Whanganui's failed wastewater treatment plant was doomed from day one, according to a confidential report that has just been made public.
The district council has released the report, which formed the basis of its legal case against the plant's designer MWH Global, now that it has settled with the company.
Prepared by an independent adviser, the report found the $15 million plant had significant design failings.
It said one of them was the attempt to combine all the treatment processes in one lagoon, which had never been tried before.
Another was significantly underestimating the sludge storage volumes and the aeration energy required.
The plant was closed in 2013 and construction of a new $41 million replacement is due to begin in July.