Chemicals found in sewage sludge are a mirror of chemical use in society. Up to 100 000 chemicals are on the market in Europe. One problematic group are the organohalogen compounds (OHCs) containing chlorine, bromine, fluorine or iodine. Some are persistent, bioaccumulative and toxic and have been banned. However, multiple new OHCs have been introduced as replacements, faster than targeted analytical methods can be developed for them. The concept of the OHC ‘iceberg’ is that we are only measuring a fraction of total OHCs in a sample. Without knowing the total size of the iceberg, it is impossible to know how much is missed by targeted analysis. The aim of this project is to characterize the total and known extractable OHCs entering (influent), retained by (sludge) and passing through (effluent) a sewage treatment plant (STP) using a combination of combustion ion chromatography and targeted analytical approaches (for total and known OHCs, respectively). This paired methodology enables determination of the fraction of extracted OHCs not accounted for by known substances (the unseen part of the iceberg). Suspect screening will then be used to identify, and when possible, quantify OHCs that make up the missing fraction. Novel OHCs will be further investigated in recipient water and fish collected near the outfall, and at sites upstream of the STP to evaluate their bioaccumulation potential, and identify potential sources (e.g. landfills, hospitals, urban run-off, households).