| To achieve safe,stable and efficient performance of the sewage treatment process,improve the quality of effluent,and reduce operating costs has become designers and operators' goals for a large number of wastewater treatment plants(WWTP).However,due to the complexity and variability of the wastewater treatment process,the traditional methods of process optimization often rely on engineering experience,need a long time,cost a lot,and maybe bring unpredictable risks in the future.As a new tool,mathematical simulation technology can be used to solve these problems.In this study,a process model was established based on BioWin software which can reflect the current operational status of a WWTP in Beijing,and used this model to optimize the operation parameters and propose the upgrading programme for this plant.According to performance and operation data analysis of the WWTP,the carbon insufficiency and water quality fluctuation in influent were the main causes leading to the TN and TP in effluent couldn't meet the discharge standard.In addition,design flaws worsened the removal of nitrogen and phosphorus.The plant's historical monitoring data was examined preliminary and checked for material balance.The sludge production was 0.87gTSS/gBODremoved,or 0.27gVSS/gCODremoved after verification.Sewage components parameters were determined after experimental testing and calculations,some of them were greater than the software default values: the fraction of readily biodegradable COD symbols Fbs was 0.191,the fraction of non-colloidal slowly biodegradable COD symbols Fxsp was 0.880 and the fraction of particulate unbiodegradable COD symbols Fup was 0.150,and others were less than the software default values.Nitrogen and phosphorus components were also different from default values.The normalised sensitivity analysis method was used to analyze the model parameters in BioWin software,and there were 11 parameters,including seven dynamics parameters,two chemometrics parameters and two switch parameters had the most prominent effect on the model output.Combined with the sensitivity analysis of model parameters,the model had a further correction.Finally,the sludge emission amount was adjusted to 650m3/d,the maximum specific growth rate for phosphorus accumulating organisms(PAOs)and ammonia oxidizing bacteria(AOB)adjusted from 0.95(default value)to 0.97 and from 0.9(default value)to 0.8 respectively,two physical and chemical parameters: the dissolved phosphate concentration when ferric salt was added in the equilibrium state of pH=7 adjusted from 0.01(default value)to 0.05 and ratio of iron and phosphorus adjusted from 1.6(default value)to 2.0.After correction of the model,the simulated values fitted the measured values better,and the model can reflect the actual performance of the WWTP.The adjusted model was used for scenario analysis of the plant optimization and modification,meanwhile the single factor analysis method and the composite factor analysis method were utilized.In the key control parameters,according to the current control of HRT,the distribution of biomass in each reaction unit could not meet the requirements,but it could be achieved by adjusting the pool volume,and optimization of pool volume was determined as follows: the volume of anaerobic tank,anoxic tank,aerobic tank and membrane tank were 2900m3,5900m3,4000m3,2000m3,respectively.The influence of the reflux ratio should be combined with other conditions to analyse,the mixed liquor return ratio of anoxic tank to anaerobic tank should remain unchanged at present,the return ratio from aerobic pool to the anoxic tank should be controlled in 200% ~ 300%,the return ratio from membrane pool to aerobic pool can be appropriately reduced under the condition of the flushing strength was enough and sludge concentration was not too high in membrane pool.It was also recommended that the amount of sludge discharge should be controlled between 650~1000m3/d and could appropriate increase sludge discharge in the currently running.The DO concentration in the aerobic tank and in membrane tank should be controlled between 1.5~2.5mg/L,5.0~6.0mg/L,respectively.The TN in effluent can meet the national standards and the Beijing Local B standards when using step feed process,and the influent flow distributed at 1:1 for anaerobic tank and anoxic tank.In order to make the effluent TN meet standards,the carbon source dosage should be controlled between 1200~1600m3/d(1L sodium acetate is equivalent to 269000 mg COD).In addition,the sodium acetate adding point changed to the beginning anoxic pool could reduce carbon source dosage.To ensure the effluent TP meet the national level A(TP<0.5mg/L),the chemical phosphorus removal dosage should be control above 2000L/d(the concentration of ferric iron is 140000mg/L)and changing the adding point to the front of membrane pool could further reduce the dosage due to the performance of biological phosphorus removal could be played fully.Moreover,employing chemical phosphorus removal to supernatant from sludge storage tank could save about 150 thousand yuan of dosage expense a year.Two sets of optimization and modification scheme were finally put forward through the mathematical simulation.The first scheme was as follows,setting the volume of anaerobic tank,anoxic tank,aerobic tank and membrane tank were 2900m3,5900m3,4000m3,2000m3,respectively,changing the MBR mixed liquor return to anoxic tank with 200% reflux ratio,the carbon source dosage was 360L/d and the dosage for phosphorus removal was 800L/d,applying 210L/d ferric salts to supernatant from sludge storage tank for phosphorus removal,increasing the amount of sludge discharge to 780m3/d,changing DO level of aerobic pool to 2.0mg/L,and with other conditions unchanged.The second scheme was as follows,changing the MBR mixed liquor return to anoxic tank with 200% reflux ratio and the return of anoxic tank to anaerobic tank with 200% reflux ratio,the carbon source dosage was 900L/d and the chemical phosphorus removal dosage was 2000~2300L/d,applying 210L/d ferric salts to supernatant from sludge storage tank for phosphorus removal,increasing the amount of sludge discharge to 650~1000 m3/d,changing DO concentration of aerobic pool to 2.0mg/L,and with other conditions unchanged.The simulation results showed the effluent quality under conditions of both schemes could meet the national level A standard,and also meet Beijing local B standard,moreover,either of them could reduce energy consumption and operation cost. |
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