The Chemical Investigations Programme (CIP) brings together water companies in England and Wales with regulators to investigate a range of chemical substances, via the UK Water Industry Research platform (UKWIR). Building on CIP3, the CIP4 Sludge Investigation will assess the fate of chemical substances across the whole sewage sludge treatment train for different sludge digestion types. In addition to ubiquitous substances found in biosolids in CIP3, this project will analyse for microplastics and 54 different per/polyfluoroalkyl substances (PFAS). This project will be pivotal to the future direction of this critical biosolids outlet, and by association, the net resource recovery possible by the UK sector's current digestion asset base. For example, the Environment Agency will use CIP outcomes to understand and assess risks posed by chemical substances as it looks to replace the Sludge Use in Agriculture Regulations in England. This project will also inform wider discussion on the use of biosolids in agriculture, which is receiving increased global media and public scrutiny. As project lead, AtkinsRealis tasks include a global review of the scientific literature and the interpretative reporting of analytical results. Sampling and laboratory analysis tasks are led by RPS. The presentation will update the sector with key project insights, following its mid-point review.