Enhancement Of Hydrogen And Methane Production By Enteromorpha With Integrated Two-phase Anaerobic Bioreactor

With the low-carbon transition and environmental governance being gradually adopted as a global consensus,China has adopted the"dual carbon"strategy as a long-term plan for future energy restructuring.In terms of marine environmental management,red tides caused by algae such as Enteromorpha are a natural disaster that needs to be addressed urgently.Therefore,the treatment and energy utilisation of Enteromorpha is an effective solution to combine environmental management with the"dual carbon"strategy and has good prospects.However,due to the high water content of Enteromorpha,some traditional methods such as pyrolysis and combustion,which are widely used in the treatment of agricultural waste such as straw,are difficult to use for the disposal of Enteromorpha.The anaerobic fermentation method is characterised by low substrate water content and high energy efficiency,making it a suitable method for the treatment and energy utilisation of algal wastes such as Enteromorpha.This thesis takes an integrated two-phase anaerobic fermentation reactor as the research object,and aims at maximising the capacity efficiency and substrate utilisation,and addresses several key issues in anaerobic fermentation treatment:the slow start-up and low efficiency of the anaerobic fermentation reactor due to the complex structure of Enteromorpha and the poor mixing effect of the fermentation,and the bottleneck of the reactor structure that needs to be optimised.The problem of the anaerobic fermentation of Enteromorpha is studied in depth.Firstly,in order to solve the problem of difficult microbial utilisation due to the complex cell wall structure of Enteromorpha,study conducted pretreatment tests on Enteromorpha using autoclave pretreatment,microwave pretreatment and frozen Na OH/urea pretreatment with the aim of improving the biogas production of Enteromorpha fermentation.It was found that the frozen Na OH/urea method is a new pretreatment method suitable for using under cold conditions in the north China,and the cumulative fermentation gas yield at-16℃ increased by 9.67 m L·(g _Enteromorpha)^-1compared to the control group,while the microwave pretreatment increased the cumulative fermentation gas yield by 14 m L·(g _Enteromorpha)^-1compared to the control group.The highest net production rate was found for the microwave pretreatment at room temperature,with an increase of 77.5%compared to the frozen Na OH/urea pretreatment.The microwave pretreatment at room temperature was found to be more suitable for destroying the structure of Enteromorpha and improving the efficiency of bacterial decomposition.Secondly,to address the problems of slow start-up and complicated regulation of integrated two-phase anaerobic fermentation system,this study proposed a fast start-up strategy of molasses-Enteromorpha replacement,and after stable operation,the fermentation capacity efficiency was enhanced by the method of injecting nanoparticles.The rapid start-up of the reactor was completed within 20 days by using an Integral Biological Reactor（IBR）in two phase,and stable operation was achieved by 50 days of substrate replacement with Enteromorpha as the substrate.The results of the four nanoparticle enhanced fermentation showed that Ni nanoparticles had the best enhancement effect on the IBR,increasing the average daily gas production to3.53 L/（L·d）for the acid-producing phase and 1.38 L/（L·d）for the methanogenic phase.TEM and other results showed that nanoparticles with larger specific surface area are more effective in catalysis,it also found that magnetic nanoparticles could catalyse the acetic acid pathway by enhancing electron transfer,and Co/Ni nanoparticles could further improve the catalytic efficiency by enhancing hydrogenase activity,the dominant group of bacteria for substrate fermentation with Enteromorpha was identified by high-throughput sequencing,and functional groups associated with enhanced electron transfer and hydrogenase activity after nanoparticle addition were also identified.Finally,CFD flow field simulation of IBR was carried out to optimise the design of the reactor mixing structure in response to the poor mixing effect and unreasonable flow pattern of Enteromorpha particles found during the operation of the conventional anaerobic fermentation reactor.The flow field changes caused by the arrangement of the vertical and tangential return vents of the methanogenic phase of the UASB were investigated.The simulation found that the tangential return vents have a more uniform sludge distribution at different heights,which can form vortices to reduce sludge settling and the mixing effect is better than that of the vertical return vents.CFD simulations were carried out to investigate the effect of the stirring paddle configuration on the mixing effect of the acid producing phase of the CSTR.The results showed that the lower stirring paddle stirred the sludge and substrate more adequately at the same speed.The speed comparison showed that the wheel type stirrer was suitable for a stirring speed of 100 r/min,which improved mixing efficiency and reduced sludge loss.The results showed that the average sludge concentration at different cross-sections was 6.7%lower for the same sludge volume than for the four-bladed paddle.The streamline analysis showed that the wheel-shaped paddle was more effective in generating vortex flow,which was more conducive to the uniform mixing of the Enteromorpha substrate with the anaerobic sludge.In summary,this study was conducted to investigate three important aspects of the anaerobic fermentation of Enteromorpha,namely the pretreatment of the substrate,the start-up and operation of the reactor and the need to optimise the structure of the reactor.This study provides a theoretical basis and an important reference for the anaerobic fermentation of Enteromorpha.