A COMPARISON STUDY OF TWO METHODS FOR PERFORMING NITRIFICATION INHIBITION TESTS

Control and monitoring of nitrification processes occurring in waste water treatment plants (WWTP) are crucial in preventing shock loads and inhibition of the sensitive nitrifying activated sludge population. The aim of this study was to compare two different methods for testing nitrification inhibition: an automated titrimetric Activity and NITrification Analyser (ANITA) and the traditional manual batch tests using 250 mL shakeflasks. Allyl thiourea (ATU), 3,5 dichlorophenol (3,5 DCP) and Dimethylformamide (DMF) were chosen as typical nitrification inhibitors. Activated sludge from a local WWTP was used for all the tests performed. In addition, classical photometric measurements of ammonia, nitrite and nitrate were performed for both methods. The Effective Concentration (EC) of toxicant which inhibits 50% (EC50) of the nitrification activity of the microbial population present in the system, was determined for the three test compounds chosen. The EC50 values obtained for ATU using both the ANITA (0.89 mg L−1), and batch shakeflasks (0.63 mg L−1) were in good agreement. For 3,5 DCP, EC50s obtained were 4.4 and 4.1 mg L−1 using the ANITA and the batch shakeflasks respectively, which are also in good agreement. The results obtained for DMF are not in agreement and the reason for the differences are discussed. In addition, results obtained using the ANITA compare well with results previously reported by other authors who used different methods (EC50s in the range of 0.5 to 2.0 mg L−1 for ATU and 0.5 to 3.6 mg L−1 for 3,5 DCP). In summary, the ANITA provides an automated, simple and sensitive method for testing nitrification inhibition, and it can be used to replace the manual testing methods provided the current equipment is improved.