In bioreactor processes, the on-line monitoring of key parameters relies on the use of adequate sensors. In this article, we implement model-based software sensors to monitor nitrate concentrations in an algae batch process. Nitrate concentration is a crucial component in algae growth, and its assessment is used to achieve optimal growth conditions or to determine optimal harvest times. Batch cultivation of the algae Chlamydomonas reinhardtii is carried out in a Labfors 5 airlift photobioreactor provided by INFORS-HT. By means of…