occasional-use compost toilet - help!

The house we bought a year ago has mains water and a septic tank system, and all mod cons inside, which we've been using more or less happily, and ocassionally thinking about how wasteful it all is. We do conserve water as much as possible, but frankly we live in a very wet area. Well, too wet as it turns out! We've been having bad rainstorms, and the septic system is backing up. Not into the house, yet. But the manhole covers near the back door are at the brim, the problem appears to be that the septic tank simply can't drain the liquid away as the ground is so sodden, or even worse-case scenario, the ground water might even be backing up into the tank. So since we discovered this today, we haven't been able to flush or put anything down the drain. Definitely think we need to work out a compost toilet back-up! (We are trying to figure out if there is anything that can be done to fix the septic, aside from wait until the ground dries out, errr, never, winter in Wales!) We are hoping our current system is fixable and that it will continue to be our primary system, but this experience definitely tells me we need a Plan B, which would be for occassional use, although perhaps in future we could transition more use away from the septic.
Anyway. There is SO MUCH info out there about different systems. The only one I have personal experience of building and maintaining is at my folks' cabin in the woods - that was weekend use, and it was a very shallow pit, surrounded by stones, with an open hut on top, every few years we just shoveled it out and put the compost in the forest. At the other end of the spectrum are the complicated urine-diverter, dual-chamber, super-engineered toilet castles. I think I need something in between.
For ocassional use, would it be ok to just dig a pit (how deep) as long as I know it's above the water table (we have a high water table on part of the property but we're on a hill, so uphill a bit should be ok I think?) and put a little shed on that?

Would anyone like to share some ideas with me for a simple system? This would probably all be against building regs - I know composting toilets are allowed but I believe they need to be of the very complicated variety which I'm not into right now. Just hoping to fly under the radar on this one.

Sawdust bucket, in your normal bathroom when you need it. Easy enough for a couple. You may need a bit more if you have a lot of guests.

Thanks, that would certainly be an easy solution! So then what would you suggest doing with the contents of the bucket? Burying it in a hot compost heap? (having issues getting my compost to heat up consistently), burying it in the ground? Urine I'm not as worried about to be honest, obviously urine+sawdust could go right on the compost, and we usually have a pee bucket outside the backdoor anyway that gets diluted and poured on the garden. But what about feces? Seems like this is the tricky one to dispose of in the absence of a fully thought-out composting toilet set up.

We are thinking about a treebog, having seen plenty of these in action, and I think we have a place that could be ok for it, not too far from the house, on a hill so not right on the water table. For occassional use I guess it doesn't matter too much anyway, and then if it seems like it's working pretty well we could try to use it as our main one and just use the indoor loo for really bad weather...

So we still can't use our drainage system Now we think there is a blockage in the system, but my husband spent almost all day rodding the drain and it hasn't solved the problem so it seems like we'll have to get a pro In the meantime we have been peeing in a bucket outside during the day and peeing in a bucket of wood shavings at night. That is all working fine, I've just put the woodshavings on the compost heap. We are lucky that the cottage next door is unoccupied and belongs to my parents in law and its system if fine so we have been going over there to poop, but that is quite a pain especially with two small kids. I dont want to poop in the bucket. I wouldnt mind emptying it too much, well at least not on a short-term basis, but I don't know what would be a good solution - can't put that on the normal compost heap obviously. What about burying it in a hole? Possible though a lot of work. Would it be safe?

I also think a sawdust bucket would work for you, even for poop. It would be ready to use today, unlike an outhouse that you would have to build. And you can quit using it as soon as your septic system gets back in working order, if you like. Your existing compost pile is very likely able to handle the poop and sawdust buckets, if you cover them with some cover material.

The bible of the sawdust bucket toilet is:
http://humanurehandbook.com/

You should buy the ebook for $10. Well worth it! One of my favourite books, a real cult classic.

Yeah we are using a sawdust (well, wood shavings) bucket at the moment, pee only though. I will get the book, been meaning to read it for ages anyway though it wasn't urgent before! From discussions of it that I have read though, I've gotten the impression that it's vitally important to compost the poop at hot compost temperatures in order to be safe. My compost heap isn't that big and it doesn't really get that hot, so I'd be concerned about that.

I'm staying at my inlaw's place in South Wales at the moment. They have very similar problems with their septic system.

I set up a simple composting toilet in their old privy building- just a box with a toilet seat and a bucket of sawdust beneath. When the house is crowded or the water table is high it is very useful.

All deposits get put straight into the ordinary garden/kitchen compost heap. Poo in a heap of sawdust breaks down really fast and doesn't smell. We just try to pull other material over the top which helps it heat up and keeps critters off. Straw is good, or garden hedge trimmings.

Have a read of the Humanure Handbook.

We get our sawdust free by the cubic meter bag from a local chestnut coppice worker.

Thanks Michael. I so dearly wish that we had a collection of old outbuildings, an old privy would be such a boon! I will get a copy of the book for sure, and hopefully be able to figure out plans for building an outdoor loo in the next few months...

You should be able to get the Humanure handbook as a free PDF.

We use composting loos regularly, including for hosting large events (200+ guests for an opera in the garden last summer). You get great compost, just avoid pine shavings as they are slower to break down and less absorbent.

The humanure is an element that heats the compost up, so you don't need to already have hot compost. Just chuck it on and cover well