In this study, we assessed the efficiency and effectiveness of the Vrishabhavathy Valley Treatment plant (VVTP) in Bengaluru city, which is the oldest STP in the city. Since VVTP treats both raw sewage and polluted river water, with the latter constituting 80% of the influent, we sampled water quality at locations upstream and downstream of the plant to evaluate overall efficacy as well. We found that VVTP is able to reduce biochemical oxygen demand (BOD5) by only 47%. This low efficiency can be attributed to the high and variable levels of chemical oxygen demand, consistent with episodic industrial discharges. Moreover, the mean values of pH, dissolved oxygen, total suspended solids, BOD5, nitrates, faecal coliforms and faecal streptococcus did not change significantly between upstream and downstream locations. Treating river water using an STP is clearly not an efficacious way of improving river water quality. Thus, before setting up new STPs, sewerage boards need to invest in building the underground drainage network to bring raw sewage to existing STPs.