Anaerobic digestion process

Claim 1

A process for treating a liquid slurry of organic material to produce a gaseous product comprising a major amount of methane gas and a minor amount of carbon dioxide gas, said process involving anaerobic digestion in a stage 1 digestor and a stage 2 digestor and maintaining said liquid slurry in a well mixed condition under temperature conditions effective in producing said gaseous product, said liquid slurry being maintained in each digester with an air space above defining an interface with said liquid slurry being maintained at a volume-to-interface ratio of at least 100 gal. per ft. 2 , and a total combined digestor liquid retention time of at least 24 hours whereby the faster molecular diffusion rate of methane gas as compared with carbon dioxide gas enables the passage through said interface of substantial amounts of the methane gas evolving from said slurry while minimizing the passage of carbon dioxide gas through said interface to achieve a gaseous product having a major amount of methane gas and a minor amount of carbon dioxide gas and a spent sludge containing said carbon dioxide, said anaerobic digestion taking place initially by introducing said liquid slurry into a stage 1 digester for a first holding time effective in producing said gaseous product, removing said gaseous product from said stage 1 digester, with the resulting sludge from the stage 1 digester being transferred to a stage 2 digester wherein said anaerobic digestion continues for a second holding time effective in producing said gaseous product, removing said gaseous product from the stage 2 digestor and transferring the spent sludge from the stage 2 digester to a stripping zone wherein the pH of the sludge is adjusted to from 6.0 to 6.5 by the addition of acid, stripping said carbon dioxide from said spend sludge, and recycling said spent sludge from said stripping zone to said stage 1 digester.