Complete newb to biogas.

Hi all and thanks for letting me join.
So as a bit of background I have been living an off grid/semi off grid lifestyle for the last 5 years.  Based in the South of England in a 30 foot American caravan(trailer). As an electrician, that side of things is covered with a 10 KW solar/battery system and a 6kw diesel generator for the British winter, that seems to last from September to July. I have built a septic tank which works really well. But this year I have been looking into, via YouTube, biogas.
As I live on my own with a mate in a static caravan behind me, turning our combined waste, food and toilet, really appeals to me.  Even if it will just run the fridges. So just a couple of questions.

I've seen in several posts and videos that toilet paper cannot go in... But no one ever says why...  So???

How much flushing water is too much. Both of our toilets are trap door types where it only sends as much water down the pan as you let it.

Obviously heat is required for the gas monkeys to brew.  Is below ground better. Could a heat belt like you use for beer/wine brewing be used.

Probably loads more to ask later

Cheers all

Interesting project Steve. I'm afraid I know very little about biogas, but did a bit of digging and found this document from the international renewable energy agency which gives a simple overview. The systems they illustrate appear to be underground. Although I don't suppose it is technically necessary, it maybe easier to construct as the walls will be supported by earth. They appear to be pressurised chambers. I guess since the bacteria need heating a form of insulation would be good too.
The only clue I found about adding paper is that as a bacterial process the bacteria can't digest cellulose (you normally need fungi for that) and it is described as a liquid process. So the paper will probably just block the system up and I suspect it will sludge up. You could look at a bidet system and wash rather than wipe perhaps - here is a spendid read about toilet paper alternatives :)
No idea on the amount of water, maybe someone with more experience will chip in.

Steve Holder wrote:
Obviously heat is required for the gas monkeys to brew.  Is below ground better. Could a heat belt like you use for beer/wine brewing be used.

There is a backyard biogas digester made by an Israeli company who sell world-wide. The website has some useful information, though the site is not easy to navigate. One link of interest I found in round-about ways is the "Booster" tent for cooler climates. One could make some kind of little greenhouse, well insulated at back, sides and bottom.

https://landing.homebiogas.com/booster/

The toilet they sell, btw, is a standard ship toilet you can get in the UK for about £90.

This video "True review: Homebiogas with bio toilet" has some useful general info in the comments.
https://youtu.be/tdkmqxy-Tno?si=5tIYTvryDYHKtQdf

A bidet would be good way to deal with the paper issue as Nancy said. As you need water to flush waste into the tank, you may as well use that black water for the flush.

Another useful bit of info that came from a video where the HomeBiogas guy said that you can easily convert any LPG or natural gas cooker to run on the low pressure bio gas by simply removing the nozzle of the burner which constricts the flow. It's the second segment of the video.

https://youtu.be/d3o_elkOgUQ?si=eI93Ej3CIakDHqgj