Denoise existing render (image sequence)?

Does the denoise process depend on information gathered during the render, or can it be run as a post-process on existing renders, as a compositing node?

Sorry for the stupid question.

Denoise appears to work “on the fly” or during the render. RenderMan uses a post process on the entire frame, but as each frame is rendered, Cycles includes the denoising during the render stage. I think both work on a similar principle, where they are using a multi layerd image to do the denoising, once you have rendered the frame, save it as a sequence or a video file, the information is not available for the renderer to use anymore as a post process.