The hunt is on for the landowners or businesses responsible for illegally dumping waste down drains in Somerset.
The Environment Agency (EA) said harmful substances were found at the sewage treatment works in Evercreech.
If the chemicals and oils had ended up in local rivers and watercourses they could have threatened wildlife.
Now the EA has teamed up with Wessex Water to try to find the culprits and to warn others of the risk of prosecution and fines for this offence.
Sewage treatment manager at Wessex Water James Dryden said: "Recent pollution incidents in Evercreech have been challenging and costly to eradicate.
"To prevent any further harm we will be visiting landowners and businesses in an attempt to locate the source of the pollution."