Nitrification inhibition test of salty wastewater containing Tetrakishydroxymethyl phosphonium sulphate (THPS), formaldehyde and methanol using salt-adapted nitrifying bacteria, an alternative test method to ISO method

Nitrification inhibition test is a standard method to test if the chemicals or samples received by wastewater treatment plant are toxic to the nitrifying bacteria in the wastewater treatment plant. Nitrification inhibition test based on ISO method ISO9509 or modified ISO9509 method (REFLAB method) can’t differentiate the toxicity between salt and toxicant in the sample, which is unrealistic for regulation of salty wastewaters that are treated in wastewater treatment plants with sufficient volume to dilute salts to harmless levels. To overcome the toxicity due to salinity to the nitrifying bacteria, salt-adapted nitrifying bacteria were grown on the Z400 MBBR carriers. The aim of this work was to validate and compare the nitrification inhibition of samples measured from salt-adapted nitrification inhibition test method (DTU method) with REFLAB method. The new salt-adapted nitrification inhibition test was validated by investigating the statistical uncertainty on the use of new test method with the existing REFLAB method.
The inhibition concentration of formaldehyde and methanol were similar when nitrification inhibition experiment was conducted using DTU method and REFLAB method. Lower inhibition concentration of Tetrakishydroxymethyl phosphonium sulphate (THPS) was observed with the DTU method. This means it is more sensitive to THPS and thus detects it at lower concentrations. The difference between the methods might be due to sorption of chemicals into the sludge as the biomass concentration in the DTU method is lower. The new methods standard deviation of nitrification inhibition around the 50% inhibition level was below 3%. The method is thus well suited to replace the REFLAB method for salty water samples as both repeatability and sensitivity is similar or better.