The incident occurred last December when a leak of mains water at the mill entered a waste lubricant sump. The sump overflowed into the site drainage system and collected in a lagoon.
The company had no procedure for checking the contents of the lagoon, Hartlepool magistrates were told. As a result the lagoon overflowed into a nearby watercourse, the Stell.
The pollution was traced to the site after a National Rivers Authority office