Study On The Biological Mechanism Of Simultaneous Hydrolysis And Nitrification Of Urea And The Nitrogen Removal Performance Of Coupled Anammox

China is a large urea production country and its annual output has consistently ranked first in the world.In 2020,the domestic urea output has reached more than 70 million tons,and 1 ton of urea produced will produce about 0.5 tons of wastewater.As China’s requirements for environmental protection are getting more stringent,the maximum concentration of urea discharged from the urea plant is now reduced from the previous100mg/L to 10 mg/L.The most mature and popular treatment method in the world is the thermodynamic hydrolysis method,the urea and ammonia nitrogen content of the treated wastewater can meet the discharge standards,but the treatment cost is very expensive.Only used by medium and large urea plants generally.Half of the domestic urea production comes from small and medium-sized urea companies,and most of the processes they use are the aqueous solution full-cycle method.The urea and ammonia nitrogen content in the discharged wastewater could be partly removed,but can not meet discharge standard.The use of thermodynamic hydrolysis not only has high investment costs but also high operating costs.However,the use of biological hydrolysis method not only enables the discharge of wastewater to meet the standards,but also has extremely low operating and infrastructure costs.This study uses a combined biological process of urea SBR-anammox,first to hydrolyze all the urea in the SBR to NH₃ and CO₂ under aerobic conditions using autotrophic bacteria,and then the AOB in the SBR converts part of the NH₃into NO!",the activity of NOB in the system is inhibited,only NO!"and NH_#^$in the effluent,and the ratio of NO!"/NH_#^$in the effluent is about 1-1.32 by controlling the dissolved oxygen and alkalinity,and finally effluent that meets the ratio enters the ANAMMOX reactor for nitrogen removal,so as to realize the harmless treatment of urea wastewater.This study first studied the mechanism of simultaneous hydrolysis and nitritation of urea in SBR.In terms of urea hydrolysis performance,SBR can treat up to 2000 mg/L urea wastewater,and the effluent urea is below 10 mg/L.In the normal operation stage,the removal rate of urea is as high as 98%,and the removal load rate of urea（calculated as Kjeldahl nitrogen）reaches 1.868 kg N/（m³·d）,And in terms of nitritation capacity,the nitrite accumulation rate in SBR also increases to 0.9847 kg N/（m³·d）with the increasing concentration of influent urea wastewater,so that the ratio of NO₂²/NH₄⁺ in the effluent of SBR could be maintained above 1.In terms of urea hydrolysis mechanism,through the study of urea hydrolysis characteristics and nitrification performance in SBR,it is found that bacteria responsible for urea hydrolysis in the SBR are Gemmatimonadetes,Nitrosospira and Nitrosomonas.nitrosa,the latter two are known as AOB.Among them the acidic environment SBR with artificially adjusted p H are enriched with Nitrosospira and Gemmatimonadetes.In this case,the urea hydrolysis capacity of SBR can reach up to2000 mg/L.It is speculated that the mechanism of urea hydrolysis is through accumulating a large amount of urea inside cell,then it is hydrolyzed into NH₃ and CO₂ by intracellular urease.Nitrosospira then directly converts the NH₃ produced in the cell into NO!",while Gemmatimonadetes obtains energy from urea hydrolysis and transfer the intracellular NH₃to the bulk for following utilization by AOB;in the urea SBR without artificial p H adjustment,Nitrosomonas.nitrosa and Gemmatimonadetes are usually largely enriched.In this case,when the influent urea concentration reaches 1000 mg/L the urea hydrolysis capacity of the system has reached the upper limit,and continuing to increase the influent concentration will increase the urea in the SBR effluent,and the nitritation performance of the system will also decrease.Through the analysis of continuous sampling in typical cycles,it is found that the ammonia nitrogen and p H in this system have a tendency to rise sharply and then decrease,so it is speculated that the urea in this system is hydrolyzed by extracellular urease.The analysis of the results of high-throughput sequencing also confirmed the above speculations.In the SBR that has not been manually adjusted to an acidic environment,the enriched dominant bacteria are Nitrosomonas and Gemmatimonadetes,with relative abundances of 7.3%and 17.4%,respectively.After p H adjustment,the dominant bacteria change into Nitrosospira and Gemmatimonadetes,and the relative abundance was 36.4%and 14%.Meanwhile,the relative abundance of Nitrosomonas dropped to 1.09%.This conclusion indicates that enriching Nitrosospira under an acidic environment can increase the treatment capacity of urea wastewater.In study of nitrogen removal performance of urea SBR coupled ANAMMOX,the combined process achieved a total nitrogen removal rate of 1.08 kg N/（m³·d）,which is equivalent to a urea treatment capacity of 2.313 kg N/（m³·d）,The final influent nitrogen concentration reaches 550 mg/L,the total nitrogen removal rate is stable at 82-85%,and the ammonia nitrogen removal rate maintained at 96-98%,which shows that it is feasible to use SBR coupled ANAMMOX process to achieve nitrogen removal of urea wastewater.