Combining submerged membrane technology with anaerobic and ...

Materials and methods2.1.3. Phosphorus compounds2.1.3.1. OrthophosphatesOrthophosphate concentration in wastewater is determined following the method4500-P-E described in Standard Methods for the Examination of Water and Wastewater(APHA-AWWA-WPCF, 1998).Ammonium molybdate and antimony potassium tartrate react with orthophosphate inacid medium to form phosphomolybdic heteropolyacid. This compound is reduced byascorbic acid into molybdate blue.Reagents preparationReagent A: Sulphuric acid 5N.Reagent B: Solution of antimony potassium tartrate. 1.3715 g ofK(SbO)C 4H 4O 6∙0.5H 2O are dissolved in 500 mL of distilled water. This solution must bekept in a bottle with glass top in order to be preserved.Reagent C: Solution of ammonium molybdate. 20 g of (NH 4)6Mo 7O 24∙4H 2O aredissolved in 500 mL of distilled water. This solution must be kept in a bottle with glass topin order to be preserved.Reagent D: Ascorbic acid 0.01M. This solution is stable for one week.Combined reagent: To prepare 100 mL of the combined reagent, the reagents A to Dare mixed according to the following volumes: 50 mL of reagent A, 5 mL of reagent B, 15mL of reagent C and 30 mL of reagent D. The mixture must be stirred after the addition ofeach reagent, following the mentioned order. This combined reagent is stable for 4 hours.Determination procedureA sample of 5 mL is taken and one drop of phenolphthalein indicator solution (0.5-1 gphenolphthalein in 1 L of ethanol at 80% concentration) is added. If red color appears,reagent A (H 2SO 4 5N) is added (drops) until the red color disappears. Then, 0.8 mL of thecombined reagent is added and the mixture is stirred with a vortex stirrer. After 10 minutesbut before 30 minutes, the absorbance at 880 nm is measured with a spectrophotometerCecil CE 7200. The quantification is done with a 6-8 points calibration curve in the range of0-1 mg P-PO 43-·L -1 , using KH 2PO 4 as standard (figure 2.6).71