You'll definitely need boiling water, since the hot temperature helps loosen up the grease a little. You should also find a strong degreasing soap or dish detergent. Baking soda and vinegar should also help. First you can put the filters in the sink, cover them with baking soda and pour some vinegar on them. Wait for about half an hour, then with a brush remove all the grease and dirt you can. If after that the filters are still greasy, you can try the degreasing soap. While the filters are in the sink just pour lots of hot water on top of them and add the soap. Wait for a while and after that try to scrub off the dirt. It works every time.