The remaining liquid from secondary treatment is now called the secondary effluent, and disinfected with sodium hypochlorite (a form of chlorine) which kills bacteria . Right before the effluent is sent to the outfall tunnel, sodium bisulfite is also added to dechlorinate the water so that chlorine levels in the final effluent will not harm the marine environment. The effluent is then sent to the outfall.

Detail of a basin end of the disinfection system. Image credit: MWRA