[oil pan 4]

I not as easy as just water washing it. Because then you have to get the water back out. In natural gas processing this is done with an ethylene glycol tower. Gas goes up, dehydrated glycol runs down picking up water.
Once you get out the water and all the corrosive stuff the best bio gas from herbivore poo still has around 25% CO2. Which burns meh.If the bio gas came from only decomposing vegetation then it could be anywhere from 50% to 75% CO2.
If bio methane is being used to power vehicles it's been heavily processed. All the stuff you don't want removed.

The idea of a farmer using it to power mobile machines is laughable. I'm sure they can make it and use it for heating but just try to talk a farmer into putting home made bio gas in his $250,000 tractor that has to be reliable. Good luck with that.

Where I work we use bio gas. One 1,500hp boiler runs 25% bio gas. It saves about $1,000 a day in natural gas. But this boiler is 11 or 12 years old and has been retubed 3 times. Because the bio gas is more corrosive. $25,000 to retube the boiler every few years is nothing.