(US5874263-A)      Organic acids and their salt (A) are produced by anaerobic digestion of biomass. The biomass is continuously flowed from a region (1) of fresh biomass to a separate region (2) of digested biomass, and concentration of (A) in the two regions is controlled to maintain digestion. Concentration of (A) is less in region (2) than in region (1). Also claimed is apparatus for obtaining (A) from biomass, comprising 2 regions in series with means of transporting biomass between the regions, of supplying biomass to the regions, of removing biomass from the regions and of washing (A) from biomass.    (A) is concentrated in region (1) by flowing an aqueous stream (S) through region (2) to wash (A) from it into (1). (S) flows in (1) in countercurrent to the biomass, or biomass and (S) pass countercurrent in an extraction apparatus comprising a hollow column containing baffles to prevent channelling of liquid through the biomass. Some of the biomass from region (1) may be recycled to the same region and another part passed to region (2). Total residence time in the regions is 20-60 days. The two regions comprise fermentation reactors. A neutralising agent, e.g. a carbonate, bicarbonate or hydroxide, especially calcium carbonate, may be added to (1) or (2) to maintain pH above 4.8. A methanogen inhibitor, e.g. 2-bromoethanesulphonic acid, may be added.    USE -   (A) comprises acetic, butyric and propionic acid and/or their salts, especially calcium salts. The biomass is e.g. municipal solid waste or sewage sludge.    ADVANTAGE -   High biomass digestion and high product concentrations are obtained, with reduced product recovery cost.    (EP918876-A2)      Organic acids and their salt (A) are produced by anaerobic digestion of biomass. The biomass is continuously flowed from a region (1) of fresh biomass to a separate region (2) of digested biomass, and concentration of (A) in the two regions is controlled to maintain digestion. Concentration of (A) is less in region (2) than in region (1). Also claimed is apparatus for obtaining (A) from biomass, comprising 2 regions in series with means of transporting biomass between the regions, of supplying biomass to the regions, of removing biomass from the regions and of washing (A) from biomass.    (A) is concentrated in region (1) by flowing an aqueous stream (S) through region (2) to wash (A) from it into (1). (S) flows in (1) in countercurrent to the biomass, or biomass and (S) pass countercurrent in an extraction apparatus comprising a hollow column containing baffles to prevent channelling of liquid through the biomass. Some of the biomass from region (1) may be recycled to the same region and another part passed to region (2). Total residence time in the regions is 20-60 days. The two regions comprise fermentation reactors. A neutralising agent, e.g. a carbonate, bicarbonate or hydroxide, especially calcium carbonate, may be added to (1) or (2) to maintain pH above 4.8. A methanogen inhibitor, e.g. 2-bromoethanesulphonic acid, may be added.    USE -   (A) comprises acetic, butyric and propionic acid and/or their salts, especially calcium salts. The biomass is e.g. municipal solid waste or sewage sludge.    ADVANTAGE -   High biomass digestion and high product concentrations are obtained, with reduced product recovery cost.    (US5962307-A)      Organic acids and their salt (A) are produced by anaerobic digestion of biomass. The biomass is continuously flowed from a region (1) of fresh biomass to a separate region (2) of digested biomass, and concentration of (A) in the two regions is controlled to maintain digestion. Concentration of (A) is less in region (2) than in region (1). Also claimed is apparatus for obtaining (A) from biomass, comprising 2 regions in series with means of transporting biomass between the regions, of supplying biomass to the regions, of removing biomass from the regions and of washing (A) from biomass.    (A) is concentrated in region (1) by flowing an aqueous stream (S) through region (2) to wash (A) from it into (1). (S) flows in (1) in countercurrent to the biomass, or biomass and (S) pass countercurrent in an extraction apparatus comprising a hollow column containing baffles to prevent channelling of liquid through the biomass. Some of the biomass from region (1) may be recycled to the same region and another part passed to region (2). Total residence time in the regions is 20-60 days. The two regions comprise fermentation reactors. A neutralising agent, e.g. a carbonate, bicarbonate or hydroxide, especially calcium carbonate, may be added to (1) or (2) to maintain pH above 4.8. A methanogen inhibitor, e.g. 2-bromoethanesulphonic acid, may be added.    USE -   (A) comprises acetic, butyric and propionic acid and/or their salts, especially calcium salts. The biomass is e.g. municipal solid waste or sewage sludge.    ADVANTAGE -   High biomass digestion and high product concentrations are obtained, with reduced product recovery cost.