Options on how to compete the clean-up of a vast toxic sludge spill at an American power plant - and how much it will all cost - are being considered by the country's environmental watchdogs.

In December 2008 a dike at a coal-fired power station operated by the Tennessee Valley Authority in Kingston gave way releasing vast quantities of ash sludge onto surrounding land and the neighbouring river, destroying several homes in its way.

At the time, the event was called the worst environmental disaster in American history.

The clean-up project is ongoing, but is now moving towards what the Environmental Protection Agency has called the 'non-time-critical' stages, giving the regulator a window to look at the options.

The EPA's Engineering Evaluation and Cost Analysis (EE/CA) Report was published this week, with what boils down to three distinct options.

The first two are both colossal dig-and-dump schemes, involving 2.8m cubic yards or 6.8m cubic yards respectively.

The first of these would involve excavating the now-contaminated land beyond the confines of the breached holding pool and disposing of it offsite, while the second option adds excavation of parts of the containment cell onto that.

Both these options would see all or part of the containment pond sealed and capped.

The third option on the table would be to be similar to the third, but with on-site disposal of the contaminated material.

All three alternatives restore the area to pre-spill conditions and have different levels of foundation treatment so the perimeter dike containment system will be protective and stable over the long-term.

The EPA has put the report out to public consultation and says it would welcome feedback.

It says that while producing the report it considered several other options but dismissed them as not viable.

Sam Bond