| Hydrogen is the fuel of the future mainly due to its high conversion efficiency, recyclability and nonpolluting nature. Biological hydrogen production processes are found to be more environment friendly and less energy intensive as compared to traditional hydrogen production processes. It can be divided into two main categories, photosynthetic and fermentation process. The fermentative bacteria ferment organic wastes to H2 and organic acids, but cannot utilize the organic acids as electron donors. Accumulation of these organic acids restrained hydrogen production and substrate adequate utilization. However, the photosynthetic bacteria can use these small-molecular organic acids as electron donors for the hydrogen production at the expense of light energy. Therefore, a more promising method of hydrogen production was combining the fermentative and phototrophic hydrogen production. And phototrophic hydrogen production from volatile fatty acids (VFAs) using phototrophic bacteria was the key approach of the hybrid system.In this paper, hydrogen production by photosynthetic bacteria (PSB) Rh.Palustris Z02 from individual VFAs, i.e. acetate, propionate, butyrate, lactic acid, which were the main effluent from the fermentative H2 production reactor, was investigated. Bacteria is a key factor of biohydrogen production process, so the growth feature of PSB Rh.Palustris Z02 using VFAs were investigated. The modified Gompertz model was determined to describe the growth kinetics of Rh.Palustris Z02 by models comparison. And the growth factor was investigated also. The result showed that pH7.0, temperature 30℃and light intensity 2300 lux was the most suitable condition for the growth of Rh.Palustris Z02.Reaction kinetics of microorganism is research on relationship of all kinds of environmental factors and microorganism metabolism. Reaction parameters was vital for hydrogen production. In this paper, several reaction kinetic models were used to describe the growth of hydrogen producing microorganisms, consumption of VFAs, formation of product ,pH change and VFAs degradation in this work. The experimental data were subjected to numerical simulation to estimate the unknown kinetic constants for the substrate utilization, microbial growth and product formation in this hydrogen-producing process. Based on monad equation, Andrew substrate inhibitory model was used as the reaction kinetics equation of VFAs inhibition because higher concentration of VFAs was detrimental to hydrogen production. The results showed that the experimental data could be described by the proposed kinetic models with good agreements.The effects of pH, temperature and light intensity on the maximum hydrogen yield (mol/mol) were evaluated using a response-surface methodology (RSM). Experimental results showed that pH, temperature and light intensity all had an influence on hydrogen production. The maximum hydrogen yield of 1.90 mol H2/mol acetate, 2.38 mol H2/mol propionate, 3.02 mol H2/mol butyrate, 1.75 mol H2/mol lactic acid was estimated under the optimum conditions of pH 7, temperature 30℃and light intensity 6700 lux. The effect of pH and temperature were significant, but light intensity was insignificant with pH and temperature by ANOVA analysis. Since the absence of carbonate limits VFAs utilization and hydrogen production, the effect of various NaHCO3 concentrations on hydrogen production from VFAs were evaluated. The results obtained in this research showed that the phototrophic hydrogen production from propionate and butyrate required carbonate as an electron acceptor. Furthermore, two main inhibitory factor of phototrophic hydrogen production (oxygen and NH4+-N2) were investigated. the results showed that the inhibitory effect of NH4+ for phototrophic hydrogen production from VFAs was obvious. A large amount of O2 in gas phase was toxic for phototrophic hydrogen production. However, a small quantity of O2 can promote hydrogen production. When Argon gas and a small quantity of O2 mixed together, the highest hydrogen yield was achieved.Effects of individual initial VFAs concentration (acetate and propionate ranging from 0.015 mol/l to 0.090 mol/l, and butyrate ranging from 0.005 mol/l to 0.090 mol/l) on phototrophic hydrogen production were evaluated by using PSB Rh.Palustris Z02. Experimental results indicated that VFAs concentration had a substantial effect on phototrophic hydrogen production. Hydrogen yield, cumulative hydrogen production volume and hydrogen conversion efficiency were evaluated at various initial VFAs concentration. The quadratic model was calculated for hydrogen yield and hydrogen conversion efficiency when initial VFAs concentration was variable. A modified cubic model was calculated for accumulative hydrogen production. The results demonstrated that H2 conversion efficiency as a good indicator is more effective than the other two indices, hydrogen yield and cumulative hydrogen production volume. The observation demonstrated that higher VFAs concentration was detrimental to phototrophic hydrogen production. There was an optimal VFAs concentration within trial stretch for three VFAs individually. The optimal VFAs concentration of H2 conversion efficiency followed the order of acetate > propionate > butyrate. Furthermore, hydrogen production from a mixture of the butyrate and acetate, which were the main effluent from the fermentative H2 production reactor, was investigated. The ratio of acetate to butyrate (HAc/HBu ratio) had a substantial effect on phototrophic hydrogen production from mixture VFAs. HAc/HBu ratio on the maximum hydrogen conversion efficiency was evaluated using RSM. Experimental results showed that acetate concentration and butyrate concentration had an influence on hydrogen conversion efficiency. The effect of acetate concentration and butyrate concentration on hydrogen conversion efficiency were significant. A maximum hydrogen conversion efficiency of 43.88% was estimated at HAc/HBu ratio of 3.792, when acetate concentration was 0.0383 mol/L.In order to enhance hydrogen yield from VFAs, phototrophic hydrogen production from VFAs by immobilized Rh.Palustris Z02 were investigated. The results indicated that hydrogen yield of immobilized cells is higher than that of free cells, and sodium alginate is a suitable immobilized carrier for hydrogen production. Furthermore, pH change, hydrogen conversion efficiency and the effect of initial concentration of VFAs on hydrogen production were analyzed. The results demonstrated that higher concentration of VFAs was still detrimental to hydrogen production. However, the optimal VFAs concentration of immobilized cells was higher than that of free cells, the optimal initial concentration decreasing with carbon atomicity of VFAs increasing also, and following the order of acetate(0.043 mol/l)>propionate(0.029 mol/l)> butyrate(0.022 mol/l). The highest hydrogen conversion efficiency of 65.3% was achieved from acetate by immobilized cells. For acetate, propionate and butyrate, hydrogen content of biogas is increasing with carbon atomicity of VFAs increasing. But, initial VFAs concentration and immobilization of cells has no effect on hydrogen content.Finally, the mechanism and the feasibility of the hybrid biohydrogen production system of using fermentative and photosynthetic bacteria was analyzed and verified individually. Three hybrid approaches were tested, two-stage tandem connection biohydrogen production system was the best among three methods.
|
PDF Full Text Request