A continuous process and apparatus for preparing organic acids and their salts from biomass. Biomass flows from a region of fresh biomass and high acid and acid salt concentration to a region of digested biomass and low acid and acid salt concentration in a fermentation apparatus under anaerobic conditions. Organic acids and acid salts produced by the process of the present invention are volatile fatty acids such as acetic, butyric and propionic acids and their salts such as calcium acetate, calcium propionate, and calcium butyrate. The apparatus of the present invention contains at least two fermentation reactors in series to increase the biomass residence time in the reactors. The fermentation reactors of the present invention are imbedded in the ground with earthen berms as support for sides having three layers. The core of the reactors are covered with a flexible covering to maintain an anaerobic environment.
