The decrease of the concentrations of the indicator microorganisms of faecal pollution from the discharge point of sewage was studied in the marine area affected by the sewage discharges through three submarine outfalls located in the Southern Mediterranean coast of Spain. A noticeable decrease of the concentrations of the microbial indicators was observed after sewage disposal to the sea. Although the physical dilution affects the bacterial population levels, this process is not enough to justify the microbial decrease observed; for this, the biological inactivation process plays an important role in the microbial die-off in seawater. A bacterial reduction higher than 98% during the wastewater ascent towards the marine surface was achieved. These values imply a high bacterial inactivation during the initial dilution in addition to the physical process. Also, a fast decrease of the bacterial concentrations was observed from the upwelling point of wastewater at the sea surface; however faecal streptococci show a clearly longer survival than coliforms in the sea environment. On the other hand, the highest values of T90 and K were observed in summer.

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