Inner Circulation And Multi-Partition Driving Autotrophic Nitrogen Removal In A Novel Single-Stage Partial Nitrification-Anammox System

The traditional biological nitrogen removal technology with nitrification-denitrification as the core is used in most municipal wastewater treatment in China.There are many prominent problems such as insufficient carbon source,lack of alkalinity,long process flow,large aeration energy consumption,and low nitrogen removal efficiency.Under the background of the implementation of the "double carbon strategy" in China,the single-stage autotrophic nitrogen removal process based on partial nitrification-anaerobic ammonia oxidation has become a research and application hotspot due to its high efficiency and low consumption.As one of the representatives of new biological nitrogen removal technology,single-stage autotrophic nitrogen removal process has made more achievements in the research and application of side flow process for high ammonia nitrogen wastewater treatment,such as sludge digester and landfill leachate in anaerobic pools.However,the treatment of low and medium concentration ammonia nitrogen wastewater,especially the low ammonia nitrogen wastewater of the mainstream process of municipal wastewater,mostly stays in the laboratory stage,and the technology is not mature enough.There are problems such as long start-up cycle,poor treatment effect or difficult long-term stable operation.To achieve the engineering application of low-concentration ammonia nitrogen wastewater in single-stage autotrophic nitrogen removal process,further research is needed.On the basis of summarizing the research and application of wastewater biological nitrogen removal theory and technology,from the perspective of optimizing the flow field in the reactor and constructing dissolved oxygen and different sludge partitions to improve the activity and abundance of nitrogen removal functional microorganisms in sludge and strengthen the synergistic interaction of nitrogen removal functional microorganisms,a new single-stage partial nitrification-anaerobic ammonia oxidation system——AIPBR reactor with flow field optimization,energy saving,high efficiency and stable operation was developed.The long-term operation performance of the process applied to simulate medium and low ammonia nitrogen wastewater treatment were investigated,and the nitrogen removal performance,sludge characteristics and the construction and stable operation of autotrophic nitrogen removal system were explored.The changes of microbial community structure and the strengthening mechanism of autotrophic nitrogen removal were comprehensively analyzed.The main results were as follows:（1）Based on the self-developed single-stage PN/A system,the single-stage autotrophic nitrogen removal system was successfully constructed within 36 days after long-term monitoring under medium ammonia nitrogen（NH₄⁺-N=250 mg/L）,combined with HRT and aeration regulation.After 215 days of continuous operation,the mixed system of deep red flock-granular sludge was gradually established.NRR increased to 0.84±0.02 kg-N/m³/d,AOR reached 0.64±0.02 kg-N/m³/d,and NOR remained extremely low（<0.01 kg-N/m³/d）.The diversity of microorganisms in the new single-stage PN/A system decreased continuously,and the main microbial species included Chloroflexi,Bacteroidota,Proteobacteria,and Planctomycetes.The relative abundance of AOB increased from 4.90%to 9.95%for Nitrosomonas.Candidatus Brocadia was dominant in AnAOB,and the relative abundance increased from 1.06%to 6.41%.NOB was dominated by Nitrospira,and its relative abundance was less than 1%.There were obvious DO zones in the inner and outer layers of the reactor.The DO concentration in the inner layer was higher,and the difference of DO concentration between the inner and outer layers remained in the range of 0.05-0.1 mg/L.The sludge separator realizes the division of flocculent sludge and granular sludge.Due to its high density,the granular sludge mainly slides down the outer wall of the diaphragm to the bottom of the reactor with low DO concentration to form a granular sludge active area,while most flocculent sludge circulates in the annular channel between the inner cylinder with high DO concentration and the separator to form a flocculent sludge active area.The double inhibition of NOB growth by low DO（<0.3 mg/L）and high FA concentration in the system was the key to achieve stable maintenance of single-stage autotrophic nitrogen removal.Internal circulation and multi zoning promote the granulation of sludge,strengthened the activity of denitrification functional bacteria,helped to quickly build a single-stage autotrophic nitrogen removal system with the symbiosis of flocculent sludge and granular sludge,realized the benign interaction between various functional microorganisms,and ensured the suitable growth and metabolism environment.（2）Based on the successfully constructed single-stage autotrophic nitrogen removal system of flocculent sludge and granular sludge,the key process parameters and stability of the new single-stage PN/A system were investigated under low ammonia nitrogen（NH₄⁺-N=50 mg/L）.When HRT was 4 h and aeration rate was 0.6 L/min,the nitrogen removal performance of the system was the best.ARE and TNRE were 77.29±1.68%and 54.94±1.71%,respectively,and NRR was 0.17±0.01 kg-N/m³/d.However,11.05±1.34 mg/L NO3--N was accumulated in the effluent,because the low FA concentration could not effectively inhibit NOB growth.After 45 days of continuous operation,the sludge changed from dark red to reddish brown.The Heme c concentration and the granulation degree decreased significantly.The biomass retention capacity of the system and the sludge concentration decreased.Compared with the medium ammonia nitrogen condition,the AOB activity was enhanced,AnAOB activity was significantly decreased,and NOB activity was significantly increased.The relative abundance of Nitrosomonas and Candidatus Brocadia decreased to 5.13%,while the relative abundance of Nitrospira increased to 4.25%.In summary,a novel single-stage PN/A system was developed.Under the condition of medium ammonia nitrogen,a single-stage autotrophic nitrogen removal system based on the symbiosis of flocculent sludge and granular sludge was constructed rapidly.The partition of dissolved oxygen and different forms of sludge was realized,and the symbiotic system of AOB-AnAOB synergistic interaction was formed,which strengthened the autotrophic nitrogen removal performance of the system.Under the condition of low ammonia nitrogen,the inhibition effect of the system on NOB activity was significantly weakened,the NOB activity was enhanced,and the nitrogen removal performance was difficult to further improve.Therefore,it is necessary to further study the inhibition or elimination strategy of NOB activity when using the new single-stage PN/A system to treat low ammonia nitrogen wastewater.This study provides a theoretical basis and technical support for the application of single-stage autotrophic nitrogen removal system in the treatment of medium concentration ammonia nitrogen wastewater,and provides a new idea or direction for the treatment of low concentration ammonia nitrogen wastewater.