In regard to gas buses in China:
http://www.paulnoll.com/China/Commerce/trans-gas-bus.html
this shows a natural gas bus with the bag on top. (not necessarily from human waste)
I can't find an online source about them making gas from waste, but from my own research I'll say that it's entirely possible, and more likely to be agricultural wastes than human waste.
http://www.informinc.org/chinacrisis.php
the "China's Methane Resources" heading about halfway down the page here talks about China inceasing dependence on natural gas, but not from methane digesters. The next heading down is also relevant, which says that china has 2500 of 500000 buses running on natural gas.
So we see that China is running buses on natural gas with bags of uncompressed gas on the roofs of the buses. Most of it is mined from what I read.
http://english.cri.cn/3130/2007/04/12/
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this short article is about methane production from agricultural waste in rural China for household use.
http://www.motherearthnews.com/Alternative-Energy/1975-05-01/Don-Judy-Mosers-Methane-Powered-Bus.aspx
this article is about a methane powered bus in the USA.
apparently they power a small fridge on methane generated from their own waste.
and I can't resist posting the wikipedia article on methane generation.
http://en.wikipedia.org/wiki/Biogas
and if anyone is still interested I'll add what else I know.
the CO2 in the biogas is typically "washed" from the gas by running the gas through water.
China is innovative in not compressing the gas. Other countries, France for instance, compress it.
And that fridge that runs on a small amount of methane from a family's own waste? well, guess what. Fridges used to run on gas or kerosone. They can essentially run just on heat energy. So if anyone can find some heat energy falling from the sky and focus it on the right part of the fridge, they can run a fridge on the sun while the sun shines.
I'm working on doing this myself, looking at ways of focusing the sun with aluminium reflectors made from drink cans.
This focused heat energy can also power a stirling engine, which is a closed cycle external combustion engine that provides motion energy. Look it up on wikipedia. A lot of little-explored possibilities here.