Study On Technique Of Two-Stage SBR Process For Phosphorus And Nitrogen Removal And Its Operational Controlling Parameters

At present, biological phosphorus and nitrogen removal process used widely is a combined process in the wastewater treatment engineering; it is based on the traditional theory. In this system, because the different function microbes grow in the same system, especially mixed growth of the long generation time autotrophic nitrifying bacteria and heterotrophic bacteria, some problems will inevitably arise , such as inconsistency of sludge retention time(SRT)between nitrifying bacteria and polyphosphate accumulation organisms(PAOs), the competition for carbon source between the denitrification and phosphorus releasing processes, the influence of nitrate in anaerobic zones on the phosphorus removal and so on. These problems make it hard to synchronously remove phosphorus and nitrogen effectively in usual. In order to alleviate these paradoxes, people have done many efforts to improve the process structure, and developed many new processes to prevent nitrate from going into the anaerobic zones. However, these processes not only increase the complexity of the structure and control of the processes, but also increase the foundational construction and the run costs at the same time. In addition, these processes can not solve the problems of sludge age very well between nitrifying bacteria and PAOs, because the paradoxes are the inherent of the mixed microbial growth system and can not be settled by the system itself. On the other hand, discharged total phosphorus (TP) of biological phosphorus removal system which adopt sedimentation to separate solid and liquid can not reach the first rank "a"level (TP≤0.5mg/L) of the current national standards, because of the phosphorus releasing of rich phosphorus sludge before discharge and the existing of rich phosphorus sludge particles inevitably in the wastewater treated. At present, how to settle these paradoxes is an important issue desiderated to be resolved, to explore economical and high efficiency simultaneous phosphorus and nitrogen removal process running under the normal atmospheric temperature, to find the potential of conventional biological phosphorus and nitrogen removal process, to fit the increasing demands in the wastewater discharge standard. In this work, by analyzing the current condition of the biological phosphorus and nitrogen removal process and the problems of the processes, considering the environment requirement of the biological phosphorus and nitrogen removal in general, integrating the flexible characteristic in the running of SBR process, two-stage SBR system used in series. By controlling SRT, make nitrifying bacteria and PAOs grow optimally in the two-stage reactor respectively, which can form the running pattern of removing phosphor(removing most of the organic materials at the time) first, and nitrogen removal in succession. These measures can settle the problem between autotrophic nitrobacteria and heterotrophic bacteria sludge age, and reduce the TP concentration of outlet sufficiently, and can reach the optimum phosphorus and nitrogen removal finally. The two-stage series SBR reactor results from the experiences of AB process. In each of the reactors we can cultivate different obligate bacteria, which fit to degrade different substrate so that the substrates can be degraded sufficiently under different appropriate biochemical condition. Peng Yongzhen et al. employ two-stage SBR process to treat chemistry industry wastewater, the process can incubate the activated sludge which is feasible in treating acetic acid and aromatic organic material, this kind of activated sludge can solve the problem of "glucose effect"and improve the treatment efficiency remarkably. Fang Shi and Zeng Wei adopt two-stage SBR process to treat monosodium glutamate and petrochemical wastewater and get good effect on the organic and nitrogen removal. But objective of the two-stage SBR system is to optimize phosphorous and nitrogen removal, to make nitrifying bacteria and PAOs grow optimally in the two-stage reactor respectively, and remove phosphorous anddenitrify effectively during the process of the organic removal. There are no reports about this issue. The traditional auto-controlling way of SBR system was mainly a fixed time program; its limitation was that it could not adjust the system's operation state according to the inlet water quality; so it is difficult to use self-adjust auto-controlling. Therefore, it is system's steady, economic and effective operation guarantee to look for steady and available process controlling parameters for two-stage SBR system which can reflect system state timely. Under the condition of two-stage SBR system stable operation, the simulated urban wastewater is used to study three aspect contents. They are summarized as follows: 1. Superiority and performance of the two-stage SBR process for phosphorus and nitrogen removal The feasibility and its controlling operation condition of the optimized phosphorus and nitrogen removal in different stages of two-stage SBR system is studied comprehensively and systematically, the compassion of experiments with traditional SBR process combined with mechanism analyses are used as study methods. The superiority and performance of the two-stage SBR system for phosphorus and nitrogen removal are studied systemically. The results indicate that: (1) Traditional single-stage SBR system is difficult to resolve the inherent paradox of the system in the disharmony of SRT between nitrifying bacteria and PAOs, the inhibition of nitrification by a great lot PAOs increment with using PHB in the cell and largely absorbing phosphorus at the aerobic initial phase and so on. It results in the system effective phosphorus and nitrogen removal hard at the same time. (2) Two-stage SBR process is used to make the optimum growth of PAOs, and nitrifying bacteria in two-stage reactor respectively by controlling SRT successfully, which can complete better the two paradox biological treatment process, viz. phosphorous and nitrogen removal in two-stage respectively. Underthe condition of better outlet water quality, treatment efficiency of two-stage SBR is twice as one of traditional SBR. (3) The SBR biological phosphorus removal system of sedimentation separation: ①TP of the outlet can less than 1.0mg/L, but can not less than 0.5mg/L; ②at the beginning of the sedimentation, POAs can release phosphorous, and its rate increase about 10min of sedimentation and then basically maintains constant; ③after 20min sedimentation, TP increase resulted from releasing phosphorous is large than TP decrease resulted from suspension removal by the sedimentation; ④settlement time is shorten in satisfying settlement requirements, and suggesting choice time is about 30~40min. (4) Two-stage SBR system is easy and steady to realize the final outlet TP satisfied the first rank "a"level (TP≤0.5mg/L) of National Standards. It does not been realized by using traditional biological phosphorous removal system at present. (5) In the nitrogen removal stage of two-stage system (SBR2), heterotrophic bacteria of degrading difficult degradable organics material can be cultivated besides predominant nitrifying bacteria. SBR2 system can remove little remained difficult degradable organics material further based on the most of the organics removed in the SBR1, and the COD concentration of the discharged water is lower than traditional single stage SBR system. (6) Increasing the biodegradability of organics material and P load of the inlet appropriately can induce anaerobic phosphorous releasing and aerobic phosphorous adsorption of POAs, and can increase removal load of the systematic phosphorous. (7) The remained liquid of the former period in reactor has adverse impacts on the next period metabolizing process of the POAs. Reduction of the remained liquid can enhance anaerobic phosphorous releasing and aerobic phosphorous adsorption ability of POAs, and can increase stability of the system and the phosphorous removal load. 2. Study of controlling parameters of phosphorousremoval stage The change regulations of pH, ORP in the anaerobic phosphorous releasing process and pH, DO, ORP in the aerobic absorption phosphorous process are studied by comprehensive and systemic experiments and profound mechanism analysis. The general rules of pH and ORP as well as pH, DO and ORP have been concluded in the two processes, which are summarized as follows: (1) There is good correlation between anaerobic phosphorous releasing and the variation of pH and ORP in the mixed liquid; in addition, under the different inlet C/P conditions, both parameter of pH and ORP could offered the evident and steady signal in the end of phosphorous releasing process, that is in the end of anaerobic phosphorous releasing process, the pH curve from increase to basic parallel with the time coordinate appears inflexion; decreasing rate of ORP curve reduces obviously and ORP curve appears inflexion and ORP≈-190~-200mV, This discovery is significant for real time controlling of the anaerobic phosphorous releasing process. (2) In the aerobic phosphorous absorption process, there is a good correlation between DO, ORP varying and the PHB degradation in the PAOs cell. Degradation degree of the PHB in the cell can be reflected by varying of DO and ORP. The "peak"appeared simultaneously in both dDO/dt curve and dORP/dt curve suggests the end of PHB degradation. (3) The pH curve rises quickly during the aerobic PHB degradation. When the PHB degradation is finished under the sufficient or more sufficient carbon sources condition, the pH curve stops rising and emerges the "protruding point"or "flat", and corresponding dpH/dt curve emerges the straddling "zero point"or "dpH/dt≈0 flat"; additionally, the characteristic point with the "peak"in the dDO/dt and dORP/dt curve emerge at the same time. When the PHB degradation finishes under the condition of insufficient carbon sources, the rising rate of pH curve reduces, but the characteristic point ("protruding point"or "flat") of stopping increase above pH curve is behind "peak"of dDO/dt curve and dORP/dt curve. The pH varies mainly depend on the dynamic relationshipbetween the degree of nitrification reaction and the blow of CO2 after appearing the pH characteristic point. (4) During the PHB degradation, the phosphorous absorption rate of PAOs is almost constant. When the PHB degradation ended, the PAOs absorption rate doesn't decrease right now, but decreased gradually in a period. The effective phosphorous absorption time of this period is 40 minutes more or less under the condition of existing phosphorous for the mixed liquid. (5) Under the carbon sufficient condition, {①"fast-increasing zone"of DO and ORP curve;②"peak point"of the dDO/dt and dORP/dt curve; ③characteristic point ("protruding point"or "flat") of pH curve; or ④characteristic point of dpH/dt=0 in dpH/dt curve} appear at the same time. They served as indicator of the approach end of phosphorus absorption together (PO43--P<1mg/L in mixed liquid). Aeration process is finished in short time (about 3~10min, and if C/P is high, use lower limit) after these characteristic points appear; in this time, phosphorous absorption effect in mixed liquid is PO43--P≈0mg/L; if carbon source is basically sufficient, aeration time is properly prolonged (not more than 40min) and aeration process is finished according to the inlet C/P after four characteristics appear at the same time, which also can reach phosphorous absorption effect of PO43--P≈0mg/L in mixed liquid; if carbon source is insufficient, according to ①and ②c haracteristics, it is the best optimum controlling plan that aeration is finished in 40min after the ①and ②c haracteristics appeared . To biological treatment system for removing phosphorous only, carbon source of the urban wastewater can satisfy or basically satisfy the requirements of treatment, and total alkalinity close to the simulated wastewater used in this work. Therefore, if the same aeration manner is adopted as this work (used in engineering usually), it has far-reaching significance in the treatment urban wastewater under the sufficient and basically sufficient carbon source conditions. (6) The total alkalinity of the mixed liquid is decreased a little in the anaerobic phosphorous releasing process, and a little alkalinity is also consumedin aerobic phosphorous absorption process. These two processes of consuming alkalinity mainly occur in the unsteady anaerobic phosphorous releasing and aerobic phosphorous absorption. As anaerobic phosphorous releasing process reaches steady and the aerobic phosphorous absorption process reaches steady before the nitrification occurs, pH and total alkalinity reach steady simultaneously. If a certain nitrification is appeared in the end of aerobic period, the total alkalinity decreases further. 3. Study of controlling parameters of nitrogen removal process On the basis of the national and foreign research achievements, the further study of SBR biologic nitrogen removal is performed about the general rules of pH, DO, ORP in the full course nitrification process and that of pH, DO in the full course denitrification process, and we have gotten some new conclusions that are different from others's opinions, in addition, the adequate mechanism analysis is performed, and achievements are improved. In this stage, tests which make steady full course nitrification process transit to short course nitrification process are performed by controlling temperature and DO. Main results are summarized as follows: (1) NH4+-N is a limitation factor of ammonia oxidation, and its threshold concentration is very low, about 3~5mg/L (use the lower limit if temperature is high). (2) In the steady nitrification stage, pH curve linear decreases, and flat appears in DO curve; Approached to ammonia oxidation end, pH curve appears "valley point"; in the end of the ammonia oxidation, increase extent of DO curve is large; the characteristics of pH and DO are obvious and available, and appearing interval is not more than 5min, which can be used as indicator of ending aeration process together. When characteristic of pH appears, although a little NH4+-N even is not oxygenated in mixed liquid (<2mg/L in general), it can be removed by denitrifying bacteria assimilation in the denitrification process. Therefore, combination of appearing pH characteristics with initial fastincreasing signal of DO can be used as the sign of the aeration end. Experimental results indicate that they not only have no impacts on outlet water quality, but also shorten reaction time of nitrification effectively. (3) At the beginning of nitrification, ORP increases quickly; its increasing rate reduces gradually and tends to steady after 10~15min. In the end of ammonia oxidation, increasing of ORP accelerates and there is peak in dORP/dt curve (same step with dDO/dt curve). However, this characteristic is not obvious sometimes; therefore only using ORP as nitrification controlling parameter is not suitable. (4) In the steady denitrification process, pH and ORP increases and decreases with high rate respectively. In the end of denitrification process, "protruding"and "knee point"appear in pH and ORP curves one after the other. The characteristics of pH and ORP are obvious and available, and appearing interval is not more than 3min (latter is behind former); their combinations can be used as sign of the denitrification end. (5) In the unsteady domestication stage of nitrifying sludge and denitrifying sludge, normal varying rule of pH and DO can be used as controlling parameter of nitrification together, and normal varying rule of pH can be used as denitrification controlling parameter. (6) Disappearing of NO2--N accumulation in denitrification process combined with steady appearing of normal varying rule of pH and ORP parameter at the same time in denitrification process can be used as maturity indicators of denitrifying active sludge. (7) Under the condition of the lower inlet NH4+-N concentration (not more than 45mg/L in mixed liquid), mature active sludge cultivated in normal temperature (23~25°C) has ability to nitrify in full course, and active sludge cultivated in the lower temperature has ability to nitrify in full course after it is cultivated in the higher temperature (28~32°C) with shortcut nitrification steadily. These two kinds of active sludge are different; the former has strong adaptability to high temperature and low DO concentration.Two-stage SBR phosphorous and nitrogen removal process settles paradoxes of phosphorous and nitrogen removal basically, and its efficiency is higher than that of the traditional SBR system. The reaction time of SBR1 and SBR2 in different stages can be simply controlled by varying rule of pH, DO and ORP; auto-controlling problems of process are solved, which ensure process operating steadily, economically and efficiently.