At Scunthorpe magistrates court on 7 November, Corus UK - formally British Steel - pleaded guilty to three counts of inadequate maintenance and written instructions in contravention of its integrated pollution control (IPC) authorisation.
The incident, at Dawes Lane coke ovens on 29 June 1999, was caused by a blockage in a process which cleans up exhaust gases for use as fuel elsewhere on site. One stage involves passing the gas through a water shower to remove ammonia. In the event of a blockage, emergency vents - called battery bleeders - are designed to open automatically to prevent an explosion.
Speaking after the trial, Dr Alan Walters, who heads the Environment Agency's inspection team, said the blockage was caused by a "catalogue of errors" by staff at the works. A section of the ammonia washers was allowed to fill up with water, blocking the gas inlet.
First, staff could not tell whether the water pumps were working because a warning light in the control room had failed. This caused maintenance workers to fail to reactivate a pump removing water from the washer.
Second, a faulty water level indicator meant that the level in the washer was not correctly shown, even when dangerously high. And third, the pumps in the washer had been wrongly reconfigured following unrelated routine electrical work.
On the first count Corus was fined £17,000, with the other two charges attracting £10,000 each. The three offences were contrary to section 6(1) of the Environmental Protection Act 1990. A further three charges were withdrawn. The company was also ordered to pay the Agency's full costs of £51,960.
The incident was aggravated by a failure to take effective action. Dr Walters said the emergency vents remained open for 80 minutes. The combination of broken warning lights and staff leaving the control room led to further delays.
The result was a "very large and very yellow" cloud containing carbon monoxide, hydrogen, methane, hydrogen sulphide and polyaromatic hydrocarbons. Although the cloud was visible from Scunthorpe town centre, it did not pass overhead.
Following an initial investigation, the Agency issued Corus with an enforcement notice requiring five remedial measures and a variation notice requiring six improvements. Another notice followed last November after Corus failed to supply inspectors with sufficient information on the content of the emission. Appeals by Corus against these notices were withdrawn without being heard.