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LCA part3

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is still as isolated from its biotic surroundings
as it was before it was opened). Regulated use of waste materials on soil substrates, as when mining
wastes are applied under permit for the foundation of roads or buildings, do not constitute emissions
either, although subsequent emissions from the new road structure should be duly recorded. At
controlled landfill sites, only the emissions out of the site are recorded, in the way specified above. At
non-regulated landfill sites, inflowing wastes are also treated in their entirety as emissions. For partially
controlled sites, a mix may be applied.
Finally, there is the question of how to treat short and long carbon cycles. Short carbon cycles should
preferably be regarded as cycles and thus, at the systems level, the fixation of CO2 during tree growth
should be subtracted from the CO2 emitted during waste treatment of discarded wood and any CH4
emissions should be quantified. For long carbon cycles, CO2 and CH4 emissions should be recorded in
their entirety, without further balancing against prior fixation.
- Elaborate the specification of temporal aspects in a steady-state modeling context, especially for
landfill and long-cycle systems, inter alia in relation to temporal aspects of Impact assessment.
- Detail the boundaries between Technosphere, Biosphere and Lithosphere in a more systematic
Part 3: Scientific background 92 May 2001
3.3 Flow diagram
The flow diagram provides an outline of all the major unit processes to be modeled, including their
interrelationships. It is helpful in understanding and completing a system to describe the system using a
process flow diagram.
With respect to the topic of flow diagram ISO states only that “it is helpful to describe the system using
a process flow diagram showing the unit processes and their interrelationships”. In addition to this initial
flow diagram, preparation of ‘specific’ flow diagrams is discussed as the first step in the paragraph on
Preparing for data collection (6.2): "drawing of specific process flow diagrams that outline all unit
processes to be modeled, including interrelationships".
Drawing up an initial flow diagram can be regarded as a practical aid to data collection. As it will only
become apparent which economic flows and delivering processes are relevant to a particular study as
data are retrieved, compiling a definitive flow diagram will be a highly iterative process. Initially, the
diagram will cover only the process delivering the functional unit and the adjacent processes supplying
the main raw materials and treating the principal waste flows, and their respective interconnections.
Even after all the relevant data have been collected, though, a truly comprehensive diagram will in
practice be impossible to compile. Given the common occurrence of process loops and multi-outputs,
such a diagram would simply include too many unit processes in overly complex interrelationship. In
most cases, then, the only workable solution is to draw up a basic flow diagram showing the main
constituent unit processes, from which to ‘zoom in’ to the underlying unit processes, represented
separately in partial flow diagrams of their own.
Wegener Sleeswijk & Huppes (1994) propose the following conventions for constructing flow diagrams.
- unit processes are represented as boxes;
- economic flows are represented as arrows between such boxes;
- economic flows enter a unit process at the top of the box and leave a unit process at the bottom of
the box;
- the main direction of flow through the flow diagram is top-down, although recursive flows are in the
reverse direction;
- all boxes contain text labels with the name of the process, e.g. ‘production of sulphuric acid’;
- arrows are labeled only in so far as their name is required for an unambiguous understanding of the
flow diagram;
- a flow diagram should preferably not contain more than 20 boxes;
- use of partial flow diagrams may be useful; for each box, it should then be clearly indicated whether
it represents an undivided unit process or an aggregated unit process, possibly elaborated
elsewhere in a partial flow diagram.
These rules should be seen as recommendations for an optimum use of flow diagrams rather than as
binding guidelines.
For reporting flow diagrams Lindfors et al. (1995a) provide the following recommendations:
Heijungs et al. (1992)
The life cycle consists of interlinked economic processes, each process input coming either from another
process or directly from the environment and each process output flowing either to another process or to
the environment, at least after allocation. Economic processes are taken to comprise resource extraction,
production of materials and components, product manufacture, product use and associated waste
processing, including recycling and reuse, and a variety of ancillary processes, such as transport and
electricity generation. Any processes omitted should also be specified.
In practice a summary flow diagram will first be compiled that includes only the key processes of resource
extraction, product manufacture and associated transport. The interconnected processes underlying each
of these are then represented separately in partial flow diagrams, allowing the practitioner to ‘zoom in’ to
the items of interest.
Part 3: Scientific background 93 May 2001
- The studied systems should be defined and reported using flow diagrams at the maximum level of
detail used in the study.
- Sub-systems may be aggregated to higher levels if appropriate, as long as detailed descriptions are
provided (e.g. in an appendix).
On this issue Meier et al. (1997) recommend that the process flow diagrams reported describe the entire
system under study and include system boundaries, major inputs, products and co-products, as well as
the main production sequence, ancillary materials and energy/fuel production.
No specific developments are foreseen for this topic.
Concluding, we recommend:
- to distinguish between an initial flow diagram, at the level of aggregated processes for each life cycle
stage, and a detailed flow diagram, at the level of (possibly) unit processes;
- to denote (aggregated) processes by boxes and economic flows by arrows and adopt the other
recommendations of Wegener Sleeswijk & Huppes (1994);
- to exclude environmental flows from flow diagrams, for pragmatic reasons; and
- to draft the flow diagram or diagrams as an iterative process during the data collection step.
No specific research is recommended.
3.4 Format and data categories
A key task of the Inventory phase is the collection of process data. This usually involves large quantities
of data in electronic form, retrieved in part from databases set up by others. To render these comparable
and mutually consistent a standard data format must be developed. All the various data categories
should be assigned a specific place in this format and a general description given of each to facilitate
and guide data entry and retrieval.
Ideally, the data formats used for data exchange (paper version of technical software format) and for
(software) processing should be identical. However, as software packages differ in terms of underlying
data model (often unspecified), no overall format for data processing can be recommended. Such a
format can be drawn up for data exchange, however, reducing substantially the efforts required for
processing with specific software.
Although ISO 14041 (1998E) does not distinguish a separate ‘format’ step, clause 5.3.4 covers
‘Description of data categories’ which is clearly related. This, in Annex A4 of ISO 14041 (1998E), an
example is given of a Data Sheet for a Unit Process. This Annex is provided for purely illustrative
purposes. In the accompanying text it is stressed that there are no fixed rules for either the number of
data categories or the amount