Yeah, my first thought was, how do they have six month old samples of waste water to test? Do they store samples regularly? And since PCR can pick up extraordinarily tiny amounts, how do they rule out the possibility that someone involved in storing and re-accessing the wastewater didn't contaminate it? In fact, how do they explain the virus remaining intact enough to be detected over that very long period of time?How sound is that study? I remember there was talk about Covid-19 tests potentially picking up other coronaviruses. Would that be possible here? I dunno, December sounds pretty extreme, and I'm not sure its not being very contagious would explain the later data.
Edit: I now see that a pilot project to monitor waste water was set up about a year ago, before anyone knew about the virus. It's reported that October and November samples tested negative, while months after that tested positive. I also found a preprint of an earlier study by the Italian group, testing samples of wastewater from February on. Apparently, these samples are stored frozen after collection, which should inhibit degradation of the virus. Still, I wonder if failure to detect the viral RNA in samples earlier than December could have been the result of more degradation in the earlier samples.