Why do you want to get rid of sponges, do they cause mechanical issues like drop in flow rate or etc? Because they basically provide you with free and effective filtration, with that much of sponge, you might need to keep an eye on bioload after removing them.
You can use boiling water if the pipes are rated for boiling water temperature. But dont inhale the vapor afterwards as it might have toxic chemicals. After that you can scrape the inside by a long pipe/drain cleaning brush.
Something like this;
https://www.amazon.com/dp/B002C4XFCM...a-314350059250