Basic Research On Algae Resource Utilization Based On Hydrothermal Carbonization-Anaerobic Digestion

Recently,huge amounts of algae residues are generated from algae blooms including red and green tides caused by marine eutrophication,as well as offshore seaweed farming.At present,the algae residues utilization rate is low,which has caused increasingly serious environmental problems.Algae resource utilization is an effective way to dispose of algae residues.However,the common methods of algae resource utilization often have some disadvantages such as high water and energy consumption,high risk of environmental pollution,low efficiency,and low sustainability.Thus,this dissertation developed a novel combined process of hydrothermal carbonization(HTC)and anaerobic digestion(AD)to efficiently treat the wet algae residues,and the related mechanisms were explored.Specifically,improving the HTC process to increase the yield and quality of hydrochar,reduce water consumption and adjust the HTC process water components to facilitate the production of methane(CH4);in-situ using algae-based hydrochar to promote the AD efficiency of HTC process water,and adding the Fe-modified hydrochar(Fe-HC)to achieve in-situ biogas upgrading;and using the intermittent fed-batch AD technology to enhance the AD performance of HTC process water.The main contents and findings are as follows:(1)Improved the algae HTC process based on HTC process water recirculation.This study selected Laminaria as the representative of macroalgae for HTC,optimized the HTC parameters,assessed the effects of process water recirculation on solid and liquid products from HTC,and clarified HTC process water recirculation parameters.The results showed that reaction temperature 220?,reaction time 2 h,and material/water ratio 1:20 are the optimal Laminaria HTC parameters;recycling process water had a positive effect on the yield,carbon recovery rate and high heating value of hydrochar,which brought the enhancement by 27.8%,41.5%and 11.4%,respectively,from initial cycle to round 10,but there was no significant difference between the round 12 and round 10;the volatile fatty acids(VFAs)predominantly accumulate with process water recirculation,which increased 19.5 times from the initial cycle to round 12,the corresponding proportion of VFAs on chemical oxygen demand(COD)increased from the initial 13.8%to the highest 57.7%;the soil seed germination index(GI)results indicated that the process water recirculation could partly alleviate the toxicity of process water,and the process water from round 10 had the least toxicity;and the AD experiments also confirmed that HTC process water recirculation could modify the components of process water to promote CH4 production,a 12.3%increase was observed in the round 10,compared with initial process water.(2)Enhanced the AD performance of HTC process water based on the in-situ utilization of hydrochars.The Chlorella-based hydrochar(HC-C)and Laminaria-based hydrochar(HC-L)were prepared by HTC process using microalgae marine Chlorella and macroalgae Laminaria as raw materials in this study,and the effects of HC-C and HC-L on AD of Laminaria HTC process water were investigated through batch experiments and continuous reactor experiments.The batch experiments results showed that the hydrochar/sludge=1:1 was the optimum HC-C and HC-L addition ratio in this study,compared with the control,the HC-C and HC-L addition enhanced CH4 yields by 30.6%and 37.6%,respectively.The continuous reactor experiments results showed that the AD process stability of hydrochars supplemented reactors were performed well under the increase of organic loading rate from 2.6 to 6.5 g COD/L/d,and HC-C and HC-L addition could significantly enhance the daily CH4 yield rate by 36.0%and 31.4%,respectively,which were in good agreement with the batch experiment results;the HC-C and HC-L addition could significantly accelerate the consumption of VFAs,especially the oxidation of propionic acid;the possible mechanisms of HC-C and HC-L on the enhanced AD were similar,namely,increasing sludge granulation,promoting the Methanothrix relative abundance and key enzyme(F420 and DHA)activities,and further facilitating potential direct interspecies electron transfer(DIET)between methanogens and electrogenic organic-oxidizing bacteria.However,the HC-C and HC-L addition could not significantly improve the CH4 proportion in biogas.(3)Developed a novel Fe-modified hydrochar(Fe-HC)-based process for promoting in-situ biogas upgrading in AD of HTC process water.The Fe-HC composite material was synthesized by the one-step HTC process of the mixture of zero-valent iron(ZVI)powder and Laminaria in this study,to investigate the enhancement performance of in situ biogas upgrading,and the related mechanism was explored.The results showed that the core-shell structure particles(iron oxides shell and Fe0 core)were formed and well-dispersed on the cracks and pores of Fe-HC,and the electron exchange capacity of Fe-HC is about 1 times higher than that of unmodified hydrochar;the daily CH4 yield rate in the Fe-HC added reactor was 216.6 mL/g CODfed,which was nearly 60%higher than the control reactor,and the CH4 proportion in biogas was 90.3%.The conductive Fe-HC increased relative abundances of Trichococcus and hydrogenotrophic methanogens(Methanosarcina and Methanobacterium)and promoted the electron transport system activity,heme c content in extracellular polymeric substances,and sludge granulation;these results implied the Fe-HC-promoted biogas upgrading might be ascribed to the enhancement of hydrogenotrophic methanogens-mediated carbon dioxide(CO2)reduction and potential DIET-mediated CO2 reduction,and trace FeCO3 precipitation.Furthermore,the Fe loss of Fe-HC was just 3.3%after 50 days,indicating Fe-HC had the sustainable enhancement effects in AD process.In addition,the effluent from the Fe-HC supplemented reactor was rich in nutrients,including K+,I-,gibberellin(GA3),etc.,and low in human potential pathogenic bacteria and heavy metal content,indicating it had excellent potential as a soil amendment.(4)Established a new process for promoting the AD performance of HTC process water based on the intermittent fed-batch AD.The Laminaria HTC process water was used as the substrate in this study to evaluate the enhancement of the fed-batch process for AD of HTC process water to produce CH4 and high-concentration liquid fertilizer.The results showed that the CH4 yields during 10 rounds of fed-batch cultivation were relatively stable,ranging from 177-210 mL/g CODadded;the fed-batch AD process could significantly shorten the fermentation period,which was shortened from 14 days to 4 days after round 7,and the lag phase was shortened to less than 0.4 day;the inorganic and organic components in the digestate broth gradually accumulated as the fed-batch cultivation proceeded;the GI increased significantly with the increase of fed-batch cultivations,and the correlation analysis showed that the contents of NO3-N and GA3 in the digestate were highly significantly correlated with GI(p<0.01),the digestate from round 10 caused the highest GI,and amended coastal saline soil,further confirming that the fed-batch AD process could adjust the quality of the digestate and obtain the storable and transportable concentrated digestate for soil amendment.In conclusion,this dissertation developed a novel combined process of HTC and AD to efficiently treat algae residues,which could achieve high-efficiency resource recovery of liquid,gas,and solid products during the process.This process can not only solve the possible environmental risks caused by algae residues but also produce renewable energy to achieve sustainable development,which provided a new idea for rapid and efficient resource utilization of algae residues.