A simple method, reported in unpublished Israeli work, is to have an inhibition function I(C) = max(0, 1 - a C).
Some published work suggests that you can ignore inhibition, unless you propose to model adaptation to salinity:
C Dahl, C Sund, GH Kristensen and L Vredenbregt, 1997, "Combined biological nitrification and denitrification of high-salinity wastewater", Wat. Sci. Tech. 36(2/3) 345-352