Distribution and behavior of dissolved species of nitrogen and phosphorus in two coastal Red Sea lagoons receiving domestic sewage

Abstract EN

Two sewage treatment stations are daily dumping about 100,000 m3 of domestic sewage into two coastal lagoons off the city of Jeddah.

Field measurements and laboratory experiments were undertaken to study the behavior and impact of the effluent on the hydrochemistry of the receiving water body.

Physical and chemical characteristics of the receiving water suffered important modifications.

Salinity and pH were lowered at the discharge points and increased with distance from it.

Concentrations of the different species of nitrogen and phosphorus were largely in excess to those measured in Red Sea water out of the impact of land sources.

Ammonium and organic nitrogen are the major constituents of total nitrogen representing more than 90% of the total nitrogen concentration at the stations in proximity of the effluents.

Reactive phosphate is the major phosphorus species representing between 68 and 94% of the total phosphorus concentration.

Flux estimates showed that approximately 2000 kg of nitrogen and 800 kg of phosphorus are discharged in the area every day.

Despite the laboratory evidence of the liberation of dissolved nitrogen (46%) and phosphorus (36%) from the particulate fraction within one week, and the presence of active processes of transformation between the different species, field data showed an almost conservative behavior.

Deviation from this behavior was only observed when environmental conditions were highly reducing.

The short residence time (~1.5 day) and the relative slowness of the biological elimination and transformation rates are the most probable reasons for the apparent conservative behavior.