Denmark's National Inventory Report 2005 - Submitted under the ...

The average fraction of sludge treated anaerobe is considered fairly constant based on the available
data. Furthermore, the average fraction of industrial influent load has reached a constant level
from the year 1997 and forward (Annex 3.E, Table 3.E.3). This in addition to the intensive technological
upgrading of the Danish WWTPs from 1987 to 1996 is indicative of an optimised and stabilised
situation regarding WWT processes. It seems reasonable to assume a constant emission factor
of 0.15 kg CH 4 / kg BOD based on the ww fraction of sludge treated anaerobic.
Data gap filling procedures for arriving at gross emissions from 1990 to 2003 are given in Annex
3.E. For additional information see Thomsen & Lyck (2005).
Activity data and EF for calculation of the amount of recovered or not emitted CH 4 potential
Available activity data and EF for calculating the amount of recovered and flared CH 4 potential
(theoretical negative methane emission) is given in Table 8.16 and in the text below.
Table 8.16. Sludge in per cent of the total amount of sludge and tonnes dry weights (dw) according to disposal
categories of relevance to CH 4 recovery.
Unit Year
per cent
total tonnes dw
204
Combustion
internal
Combustion
external
Biogas Other*
1987 24.6 18.5
1997 15.5 6.2 1.5 0.8
1999 7.4 14.8 1.9 9.1
2000 15.0 9.2 1.6 14.4
2001 14.8 6.3 1.0 11.3
2002 11.4 4.4 0.9 10.0
1987 23330 11665 7667
1997 23500 9340 2338 1211
1999 23008 9845 2972 14140
2000 11734 23591 2476 22856
2001 23653 14532 1588 17883
2002 15932 6120 1262 13989
*The category “Other” represents sludge which is combusted in cement furnaces and is used in further combusting processes for the
production of sandblasting products.
The IPCC GPG background paper (2003) estimates the maximum methane producing capacity to
be 200 kg CH /tonnes raw dry solids, which is also the emission factor (EF), as the methane con-
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version factor (MCF) is equal to unity for biogas process (EF= B * MCF). The fraction of the gross
o
CH emission, not emitted in reality, is then the dry weight of the category biogas multiplied by an
4
EF of 200 kg CH /tonnes raw dry solids. For comparison, the biogas yield, i.e. EF is given to be
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within 250 to 350 m 3 /tonnes organic solids for sewage sludge in a report on biogas systems by IEA
Bioenergy (x, undated). The density of methane gas is 0.715 kg/m 3
at standard conditions, which
gives an average EF of 214.5 kg CH /tonnes raw dry solids. The same EF is used for calculating the
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theoretical methane potential not emitted by the remaining disposal categories given in Table 8.16.
As seen from Table 8.16 there are gaps in the data. See Annex 3.E for details concerning data gapfilling.
8.3.3 Methodological issues related to the estimation of N O emissions
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While CH is only produced under anaerobic conditions, N O may be generated by nitrification
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(aerobic process) and denitrification (anaerobic process) during biological treatment. Starting material
in the influent may be urea, ammonia and proteins, which are converted to nitrate by nitrification.
Denitrification is an anaerobic biological conversion of nitrate into dinitrogen. N O is an
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