Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

CH 4 EM IS SI ONS FROM AN IMAL
MANUR EACKNOWLEDGEMENTS
This paper was written by Grietje Zeeman and Sybren Gerbens (Wageningen University, The Netherlands).

ABSTRACTThe paper identifies uncertainties in the default emission factors, Bo (biodegradability of manure) and MCF(methane conversion factor), and gives suggestions for improvement of these values. The maximum
biodegradability, Bo, is expressed in m³ CH4 produced per kg VS (volatile solids). The determination of VS
directly includes possible errors, when volatile components are present. Manure will always contain a fraction ofvolatile fatty acids (VFA). The latter will result in an over estimation of the Bo value. Bo determination should
not only include VS analysis but also total chemical oxygen demand (COD) and VFA. Standardisation of the Bo
determination, including sampling methods is recommended. The MCF does not only depend on the manure
management system used, but also on the temperature of the stored manure, and the handling of the system forexample the mean percentage of manure left over (=inoculation) after ‘emptying’ (use). It is therefore
recommended to collect more data on this item, and re-estimate default MCF values with the aid of model
calculations. Specifying the MCF values for digesters is recommended because, with the now recommended
MCF defaults values (10 percent), controlled anaerobic digestion is expected to always include a relatively highmethane emission, which could prevent the implementation of the method. Anaerobic digesters can beconstructed in such a way that no or hardly any methane emission occurs.

KEYWORDS
CH4, emission, animal manure, biodegradability, Volatile Fatty Acids, Volatile Solids, Methane Conversion
Factor

CH4 Emission from Animal Manure339

Background Paper

1

INTRODUCTION

1.1

Aim of the workshop

The aim of the sectoral meetings is to define guidelines on good practice. Issues to be covered will include a
process to identify preferred methods, good practice in data collection and emission factor
development/selection/verification, good practice in inventory calculations, and preferred approaches toestimating uncertainties as well as providing transparent documentation, including type and level of presentation.

1.2

Background papers

Two background papers have been produced for the sector workshop methane emission from animal manure.
The first paper was prepared by Gibbs et al (1999) and gives an overview of all factors and uncertainties
involved in the determination of methane emissions from animal manure.
The present paper tries to identify uncertainties in the default emission factors, Bo and MCF, and gives
suggestions for improvement of these values. Furthermore, standardisation of certain determinations as includedin the IPCC Tier 2 method is recommended.

2

METHODOLOGICAL ISSUES

2.1

Framework for the best practice method

According to the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC Guidelines)
the total annual methane emissions for animal type i in a particular climate region is the sum of annual emissions
over all applicable manure systems j:
C H 4i = ∑ j B oi • V S • M S % ij • M C F ji

V S i = N i • v si

Where:
CH4:

Methane emissions (m3/yr)

Bo:

Biodegradability of manure (m3 CH4/kg VS)

MCF:

Methane conversion factor (%)

MS%:

Manure management system usage (%)

VS:

Total volatile solids produced annually (kg/yr)

N:

Animal number (heads)

vs:Average annual volatile solids production per head (kg/head/yr)

It is clear from the IPCC formula above that five important parameters should be known in order to determine
the occurring methane emissions. These parameters are evaluated below.
Methane emissions from animal waste•

depends very strongly on the specific manure management system applied as well as the conditions and themanner of how the system operates

Animal number (heads) per type and class of animals (N)•which disaggregation circumstances should be applied under what circumstances, and•if data are not available what methods should be considered.

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Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories

Manure production (VS/head/year) (VS)The manure production per animal kept under certain conditions will based on feed intake and digestibility of thefeed, which in turn depends on:•

region where the animal is kept, and•type and class of animal.

Methane production Potential (Bo = m³CH4/kgVS)This depends on feed intake and digestibility of the feed, which in turn depends on:•

region where the animal is kept, and•type and class of animal.

Methane conversion factor (MCF)
The first three parameters are discussed by Gibbs et al. (1999). The last two parameters- Bo -value and MCFvalue will be discussed in more detail in this paper.

2.1.1

Methane Production Potential (Bo = m³CH 4 /kgVS)

Within the IPCC Guidelines two tiers are distinguished. The first tier is to be used when no or hardly any
country- specific data are available. Otherwise the second tier is to be used.
(a) IPCC Tier 1(Default Bo values)
IPCC default values are based on the values as reported by Safley et al (1992). The values are illustrated Table 1.
TABLE 1PRODUCTION OF VS AND MAXIMUM ANAEROBIC BIODEGRADABILITY OF THE VS
RegionDairy

Non-dairy

Buffalo

Swine

Bo a

VSb

Bo a

VSb

Bo a

VSb

Bo a

VSb

North America

0.24

5.2

0.17

2.4

0

0

0.45

0.5

Western Europe

0.24

5.1

0.17

2.7

0.1

3.9

0.45

0.5

Eastern Europe

0.24

4.1

0.17

2.7

0.1

3.9

0.45

0.5

Oceania

0.24

3.5

0.17

3

0.1

3.9

0.45

0.5

Latin America

0.13

2.9

0.1

2.5

0.1

3.9

0.29

0.3

Africa

0.13

1.9

0.1

1.5

0.1

3.9

0.29

0.3

Middle East

0.13

1.9

0.1

1.5

0.1

3.9

0.29

0.3

Asia

0.13

2.8

0.1

2.3

0.1

3.9

0.29

0.3

Indian Subcontinent

0.13

2.6

0.1

1.4

0.1

3.1

0.29

0.3

a

3

in m CH4/kg VSb

Average VS production for per head per day for the average animal (kg VS/(head.day))

Source: IPCC 1997

The determination of the Bo value is an important aspect with respect to the reliability of the method. In
literature, large variations can be found for Bo values for both cattle and pig manure. The Bo values as adapted
for the US are also used in the IPCC Guidelines for developed countries as it is assumed that typical diets are
similar. The values for developing countries are modified, considering that there is less energy in the feed used.
Possible errors during Bo determination
The maximum biodegradability, Bo, is given as m³ CH4 produced per kg VS. The determination of VS directly
includes possible errors. The VS measurement includes drying and incineration steps. The first gives rise topossible errors when volatile components are present. Manure will always contain a fraction of volatile organic
components, viz. volatile fatty acids (VFA). The quantity of VFA will depend on the storage conditions of theused manure. According to Derikx et al. (1994) large losses of VFA can be expected during dry matter

CH4 Emission from Animal Manure341

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determination. Figure 1 presents the percentage recovery of VFA after dry matter determination as a function of
the pH for pig, cattle and poultry manure. The results show that at pH values of 7.5, a normal pH for influent
animal manure, ca 75% of the VFA is lost.

Figure 1
Percentage recoveryof VFA after dry matter
determination of pig manure (A),cattle manure (B) and poultry
manure (C)As an important part of the VFA will be lost during thedetermination of the VS, while it will be completely
converted to CH4 gas during the biodegradability test, anover estimation of the Bo value will occur. The
magnitude of the error will depend on the fraction of
VFA present in the influent manure, which in turn will
depend on the conditions during the storage of the
manure. Zeeman (1991) presents VFA concentrations
for cow manure between 9 and 16 g COD/l, at VSconcentrations between 56 and 81 g/l and VFA
concentrations for pig manure of 30 g COD/l, at a VS of
78 g/l. V. Velsen (1981) shows VFA concentrations for
pig manure lying between 8.7 and 15.9 g COD/l at VSconcentrations between 38 and 65 g/l. This means thatan error between 10 and 30% in the VS measurement
can be made.
Evaluation of the default values based on theoreticalconsiderations.
The suspended solids in animal manure mainly consistof carbohydrates ((hemi) cellulose), some proteins andeven less lipids. Table 2 presents some literature values
on the composition of cow and pig manure

Source: Derikx et al., 1994.

TABLE 2COMPOSITION (IN PERCENTAGE OF THE TOTAL SOLIDS) OF ANIMAL MANURE, AS USED BY SEVERAL RESEARCHERS
LipidsProteins

Carbohydra
tes

Anorg
residue

References

Cow

6.1

13.7

59.9

20.3

1

Cow

6.1

15.0

62.1

16.9

1

Cow

7.5

15.6

14.5

19.3

8.2

29.0

2

Cow

3.5

15

17.o

19.0

6.8

28.0

3

Cow

4.0

15+

25.0

2.0

9.0

16.0

6

Pig

12.3

16.0

10.3

17.1

3.7

17.3

2

Pig

7.7

20.9

22.9

20.8

10.1

17.6

4

Pig

7.0

28.9

27.0

5

53.8*

Cellulose

Hemicellulose

Lignin* cellulose, hemi-cellulose, lignin; + (total-N)-(NH4+-N)*6.25
1. Steiner (1983); 2. Wellinger, (1984); 3.Varel, Isaacson and Bryant, 1977), 4. Hobson, Bousfield and Summers (1974); 5. Temper (1983);
6. Robbins, Gerhardt and Kappel (1989).

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One gram of VS for ‘biological’ sludge's is normally considered to be equal to 1.4 g COD. Considering theconversion factors for the different polymers shown below and the composition as shown in Table 3, this overall
conversion factor is a reasonable estimate for animal manure, provided that fresh manure is involved with no or
hardly any VFA present.
TABLE 3
CONVERSION FACTORS FOR DIFFERENT POLYMERS TO COD
Polymer

Structure formula

Equivalent to

1 g carbohydrates

C6 H12 O6

1.07 g COD

1 g lipid
1 g protein

2.91 g COD (Sayed, 1987);(C4 H1.6 O1.2 N)x)

1.5 g COD (Sanders et al. 1996)

Hashimoto (1984) uses pig manure with concentrations of 33.5-77.1 g VS/l and 38.5 - 96.3 g COD/l. Based on
the results of continuous experiments, a Bo value of 0.48 m³ CH4/kgVS is determined. Based on the reported
COD and VS concentrations, a COD/VS ratio between 1.15 and 1.25 can be calculated. This means that the Bo
should be equal to 0.44 - 0.4 m³ CH4/kg COD. The latter is impossible as the theoretical maximum methane
production is 0.35 m³ CH4/kg COD. Also the results of the effluent COD measurements as reported by
Hashimoto (1984) show that the reported gas production (m³ CH4/kgVS) is a factor of 2 higher than can be recalculated based on the removed COD. Hashimoto (1983) also determines a Bo value of 0.49 m³ CH4/kgVS
based on batch experiments at 35 and 55°C, using a similar pig manure with a COD/VS ratio of 1.01-1.2 .
Chen (1983) makes use of the above-mentioned Bo values. Stevens and Schulte (1979) presents results of anaerobic
digestion of pig manure at temperatures of 22.5-40 °C. The used manure has a high COD/VS ratio of 1.9-2.1.
Reported gas productions at different conditions differ between 0.29-0.68 m³ CH4/kgVS. Based on the reported
COD effluent values, again much lower gas production values are re-calculated, viz. 0.24-0.36 m³ CH4/kgVS.
As the IPCC Bo default values, presented in Table 1, are for a large part based on the above cited references, reestimation of the default Bo values should be considered.
Zeeman (1994) estimates the anaerobic biodegradability of swine manure to be 70% and that of dairy manure to be
50% of the total COD based on results of laboratory research (Zeeman, 1991 and Velsen, 1981). Thesebiodegradability values are based on the highest removal efficiency during continuous experiments. Considering that
1g VS=1.4 g COD, this will result in a Bo value of 0.25 and 0.34 m³ CH4/kgVS for dairy and pig manure, respectively.
The IPCC default values are estimated at 0.24 and 0.45 m³ CH4/kgVS, respectively. Especially the latter isrelatively high. Considering the above mentioned value of 1.4 for conversion of VS to COD, this would meanthat 0.45/1.4/0.35 ●100%=92% of the influent COD of pig manure would be biodegradable.
Sensitivity analysis
Gerbens (1999) calculated 14.2 Tg global CH4 emission per year for major contributors (dairy, non-dairy,
buffalo and swine) of animal manure based on the IPCC defaults values and for assumed temperate climaticconditions in North America (NA), Western Europe (WE), Eastern Europe (EE), Oceania (OC), and Asia (AS)and warm climatic conditions in Latin America (LA), Africa (AF), Middle East (ME) and the Indian
Subcontinent (IS). Considering the above suggested lower Bo value of 0.34 instead of the IPCC default value
0.45 m³/kg VS for pig slurry, the total global methane emission will become 13.1 Tg/year instead of 14.2
Tg/year. When the default Bo value for pig manure in developing countries is proportionally lowered to 0.22 m³
CH4/kgVS instead of the IPCC default value of 0.29 m³ CH4/kgVS, a global methane emission from animal
manure of 12.6 instead of 14.2 Tg/year is arrived at.
Recommendations
The above discussions lead us to the following recommendations:•

Determination of the (corrected) COD/VS value for ‘fresh’ manure by reviewing literature for total COD,
VS and VFA concentration (if available) and recalculating the default Bo values.

•

Standardising the determination of Bo, including analysis of total COD, VFA and VS.

•

Determination of the Bo values for different types of 'fresh' animal manure under standardised conditions.

(b) IPCC, Tier 2
Bo value for each representative animal type should defined. The IPCC Tier 2, includes a determination of the Bo
value for each representative animal type defined. Country specific data should be used where feasible.
Otherwise the default value (Tier 1) are to be used.

CH4 Emission from Animal Manure343

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Recommendations
In order to be able to develop reliable country specific data for the Bo value, a standardised method for thedetermination of Bo is to be developed. A standardised method should include:•

Sampling method;•

Number of samples to be taken;•

Type of analysis to characterise the manure sample, and•Procedure for the Bo determination.

2.1.2

Methane Conversion Factor (MCF)

Methane emissions from animal waste strongly depend on the specific manure management system applied andalso on the conditions and the manner the system operates. The manure management systems as identified within
the IPCC Guidelines are presented in Table 4.
TABLE 4CHARACTERISTIC OF THE IPCC METHOD FOR IDENTIFIED MANURE MANAGEMENT SYSTEMSManagement system

Characteristics

Storage time

Pasture/range

The manure from pasture- and range-grazing animals isallowed to remain as it is, and is not handled at all.

Daily Spread

Manure is collected in solid form by some means, such as
scraping. The collected manure is applied to fields regularly.

Solid Storage

Manure is collected in solid form by some means, such as
scraping.

DrylotIn dry climates animals may be kept on unpaved feedlots where
the manure is allowed to dry until it is periodically removed.
Upon removal, the manure may be spread on the fields.

Liquid/slurry

These systems are generally characterised by large concretelined tanks built into the ground≥ 6 months

Anaerobic lagoon

Anaerobic lagoon systems are characterised by flush
systems that use water to transport manure to the lagoons.
The water from the lagoon may be recycled as flush water orused to irrigate and fertilise fields

30 days to >200 days

Pit storage

Liquid swine manure may be stored in a pit while awaiting
final disposal. The pits are often constructed beneath theswine building

two categories:

long period of time(months)<1 month> 1 month

Anaerobic Digester

Produce CH4 gas for energy. The amount of CH4 produced
depends on the operating characteristics of the digester andthe characteristics of the manure.

Burned for Fuel

Manure is collected and dried in cakes and burned forheating or cooking. This system is common in Asia and theFar East. In India it is estimated that two-thirds of cattle
manure is burned for fuel.

The actual methane emission will not only depend on the system and the climatic conditions, but also on themanner of management. Moreover the climatic conditions are not always representative of the temperature
conditions in the manure. The latter is especially the case when pit storages for pig manure are involved as thesesystems are constructed beneath the swines in the buildings. This means that the manure temperature will be
determined by the climatic conditions in the stable more than by the conditions outside.
In order to identify possible errors in the estimated MCF default values for animal manure, it is necessary toinvestigate the major manure methane emitting management systems in more detail. As shown in Table 5, thewet manure systems (liquid/slurry tanks, pit storage and anaerobic lagoons) are the most important methane
emitting manure management systems. All three systems can be considered as non-optimised anaerobic reactors,
operated in a fed-batch mode. Fed-batch means that the storage/reactor is filled in time, until it is completely full.

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After filling, the reactor is emptied leaving only some left over 'stored' manure (inoculum). The fraction of‘stored’ manure which is left over after 'emptying' the storage, will, amongst others, largely determine how muchmethane will be produced during the filling period (Zeeman, 1991). If the storage could be completely emptied
after filling, so that no methanogenic bacteria are left, methane emissions could be considerably reduced, unlessvery long storage times are applied. When a high fraction of inoculum is left, the methane emission will increase.
Zeeman (1991 and 1994) assumes, on the basis of practical experience, that always ca.15 percent of the manurestorage cannot be emptied, leaving a (methanogenic) inoculum in the storage. Especially when pig manure isused, solids will settle to the bottom of the storage, increasing the solids detention time as compared to the liquid
detention time resulting in an even higher methane emission.
Default MCF values (Anaerobic lagoons)
The anaerobic lagoon is designed to have a longer SRT (sludge retention time) than HRT (hydraulic retentiontime), which can result in a high MCF value. When the SRT becomes infinite, the MCF will approach 100%
irrespective of the temperature applied. The IPCC method assumes an MCF factor of 90%. The HRT in
anaerobic lagoons can however vary between 30 and >200 days. It is not sure whether the high MCF values will
always occur. At the lower HRT’s also the SRT will decrease, resulting in lower MCF factor as compared to thefactor of 90 percent as proposed by the IPCC Guidelines especially when low temperature conditions prevail.
Zeeman (1994) calculates a MCF value of only ca. 50 percent, when a temperature of 10-15°C and a storage
time as long as 360 days (HRT=SRT) prevails.
Sensitivity analysis
Gerbens (1999) calculated a 14.2 Tg global CH4 emission per year for major contributors (dairy, non-dairy,
buffalo and swine) of animal manure based on the IPCC defaults MCF values and for assumed temperate
climatic conditions in NA, WE, EE, OC and AS and warm climatic conditions in LA, AF, ME and IS.
Considering default MCF values for anaerobic lagoons of 70% instead of 90 %, a global CH4 emission fromanimal manure of 13.7 instead of 14.2 Tg/year was arrived at.
The MCF factor does not depend on the manure management system used, but also on the temperature of thestored manure, and the handling of the system. Table 5 provides the MCF values for the different systems at
different temperature conditions as used in the IPCC Guidelines.
TABLE 5METHANE CONVERSION FACTORS (PERCENTAGE) FOR DIFFERENT MANURE MANAGEMENT SYSTEMS AND CLIMATES FORDAIRY CATTLE, FOR NON-DAIRY CATTLE AND FOR BUFFALO
Manure Management System MCFs for dairy cattle, for non-dairy cattle and for buffalo
ClimateLagoonLiquid/slurrySolidstorage

DrylotPasture
range

Daily
SpreadDigesterBurned for
fuel

Other

Cool

90

10

1

1

1

0

10

10

1

Temperate

90

35

1.5

1.5

2

0.5

10

10

1

Warm

90

65

2

5

2

1.0

10

10

1

Solid
storage

DrylotPit <1month

Pit >1month

Daily
SpreadDigester

Other

Manure Management System MCFs for Swine
ClimateLagoonLiquid/slurry

Cool

90

10

1

1

5

10

0.1

10

1

Temperate

90

35

1.5

2

18

35

0.5

10

1

Warm

90

65

2

5

33

65

1

10

1

Source: IPCC 1995/1996

Liquid/slurry tanks and pits
According to the IPCC Guidelines the liquid/slurry storage tank has a storage time ≥ 6 months. The default MCF
factors chosen are 10, 35 and 65 percent for mean temperatures of 10-15 oC (low), 15-25oC (temperate) and >25°C (warm) respectively. The pits are divided into those with a storage capacity < 1 month and those with astorage capacity > than 1 month. The default MCF factors chosen are also 10, 35 and 65 % for mean temperatureof 10-15oC (low), 15-25 oC (temperate) and > 25°C (warm) respectively for >1 month storage capacity and 15,
18 and 33% for storage capacities of<1 month. Zeeman (1994) estimated some of these MCF values differently

CH4 Emission from Animal Manure345

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as presented in Table 6.
TABLE 6
COMPARISON BETWEEN IPCC DEFAULT MCF VALUES AND MCF VALUES AS OBSERVED BY ZEEMAN (1990)
Manure management Strategy

Climatic condition

MCF (%) (IPCC, 1997)

MCF (%) (Zeeman, 1994)

Liquid/slurry

Cool

10

39

Temperate

35

45

Warm

65

72

Cool

5

0

Temperate

18

0

warm

33

30

Cool

10

39

Temperate

35

45

Warm

65

72

Pit < 1 month

Pit > 1 month

For methane emissions calculated by Gerbens (1999), the use of MCF values as observed by Zeeman (1990),
would result in a global CH4 emission estimate of 16.3 Tg instead of 14.2 Tg. Therefore a significant (15%)
increase is established by adapting the MCF values derived by Zeeman (1994).
Considerations
Zeeman (1994) calculated a higher MCF factor at low temperature conditions and a somewhat higher MCF value
at median and high temperature conditions, considering 180 days storage time and 15 percent of the stored
manure (inoculum) to be left over after emptying. When more or less manure is left over after digestion, methane
emissions will considerably change. In countries with low temperature winters, no manure will be applied on thefield during the winter period. Therefore before winter starts the manure storage will be emptied as much as
possible provided that enough storage capacity is available. The emptying in summer will depend on thepossibility to use the fertilisers on the fields. The emptying of manure tanks in warm climates will presumably bemainly controlled by the possibility to use the manure on the fields. It is very important to gain insight into theway of management of these systems in warm climate countries. When a large fraction of manure is left in thestorage, it implies that the system is operated more as a CSTR (continuously stirred tank reactor) than a fedbatch system, then very high methane emissions can be expected, even in pit storages of less than 1 month.
Recommendations
As the way of management of the storage can influence the MCF factor substantially, it is recommended tocollect more data on this item, especially for high temperature countries.
•

Based on these data collection, the mean percentage of manure left over (=inoculation) after ‘emptying’(use) is to be estimated.

•

Default MCF values can be re-estimated based on the estimated percentages of inoculation, the definedstorage period and the temperature conditions with the aid of model calculations (Zeeman, 1994).

Digesters
The IPCC Guidelines includes a MCF default value for anaerobic digesters of 10 percent. The gas collection, gasuse, the gas flaring facilities and also the manner and period of storing digested manure will, however, to a large
extent determine the actual methane emissions. Whether the above-mentioned facilities are included or not, thiswill largely depend on the scale in which digestion is performed. In principle, zero emissions could be achieved.
It is therefore recommended to develop different default values depending on the type of facilities included. The
following categories are recommended:•

presence of a gas collection system with a buffer capacity larger than the volume of daily gas production;•

presence of a gas flare for flaring excess gas, and•presence of a gas tight cover for collecting and using the gas produced during the storage of digested manure.
The best way is combining the gas collection system with the storage of the animal manure.

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Zeeman (1994) calculated that when pig manure is digested in a CSTR at 30°C and 20 days digestion time, thecontrolled methane production is 18.4 m³/ m³, while additional 2.4 m³/ m³ is produced during storage underDutch conditions.
Specifying the MCF values for digesters is also recommended because, with the now recommended MCF
defaults values, it is suggested that controlled anaerobic digestion should always include a relatively highmethane emission, which could prevent the implementation of the method. However as discussed above,
anaerobic digesters can also be constructed in way that no or hardly any methane emission will occur. Moreover,when the use of gas for replacing fossil fuel or wood is included, the reduction in total global warming potential
is substantial (Gerbens and Zeeman, 1999). Both aspects makes controlled anaerobic digestion a highly attractive
method for reducing emissions of greenhouse gases (Gerbens and Zeeman, 1999).

3

CONCLUSIONS•

Manure will always contain a fraction of volatile fatty acids (VFA), which will result in an over estimation of
the Bo; value when based on VS. Bo determination should, next to VS, include total COD and VFA analysis.

•

Standardisation of the Bo determination, including sampling methods is recommended;•

The actual MCF factor will vary with the manure management system, the temperature of the stored manure,and the handling of the system. The mean percentage of manure left over (=inoculation) after ‘emptying’(use) is of crucial importance;•

It is recommended to collect more data on percentage inoculation in storages and re-estimate default MCF
values with the aid of model calculations;•

Specifying the MCF values for digesters is recommended because, with the now recommended MCF
defaults values (10 percent), it is suggested that controlled anaerobic digestion should always include
relatively high methane emissions, and•Anaerobic digesters can be constructed such that no or hardly any methane emission will occur.

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Background Paper

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