Mechanism Of Acetate Production By Homoacetogens In Anaerobic Digester

This study investigated the contribution of homoacetogenic bacteria during the anaerobic fermentation and proposed the methods for improving the acetate percentage from the organic matters by anaerobic fermentation. By the influence of CO₂ concentration on the acetate yield, a strategy of enhancing acetate production was developed. A novel method for the homoacetogens enrichment was developed based on the formate as carbon source after the study of the enrichment methods. Comparing the production of volatile fatty acids before and after the inoculum of homoacetogen enriched sludge, the idea that homoacetogen is favorable of the acetate production was demonstrated. The detailed contents were as follows:1. The syntrophic acetogenic bacteria and the homoacetogenic bacteria are belonged to syntrophic relationship in the anaerobic fermentation system. The syntrophic acetogen provides substrate for the homoacetogens and homoacetogen decreased hydrogen partial pressure for the syntrophic acetogens. Under the conditions of high hydrogen partial pressure, the syntrophic acetogenesis can not be occurred while under the condition of absent of CO₂, the homoacetogenesis was not happened. The results showed that change of the composition of the headspace gas can control the acetogenesis process. When the fermentation was run at 1g/L glucose as substrate, 1 1g VS/L seed sludge, pH at 7.0 and the mesophilic conditions, the contribution of the acetogenesis was 14.1% for the total acetate production and 6.36% 2. The effects of CO₂ concentrations in headspace of anaerobic fermentation system on the volatile fatty acids generation, substrate degradation and homoacetogen change were investigated. The results showed that low concentration of headspace CO₂ was helpful for the substrate degradation. The substrate degradation efficiency of low CO₂, high CO₂ and control are over 93% at 16h, 87.6% and 88.3% at 20h, respectively. The high CO₂ concentration improved the acetate accumulation with the conversion yield at 8.2 mmol/g COD at the end of fermentation, which was 1.52 and 1.87 folds of the control and low CO₂ reactors, respectively. There is a positive relationship between CO₂ concentration and the quantity of homoacetogen. The fhs gene copy numbers are 9.83×106/mL, 5.32×108/mL, 6.97×107/mL in the low CO₂, high CO₂ and the control reactors, respectively. The homoacetogen can be enriched from a mixed culture by a high CO₂ concentration in the headspace.3. The method of homoacetogenic bacteria enrichment was studied in mixed strains. The results showed that the homoacetogenic bacteria could be enriched when the carbon source was sodium formate. When the concentration of sodium formate were 3.25g/L, 6.5g/L and 13g/L, the homoacetogenic bacteria were 2.93×109 /mL, 6.09×109 /mL and 6.08×109 /mL, respectively, which were 5.68%, 12.01% and 11.68% of the total bacteria. The growth rate of homoacetogenic bacteria and utilization of sodium formate were the fastest during the enrichment period when the sodium formate was 3.25g/L.4. The effect of inoculum with homoacetogenic enriched sludge on the acetate yield was investigated. The results showed that the yield of acetate in the reactors which homoacetogen enriched sludge was 1.38 folds higher than the reactors without homoacetogen enriched sludge. The yield of acetate which filled with CO₂ in the reactors with homoacetogen enriched sludge was 1.77 folds higher than the CO₂ absent group. Based on the theoretic calculation, the inoculum with homoacetogen in coupled with CO₂ filling is a promising and feasible strategy for the pollution control.