This has been all over the breaking news, although none of the news stories I found link to the actual study, so here's Reuters' take:
The Italian National Institute of Health looked at 40 sewage samples collected from wastewater treatment plants in northern Italy between October 2019 and February 2020. An analysis released late on Thursday said samples taken in Milan and Turin on Dec. 18 showed the presence of the SARS-Cov-2 virus.
“That COVID-19 could have been circulating in Italy is possible,” said Rowland Kao, a veterinary epidemiology and data science professor at Scotland’s Edinburgh University.
“(This finding) does not on its own, however, tell us if that early detection was the source of the very large epidemic in Italy, or if that was due to a later introduction into the country.”
Samples positive for traces of the virus that causes COVID-19 were also found in sewage from Bologna, Milan and Turin in January and February 2020. Samples taken in October and November 2019 tested negative.
So it is corroborating evidence that the virus may have been circulating in Europe somewhat earlier (December 18 in Italy) than the confirmed (individual) cases.
It seems that study has been published in a peer reviewed venue. They mention Milan and Turin in the abstract and find Dec 18 as the first positive sample, so it's almost certainly the same study that was discussed in the press.
Furthermore the CDC has published a paper from Italian authors finding a similar case to the French one, but in Italy, i.e. a Dec 2019 retrospective detection:
We identified severe acute respiratory syndrome coronavirus 2 RNA in an oropharyngeal swab specimen collected from a child with suspected measles in early December 2019, ≈3 months before the first identified coronavirus disease case in Italy.
The sequence (SARS-CoV-2_Milan_Dec2019 [GenBank accession no. MW303957]) was identified in a specimen collected from a 4-year-old boy who lived in the surrounding area of Milan and had no reported travel history. On November 21, the child had cough and rhinitis; about a week later (November 30), he was taken to the emergency department with respiratory symptoms and vomiting. On December 1, he had onset of a measles-like rash; on December 5 (14 days after symptom onset), the oropharyngeal swab specimen was obtained for clinical diagnosis of suspected measles. This patient’s clinical course, which included late skin manifestations, resembles what has been reported by other authors; maculopapular lesions have been among the most prevalent cutaneous manifestations observed during the COVID-19 pandemic, and several studies have noticed a later onset in younger patients (7).