Assessing effluent fluoride concentrations following physicochemical wastewater treatment

Abstract

The anticipated fluoride concentrations in untreated wastewater, as well as their removal during physicochemical treatment, were determined in order to assess potential risks to the aquatic ecosystem of the St. Lawrence River due to proposed fluoridation of Montreal's drinking water. Monitoring of the raw wastewater yielded average daily fluoride concentrations of 0.22–0.58 mg/L, but discrete sample concentrations ranged from 0.20 to 1.11 mg/L. This indicates that significant massive point discharges occur in the Montreal Urban Community (MUC) sewer system. Jar tests suggest that no precipitation of fluoride would occur following ferric chloride and polyelectrolyte addition at doses similar to those used on the full-scale wastewater treatment plant. This is confirmed by thermodynamic analysis of the solubility of various fluoride species. Full-scale tests, conducted by applying a continuous fluoride dose to the plant influent, confirmed that no removal would result from the treatment process. However, the plant did dampen the shock loads of fluoride from industrial sources. The predicted average raw wastewater fluoride concentration, including ground water and unidentified industrial discharges, was calculated to be 0.83 mg/L. Unidentified sources contribute 132 kg/d (~ 40% of the total). The average fluoride flux in the St. Lawrence River, as a result of fluoridation of the drinking water, would increase by 1.2%. Key words: fluoridation, physicochemical wastewater treatment, St. Lawrence River, mass balance analysis.