The final phase of a £2bn sewage treatment programme near Looe in Cornwall is beginning this week.
Construction of a £5m project at Polperro will see a sewage screening station at Scilly Cove and a 330m (1,083ft) long outfall pipe to the sea.
The scheme, described as "one of the final pieces of the jigsaw" of South West Water's Clean Sweep programme, will take more than a year to complete.
The work will improve sea water quality in the area, SWW said.
The programme has seen the modernisation of sewage treatment and discharge around the south west of England, removing almost 250 untreated sewage outfalls.