Most sewage sludge in the UK is treated and spread on farmland, and there is growing concern about the microplastics it carries. To manage this, water companies first need a reliable way to measure microplastics in sludge. This work is part of the CIP4 Microplastics and Advanced Thermal Conversion project, funded by UKWIR through the Chemical Investigations Programme and delivered with AtkinsRéalis, Aston University and other partners.

Sludge is hard to analyse because its own organic matter, such as fats and proteins, can break down into the same compounds that are used to identify plastics. This can make plastic levels look higher than they really are.

We are comparing three approaches for measuring microplastics in sewage sludge digestate by pyrolysis-GC/MS: (1) a single shot, where the dried sludge is heated once at 600 °C; (2) a double shot, where a first step at 300 °C removes loosely held organic compounds before the 600 °C step; and (3) digestion of the organic matter before analysis. All three use a certified standard of 12 common plastics, tests with pure plastics, and spiked samples to check accuracy. The aim is to recommend the most reliable and practical approach for monitoring microplastics in sewage sludge.