I have seen some continus digesters used on dairy farms for over the last 25 years. The most common designs were a sloped concrete trench, covered with a heavy plastic sheet, sealed with dirt piled on the lip of the plastic. You add your new "feedstuff" on the top of the slope and as you push it under the lip of the digester, digested material appeared out the far end flap. You knew you had gas when the plastic bubbled up. The gas was removed through a iron pipe with a simple filter going through the side of concrete wall, then angled up into the bubble area(saved from cutting the plastic).
I remember that they had a rule of pumping out all the gas before the daily reload. Then reloading the unit. Careful about making sure the plastic was sealed again, then they pumped out the new air. They had trouble with too much air in with their fuel. They used old propane tanks and pumps to store the fuel.
Then they ran the fuel through a modified lpg Catipillar engine/generator and sold the electricity back to the utility. The trouble it took 100 cows' material to feed the methane generator. But, it paid for itself in 6 years.
You will be surprised by how much methane you can make in a day, and don't be surprised that the digester can heat up to 150-170f. The key is to have a good steady supply of new "feedstuff".