SARS-CoV-2 detected in waste waters in Barcelona on March 12, 2019

This is a two month old paper. People have generally agreed it was most likely a false positive due to it only detecting a partial match with a very faint signal and there being no other supporting evidence.

From https://theconversation.com/was-coronavirus-really-in-europe...:

"In coronavirus testing, scientists typically screen for more than one gene. In this case, the researchers tested for three. They had a positive result for the March 2019 sample in one of the three genes tested – the RdRp gene. They screened for two regions of this gene and both were only detected around the 39th cycle of amplification. (PCR tests become less “specific” with increasing rounds of amplification. Scientists generally use 40 to 45 rounds of amplification.)

There are several explanations for this positive result. One is that SARS-CoV-2 is present in the sewage at a very low level. Another is that the test reaction was accidentally contaminated with SARS-CoV-2 in the laboratory. This sometimes happens in labs as positive samples are regularly being handled, and it can be difficult to prevent very small traces of positive sample contaminating others.

Another explanation is that there is other RNA or DNA in the sample that resembles the test target site enough for it to give a positive result at the 39th cycle of amplification.

Further tests need to be carried out to conclude that the sample contains SARS-CoV-2, and a finding of that magnitude would need to be replicated separately by independent laboratories."

Previously discussed and (mostly?) debunked at https://news.ycombinator.com/item?id=23652804

> These results encouraged the researchers to analyse some frozen samples between January 2018 and December 2019, with the shocking results of the presence of SARS-CoV-2 genome in March 2019, before any notification of COVID-19 cases in the world. “All samples were negatives regarding the SARS-CoV-2 genome presence except for March 12, 2019, in which the levels of SARS-CoV-2 were low but were positive, using two different targets”, says the researcher.

Smells like a false positive.