Mg²⁺ And Struvite（MAP）-Mediated Granulation Process Of Anammox And The Induced Strategy For Flotation Control

As the world population continues to increase,more and more frequent human activities have profoundly changed the availability and mobility of nitrogen in large areas of the earth,affecting the global nitrogen cycle and having a profound impact on human health and natural ecosystems.Nitrogen pollution of water bodies,especially ammonia nitrogen pollution of water bodies,is an important environmental pollution problem of human world.Anaerobic ammonia oxidation（anammox）,as a promising autotrophic nitrogen removal process,has received widespread attention because of its high efficiency,energy saving,no need for organic carbon source and its suitability for treating wastewater containing high concentration of ammonia nitrogen.However,the slow growth rate of anammox bacteria,long multiplication time,environmental inhibition,and the tendency of sludge flotation and loss during operation have limited the practical application of anammox process.The granulation of anammox can improve the settling performance of sludge,maintain the biomass of reactor and help the bacteria to overcome the adverse external environmental effects,which is one of the effective methods to solve the application limitation of anammox process.In this study,four identical EGSB reactors were set up to control the Mg and P concentrations in different reactors（R1,R2,R3,R4）and the reactors were operated stably for 200 days.The granulation process of anammox sludge mediated by different factors was investigated by examining the changing characteristics of physicochemical characteristics,the activity and microbial community of anammox sludge under Mg²⁺and struvite（Mg（NH₄）PO₄·6H₂O,MAP）mediated conditions.The metabolomics and metatranscriptomics techniques were also combined to analyze the different metabolic characteristics of anammox sludge mediated by Mg²⁺and MAP,and to investigate the differential metabolic mechanisms involved.The main conclusions obtained were as follows:（1）The presence of Mg²⁺and MAP production did not affect the long-term nitrogen removal performance of the reactors,and the average nitrogen removal rate（NRR）of the four reactors was 1.33 kg N·m^-3·d^-1,while the average nitrogen removal efficiency（NRE）was>75%.During the long-term operation,the density,particle size and concentration of the sludge in the Mg²⁺-mediated reactor did not change much,but the specific anammox activity（SAA）,heme c content increased significantly,and its apparent morphology showed a deeper carmine color.The adsorption of Mg²⁺surprisingly promoted the activity of anammox bacteria.The generation of MAP crystal precipitation can accelerate the growth rate of anammox sludge particle size,density and sludge concentration,and effectively promote the rapid granulation of anammox sludge.But additional MAP microcrystal injection does not enhance these effects,and excessive MAP generation can also inhibit the activity of anammox bacteria and reduce the percentage of sludge biomass.（2）In order to obtain MAP with relatively high purity and co-growth with anammox sludge,the concentration of Ca²⁺and other precipitation-prone ions in the feed water should be reduced as much as possible,and the p H should be controlled in the range of 8.05-8.15,so as to maintain the relatively stable activity of anammox sludge while generating as much MAP in the reactor as possible to achieve simultaneous nitrogen and phosphorus removal.The generated MAP will fuse with the anammox sludge to form a unique granular structure,and effectively increase the density and settling performance of sludge,so that the sludge can be better retained in the reactor.And it will also maintain the stable operation of the reactor.It is an effective strategy to control sludge flotation even at high flow rate of 1.27 mm·s^-1 and will is beneficial to the practical application of anammox process.On the contrary,the presence of high concentration of Mg²⁺(80 mg·L^-1)help to increase the activity of sludge,though,makes the decrease of its density,which intensifying the flotation of anammox sludge.（3）Both the presence of Mg²⁺and the production of MAP significantly affected the microbial community succession and metabolic processes in the anammox system.The highest abundance of total anammox bacteria was found in the sludge under the effect of Mg²⁺,and its anammox capacity was mainly responsible for the higher activity of Candidatus Brocadia,which might be the reason for the increase activity of sludge.In contrast,the blockage of mass transfer resulting from the production of MAP,which combined the effect of high p H,led to the shift of core functional bacteria from Candidatus Brocadia to Candidatus Kuenenia.In terms of metabolic processes,the presence of Mg²⁺may enhance the import of Mg²⁺to anammox cells,strengthen intracellular signaling.It may also enhance the activity of inorganic carbon metabolism and nitrogen metabolism,activate the metabolic pathways of purines and pyrimidines,thus promote the secretion of EPS and intercellular communication,leading to an increase in sludge activity.The production of MAP,on the contrary,inhibits inorganic carbon metabolism but promotes organic carbon and nitrogen metabolism.However,it can also enhance the metabolism of purine,pyrimidine and EPS secretion in sludge,so that the sludge can achieve rapid growth.