A piece of equipment at the Colchester Sewage Treatment Works failed, causing a foul smell in the air for weeks.
Anglian Water has apologised and promised a replacement part will fix the problem by tomorrow at the latest.
The spokesman said: “It is the result of some work we are doing for a £20million upgrade, due to be completed by December 2013, replacing and refurbishing a lot of equipment on the site.
“The smell was caused by a piece of equipment which was due to be replaced but gave out and was a part of the treatment process.
“We are very sorry for the inconvenience and the smell it has caused.”
The environmental team at Colchester Council has been made aware of the problem.
A spokesman said: “Our environmental protection officers have visited the site and believe that Anglian Water are doing all they can to resolve the problem.
“We are aware that they are experiencing problems with a piece of equipment, for which a replacement has been ordered.
“Should residents wish to report an odour complaint they can contact the council or alternatively they can discuss their complaint with Anglian Water directly.”
Businesses and residents complained of nausea, sickness and a loss of trade as a result of the odour.