Soil was used as seed without pretreatment for the fermentation of lactate in order to produce H 2. When the pH was kept at a constant level, H 2 production occurred in three steps. The first was the production of small amounts of H 2 during the initial growth of the microflora. Then, after a reasonably long period of inactivity during the cultivation stage, a large amount of H 2 was produced in additional two steps. It was found that the reduction of the pH after the initial growth phase had a positive effect on the H 2 yield. After cell growth at pH 6 and cultivation at pH 5, a total H 2 yield of 154 mmol l -1 was achieved. It was found that acetate was essential for the rapid H 2 formation. On the other hand, even small initial concentrations of butyrate impeded the H 2 formation.
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