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Low aliphatic alcohols or organic solvents, especially ethanol are continuously produced by fermentation from sugar containing nutrient substrates. The process includes two fermentation steps or stages in which the substrate is subjected to the effects of microorganisms such as yeast. The first ferm

Low aliphatic alcohols or organic solvents, especially ethanol are continuously produced by fermentation from sugar containing nutrient substrates. The process includes two fermentation steps or stages in which the substrate is subjected to the effects of microorganisms such as yeast. The first fermentation stage has a volume of 10 to 20% of that of the second fermentation stage. By adjusting the environmental conditions in each stage a large cell growth with a small portion of the total supplied substrate quantities takes place in the first activation stage and a high product formation rate is achieved with a considerably larger portion of the entire supplied substrate quantity in the second production stage. A partial outflow stream from the first stage is microfiltered and the permeate as well as the unfiltered outlet flow of the first stage, is directed into the second stage. The outlet flow of the second stage is microfiltered and the concentrate is partly discharged as a product output stream and partly returned to the second stage. The transfer flow and return flow streams and the discharge of the permeate and the product streams from the second stage are adjusted so that, for dwell times of the cell mass of up to two hours in the first stage and up to 15 hours in the second stage, an average cell age of 40 to 100 hours is achieved with a corresponding high yield output.

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A process for the continuous production in a two stage fermenter system of low aliphatic alcohols or organic solvent products, from sugar containing substances providing a raw material substrate quantity, comprising the following steps, (a) supplying about 5 to 20% of said substrate quantity into a

A process for the continuous production in a two stage fermenter system of low aliphatic alcohols or organic solvent products, from sugar containing substances providing a raw material substrate quantity, comprising the following steps, (a) supplying about 5 to 20% of said substrate quantity into a first activating fermentation stage and supplying about 95 to 80% of said substrate quantity into a second production fermentation stage, (b) introducing an oxygen suppyling gas into both stages in quantities sufficient to adjust an oxygen partial pressure in said first stage at approximately 3% and an oxygen partial pressure in said second stage at approximately 1%, (c) maintaining a temperature within the range of about 30°C. to 35°C. in said first stage and a temperature of about 35°C. to 40°C. in said second stage, (d) passing a portion of an output flow from said first stage through first microfilter means (3) to produce a first concentrate and a first permeate and feeding said first permeate and the remainder of said output flow into said second stage, (e) passing an output flow from said second stage through second microfilter means (4) to produce a second concentrate and a second permeate, back-feeding a portion of said second concentrate partially into said second stage (at 12′) while removing another portion of said second concentrate (at 16) from said system as a product stream, and removing said second permeate from said system (at 14), (f) adjusting said feeding of said first permeate and of said remainder from said first stage into said second stage, said back-feeding of said second concentrate, and said removing of said other portion of said second concentrate and of said second permeate in such a manner, that the following conditions are satisfied: cell mass residence times in said first stage are within the range of about 0.5 to about 2.0 hours, cell mass residence times in said second stage are within the range of about 5 to about 15 hours, and an average cell age is within the range of about 40 to about 100 hours, and (g) using for said first stage a fermenter having a given volume and using for said second stage a fermenter having a volume about five times to about ten times said given volume of said first stage fermenter, whereby said first stage is operated as a non-sterile activating stage for substantially promoting cell growth and activation while said second stage is operated as a non-sterile producing stage.