Achieving Advanced Nitrogen Removal In Mainstream Anammox Process Through Multiple Nitritation

The partial nitrification-anammox（PN/A）process has attracted wide attention in recent years.However,there remain many challenges associated with the practical application of PN/A process in sewage treatment,with the accumulation of nitrite oxidizing bacteria（NOB）being the main challenge frequently reported.NOB accumulate easily and compete with anaerobic ammonium oxidation bacteria（An AOB）for nitrite,leading to an increase in effluent nitrate concentration and poor nitrogen removal performance.Furthermore,the abundance of An AOB decreased during long-term operation,which is unfavourable to system stability.To solve the above problems,the effects of different nitrite production rates on the competition mechanism between NOB and An AOB;the mechanism of synergistic nitrogen removal between NOB and An AOB were investigated.Finally,the effects of different influent chemical oxygen demand（COD）concentration on the succession rule of endogenous partial denitrification bacteria and the synergistic nitrogen removal with An AOB were investigated.The main research results are as follows:（1）It was proposed that nitrite production rate was the key factor affecting the competition between An AOB and NOB for nitrite under intermittent aeration condition.With the dissolved oxygen（DO）increased from 0.4 to 0.6 mg/L,although the abundance of Nitrospira increased to 1.5×10¹⁰copies/L,the abundance of Nitrosomonas increased from 0.14%to 0.22%,providing more nitrite for An AOB and promoting its enrichment（grew by 77.4%）.Adding extra nitrite of 7.14 mg N/（L·h）during the aeration phase to reactor could further increase An AOB abundance by117.6%,which was higher than the control reactor（40.2%）.The results suggested that An AOB had a competitive advantage for nitrite over NOB with increasing nitrite production rate.（2）It was proved that low DO concentration might induce Nitrospira to perform metabolic functions and coordinate with Candidatus Brocadia for nitrogen removal.In this study,the mainstream PN/A reactor was started by inoculating the nitrification/denitrification activated sludge and anammox granular at the inoculation ratio of 25:1,and operated under intermittent aeration（DO concentration of 0.10-0.12mg/L）.Total inorganic nitrogen（TIN）removal efficiency reached 96.4±1.7%,indicating adcanced nitrogen removal.Mass balance analysis showed that 92%of nitrogen was primarily removed via anammox and nitrite oxidization was effectively inhibited.However,the abundance of Nitrospira,the main NOB in PN/A system,increased significantly from 0.70%to 5.49%,showing similar upward trends with Candidatus Brocadia（dominant An AOB）.Metagenomic sequencing results showed that Nitrospira had a high abundance of amo（ammonia→nitrite）gene,which indicated that Nitrospira had the potential of partial nitrification,which could provide substrate for Candidatus Brocadia,promote the growth and enrichment of An AOB,and improve the nitrogen removal performance of the system.For the partial nitrification function of Nitrospira,low DO concentration might be the major trigger.（3）The influence of influent COD concentration on endogenous partial denitrification bacteria succession was exploredThe PN/A system was operated under the condition of intermittent aeration with low DO concentration（DO:0.10-0.12mg/L）.When the influent COD concentrations decreased from 174.8 to 70.4 mg/L,TIN removal efficiency decreased from 96.1±2.0%to 82.4±6.4%,but after long-term operation,the nitrogen removal efficiency became poor.This could be due to carbon sources.When COD concentration increased to 97.0mg/L,the TIN removal efficiency recovered（91.0±3.3%）.With influent COD concentration decreasing from 174.8 to 97.0 mg/L,the abundance of endogenous partial denitrification bacteria Candidatus Competibacter decreased from 7.4%to 0.5%,while the abundance of Thauera increased significantly to 8.1%.This indicated that endogenous partial denitrification dominant bacteria succession,which would continuously provide nitrite for An AOB and promote its growth and enrichment.In addition,the abundance of Nitrosomonas increased to 3.0%to provide more substrate for An AOB.Under low DO and low influent COD concentrations condition,the system achieved efficient and stable nitrogen removal.Overall,the process has great potential to achieve cost-effective and energy efficient municipal wastewater treatment.