Dynamic Forces Between Anaerobic Granular Sludge And Biogas Bubbles

Anaerobic digestion technology is a sustainable organic wastewater treatment technology.While degrading organic pollutants,it can also produce clean energy such as CH4.It has been widely used in recent decades.Among them,the up-flow anaerobic sludge bed reactors(UASB)are the most studied and widely used high-speed anaerobic reactor,which has a typical gas-liquid-solid three-phase flow system.The gas production performance of UASB is influenced by many factors.Extracellular polymeric substance as the last contact material when biogas bubbles is detached,plays a key role in the interaction between anaerobic granular sludge(AGS)and biogas bubbles.Therefore,discussing the dynamic force between the sludge and single bubble is of great guiding significance for deeply studying the bubble behavior in the reactors and enriching the gas-liquid mass transfer theory of the reactor.In this paper,based on the study of the effect of organic loading rate(OLR)and exogenous polystyrene nanoparticles(Pd-PS)on the performance of anaerobic reactor methane production,the bubble colloidal probe technology of atomic force microscope(AFM)was used to measure the dynamic force between different types of AGS and simulated biogas bubbles(CH4 and CO2)at the level of single particle and single bubble,providing new insights into the bubble generation and movement of the reactors.The main research results are summarized as follows(1)The influence of different OLR on the foaming performance of anaerobic digestion system was analyzed.With the increase of OLR,the CH4 production in the reactor was significantly reduced,and the H2 production was gradually increased.Due to the insufficient buffer capacity of the system,anaerobic foaming occurred.As the reaction load increased,the surface tension in the solution gradually decreased,and the viscosity gradually increased,affecting the formation of bubbles.Total volatile fatty acids accumulated significantly with the increase of OLR,and the protein concentration also gradually accumulated.The soluble protein was related to extracellular polymeric substance,which was conducive to the formation of bubbles.With the improvement of OLR,Firmicutes had become a typical indicator microorganism for foam formation.In addition,the mass reproduction of Clostridium bacteria made butyric acid the most important volatile fatty acid,which accumulated in the system and aggravated the foaming phenomenon of the anaerobic reactor.(2)The promotion mechanism of Pd-PS on gas production performance of anaerobic digestion system was clarified.As the concentration of Pd-PS increased,the cumulative methane production of the system increased.The protein secondary structure of the extracellular polymeric substance of sludge was characterized,and it was found that the addition of Pd-PS reduced the protein concentration,and exposed the hydrophobic groups in the sludge through the loose structure of the protein,which was conducive to the aggregation and stable existence of granular sludge.The addition of Pd-PS increasd the hydrophilic functional groups such as hydroxyl groups and amino groups in the extracellular polymeric substance,and interacted with humic acids,resulting in the reduction of humic acids and changing the composition of the extracellular polymeric substance.Analysis of the microbial community found that Pd-PS can promote the production of CH4 through the decomposition of acetic acid and the synthesis of H2 and CO2.In addition,the relative abundance of Geobacter was increased,which verified that Pd can promote direct interspecies electron transfer.(3)The key role of AGS extracellular polymeric substance on bubble desorption was revealed.The results of the AFM experiment showed that for the two simulated biogas bubbles(CH4 and CO2),AGS in UASB treating wastewaters of protein factory(AGSp)had greater adhesion to CH4 and CO2 bubble than AGS in UASB treating wastewaters of beer factory(AGSb).The difference in the normalized adhesion of the two AGS to CH4 or CO2 bubbles was related to the surface properties of AGS.Different types of AGS can cause differences in the amount or rate of bubble generation.Compared with the extracellular polymeric substance of AGSp,AGSb’s extracellular polymeric substance had a higher protein content and a higher protein concentration/polysaccharide concentration ratio(PN/PS).The surfaces of AGSb and AGSp were negatively charged,and there may be electrostatic repulsion between the biogas bubbles.But the C=O content of AGSp is lower than that of AGSb,which was consistent with the oil contact angle data.It indicated that the surface of AGSp was more hydrophobic,and there was greater adhesion between the hydrophobic biogas bubbles.