Long trend reduction of phosphorus wastewater loading in the Seine: determination of phosphorus speciation and sorption for modeling algal growth.

The lower Seine River is severely affected by the release of the treated wastewater from the 12 million inhabitants of the Paris agglomeration. Whereas urban effluents were the major source of phosphorus pollution in the late 1980s, the ban on polyphosphates from detergents in 1991 considerably reduced the phosphorus (P) loading to the Seine River and was followed in 2000 by the implementation of phosphorus treatment in the largest wastewater treatment plant of Paris conurbation (Seine Aval). Phosphorus discharged to the rivers from domestic wastewater was reduced by 80 %, significantly decreasing phytoplankton biomass in the large branches of the Seine River. Considering that phosphorus treatment (the use of ferric salts in the P treatment line) might change the adsorption of ortho-phosphates on suspended matter, we experimentally studied again their sorption processes in these new conditions. We found parameters of the Langmuir equation (Pac = 0.003 mgP mgSS-1 ; Kps = 0.04 mgP L-1 ) that significantly differed from the values previously considered for modeling of the whole Seine, especially for Kps (Pac = 0.0055 mgP mgSS-1 ; Kps = 0.7 mgP L-1 ). Using the Seneque-Rivertrahler modeling approach, we showed a better agreement between P observations and simulations with the new P sorption parameters, with slight effect on the simulation of the development of phytoplankton in the water column.