Removing impurities is essential in the Muslim’s life. Jurists have detailed the rulings of removing different kinds of impurities taking into consideration the fact that this religion imposes no hardship on its followers.
Dr. Waleed ibn Khaled Basyouni said:
If the blood is removed from the knife with water or anything else, the impurity is gone. This is also why we are ordered in sharia to wash the private parts with water or to wipe with something solid and dry, like a stone or toilet paper, after using the bathroom. If the material used is pure and clean in itself, it removes the impurity.
If any traces of the impurity are left on the knife, and it is then used to slaughter something permissible like a sheep that would not make the latter slaughter haram. Rather, it should be cleaned after being slaughtered as it is in countries where people eat pig meat and slaughter them in their slaughterhouses, even though Muslims might buy from these places as well.