An appeal is being considered after a water company put treated sewage into a Bedfordshire brook.
The Environment Agency claimed fish died when Anglian Water put activated sludge - sewage treated with bacteria - into Pix Brook at Stotfold in 2002.
Luton Crown Court heard the company had a permit to discharge treated sewage into the stream.
The judge agreed the permit did not say what could be dumped and did not specifically cover "activated sludge".
Officers, who are now considering whether to appeal, say at the very least the wording of the permit which allows dumping needs to be revised.
An Anglian Water spokesman said the company was pleased with the outcome of the hearing and regretted the case had reached the prosecution stage.
The material was put into the stream, under a permit issued by the Environment Agency, from one of six sewage treatment tanks after a breakdown at a nearby treatment works.