The majority of wastewater plants in Northern and Central Europe now apply tertiary treatment, although elsewhere in the EU, particularly in the south-east, the proportion of primary and secondary treatment remains higher. This has led to a reduction in discharge of nutrients, biological oxygen demand — a measure of organic pollution — and of ammonia to receiving waters. The emission of some hazardous chemicals has also been reduced. To mitigate point-source pollution by micropollut ants in Switzerland, for example, the large mem plants in areas of concern are to be upgraded, with a furthe treatment step in addition to tertiary treatment. The correspond ing legal basis is currently being established. Contaminant transport pathways are complex and the ultimate fate of urban pollutants highly variable, depending, among other things, on the mode of wastewater collection and treatment.