How Many Small Agglomerations Do Exist in the European Union, and How Should We Treat Their Wastewater?

Abstract

The European Union (EU)’s legislation on urban wastewater requires all agglomerations with a population equivalent (PE) above 2000 people to undergo a secondary (mechanical/physical and biological) wastewater treatment. Agglomerations below 2000 PE, though, fall outside the scope of the current EU’s legislation. As such, their regulation is heterogeneous across the various EU member states, and there is no systematic collection nor reporting of data enabling an estimation of their actual significance as a source of pollution for the receiving water bodies. Here we present a spatial model to delineate agglomerations in a GIS, based on population distribution and land cover. From the model results, in the EU, we identify 364,650 agglomerations with 2000 PE or less, housing a cumulative population of about 75 million inhabitants. We then calculate the organic matter and nutrient loads these agglomerations can discharge, assuming they presently undergo primary wastewater treatment, and the reduction of loads that can be expected under different treatment scenarios, together with the corresponding treatment costs based on a simple cost model. Using a conventional shadow price for the organic matter and nutrients removed, we show that all treatment scenarios show a benefit-to-cost ratio (B/C) above (or close to) 1. However, only a scenario of secondary treatment applied to all agglomerations above 1000 PE provides sufficient safety margins on the B/C. This suggests the opportunity to expand the scope of the current legislation down to agglomerations of this size, while addressing smaller agglomerations depending on their actual impacts on the receiving water bodies, through “appropriate treatments” defined by the local authorities.