Resource recovery specialist Axion Consulting has voiced concerns that proposed sampling sizes and techniques for the Materials Recycling Facility Code of Practice could lead to unacceptable margins of error and inaccurate data on material input/output composition.
The Manchester-based company, argues that a “one size fits all” approach is wrong as sample sizes should be determined according to the number of individual items in each sample taken – not by weight alone.
In response to the government’s public consultation on the Code, Axion Director Keith Freegard said that as well as deciding on the most appropriate and statistically-accurate sampling techniques, MRF operators must focus on exactly what they want to achieve from each particular sampling exercise.
For example, the “correct” sampling regime for measuring the long run average composition of a MRF’s mixed infeed stream would be significantly different from the regime required to verify the purity of each individual output material load sold to a reprocessor as part of a quality-based, commercial sales contract.
Keith Freegard, Axion Director – “Sample requirements can be very different for different material streams. I think a one size fits all approach is wrong; it’s more about asking yourself the right questions and then designing an appropriate and accurate sampling system”
The Code is intended to improve MRF output quality by making operators of MRFs over a certain size sample and measure the composition of their inputs and outputs. Results from these tests would then be made available for businesses and local authorities in the supply chain.
Keith – “Sample requirements can be very different for different material streams. I think a one size fits all approach is wrong; it’s more about asking yourself the right questions and then designing an appropriate and accurate sampling system that considers whether the calculated errors that arise are acceptable set against the commercial importance and intended use of the results.”
Stipulating purely weight-based sample sizes is not the solution, he argued, because the average weight of individual items varies widely across the material types found in the UK waste stream. Sampling techniques have to be tailored according to the material type and the operational characteristics of individual MRFs.
“For example, 20kg of very light plastic bottles contain many more individual items than 20kg of glass jars or steel tins. The correct statistical approach is to use pre-determined numbers of items in each sample to give a known accuracy of the result within set error limits and confidence levels,” said Keith.
He added: “Crucially, it’s really important to ask yourself a simple question: Why am I taking this sample? Because then you can decide if the sample size and frequency you have chosen will give a result that helps you answer that question within an agreed range of accuracy.”
The Code is set to be delivered through changes to the Environmental Permitting Regulations (England and Wales) which will come into force on April 6 2014.