CN110255820B - Low-carbon-source sewage biological denitrification system and method adopting plug-flow anoxic tank - Google Patents
Low-carbon-source sewage biological denitrification system and method adopting plug-flow anoxic tank Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F1/00—Treatment of water, waste water, or sewage
- C02F1/52—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F3/00—Biological treatment of water, waste water, or sewage
- C02F3/30—Aerobic and anaerobic processes
- C02F3/302—Nitrification and denitrification treatment
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F2101/00—Nature of the contaminant
- C02F2101/10—Inorganic compounds
- C02F2101/105—Phosphorus compounds
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- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F2101/00—Nature of the contaminant
- C02F2101/10—Inorganic compounds
- C02F2101/16—Nitrogen compounds, e.g. ammonia
Abstract
The invention belongs to the field of water environment protection, and particularly provides a low-carbon-source urban sewage biological denitrification system and method adopting a plug-flow anoxic tank, belonging to the integration of activated sludge and a biomembrane process. The method is an important improvement aiming at the traditional denitrification process based on anoxic-aerobic organisms, the anoxic tank is transformed into a plug flow type reaction tank and is divided into three to four sections, elastic filler is added into the anoxic reactor to maintain high sludge concentration, and the anoxic tank adopts longer hydraulic retention time. The anoxic tank is actually a plug-flow reaction mode formed by connecting three to four complete mixing reactors in series. The method is suitable for improving the traditional biological denitrification process (the denitrification principle is based on the combination of anoxic denitrification and aerobic nitrification), the requirement on the carbon-nitrogen ratio of the sewage can be reduced to 3.0-3.2, the denitrification rate of the traditional process can be maintained, and the denitrification rate is slightly improved. Compared with the traditional biological denitrification process, the method only needs to modify the anoxic tank and is suitable for upgrading and modifying the existing process. The method has the advantages of reduced aeration energy consumption, reduced or even eliminated carbon source supplement, low operation cost and good economy.
Description
Technical Field
The invention belongs to the field of water environment protection, and particularly relates to a biological nitrogen removal system and method for low-carbon source sewage by adopting a plug-flow anoxic tank.
Background
In the town sewage treatment process, the removal of total nitrogen and total phosphorus respectively needs to meet certain organic matter amount, and the nitrogen and phosphorus removal of the town sewage is difficult because the concentration of the organic matter of the inlet water of the current town sewage plant is generally low. Urban sewage plants usually focus on removing total nitrogen, while the removal of total phosphorus can be completed to a certain extent by adding drugs, and how to improve the total nitrogen removal rate of sewage with low carbon-nitrogen ratio is a key problem in current urban sewage treatment.
The traditional biological nitrogen and phosphorus removal method is ANThe method is characterized in that the total nitrogen removal rate of the process is difficult to improve, the demand on a carbon source is high, and the carbon-nitrogen ratio is generally required to be not less than 4.0. The process comprises the following steps: a. theNO process effluentThe method firstly enters an anoxic pond (A pond), the organic matters of inlet water and the nitrate which flows back (containing sewage internal reflux and sludge reflux) are subjected to denitrification reaction, the denitrification rate of the process depends on the reflux ratio, and if the sum of the sewage internal reflux and the sludge reflux is 350%, the total nitrogen removal rate can theoretically reach (350 ÷ 450). times.100% ~ 77.8%. If the denitrification rate needs to be further improved, the reflux ratio is only further increased, but the overlarge reflux ratio increases the volume of the reactor and increases the power consumption on the one hand, and on the other hand, the limitation of oxygen deficiency and oxygen deficiency is unclear, the two reactors of the oxygen deficiency pool and the aerobic pool are closer to a reactor in a completely mixed flow state as a whole, the denitrification rate cannot be further improved at all, and the total nitrogen removal rate in the actual operation of the process can only reach about 70 percent generally. In addition, each reactor is in a completely mixed form, and in order to maintain the nitrogen removal by the anoxic reaction (denitrification), the concentration of organic matters in the anoxic reaction tank cannot be too low, and the organic matters are necessarily removed by oxidation in the aerobic tank. This part belongs to an ineffective carbon source for denitrification, and the carbon source accounts for more than 1/3 of the organic matters in the wastewater inlet water (the specific value is related to parameters, the denitrification rate is low when the reflux ratio is low, the organic matters are wasted, and vice versa), so that the requirement of the denitrification process based on the traditional biological denitrification theory (anoxic-aerobic) on the carbon-nitrogen ratio is obviously higher than the theoretical value. The carbon-nitrogen ratio of the process is high, and if the carbon-nitrogen ratio is lower than 4.0, the carbon source is required to be supplemented frequently to maintain denitrification efficiency. The related specifications of China at present specify that the carbon-nitrogen ratio and the carbon-phosphorus ratio of sewage are not less than 4 and 17 respectively when nitrogen and phosphorus are removed; and meanwhile, when nitrogen and phosphorus are removed, the requirements of the carbon-nitrogen ratio and the carbon-phosphorus ratio are required to be met simultaneously.
In fact, for a pure denitrification process, in the traditional ammoniation, nitrosation, nitrification and denitrification processes, a denitrification reactor is arranged in front, and a denitrification carbon source at least needs to simultaneously meet the following conditions: NO3 -Reduction, cell proliferation and aerobic respiration processes. Theoretically, 1molNO3 -(14g) Reduction to N2The reduction of the valence of gas, N from +5 to 0 requires 1.25mol of O2The corresponding organic matter (i.e., 40g ThOD), if according to 40g BODLCalculated, i.e. corresponding to BOD5Is 27.4g (oxygen consumption rate constant k)1Take 0.1d-1I.e. BOD5＝0.684BODLTo BODLInstead of the theoretical oxygen demand), the oxygen equivalence coefficient of denitrification is only 1.96gBOD in the case of complete utilization of the carbon source by denitrification5/gNO3-N. However, considering that 0.45g of cells are formed in the whole process (0.64 g of reduced oxygen, 1.42 of cell oxygen equivalent coefficient), the denitrification oxygen equivalent coefficient is 2.60gBOD5/gNO3N (Shendazole, New technology for biological wastewater treatment, environmental science publishers, Nanqi, Water pollution control principles and technology, Qinghua university publishers, Jiang Yong, environmental engineering, advanced education publishers). Considering further that a small amount of oxygen molecules is still required during the anoxic process, aerobic respiration is not completely stopped, and this oxygen also consumes organic matter, about 0.1-0.4g BOD is additionally required5/gNO3-N. Considering all the above aspects, the oxygen equivalent coefficient of denitrification is actually 2.7-3.0gBOD5/gNO3about-N (approximately 3.7-4.1gCOD/gNO in terms of common domestic sewage)3-N), substantially lower than the actual need of about 4.0g BOD in conventional denitrification processes5/gNO3-N. The reason for the increased amount of organic matter required in the actual process is that a considerable part of the organic matter in the process is not utilized by denitrification but by heterotrophic microorganisms, which are directly converted into CO2And H2O, this portion of the organic matter is effectively "wasted". Currently conventional processes, e.g. ANthe/O process, the A/A/O process (inverted A/A/O process), the SBR process and various deformation processes thereof, the UCT process and the like (the processes remove ANand/O, generally considering phosphorus removal), because the anoxic reactor is mainly completely mixed in nature, the phenomenon that organic matters are consumed by heterotrophic microorganisms exists to different degrees. No matter how optimally designed, the carbon-nitrogen ratio requirement of the above conventional process flow is usually 4.0, which is significantly higher than the oxygen equivalence factor actually required for denitrification. In addition, it is generally difficult for the theoretical total nitrogen removal rate of the above process to exceed 75% (if the reflux ratio is too large, the theoretical total nitrogen removal rate may be increased, butThe disadvantages of large capital investment, large operation energy consumption and the like are easily caused), and the total nitrogen removal rate in the actual engineering is even lower than 70 percent.
Based on the common carbon source of the inlet water of the current urban sewage treatment plant is not enough, the carbon-nitrogen ratio can rarely meet the requirement of not less than 4, when the carbon-nitrogen ratio of the inlet water can not meet the requirement, the total nitrogen removal rate is obviously reduced, and the outlet water can not reach the standard easily. Therefore, it is very urgent to find a high-efficiency and low-cost denitrification technology suitable for low carbon sources. The main counter measures adopted at present include various methods such as changing a water inlet strategy, anaerobic ammonia oxidation, shortcut nitrification and denitrification, endogenous denitrification, hydrogen autotrophic denitrification and the like, and an external carbon source substance is often selected under the condition of no good counter measures. The added carbon source is simplest, but the cost is high, the sewage plant is difficult to bear, and the added carbon source needs to be accurate, too much or too little is not suitable, so that the problem that the reaction is difficult to control still exists; the water inlet strategy is changed, measures such as step-by-step water inlet (multipoint water inlet) and the like are usually adopted at present, but the process flow is more complicated, a plurality of structures are provided, and the operation cost is obviously increased; the anaerobic ammonia oxidation technology can fundamentally solve the problem of insufficient carbon source, but the anaerobic ammonia oxidation needs very harsh conditions, is generally suitable for high-ammonia nitrogen industrial wastewater, is difficult to be applied to the field of urban sewage treatment, and the existing denitrification process developed based on the anaerobic ammonia oxidation method is not a mature process for urban sewage treatment; the multi-stage AO series process is similar to step-by-step water inlet, a plurality of anoxic tanks and aerobic tanks are connected in series, water inlet is divided into multiple paths, water enters each anoxic tank, the process has a good denitrification effect, the denitrification removal rate of the process depends on the series stages of AO, the denitrification effect is better when the series stages are more, the process structures are more and the operation cost is obviously increased when the series stages are too many.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a low-carbon-source sewage biological denitrification system and method adopting a plug-flow anoxic tank, so as to solve the dependence and the requirement of the existing biological denitrification system on a carbon source and achieve the purpose of reducing the requirement of the carbon source.
The invention realizes the purpose through the following technical scheme:
a low-carbon source sewage biological denitrification system adopting a plug-flow type anoxic tank comprises the plug-flow type anoxic tank and an aerobic tank, wherein the plug-flow type anoxic tank is formed by connecting a plurality of mutually separated anoxic reaction sections in series, the volume of the first anoxic reaction section is large from water inlet to water outlet, and the volumes of the later anoxic reaction sections are sequentially reduced; because the flow state of water in each section still belongs to the complete mixing property, each anoxic reaction section still belongs to the complete mixing type reaction tank per se, namely a plurality of complete mixing type reaction tanks are connected in series to form a plug flow type anoxic tank.
Further preferably, the plug-flow type anoxic tank is divided into 3-4 sections, and the volume ratio of each section from water inlet to water outlet is 5: 2.5-3: 1.5-2: 0 to 1, so that when sewage flows into the first section of the anoxic tank, BOD (biochemical oxygen demand)5The concentration of the nitrogen and the nitrate nitrogen are close to those of the anoxic reactor in the traditional process, and the denitrification reaction of most of carbon sources and the nitrate can be realized in the first section of the anoxic tank; when the sewage enters the second section of the anoxic tank, BOD5Has been significantly reduced and is further removed in a second stage; further, the wastewater enters the third (or even fourth) section of the anoxic tank to completely remove BOD5And nitrate nitrogen, so that the effluent of the anoxic pond can maintain low BOD at the same time5And nitrate nitrogen concentration (BOD for complete nitrate removal5Concentration is not zero).
A biological nitrogen removal method for low-carbon source sewage by adopting a plug-flow anoxic tank is suitable for improving various traditional biological nitrogen removal (or biological nitrogen and phosphorus removal) processes comprising an anoxic-aerobic reactor, and comprises but is not limited to the following processes: an A/A/O process, an A/O process, Bardenpho, UCT, etc., comprising the steps of:
(1) preparation:
a plug flow type anoxic tank consisting of a plurality of completely mixed reaction tanks which are separated from each other is adopted to replace an anoxic tank in the traditional process, and elastic filler is added into the plug flow type anoxic tank to improve the sludge concentration in the plug flow type anoxic tank; the surface of the filler can form a biological film, when a large amount of biological films are formed, the concentration sum of the biological film sludge and the suspended activated sludge in the reaction tank can reach more than 10000mg/L, and the reaction state of the anoxic reaction tank is greatly improved; by adopting the filler, the anoxic microorganisms can be relatively stably retained in the anoxic tank, which is also very important for maintaining higher reaction efficiency of the anoxic tank;
(2) treating:
the original sewage (anaerobic treated sewage if containing phosphorus removal) and the returned nitrified liquid containing nitrate radical are conveyed to a plug flow type anoxic tank for treatment, the hydraulic retention time of the sewage in the plug flow type anoxic tank is prolonged to 3.0-5.0h in the treatment process, and the effluent after the anoxic reaction is ensured to maintain lower BOD5The total reflux ratio is larger, and the hydraulic retention time value is higher. When the sewage flows into the first section of the plug-flow anoxic tank, the concentration of nitrate nitrogen is close to that of the anoxic reactor in the traditional process, and BOD is5The concentration is slightly lower than that of the traditional process. The denitrification reaction of most carbon sources and nitrates can be realized in the first section of the anoxic pond. When the sewage enters the second section of the anoxic tank, BOD5Has been significantly reduced and is further removed in the second stage. Further, the wastewater enters the third (or even fourth) section of the anoxic tank to completely remove nitrate nitrogen and further reduce BOD5The concentration of the effluent of the anoxic pond can simultaneously maintain the low concentration of nitrate nitrogen and the low BOD5And (4) concentration. When the sewage enters the second section and the following sections of the anoxic tank, the sewage is polluted due to BOD5The concentration of nitrate nitrogen and the concentration of nitrate nitrogen are both low, and the reaction is difficult to carry out according to the conditions of the traditional process, and the effect of the reaction can be ensured by adding a filler in the reactor and increasing the hydraulic retention time.
Further preferably, the total sludge concentration (including biological membrane and activated sludge) in the anoxic tank is maintained to be more than 8.0g/L, and the attached sludge (biological membrane) concentration is maintained to be more than 7.0g/L by filling the elastic filler in the anoxic tank.
Further preferably, the preparation stage of step (1) further comprises adding an elastic filler into the aerobic tank to maintain a high microorganism concentration and avoid forming an excessively high suspended activated sludge concentration; when the aerobic tank adopts the elastic filler, the microorganisms of the aerobic tank are mainly nitrifying bacteria, and the nitrifying bacteria participate in circulation, so that suspended sludge (activated sludge) can circulate between the anoxic tank and the aerobic tank, and if the concentration of the activated sludge in the circulating nitrified liquid is too high, organic matters are easily consumed in the anoxic tank, so that the concentration of the suspended sludge adopted in a process system is lower and can be controlled to be below 2.5 g/L.
Further preferably, in the treatment stage of the step (2), a small amount of oxygen is supplied to the completely mixed reaction tank in the second stage and the subsequent stages to meet the normal progress of the denitrification reaction; in the first section of the anoxic tank, the returned nitrifying liquid contains dissolved oxygen, so that the first section does not need to be supplemented with oxygen. However, in the second and subsequent stages, since the dissolved oxygen is consumed, the anoxic reaction tank is not suitable for operating in a completely oxygen-free state, and thus, a trace amount of oxygen is required to satisfy the normal operation of the denitrification reaction. Although the denitrification reaction does not involve oxygen, certain enzymes of the denitrifying bacteria need oxygen to be metabolized normally, so that a proper amount of supplementary oxygen is needed, which is obviously different from the common complete mixed anoxic tank.
Further preferably, the method also comprises a reflux stage of the step (3), wherein a part of the sewage treated by the plug-flow type anoxic tank is refluxed to a preposed anaerobic tank, when the carbon-nitrogen ratio of the inlet water is more than 3.2, part of carbon source is allowed to be wasted in the outlet water of the anoxic tank, and a higher reaction rate can be obtained in a later aerobic treatment stage.
The principle of the invention is as follows:
since the biochemical reaction of denitrification belongs to the first-order reaction kinetic property, the reaction rate is obviously reduced when the carbon source is insufficient, and the structure and parameters of the reactor are important in order to ensure that the denitrification reaction of organic matters and nitrate (including nitrite) can be completely carried out in the anoxic pond. The main factors influencing the reaction rate are the concentration of microorganisms, the substrate concentration (referred to herein as BOD)5And nitrate) and temperature, and will depend on the substrate concentration given both the microorganism concentration and temperature. That is to say, in an anoxic reaction tankBesides the requirement of high sludge concentration, a certain amount of organic matters and nitrates are required, and the denitrification reaction rate is higher.
If a complete mixed reaction tank is adopted and higher BOD is maintained5The concentration (the same is true in the traditional process) can easily meet the requirement and can reach lower concentration of the effluent nitrate, but the disadvantage is that a large amount of organic matters flow to the aerobic reactor along with the effluent, so that a large amount of carbon source is wasted. Therefore, the invention can obviously overcome the defect by modifying the anoxic reaction tank into the plug flow type reactor. When the reactor is a plug-flow type reaction tank, the first stage of plug-flow is basically equivalent to that of the traditional process, and the efficiency of each stage in the second stage and the following stages is gradually reduced, so that in order to maintain the total reaction efficiency, the sludge concentration needs to be increased, and the hydraulic retention time (namely the volume of the reactor is increased) can be increased.
The invention adopts two methods simultaneously: the method of increasing the sludge concentration and the hydraulic retention time can ensure that nitrate nitrogen is almost completely removed at the tail end of the reaction tank even if the anaerobic reaction tank does not have high organic matter concentration, and can maintain the low BOD of the effluent5And (4) concentration. The plug-flow anaerobic reaction tank can receive sewage with a lower carbon-nitrogen ratio, and when denitrification is completed in the anaerobic reaction tank, the concentration of organic matters is very low, so that a carbon source is mainly utilized by a denitrification process (residual sludge is formed and aerobic respiration is still inevitable and needs to consume an additional carbon source). Thus, when the sewage flows into the aerobic tank, the sewage is polluted because of BOD5The concentration is already low, so that the carbon source loss in the aerobic tank is very low, thereby reducing the carbon source requirement of the system.
Regarding the carbon nitrogen ratio of the sewage: because the complete reaction on nitrate can be realized in the anoxic tank, the total carbon-nitrogen ratio of the sewage entering the anoxic tank at least meets the theoretical value of 2.7-3.0. From the foregoing analysis, the theoretical minimum oxygen equivalent coefficient for denitrification is only 1.96gBOD5/gNO3N, but considering that phosphorus removal in this process still inevitably requires part of the carbon source (because excess sludge must be discharged) and aerobic respiration is not completely stopped, the actual requirement is reached2.7-3.0gBOD5/gNO3And N, considering that a safety margin is left, the denitrification efficiency can be completely guaranteed by reaching 3.0-3.2. In addition, in view of carbon source saving, about 1/3% of carbon source is wasted in the conventional process, and if the proportion of the carbon source wasted in the conventional process can be reduced to below 10-15%, 80% of the carbon source is required at most in the conventional process to meet the denitrification requirement, and the total carbon-nitrogen ratio is required to meet 3.0-3.2. If the carbon-nitrogen ratio of the original sewage is lower than 3.0, when the operating process parameters are not in the optimal state, the process of the invention may still need to add a carbon source; if the carbon-nitrogen ratio of the original sewage is lower than 2.7, an additional carbon source is required to be added. Even so, the carbon source to be added has been significantly reduced compared to the conventional process.
Regarding hydraulic retention time: in general, the time required for the anoxic reaction of the biological denitrification (or denitrification and dephosphorization) process is in the range of 0.5-3.0h (this is also the A in the specification)NThe time specified by the/O process and the a/O process), but considering the situation that the town sewage is mainly subject to denitrification, the hydraulic retention time of the anoxic tank of the traditional process is preferably a large value such as 2.0-3.0 h. The reaction time of the plug-flow type anoxic tank is properly increased, and the suggested time of the invention is 3.0-5.0 h. If the intermediate value is 4.0h, the increase is more than 50 percent compared with the traditional process, which can basically meet the requirement of denitrification.
The invention has the beneficial effects that: the invention relates to a low-carbon source sewage biological nitrogen removal system and a method adopting a plug-flow type anoxic tank, which are an improvement of the traditional biological nitrogen removal (phosphorus removal) process based on an anoxic-aerobic process. Through the improvement, the higher denitrification rate can be realized on the premise of low carbon source, and the biological phosphorus removal function is not eliminated. Theoretically, no additional carbon source is needed under the condition that the carbon-nitrogen ratio is as low as about 3.0-3.2, the cost for adding the carbon source is obviously saved, and compared with the traditional process, the denitrification rate is also improved. The method has simple process, only needs to transform the anoxic tank, and is suitable for upgrading and transforming the existing process. The invention reduces the energy consumption of the gas, saves or even cancels the supplement of carbon source, thus having lower operation cost and better economy. In addition, since the anoxic tank volume is increased, although energy consumption is increased, since oxygen consumed by organic matter is significantly reduced, oxygen demand for aeration is also reduced, and energy consumption for aeration is reduced. The influence of the positive and negative aspects is combined, and the energy consumption cost in actual operation is almost unchanged. But considering the condition of obviously reducing the addition of carbon source, even without adding carbon source, the cost of the medicament is obviously reduced, so compared with the traditional process, the operation cost is reduced.
Drawings
FIG. 1 is a flow diagram of the process of the present invention.
Detailed Description
The present application will now be described in further detail with reference to the accompanying drawings, wherein it is to be noted that the following embodiments are provided only for illustrating and explaining the present invention and are not to be construed as limiting the flow, configuration, parameters and the like of the process of the present invention; the specific parameters given in the embodiments are merely examples for illustrating the present invention, and are not necessarily parameters. The embodiment is only used for illustrating a specific application of the biological denitrification method for the low-carbon-source urban sewage by adopting the plug-flow anoxic tank. In fact, the method can have various deformation forms, as long as the method realizes the biological denitrification process based on the anoxic-aerobic alternate operation, the completely mixed anoxic tank is improved into the plug flow type anoxic tank and is divided into three sections or four sections, and the invention is in the scope of protection regardless of the structure and the mode of the anoxic tank and regardless of whether filler is added into the anoxic tank.
The method is suitable for improving various anoxic tanks of biological denitrification (or biological denitrification and dephosphorization) processes comprising an anoxic-aerobic reactor, such as an A/A/O process, an A/O process, Bardenpho, UCT and the like. The following is detailed only with an example of the A/A/O process.
The utility model provides an adopt low carbon source sewage biological denitrification system of plug-flow type oxygen deficiency pond, as shown in figure 1, according to the rivers direction, includes anaerobism pond, plug-flow type oxygen deficiency pond, good oxygen pond and sedimentation tank in proper order, wherein: the plug-flow type anoxic tank is formed by connecting 3-4 mutually separated anoxic reaction sections in series, and the volume ratio of each section is 5: 2.5-3: 1.5-2: 0-1, namely the first section has larger volume and the volume is reduced in sequence. The aerobic tank is provided with sewage (nitrifying liquid) which internally flows back to the water inlet end of the anoxic tank, the anoxic tank is provided with sewage which internally flows back to the water inlet end of the anaerobic tank, and the total reflux ratio is 250-400%; the sedimentation tank is provided with sludge which flows back to the water inlet ends of the anaerobic tank and the anoxic tank, the sludge reflux ratio is determined according to calculation, and if the value is 50% (mostly, the value is about 50% according to the calculation result), the total reflux ratio is 300% -450%. If the sludge completely flows back to the anaerobic tank, the anoxic tank does not flow back to the anaerobic tank; if the sludge only partially flows back to the anaerobic tank, the sewage in the anoxic tank properly flows back to the anaerobic tank.
The method for biologically denitrifying the low-carbon source sewage by using the biological denitrification system comprises the step of treating the wastewater to be treated by sequentially passing through an anaerobic tank, a plug-flow anoxic tank, an aerobic tank and a sedimentation tank. As will be described in detail below with reference to specific cases, for convenience of analysis, it is assumed that TN of the raw wastewater is Kjeldahl nitrogen, the carbon-nitrogen ratio just satisfies 3.0, and if TN is 45mg/L, BOD is obtained5The concentration was 135 mg/L.
(1) The raw sewage is treated by an anaerobic tank and then treated by anaerobic treatment, namely BOD55% of degradation, the ammonification reaction of the anaerobic tank can be carried out thoroughly, and the water quality of the effluent of the anaerobic tank is as follows: BOD5128mg/L and 45mg/L ammonia nitrogen. Since the anaerobic tank is a conventional process, it will not be described in detail herein.
(2) The sewage treated by the anaerobic tank enters a plug flow type anoxic tank for treatment; wherein, elastic filler is added into the plug flow type anoxic tank to improve the sludge concentration in the plug flow type anoxic tank. Due to the filling material, the anoxic microorganisms can be relatively stably retained in the anoxic tank, which is also very important for maintaining high reaction efficiency of the anoxic tank. Through the improvement, the effluent of the anoxic tank has the solubility BOD5The concentration can be reduced to below 5-8 mg/L; BOD in this case5At concentrations, the nitrate nitrogen concentration may also be as low as 1-2mg/L or less.
When the sewage flows into the first section of the anoxic tank, BOD5Concentration of (D) and concentration and transmission of nitrate nitrogenThe anoxic reactor of the traditional process is close to that of the anoxic tank, and the first section of the anoxic tank can realize the denitrification reaction of most of carbon sources and nitrates; when the sewage enters the second section of the anoxic tank, BOD5Has been significantly reduced and is further removed in a second stage; further, the wastewater enters the third (or even fourth) stage of the anoxic tank to completely remove nitrate nitrogen and further remove BOD5Can ensure that the effluent of the anoxic pond can simultaneously maintain very low nitrate nitrogen concentration and lower BOD5And (4) concentration.
In order to ensure the normal operation of the denitrification reaction in the plug-flow type anoxic tank, an oxygen supply system is arranged in the completely mixed reaction tank of the second section and the following sections of the plug-flow type anoxic tank, and the first section of the anoxic tank does not need to be supplemented with oxygen because the returned nitrified liquid contains dissolved oxygen. However, in the second and subsequent stages, since the dissolved oxygen is consumed, the anoxic reaction tank is not suitable for operating in a completely oxygen-free state, and thus, a trace amount of oxygen is required to satisfy the normal operation of the denitrification reaction. Although the denitrification reaction does not involve oxygen, certain enzymes of the denitrifying bacteria need oxygen to be metabolized normally, so that a proper amount of supplementary oxygen is needed, which is obviously different from the common complete mixed anoxic tank.
In addition, the total hydraulic retention time of the plug-flow type anoxic tank is longer than that of the traditional process by 0.5-3h and needs to be prolonged to 3.0-5.0 h. If the hydraulic retention time is increased by more than 50 percent compared with the traditional process according to the intermediate value of 4.0h, the requirement of denitrification can be basically met. If the middle value of the three-section plug-flow type anoxic tank is 4.0h, the three sections can take values of 1.6h, 1.2h and 1.2h respectively, the retention time of other tanks can meet the original process requirements, and the specific value is not limited.
(3) And the sewage treated by the plug flow type anoxic tank enters an aerobic tank. When nitrate nitrogen has been substantially removed and BOD5The effluent of the anoxic tank with low concentration liquid enters an aerobic tank for aeration, and nitrification of ammonia nitrogen is mainly carried out in the aerobic tank (if the phosphorus removal function is enhanced, the phosphorus absorption is still finished in the aerobic process), and the BOD of the influent water5Low concentration of organic substancesThe oxidation is no longer the main process, so the carbon source loss amount of the aerobic pool is less.
Elastic filler is required to be added into the aerobic tank to maintain high microorganism concentration and avoid forming overhigh suspended activated sludge concentration; when the aerobic tank adopts the elastic filler, the microorganisms of the aerobic tank are mainly nitrifying bacteria, and the nitrifying bacteria participate in circulation, so that suspended sludge (activated sludge) can circulate between the anoxic tank and the aerobic tank, and if the concentration of the activated sludge in the circulating nitrified liquid is too high, organic matters are easily consumed in the anoxic tank, so that the concentration of the suspended sludge adopted in a process system is lower and can be controlled to be below 2.5 g/L.
(4) And the sewage treated by the aerobic tank enters a sedimentation tank for sedimentation, so that the whole denitrification process is completed.
When the total reflux ratio is 300%, the quality of the effluent of the aerobic tank, namely the nitrified liquid reflux water, is estimated according to the following steps (if the initial estimation deviation is larger, iteration can be continuously corrected until the data is within the error range): BOD55-8mg/L, 1mg/L ammonia nitrogen and 11mg/L nitrate nitrogen. The water quality after the nitrification liquid reflux and the anaerobic tank inlet water are mixed (namely the water quality of the anoxic tank inlet water, the total reflux ratio is 300%, and the weighted calculation concentration) is as follows: BOD535.8-38.0mg/L, 12mg/L ammonia nitrogen and 8.3mg/L nitrate nitrogen. Nitrate nitrogen is completely removed in the anoxic reaction process (calculated according to the actual required carbon-nitrogen ratio of 3.0), the ammonia nitrogen change is ignored, and then the effluent quality of the anoxic tank is as follows: BOD510.9-13.1mg/L, 12mg/L ammonia nitrogen and almost 0 nitrate nitrogen (because the plug flow type anoxic tank reaches the tail end, the BOD of the anoxic tank5Can be maintained at a level of approximately more than 10mg/L, ensuring the effect of the anoxic tank). In the course of aerobic reaction, BOD5Further degrading by 50 percent, and converting 90 percent of ammonia nitrogen, so that the effluent quality of the aerobic tank is as follows: BOD55.5-6.5mg/L, 1mg/L ammonia nitrogen, 11mg/L nitrate nitrogen, in substantial agreement with the initial assumptions. The carbon source which can be utilized by biological phosphorus removal is at most 14mg/L, so that 1mg/L of phosphorus can be removed, and the total phosphorus concentration of the urban domestic sewage is generally obviously greater than 1mg/L, so that the phosphorus removal mainly needs to be completed by adding medicines. From this it can be concluded: the method finishes the carbon-nitrogen ratio of 3.0The method can meet the requirements, and under the specified parameters and conditions, the TN removal rate is the highest 73% (excluding the residual sludge discharge) or the TN removal rate is the highest 76% (considering the residual sludge discharge).
When the total reflux ratio is 450% (since the hydraulic retention time of the anoxic pond takes a larger value, the total reflux ratio can also take a larger value as appropriate), the quality of the nitrified liquid reflux water is estimated as follows: BOD55-8mg/L, ammonia nitrogen 1mg/L and nitrate nitrogen 8 mg/L. The water quality (namely the water quality of the inlet water of the anoxic tank, the total reflux ratio of 450 percent and the weighted calculation concentration) after the nitrification liquid reflux and the inlet water of the anaerobic tank are mixed is as follows: BOD527.4-29.8mg/L, 9mg/L ammonia nitrogen and 6.5mg/L nitrate nitrogen. Nitrate nitrogen is completely removed in the anoxic reaction process (the carbon-nitrogen ratio is calculated as 3 according to actual needs), the ammonia nitrogen change is ignored, and then the effluent quality of the anoxic tank is as follows: BOD57.9-10.3mg/L, 9mg/L ammonia nitrogen and almost 0 nitrate nitrogen (because the plug flow type anoxic tank reaches the tail end, the BOD of the anoxic tank is5The concentration of (2) is maintained at about 8mg/L or more, the effect of denitrification in the anoxic tank can be basically ensured, but it may not be appropriate to further increase the reflux ratio). In the course of aerobic reaction, BOD5Further degrading by 40%, and converting 90% of ammonia nitrogen, wherein the effluent quality of the aerobic tank is as follows: BOD55-6mg/L ammonia nitrogen, 1mg/L ammonia nitrogen and 8mg/L nitrate nitrogen, which is basically consistent with the initial hypothesis (in fact BOD in the aerobic tank)5Whether 40 percent of the wastewater can be degraded or not has no substantial influence on the process, even if the wastewater cannot be degraded, the denitrification is not influenced, and BOD of an aerobic tank5Low removal rate is rather advantageous for denitrification). The carbon source which can be utilized by biological phosphorus removal is about 10mg/L, approximately 0.6mg/L of phosphorus can be removed, and as the total phosphorus concentration of the urban domestic sewage is generally remarkably greater than the value, the phosphorus removal mainly needs to be completed by adding medicines. From this it can be concluded: the carbon-nitrogen ratio of 3.0 in the method can completely meet the requirement, and the TN removal rate is 80 percent at the maximum (excluding the residual sludge discharge) or the TN removal rate is 83 percent at the maximum (considering the residual sludge discharge) under the specified parameters and conditions.
Comparing the total reflux ratio of 300% to 450%, it was found that by further increasing the reflux ratio to achieve an increase in the denitrification rate, which could be technically achieved but is not economical, the present process could maintain the same or a slight increase in the denitrification rate as compared to the conventional process.
If the requirement of the method for further reducing the carbon-nitrogen ratio of the raw sewage is expected, the aim can still be achieved by optimizing the parameters and operation of the plug-flow anoxic reactor, so that the anoxic reaction tank of the method has the possibility of further improvement in the technology.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention.
Claims (5)
1. A biological denitrification method for low-carbon source sewage by adopting a plug-flow anoxic tank is characterized by comprising the following steps:
(1) preparation:
constructing a low-carbon-source sewage biological denitrification system, wherein the low-carbon-source sewage biological denitrification system comprises a plug-flow type anoxic tank and an aerobic tank which are sequentially connected, the plug-flow type anoxic tank is formed by connecting a plurality of completely-mixed anoxic reaction sections which are separated from one another in series, the volume of the first anoxic reaction section is large, the volumes of the later anoxic reaction sections are sequentially reduced from water inlet to water outlet, and elastic filler is added into the plug-flow type anoxic tank to improve the sludge concentration in the plug-flow type anoxic tank and maintain the sludge concentration sum in the plug-flow type anoxic tank to be more than 8.0 g/L;
(2) treating:
conveying the sewage to be treated to a low-carbon-source sewage biological denitrification system for treatment, prolonging the hydraulic retention time of the sewage in a plug-flow type anoxic tank to 3.0-5.0h in the treatment process, refluxing the nitrifying liquid in an aerobic tank to the water inlet end of the plug-flow type anoxic tank, and performing trace oxygen supply in a fully mixed reaction tank of the second section and the later sections of the plug-flow type anoxic tank so as to meet the normal operation of denitrification reaction.
2. The biological nitrogen removal method for the low-carbon-source sewage by using the plug-flow type anoxic tank as claimed in claim 1, wherein in the step (1), the plug-flow type anoxic tank is divided into 3-4 anoxic reaction sections, and the volume ratio of each section from water inlet to water outlet is 5: 2.5-3: 1.5-2: 0 to 1.
3. The method for biologically denitrifying low-carbon-source sewage by using a plug-flow anoxic tank according to claim 1, wherein in the step (1), the concentration of the attached sludge in the plug-flow anoxic tank is maintained to be more than 7.0 g/L.
4. The method for biologically denitrifying low-carbon-source sewage by using a plug-flow anoxic tank according to claim 1, wherein the preparation stage of the step (1) further comprises adding an elastic filler into the aerobic tank.
5. The method for biologically denitrifying low-carbon-source sewage by using a plug-flow anoxic tank according to claim 1, further comprising a reflux stage of step (3) including refluxing a portion of the sewage treated by the plug-flow anoxic tank to a preceding anaerobic tank.
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