in Table 6.9, Chapter 6, Volume 5 of the 2006 IPCC
Guidelines.

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305
306

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

307
308

EQUATION 6.3
TOTAL ORGANICALLY DEGRADABLE MATERIAL IN DOMESTIC WASTEWATER∙∙ ∙ 0.001 ∙ 365

309
310
311
312
313

EQUATION 6.4
TOTAL ORGANICALLY DEGRADABLE MATERIAL IN INDUSTRIAL WASTEWATER∙ , ∙ 365,
Where:

314
315

TOWj

=

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

316
317

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

318

i

=

industrial sector

319

Pj

=

population whose wastewater treated in CW*

320

BOD

=

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

321

I

=

correction factor for additional industrial BOD discharged into sewers (for322

collected the default is 1.25, for uncollected the default is 1.00 as given in the 2006

323

IPCC Guidelines)

324
325

CODi

=

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

326

Wi,j

=

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

327
328

* Population should be subtracted from total population used in an Equation 6.3 in Chapter 6, Volume 5 in the 2006 IPCC
Guidelines to avoid double-counting.

329

6.2.2

330
331
332
333
334
335
336
337
338
339

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.

340

6.2.3

341
342
343
344
345

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.

346
347



Time series consistency

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 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.

Uncertainties

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, and on other

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349

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

350
351



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

352



Different plant species applied in CWs that are involved in gas exchange.

353
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%

354
355
356

* Uncertainty of MCF calculated as 95% confidence interval is shown in Table 1 in Annex. Uncertainty of COD loadingfrom 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 Tables 6.7 in Chapter 6 in Volume 5 of the 2006 IPCC Guidelines.

357

6.2.4

358
359
360
361
362
363
364

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:

365
366
367



368
369
370
371
372
373

QA/QC, Completeness and Reporting

Activity Data
 Make sure that the sum of wastewater flows of all types of wastewater treatment processes including CWs
equal 100 percent of wastewater collected and treated in the country.
Inventory compilers should compare country-specific data on BOD in domestic wastewater to IPCC default
values. If 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.

374
375
376
377



Inventory compilers should confirm the agreement between the units used for organically 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 accountwhen comparing the emissions.

378
379



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.

380
381



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

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COMPLETENESS

383
384
385
386
387
388
389
390

Completeness can be verified on the basis of the degree of utilization of a treatment or discharge system orpathway (T) for all wastewater treatment system 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
and pathways, including collected and uncollected, as well as treated and untreated. Constructed wetlands and
SNTWs are under treated and collected pathway. In general, the amount of vegetation harvested from CWs isvery small. 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 data used in therelevant sectors.

391
392
393
394
395
396

Completeness for estimating emissions from industrial wastewater depends on an 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, the 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.

397

REPORTING

398
399
400

Methane emission from CWs for wastewater treatment is reported in waste sector under the categories ofdomestic or industrial wastewater. Methane emission from CWs treating collected runoff from agricultural land
is to be reported under the category of industrial wastewater.

401

6.3

NITROUS OXIDE EMISSIONS FROMCONSTRUCTED WETLANDS

403

6.3.1

Methodological issues

404
405
406

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 CWs andemission factor.

407

Three tier methods for N2O from this category are summarized below.

408
409

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.

410
411

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.

412
413

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.

414
415
416

The methodology provided assumes typical vegetation harvesting practices. However, the amount of vegetation
harvested from CWs (studied until now) is generally very small and the harvested plant biomass is commonly
not recorded so the harvesting practice is not considered as an influencing factor in the estimation of emissions.

417
418
419

Emissions from SNTWs treating uncollected wastewater are estimated using the same methodology. Indirect
N2O emissions from domestic wastewater treatment effluent that is discharged into aquatic environments has
already been covered in the 2006 IPCC Guidelines.

420

6.3.1.1

421

A decision tree for domestic or industrial wastewater is shown in the Figure 6.4.

422
423

The general equation to estimate N2O emissions from CWs treating domestic or industrial wastewater is shownin Equation 6.5.

402

C HOICE OF METHOD

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426

EQUATION 6.5
N2O EMISSIONS FROM CONSTRUCTED WETLANDS∙∙ 44⁄28,∙ 44⁄28

∙

,

427
428

Where:
N2O emissions=

N2O emissions in inventory year, kg N2O/yr

429
430

Nj

=

total nitrogen in domestic wastewater entering CWs in the inventory year, kg
N/year

431
432

Ni,j

=

total nitrogen in industrial wastewater entering CW in the inventory year, kg
N/year

433

EFj

=

emission factor, kg N2O-N/kg N

434
435

If more than one type of CW is used in an industrial sector this factor would
need to be a Ni,j-weighted average.

436

i

=

industrial sector

437

j

=

type of CWs

438

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

439

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440

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 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 or
measured 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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6.3.1.2

C HOICE OF EMISSION FACTORS

443
444
445
446
447
448
449
450
451
452
453

The 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 literatures and influenced by the extent of nitrification anddenitrification taking place in CWs, the coverage of vegetation in CWs and climatic conditions. There wasinsufficient actual measurement data of 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 N2O-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).

454

6.3.1.3

455
456
457
458
459

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 constructed wetland system in the inventory year (kg N) can be useddirectly. The equations for determining TN for domestic and industrial wastewater are:

C HOICE OF ACTIVITY DATA

460
461
462

EQUATION 6.6
TOTAL NITROGEN IN DOMESTIC WASTEWATER
∙
∙
∙
∙

463
464
465

EQUATION 6.7
TOTAL NITROGEN IN INDUSTRIAL WASTEWATER
∙ ,
,

466
467

Where:
Nj=

total nitrogen in domestic wastewater entering CW in inventory year (kg N/year)

468

Ni

=

total nitrogen in wastewater from industry i entering CW in inventory year (kg N/year)

469

i

=

industrial sector

470

Pj

=

human population whose wastewater entering CWs

471

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

472
473

FNPR

=

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

474
475
476

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)

477
478

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

479
480

TNi

=

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

481

Wi,j

=

flow rate of industrial wastewater entering CW, m3/yr

482
483
484
485

Ni is a function of total N concentration and flow rate which can be estimated by multiplying industrial product P(tons/yr), wastewater generation (m3/ton) (Table 6.9, Chapter 6, Volume 5 in 2006 IPCC Guidelines) and N
content in Table 6.6 of this supplement.

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

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
well compacted 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)

486

6.3.2

Time series consistency

487
488
489

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.

490

6.3.3

491
492

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

Uncertainties

493
TABLE 6.7
NITROUS OXIDE METHODOLOGY DEFAULT UNCERTAINTIESParameter

Default valueRange

0.0013 for SF
0.0079 for HSSF
0.00023 for VSSF

± 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%

494
495
496

* Uncertainties of emission factors calculated as 95% confidence interval is shown in Table 6A1.1 in Annex. Uncertainty of
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.

497

6.3.4

498
499
500

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
associated with domestic and industrial discharge either collected into the collection system and treated in CWs

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502

or uncollected and discharged into SNTWs. This estimate can be seen as conservative and covers the entire
source associated with domestic and industrial wastewater discharge.

503

REPORTING

504
505
506
507
508

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 agriculturalland and landfill leachate are to be reported under the category of industrial wastewater. If agricultural runoff iscollected and treated by CWs or SNTWs, the amount of nitrogen flows into CWs/SNTWs must be subtracted to
avoid double counting.

509

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Cooper, P. F., Job, G.D., Green, M.B., and Shutes, R.B.E. (1996). "Reed Beds and Constructed Wetlands for
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Cooper, P. F. (2005). "The performance of vertical flow constructed wetland systems with special reference tothe significance of oxygen transfer and hydraulic loading rates." Water Sci. Technology 51(9): 81-90.

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Crites, R. W., Middlebrooks, E.J., and Reed, S.C. (2005). "Natural wastewater treatment systems." CRC Press:
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Ehrig, H. J. (1983). "Quality and quantity of sanitary landfill leachate " Waste Management & Research 1: 53-68.

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Fey, A., Benckiser, G., and Ottow, J.C.G. (1999). "Emissions of nitrous oxide from a constructed wetland usinga groundfilter and macrophytes in waste-water purification of a dairy farm." Biol. Fertil. Soils 29: 354-359.

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Garcia, J., Capel, V., Castro, A., Ruiz, I., and Soto, M. (2007). "Anaerobic biodegradation tests and gas
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Hunt, P. G., Stone, K.C., Matheny, T.A., Poach, M.E., Vanotti, M.B., and Ducey, T.F. (2009). "Denitrification of
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Inamori, R., Gui, P., Dass, P., Matsumura, M., Xu, K. Q., Kondo, T., Ebie, Y., Inamori, Y. (2007). "Investigating
CH4 and N2O emissions from eco-engineering wastewater treatment processes using constructed wetland
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Inamori, R., Wang, Y., Yamamoto, T., Zhang, J., Kong, H., Xu, K., and Inamori, Y. (2008). "Seasonal effect on
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Jia, W., Zhang, J., Li, P., Xie, H., Wu, J., and Wang, J. (2011). "Nitrous oxide emissions from surface flow andsubsurface flow constructed wetland microcosms: Effect of feeding strategies." Ecological Engineering 37(11):1815-1821.

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Johansson, A. E., Klemedtsson, K., Klemedtsson, L., Svensson, B.N., and Dass, P. (2003). "Nitrous oxide
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Kjeldsen, P., Barlaz, M.A., Rooker, A.P., Baun, A., Ledin, A., and Christensen, T.H. (2002). "Present and longterm composition of MSW landfill leachate- a review." Critical Reviews in Environmental Science andTechnology 32(4): 297-336.

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Annex 6A.1 Estimation of default emission factors for CH4 andN2O in constructed wetlands for wastewater treatment

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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”, “CO2”, “methane”, “CH4”, “nitrous oxide”
and “N2O” were searched.

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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 and Kløve, 2007; Liu et al., 2009),
six CWs (mesocosms in Xue et al. (1999) paper, Donaumoos, Genarp, Görarp, Ormastorp, and Hovi) treatedwaters of agricultural non-point pollution (Xue et al., 1999; Wild et al., 2001; Stadmark and Leonardson, 2005;
Søvik et al., 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).

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Regarding the vertical subsurface flow (VSSF) CWs, there were only 4 measurement periods presented for 3
CWs from which CH4 emission data and ratios could be calculated: Kõo in Estonia (Teiter and 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 N2O emission, additionally laboratory microcosm experiments with different plant species from
Ibaraki, Japan (Inamori et al., 2008; Wang et al., 2008) were included.

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For CH4 fluxes from horizontal subsurface flow (HSSF) CWs we could use data from two system in Estonia
treating domestic wastewater, Kodijärve and Kõo (Mander et al., 2003, 2008; Teiter and Mander, 2005; Søvik etal., 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 etal., 2007), a HSSF treating wastewater from a peat extraction area in Kompsasuo, Finland (Liikanen et al., 2006),
a 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,
also a CW for dairy farm wastewater treatment in Friedelhausen, Germany (Fey et al., 1998) has been included.

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Tanner et al., (1997) presented estimated values for inflow total organic carbon (TOCin), 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.

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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 WETLANDSEmission factor CH4-C/TOC (%)

SF
HSSF
VSSF

Emission factor N2O-N/TN (%)

AverageStandard
Error

Median

2.5%

97.5%

AverageStandard
Error

Median

2.5%

97.5%

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

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Table 1 presents values of emission factors calculated based on literature sources described above.

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In Figure 1, correlation between the inflow TOC loading and CH4-C emission and between the inflow TN
loading and N2O emission in SF, HSSF and VSSF CWs is presented.

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Figure 6A1.1

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

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