Mathematical Modeling And Anabolic Rule Of Tightly And Loosely Bound Extracellular Polymeric Substances In Aerobic Biological Treatment System

The content of TB-EPS and LB-EPS has a very important effect on the settlement and stability of the sludge in the aerobic biological treatment system such as activated sludge（AS）and aerobic granular sludge（AGS）.In the context of smart water,it has become an inevitable trend to analyze and predict complex biological reaction processes using mathematical models.However,the biological mechanism of the generation and consumption of TB-EPS and LB-EPS and their quantitative variations are not clear,so the existing mathematical model simulates EPS as a whole component,instead of dividing EPS into TB-EPS and LB-EPS for simulation.Therefore,this study took activated sludge system and aerobic granular sludge system as the research object,and expanded the extend unified model（EUM）based on the anabolic theory of microbial capsule and mucus layer.The generation and consumption mechanism of TB-EPS and LB-EPS in activated sludge system and aerobic granular sludge system was analyzed,and the quantitative variations of TB-EPS and LB-EPS was simulated.In order to quantitatively study the generation and consumption mechanism of TB-EPS and LB-EPS,based on the capsule and mucus layer theory in microbial anabolism and the extend unified model（EUM）of microbial products simulation,mathematical models of different generation and consumption mechanisms of TB-EPS and LB-EPS were established（named EUM-TL1 and EUM-TL2,respectively）.The generation and consumption paths of TB-EPS and LB-EPS in EUM-TL1 were assumed as follows:microorganisms use exogenous substrate S_S to synthesize TB-EPS,which can be further hydrolyzed to produce LB-EPS,and finally LB-EPS hydrolyzes to produce biomass-associated products（BAP）.（S_S→TB-EPS→LB-EPS→BAP）;The generation and consumption paths of TB-EPS and LB-EPS in EUM-TL2 were assumed as follows:microorganisms use exogenous substrate S_S to synthesize TB-EPS and LB-EPS simultaneously,and TB-EPS can be further hydrolyzed to produce LB-EPS,and finally LB-EPS hydrolysis produces BAP（S_S→TB-EPS→LB-EPS→BAP,S_S→LB-EPS→BAP）.Model verification results showed that EUM-TL2 had better simulation effect on TB-EPS and LB-EPS,and the generation and consumption path of TB-EPS and LB-EPS was more conducive to describing the generation and consumption mechanism of TB-EPS and LB-EPS.EUM-TL2 mainly considered the biological reaction process of heterotrophic bacteria X_H,the newly introduced parameters include k _TB,O(0.12 g COD X_LB-EPS/g COD S_S)and k _LB,O(0.1 g COD X_LB-EPS/g COD S_S)can well simulate the electron inflow ratio of TB-EPS and LB-EPS in exogenous S_S,which was about 12%and 10%,respectively.The newly introduced parameters k_hyd,TB(0.015h^-1)and LB-EPS k_hyd,LB(0.03h^-1)can better reflect the hydrolysis rates of TB-EPS and LB-EPS.The pollutant removal and sludge concentration during the start-up stage of the biological treatment system did not reach relative stability,and the concentration of TB-EPS and LB-EPS at this stage have an important impact on the system.In order to predict the variations of TB-EPS and LB-EPS and removal of carbon and nitrogen during the unsteady state of AS system,and obtain the typical parameter values of AS system under different conditions,firstly,the EMU-TL2.1 model was established by introducing nitration process based on EUM-TL2,which nitration process was not included.In order to verify the simulation effect of EUM-TL2.1 on activated sludge system with different influent components and reactor types,the model was verified by using data such as TB-EPS and LB-EPS in a completely mixed reactor with different influent carbon sources（sodium acetate,glucose and starch）and a sequential batch reactor（SBR）with different aeration conditions.The results showed that the simulation results of EUM-TL2.1 on the removal of TB-EPS,LB-EPS and pollutants were in good agreement with the experimental results,which proved that the model can better simulate the changes of TB-EPS and LB-EPS during the start-up and operation of AS reactors.The simulation results of TB-EPS,LB-EPS and removal of carbon and nitrogen by EUM-TL2.1 were in good agreement with the experimental results,which proved that the EUM-TL2.1 can simulate the change rule of TB-EPS and LB-EPS in activated sludge reactors with different operating modes.Model analysis showed that compared with TB-EPS,LB-EPS was more affected by the type of influents carbon source,and microorganisms were more likely to use sodium acetate to produce TB-EPS and LB-EPS,and the hydrolysis rate constant of TB-EPS and LB-EPS was higher.Aeration rate had little effect on the variation trend of TB-EPS and LB-EPS in activated sludge SBR system,but had certain influence on the concentrations of TB-EPS and LB-EPS.The concentrations of TB-EPS and LB-EPS in high aeration reactor were generally higher than those in low aeration reactor,indicating that appropriate increase of aeration could promote the synthesis of TB-EPS and LB-EPS.The influence analysis of aeration-related parameters,total transfer coefficient（k _La）and interfacial dissolved oxygen concentration（C_S）on TB-EPS and LB-EPS also proved this rule.In order to ensure a high removal rate of S_S and S_NHand a stable active biomass,when C_S was 2 mg/L,k _La can be set at 30-50 h^-1,when k _La was 50 h^-1,C_S can be set at 1-1.5 mg/L.TB-EPS and LB-EPS are very important for the start-up and stability of the aerobic granular sludge system.Meanwhile,the particles in the aerobic granular sludge system are easy to form anoxic zone so as to achieve simultaneous nitrification and denitrification.Therefore,in the mathematical simulation of aerobic granular sludge system TB-EPS and LB-EPS,denitrification process was introduced on the basis of EUM-TL2.1 model,and EUM-TL2.2 was established.Biofilm reactor coupled with SBR reactor was used to simulate the operation of aerobic granular sludge SBR reactor,and the model was verified with reactor data under different aeration conditions.The results showed that the simulation results of TB-EPS,LB-EPS,particle size and pollutant removal by EUM-TL2.2 were in good agreement with the experimental results.Different aeration had little effect on the change trend of TB-EPS and LB-EPS in aerobic granular sludge system,which showed a change rule of first increasing and then decreasing to stable.Different aeration rates had a certain effect on the concentration of LB-EPS,but had no significant effect on the concentration of TB-EPS.In the aerobic granular sludge reactors with apparent gas velocities of 1.74 cm/s,2.34 cm/s and 2.91 cm/s,the concentration of LB-EPS in the particles of 2.34 cm/s reactor and its proportion in the particles were higher than those in the other two groups at the same period.The model predicted that the maximum concentration of LB-EPS reached 77.74 COD X_LB-EPS/mg COD VSS at about 210 h.After the 2.34 cm/s reactor reached the stable state,the particle size reached 613μm,which was higher than that of the other two groups of reactors,indicating that either too high or too low aeration rate was not conducive to the generation of LB-EPS,LB-EPS had a significant effect on particle size,and LB-EPS was conducive to the increase of particle size.