Will anything but a conventional toilet work in this scenario?

I am fitting out my unused garage as a living space. I intend to rent it out initially as a vacation/short-term rental and use the funds to transform my 0.6-acre, heavily treed/shaded property into a permaculture forest as much as possible. In about 10 years, when I'm an empty-nester, I would move into the remodeled garage and turn my current house into the rental. As such, I am designing one space for two different objectives: for renters now, and for me later. To align these objectives somewhat, I am planning to market the rental towards people with an interest in experiencing alternative living solutions.

Toilet options I know of:

1- Install a water-using toilet.
Pros: no maintenance, comfort/ease for occupants.
Cons: Cost - Zero attention to earth care - a standard toilet will consume water and create waste; my house is currently connected to a septic system which is now over 70 years old but still seems to be going strong; if I add one more toilet, I will be required to connect to the city sewer. I haven't priced it out yet, but my guess is that it will cost me $20-30k to connect the new toilet to the sewer system.

2- Install a composting toilet. In spite of what the city staff first told me, upon pushing back, it appears that I legally AM allowed to install a composting toilet, either within the space or separately outdoors. (The code does state that I must have a greywater connection. I am still trying to figure out why this is; any system I have looked at does not appear to create greywater runoff.)
Pros: Earth care - no water use, beneficial use of human byproduct.
Cons: Not very "exciting" or inspiring - feels like RV life more than innovative green life - so it may deter more renters than it attracts, undermining my imperative to earn money to fund the permaculture project; the compost toilet chambers would need to be changed after each rental (no one is going to want to vacation in a house with someone else's pee and poo in the chambers) - I am not exactly desiring to deal with other people's poo and pee on a regular basis.

3- Install a HomeBiogas toilet.
Pros: A somewhat normal-feeling toilet experience, since pee/poo is channeled out of the building; feels super innovative and exciting; provides free cooking fuel (SO cool); transforms human byproduct into fertilizer/compost aid.
Cons: I'm not clear yet on how the system works and don't know if it's fitting in a situation where there may be days or even weeks without the system being "fed," so I don't really know if it would work will in an AirBnB setting; the outdoor component of the system is pretty large and has several location restrictions: must be within 10 feet of kitchen for gas line to stove, must be on level ground, must be in sun, must be far enough from windows that smell will not be an issue. I simply don't have a location that fits those bills,   As much as I wish this could be my solution, I don't know if it's possible - I have thought about housing it in an insulated box, lined with ecofoil, with a side that tilts out in the morning and reflects sun off the ecofoil into the box, but there is really nowhere within 10 feet of the kitchen that gets enough sun for even that solution.

I'm really hoping there are some solutions out there I haven't considered. Thanks for your thoughts. I am happy to post more details about my property/structure/layout and respond to any questions.

Elyce

Grey water is showers, hand basins and kitchen sink and laundry.
There are composting toilets that actually dont just fill a bucket, there are other toilets that burnt the waste  and leave you ash.
Look ar what is in Swenden.

Would upgrading/replacing your current septic system be an option rather than connecting to the mains sewers? If water use is not a problem (there are ways of using grey water/roof runoff that reduce the need for drinking water quality water to be used as flush) then that could be a possible alternative.
We looked at a flush toilet combined with a solid separator: the Aquatron is a non powered centrifugal which uses the action of the water swirl to separate most of the solids out into a composting chamber, the water can be treated as grey water then (or you could use your existing system perhaps). Here's a link for more information. You do end up with humanure to process however, but not on a weekly basis! The main advantage is that the front end is unchanged. A modern low water usage WC is all the occupants will see. The main disadvantage is that you do need quite a height drop for the chamber, which can be tricky unless you have a sloping site. When we finish our house extension and new bathroom (not started yet), this is still an option I quite fancy.

Aquatron installation

We rented our house out once that had a commercial composting toilet in it. It freaked people out which severely reduced the number of potential renters.

We have some vacation cabins, so I want to say that folks just can't read instructions.

Use something that does not need instructions and works like what the folks would have in their homes.

Great idea, by the way.

We have some vacation cabins, so I want to say that folks just can't read instructions.

I have heard this observation world wide, I have trouble comprehending why.
No wonder ladders are covered with safety warnings, so the manufacturer can honestly say"we gave you 9 sets of instructions...."

John said, "I have heard this observation world wide, I have trouble comprehending why.

I would give some examples though I don't want to get off-topic.

Maybe these folks are there to have fun and don't want to both with the little things ...

Also, I find it "funny" what is found in toilets when they have to be plunged ... bars of soap, a bottle of shampoo, and who know what else.

Nancy Reading wrote:Would upgrading/replacing your current septic system be an option rather than connecting to the mains sewers? If water use is not a problem (there are ways of using grey water/roof runoff that reduce the need for drinking water quality water to be used as flush) then that could be a possible alternative.

In my locale, the governing bodies are very eager to decommission septic systems. Adding a toilet triggers a county survey, which in turn triggers the city to require connection to the sewer. I will look more closely at the AquaTron, thanks for suggesting it. I am confused by their website if it generates black water and/or sludge.

I think there is minimal sludge in the Aquatron liquid discharge, but some post treatment would obviously be required - there is still urine in this water potentially. There are some lab test results on the website, but I guess it would depend on your grey water system and local regulations as to what is required. They do offer a UV lamp option if lower pathogen level are required.

Elyce,

The website vermifilter.com shows several DIY flush compost toilet designs that take advantage of some of the latest technical papers in the field of "vermifiltration".  The DIY design at vermicompostingtoilets.net was based on the work of Anna Edey, whose design and testing results are documented in her Green Light at the End of the Tunnel book.  Hawaii is also currently looking into vermifilter designs as a drop in replacement for household systems (see the end of this presentation).

It's also probably worth mentioning that vermifilters can treat the output of a septic tank and can be made to treat to a level that is safe for surface discharge.  See, for example, the Wikipedia article on Vermifilter.

I would add to Nancy's comment that Aquatrons can get clogged (e.g. with toilet paper), and the fairly dry solids pile means you periodically have to manually knock the pile over.  I agree though, its a pretty clever device.