Monitoring of phosphorus discharge in a sewage treatment plant with a phosphate automated analyzer

The removal of phosphorus in the sewage treatment plant is commonly done with precipitation with ferric iron in the form of ferric chloride. Ferric iron is added to the sludge recycling stream to assure a good mixing with the wastewater in the biological reactor. The precipitated phosphate is removed in the secondary settling tanks with the excess of biological sludge. This study installed and optimized the operation of a phosphate automated analyzer in the wastewater treatment plant with the objective to use the phosphate concentrations to adjust the dosing of ferric chloride and minimize its consumption. The study tested three sampling points for the phosphate automated analyzer: the sludge recycling stream to the biological reactors, the outlet of the biological reactor and the final discharge effluent. The objective is to identify the most stable sampling point to determine the necessary amount of ferric chloride to comply with the phosphorus legal limit in the outlet stream discharge into the sea. This study concluded that the phosphate analysis in the final discharge effluent is the best location for a stable and reliable phosphate determination. Furthermore, the phosphate concentration in the final discharge effluent is proportional (97–98%) to the total phosphorus concentration, so the phosphate analyzer can be used for the monitoring of phosphorus assuring the compliance with the legal discharge limit.