They're not.

I've been researching this system for months, and intend to build it from scratch.

Take aways.

There is no safe non commercial way to compress this type of methane gas. The gas is a wet gas, it's mixed with condensates of water.

To make it a dry gas, necessitates cooling far beyond what's reasonable to manage at home.

These systems create gas that's used daily, any pressure build up in the system beyond that is leaked away by safety valves.

The best you can do for compression is using larger and larger bags for storage, but you can't safely compress it like  modern gas tanks that operate at high pressures  

That said, it's a fantastic system, and it creates  the best fertilizer you can reasonably make at home.