On 22 April last year, the Environment Agency received reports of sewage flowing out of a manhole cover in Broadwater Crescent, Stevenage.
An officer went to the scene and found children from a nearby school paddling through the sewage on their way home, having to walk in the road to avoid unpleasant debris littering the pavement.
The officer phoned Thames Water and was told a contractor had gone to unblock the sewer, but found they needed larger equipment.
The officer returned to the site the next day, but found the sewer was still blocked.
Sewage had also entered Stevenage Brook via surface water drains and samples showed it had ammonia levels 15 times above normal.
Appearing before Stevenage magistrates on 19 January, Thames Water pleaded guilty to causing sewage to enter controlled waters contrary to section 85(3) of the Water Resources Act 1991. It was made to pay £1,172 costs on top of the £12,000 fine.