Comparison Of Autotrophic And Heterotrophic Denitrification Processes For Nitrate Removal From Micro-Polluted Surface Water

The Qiantang River diversion project,which was built in 1986,diverts 400000 tons water per day from the Qiantang River into the West Lake after treatment by the surface water pretreatment plant.The water diversion project has greatly improved the water environment condition of the West Lake,and the removal efficiency of phosphorus is relatively high after the existing pretreatment technology of coagulation and sedimentation.However,the pretreatment technology has no effect on total nitrogen removal,and the content of total nitrogen is higher than 2.0 mg/L?nitrate nitrogen accounts for 80%⁹0%of total nitrogen?.When the pretreated water,in which the total nitrogen are out of standard enter the West Lake,it will boost the growth of algae and greatly affect the landscape effect of the lake.Therefore,in order to further improve the water quality of the West Lake to grade IV in Chinese National Surface Water Environmental Quality Standard?GB3838-2006?,it is necessary to remove nitrogen from the water diversion of the Qiantang River.Based on the above background and the water quality characteristics of low phosphorus and high nitrogen after coagulation and precipitation pretreatment of Qiantang River diversion water source,systematic comparative tests of autotrophic and heterotrophic denitrification biofilter processes for nitrate removal from micro-polluted surface water were investigated in this study.Besides,technology and economic analyses of the two processes were also carried out,and a selected optimal denitrification process was constructed as demonstration project.This study could provide theoretical and technical support for nitrogen removal from the water diversion of the Qiantang River,and the main conlusion are as follows:?1?The optimal molar ratio of S₂O₃^2-and NO₃^-?S/N ratio?and mass ratio of COD and NO₃^-?C/N ratio?were 1.2 and 7 for the autotrophic denitrification biofilter?ADBF?and the heterotrophic denitrification biofilter?HDBF?,respectively,when the ADBF and the HDBF treated synthetic phosphorus-limited?P-limited?nitrate micro-polluted surface water at low water temperatures?13±2 ??.TN removal efficiencies of the ADBF were 20.2±7.6%,66.4±22.7%and 80.3±12.8%,and utilization efficiencies of S₂O₃^2--S were 99.5±0.8%,98.9±0.9%and 93.2±3.4%at S/N ratios of 1.0,1.2 and 1.5,respectively.TN removal efficiencies of the HDBF were 35.9±15.3%,63.4±13.1%and 52.5±15.9%,and utilization efficiencies of carbon sources were 36.8±9.3%,43.8±13.1%and 30.0±17.0%at C/N ratios of 5,7 and 9,respectively.?2?The appropriate addition of phosphorous sources could greatly enhance the resistance of the P-limited ADBF and HDBF to hydraulic loading shock.When water temperature and sulfur?or carbon?source were nonrestrictive factor and treating synthetic P-limited nitrate micro-polluted surface water,TN removal efficiencies of the ADBF were 79.8±10.5%and69.6±9.2%,and TN removal efficiencies of the HDBF were 66.7±9.2%and 31.9±5.3%at hydraulic retention time?HRT?of 1 and 0.5 h,respectively.However,when phosphorus source was nonrestrictive factor,TN removal efficiencies of the ADBF increased to 91.7±3.1%and92.2±2.5%,and TN removal efficiencies of the HDBF sharply increased to91.0±3.3%and 93.3±1.0%at HRT of 1 and 0.5 h,respectively.?3?The lack of phosphorus inhibited heterotrophic denitrification more greatly than autotrophic denitrification,which was related to the relative abundances of dominant functional bacteria in two denitrification systems.The dominant functional genus in autotrophic denitrification system was Thiobacillus,whose relative abundance was less affected by phosphorus limitation.However,dominant functional bacteria in heterotrophic denitrification system were denitrifying phosphate-accumulating organisms,their growth and metabolism were inhibited and relative abundances were greatly reduced when phosphorus source was insufficient,thus,this greatly influenced denitrification performance of the HDBF.?4?Nitric oxide?N₂O?emission flux and global warming potential?GWP?of the ADBF were both significantly less than those of the HDBF.N₂O emission fluxes of the ADBF were 0.60±0.25 and 0.63±0.15 mg N₂O-N×m^-3×d^-1,and GWP values were 0.086±0.022 and 0.013±0.007 g CO₂×d^-1 at water temperatures of 28±2 and 17±2 ?,respectively.However,N₂O emission fluxes of the HDBF were 3.52±2.09 and 1.26±0.70 mg N₂O-N×m^-3×d^-1,and GWP values were 0.162±0.056 and 0.046±0.036 g CO₂×d^-1 at water temperatures of 28±2 and 17±2 ?,respectively.?5?The resistance to hydraulic loading rate?HLR?for heterotrophic denitrification was stronger than autotrophic denitrification.When treating real nitrate micro-polluted surface water in pilot-scale,NO₃^--N and TN removal efficiencies of the ADBF decreased from 86.2%to 51.3%and from 65.0%to 34.3%with HLR increasing from 6.0 to 7.5 m³×m^-2×h^-1,respectively.However,NO₃^--N removal efficiencies of the HDBF reached up to 87.5%,87.3%and 87.1%,and TN removal efficiencies reached up to83.1%,90.3%and 89.0%at HLRs of 6.0,7.5 and 8.6 m³×m^-2×h^-1,respectively.?6?The effect of backwashing on denitrification performance of the ADBF was significant,and there would be a long period of substandard water discharge after backwashing.However,the effluent TN of the HDBF could still meet the requirement of grade IV in Chinese National Surface Water Environmental Quality Standard?GB3838-2006?after backwashing.Besides,denitrification performance of the HDBF could completely recover in a short time?12⁴8 h?,which provided a guarantee for the system to maintain high-efficient denitrification efficiency during the next operation cycle.?7?From the perspective of technical and economic analyses,the HDBF was more suitable to treat nitrate micro-polluted surface water in order to achieve the goal of demonstration project.The long-term operation effect of the demonstration project with the capacity of 5?10⁴m³/d showed that the effluent TN of the full-scale HDBF could reach grade IV or even grade III or II in Chinese National Surface Water Environmental Quality Standard?GB3838-2006?when the water temperature was in a suitable temperature range?>18 ??.In addition,the water quality could be guaranteed by adding 0.1 mg PO₄^3--P/L to the influent at low water temperatures?13±2 ??.