Grey-box models, which combine the explanatory power of first-principle models, with the ability to detect subtle patterns from data, are attracting great attention in various sectors. Simulating the dynamic process model with software sensors in parallel to the real plant, allows the automation system to observe all the modeled inner states of the plant even if they cannot be registered by sensors. In this work, software sensors serve to monitor the ammonia concentrations in the aerobic tanks of the plant line and in the anoxic tank. In addition, the software sensor tracks the maximum nitrification rate.