Norovirus (NoV) is one of the most important aetiological agents of acute gastroenteritis both in developed and developing countries. NoV is shed in high concentrations by infected persons and contaminates recreational and drinking water through sewage discharge into the environment. The aim of this study was to determine the prevalence, genotypes and removal ratio of NoV by PCR, seminested-PCR and quantitative PCR (qPCR) assays in a sewage treatment plant in Rio de Janeiro city, Brazil, during one year of surveillance. NoV was detected in 7 (15%), 14 (29%) and 28 (58%) samples using PCR, seminested-PCR and qPCR, respectively. The mean removal ratio for the activated sludge process was 0.6 log10 for NoV genogroup I (GI) and 0.32 log10 for NoV genogroup II (GII). The peak NoV concentration was detected in the coldest months, with 53,300 genomic copies/litre. Nucleotide sequencing and phylogenetic analysis revealed that five strains clustered with GI strains and six with GII strains. This study demonstrates that NoV spreads into the environment despite the sewage treatment process and remains a source of waterborne outbreaks of acute gastroenteritis.