SUbstance flow analysis of the recycling of small waste electrical ...

60 Substance flow analysis of the recycling of small WEEE
determine and express the uncertainty of any kind of measurements. Without the
specification of the uncertainty, measurement results cannot be compared with each other or
with reference values (DIN V ENV 13005). According to this guide, the uncertainty of a
measurement results consists of several components, which can be classified into two types
according to the method used to quantify them:
A. Components calculated with statistical methods
B. Components determined in another way.
The standard deviation (square root of the variance) is calculated to quantify the components
of type A. For components of type B, a variable is determined to approximate the standard
deviation, the existence of which is assumed.
In this thesis, the standard deviation of every component used to calculate a mass flow
(mass, metal concentration, transfer coefficient) was either calculated or estimated. If the
standard deviation could not be calculated, the radius (half the width) of the 95% confidence
interval of the variable was estimated. Based on the assumption that the distribution of the
uncertainty was normal, the standard deviation of components was assumed to be half of the
radius of the 95% confidence interval. The variation coefficient, which is the standard
deviation divided by the value of the variable, can also express the uncertainty. The
uncertainties of the components were combined according to Gauss’s law of error
propagation by summing the square of the standard deviations of the components (DIN V
ENV 13005). The resulting uncertainty is also expressed as a standard deviation.
3.4. Data compilation
The compilation of the data gathered through the methods presented in 3.3 requires the
following steps:
1. A qualitative description of the system, defining the collection, treatment and reuse
processes and modelling the material and substance flows from one process to the
next one;
2. The integration of the quantitative data on the flows of sWEEE into the system model;
3. Data reconciliation to calculate observable unknown quantities and to eliminate
discrepancy of redundant information (Reuter et al. 2005);
4. Calculation of the flows of precious metals based on the concentration of precious
metals in the material flows.
These four steps were realised by the STAN freeware (TU Vienna 2008), which helps to
perform material flow analysis (Cencic & Rechberger 2008). After drawing the processes and
material flows in the graphical interface of STAN, the data on the flows with their