Check Valve

The fire trap as you have described would work but would need to hold some amount of pressure. I am thinking one in-line one-way check valves with steel wool stuffed in a larger pipe in between might also work and hold more pressure.

Steel wool packed pipe       No steel wool 
________________________   _______________
________________________SX>_______________ <----- Flow of gas 
                         X>="One-way" check valves 
                         S="spring" 

Steel wool has the advantage of slowing down an explosion, allowing the pressure to build slowly, this would allow the one way check valve to close. The pipe may need to be the next size larger than the normal carburetor feed line so that when the steel wool is added it doesn't restrict the flow. Use as coarse a steel wool as you can find that will fit the pipe. You don't want it burning or melting. Small pipes can hold an extremely large amount of pressure. This technique has the advantage of minimizing the water adsorbed in the methane gas. The explosion trap should be the last thing in line as it goes to the carburetor. It should be as close to the carburetor as is possible.

Water would be partly removed with appropriate water traps upon compression in the previous stages. As gas cools off (usually from expansion of lower pressure) it will drop out water, so after the regulator would be another opportunity for a water separator. A water separator is nothing more than a large chamber (as compared to the input pipe size) with a drain in the bottom. Gas comes in fast mixed with water, its flow rate slows down enough for the water to fall out, and puddle in the bottom. The drain is opened once in a while to let the water out.

Offered by Mike.