The company pleaded guilty before Bury St Edmunds magistrates on 18 April to causing polluting matter to enter the river, contrary to section 85(1) of the Water Resources Act 1991.
The court heard the firm holds an environmental permit which allows it to discharge treated effluent to the river. But in July 2007 it released untreated effluent which inflicted severe damage on the quality of the river.
A problem at the effluent treatment plant meant a tank had to be emptied into a treatment lagoon via a buffer tank. An employee set the transfer in motion and then left the site for two hours. The rate was too fast, causing the buffer tank to overflow into the river. About 80 cubic metres were estimated to have been discharged. Over 5,000 dead fish were collected along 1.5km of the river.
Muntons said in mitigation it did not realise there had been an overflow an it had now fitted an alarm and shut-off valve to prevent a recurrence.