A modified nitrification inhibition test for high-salinity wastewater

Nitrification inhibition is a standard test for industry wastewater received by wastewater treatment plants (WWTPs). Standard tests such as ISO9509 cannot differentiate the effect of toxicity from relevant chemicals and the temporary inhibition of bioactivity that is induced by a change in osmotic pressure, which occurs if a sample has a high salt concentration. This, however, is unrealistic, as salt does not result in nitrification inhibition on real WWTPs, as their nitrifiers adapt to salt concentration in the same way that nitrification occurs in the oceans. To provide a more realistic inhibition test, we cultured nitrifiers on biofilm carriers in wastewater constantly high in salt content and fortified the salinity of the tested samples to the salt concentration used for culturing, thus eliminating the salt effect. The salt tolerance test was compared with the existing ISO9509 method on statistical uncertainty with results for the common type of salty wastewater from oil extraction and a solution of single chemicals relevant to this wastewater. The nitrification inhibitions of formaldehyde and methanol were similar between the salt-adapted method and the ISO9509 method, while lower inhibition at biocide tetrakis(hydroxymethyl)phosphonium sulphate (THPS) was observed with the salt adapted method. The new methods standard deviation of nitrification inhibition around the 50% inhibition level was below 3%. Thus the method is well suited to test nitrification inhibition in salty water samples as both repeatability and sensitivity are similar or better than the ISO9509 method.