Process For Treating Citric Acid Production Wastewater By Aerobic Granular Sludge

With the continuous improvement of the standard discharge requirements of fermentation industrial wastewater,the efficient treatment of high concentration organic wastewater produced in the process of citric acid production becoms important.Due to its excellent settling performance and high efficiency and energy saving,aerobic graular sludge（AGS）has great advantages in treating industrial organic wastewater,So far,there is no research on the treatment of citric acid wastewater by AGS technology.Taking the anaerobic effluent from the citric acid production wastewater treatment plant as the raw water in the present work,the feasibility,the stability and the scale-up of the AGS treatment process were studied.The main research contents and conclusions of this paper are as follows:（1）Cultivation of AGS with synthetic wastewater and citric acid production wastewater.AGS was cultivated in a 9L SBR reactor with anaerobic granular sludge as the seed sludge and synthetic wastewater as influent.After 40 days of operation,the reactor reached a stable state,the biomass concentration in the reactor was 6.8 g/L,and the SVI₅value was less than 30 m L/g（far less than the requirement for AGS of 50 m L/g）,showing good settling performance.The AGS was shaped smooth spherical or ellipsoidal,with an average particle size of 2 mm,with yellow color and a large amount of gelatinous material on the AGS surface.The removal rates of COD,TN,NH₄⁺-N and TP in the wastewater were all above 80%.Contrastly,when the anaerobic effluent of citric acid wastewater was used as the influent to cultivate AGS,the SVI₅and SVI₃₀in the reactor decreased to 29 m L/g and 26 m L/g respectively after 60 days of operation.After 78 days,the shape of the reactor became regular,and the average particle size was 1.2 mm.It was showed that the removal rates of COD,TN,NH₄⁺-N and TP reached 84%,80%,96%and 97%respectively,and the effluent pollutant concentration reached the indirect discharge standard of citric acid wastewater（GB19430-2013）.The feasibility of treating citric acid production wastewater by AGS technology was verified.Compared with the process of cultivating AGS with synthetic wastewater,the citric acid production wastewater has the complex and refractory components which are difficult to be biodegraded.Therefore,the growth rate of microorganisms in the AGS was relatively slow,the start-up time and the time of reactor reaching stable operation were prolonged.The resulting average particle size of AGS was smaller and the color was darker.However,the removal efficiency of pollutants,especially NH₄⁺-N and TP,obtained better performance.（2）The effect of sludge load on the stablity of AGS reactor operationIn a 9-L SBR reactor,the mature AGS cultivated by the synthetic wastewater was used as the seed sludge to treat the anaerobic effluent of citric acid wastewater.After a period of operation,the reactor operated stably and the effect of pollutant treatment is good.The initial influent sludge load is low,and the operation of the reactor is stable.With the gradual increase of influent sludge load,the sludge volume index（SVI）increases,and the performance of AGS reactor becomes worse.The method of reducing the sludge load can restore the stability of AGS reactor.Reactor run in the first 10 days,due to the sudden change of wastewater composition,inoculated aerobic granular surface microorganisms need a conversion to adapt to the process of granular sludge obvious change in macro morphology,particle size smaller,particle surface color from yellow to brown,after a period of running,the morphology of granular sludge basically remained stable.After 40 days of operation,the biomass concentration in the reactor decreased from 12 g/L to about 7.5 g/L.When the influent volume load was set at 1.4 kg COD/（m³·d）and the hydraulic retention time was at 12h,the removal rates of COD,TN,NH₄⁺-N and TP in AGS reactor reach 82%,72%,92%and92%,respectively.The effluent indexes could all meet the indirect discharge standard of citric acid production wastewater.Compared with the traditional activated sludge process,it exhibited considerable advantages.At the beginning of operation,the filamentous bacteria grow rapidly after the influent sludge load is increased,the surface structure of AGS is loose and easy to disintegrate,and the sludge settling performance becomes worse,which is not conducive to the stable operation of the reactor.After appropriately reducing the sludge load of influent,the growth of filamentous bacteria was inhibited.Under the action of hydraulic shear force,the surface structure of particles became compact and the stability began to recover.（3）Pilot scale anaerobic treatment of citric acid wastewater by AGS technologyIn a 200 L pilot scale sequential batch reactor,matured AGS was obtained in about 45 d of cultivation.At the 78th day,the biomass in the reactor reached 8.124 g/L.SVI₅and SVI₃₀dropped to 45 m L/g and 32 m L/g,respectively,showing good settling performance.The resulting AGS has a particle size distribution between 0.5-2 mm（>80%）,a regular shape,and a yellow-brown color.Compared with the small scale culture process,the removal efficiency of pollutants in the reactor is lower after 40 days of operation.When the influent volume load is 0.41 kg COD/（m³·d）,the removal rates of COD,TN,NH₄⁺-N and TP in the wastewater are60%,45%,60%and 58%,respectively.The main reason for the low removal efficiency of pollutants is that the reactor does not reach the stable operation state and the AGS is not active enough.Feeding model based on COD specific consumption rate was used in subsequent experiments,,in the anaerobic period,the influent volume load is adjusted according to the specific rate of COD absorption of AGS.Feeding model based on COD specific consumption rate can not only accelerate the formation of AGS,but also inhibit the formation of flocculent sludge,which is conducive to the long-term stable operation of AGS reactor.The results show that after 78 days of operation,when the influent volume load is 1.23kg COD/（m³·d）,the removal rates of COD,TN,NH₄⁺-N and TP in the wastewater are 80%,70%,85%and 75%,respectively,which is significantly improved compared with that before.