Anglian Water has been fined £18,000 for pumping polluted water into the North Sea off Suffolk.
The sample taken at the new treatment centre
Magistrates at Lowestoft said the way the company had dealt with the problem was "woefully inadequate".
The fine comes two days after Anglian Water, based in Huntingdon, was fined more than £17,000 after sewage effluent entered the River Great Ouse in Norfolk.
Lowestoft magistrates heard on Thursday that on 3 July last year the Environment Agency took a routine water sample from a new £75m water treatment centre at Corton in Suffolk.
The sample breached European water discharge regulations with more than three times the permitted levels of solids.
A failure at an Anglian Water pumping station meant that over two days effluent from a vegetable processing factory had overflowed into the domestic sewer.
Anglian Water, which pleaded guilty, said it deeply regretted the lapse, which had happened while the new plant was still in its commissioning process.
The company, which has had 65 convictions in the 12 years since water privatisation, said it had invested billions of pounds to ensure river and sea water is cleaner than it has ever been.
It was stressed in court that there had been no significant environmental impact from the latest incident.