Pilot Process For Aerobic Granular Sludge Treatment Of Anaerobic Effluent From Citric Acid Production Wastewater

The requirements for the discharge standards of citric acid production wastewater in China are becoming increasingly strict,and efficient treatment of high concentration organic wastewater generated in the production process has become particularly urgent.At present,anaerobic AAO biological treatment process is commonly used for citric acid production wastewater,but there are shortcomings such as long treatment process,large land occupation,and high energy consumption.The adoption of efficient and energy-saving processes has become an inevitable demand to comply with the national dual carbon goals.The aerobic granular sludge process is highly favored due to its characteristics of rapid sedimentation,efficient treatment,space saving,and low energy consumption.Taking the anaerobic unit effluent of citric acid production wastewater as the treatment object,the feasibility of the aerobic granular sludge process was explored through process parameter optimization.For the first time,a 30 m³pilot scale aerobic granular sludge reactor has been developed for citric acid production wastewater.The stability,sedimentation,and bed expansion characteristics of the aerobic granular sludge process after scaling up have been studied,laying a foundation for future practical applications.The following are the main research contents and conclusions:（1）Design and construction of a 30 m³pilot aerobic granular sludge reactor and pre-treatment equipmentIn combination with the settling performance of aerobic granular sludge,hydraulics principle and process conditions,a full-automatic 30 m³pilot AGS reactor was designed and built for the first time according to the characteristics of citric acid production wastewater to cultivate and stabilize aerobic granular sludge.To solve the problem of high concentration of suspended solids in the influent,a vertical flow settling tank was added in front of the pilot AGS reactor,which can stabilize the concentration of suspended solids below 350 mg/L and meet the influent water quality requirements of the pilot aerobic granular sludge reactor.（2）Flocculation pretreatment of reactor inlet waterThe high concentration of suspended solids in the influent of aerobic granular sludge can have adverse effects on the stability of AGS.In order to better reduce its negative impact,the influencing factors and levels of suspended solids flocculation and sedimentation in the influent of the reactor were studied and analyzed.The correlation between influent turbidity and suspended solids concentration in the reactor was analyzed.A single factor analysis was conducted on the p H,dosage of polyaluminum chloride（PAC）,and dosage of cationic polyacrylamide（PAM）during the flocculation process of the reactor inlet,and the optimal influencing level was determined.On this basis,Box Behnken optimization analysis and demonstration were conducted,and the optimal process conditions were obtained:p H is 8;The dosage of 10%PAC is 0.44%±0.11%?SS;The dosage of 0.1%cationic PAM is 0.35%±0.03%?SS.Under these conditions,after 15 minutes of flocculation and sedimentation process validation,the turbidity of the supernatant in the influent of the reactor decreased to69.9±1 NTU,the corresponding suspended solids concentration decreased to 46.94-151.06mg/L,and SVI₁₅decreased to 100 m L/g,meeting the requirements for the influent water quality of the AGS reactor.（3）Process debugging of pilot aerobic granular sludge reactorUsing anaerobic granular sludge from a certain fermentation enterprise as inoculum sludge,citric acid production wastewater treated by anaerobic units was used as reactor inlet,and aerobic granular sludge was cultivated in a 30 m³AGS reactor.After 126 days of cultivation,small aerobic granular sludge with a particle size of about 0.4mm began to appear in the reactor.After continuous operation until the 230th day,regular yellow brown aerobic granular sludge with a diameter of 0.5-1 mm was obtained through selective pressure screening.At this stage,the biomass concentration steadily increased to 5.5 g/L,SVI₅decreased to 50 m L/g,SVI₃₀stabilized at 35 m L/g,SVI₅/SVI₃₀remained at 1.4,and remained basically unchanged thereafter.The settling performance of small aerobic granular sludge cultivated in a 30 m³pilot AGS reactor is relatively good.After 250 days of stable operation,the COD specific consumption rate of aerobic granular sludge gradually increased from 8g COD/（g TSS?h）in the initial start-up stage to 24 g COD/（g TSS?h）.At the same time,after 130days,the degradation of NO₃-N during the hypoxia period was observed,and the degradation of TN gradually improved during the cycle.Finally,the removal rates of COD,NH₄⁺-N,TN,and TP in wastewater by the reactor reached 80%,95%,70%,and 80%,respectively.（4）Exploring the sedimentation and bed expansion behavior of aerobic granular sludgeFirstly,a single particle equilibrium settling velocity model was established through Newtonian mechanical equilibrium,successfully verifying the reference ability of the model.Further analysis of the relationship between particle size and drag coefficient shows that the particle Reynolds number of aerobic granular sludge with different particle sizes is in the range of 1<Re_p<50,and the drag coefficient equation is fitted,C_D=6.643Re_p^-0.146.Subsequently,the process of aerobic granular sludge bed expansion and fluidization was simulated in the constructed experimental device.The relevant parameters of the Richardson&Zaki model under monodisperse particle conditions were measured,and the relationship between the critical fluidization velocity（u_f）and the equilibrium settling velocity（u_t）of particles was obtained,u_f/u_t=0.522.The analysis of the sedimentation rate model provides a theoretical basis for the screening and retention of different particle sizes in the selective sludge discharge process,as well as a design basis and operational guidance for the further industrial scale up of aerobic granular sludge reactors.