According to Veolia, Unite represents approximately a quarter of the workforce, but Unite has previously disputed this figure, claiming it represents 80%.
Mr Sweeting said Unite members wanted to be part of negotiations on subjects such as pay and conditions.
Veolia said it already offered them “a voice and seat at the table”, but they rejected the offer to continue pursuing recognition.
The company said it would not grant this, because rival union GMB would retaliate with its own strikes.
Mr Sweeting called this “absolutely farcical”, because the majority of the workforce had “collectively signed that they will support recognition of their colleagues”.
However, GMB has protested against Unite being recognised, raising a complaint to the Trades Union Congress and a counter dispute with Veolia.
Veolia said Unite should “end this feud against a rival union and take the seat, rather than holding rallies and protests”.