Biogas, methane digester:
https://www.maasenergy.com/how-digesters-work
https://www.biogassa.co.za/index.php...stic-digesters
Basically you want to poop in a bag!
A large bag so the organic waste sits about fermenting for around a month.
The other end of the bag needs to drain into std sewage systems IMHO, or its just a **** idea!
Other food and garden waste can also be added.
Then you would need a compressor to fill gas bottles if you want to run a car on it.
(or just use it @ home. cookers, water heaters, etc.)
Some filtering of other gasses is reqd, but nothing high tech.