The final decision to buy the farm, for pounds 1.2m, was made four months ago by the council's chief executive and leaders of the three main party groups.
A wider group of councillors got their first chance to review the deal only this month at a meeting of the policy committee.
If the plan to dump millions of tons of refuse at Aller Barton farm, Cullompton, is to go ahead, the county will have to give its subsidiary, Devon Waste Management, planning permission and an operating licence.
Opponents, including Roger Giles, an independent councillor, fear that may be a foregone conclusion given the decision to buy the farm.
'The public will believe that the planning process has been prejudiced,' Mr Giles wrote to the county's chief executive, Richard Clark.
What angers opponents further is that the purchase was being finalised behind closed doors as the council began consulting people about the future of waste disposal in the county. Alan Peters, chairman of the Cullompton-based protest group that is fighting the plan, said: 'It's sheer vandalism. It will destroy a beautiful area of the countryside.'
More than 70 dustcarts a day will travel to the partly wooded farm near the Blackdown Hills. Opponents say the nearby river Culm would be at risk of pollution.
The secrecy flows from government legislation forcing counties to run waste disposal operations on a commercial basis, at arm's length from the rest of the council.
Devon Waste Management, the county's waste disposal operating arm, had realised that its existing dumps were fast running out of space. Richard May, its chief executive, said that in order not to be outdone by private sector rivals, it had to find a new site to take 100,000 tons a year of household waste from the Exeter area. Aller Barton farm emerged as the most suitable site where a landowner, Joy Denning, was willing to sell.
But the council had to become involved in the decision to purchase because Devon Waste Management is not yet trading as an independent entity and has no capital of its own.
The council, which has no political party in overall control, had hoped this would have happened about two years ago. But the Department of the Environment has refused to allow the company to have property, rights and liabilities legally transferred to it from Devon.
The council and the department's lawyers are still disputing the allocations of potential pollution liabilities between Devon and its waste subsidiary.
Mr Clark, the council's chief executive, and the leaders of the three main parties agreed to provide the pounds 1.2m by buying extra shares in Devon Waste Management.
There will be an extra pounds 700,000 royalties payment for Mrs Denning if the waste disposal plan gets final approval. Most of the 500-acre farm would be sold as agricultural land, with only 64 acres kept for the dump and landscaping.
Mr Clark insists that the purchase does not prejudice the county's decision on whether to grant planning permission and an operating licence for Aller Barton. If they are not awarded, the farm will be resold.
Devon Waste Management said the rubbish will be dumped in a natural amphitheatre, screened by higher ground and trees. Advance engineering works would ensure no pollution leaks from the site. The protesters say their fight goes on.
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