The Environment Agency received complaints about a tributary of the Siston brook being discoloured and smelly in November 2007.
An inspection of the nearby abattoir showed there had been a back-up in its sewers which caused an overflow into surface water drains.
The company confirmed a sewer had been blocked the previous night and contractors had been brought in to clear it. The sewer was privately owned and not the property of the local sewerage undertaker Wessex Water.
The Agency told the court water samples showed the stream was highly polluted where it ran through a visible amenity area.