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

intermediate of both processes. Danish investigation indicates that N 2 O is formed during aeration
steps in the sludge treatments process as well as during anaerobic treatments; the former contributing
most to the N 2 O emissions during sludge treatment (Gejlsberg et al, 1999).
Methodology - Direct N 2 O emission
A methodology for estimating the direct emission of N 2 O from wastewater treatment processes has
been derived. The EF is derived from a factor of 3.2 g N 2 O/capita per year (Czepiel, 1995) multiplied
by a correction factor of 3.52 to account for the industrial influent load. The average resulting
emission factor for direct emission of N 2 O is (3.52*3.2) 11.3 g N 2 O/capita per year.
The correction factor of 3.52 is derived from the difference in average nitrogen influent load at
large and medium size WWTPs divided by the influent load at large size WWTPs (cf. Annex 3.E,
Table 3.E.10). This approach is based on the assumption that the large size WWTPs receive industrial
wastewater while the medium size mainly receive wastewater from households (cf. Annex
3.E, Background).
Until better data is available, simple regression of the relation between industrial influent load in
percent and the EF is used for the years 1990 to 1997, after which the industrial contribution to the
influent load is assumed constant and the EF of 11.3 g N 2 O/capita per year is used in the calculations.
The influent load of nitrogen is assumed to increase similar to the industrial influent loads of
BOD given in per cent in Table 8.17. The estimated Danish emission factors as function of the increase
in industrial influent load in the Danish WWTPs are given in Table 8.17.
The direct emission from wastewater treatment processes is calculated according to the equation:
( ,
, = 1 ⋅ ) ⋅ ()
2
2,
,
where N pop is the Danish population number, F connected is the fraction of the Danish population connected
to the municipal sewer system (0.9) and EF N2O.WWTP.direct is the emission factors given in Table
8.17.
Activity data and EF for calculating the direct N 2 O emission
Table 8.17. EF and activity data used for calculating the direct emission of N 2 O from wastewater treatment
processes at Danish WWTPs.
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
N-pop (1000) 5140 5153 5170 5188 5208 5228 5248 5268 5287 5305 5322 5338 5351 5383
F-connected 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9
% industrial load 2.5 2.5 2.5 5.0 15.5 23.9 32.3 40.7 48.0 41.0 42.0 38.0 38.0 42.0
Danish EF* 3.8 3.8 3.8 4.2 6.2 7.8 9.4 11.0 12.3 11.0 11.2 10.0 10.4 11.3
*where data are available for calculating EF, the average value, of the calculated by national data and by regression, is given.
The industrial loads of wastewater influent loads given in Table 8.17 for years 1990-2003 have been
estimated from the original and registered data (Table 3.E.3, Annex 3.E). For the year 1990 to 1992
the industrial influent load is set to an average of 2.5 %. From the year 1993 to 1997 the percentages
are assumed to continue to increase as shown in Table 8.17. The Danish emission factors are based
on a regression of per cent industrial loads versus the corrected emission factors given in Table
3.E.10 in Annex 3.E. The average fraction of industrial nitrogen influent is considered constant
from the year 1997 and forward. This is consistent with a fairly constant fraction of industrial
wastewater influent from 1997 and forward.
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