Using ffmpeg to denoise (reduce film grain)
If you are patient enough, use the nlmeans filter (it needs more time to denoise).
Alternatively, you can use atadenoise or hqdn3d video filters for fast denoising.
There are also other filters you could try, like vaguedenoiser.
I would strongly advise you to pick a couple of representative frames from videos and then try to denoise it with those filters first, experimenting with different options of these filters, and picking the one that works best for your case.