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

2. Background and definitions 37
The ‘simple delay’ method consists of assuming that after a fixed lifetime, the devices are
discarded, i.e. the amount of WEEE generated at a time t is equal to the amount of EEE sold
at the time ‘t minus lifetime’.
By the ‘distribution delay’ method, also called ‘survival analysis’ by Wen et al. (2009), a
lifetime distribution is assumed. The lifetime distribution is defined as the probability density
function that EEE that entered the use phase at time t 0 exits use as WEEE at time t 1 . Several
authors mentioned that the lifetime distribution of EEE is best approached with Weibull
functions (Davis et al. 2007; Mueller et al. 2007; Terazono et al. 2006). However, plotting
Weibull distributions requires knowing a shape and a scale parameter (Davis et al. 2007;
Mazhar et al. 2007), which would depend on the equipment types. No data on these
parameters are available in the literature. According to Mueller et al. (2007), normal
distribution functions can also fit well to model the lifetime distribution of EEE. This
distribution was used in this research, taking as mean the average lifetime and as standard
deviation 30% of the average lifetime (variation coefficient of 30% as modelled by Bilitewski
et al. (2008)). The estimated WEEE generation for 2007 is the sum of the amounts of WEEE
generated by the discarding of the EEE sold in the years before 2007.
The ‘batch leaching’ method consists of dividing the stocks of EEE in households and
businesses by the average lifetime. The stocks are determined by multiplying the penetration
rates in the households (businesses) by the number of households (businesses). Penetration
rates are defined as the average number of appliances per capita or per household. This
method was applied by EEA (2002), Kumar & Shrihari (2007), Rotter & Janz (2006) and
Fraunhofer IZM (2007).
More information on the other methods listed in Table 6 can be found in the publications of
EEA (2002) and Terazono (2007).
2.4.2. Collection
The separate collection of WEEE conditions the recycling and reuse of WEEE. WEEE can be
collected on a voluntary basis or in order to fulfil legislative regulations, driven by economic
and/or idealist motivations (for environmental protection). Collecting large amounts of WEEE
to divert it from being disposed of is one of the main objectives of the majority of the current
legislations.
In practice, the channels used to collect WEEE differ both in terms of the applied models and
the persons or organisations responsible for organising and/or financing the operations. The
following models are available for collection:
1. Bring systems, with permanent collection centres with fixed opening times, containers
on the streets, or temporary collection events;