Non-steady-state DO environment:
The non-steady-state DO environment refers to the unstable DO concentration in the reactor during the process. This environment can create a good process of aerobic nitrification / hypoxia denitrification, which can significantly improve the system's removal of organic matter while saving energy. And denitrification effect. Activated sludge method is widely used in wastewater treatment. In traditional aerobic treatment, continuous and stable aeration can maintain a stable dissolved oxygen (DO) environment in the reactor to meet the oxygen demand for nitrification, but it cannot achieve anoxic denitrification, so it can reach a certain level. Ammonia nitrogen removal effect, but the total nitrogen removal effect is poor. The non-steady-state DO environment refers to the unstable DO concentration in the reactor during the process. This environment can create a good process of aerobic nitrification / hypoxia denitrification, which can significantly improve the system's removal of organic matter while saving energy. And denitrification effect.
SBR, oxidation ditch, mixed pulse and other processes all have the characteristics of an unstable DO environment during operation. SBR makes the entire working cycle run in intermittent aeration mode, which can achieve aerobic and anoxic environments in the same reactor. The aeration tank of the traditional oxidation ditch process is a circulating ditch, where the mixed liquid of sewage and activated sludge flows, and the rotary aeration is used to provide dissolved oxygen and keep the mixed liquid in a completely mixed state; because the aeration device is only installed in the oxidation ditch In one or more places, the DO concentration in the reactor changes frequently. Pulse aeration is based on the aerobic activated sludge method to change continuous aeration to pulse aeration, so as to achieve an environment with high and low DO concentrations, and improve the efficiency of nitrogen and phosphorus removal.
Based on the analysis of the non-steady-state DO concentration variation in the reactor, the biological treatment effect and research status of the wastewater in the non-steady-state DO environment were reviewed, and its research and development trends were prospected in order to provide an energy-saving Ways to consume and achieve high standards of treatment.
1. Unsteady-state DO change law
In the SBR process, the system's water intake and precipitation period is anoxic, and the DO is low. When the aeration reaction starts, the DO increases, but due to the higher sewage load, the DO does not increase much; as the organic matter in the sewage is degraded As the oxygen demand of the microorganisms decreases, the increase in DO increases. Subsequently, during the non-aeration stage of precipitation and effluent, DO begins to decrease, decreases to the DO concentration in the initial anoxic stage, and continues to the next aeration stage. Under normal circumstances, the DO is higher than 1.5 mg / L during the aeration phase, and when the aeration is stopped, the DO drops below 1.5 mg / L.
In the traditional oxidation ditch, the DO gradient is easily formed due to the use of a surface aerator for oxygen supply and push flow. The DO concentration in the region near the aeration point is high, and the farther away from the aeration point, the lower the DO concentration. Therefore, alternating aerobic and hypoxic regions are formed in the same trench, which is more obvious under multi-ditch conditions. Among them, the DO at the near aeration point area can reach 2.25mg / L, and the DO far from the aeration point area drops below 0.5 mg / L, or even 0.
In the pulse aeration mode, within a working cycle, the DO suddenly increases during the aeration, and then stabilizes in a certain range. At this time, the system is aerobic; the DO will decrease sharply at the moment of stopping the exposure, and then maintain a certain level. Low value, the system is in an anoxic or anaerobic state. The frequent alternation of aeration / stop exposure strengthened the system's aerobic-hypoxic-anaerobic environment. At the end of the exposure, the DO is low or 0. When the aeration is started, the DO will increase at the maximum rate and can reach a higher dissolved oxygen level. S.Lochmatter et al. Found that in the pulse aeration mode, DO increased rapidly during aeration, reaching 50% of saturated dissolved oxygen, and remained relatively stable during the aeration period; when the exposure was stopped, DO rapidly decreased to 0, and Continue until the start of the next cycle of aeration.
2. Biological treatment effect of wastewater under non-steady-state DO environment
2.1 Analysis of removal of organic matter and denitrification effect in wastewater under unsteady DO environment
In wastewater treatment, continuous and stable aeration can easily lead to long-term accumulation of nitrifying bacteria and inhibit the occurrence of denitrification. The periodic aerobic, anoxic, and anaerobic environment under non-steady-state DO environment can make a suitable DO gradient distribution inside the activated sludge floc. During periods or areas where DO concentration is high, nitrifying bacteria oxidize ammonia nitrogen to nitrate nitrogen and nitrite nitrogen, while during periods or areas where DO concentration is low, the reaction tank is in an anoxic state, and microorganisms use organic matter as hydrogen. The donor denitrifies nitrate nitrogen, reduces it to N2 or NxOy, and discharges it into the atmosphere, thereby achieving the purpose of denitrification. At the same time, in the anoxic phase, NO3--N and NO2--N can replace the molecular O2 as an electron acceptor, and continue to oxidize organic pollutants in sewage, thereby reducing the organic load in the aerobic phase. In the SBR process for treating slaughtering wastewater, the COD and TN removal rates in the wastewater can reach 97% and 94% when the aeration is 50 minutes and the exposure is stopped for 50 minutes. By changing the operation mode of the CASS process and adopting aerobic pulse aeration, the removal rate of organic matter and nitrogen can reach more than 80% when the aeration and stopping time are respectively 5 and 5 minutes. In addition, by controlling the concentration and distribution of DO in the oxidation ditch, simultaneous nitrification and denitrification biological denitrification can be achieved in the channel outside the oxidation ditch, and the TN removal rate can reach up to 86%. G. Yilmaz et al. Pointed out through aerobic activated sludge experimental research that the DO rapidly decreased during the stop-exposure stage, and the compact sludge bed structure that resulted in flocculation of sludge particles formed a completely anoxic environment. Nitrification and denitrification further improve the system's denitrification efficiency. With non-steady-state aeration, when the DO is reduced from 3.5 to 5.0 mg / L to 0.5 to 1.2 mg / L, the system's nitrogen removal rate can reach 94.9%, and no additional carbon source is required. This is because the degree of denitrification depends on the supply of organic carbon during the anoxic stage. The non-steady-state DO environment is conducive to saving carbon source consumption, allowing more carbon sources to be used for denitrification and denitrification, thereby improving the overall system's nitrogen removal efficiency. .
2.2 System energy consumption under non-steady-state aeration
Recent studies have shown that the non-steady-state aeration method can save a large amount of aeration energy consumption on the basis of effluent water quality standards, and this aeration method can be applied to the transformation of ordinary aerobic activated sludge method. . Compared with continuous aeration, the non-steady-state aeration method shortens the aeration time, and the aeration volume can save 33% compared with continuous aeration. In addition, due to the DO concentration gradient in the non-steady-state DO environment, the driving force for oxygen transfer is large and the dissolved oxygen efficiency is high at each aeration, so the actual energy consumption saved will be higher. H. Doan et al. Carried out experiments using non-steady-state aeration to modify a common aerobic continuous aeration process. The experimental results show that when an appropriate stop / aeration time cycle is set, during a complete experiment, Based on the complete removal of BOD5, compared with ordinary aerobic continuous aeration, non-steady-state aeration can save about 27% to 58% of energy.
Because MBR has higher energy consumption than traditional processes, its promotion and application are limited. The use of pulse aeration can reduce the MBR membrane scrubbing air volume, and can reduce the MBR energy consumption by switching between high and low DO concentrations. In the biological contact oxidation method, suspended matter in the water and aging biofilm adhere to the surface of the filler due to continuous aeration, which hinders the material exchange between the water body and the biofilm. Using pulsed aeration, short-time, high-intensity aeration can effectively flush the surface of biofilm, promote biofilm renewal and material exchange, and its effect is better than traditional continuous aeration, which can save 60% ~ 80% of air volume.
2.3 Residual sludge volume under non-steady-state DO environment
In most wastewater biological treatment processes, the large amount of residual sludge produced by microbial growth is an important issue in wastewater treatment. Non-steady-state DO changes can provide microorganisms with an alternating aerobic and anaerobic environment, so that ATP obtained by bacteria in the aerobic phase cannot be used to synthesize new cells immediately, but in the anaerobic phase as a cell life-sustaining activity The energy is consumed, the microbial decomposition and the anabolic are relatively separated, the net synthesis of bacteria is reduced, and the sludge production is reduced. Through comparative experiments, SJ Jung et al. Concluded that the use of non-steady-state aeration methods to create an aerobic / anaerobic microbial growth environment can reduce the amount of remaining sludge, and the shorter the aeration / stop exposure cycle, the remaining sludge amount The reduction is more pronounced. Huang Tianyin and other studies found that during intermittent aeration, cells rely on low molecular oxygen released by nitrate in the mixed solution to breathe. Because the redox potential of the inorganic electron acceptor is higher than oxygen, the ATP released during hypoxic breathing is also less.
Changes in the aerobic and anaerobic microbial environment lead to the death of obligate aerobic or anaerobic microorganisms and are used by other bacteria, which increases the rate of oxidation of the sludge cells themselves, thereby reducing the remaining sludge production. The experimental research results of immersed membrane bioreactors by JJ Chang et al. Show that compared with the steady-state DO environment, the non-steady-state DO environment not only enables the bacterial microorganisms to adapt to the aerobic / hypoxic environment, but also greatly improves the sludge. The number of bacteria in the bacteria, thereby enhancing the bacteria's own oxidative and metabolic capabilities, and reducing the remaining sludge production.
3. Research Prospects of Biological Treatment of Wastewater in Unstable DO Environment
In the biological treatment of wastewater, a large amount of greenhouse gas N2O is generated during the nitrification and denitrification denitrification processes, and the production amount is closely related to the DO concentration. At present, there is no clear-cut theory about the impact of non-steady-state DO environment on N2O emission flux. JH Ahn and others investigated the N2O emission flux of 12 sewage treatment plants in the United States and found that frequent alternating aerobic / hypoxic environments will increase N2O emission flux. The unsteady DO environment has high dissolved oxygen efficiency and strong oxygenation capacity, and higher DO concentration is the main reason for N2O production during nitrification. Y. Kimochi et al.'S experiments on the treatment of urban sewage through non-steady-state aeration point out that compared with traditional continuous aeration, the non-steady-state DO environment created by non-steady-state aeration can reduce N2O emissions in the system, and the aerobic stage The shorter the time, the lower the N2O emission flux. ZhenHu et al. Proposed through research that the aerobic / anaerobic environment created by non-steady-state aeration is conducive to saving carbon source consumption, making the supply of anoxic denitrifying carbon source more sufficient, and thus reducing N2O emissions. Therefore, how to control the appropriate DO concentration and aerobic / hypoxic phase ratio is the key to reducing N2O emission flux under non-steady-state DO environment.
In addition, there are a large number of SMPs (dissolved microorganism products) in the biological treatment of wastewater, which have poor biodegradability, which not only makes the organic content of the system effluent unable to meet the standards, but also affects the performance of activated sludge. At present, there is no certainty about the impact of non-steady-state DO environment on SMP production. Fan Ji et al. Conducted experiments on domestic sewage treatment with SBR and pointed out that compared with traditional aerobic conditions, the non-steady-state DO environment created by the aerobic / hypoxic operation mode can reduce the output of SMP in the effluent because the microorganisms are in the aerobic environment. The reduction in time has led to a reduction in the amount of SMP produced.
According to the test by using the SBR process to treat synthetic wastewater, it is pointed out that the unsteady DO environment has high dissolved oxygen efficiency and strong oxygenation capacity, which not only shortens the reaction time in the anoxic section, but also increases the DO level in the reactor. The concentration caused an increase in the amount of SMP produced. In the study of MBR process, SG Lu et al. Pointed out that although the reduction of biomass activity leads to a decrease in the amount of SMP produced at a lower DO concentration, the biodegradability of SMP will decrease; when DO increases to 3 mg / L When the DO concentration is higher, the metabolic activity of the microorganisms in the reactor is enhanced, and the amount of SMP produced must be higher than that under low DO concentration. The difference between SMP production and degradation at different DO concentrations. Therefore, in an unsteady DO environment, how to control the appropriate DO concentration and establish a controllable SMP generation and degradation model under an unsteady DO environment to optimize the wastewater treatment effect and ensure the quality of the effluent water will become increasingly important .