A nature-based solution to mitigate the impact of reclaimed water discharge into a river under water scarcity conditions

Abstract

Water scarcity in Mediterranean regions is a growing concern. Alternatives are necessary to meet the water demand from urban areas and to maintain ecological flows in rivers due to the decreased availability and the poor quality of water that is found during drought events. In these conditions, flow can be sustained through discharges from sewage plants. However, if river dilution factors are low, the discharges can endanger aquatic ecosystems. To mitigate these impacts, tertiary treatments are integrated into wastewater treatment plants to improve effluent quality. In this study, the impact of a wastewater treatment discharge in a river has been evaluated before and after the implementation of a nature-based tertiary treatment under different river dilution conditions. The water quality of the river as well as of the discharged effluent was assessed through the concentration of small-suspended sludge particles (d < 100 μm). The main findings of the manuscript were: (1) The concentration of small-suspended particles in the river, 50 m downstream the WWTP point of discharge was found to be lower, especially during periods with a low dilution factor, after the nature-based tertiary treatment was put in operation.

This was due to the effectiveness of the nature-based tertiary treatment in eliminating small particles (85.4 %) whereas large sludge particles (d > 100 μm) are also reduced (69.8 %) by sedimentation. (2) A predictive model to assess the impact of the treated wastewater discharge into the river was developed and calibrated. The model considers different percentages of water treated by the tertiary system in water scarcity scenarios.