| Orbal oxidation ditch process has been studied and applied widely because itexhibits advantages of extended aeration process and simultaneous nitrification anddenitrification process. Simultaneous nitrification and denitrification is the majorapproach for nitrogen removal in the process, and the distinguish feacture of the processis the0-1-2(mg·L-1) oxygen distribution in outer-middle-inner channels, respectively,which makes it easy to control the oxygen concentrations. However, with the changes ofwater quality and quantity, the operation condition deviates a lot from the designedcondition （e.g. the just case under this study）, and it is still unclear how to effectivelyoperate the oxidation ditch in such conditions.In this project study case, the effect of nitrogen removal of oxidation ditch reliesheavily on the ammonia removal so that nitrification has become a bottleneck factor forthe process operational efficiency, while the effect of denitrification is not so obvious.Nevertheless, present oxidation ditch technology of operation theory is basicly from thesimultaneous nitrification and denitrification, but it is not clear on the operation strategythat Orbal oxidation ditch enhances nitrification effect alone. Aiming at this problem,based on the analysis of traditional water quality and sludge parameters, the projectposts a test on the dynamics with a respirometer to discuss the methods to improveoxidation ditch’s nitrification capability by presenting related quantitative operationchart.The main results of this research are as follows:1. Some parameters in the oxidation ditch have been long-term monitored including ammonia nitrogen concentration of influent and effluent, sludgeconcentration（MLSS）, ammonia nitrogen concentration in the outer channel; throughthe comparative analysis, the importance of outer channel for the nitrification of thewhole system can be verified; The main reason of the low nitrification capability of theOrbal oxidation ditch process is the fail operation of the outer channel; and change ofMLSS has insignificant effect on nitrification. Respirometry was used in this study, andthe results showed that autotrophic bacteria concentration in the Orbal oxidation ditch is93.08mgCOD·L-1, which accounted for1.79%of MLVSS concentration.2. The concentration and distribution of dissolved oxygen （DO） in outer, middleand inner channel has been measured verticaly and horizontaly. An obvious verticalstratification of DO in outer channel cross section was found when it was deeper than0.5m, while the distribution of DO in middle and inner channel was more uniform. Theaverage DO of each channel was estimated as0.08mg·L-1,0.47mg·L-1and1.40mg·L-1,respectively. Velocity and oxygen distribution were improved effectively by addingbaffles, the great influence of the velocity distribution in cross section on DO level wasconfirmed, the distribution trend of velocity and DO was consistent in the aerationsection of outer channel, both exhibited significant stratification, the low DO in theouter channel and the limit of equipment oxygen transfer ability may be ascribed to theuneven flow velocity distribution in the disc section.3. An experiment has been carried out to test the theoretical nitrification capabilityof outer channel depending on the nitrification rate of kinetic parameters, and theaverage nitrification rate of outer channel is5.94mg·L-1·d-1. This result indicates thatimproving the outer channel DO could improve the nitrification capability of the system.Under the current operational conditions (raw water’s ammonia-N concentration of110mg·L-1and COD concentration of300mg·L-1), even if the outer channel DO reaches upto0.61mg·L-1（much higher than the oxygen capacity of current system）, the ammonianitrogen removal ability is only10mg·L-1; therefore, only improving the oxygen supplycapacity of the equipment can not meet the requirement of effluent standards. From thedistribution of oxygen consumption, sludge endogenous respiration has a large oxygendemand, accounting for43.68%of the total oxygen demand,2folds as that undernormal condition. Therefore, too high MLSS will worsen the system’s insufficient on oxygen supply capability, so that the trade-off between DO and MLSS becomes a keyissue. In order to resolve this problem, this study, based on measured data, will carry outrelevant theoretical calculation to set forth the first set of control diagram to enhancenitrification capacity, with optimal control interval for the above two factors.4. From the point of view of the hydraulic conditions of oxidation ditch, the twoparameters of nitrification fraction （fn） and nitrification hydraulic retention time （HRTN）are applied. The nitrification fraction of the oxidation ditch’s outer channel is0.14,which is low to some extent, so that the aeration volume ratio of the total volume shouldbe increased. When the amount of ammonia nitrogen removal （SNE） is constant, toshorten HRTNcould cut the fndemand of the system （aeration volume ratio decreased）.In this way, measures to be taken include strengthening the nitrification performance,increasing the autotrophic bacteria concentration, decreasing the flux load or increasingF/M, etc. When the nitrification rate is constant in the system, to increase fn（aerationvolume ratio increased） could make HRTNgrow, which could be achieved byimproving DO distribution, increasing aeration area （by adding more baffles） orincreasing the oxygenation capacity of aerator. At the same time, according to these twoparameters, the study estabilished another control diagram to enhance nitrificationcapacity which can quantitatively select the nitrification hydraulic retention time andnitrification fraction.With the data from the respirometer as basic parameters, this study performssuccessful analysis on the operational status quo of Orbal oxidation ditch and establishsrelevant parameters and methods through quantitative evaluation on nitrificationperformance, giving a good example for the actual operation and managementapplication with respirometers in sewage treatment plants. Based on the above studyand analysis, this work proposes two sets of control diagram to enhance nitrificationcapacity, which will be of significance to the operation management of sewagetreatment plants. |
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