All these test kits would check for the presence of small fragments of RNA. If you are applying the test on lots of sewage, it is likely to find some weird false positive that is not COVID-19 in the first.
I would find it even more interesting if there were specific agents in sewage, irrelevant from COVID-19, that would cause a false positive.
I'm glad that I'm not the only one scratching their heads slightly. According to the article " claims to have found the presence of SARS-CoV-2 genomes in a Barcelona sewage sample from 12 March 2019[1]. Further reading of [1] says that zilch (zero, nill, nothing) was found in samples taken on the 11 March 2019 (the day before) and that equally zilch was found on 13 March 2019 (the day afterwards)[2] and yet the article casually makes the assertion " have found the presence of SARS-CoV-2 genomes in a Barcelona sewage sample from 12 March 2019". Uhmm yeah, no.
[2] "archival WWTP samples from January 2018 to December 2019 (Figure 2). All samples came out to be negative for the presence of SARS-CoV-2 genomes with the exception of March 12, 2019".
I'm not an expert (or anything even close to being one in this domain) but let's not post anything that has yet to be properly peer reviewed. Not a dig at the submitter, but an eye-roll at the article.
I would find it even more interesting if there were specific agents in sewage, irrelevant from COVID-19, that would cause a false positive.