According to the French Ministry of Higher Education, Research and Innovation, there is a strong correlation between the genome of the coronavirus in sewage and the number of cases in the population.
Assaying by PCR (or other methods of nucleic acid amplification) in wastewater is therefore a simple method of monitoring the progress or the regression of the epidemic.
Apparently water has been tested in Canadian capital for virus since summer. Around Thanksgiving they predicted disaster. Instead Ottawa cases on decline right now bucking national trend. Oh, all those scientists with their predictions...