Development Of An Acidogenesis And Homoacetogenesis Coupling System For Wastewater Anaerobic Fermentation And Its Conditions Optimization

Wastewater treatment is shifting from single pollution control to paying equal attention to both pollution control and waste resource utilization. Research about using wastewater to produce bioenergy and biochemical materials is increasingly attracting widespread attention. Volatile fatty acids can be produced by wastewater anaerobic fermentation and be supplied to produce highly valuable biochemical products by further fermentation, which provides a new way for achieving wastewater resource utilization. Acetate is produced by anaerobic fermentation and can be easily converted to methane, and carboxylate accumulation is reduced. This study treated the anaerobic activated sludge by heat to kill methanogen, and constructed a novel acidificationhomoacetogenesiscoupling system. In this case, hydrogen inhibition during wastewater acidification is relieved resulting from hydrogen consumption by homoacetogen, which enhances acetate production.Simulated wastewater containing 5 g/L glucose was acidified by heat-treated sludge and non-heat-treated sludge, respectively. It is indicated that acetate yield of the former is 3.1-fold higher than the latter at an initial pH 8.0. Then, the acidification-homoacetogenesis coupling system was used to produce acetate from simulated wastewater containing 30 g/L glucose and pretreated sludge liquor containing 34 g/L COD. Acetate yields in the coupling system are 52-87% and 29-39% higher than that in ordinary acidification systems, respectively. Enhancement of acetate production is attributed principally to homoacetogenesis, relief of the products (H₂ and CO₂) inhibition to acidification and syntrophic acetogenesis, enhancement of the conversion of substrate and the oxidation of ethanol.The single-factor experiments were conducted to study the effects of substrate concentrations, initial pH and inoculum concentrations on acetate production in the acidification and homoacetogenesis coupling system. The results show the variation of a condition in the acidification phase affects acetate production not only in the acidification phase itself but also in the homoacetogenesis phase. With the increase of substrate concentration, acetate concentrations usually increase. But at 192 h, the acetate concentration at 30 g/L of substrate is 27% higher than that at 40 g/L of substrate. Ethanol-type fermentation mainly takes place at initial pH 5 while butyrate-type fermentation at initial pH 6 and 7. But acetate is the dominant product at initial pH 8-11. The optimal initial pH is 10 for acetate production in the coupling system. Such an effect of initial pH and substrate concentration can be attributed principally to the difference of inhibition degree of non-dissociated acids on different microbes. At 2-8 g/L of inoculum sludge, the optimum sludge inoculum size in the syntrophic acetogenesis phase and the homoacetogenesis phase is both 4 g/L. At that sludge inoculum size, the maximum acetate production and yield can be achieved. The variation of acetate yield may be attributed to different metabolic pathways and the imbalance between hydrogen production and hydrogen consumption in the coupling system with different inoculum concentrations. Based on above single factor tests, uniformity design test was used to obtain the optimization conditions as follows: fermentation time 10 d, initial pH 11, substrate concentration 40 g/L and inoculum sludge 6 g/L. At these conditions, the maximum acetate production, high acetate yield and high acetate production rate could be achieved.The contribution of homoacetogenesis to acetate production should reach up to 33% by calculation according to acidification reaction metrology. But the acetate production in the homoacetogenesis phase accounted for only 5-9% in the coupling system. This may be resulted from the serious imbalance between hydrogen production and hydrogen consumption, and mass transfer obstruct between two phases. Gas circulation and fed-batch fermentation were applied for enhancing acetate production. The fed-batch method helps to balance hydrogen production in the acidification phase and hydrogen consumption in the homoacetogenesis phase of the coupling system, and helps to reduce the shock loading of organics at the beginning of the fermentation. Gas circulation enhances mass transfer from acidification phase to homoacetogenesis phase, hence resulting in increasing substrate concentrations in homoacetogenesis phase and decreasing the hydrogen partial pressure in the acidification phase. The results show that the acetate yield in the fed-batch test with gas circulation is about 44% higher than that in the batch test without gas circulation. Acetate can be directly from glucose conversion, H₂+CO₂ homoacetogenesis and ethanol oxidation. Acetate from glucose conversion accounted for 49-80%. When vacuum degree in the acidification phase was 28 kPa, H₂+CO₂ homoacetogenesis contributed more than 30% of acetate production for fed-batch tests, and ethanol oxidation contributed more than 40% of acetate production for batch tests.To investigate the operation stability of the acidification-homoacetogenesis coupling system and the microbial population dynamics under acidification condition, semi-continuous long term run was conducted and molecular ecology techniques were used to analyze acidification microbial population dynamics. The stable status of the semi-continuous run test can be attained after 100 d, at which organic acids and ethanol concentrations and pH are almost constant. When substrate loading rate is 4-6 g/(L·d) and HRT is 20 d, acetate yield in the coupling system is 0.4-0.48 g/g. When the loading rate increases from 4 g/(L·d) to 6 g/(L·d), Shannon-Weiner index decreases from 0.71 dit to 0.50 dit in the acidogenesis phase. When the loading rate is adjusted back to 4 g/(L·d), the Shannon-Weiner index then increases to 0.75 dit. There is a positive correlation between biology diversity and acetate yield. The Shannon-Weiner index in the homoacetogenesis phase is only 0.21 dit, which is much lower than acidogenesis phase. There are Clostridium and Fusobacterium in two phases of the coupling system all the time, and they represent a higher proportion. Through quantitative PCR, homoacetogens in samples of the acidification phase and homoacetogenesis phase are 6.3×10⁸ cells/mL and 8.9×10⁸ cells/mL, respectively. These are 2-3 orders of magnitude higher than the ordinary anaerobic digestor. In the coupling system, the activities of key enzymes are as follows: the activities of acetate kinase (AK) and phosphotransacetylase (PTA) in the acidification phase are 1.5-2.5 U/mg and 0.17-0.33 U/mg, respectively; the activities of AK and PTA in the homoacetogenesis phase are 1.0-1.7 U/mg and 0.11-0.22 U/mg, respectively. There is a positive correlation between key enzyme activity and acetate production rate.