A high yield is not always better, and a low yield does not always indicate a problem. The result must be consistent with the technology, input quality and actual output streams.
What is process yield?
Yield describes the share of input material that becomes an appropriate process output. It must always be interpreted in relation to the technology, material specification and classification of the individual streams.
What can reduce yield?
- contamination in the input material;
- other polymers;
- moisture and foreign matter;
- labels, metals and other components;
- process rejects;
- losses during washing, separation and filtration.
Useful output material
Not every stream leaving a line should be treated in the same way. What matters is whether the material meets the relevant specification and is genuinely intended for further use.
Why does yield matter to an auditor?
Yield helps assess the plausibility of the balance. If the technology would be expected to generate significant rejects, but the records indicate almost 100% output material, an explanation may be needed.
Do not optimise the figure at the expense of quality
Trying to maximise yield by including material that does not meet the product specification can reduce quality and undermine the reliability of the system. The result should reflect the actual effectiveness of the process.
How should streams be documented?
Good practice is to distinguish clearly between the main product, other qualifying outputs, rejects, losses, by-products and inventory changes.
Key takeaway
Yield is an indicator, not an end in itself. It is most useful when it can be reproduced from records and explained in the context of the actual process.