The company appeared before Bedford magistrates on 17 June to plead guilty to polluting the Harrowden Brook with sewage, contrary to section 85(1) of the Water Resources Act 1991.
The incident happened in December 2003 when a main sewer burst, flooding the field near Wilstead with sewage and turning the brook turbid and grey. The Environment Agency said that there was no evidence of a fish kill but ammonia levels rose 30-100 times above the normal level.
Agency prosecutor Anne-Lise McDonald told the court: "There is a history of the rising main bursting at this site and saturating the field, in fact three occasions since November 2001. Anglian Water have already received a formal warning followed by a formal caution for the last two occasions, both of which had a significant impact on the environment."
Anglian was fined £18,000 and ordered to pay costs of £1,748. It was the company's first water pollution fine this year.