Biogas has:

• NO smell
• NO smoke
• NO carbon monoxide, and 
• Produces Carbon Dioxide and Oxygen 

The biodigester is always filled with water, then you add waste and food to let off the GAS!

FYI: fats make a lot of Biogas!

Fats, grease floats to the top, egg shells settle to the bottom so their not in the filter.

• You would also need to know that if you put paper into the biodigester it would take more maintenance. 

If it is a question whether some food scrapes are more preferable than others- it all comes down to what you want to do with your Biogas. If you want a wicked amount of gas- use fats! Think about what is the use, what is the product? But there isn't really anything the biodigester would reject. The only thing you don't want to put into your Biodigester are things that kill bacteria.

Does the tube have a permanent pressure? Yes, you cannot start a Biodigester off of just vegetables you need something like lake much, pond mud, guts of an animal and even manure is great to help make those pH levels neutral.