KR100217358B1 - Process for the biological removal of nitrogen and phosphorus - Google Patents

Abstract

The present invention relates to a wastewater treatment method for the removal of eutrophication substances such as organic matter, nitrogen, phosphorus, etc. in the wastewater with a high removal rate. According to the present invention, the organic matter is removed from the anaerobic tank and the phosphorus in the form of polymer is eluted in the form of phosphorous acid, the phosphorus is excessively absorbed into the microorganism in the first aeration tank, and the excess phosphorus-containing sludge is removed through a sand filtration tank, An improved method for biological removal of nitrogen and phosphorus is provided, including nitrification followed by denitrification in a denitrification tank, discharge of the supernatant from the final settling tank and returning the settling sludge to a pre-process.

Description

Biological Removal Methods of Nitrogen and Phosphorus

1 is a schematic representation of the process of the present invention for the removal of nitrogen and phosphorus from wastewater using microorganisms.

The present invention relates to a method for treating municipal sewage or industrial wastewater, and more particularly, to a wastewater treatment method for removing not only organic matter but also eutrophic substances such as nitrogen and phosphorus using microorganisms.

Existing Bottoms Currently Running The wastewater treatment system is mainly operated by the standard activated sludge process, but the removal of organic matter is 90% by the standard activated sludge process. Abnormal treatment is possible, but the removal rate is 15 to 40 for nitrogen and phosphorus, respectively. , 20 to 40 It only causes nitrogenous or phosphorous appeal and eutrophication and red tide of the sea area. Therefore, in recent years, the focus has been on the development of a process for increasing the removal rate of nitrogen and phosphorus as a water pollution prevention method, and the process of adding physicochemical treatment to the existing activated sludge process called secondary treatment and the existing activated sludge Processes for removing nitrogen and phosphorus using microorganisms with improved process have been developed.

Generally, ha Nitrogen in the wastewater is mostly ammonia nitrogen and organic nitrogen, and a small amount of nitrite and nitrate nitrogen is present. The composition varies slightly depending on the type of wastewater, the season or the day. Nitrogen removal in conventional conventional treatment plants is carried out by precipitation of organic nitrogen associated with sedimentable substances in the primary sedimentation basin, but soluble and colloidal organic nitrogen and soluble inorganic nitrogen are hardly removed and activated ammonia and organic matter in activated sludge baths Nitrogen is oxidized to nitrite and nitrate nitrogen, some of which are used as part of microbial cell constituents.

At present, the physicochemical higher-level treatment process for removing nitrogen is sufficient to raise the pH of the wastewater to be treated to 11 or more to degassing the ammonium ions into ammonia to remove them in a gaseous state. There is a breakdown point chlorine treatment method for removing ammonia nitrogen in solution by nitrogen gas, and an ion exchange resin method for removing ammonium ions through an ion exchange resin having excellent selective ion exchange properties. Not only does it take much time, but it is also difficult to use in actual large scale processes due to the control of pH, toxicity of injecting gas, and difficulty in regeneration.

In addition, as a biological higher-order treatment process using a microorganism in the aerobic state using a microorganism such as Nitrobacter, Nitrosomonas (Nitrosomonas) It is a process that oxidizes organic nitrogen or ammonium salt nitrogen in waste water in the form of nitrate, and then reduces and removes nitrate nitrogen with nitrogen gas in the presence of Bacillus, Pseudomonas and Micrococcus under anoxic conditions. Although used in the treatment plant, there is a demand for optimization of the process and improvement of efficiency due to the reliability of the treatment and instability of load variation. Also, ha Phosphorus in wastewater was The degree of the removal of the untreated phosphorus is high, and the physical high-level process for removing the untreated phosphorus includes a process using a polymer membrane and an ion exchange resin, but these processes are expensive and have technical and regeneration difficulties. As a chemical higher order process, ortho-phosphate is agglomerated using lime and metal salt flocculant and then removed by gravity sedimentation. However, this method requires a large amount of chemicals and dehydration of sludge causes a large scale. Not suitable for the device

The process of removing phosphorus biologically is a process of removing phosphorus using intracellular phosphorus accumulating microorganisms such as Acinetobacter, Bacillus, Pseudomonas, Micrococcus, etc. By culturing the microorganisms under anaerobic conditions, the microorganism in the anaerobic state is unable to obtain energy through a normal tricarboxylic acid cycle as in the aerobic state, so that the polymer phosphate is used. Microorganisms metabolize using the energy released by the hydrolysis of macromolecular phosphoric acid and converting it into phosphate. This energy is used to actively transport extracellular organisms into cells. Hydroxy butyrate (poly- -hydroxybutyrate) and when it is aerobic, it is sent to the normal tricarboxylic acid circuit to obtain adenosine triphosphate, which absorbs extracellular phosphoric acid with this energy and stores it as polymeric phosphoric acid. This happens. Phosphorus content in cells is considerably low when the growth of microorganisms is fast, but increases under conditions where the growth of microorganisms is poor due to nutrient imbalance. In other words, an inverse relationship is established between the synthesis of the polymeric phosphoric acid and the nucleic acid. Phosphorus biological removal is the accumulation of excess phosphorus in activated sludge and removal of this sludge. The accumulation of intracellular phosphorus is over-plus because it is necessary to create a temporary compression state in order to accumulate macromolecule phosphorus intracellularly. And by a mechanism called Luxury-Uptake.

The over-plus microorganism is a phenomenon in which phosphorus is rapidly ingested after the supply of phosphorus is low and the phosphorus concentration is high again. The up-take takes phosphorus into the cell in a state in which a limited element other than phosphorus is restricted. When there is enough energy to move, it means excessive phosphorus. Intracellular accumulation of phosphorus occurs when the supply of substrate from the outside is greater than the amount of substrate required, and the stored compound is used when the supply of energy from the outside is less than the cell minimum. As described above, the biological phosphorus removal method utilizes such excess phosphorus by using phosphorus accumulating microorganisms to release the macromolecular phosphoric acid stored in the anaerobic state and ingest excess phosphoric acid as an intracellular storage material for growth in the aerobic state. As a treatment process for removing phosphorus through the removal of phosphorus, it is advantageous for a large amount of treatment, but the stability of the treatment is insufficient.

Conventional processes for the simultaneous removal of nitrogen and phosphorus have been developed such as PhoStrip, Bardenpho, Anaerobic / Aerobic (A ₂ / O), UCT processes, etc. As a process for stably removing phosphorus, a part of the return sludge is eluted from the final sludge of the activated sludge process in an anaerobic tank, and the coagulation and precipitation of lime is added to remove phosphorus. However, when nitrification is made in the aeration tank, the release of phosphorus is inhibited due to the effect of nitrogen oxides, chemical costs are high, and microorganisms of activated sludge are killed.

The anaerobic / aerobic (A ₂ / O) process installs anaerobic, anaerobic and aeration tanks where the return sludge is mixed with the incoming wastewater in the anaerobic tank, resulting in the absorption of organics and release of phosphorus in the anaerobic state, followed by the nitrification process in the aerobic tank. The wastewater passed through the wastewater is returned and denitrification occurs.In the aeration tank, the process of removing phosphorus by sludge disposal after excessive ingestion of phosphorus in microorganisms is relatively simple but difficult to set operating conditions and difficult to remove stable phosphorus. Have.

In the UCT process, an anaerobic tank, an anoxic tank, and an aeration tank are installed to bring the return sludge into the anoxic tank to increase the efficiency of phosphorus removal, but the sedimentation and concentration of sludge is reduced.

In the modified Bardenpho process, the reaction tank is installed in the order of anaerobic tank, first anaerobic tank, first aeration tank, second anaerobic tank, and second aeration tank, and the return sludge flows into the anaerobic reactor, and denitrification reaction occurs in the first and second anaerobic tanks. Phosphorus is removed through the excess sludge treatment of the aeration tank. This process has a very high nitrogen removal rate, a high proportion of phosphorus in the sludge and a relatively stable treatment [Barth, E.F., Project Officer, Summary Report: Workshop on Biological Phosphorus Removal in Municipal Wastewater Treatment, US EPA (1982); and Barnard, J. L .: "Biological Nutrient removal without the addition of chemicals", Water Research 9 485 (1975).

Among the methods developed above, regarding the PhoStrip process, Gilbert V Levin et al. Are disclosed in US Patent No. 4,141,822 and Korean Patent Application No. 89-908 (Notice No. 91-3004) to Kim Chang-Hee, The anaerobic / expiratory method is disclosed in Korean Patent Application No. 82-1616 (Notice No. 87-107) to Matsuo Yoshidaka et al.

Accordingly, the present inventors first make the microorganisms ingest the phosphorus in the primary aeration tank in order to enhance the removal rate of phosphorus as well as organic matter and nitrogen while taking advantage of the modified Badenpo process, which has a high removal rate of nitrogen and enables stable treatment. The removal of the sludge contained by filtration eliminates the influence of nitrogen oxides on the removal of phosphorus and then denitrification in an anoxic bath via nitrification to improve the existing process to complete the present invention.

Therefore, an object of the present invention is to provide an efficient and economical It is to provide a method for biological removal of nitrogen and phosphorus in the waste water. That is, an object of the present invention is to provide a combination of anaerobic-aerobic activated sludge process by microorganisms, It provides a method for efficiently and economically removing contaminants such as organic matter, nitrogen and phosphorus in waste water under high removal rate.

Still another object of the present invention is to provide a method of using biogas generated during anaerobic treatment using an anaerobic treatment tank as a main process for removing organic matter.

Hereinafter, the present invention will be described in detail.

The present invention improves the modified Badenpo process, by connecting the reaction treatment tank in the order of anaerobic treatment tank, the first aeration tank, nitrification tank, denitrification tank, re-aeration tank and settling tank To remove organic matter, nitrogen and phosphorus in the waste water Provide a wastewater treatment method.

In the present invention, by treating for a short contact time of 0.5 to 1.5 hours to prevent nitrification in the first aeration tank during the removal of phosphorus, by installing a sand filtration tank to prevent elution of phosphorus due to the anaerobic filtration layer during the sludge removal process Rapid sand filtration and nonwoven fabrics placed on the top surface of the sand filtration layer facilitate treatment of sludge and reduce power and time due to backwashing. In the nitrification tank, residual organic matter is removed and nitrification is carried out. In the anoxic tank denitrification tank, denitrification reaction is carried out. At this time, a return sludge recycled from the final precipitation tank was used as the electron acceptor. The nitrification tank and the denitrification tank are separated into two or more reaction tanks, respectively, and performing in two or more stages is preferable to increase the treatment efficiency in each treatment tank. In the reaeration tank, aeration is carried out to prevent the dissolution of residual phosphorus in the final sedimentation basin and to protect the water quality of the effluent.The sludge of the final sedimentation basin is recycled to minimize the amount of surplus sludge, and the biogas generated in the anaerobic tank By heating and sterilizing the sludge supplied to the anoxic tank, the sludge is easily used as an electron acceptor and the temperature of the anoxic tank is controlled.

Features of the present invention will be described in detail with reference to the process diagram shown in FIG.

From the return sludge (17) and the sand filtration tank (6) where the pretreatment effluent (1), such as municipal sewage, alcoholic waste, and food wastewater, first treated in the first settling basin (not shown) is recycled in the final settling tank (16). Into the anaerobic treatment tank (2) with the backwash water (8) to be recycled and mixed. Here, the organic matter is removed, and under the anaerobic conditions, the macromolecular phosphorus in the microbial cell is converted into the phosphate form and released. The conditions in the reactor are preferably free of oxygen and bound oxygen, which are electron acceptors. Concentration is 1-3 Of It is preferred that the redox potential value, which is below and used as a measure of anaerobic level, is maintained between -200 and -300 mA. In addition, the anaerobic treatment tank (2) is separated into two or more reactors, not only reduces the effect of nitrogen oxide and free oxygen contained in the return sludge, but also reduces the volume of the entire reactor, It is preferable to separate and connect in series, and in the anaerobic treatment tank Or more, preferably 60 The residence time is long enough, preferably 4 to 10 hours, to remove internal and external organic matter, and the biogas generated therein is used for the return sludge treatment flowing into the denitrification tank.

Effluent 3 from the anaerobic treatment tank is introduced into the first aeration tank 4, the total concentration in the aeration tank is 2 to 3.5 Of While maintaining so as to be in contact for 0.5 to 1.5 hours, preferably 60 minutes to 80 minutes so that the phosphorous acid in the effluent (3) is ingested in the form of polymeric phosphoric acid in the microorganism body to over-accumulate. At this time, if nitrification occurs in the aerobic state, the removal efficiency of phosphorus is lowered so that the residence time does not exceed 1.5 hours. In the first aeration tank 4, the organic matter remaining in the anaerobic treatment tank effluent 3 is removed by simultaneously absorbing excessive amounts of phosphorus in the wastewater to which microorganisms are treated, and simultaneously removing the organic matter. To process

The outflow water 5 from the first aeration tank 4 flows into the sand filtration tank 6. The sand filtration tank 6 is a step for separating and removing the microorganism sludge containing an excess of phosphorus in the first aeration tank step to cover the nonwoven fabric on the upper surface of the conventional sand filtration device, the conventional backwash Instead of the process of removing the containing sludge, two or more layers, preferably two to four layers of nonwoven layers, reduce the time and power required for conventional backwashing, as well as 45 to 65 remaining unfiltered on the nonwovens. It is easy to remove the microbial sludge and the used nonwoven fabric can be reused after washing, so the power and the number of backwashes required for conventional backwashing are reduced, and the sludge removed is also easy to concentrate. The excess phosphorus containing sludge filtered on the nonwoven fabric of the sand filtration tank 6 is disposed of as waste sludge 7 and the remaining sludge is recycled to the anaerobic tank 2 as backwash water 8.

In still another aspect of the present invention, a precipitation tank may be used instead of the sand filtration tank to remove excess phosphorus-containing sludge.

The nitrification tank 10 flows in with the outflow water 9 passed through the sand filtration tank 6 and the return sludge 17 recycled from the final settling tank 16 together. The nitrification tank 10 is preferably separated in two or more stages to increase the nitrification efficiency, in which case the first stage is 2 Of The aeration rate is adjusted to maintain the above oxygen concentration, and at the last stage, the aeration rate is lowered to lower the concentration of dissolved oxygen in the treated water so as to flow into the denitrification tank 12 which is the next step. The residence time in the nitrification tank is 2 to 6 hours, preferably 2.5 to 4 hours. In the nitrification tank 10, ammonia nitrogen and nitrous acid are converted to ammonia nitrogen using nitrifying bacteria such as nitrosomonas and nitrobacter. 75 to 90 of the organics present in the treated water converted to nitrogen and introduced into the nitrification tank Remove the abnormality.

The treated water 11 which has passed through the nitrification tank 10 is introduced into the denitrification tank 12 together with the return sludge 17 of the final precipitation tank 16. In the denitrification tank 12, in order to reduce and remove nitrogen in the state of nitric oxide, the inflow of outside air is prevented and operated under anoxic conditions without dissolved oxygen, and when free oxygen is present, respiration of microorganisms using combined oxygen such as nitrogen oxide Rather than breathing with free oxygen, the free oxygen acts as a deterrent to the denitrification reaction, so the denitrification tank 12 must be completely sealed, and the gas outlet and agitator are installed on the upper part of the denitrification tank 12 to completely mix. This can be done and at the same time facilitate the discharge of the generated gas. In the denitrification tank 12, nitrogen in the form of nitric oxide is reduced and removed in the form of nitrogen gas for a contact time of 2 to 4 hours. In addition, 70 to 85 using the biogas generated in the anaerobic treatment tank 2 when the return sludge 17 is supplied to the denitrification tank 12. When heat sterilization is performed to a degree of temperature, when denitrification is carried out using microorganisms in sludge such as Pseudomonas, Micrococcus or Bacillus, the microorganisms in the conveying sludge are destroyed and the microbial cell constituents are transferred to the electron acceptor for reducing nitrate nitrogen It is used as a heat source for maintaining the temperature of the anaerobic tank while being easy to use.

The denitrification tank effluent 13, which has undergone the denitrification tank process, flows into the reaeration tank 14 to stabilize the water quality of the effluent and remove dissolved odors by adding dissolved oxygen to prevent anaerobic conditions in the final sedimentation tank 16. To undergo aeration for 30 to 60 minutes.

Subsequently, the reaeration tank effluent 15 is separated into a return sludge 17 which is introduced into the final settling tank 16 and recycled to the preceding process step by the gravity settling process and the final settling tank effluent 18 discharged from the final settling tank. The conveying sludge 17 is an anaerobic treatment tank 2, a nitrification tank 10, and a denitrification tank 12, respectively, without waste sludge. , 45 to 60 And 10 to 20 By conveying the total amount of sludge, it is possible to improve the removal rate of nitrogen and phosphorus as well as reduce the amount of waste sludge.

The present invention was performed to remove nitrogen in order to compensate for instability, which is a disadvantage of the conventional nitrogen and phosphorus biological removal process to provide a stable treatment efficiency, and to carry out the nitrogen removal process and to place the non-woven film on the upper surface of the sand filtration tank containing excess phosphorus One sludge can be removed easily and quickly, reducing the power and time of the backwashing process used in the conventional removal process, thereby reducing the overall cost and preventing re-elution of phosphorus by anaerobic treatment during final precipitation.

Another feature of the present invention is that the sludge is recycled from the final settling tank 16 and introduced into the denitrification tank 12 by sterilizing and supplying the biomass generated from the anaerobic treatment tank 2 to supply the sludge. It is easy to use as a receptor and by recycling the total amount of the sludge discharged from the sedimentation tank 16 to increase the efficiency and stability of the treatment.

As described above, the present invention has a high removal efficiency of phosphorus as well as a nitrogen removal rate as compared to the conventional method for treating nitrogen and phosphorus, and it is easy to remove sludge containing excess phosphorus in a sand filtration tank, and anaerobic treatment tank. It is possible to reduce the cost of energy by using organic materials and to use biogas, and to convey the total amount of sludge flowing out of the final sedimentation tank to increase the treatment efficiency and stability as well as to facilitate the treatment of sludge. Elution of residual phosphorus is prevented and the quality of the effluent is improved.

This invention will be described in more detail based on the following examples. The following examples are intended to specifically illustrate the invention and are not intended to limit the scope of the invention.

Example 1

In this embodiment, the municipal sewage having the composition shown in Table 1 is used as treated water, and treated according to the treatment apparatus configuration as shown in FIG. 1 to remove organic matter, nitrogen, phosphorus, etc. according to the conditions in each treatment tank. I found out. The anaerobic tank consists of two hermetically sealed agitators and the capacity of each tank is four ports to make 20-60 The organic material and sludge were mixed smoothly. The first aeration tank is supplied with air by a diffuser and has four ports as a single tank, giving a capacity of 10-30. A nonwoven layer was placed on top of the sand filtration tank to remove excess phosphorus-containing sludge remaining on the top surface of the nonwoven fabric together with the nonwoven fabric and the nonwoven fabric was washed and reused. Nitrifiers supply air by the diffuser and have a capacity of 10-40 with four ports. Reactors were used in series connection in three stages. Anaerobic denitrification tank has a capacity of 4 to 10 ports. Reactors were connected in series in two stages, and a reaeration tank and a final settling tank were installed. The total sludge flowing out of the final settling tank was recovered and re-supplied to the anaerobic tank, nitrification tank and denitrification tank, respectively. The experiment of this example was carried out for 25 days, and the decrease rate of BOD (biological oxygen demand) of the treated water was investigated while changing the residence time as shown in Table 2 to determine the efficient organic removal and phosphorus release in the anaerobic tank. The results are shown in Table 2 below.

As can be seen in Table 2, the BOD reduction rate according to residence time is 49.3 to 56.4 at a residence time of about 3 to 4 hours. The reduction rate of was found to be the most efficient.

In the results of the present embodiments, the efficiency is expressed as an optimum concept between the efficiency per unit time and the material to be removed as the removal rate with respect to the unit time.

Example 2

In the present embodiment, in order to determine the change in the removal rate according to the residence time in the first aeration tank, Example 1 except that the treatment is performed for 14 days while changing the residence time in the first aeration tank as shown in Table 3 below. The same treatment process was carried out. The results are shown in Table 3 below.

From the results in Table 3, it can be seen that the efficiency of the phosphorus removal rate in the first aeration tank according to the residence time is the best at 60 to 80 minutes.

Example 3

In this embodiment, in order to determine the change in the removal rate of the unfiltered sludge according to the number of use of the nonwoven fabric located in the sand filtration tank in the sand filtration process, except that the number of nonwoven fabric in the top of the sand filtration tank is changed to 1 to 5 The same treatment process as in Example 1 was carried out. The results are shown in Table 4 below.

As can be seen in Table 4, the removal rate of the unfiltered sludge was shown to have the highest efficiency when using two to four nonwoven fabrics.

Example 4

In this embodiment, in order to determine the change in the removal rate of BOD and ammonia nitrogen with the residence time in the nitrification tank, the residence time in the nitrification tank was changed as shown in Table 5, except that The same treatment process was carried out. The results are shown in Table 5 below.

As can be seen in Table 5, the treatment efficiency was the highest when the residence time of the nitrification tank is 2.5 to 4 hours.

Example 5

This example is to determine the change in the removal rate of total Kjeldahl nitrogen with the residence time in the denitrification tank, the same as in Example 1 except for changing the retention time in the denitrification tank as shown in Table 6 below. The treatment process was carried out. The results are shown in Table 6 below.

It can be seen from Table 6 that the treatment efficiency is the best when the residence time in the denitrification tank is 2.5 to 4 hours.

Example 6

The residence time in the anaerobic tank is 3.5 hours, the residence time in the first aeration tank is 1 hour, the residence time in the nitrification tank is 4 hours, the residence time in the denitrification tank is 3.5 hours, and the residence time in the reaeration tank is 1 hour. The wastewater was treated as in Example 1 above. As a result, the change of the composition of the treated water before and after the treatment according to the present invention is shown in Table 7 below.

Comparative Example 1

Wastewater was treated with the same residence time and conditions as in Example 6 except that it was treated by an activated sludge process instead of the process of the present invention. As a result, the removal rate of components such as organic matter, nitrogen and phosphorus in the treated water is shown in Table 8 below.

As can be seen from the results of the above examples and comparative examples, the wastewater treated according to the present invention has a very low content of nitrogen and phosphorus as well as organic matter, whereas the wastewater treated by the conventional activated sludge process has a content of organic matter. Was relatively low, but the nitrogen and phosphorus contents were about 20 to 30 compared to before treatment. Only was removed. Thus, the present invention provides an improved method for biological removal of nitrogen and phosphorus with superior effects and economics over conventional wastewater treatment methods.

While the present invention has been described above in connection with specific embodiments, it should be understood that changes, modifications, and modifications apparent to those skilled in the art without departing from the scope of the present invention fall within the scope of the present invention. .

Claims (9)

In the wastewater treatment method for removing organic substances, nitrogen and phosphorus-containing eutrophication, the organics are mixed by introducing the effluent from the first settling basin, the return sludge recycled in the final settling tank and the excess phosphorus containing sludge recycled in the sand filtration tank into the anaerobic tank. Removing and eluting phosphorus in the form of a polymer in the form of ortho phosphate; Flowing the effluent of the anaerobic tank into the first aeration tank to overabsorb phosphorus on the microorganism; Filtering out excess phosphorus-containing sludge of the first aeration tank in a sand filtration tank and returning a portion thereof to the anaerobic tank; Introducing the nitrate through the sand filtration tank into the nitrification tank with the return sludge recycled in the final settling tank to nitrify the effluent; Introducing nitrified treated water into a denitrification tank with recycled sterilized return sludge in a final settling tank to remove nitrogen from nitrogen oxide to nitrogen gas under anoxic conditions; Supplying oxygen by introducing the treated water that has undergone the denitrification into a reaeration tank; The discharge of the reaeration tank flows into the final sedimentation tank after gravity precipitation treatment to discharge the supernatant and returning the settling sludge, wastewater treatment method to increase the removal rate of nitrogen and phosphorus. The method of claim 1, wherein the anaerobic bath consists of two or more stages. The method of claim 1 wherein the contact time in the first aeration tank is from 0.5 to 1.5 hours. The method of claim 1, wherein the precipitation tank is used to remove excess phosphorus sludge in place of the sand filtration tank. The method of claim 1, wherein at least two layers of nonwoven are placed on the top surface of the sand filtration bath. The method according to claim 5, wherein the remaining sludge is returned to the anaerobic tank except for the sludge remaining on the upper surface of the nonwoven fabric of the sand filtration tank. The method according to claim 1, wherein the denitrification tank is composed of two or more stages. According to claim 1, wherein the total amount of sludge discharged from the final settling tank is 30 to 45 to the anaerobic tank, nitrification tank and denitrification tank, respectively , 45 to 60 And 10 to 20 How to bounce at a rate. According to claim 8, 70 to 85 using the biogas generated in the anaerobic tank sludge flowing into the denitrification tank Heat treatment to a temperature of.