The incident occurred in June last year when United was operating the works at Cross Hands, near Carmarthen, on behalf of Welsh Water. Vandals entered the works and caused a discharge of sludge, polluting 900 metres of the Nant Glas stream and river Gwili. Some 560 trout died.
The company pleaded guilty before Llanelli magistrates on 26 August to causing polluting matter to enter controlled waters, contrary to section 85(1) of the Water Resources Act 1991, and to causing waters to be poisonous or injurious to fish, contrary to section 4 of the Salmon and Freshwater Fisheries Act 1974.
United was fined £2,000 on each charge and ordered to pay costs of £1,179.
The Environment Agency, which brought the case, said the incident could have been avoided if the site had been made secure, the tanks bunded and valves locked shut.
United issued a statement saying the vandals had been determined to cause damage and had cut through perimeter fencing to gain access. It added that security measures have since been improved further.
The offences bring United's total for 2004 to seven, and its total fines to £35,200.