Thanks for the conversation. I wanted to clarify some aspects of biogas and biogas digesters. First, they don't need to be made of concrete. They can be made from salvaged IBC tanks, 55 gallon plastic barrels, and there is a model that is made by Puxin that has several models of mobile biogas digesters. http://puxinbiogas.en.alibaba.com/product/702824597-220434916/Mobile_biogas_digester.html
(Puxin has made many of the problems of previous digestes moot.)
Actually I think this is where biogas shines and it seems that many of you DIY folks would be well served to explore this more for the betterment of all of us! Second clarification is that different digesters have different flowthrough and therefore different slurry composition depending on the time it has "composted" in the digester. There are many ways and strategies to make sure that new waste that comes in spends the maximum amount of time in the digester therefore insuring that the resulting slurry is as pathogen free as possible. To make sure that this happens, most people/institutions collect slurry and store it in a holding tank as TIME is one of the factors in slurry decontamination. Another one is HEAT and we know how to build sun ovens and solar hot water heaters and so applied to this technology we have yet another way of decontamination. Possibly the slurry could be dried thereby further reducing the space it takes up while still remaining a good fertilizer.
Perhaps one of the wrenches in the biogas plan is the amount of water that is used. I am not familiar with all mobile systems, but our model uses water to pressurize the gas. I know some people collect the gas in bags and weight them with a stone to pressurize. Digesters would need to be next to showers/kitchens, but this is assuming a lot about water useage. I'm sure that from handwashing alone enough water could be recovered for use in the system.
I think the solution could be a BOTH/AND scenario regarding biogas vs. compost toilets. I wanted to bring biogas to the table and expand upon the conversation here because it seems like most did not know that you can build biogas systems from salvaged materials and absolutely you don't need concrete.