IHSS&IWA26 / BRNO / CZECHIA / 23–28 August 2026 BOOK OF ABSTRACTS 120 NOM and Aquatic Systems Thursday, 27 August 2026 / Hall C SL57 The Curious Role of Inorganic Ions on the Coagulation of Natural Organic Matter Peter Jarvis, Sam Nyarko, Bruce Jefferson and Irene Carra Cranfield University, Cranfield, Bedford, MK43 0AL. Corresponding author e-mail – p.jarvis@cranfield.ac.uk The coagulation of natural organic matter (NOM) using metal salts remains the principal mechanism for its removal in most surface water treatment works (WTWs) across the globe. Indeed, this can provide highly effective removal of disinfection by-product (DBP) precursors when operated under optimised conditions, requiring manipulation of coagulant dose and pH to ensure effective charge neutralisation. In an increasing number of cases, additional removal of NOM is being required due to stricter regulation associated with DBPs, higher levels of NOM being seen in source waters, and changes in the nature of the organic matter that make it harder to coagulate. This has driven application of suspended ion-exchange (IEX) processes located upstream of coagulation that act to reduce the load of NOM reaching the downstream coagulation phase. This combination of treatment leads to highly effective reductions of dissolved organic carbon (DOC), and reduced levels of a range of DBPs. Another (perhaps unintended) consequence and benefit of applying upfront IEX treatment for NOM removal has been an improvement in clarification observed at WTWs following coagulation, which has driven longer filter run times as particle loads have decreased [1]. This process stability appears to be driven by the formation of very large and robust floc particles that form during coagulation of water that has been treated by IEX. We have now observed this phenomenon for surface water sources from upland and lowland catchments. Possible explanations for these observations relate to changes in the nature of the NOM characteristics and balance following IEX, and/or the role of background inorganic ions on the coagulation and floc formation process [2]. The aim of this work was to explore the role of both the organic matter and inorganic ions on the coagulation process, particularly in relation to floc formation and character. Through a series of jar test experiments on a range of water sources that were treated with and without IEX pre-treatment, floc characteristics were compared, in addition to water quality characterisation. These experiments were carried out on surface waters from both upland and lowland water sources. These results showed that for IEX pre-treated water, floc size was typically double that of water without pre-treatment when the coagulant:DOC
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