I've worked with many digesters at OIT, ultimately being the one to clean up the mess...sealing was a major issue. There is a deep visceral feeling when in the middle of this type of project. A digester is an organism all to itself, and having a digester is very similar to raising a baby.
I prefer composting organic material in a hot pile, rows, or a Rocket and feeding good food scraps to animals. A digester is a very wet anaerobic compost pile.
To generate flammable gasses, I would consider syngas from gasification of woody material. Most notably from: All Power Labs
At OIT, we also had success fermenting food scraps and distilling the alcohol.
I was personally developing microbial fuel cells to inspire microorganisms to use oxygen as the terminal electron receptor. The input stream was grey water with a high BOD.
A visit to the Stahlbush farms in Oregon is a good example of a commercial scale digester, and the use of an ICE engine to provide heat and power to the farm. Most of their equipment was built off of European components and philosophies, because Europe is more developed in this discipline.
This Kickstarter is the best digester I've seen: HomeBiogas 2.0