The Environment Agency warned South Herts Waste Management of the problem in March after receiving the company's waste returns for 2003 and 2004.
Four of the company's sites - Bilsthorpe, Fiskerton, Hucknall and Lenton - had accepted more waste than they should have done, by a total of over 9,000 tonnes.
Appearing before Nottingham magistrates on 10 August, the company pleaded guilty to four charges of contravening conditions of its waste management licences, contrary to section 33(6) of the Environmental Protection Act 1990. It was fined £5,000 for each offence with £1,395 costs.
Speaking after the case, Paul Thornton, the company's managing director, expressed "disgust" at the Agency. "It was a cheap prosecution," he said. "We knew we were over and we could have closed the gates, but we made the sensible decision to keep them open. Next time I will close them and the Agency can deal with any fly-tipping [that results]."
The Agency explained that the company had breached limits at other CA sites before and its policy is to prosecute repeat offenders.
The company has since had its waste management licences modified, increasing the amount of waste each can accept to 25,000 tonnes a year.