Significance of acetogenic H2 consumption in dark fermentation and effectiveness of pH

Two identical thermophilic H(2) fermenters (R1 and R2) were operated at different pH levels between 4.7 and 5.7. In R1, several unexpected and severe drops in H(2) yield inversely proportional to increase in acetate production were experienced at pH 5.5 and 5.7. In contrast, R2 operated at pH 5 and 4.7 performed more stable H(2) production mainly through butyrate fermentation. Although the H(2) partial pressure (>50 kPa) was far above the favorable values, acetate was produced as well as butyrate in all pH levels tested. To determine whether some portion of the acetate is produced through another pathway such as autotrophic synthesis via H(2) dependent reduction of CO(2) or not, batch dissolved H(2) consumption rate tests were performed at pH 5.0, 5.5 and 6. The specific H(2) consumption rate was 488(+/-49) micromol/gVSS.hr at pH 6 and slightly higher than at pH 5 and 5.5. The results of continuous and batch experiments revealed that acetogenic H(2) consumption is more favorable at pH levels above 5.5 and is one of the reasons of instabilities in dark fermentative H(2) production.