The naturalist made an unscheduled appearance on stage at the festival last week.
Thousands of people came to the stage to watch him speak - and share some clips from his upcoming series Seven Worlds, One Planet, which documents the 'marvelous creatures, birds, mammals and animals' on all seven continents.
"That is more than a million bottles of water that have not been drunk by you," Sir David said. "Thank you. Thank you.
"There was one sequence in [television series] Blue Planet 2 which everyone seems to remember. It was one in which we showed what plastic has done to the creatures that live in the ocean.
"They have an extraordinary effect. And now, this great festival has gone plastic-free."