Combining submerged membrane technology with anaerobic and ...

Materials and methodsInterferencesChemical incompatibility makes it unlikely that NO 2- , free chlorine and nitrogentrichloride (NCl 3) will coexist. NCl 3 imparts a false red colour when colour reagent is added.The following ions interfere because of precipitation under test conditions and should beabsent: Sb 3+ , Au 3+ , Bi 3+ , Fe 3+ , Pb 2+ , Hg 2+ , Ag + , chloroplatine and metavanadate. Moreover,cupric ion may cause low results by catalizing decomposition of the diazonium salt.The determination should be promptly made on fresh samples in order to avoidbacterial conversions of NO 2- . At least filtration of the samples should be done immediatelyafter collection.2.1.2.3. NitrateNitrate concentration in wastewater is determined following the method 4500-NO 3- -Bdescribed in Standard Methods for the Examination of Water and Wastewater (APHA-AWWA-WPCF, 1998).Measurement of UV absorption at 220 nm enables rapid determination of NO 3-ions.Because dissolved organic matter also may absorb at 220 nm and NO 3-does not absorb at275 nm, a second measurement at 275 nm is used to correct the NO 3-value. If correctionvalue is more than 10% of the reading at 220 nm, this method should not be used.Determination procedurePlace 5 mL of sample (diluted if necessary to get a maximum concentration of N-NO 3-of 2.5 mg·L -1 ) and add 0.1 mL of HCl 1N. Afterwards, the absorbance at 220 and 275nm is measured in a spectrophotometer (Cecil CE 7200) with quartz or matched silica cellsof 1 cm or longer light path. The absorbance related to nitrate is obtained by subtractingtwo times the absorbance reading at 275 nm from the reading at 220 nm according toequation 2.3. The quantification is done with a 6-8 points calibration curve in the range of0-17.5 mg N-NO 3-·L -1 , using KNO 3 as standard (figure 2.5).eq. 2.3where A 220nm and A 275nm are the absorbances at 220 and 275 nm, respectively, a isthe slope of the calibration curve and b is the intercept.InterferencesDissolved organic matter, surfactants, NO 2-and Cr 6+ interfere with NO 3-determination. Moreover, various inorganic ions such as chlorite and chlorate may69