Material Flow Analysis - Description of The Method - Basic Principles

Basic Principles

MFA is based on two fundamental and well-established scientific principles, system approach and mass balance. While these principles are applied wide across science and technology, it is the way they are applied to the socioeconomic metabolism that makes MFA a special method.

System Definition:

An MFA system is a model of a process, industry sector or region of concern. Its level of detail is chosen according to the purpose of the study. An MFA system consists of the system boundary, processes, flows, and stocks. Contrary to e.g. chemical engineering where such a system would represent a specific physical setup, systems and processes in MFA can represent much larger and more abstract things as long as they are well-defined. The concept of the system is central as it allows to allocate quantitative information either as stocks within certain processes or as flows between processes. In other words an MFA system allows to graphically allocate the meaning of measurements or statistical data in form of stocks or flows that are related to certain processes in a given system.

MFA studies can be refined by disaggregating or simplified by aggregating processes.

Next to the system and the arrangement of processes and flows in between, scale and scope of the system need to be specified. The spatial scale is the geographic entity that is covered by the system. A system representing a certain industrial sector can be applied to the US, China, certain world regions, or the world as a whole. The temporal scale is the point in time or time span for which the system shall be considered. A system can represent a snapshot of stocks and flows at a certain point in time or it can contain time series which describe the temporal evolution of the system variables. The material (scope) of the system is the actual physical entity that shall be quantified. This can be a certain chemical element such as cadmium or a substance such as CO2. More general things can be quantified as well as long as some kind of balance can be established. Examples are goods such as passenger cars or other physical quantities such as energy.

Unlike in daily life, MFA requires a more precise use of the terms material, substance, or good due to the way they are affected by the mass balance principle. We refer to chapter 2.1 from Brunner and Rechberger:

• A chemical element is "a pure chemical substance consisting of one type of atom distinguished by its atomic number".
• A substance is "any (chemical) element or compound composed of uniform units. All substances are characterized by a unique and identical constitution and are thus homogeneous." From chapter 2.1.1 in Brunner&Rechberger.
• A good is defined as "economic entity of matter with a positive or negative economic value. Goods are made up of one or several substances". From chapter 2.1.2 in Brunner&Rechberger.
• The term material in MFA "serves as an umbrella term for both substances and goods." From chapter 2.1.3 in Brunner&Rechberger.

Process balance: One of the main purposes of MFA is to obtain a complete picture of the metabolism of certain elements or substances within the scope of the system. Such an analysis must also cover the stocks and flows that are not covered by financial accounting such as some waste flows, exhausts, or stocks of obsolete products. Mass balance or more general process balance is a first order physical principle that turns MFA into a powerful tool. The requirement for a balance to hold for each process facilitates a complete picture of the materials used, produced, and discarded within the various processes. Which balances hold for a given system depends on the specific processes that are considered: While for a process ‘oil refinery’ one can establish a mass balance for each chemical element, this is not possible for a nuclear power station. A car factory respects the balance for steel, but a steel mill doesn’t. Mass balance is a powerful and surprisingly versatile concept for the quantification of MFA systems.

When quantifying MFA systems either by measurements or from statistical data, mass balance and other process balances have to be checked to ensure the correctness of the quantification and to reveal possible data inconsistencies or even misconceptions in the system such as the omission of a flow or a process.