Dilution and autodepuration processes in a coastal system affected by urban wastewater discharges: Case study of the Iro River estuary (southwestern Spain)

Abstract

Dilution and autodepuration processes in an estuary of strong tidal regimen affected by sewage from a wastewater treatment plant have been studied and identified. To differentiate between both processes, three evaluations were made during different seasons. Two tests were developed in parallel during each evaluation: characterization of the estuary and microcosm trials. In both tests, the evolution of diverse microbiological and physicochemical parameters were examined. From the microbiological results, T₉₀ for the faecal coliform (FC) and faecal streptococci (FS) bacterial groups were obtained. In the estuary, dilution was identified as the most important microorganism reduction process, with T90 of 12.2–40.8 and 11.7–53.5 minutes for FC and FS, respectively. In the microcosm trials, salinity was the main autodepuration factor, with T₉₀ of 3.1–42.6 and 4.6–54.8 hours for FC and FS, respectively. The FC group proved to be the most sensitive bacteria to autodepuration processes and FS the most adaptable to the adverse conditions of the aquatic environment.