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How many square metres are you having re-plastered? I've worked with some filthy plasterers who scarcely put more on the walls than they dropped on the floor.
Heavy duty (cotton) dust sheets should protect your floors. Your plasterer does have them, doesn't s/he?
Has much wash water are they going to make? You could let that settle then flush just the liquid, but none of the solids.
All solid waste material, dried plaster, can be disposed of at your local authority recycling/waste site. Some charge for building waste that cannot be recycled, passing onto you your Landfill Tax.
Yeah, fuck your local river with plaster residues. The Environment Agency will never track you down.