Yorkshire Water Services has been fined £733,000 after a worker suffered fatal burns.
Leeds Crown Court heard that Michael Jennings was using an angle grinder to cut through corroded bolts at the bottom of a dry well when sparks from the grinding wheel ignited his overalls.
A Health and Safety Executive (HSE) investigation discovered the fire had occurred because the air in the dry well had been oxygen-enriched, due to a half-open drain valve from an adjoining section of the site.
Mr Jennings suffered whole-body burns as a result of the fire and died two days later in hospital.
He and a colleague were changing the stop valve on the end of the disused drain pipe at Tadcaster Sewage Treatment Works (pictured) when the incident happened.
The HSE also found a near-miss report had been recorded just under a year previously.
Workers had alerted local managers to the same area of the treatment works being oxygen-enriched, which led to an investigation.
However, this probe wrongly concluded that the oxygen enrichment was a result of residual oxygen rather than a leak.
This led to work continuing in the section on the basis that there was no further risk of oxygen enrichment.
The HSE said its investigation had revealed Yorkshire Water Services risk assessment and permit to work procedures had been inadequate.
It found there to be no site-specific procedures in place and the generic risk assessment template form did not include oxygen enrichment as a possible hazard.
Adding that the employees working on the day of the incident were not familiar with the site and they were not aware of the September 2014 near miss.
As a result, the staff had did not have the knowledge or experience to recognise that oxygen-enrichment of the dry well was a potential hazard when the valve was taken off or opened.
HSE inspector John Micklethwaite said: “This was a tragic and wholly avoidable incident, caused by the failure of the company to implement an adequate and effective safe system of work for work in a confined space.
“Those in control of work activities have a duty to identify hazards that could arise, to eliminate or to mitigate them, and to devise suitable safe systems of work. The risk assessment process is central to this role.
“The employer also has a duty to provide the necessary information, instruction and training to his workers, and to provide an appropriate level of supervision to ensure that the work can be carried out safely and without risks to health.”
Yorkshire Water Services Ltd of Western House, Bradford, pleaded guilty to breaching Section 2(1) of the Health and Safety at Work etc. Act 1974 and was fined £733,000 and costs of £18,818.