Sundourne Products, owners of the Bryn Posteg Landfill site at Llanidloes are considering appealing after being fined £37,500 for pollution offences.
The company was also ordered to pay prosecution costs of £46,207.03 and a £15 victim surcharge at Brecon on Monday.
The company was convicted of two charges of polluting a stream, said the Environment Agency. The agency said the firm was also convicted of two charges of exceeding emissions and failing to report it.
A spokesman for Sundorne Products said after the hearing it was considering appealing.
During a site visit by officers, on 29 January 2008, evidence was seen of system failures on site.
The agency said it had given approval for the company to install of a reed bed system to treat surface water, which would minimise the risk of further pollution incidents.