You will need to all the floor sanding first, and then do the bleaching, other wise you will be sanding out the bleached effect, if you bleach first and sand later. I sure hope you are hiring a pro to to the floor sanding, but certainly do the finishing yourself, that's the fun part anyway. In any case you will need a long pause between the floor sanding and bleaching and finishing any way.
This is the most difficult of stains to remove, and you can really only bleach it out. But in your case the sanding will have diminished the stain a bit, I hope.
So for organic stains, like urine and feces, your only hope is to use hydrogen peroxide, in a very strong and dangerous concentration. Klean-Srtip is readily available in North America, and comes with good instructions. Visit their web site at www.kleanstrip.com for info on how to use this stuff. Click on the product button, them the wood bleach link. This will generally turn the dark spots to a off white color, which you will have to blend in with a light colored wood stain to match the color of your finished wood floor. On the freshly sanded floor you can use a splash of naphtha to test for the final color of the finished wood, on a clean area.
This bleach is water based, so it will raise the grain and make the wood rough. Be sure to let the wood dry overnight. And be careful when you sand it smooth not to sand out the bleached effect. Final sand with little pressure with 120 grit sandpaper.
Or you can repair the spot. Go to the Hardwood Authority section of this web site, and click on the wood floor repair article.
Let us know how the project went, send me a photo by email if you can. We will post it on the site. Oh, heck even a snap shot sent by mail would be great, send to the afore mentioned address.
As always your Most humble servant, Joseph, the Wood Floor Doctor.