A novel sewer pipe with a double-deck structure was developed for in-sewer treatment. The purification rates were ca. 10 g-BOD/(m•d). Furthermore, the performance of the novel pipe coupled with simple post-treatments was also examined. The coagulation was the most efficient: BOD was successfully decreased to below 20 mg/L by the in-sewer treatment followed by the addition of polyaluminum chloride. The novel pipe, therefore, may be promising technology for both BOD reduction (independent usage) and alternative secondary treatment (coupled with coagulation).