Whanganui councillors have been told the city's wastewater plant is failing on a catastrophic level - but it has never worked as it was designed to.
Foul smells have plagued the treatment plant since before Christmas last year, when larger than usual amounts of trade waste entered it.
Infrastructure manager Mark Hughes says the plant has not worked properly and never met its resource consent since it opened in 2007.
Mr Hughes says the problem is fundamentally due to the design, with the main treatment lagoon a unique shape and depth compared with other wastewater plants.
He says the waste is not being treated properly by bacteria, or biomass, and flows too quickly towards the back of the pond, accumulating in 'dead zones' without oxygen.
The council has agreed to commission engineering consultants Cardno BTO to look at the options for fixing or replacing the plant.