Assessing the impacts of wastewater treatment implementation on the water quality of a small urban river over the past 40 years

This paper studies the effects of the implementation of wastewater treatment (WWT) on the water quality of small urban river systems by considering as an extreme case study (volumetric contribution of wastewaters > 50 %) the evolution of the Zenne River waters (Belgium) over the last 40 years. In urban rivers, organic matter (OM), oxygen, and nutrients are primarily controlled by wastewater releases which depend on the population and the WWT capacity in the river basin, the latter being dependent on environmental policy decisions. We introduce a novel basin-scale evaluation method that considers the evolution of annual pollutant loads at the outlet of the river basin directly as a function of WWT capacity. Based on this approach, we could prove that the load reductions observed after the implementation of WWT in the river basin was a good indicator of the global treatment efficiency of the WWT plants. We also show that high self-purification processes within the river basin may lead to reach minimum levels of OM before the completion of WWT. In addition, the effects of wet weather conditions did also change as a function of the WWT capacity going from positive effects at low capacity to negative effects at high capacity. Finally, the full implementation of WWT in urban river basins does not necessarily guarantee a good status for water quality, mostly because of the high volumetric proportion of treated wastewaters, which do not have the quality standards of river waters.