Thanks for the info, planet2 and Vidya, could be useful. I'm aware of the problem mentioned about the flooding in the spring, because I noticed that went the soil gets saturated, the water in the ground can back up and flood the septic tank. Really the septic system is obsolete because quite often, it pollutes the ground.
Vidya, the simple system you mention works, but the people are still use to the convenience of indoor plumbing, so I'm trying to come up with a solution for that if possible.
The link to magnegas also triggers me to lok into the manure biogas processor, which would also provide methane for cooking. This may turn out to be the best solution, because it still provides the convenience of indoor plumbing, and there is gas for cooking and compost also for the vegetables after due process.
Some links for people interested:
An overview report:
http://74.125.95.132/search?q=cache:...&client=safari
Info to build (it's also a lot cheaper than the commercial ones):
http://www.ruralcostarica.com/biogas.html
http://journeytoforever.org/biofuel_...sters/MD3.html