The sponge is to keep it in there snug. I just got one from J&L and believe they had another in stock.

I clean mine with vinegar spunge included.
Sponge over night only, the rest is for about 48-72 hrs
Works very well.

I clean mine about every 6 months. There is a small disc at the bottom of the impeller that needs to be cleaned. It has 3 or 4 holes in it and they must be open. If they get plugged they will trigger an alarm that is just annoying.

Tom R