This research deals with the contribution of process simulation models to the factory acceptance test (FAT) of process automation (PA) software of drinking water treatment plants. Two test teams tested the same piece of modified PA-software. One team used an advanced virtual commissioning (AVC) system existing of PA-emulation and integrated process simulation models, the other team used the same PA-emulation but basic parameter relations instead of the process simulation models, the VC-system. Each test team found one (different) error of the thirteen errors put into the software prior to the experiment; the majority of the errors was found prior to the functional test. The team using the AVC-system found three errors, the team using the VC-system found four, but the AVC-team judged 1% of the test items ‘not possible’, the VC-team 17%. It was concluded that the hypothesis that with AVC more errors could be found than with VC could not be accepted. So, for the FAT of PAsoftware of drinking water treatment plants, the addition of basic parameter relations to PA-emulation satisfied. Not the exact process behavior helped to find errors, but the passing of process thresholds.