If you are unsure of the source then get new sponges and pads
Definately agree with this.
Most people use vinegar and water to clean a used filter.
If it looks really nasty, you can soak in bleach before scrubbing, then soak for 8 hours in a concentrated solution of water and prime, or whatever dechlorinater you usually use.
What kind are you talking about, btw? Got a good deal somewhere?