Effect Of Hydraulic Conditions On The Stability Of Aerobic Granular Sludge And Its Surface Thermodynamic Mechanism

Aerobic granular sludge（AGS）has the characteristics of compact structure,excellent settling performance,strong impact resistance capacity and multi-functional microbe zonalization,which can effectively reduce the energy consumption and enhance the collaborative removal of pollutants.However,AGS still faces the bottleneck problems such as unstable structure and unclear mechanism.Focusing on the thermodynamics principle of the most stable system under the minimum free energy state,this thesis explored the changes of granular sludge characteristic indexes under different hydraulic conditions.From the perspective of microbial metabolism,the response relationship between AGS characteristic index and surface Gibbs free energy was analyzed to reveal the surface thermodynamic mechanism affecting the stability of AGS.The main results of the experiment are as follows:1.Chosen the Reynolds number as the key hydraulic parameter,the aggregation form,water-bearing characteristics,surface properties and other characteristics of AGS were investigated under different Reynolds number conditions.The experimental data showed that the contact angles of initial formed and stabilized AGS increased from50.4±2.8°and 51.1±2.4°to 60.2±1.9°and 79.1±3.2°along with the Reynolds number increased from 0 to 150.The water content decreased and the settling performance improved.Compared with the inoculated flocs,the characteristics of initial formed and stabilized of AGS had more obvious responses to the hydraulic conditions.The key indexes of granular stability were hydrophobicity,water content and SVI,and it had excellent stability when Reynolds number was controlled at 150.2.From the perspective of microbial growth and metabolism,the mass transfer and secretion characteristics of granular sludge were further studied.The mass transfer performance under different Reynolds number was studied.It was found that when the COD concentration was 500 mg·L^-1,the sludge boundary layer thickness（L_C）decreased from 375μm to 225μm with the Reynolds number increased from 100 to200,which indicated that the mass transfer performance improved.The analysis of sludge extracellular polymer substances（EPS）showed that the EPS content of the three groups increased,in which the increase of extracellular protein（PN）was more obvious.The PN content of granular sludge in the initial formed and stable granular sludge was the highest when Reynolds number was at 150.Combined with Fourier transform infrared spectroscopy（FTIR）,the key functional groups were mainlyβ-folded Amide I(1680-1630 cm^-1),Amide II(1545-1540 cm^-1)and aspartic acid(1400 cm^-1).Their change of peak value was consistent with the concentration of PN,indicated that they played an important role in the stabilization of granular structure.3.The Gibbs free energy on the surface of sludge was used as the major index to characterize the stability of granular sludge,and its change was investigated under different Reynolds number conditions.The experimental data showed that the Gibbs free energy of the stable granular sludge decreased from-113.93±2.69 m J?m^-2 to-129.75±2.15 m J?m^-2 when the Reynolds number increased from 0 to 150.The surface hydrophobicity and PN content of sludge were significantly negatively correlated with the interfacial Gibbs free energy,while the water content and SVI were positively correlated with the interfacial Gibbs free energy.To sum up,microbial metabolism and PN secretion of granular sludge increased under the condition of appropriate hydraulic parameters,and the sludge hydrophobicity increased to reduce the Gibbs free energy of sludge surface,and then the system tended to be stable.The research results can provide new viewpoints for the theoretical investigation and industrial application of aerobic granular sludge.