Here's a second link that ...

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Here's a second link that actually refers to the same institute's research, but also mentions the official Dutch governmental support health institute RIVM which did similar tests and came to the same conclusion.

https://www.h2owaternetwerk.nl/h2o-actueel/kwr-vindt-coronavirus-in-rioolwater

TO the right side of this web-page you can also follow a string of comments that still criticise the interpretation of the results (parts of RNA do not mean that the virus itself IS present in the water, but only WAS present somewhere upstream). For me irrelevant in case you want to determine where in the country the virus (re-)emerges, now or in the future.