Page 282 - Materials Chemistry, Second Edition

266 4 Life Cycle Impact Assessment

separately by experiment. If exclusively aerobic degradable organic substances are
present in the water the COD is equal to the BOD, otherwise the COD is always
larger. Therefore a consideration of the COD in the impact category, eutrophication,
under precautionary criteria always refers to the maximum possible value.
The following consideration correlates the COD with the EP:
Because the damaging impact of biodegradable carbon compounds is the result
of an ‘oxygen depletion’ 258) the oxygen demand is taken for a definition of the EP-
value (kg of phosphate equivalents). To completely oxidise a molecule of the model
biomass it is presumed that 138 molecules of oxygen are additionally necessary
besides the 110 oxygen atoms already present in the molecule (Equation 4.24). The
chemical species considered for N and P following the oxidation are NO 3 − and
HPO 4 2− which prevail in aerobic water with usual pH levels:

C 106 H 263 O 110 N P + 138O → 106CO + 122H O
2
2
16
2
+ 16NO 3 – + 1HPO 4 2– + 18H +259)
Sample Calculation

A molar ratio of n (P)/n (O ) = 1/138 results in
2
3− )
m(O )× M(PO
3− )= 2 4
m(PO
4
M(O )× 138
2
−1
(M (O ) = 32gmol −1 and M (PO 4 3− ) = 95gmol )
2
For 1 kg COD calculated as O thus 0.022 kg phosphate equivalent results.
2
Thus, somewhat artificially, the correlation of the impact of eutrophication by
oxidisable organic compounds with oxygen depletion is quantitatively achieved.
The COD of persistent (‘refractory’) compounds should actually not be included in
the calculations, as these materials do not contribute to oxygen consumption. The
COD is therefore a worst case approximation of the BOD.
For the calculation of the terrestrial EP mainly emissions into the air are consid-
ered, for the aquatic EP direct and indirect entries into the waters. 260) The majority
of continental emissions into the air reach the soil by dry and wet deposition. The
emissions into water and the settling on soil influence ecosystems which can be
over-fertilised. Soil and water are interconnected: Washing-off surplus fertilisers
from agricultural surfaces and penetration into the groundwater (nitrate pollution).
Over-fertilisation of agriculturally used grounds (technosphere, not ecosphere) is
not considered here because by definition the inadvertent eutrophication includ-
ing the unavoidable sequel impacts is discussed exclusively. This is because of
the restriction of LCA towards environmental pollution. Clearly, in the case of

258) Classical eutrophication due to P and N induces oxygen depletion as a frequent secondary effect.
259) Kummert and Stumm (1989).
260) Mauch and Sch¨ afer (1996).