Estimation of the Effect of Sewage Nitrogen Discharges on Coastal Waters: Case Study from the Mediterranean Sea

Abstract

Sewage discharges through marine outfalls are an important source of nutrients to marine waters, which cause undesired impact such as eutrophication. However, few authors have evaluated the contribution of wastewater disposal to nutrient concentration in coastal waters. We estimated how wastewater treatment plant (WWTP) discharges alter ammonium concentrations in coastal waters of the Western Mediterranean Sea. Data obtained from the literature and from the local government was used to formulate a 1D mathematical model which predicts ammonium concentrations along coastal waters with current direct discharges. The estimations were validated by comparing them to measured data and a significant agreement was found (R2=0.91). Then, the simulation of a scenario with no anthropogenic direct discharges was carried out to determine how much of the excess ammonium is due to sewage inputs. The study concludes that marine outfalls are the main driver of ammonium pollution in the studied area. Near-natural conditions could be obtained by implementing tertiary treatment to reduce nitrogen in WWTP discharges. Further research should focus on the consequences of ammonium pollution for ecosystems to efficiently evaluate the ecological status of coastal waters under the Water Framework Directive and to prioritize those coastal areas in greater need of nitrogen- removing tertiary treatment.