Research On Cyanobacteria About Optimization Of Growth Conditions And Pretreatment Of Anaerobic Fermentation For Biogas Production

Nowadays,the alternative treatment technique for municipal sludge is anaerobic fermentation.Methane,separated from the biogas producted by the anaerobic fermentation,could be ultilized as energy resources.While carbon dioxide,as the by-product of the sludge digestion,could always be released into the atmosphere and become part of greenhouse gas pollution.Considering that carbon dioxide is not utilized effectively,and the digested liquor from sludge fermentation is rich in nitrogen and phosphorus as well,this study puts forward an method of treating process to utilize cyanobacteria as a bio-catalyst,which could absorb carbon dioxide produced from sludge digestion and nutrients,such as nitrogen and phosphorus,from digested liquor,so as to covert inorganic carbon to biomass,which could be further converted to methane by anaerobic fermentation.This research concentrates on the part related to cyanobacteria of the whole process.On one hand,this research focuses on the improvement of the absorption effect of nitrogen and phosphorus through optimization of growth conditions of Microcystis aeruginosa which is the dominant algae species of the cyanobacteria bloom in Lake Taihu.On the other hand,the improvement of the effect of the anaerobic fermentation is also studied through optimization of pretreatment conditions of cyanobacteria from Lake Taihu.The full text of the research is shown as follows:1.This research investigates the effect of light intensity,N/P and aeration concentrations of CO₂ on the growth rate of Microcystis aeruginosa respectively.The results show that,the maximum of algae biomass could be obtained at the light intensity of 5000 Lux,the N/P range between 10:1 and 15:1,and the aeration concentration of CO₂ below 5%.The growth rate of Microcystis aeruginosa would be obviously influenced by light intensity when there are adequate nutrients.However,the growth rate could be restrained when the aeration concentration of CO₂ is too high or N/P is too high or too low.2.This research investigates the effect of light intensity,N/P and aeration concentrations of CO₂ on the absorption of nitrogen and phosphorus of Microcystis aeruginosa respectively.The results show that,the absorption rate of NO3--N improves with the improvement of light intensity and could reach the maximum at the light intensity of 7000 Lux.Meanwhile,the absorption rate of TP has a small change with variety of light intensity and could reach the maximum at 5000 Lux.The absorption rate of NO3--N and TP could both reach the maximum at the CO₂ aeration concentration of below 5%.The absorption of NO3--N decreases with the improvement of N/P,and Microcystis aeruginosa could reach the maximum of absorption of NO₃^--N at the N/P between the range of 10:1 and 15:1.Comparatively,the absorption of TP could reach the maximum at the N/P between the range of 10:1²0:1.3.This research investigates the effect of N/P,CO₂ aeration concentrations and p H on the growth rate and absorption of nitrogen and phosphorus of Microcystis aeruginosa respectively using response surface methodology?RSM?.The optimal combination of conditions could be reached at the N/P of 14.14:1,the CO₂ aeration concentrations of 3.59%,p H of 9.27.The predictive value of algae biomass is 3.57 g/L under this condition,while the predictive value of absorption rate of TP is 76.75%,which is proved validly after experiment under the optimal conditions.4.This research investigates the influence of NO₂--N concentration and pretreatment time on the effect of the pretreatment and anaerobic fermentation of cyanobacteria respectively.Observations of control group samples after 9 h of pretreatment are conducted through scanning electron microscopy?SEM?.The results show that,the cell structure of cyanobacteria is destroyed after pretreatment which results in the release of organic matter from cells,so that the hydrolysis rate is improved and the effect of the anaerobic fermentation of cyanobacteria is enhanced as well.The index concentrations of samples,such as SCOD,PS,PN and NH3-N,are examined and analyzed after pretreatment.The results show that,except NH₃-N,others' index concentrations experience improvenment in varying degrees,all of which are obvious.The concentrations of SCOD,PN,PS in most samples increase with the enrichment of NO₂--N concentration and the extention of pretreatment time.5.The research adds anaerobic microorganisms to cyanobacteria samples after pretreatment,in order to investigate differences of gas production.The results show that,the biogas production and degradation rate of VS of different samples usually increases with the enrichment of NO₂--N concentration at same pretreatment time.The research achieves the maximum biogas production of 142 m L/g VS at the conditions of pretreatment time of 9 h and NO₂--N concentration of 200 mg/L,while the degradation rate of VS and methane content obtain the maximum value of 49.67% and 70.4%,respectively.Therefore,the optimal condition of pretreatment of NO₂--N is obtained at the NO₂--N concentration of 200 mg/L and the pretreatment time of 9 h.