^ That. You can put Sludge in a Sludge Boiler to get stuff like Dirt, Sand, Clay, Quick Sand, Cracked Sand and Soul Sand. Sewage I'm not sure where you can put it in, but it's used to make MFR Fertilizer.
So both can help you a little early game, I used Sludge in my 1.4.7 game to avoid going...