Effects Of Phosphate On The Denitrification Performance Of Marine Anammox Bacteria In The Treatment Of High-salt Nitrogen-containing Wastewater

Nitrogen-laden saline wastewater is usually treated by biological method,but the salt stress caused by high salinity will affect the nitrogen removal performance of freshwater bacteria.Marine anammox bacteria（MAB）have high tolerance to salinity and can be used for nitrogen removal in saline wastewater.The phosphorus removal process is usually placed after the anammox process in municipal sewage treatment plants.Therefore,it is essential to study the effect of phosphate on nitrogen removal performance of MAB.In this study,the nitrogen removal efficiency of MAB in nitrogen-laden saline wastewater under various phosphate doses were investigated through long-term continuous experiment and short-term acute exposure batch experiment.The main conclusions are as follows:In the long-term effect test of phosphate on MAB biofilm,KH₂PO₄ was used as phosphorus source and its concentration varied from 0 to 400 mg/L.The phase of 5～30mg/L phosphate was the enhancement stage of nitrogen removal activity of MAB,and the time of complete ammonium removal was shortened by 0.5 h.The phase of 160～400 mg/L phosphate was inhibition stage,where the inhibition degree was proportional to the phosphate concentration.At the end of the experiment,the ammonium removal rate and nitrite removal rate decreased from 0.90 and 0.16 kg/（m³·d）in the initial stage to 0.45 and0.43 kg/（m³·d）,respectively.Therefore,it is advisable to control the influent phosphate concentration in sewage treatment plant below 160 mg/L.At 50～80 mg/L phosphate concentration,the time of complete ammonium removal returned to the original 4 h and the effluent nitrite increased with the increasing phosphate dosage.NH₄⁺-N was detected in effluent at 120 mg/L phosphate,and NO₂^--N exceeded 20mg/L.When phosphate concentration exceeded 160 mg/L,the calcium magnesium phosphate precipitation was formed in the reactor,which led to worsening nitrogen removal performance of MAB by hindering the contact between substrates and biomass.Besides,the presence of the calcium magnesium phosphate precipitation forced the bacteria to secrete a large amount of extracellular polymeric substances to protect them against the adverse environment.During the 158-day operation,MAB was still the dominant strain.When phosphate concentration was lower than 120 mg/L,the growth of MAB was promoted,but its relative abundance decreased by 15.4%at the end of experiment.The remodified Logistic model was appropriate to simulate the nitrogen removal process of MAB in saline wastewater,and the fitting results showed that appropriate concentration of phosphate was conducive to shorten the lag time of MAB.Using KH₂PO₄ and K₂HPO₄ as phosphorus sources could effectively reduce the impact of pH in the acute exposure batch test of MAB granules at high concentrations of phosphate.Compared with MAB biofilm,granules had a higher tolerance to phosphate.When phosphate concentration was 600～1000 mg/L,the ammonium removal efficiency（ARE）and nitrite removal efficiency（NRE）were 66.44%and 57.47%,respectively.When phosphate dosage reached 2000 mg/L,the surface of the bacteria was completely covered by the sediment,which seriously hindered the transport of substrate.At this time,the inhibition percentage was 49.26%,and NRE decreased to 42.47%,while ARE gradually increased due to the adsorption of NH₄⁺-N by sediment.The IC50 value calculated by fitting was 2021.16 mg/L,and the formation of sediment was the main cause of inhibition.The remodified Logistic model could simulate the nitrogen removal process of MAB well at 600～1000 mg/L phosphate,and the fitting results indicated that the lag time of MAB granule was prolonged at high concentrations of phosphate.