I used a PUR water filter I got from REI a few years back...used it for camping. I think it was 1 micron.
Instead of removing the Humungo cap, I drilled a hole and glued a standard garden hose fitting to it. It's real easy to access the liquid now.
There is some gravity filters. No need to pump.
I just ordered a cartridge. I'll let you know if it works.
Tried a gravity filter and it was too slow for me and it required elevation. with the pump I just drop the tube in and pumped into a bucket, didn't even have to move the lamp. Yes, it was a little work at about 5 minutes/gallon. filtered it twice.
Of course for a colossus you have to remove the liquid first.
But after, the MSR gravity filter filters more than 1.75 liters per minute (that's what they said, I didn't tried yet)