How does processing landfill gas work?

I've been reading about the new processing plants being built in Ohio and California, as well as sites about safety, and I was wondering:

1. How they clean the methane from 40-50% to "passable"
2. Does the gas need to be 100% methane or could it contain other chemicals like chlorine or mercury?
3. What are some effects of burning "dirty" gas for cooking or heating?
4. What is the gas per waste ratio?

I appreciate any insight. Thanks!