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CHAPTER 6

CONSTRUCTED WETLANDS WASTEWATER 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), Ulo 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 ...................................................................................................................................... 4

6

6.1.1

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22

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

6.1

6.2

23

Relation to 2006 IPCC Guidelines ..................................................................................................... 7
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, Reporting and Documentation ................................................................... 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, Reporting and Documentation ................................................................... 20

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Equation 6.1 CH4 emissions from constructed wetlands ............................................................................ 10

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Equation 6.2 CH4 emission factor for constructed wetlands ...................................................................... 1242

Equation 6.3 Total organically degradable material in domestic wastewater ............................................ 13

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Equation 6.4 Total organically degradable material in industrial wastewater ............................................ 13

44

Equation 6.5 N2O emissions from constructed wetlands ........................................................................... 16

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Equation 6.6 Total nitrogen in domestic wastewater effluent .................................................................... 18

46

Equation 6.7 Total nitrogen in industrial wastewater effluent ................................................................... 18

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Figures48

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Figure 6.1

Classification and configuration of constructed wetlands for wastewater treatment ................. 550

Figure 6.2

Wastewater treatment systems and discharge pathways ............................................................ 851

Figure 6.3

Decision tree for CH4 emissions from constructed wetlands .................................................. 11

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Figure 6.4

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

Tables

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Table 6.1

Selected factors impacting CH4 and N2O emissions in constructed wetlands .............................. 6

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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 inconstructed wetlands. .................................................................................................................... 758

Table 6.3

Coverage of wastewater types and greenhouse gas emissions from constructed wetlands ......... 959

Table 6.4

Methane Correction Factors by type of constructed wetland ..................................................... 1260

Table 6.5 Default uncertainty ranges for domestic and industrial wastewater ........................................... 1461

Table 6.6

Example of N content in some nitrogen-rich industrial wastewater.......................................... 1962

Table 6.7

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

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6 CONSTRUCTED WETLANDS-WASTEWATERTREATMENT

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6.1

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Wetland ecosystems can act as sources, sinks, or transformers of nutrients and carbon (Mitsch and Gossenlink,1993). This ability of wetlands has led to a widespread use of natural and constructed wetlands 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, constructed wetlands are engineered systems that have been designed and constructed to utilize thenatural processes involving wetland vegetation, soils, and their associated microbial assemblages to assist intreating wastewater. Synonymous terms to “constructed” include “man-made”, “engineered” or “artificial”(Vymazal, 2007). In general the size of constructed wetlands is varied from 5,000 m2 to 34,000 m2.

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"Semi-natural treatment wetlands" for wastewater treatment are natural wetland systems that have been modified
for this purpose. The modifications made within these systems usually are based on increasing the volume
reserved (i.e. dams) and constructing channels for targeting the influent and effluent. These systems can be foundin both freshwater and coastal wetlands. In general the size of semi-natural treatment wetlands is approximately
405,000 m2.

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If freshwater and coastal wetlands are modified to semi-natural treatment wetlands, compilers should check with
relevant land-use category in this supplement to avoid double-counting.

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APPLICATION OF CONSTRUCTED WETLANDS FOR WASTEWATERTREATMENT

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Constructed wetlands are used to improve the quality of water polluted from point and nonpoint sources of waterpollution, including stormwater runoff, domestic wastewater, agricultural wastewater, and mine drainage.
Constructed wetlands are also being used to treat petroleum refinery wastes, compost and landfill leachates,
aquaculture discharges, and pre-treated industrial wastewaters, such as those from pulp and paper mills, textile
mills, and seafood processing. For some wastewaters, constructed wetlands 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 constructed wetlands used for treatment of wastewater, and following paragraphhighlights the main classification of constructed wetlands.

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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 flow path (horizontal and vertical) (Figure 6.1; Vymazal 2007, 2011). Different
types of constructed wetlands 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, during the 1990s and 2000s an enhanced design approach combined vertical and horizontal flow
constructed wetlands to achieve higher treatment efficiency (Vymazal, 2011).

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Figure 6.1

Classification and configuration of constructed wetlands for wastewater
treatment
Constucted Wetlands

Sub-Surface Flow

Surface Flow
Inflow

Outflow

Inflow

Free Floating plants
Inflow

Outflow

Vertical flow

Outflow

Floating leaved plants
Inflow

Outflow

Submerged plants
Inflow

Outflow

Inflow

Horizontal flow

Outflow

Emergent plants
Inflow

Hybrid systems

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Outflow

Note: Adopted from Vymazal, 2007, 2011.

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Constructed Wetlands with Surface Flow

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Constructed Wetlands with Subsurface Flow

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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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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

Constructed wetlands with surface flow (SF), known as free water surface constructed wetlands, contain areas ofopen water and floating, 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 constructedwetlands 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 wetlands are suitable in all climates, including
the far north (Mander and Jenssen 2003).
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 in VSSF CWs percolates down through the bed and is 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, thus 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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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).

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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 nitrogen, however, the nitrified effluent from the VF bed must be recycled to the sedimentation tank(Vymazal 2011).

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The VSSF-HSSF and HSSF-VSSF constructed wetlands are the most common hybrid systems, but in general,any kind of constructed wetlands 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 constructedwetlands, the importance of which has been increasing recently. Methane is produced in methanogenesis
whereas nitrous oxide is 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 vssteady-state flow), groundwater depth, moisture of filter material (water filled soil pores (WFSP)), and presenceof aerenchymal plants plays a significant role (see Table 6.1).

Various types of constructed wetlands may be combined to achieve higher removal efficiency, especially for
nitrogen. The design consists of two stages, several parallel VF beds followed by 2 or 3 HF beds in series(VSSF-HSSF system). The VSSF wetland is intended to remove organics and suspended solids and to promote
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 most cases

No clear relationship

Higher moisture of soil or filter material(higher value of WFSP)

Clear increase

Decrease

Higher wastewater loading

Increase

Increase

Presence of aerenchymal plants

Increase/decrease (depends on
conditions)

Increase

Pulsing hydrological regime(intermittent loading)

Clear decrease

Increase (decrease in some SF
CWs)

Deeper water table (from surface) in
HSSF CWs

Decrease

Increase

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Soil temperature, oxidation reduction potential and the soil moisture (WFSP, depth of (ground) water level) are
the most significant factors affecting emissions of CH4 in constructed wetlands (Mander et al 2003; Van derZaag et al 2010). Several investigations show that a water table deeper than 20 cm from the surface of wetlands
and/or water-logged soils oxidizes most 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 fromconstructed wetlands have been observed in winter (Søvik et al 2006). Likewise, freezing and thawing cycles

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enhance N2O emissions (Yu et al 2011). Hydrological regime also plays a significant role in greenhouse gases
emissions from constructed wetlands. Altor and Mitsch (2008) and Mander et al (2011) demonstrated that theintermittent loading (pulsing) regime and fluctuating water table in constructed wetlands 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 andN loadings in and respective CH4 and N2O emissions from the main types of constructed wetlands. There is asignificant positive correlation (p < 0.05) between the initial loadings and CH4 and N2O emissions from both SF
and VSSF constructed wetlands, whereas no correlation was found for HSSF types. Seemingly, high variabilityof conditions and combination of several factors in HSSF constructed wetlands may be the reason for that. The
limited number of available data did not allow derivation of reliable relationships for HSSF constructed wetlands.
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 in SF CWs (Table 6.4) is thought to be due to the additional CH4 fromsediments accumulated at the bottom of SF wetlands.

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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

VSSF

HSSF

Influent
TOC*
mg C m-2 h-1

31 (6) a-n

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

6 (1.2) a-n

1670 (285) f,h,k,oq

31 (9) f,h,k,o-q

1118 (858) h,k,o,rw

3.6 (0.7) h,k,o,r-w* Average and standard error (in bracket).

CH4C/TOC**%
40(5.9; 7-07;
27)
1.44(0.24; 0.323.42; 18)
26.1(14.2; 0.03129.5; 11)

Influent TN*
mg N m-2 h-1

N2O-N
emission*mg N2O-N m2 -1h

127 (31) b,f-0.12 (0.03) b,f-h,j,k,m, x-aa

h,j,k,m,x-aa

919 (155)0.14 (0.03)

f,h,k,m,o-q

f,h,k,m,o-q

124 (48) f,k,o,r-0.33 (0.13)

t,w,ab,ac

f,k,o,m,r-t,w,ac

N2O-N/TN**%
0.24(0.10; 0.012.19; 25)
0.021(0.005; 0.0010.058; 21)1.0(0.5; 0.043.01; 7)** Average, and standard error, range of values and number of sites studied (in bracket)
Source: a - Tanner et al 1997; b - Wild et al 2001; c - Tai et al 2002; d - Johansson et al 2004;e - Stadmark & Leonardson 2005;
f - Søvik et al 2006; g - Søvik & Kløve 2007; h - Gui et al 2007; i - Altor & Mitsch 2008; j - Ström et al 2008; k - Liu et al 2009;l - Nahlik & Mitsch 2010; m - Van der Zaag 2010; n - Sha et al 2011; o - Teiter & Mander 2005; p - Inamori et al 2007;
q - Wang et al 2008; r - Mander et al 2005; s - Mander et al 2008, t - Liikanen et al 2006; u - Garcia et al 2007; v - Picek et al 2007;
w - Chiemchaishri et al 2009; x - Xue et al 1999; y - Johansson et al 2004; aa - Wu et al 2009; ab - Inamori et al 2008; ac - Fey et al 1999

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6.1.1

Relation to 2006 IPCC Guidelines

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This chapter is a supplement to Chapter 6 WASTEWATER TREATMENT AND DISCHARGE in the 2006
IPCC Guidelines for National Greenhouse Gas Inventories. The 2006 IPCC Guidelines include a section toestimate CH4 emissions from uncollected wastewater. The section is expanded in this supplement to cover CH4
emissions from constructed wetlands and semi-natural treatment wetlands. This Wetlands Supplement includes
guidance on estimation of nitrous oxide (N2O) emissions from constructed wetlands and semi-natural treatment
wetlands. Emission factors of CH4 and N2O from constructed wetlands and semi-natural treatment wetlands
treating industrial wastewater are the same as those treating domestic wastewater treating ones. CO2 emissions
are not included in greenhouse gases emissions from wastewater treatment as CO2 from wastewater is consideredbiogenic.

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Figure 6.2

Wastewater treatment systems and discharge pathways

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Domestic and Industrial Wastewater
Collected

Uncollected

Untreated

Rivers, Lakes,
Estuaries, Sea

TreatedStagnant
sewer

Aerobic treatment
SludgeAnaerobic
Digestion

Seweredto plant

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

Anaerobic treatment
ReactorLand
DisposalTreated

LagoonUntreated
Rivers, Lakes,
Estuarine,Sea

To ground

ConstructedWetlands, Semi‐
natural treatment
Wetlands

Landfill orIncineration

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Note: Emissions from boxes with bold frames are accounted for 2006 IPCC Guidelines. Supplement provides EF for black-colored box;
Constructed wetlands and semi-natural treatment wetlands for treatment of collected wastewater.

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Coverage of wastewater types and gases

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Chapter 6 of the 2006 IPCC Guidelines provides guidance on estimation of CH4 and N2O emissions fromdomestic wastewater with emission factors based on treatment technology. Constructed wetland in this
supplement is an additional treatment technology. The emission factors provided in this chapter coverconstructed wetlands and semi-natural treatment wetlands (collected and treated) see Figure 6.2.

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The methodology is provided for estimation of CH4 and N2O emissions from both domestic and industrial
wastewater. The indirect N2O emissions from N leaching and runoff from agricultural land are covered in the2006 IPCC Guidelines. Emissions from agro-industrial wastewater (except manure management) and minedrainage are considered as industrial wastewater. As for solid waste landfill leachate, CH4 emissions from solid
waste landfill leachate have already been considered in solid waste disposal on land in Chapter 3, Volume 5 in
the 2006 IPCC Guidelines, and are not included in section 6.2, while N2O emissions are considered in section
6.3.

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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 with
provision of methane correction factor(MCF)

Included in this supplement with
provision of EF

Industrial wastewater including
agro-industrial and dairy farm
wastewater

Included in this supplement with
provision of MCF

Included in this supplement with
provision of EF

Collected runoff from agriculturallandCollected runoff waters can beconsidered as industrial wastewater andare covered in this supplement

Collected runoff waters can beconsidered as industrial wastewaterand are covered in this supplement
Note: Indirect N2O emissions from N
leaching and runoff from agriculturalland are considered in Chapter 11,
Volume 4 of the 2006 IPCC
Guidelines and the amount of nitrogenflow into CWs must be subtracted to
avoid double counting.

Leachate from landfill

The amount of DOC leached from the
SWDS is not considered in theestimation of DOCf. Generally theamounts of DOC lost with the leachate
are less than 1 percent and can beneglected in the calculations (Chapter
3, Volume 5, 2006 IPCC Guidelines)
and not considered in this supplement

Included in this supplement

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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 constructed wetlands and an emission
factor.

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Three tiers of methods for estimation of CH4 from constructed wetlands 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 condition occurs in CWs. However, CH4 generated by constructed wetlands is not usually
recovered and combusted in a flare or energy device, and so CH4 recovery is not considered here.

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The amount of vegetation harvesting from constructed wetland is generally very small and its impact on total
emissions from constructed wetlands is considered insignificant. Moreover, the harvesting is usually not
performed on regular basis and the quantity of harvested biomass is commonly not recorded so it is notconsidered 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 constructed wetlands treating domestic or industrial
wastewater is given in Equation 6.1.

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EQUATION 6.1
CH4 EMISSIONS FROM CONSTRUCTED WETLANDS
CH4 Emissions = TOW  EF
Where:

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CH4 emissions=

CH4 emissions in inventory year, kg CH4/yr

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TOW

=

total organics in wastewater treated by constructed wetland in inventory

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year, kg BOD/yr or kg COD/yr
EF

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=

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

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Figure 6.3

Decision tree for CH 4 emissions from constructed wetlands

Start

Is CW used for
wastewater
treatment in the
country?

Use the 2006 IPCC Guidelines
for the estimation of emissions
from other wastewater
treatment system and discharge
pathways

No

Yes

Are activity dataon wastewater
treatment in CW
available?

Collect or estimate activity
data on wastewater treatment
in CW

No

Yes

Are measurement
or bottom-up data
available for CW
treatment?

Yes

Is a countryspecific method
available?

Yes

Estimate emissions
using bottom-up dataor measured data
Box 3: Tier 3NoNo

Are countryspecific EFs
available for CW
treatment?

Yes

Estimate emissions using
country-specific EF
Box 2: Tier 2

No

Is CW is a key
pathway of key
category?

Yes
Estimate country-specific
Bo and MCF

No

Estimate emissions using
default emission factors
Box 1: Tier1

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6.2.1.2

C HOICE OF EMISSION FACTORS

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The emission factor for wastewater treatment using constructed wetlands 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
EF = Bo  MCF
Where:

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EF=

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

305

Bo

=

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

306

MCF

=

methane correction factor (fraction)

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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 maximum CH4 producing capacity (Bo) for domestic and industrial
wastewater: 0.6 kg CH4/kg BOD and 0.25 kg CH4/kg COD.
The Methane Correction Factor (MCF) indicates the extent to which CH4 producing capacity (Bo) is realized ineach type of constructed wetland. It is an indication of the degree to which the system is anaerobic. The proposed
MCF for each type of constructed wetland treating domestic and industrial wastewater is provided in Table 6.4,
which is derived from literature-based analysis of CH4 conversion rates: each MCF is calculated from therelation of initial TOC loading and methane emission flux derived from references provided in Table 6.2.
TABLE 6.4
METHANE CORRECTION FACTORS BY TYPE OF CONSTRUCTED WETLAND
CW type

MCF

Range

Surface flow (SF)

0.35

0.32-0.37

Horizontal subsurface flow (HSSF)

0.1

0.064-0.227

Vertical subsurface flow (VSSF)

0.03

0.025-0.048

321
322
323
324
325
326
327
328

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 value for different vegetation types andoperating temperatures. Nevertheless, these influencing factors can be considered for the estimation using higher
tier approach. Generally semi-natural treatment wetlands are surface flow type, therefore, the default MCF of
0.35 can be used. The highest MCF must be chosen if the type of constructed wetland cannot be recognized.
Otherwise country-specific data should be used in higher tier method.

329

6.2.1.3

330
331
332
333
334
335
336

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

337
338
339
340

If industrial wastewater is released into domestic sewers, it is estimated together with domestic wastewater.

C HOICE OF ACTIVITY DATA

The equations for TOW are:

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342
343

EQUATION 6.3
TOTAL ORGANICALLY DEGRADABLE MATERIAL IN DOMESTIC WASTEWATERTOW = P  BOD  I  0.001  365

344
345
EQUATION 6.4
TOTAL ORGANICALLY DEGRADABLE MATERIAL IN INDUSTRIAL WASTEWATERTOW = COD  W  365

346
347
348
349

Where:
TOW

350

=

351

total organics in domestic or industrial wastewater treated in the constructed wetland
in inventory year (kg BOD/year or kg COD/year)

352

P

=

population whose wastewater treated in CWs*

353

BOD

=

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

354

I

=

correction factor for additional industrial wastewater discharged into sewers (for355

collected the default is 1.25, for uncollected the default is 1.00 as recommended in the3562006 IPCC Guidelines)

357

COD

=

COD concentration in industrial wastewater treated by constructed wetlands in theinventory year (kg COD/m3)

358
W

359

=

Daily flow rate of industrial wastewater treated by constructed wetlands, m3/d

360
361

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

362

6.2.2

363
364
365
366
367
368
369
370
371
372
373

The same method and data sets should be used for estimating CH4 emissions from constructed wetlands treating
wastewater for each year. The MCF for different treatment systems should not change from year to year, unlesssuch a change is justifiable and documented. If the share of wastewater treated in different treatment systems
changes over the time period, the reasons for these changes should be documented.

374

6.2.3

375
376
377
378
379
380
381

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. Tables 6.7 and 6.10 in Chapter 6 of Volume 5 provide default uncertainty
ranges for emission factors and activity data of domestic and industrial wastewater, respectively. The following
parameters are believed to be very uncertain:The quantity of wastewater that is treated in constructed wetlands or semi-natural treatment wetlands.

382
383
384
385



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

386
387



The amount of industrial TOW from small or medium-scale industries that is discharged into constructedwetlands in developing countries.

Time series consistency

For activity data that are derived from population data, countries must determine the fraction of the population
served by constructed wetland 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, of Volume 1 General Guidance and Reporting of the 2006 IPCC Guidelines can be used toestimate emissions. Emissions from wastewater treated in constructed wetlands typically do not fluctuatesignificantly from year to year.

Uncertainties

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

Different plant species applied in constructed wetlands that are involved in gas exchange.

389
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: -8%,+14%,
VSSF: -16%, +60%
HSSF: -36%, +127%

Activity data390

Human population± 5%

BOD per person± 30%

Correction factor for additional industrial
BOD discharged into sewers (I)For uncollected, the uncertainty is zero %. For collected theuncertainty is ± 20%

COD loading from industrial wastewater

± 30%

6.2.4

391

QA/QC, Completeness, Reporting andDocumentation

392
393
394
395
396
397
398

It is good practice to conduct quality control checks and quality assurance procedures as outlined in Chapter 6,
QA/QC and Verification, of Volume 1 of the 2006 IPCC Guidelines. Some fundamental QA/QC procedures
include:

399
400
401



402
403
404
405
406
407

Activity Data
 Make sure that the sum of wastewater flows of all types of wastewater treatment processes including
constructed wetlands equal 100 per cent 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.

408
409
410
411



Inventory compilers should confirm the agreement between the units used for degradable carbon in thewaste (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 when comparing theemissions.

412
413



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

414
415



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

416
417
418
419
420
421
422
423

CompletenessCompleteness 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 andsemi-natural treatment wetlands are under treated and collected pathway. In general, the amount of vegetation
harvested from constructed wetlands is very small. If vegetation biomass is removed for the purpose of

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424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443

composting, incineration and burning, disposal in landfills or as fertilizer on agricultural lands, the amount ofbiomass should be consistent with data used in the relevant sectors.
Completeness for estimating emissions from industrial wastewater depends on an accurate characterization ofindustrial sectors that produce organic wastewater and the organic loading applied to constructed wetland
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.
Reporting and documentation
It is good practice to document and report a summary of the methods used, activity data and emission factors.
Worksheets are provided at the end of this volume. When country-specific methods and/or emission factors areused, the reasoning for the choices as well as references to how the country-specific data (measurements,
literature, expert judgment, etc.) have been derived (measurements, literature, expert judgment, etc.) should be
documented and included in the reporting.
More information on reporting and documentation can be found in the 2006 IPCC Guidelines in Volume 1,
Chapter 6, Section 6.11 Documentation, archiving and reporting.

6.3

NITROUS OXIDE EMISSIONS FROMCONSTRUCTED WETLANDS

446

6.3.1

Methodological issues

447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469

Nitrous oxide (N2O) emissions can occur as direct emissions from wastewater treatment a constructed wetland
through nitrification and denitrification. Emissions are a function of the total nitrogen loaded into constructedwetland and an emission factor.

444
445

Two tier methods for N2O from this category are summarized 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, the amount of vegetation
harvested from constructed wetlands (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 estimationof emissions.
Emissions from semi-natural treatment wetlands 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.

470

6.3.1.1

C HOICE OF METHOD

471
472
473
474
475

A decision tree for domestic or industrial wastewater is shown in the Figure 6.4.
The general equation to estimate N2O emissions from constructed wetlands treating domestic or industrial
wastewater is shown in Equation 6.5.

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476
477
478

EQUATION 6.5
N2O EMISSIONS FROM CONSTRUCTED WETLANDSN2O Emissions = NEFFLUENT  EF  44/28

479
480

Where:N2O Emissions=

N2O emissions in inventory year, kg N2O/yr

481

NEFFLUENT=

Total nitrogen in wastewater treated by constructed wetlands in the inventory

482

year, kg N/year

483

EF

=

Emission factor, kg N2O-N/kg N

484

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

485

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486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533

Figure 6.4

Decision tree for N 2 O emission from constructed wetland

Start

Is CW used for
wastewater
treatment in the
country?

Use the 2006 IPCC Guidelines
for the estimation of emissions
from other wastewater
treatment system and discharge
pathway.

No

Yes

Are activity dataon wastewater
treatment in CW
available?

Collect or estimate activity
data on wastewater treatment
in CW

No

Yes

Are measurement
or bottom-up data
available for CW
treatment?

Yes

Yes

Is a countryspecific method
available?

Estimate emissions
using bottom-up dataor measured data
Box 3: Tier 3NoNo

Are countryspecific EF
available for CW
treatment?

Yes

Estimate emissions using
country-specific EF
Box 2: Tier 2

Is CW is a key
pathway of key
category?

Yes
Estimate country-specificEF

No
Estimate emissions using
default emission factors534

Box 1: Tier1

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535

6.3.1.2

C HOICE OF EMISSION FACTORS

536
537
538
539
540
541
542

The default IPCC emission factors for N2O emitted from domestic and industrial wastewater treated byconstructed wetlands are 0.0024 kg N2O-N/kg N for SF, 0.01 kgN2O-N/kg N for HSSF and 0.00021 kgN2ON/kg N for VSSF. These values are based on data provided in the literatures and influenced by the extent of
nitrification and denitrification taking place in constructed wetlands, the coverage of vegetation in constructedwetlands and climatic conditions. Good practice is to use country-specific data for emission factor, whereavailable, expressed in term of kg N2O-N/kg N loaded for domestic and industrial wastewater to be consistent
with the activity data.

543

6.3.1.3

544
545
546
547
548
549
550

The activity data for this source category are the amount of nitrogen in the wastewater treated by constructedwetlands (TN). This parameter is a function of the population served by the constructed wetland system, annual
per capita protein consumption (protein) and a factor for non-consumed nitrogen added to the wastewater fordomestic wastewater. In case of industrial wastewater, TN loading to the constructed wetland system in theinventory year (kg N) can be used directly. The equations for determining TN for domestic and industrial
wastewater are:

551
552
553

EQUATION 6.6
TOTAL NITROGEN IN DOMESTIC WASTEWATER EFFLUENTNEFFLUENT, DOM = P  Protein  FNPR  FNON-CON  FIND-COM

C HOICE OF ACTIVITY DATA

554
EQUATION 6.7
TOTAL NITROGEN IN INDUSTRIAL WASTEWATER EFFLUENTNEFFLUENT, IND = TN  W  365

555
556
557
558
559

Where:NEFFLUENT, DOM or IND

560=

total nitrogen in municipal or industrial wastewater treated in
the constructed wetland in inventory year (kg N/year)

561

P

=

human population whose wastewater treated by constructed wetlands

562

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

563

FNPR

=

fraction of nitrogen in protein, default = 0.16 kg N/ kg protein

564

FNON-CON =factor for non-consumed nitrogen added to the wastewater

565

FIND-COM

correction factor for additional industrial wastewater discharged into sewers (for=566

collected the default is 1.25, for uncollected the default is 1.00 as recommended in

567

2006 IPCC Guidelines)

568

TN

=

wetland in inventory year (kg N/m3)

569
W

570
571
572
573
574

total nitrogen concentration in the industrial wastewater treated by constructed=daily flow rate of industrial wastewater treated by constructed wetland, m3/d

TN loading can be estimated by multiplying total volume of wastewater (Table 6.9, Chapter 6, Volume 5 in 2006
IPCC Guidelines) and N content in Table 6.6.

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

15-25% of annual precipitation6

0.74 (0.01-2.50)5

Industry type
Alcohol refining

Nitrogen Fertilizer Plant
Landfill leachate

576
577
578

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

Kjeldsen et al (2002); 6 Ehrig (1983)

579

6.3.2

Time series consistency

580
581
582

The same method and data sets should be used for estimating N2O emissions from constructed wetlands for eachyear. If a country decides to change the estimation method from the default methodology (Tier 1) to countryspecific emission data (Tier 2), this change must be made for the entire time series.

583

6.3.3

584
585
586

Large uncertainties are associated with the IPCC default emission factors for N2O emissions from constructedwetlands due to limited available data.

Uncertainties

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TABLE 6.7
NITROUS OXIDE METHODOLOGY DEFAULT UNCERTAINTIESParameter

Default valueRange

0.0024 for SF
0.01 for HSSF
0.00021 for VSSF

0.0001-0.0219 for SF
0.0004-0.0301 for HSSF
0.00001-0.00058 for VSSF

Human population

Country-specific± 5%

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

588

6.3.4

589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605

± 30%

QA/QC, Completeness, Reporting andDocumentation

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
nitrogen associated with domestic and industrial discharge either collected into the collection system and treatedin constructed wetland or uncollected and discharged into semi-natural treatment wetland. This estimate can beseen as conservative and covers the entire source associated with domestic and industrial wastewater discharge.
Reporting and documentation
It is good practice to document and report a summary of the methods used, activity data and emission factors.
Worksheets are provided at the end of this volume. When country-specific methods and/or emission factors areused, the reasoning for the choices as well as references to how the country-specific data (measurements,
literature, expert judgment, etc.) have been derived (measurements, literature, expert judgment, etc.) should be
documented and included in the reporting.
More information on reporting and documentation can be found in Section 6.11, Chapter 6, Volume 1 of the2006 IPCC Guidelines.

606

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610
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Augustin J, Merbach W, and Rogasik J (1998) “Factors influencing nitrous oxide and methane emissions fromminerotrophic fens in Northeast Germany”, Biol Fert Soils 28:1–4

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Brix H (1987) “Treatment of wastewater in the rhizosphere of wetland plants - the root zone method”, Water
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614

Brix H. (1997) “Do macrophytes play a role in constructed treatment wetlands?”, Water Sci Technol 35(5):11–7.

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Cooper, P.F., Job, G.D., Green, M.B., 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 to thesignificance of oxygen transfer and hydraulic loading rates. Water Science and Technology 51, 9, 81–90.

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Crites, R.W, Middlebrooks, E.J, Reed S.C. (2005) “Natural Wastewater Treatment Systems” CRC Press: Boca
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Fey, A., Benckiser, G., et al. (1999). “Emissions of Nitrous Oxide from a Constructed Wetland using a
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Garcia, J., Capel, V, Castro A, Ruíz I, Soto M. (2007) "Anaerobic biodegradation tests and gas emissions fromsubsurface flow constructed wetlands." Bioresource Technology 98(16): 3044-3052.

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Kjeldsen, P., Barlaz MA, Rooker AP, Baun A, Ledin A, Christensen TH. (2002). “Present and long-term
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Liikanen, A., Huttunen, J. T., Karjalainen, S. M., Heikkinen, K., Vaisanen, T. S., Nykanen, H., Martikainen, P. J
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Mitsch, W.J., Gosselink, J.G., (1993). Wetlands. Wiley, New York.

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Salm, J.O., Maddison, M., Tammik, S., Soosaar, K., Truu, J., Mander, U., (2012). Emissions of CO2, CH4 andN2O from undisturbed, drained and mined peatlands in Estonia. Hydrobiologia. In press. doi:
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Søvik, A. K. and B. Klove (2007). “Emission of N2O and CH4 from a constructed wetland in southeastern
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Søvik, A. K., Heikkinen, J. A., Huttunen, J. T., Necki, J. M., Karjalainen, S. M., Kløve, B. Liikanen, A., Mander,U., Teiter, S., Wachniew, P. (2006). "Emission of the Greenhouse Gases Nitrous Oxide and Methane from
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