Performance Of The Step Feeding A/O Biofilm Nitrogen Removal Process

With the rapid development of society and the constant improvement of the standard ofliving in China, nitrogen contamination of water bodies becomes more and more serious.Conventional biological nitrogen removal processes have disadvantages such as a largeamount of excess sludge, high operating cost, large footprint and big investment. Researchersengage in the development of new biological nitrogen removal processes and theimprovement of conventional biological nitrogen removal processes actively. With theadvantages of high total nitrogen （TN） removal efficiency, less excess sludge, less investmentand lower operating cost, step feeding Anoxic/Oxic （A/O） biological nitrogen removalprocess has attracted a great deal of interest.An optimized method which can distribute the influent and the volume of each treatmentunit of step feeding A/O biofilm nitrogen removal process simultaneously was proposed inthis study for improving the performance of the process. Theoretical analysis, experimentalstudy and numerical simulation based on the biochemical reaction kinetics of the process werealso performed. According to the survey data of the quality and quantity of the wastewaterdischarged from an operating expressway service area and the theoretical analysis and theexperimental results of the step feeding A/O biofilm nitrogen removal process, the optimizedoperation strategy of the step feeding A/O biofilm nitrogen removal process dealing with theexpressway service area wastewater was proposed. The following conclusions were drawn:（1） The influent of the step feeding A/O biofilm nitrogen removal process wasdistributed by the principle of COD andNO₃^-—Nfed into the anoxic units of the processwith a fixed ratio which could make the process achieve the maximum TN removal efficiencyin theory. By comparison the effluent quality of the process under the optimization mode withthat of under the equal flow rate distribution mode, it was indicated that, the influentdistributed with the optimized method could improve the TN removal efficiency of theprocess. Flow rate distribution coefficient of the process was in relation to steps of the process,influent COD-to-TKN ratio （C/N ratio） and COD-to-NO₃^-—Nratio （α，mgCOD/mgNO₃^-—N）of the influent of the anoxic unit;（2） The maximum TN removal efficiency of the step feeding A/O biofilm nitrogenremoval process was in relation to influent C/N ratio, return ratio （R） and α. When the influentC/N ratio was relatively high, namely m≥α, the TN removal efficiency was increased with increasing of R; When the influent C/N ratio was relatively low, namely m＜α, there was acritical return ratio R_m, when R＜R_m, the TN removal efficiency of the process was increasedwith increasing of R, and when R≥R_m, the TN removal efficiency of the process was a fixedvalue and had nothing to do with R;（3） There was an optimum value of α in the flow rate distribution and the volume designof each treatment unit of the step feeding A/O biological nitrogen removal process. Bynumerical simulation based on the activated sludge model NO.1（ASM1）, α could equal to7mgCOD/mgNO₃^-—N. By experimental verification, it was proved that the value of α equaledto7mgCOD/mg was appropriate;（4） Under the equal flow rate distribution mode, R had certain effects on the TN removalefficiency of the three-step feeding A/O biofilm nitrogen removal process. When R was lowerthan3/4, the TN removal efficiency of the process was increased with increasing of R, andwhen R equaled to1, the TN removal efficiency of the process was just a little lower than thatof when R was3/4;（5） When the COD andNH+4Nconcentration of the influent were1000mg/L and100mg/L respectively, hydraulic retention time was36h, and return ratio was1/3, the TNremoval efficiency of the three-step feeding A/O biofilm nitrogen removal process under theequal flow rate distribution mode and the optimization mode were80.3%and88.8%respectively. The method proposed in this study, which optimized the flow rate distributionand the volume distribution of each treatment unit of the process simultaneously, can improvethe TN removal efficiency of the process;（6） The multi-substrate and multi-species zero-order1-D biofilm kinetic modelconstructed in this study was applicable to simulate the steady operation result of thethree-step feeding A/O biofilm nitrogen removal process, and the model can properly simulatethe effluent quality of each treatment unit of the process. When the operation parameter of theprocess was varied, the model can simulate the dynamic variation of the water quality and thebiomass in the reactor which was help for understanding the biochemical phenomenon in thereactor and had practical meaning for guiding the experiment;（7） Wastewater discharged from the expressway service area was domestic wastewaterwith high-strength COD andNH+4N, and in design of expressway service area wastewatertreatment facilities, the value of the hourly variation factor should not be decided by Code forDesign of Outdoor Wastewater Engineering（GB50014-2006） or that of conventional smallflow rate domestic wastewater. More uniform distribution of the traffic volume into the service area weakened the hourly variation intensity of the wastewater flow rate.