We have been investigating the kazubaloo system at our site. It is not cheap, but is a good clean system that doesn't require someone to empty it. You can google this solution and an excellent briefing document is available from Barnet allotments. I also found another allotment site who had installed a enviroloo - the photos are in their gallery:
http://www.slugallotments.co.uk/gallery.aspIf you look on youtube, there are various ideas for alternative solutions.
One idea which I believe is what surbie's site uses is to utilise wheelie bins so once full, you can wheel them into a sheltered corner out of the way to compost down before emptying it out on the hedges around the edges of the site. This solution is also in use elsewhere in our area at allotment sites. At our site, there is no one who really wants to have to do this :happy:
I went to a local woods where they had a straw based solution. There was an elevated toilet kiosk, under which there were straw bales piled up (I assume with a gap in the centre). You then add sawdust on top after you have used it. I assume they wheel the toilet kiosk elsewhere to start it again, allowing the original composting heap to rot down. There is a lot of info online if you google straw bale composting toilet.
If you look at my diary, I show details of the kazubaloo we are planning to install.