Nitrite Interference with Soluble COD Measurements from Aerobically Treated Wastewater.

This study aimed to determine the interference of nitrite (<inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00549-ilm01.gif"/>) with soluble chemical oxygen demand (COD) measurements from the effluent of landfill leachate treated by sequencing batch reactors (SBRs). Synthetic wastewater assimilating young and old landfill leachate was used as influent for SBR1 and SBR2, respectively. A mixture of raw wastewater and landfill leachate was used as influent for SBR3. Due to the high ammonia concentration in the influent, different levels of partial nitrification were observed in all the reactors and <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00549-ilm12.gif"/> was detected in the effluents. Theoretically, when <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00549-ilm23.gif"/> is present in the effluent, 1 mg/L of <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00549-ilm34.gif"/> accounts for 1.1 mg/L of COD (i.e., 1.1 mg COD/mg <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00549-ilm36.gif"/>) due to the oxidation of <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00549-ilm37.gif"/> to <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00549-ilm38.gif"/>. It was found that the value of 1.1 mg COD/mg <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00549-ilm39.gif"/> was 3 times higher than the values obtained in the experiments with SBR1 and SBR3. In addition, the values obtained in the experiments with SBR2 were much higher than 1.1 mg COD/mg <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00549-ilm40.gif"/>. These results suggest that the theoretical value of 1.1 mg COD/mg <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="00549-ilm02.gif"/> cannot be applied to predict the COD values caused by nitrite in the tested wastewaters. To obtain an accurate measurement of soluble COD in samples that contain nitrite, nitrite should be eliminated before the measurements.