Appearing before Halifax magistrates on 29 April, Menzolit Ltd, based in Eastwood, pleaded guilty to causing polluting matter to enter controlled waters, contrary to section 85(1) of the Water Resources Act 1991. It was ordered to pay £1,168 costs on top of the £5,000 fine.
On 6 August last year, an Agency officer visited Eastwood sewage treatment works after receiving a call from Yorkshire Water about dead fish in the Calder.
The officer found 60 dead fish in the river, as well as a smell of styrene next to Menzolit's site.
Earlier that day, the company had been washing machine belts in its yard, expecting the drainage to go into the foul sewer. Instead, the water passed into a surface water drain which led to the river.
Menzolit said that the pollution was made up of detergent, polyester compound and styrene.
In mitigation, the company said that belt cleaning now took place inside the factory, and all drains have been colour coded to stop the incident recurring.