Production of high methane content product by two phase anaerobic digestion

DETAILED DESCRIPTION I claim: 1.
In a process for production of high methane content product gas by two phase anaerobic digestion of organic carbonaceous materials wherein a first acid producing phase converts said organic carbonaceous materials to form a liquid composition of predominantly low molecular weight acids used as an influent substrate to a second methane producing phase which converts said influent substrate to form said product gas comprising predominantly methane, the improvement comprising: maintaining said first acid producing phase at substantially atmospheric pressure and desorbing substantially all free carbon dioxide as gas from said liquid composition in said first acid producing phase, thereby substantially reducing the carbon dioxide content of said influent substrate to said second methane producing phase; and pressurizing said second methane producing phase.
2.
The process of claim 1 wherein said carbon dioxide content of said influent substrate is further reduced by purging with air prior to introduction of said influent substrate to said second methane producing phase.
3.
The process of claim 2 wherein said carbon dioxide content of said influent substrate is reduced to less than about 0.
5 gram per liter.
4.
The process of claim 1 wherein said methane producing phase is conducted under a product gas pressure of about 15 to about 75 psig.
5.
The process of claim 4 wherein said organic carbonaceous materials are at a concentration below about 20 grams per liter total organic carbon in said first acid producing phase.
6.
The process of claim 4 wherein said influent substrate is retained in said methane producing phase for a hydraulic retention time of less than about 5 days; said influent substrate in said methane producing phase has a pH of about 7.
5 to about 8.
0; and said product gas comprises more than about 90 volume percent methane.
7.
The process of claim 6 wherein said pH of about 7.
5 to 8.
0 is maintained under generally steady state conditions by biological conversion of said low molecular weight acids in said influent substrate of said methane producing phase to carbon dioxide and methane