Plymouth18 sewage charges cost one utility almost £8m.
The earliest offence happened 11 years before any penalty followed.
Courts convicted South West Water on 17 counts of illegal sewage discharge across Cornwall and Devon between 2015 and 2021.
South West Water says the offences are historic and it has since invested significantly in its network.
Whether customers or shareholders ultimately absorb the £7.87m fine remains unclear.
Sources: BBC