The case was brought by the Environment Agency.
The treatment plant at Dunkeswell, near Honiton, was already scheduled for improvement works when discharges breached agreed limits between May 2013 and May 2014.
There had been 5 occasions when waste that entered the stream exceeded the limits for pollution set in the permit. On occasions concern had been expressed about the condition of the site and planned site visits were missed.
The court took into account the frequent number of visits the company paid to the site, and the steps that had been taken to rectify problems.
South West Water admitted breaching the agreed discharge limits at an earlier hearing in Exeter Crown Court.
They were fined £45,000 with costs of £5,700 and a £240 victim surcharge by Judge Phillip Wassall at a second hearing at Taunton Crown Court.
Mischka Hewins for the Environment Agency said:
There were on-going problems with sludge rising in the settlement tank, which were specifically raised on over 19 occasions and led to knock on problems with the filters. The inlet screen, which is of fundamental importance to the site, was also not working.
South West Water should have seen that problems were leading to the likelihood of breaches of permit and done more to prevent it.
The court heard the plant had been identified as in need of improvement in 2007 and by 2013 was due for investment under South West Water’s Glidepath programme.
A growth in population in the area it served put increased pressure on the Dunkeswell site and most of the problems could be traced to a failure of the screening device at its inlet.
This led to a build-up of too much sludge for the filter system which had to be repeatedly cleared by South West Water.