JPH0134677B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a wastewater treatment method including biological treatment, which is used to purify acid wastewater containing nitric acid or organic acids, general organic matter-containing wastewater, and the like. (Prior art and its problems) In recent years, a large amount of highly concentrated nitric acid-containing wastewater has been produced from various denitrification devices such as stainless steel and silicon etching processes and flue gas denitrification. However, since no effective treatment method has been established to independently purify nitric acid-containing wastewater that is satisfactory in terms of processing efficiency, equipment, and operating costs, it is not possible to treat wastewater that contains nitric acid by simply performing neutralization treatment. Currently, wastewater is uniformly treated by mixing it with other wastewater, and nutrient enrichment due to nitrogen in the treated water and salt pollution have become major problems. On the other hand, in the case of ordinary wastewater containing organic substances such as sewage or wastewater containing organic acids, BOD is generally removed by biological treatment using activated sludge under aerobic conditions. However, in such aerobic biological treatment, oxygen supply is rate-limiting, so high-load operation aimed at increasing the concentration of activated sludge is limited by the aeration rate, and bulking occurs as the sludge concentration increases. There was a problem in that swelling phenomena such as these easily occur. (Means for Solving the Problems) This invention was made in view of the above circumstances, and uses biological treatment to treat nitric acid-containing wastewater (hereinafter referred to as
It is possible to perform good _{denitrification} and desalination of organic matter-containing wastewater and organic acid-containing wastewater (hereinafter referred to as organic matter wastewater and organic acid wastewater) under high sludge concentration. Moreover, it is an object of the present invention to provide a wastewater treatment method that can perform efficient combined treatment of these wastewaters and requires low equipment and treatment costs. That is, the wastewater treatment method according to the present invention, for the above purpose, includes a dissolution step of mixing excess sludge consisting of Ca or Mg carbonate and activated sludge with acid component-containing wastewater to dissolve Ca or Mg as a soluble salt. , a solid-liquid separation step in which the activated sludge is decomposed and removed after this dissolution, and a biological treatment in which the separated liquid is biologically treated in the presence of the carbonate and activated sludge, and the carbonate is produced as a by-product from the soluble salt. a solid-liquid separation step for separating treated water after biological treatment from the carbonate and activated sludge, and a return step for sending the separated excess sludge to the dissolution step. (Operation of the invention) Biological treatment in this invention is performed in the presence of Ca or Mg carbonate and activated sludge (hereinafter referred to as MLSS). That is, the carbonate acts as a biological carrier for MLSS, and the carbonate particles surface
Because the biota of MLSS develops, the biological treatment process
It is possible to maintain a high concentration of MLSS, and as a result, it is possible to operate under a high load of organic matter.Furthermore, because it has excellent sedimentation properties, even with a high concentration of MLSS, it does not cause swelling phenomena such as bulking, and the treated water can be used in the solid-liquid separation process. can be separated with high efficiency. In addition, by adding and mixing excess sludge after solid-liquid separation to acid component-containing wastewater, Ca or Mg is dissolved as a soluble salt and transferred to the liquid phase. surplus equivalent to
Can remove MLSS. However, since the above-mentioned soluble salts are automatically converted to carbonates in the biological treatment process,
Due to its solubility, the Ca or Mg component is recycled and reused except for a small amount that is eluted and lost in the effluent treatment water. In the initial biological treatment process, Ca or Mg carbonate is removed by using Ca or Mg hydroxide as a neutralizing agent for acid-containing wastewater.
It is automatically produced as a by-product during biological processing. Therefore, after the ratio of carbonate to MLSS reaches equilibrium in the biological treatment process, the amount of neutralizing agent to be used is only a small amount corresponding to the amount of Ca or Mg that escapes into the effluent treatment water. In addition, with this treatment method, even if the acid concentration of the wastewater is high, that is, the pH is low, when it is introduced into the biological treatment process, it is neutralized by the carbonates that are present in large quantities in the process, so there is no problem. There is no problem with processing. Here _, the concentration ratio of the carbonate and MLSS at equilibrium in the biological treatment process is regulated by the sludge conversion rate of the carbonate _and organic matter. MLSS in salt treatment:
It has been found that CaCO ₃ ≒1:10 (weight ratio). For this concentration ratio, the MLSS is 10000-13000
When operating at mg/, the total solids (SS) is
Although the concentration reached 110,000 to 143,000mg/, no swelling of the sludge was observed, and it was found that the SVI was always below 4, demonstrating that CaCO ₃ functions very effectively as a biological carrier. has been done. The wastewater to be treated in this invention is HNO ₃
Various types of wastewater include acid wastewater, organic acid wastewater, and general organic wastewater that contain other acid components along with HNO ₃ , but in the case of organic wastewater that does not contain acid components, the Ca or Mg content of excess sludge is dissolved. It may be combined with the acid component-containing wastewater to be treated. As for biological treatment, existing treatment methods such as denitrification treatment, nitrification-denitrification treatment, aerobic biological treatment, etc. and combinations thereof can be selected depending on the type of wastewater, but treatment means including denitrification treatment are particularly useful. be.
In other words, denitrification treatment has the advantage of greater organic matter decomposition ability than aerobic biological treatment, and this advantage is further enhanced by increasing the concentration of MLSS using carbonate as a biological carrier. Even in the case of organic acid wastewater or general organic wastewater that does not contain HNO ₃ , highly efficient BOD removal is possible by combining it with HNO ₃ wastewater and subjecting it to denitrification treatment. After this denitrification treatment, it is preferable to perform aerobic biological treatment to decompose residual organic matter such as residual hydrogen donors, but since the residual organic matter in this case is extremely small, the amount of aeration is small. good. (Example) Hereinafter, an example of the present invention will be described based on process diagrams shown in FIGS. 1 and 2. In addition, in the following, the carbonate in the biological treatment step is represented by CaCO ₃ , but MgCO ₃ may also be used. Figure 1 shows the treatment process for _HNO3 wastewater. first
Excess sludge consisting of CaCO ₃ and MLSS in the HNO ₃ wastewater dissolution process and a small amount as a neutralizing agent
Ca(OH) ₂ is added and mixed. At this time, due to the reaction CaCO ₃ +2HNO ₃ →Ca(NO ₃ ) ₂ +H ₂ O+CO ₂ ↑ Ca(OH) ₂ +2HNO ₃ →Ca(NO ₃ ) ₂ +2H ₂ O, the Ca content is accompanied by the generation of CO ₂ . Since it dissolves in the liquid, it is used in the next solid-liquid separation process.
Only MLSS is separated and removed as a solid component. This separated liquid is then led to the denitrification process of biological treatment.
In the presence of CaCO ₃ , MLSS, and a hydrogen donor in a slightly larger amount than the theoretical amount, the mixture is stirred for the required time under non-molecular oxygen conditions (closed) to perform denitration treatment. The reaction in this case is, for example, when the hydrogen donor is CH ₃ COOH, 8Ca(NO ₃ ) ₂ +5(CH ₃ COO) ₂ Ca MLSS ------→ 13CaCO ₃ +15H ₂ O+7CO ₂ ↑+8N ₂ ↑ It is shown that NO ₃ ^- and organic matter are biologically decomposed to generate CO ₂ and N ₂ ,
Ca content precipitates as CaCO ₃ . As this hydrogen donor, in addition to CH ₃ COOH,
CH ₃ OH etc. are also used, but other wastewater BOD substances may also be used. That is, by introducing organic acid wastewater or general organic wastewater into the denitration process, the organic substances act as hydrogen donors and are decomposed, making it possible to combine HNO ₃ wastewater and these wastewaters. The liquid after the above-mentioned denitrification treatment is led to an aerobic biological treatment step, and the residual hydrogen donor is biologically decomposed by aeration. The liquid after this biological treatment is separated into solid content consisting of CaCO ₃ and MLSS and treated water in a solid-liquid separation process. Excess sludge corresponding to growth is returned to the dissolution process, and the treated water is almost completely denitrified and treated water. Since it has been desalinated and the BOD is close to 0, it can be discharged. Figure 2 shows the treatment process for organic acid wastewater. first,
In the dissolution process, excess sludge and a small amount of Ca(OH) ₂ are added and mixed to the wastewater in the same way as HNO ₃ wastewater, and the Ca content is dissolved as an organic acid Ca salt with the generation of CO ₂ . In the next solid-liquid separation step, only the MLSS is separated and removed, and the separated liquid is led to the denitrification step of biological treatment. Since _HNO3 wastewater is introduced into this denitrification process, by holding it under non-molecular oxygen conditions in the presence of _CaCO3 and MLSS for the required time, organic acid radicals ^and _NO3- are decomposed and CO ₂ and N ₂ are generated, and Ca content is precipitated as CaCO ₃ . Next, in the aerobic treatment process, the remaining organic acid Ca salt is biologically decomposed and its Ca content is removed.
Precipitates as _CaCO3 . The liquid after biological treatment is separated into effluent treated water and solids by solid-liquid separation, and excess sludge is returned to the dissolution process. In addition, in the treatment of this organic acid wastewater, instead of performing the above-mentioned denitrification treatment as biological treatment, aerobic biological treatment may be performed as shown within the dashed-dotted line in FIG. Even in this aerobic treatment, organic acids
Ca salt is decomposed and Ca content is precipitated as CaCO ₃ ,
Excess sludge obtained from the subsequent solid-liquid separation is returned to the dissolution process. However, if the organic acid concentration is high, biological treatment including the above-mentioned denitrification treatment is preferable. (Treatment test example) Test example 1 The treatment effect of _HNO3 wastewater was investigated under the following conditions. <Biological treatment tank> A 30 x 30 x 30 cm transparent PVC container equipped with a canopy, a stirrer, and an aeration pipe, with an effective volume of 17 cm. <Raw water> Calcium nitrate [Ca (NO ₃ ) ₂・4H ₂ O, special grade]
Dissolved in pure water to obtain 132,250 mg of Ca(NO ₃ ) ₂ /
Take 340 ml of the solution and add CH ₃ COOH as a hydrogen donor in an amount that gives a weight ratio of 1:1 to NO ₃ ^- , then add nutrients, and finally add pure water. The total volume was 3400ml. <Activated sludge> Using MLSS obtained from a sewage treatment plant, an acclimatization test was conducted over a long period of time, and the concentration of MLSS and CaCO ₃ reached equilibrium (in the measurement results, the weight ratio of MLSS:CaCO ₃ = 1:10). What I did. <Treatment operation> Approximately 4,000 ml of supernatant liquid from the biological treatment tank in which activated sludge (MLSS + CaCO ₃ ) has settled is extracted, and 3,400 ml of raw water is extracted.
was introduced, and the total volume was made up to 17 by adding the supernatant liquid extracted above. Note that the MLSS concentration is 10000 mg/. Subsequently, a stirrer was rotated under sealed conditions to perform denitrification treatment, followed by aeration to treat undecomposed organic matter under aerobic conditions, and then left to stand to perform solid-liquid separation. Thereafter, the above steps were repeated at a treatment cycle of 24 hours/day. The end point of the denitrification process was detected from the change in the oxidation-reduction potential, and the results revealed that the time required for denitrification was between 4 and 6 hours. In addition, SVI was always below 4. The results of the above treatment are shown in Table 1. In addition, NO ₃ ^- concentration, NO ₂ ^- concentration, BOD, COD,
Figure 3 shows the average value of the change in oxidation-reduction potential (ORP) over time. Note that t in FIG. 3 is the time point at which aeration begins. Moreover, the BOD of the treated water was almost 0.

[Table] This test result shows that HNO ₃ wastewater can be highly denitrified and desalted by applying the method of this invention. Furthermore, since the ability to decompose CH ₃ COOH by denitrification treatment is extremely large, it is clear that high-level combined treatment becomes possible by introducing general organic wastewater or organic acid wastewater in place of CH ₃ COOH. Test example 2 Acid waste water (containing HF, HNO ₃ , CH ₃ COOH) discharged from the silicon etching process is currently neutralized with slaked lime and F ^- ions are removed as poorly soluble CaF ₂ through solid-liquid separation. After that, the separation liquid [Ca
(NO ₃ ) ₂ , (CH ₃ COO) ₂ Ca] is mixed with other wastewater for treatment. Here, a two-month continuous treatment experiment was conducted in the same manner as in Test Example 1, using the mixed wastewater as the treatment target. The results are shown in Table 2.

[Table] In addition, in order to investigate the applicability of directly treating the above acid waste water (HF concentration 2000 mg/
) was added with the same activated sludge (MLSS + CaCO ₃ ) used in Test Example 1 to adjust the pH to 6, and when the F ^- ion in the treated supernatant was measured, it was found to be 5.7mg.
It was F/. As a result, it was demonstrated that there is no problem in directly treating the acid waste water using the method of the present invention. (Effect of the invention) According to the wastewater treatment method according to the present invention, activated sludge in the biological treatment process can contain Ca or Mg as a by-product.
Since carbonate is retained at a high concentration as a biological carrier, it is possible to operate under a high load of organic matter.
Various types of wastewater such as _HNO3 wastewater, organic acid wastewater, and general organic wastewater can be efficiently purified, and combined treatment of these wastewaters is also possible.Compared to conventional methods, the above-mentioned high-load operation reduces equipment and reduces electricity consumption. Moreover, since the above-mentioned by-product carbonate is reused and elution of Ca or Mg into the treated water is extremely small, the amount of neutralized product used can be extremely reduced. Excellent denitrification and desalination effects are achieved.

[Brief explanation of drawings]

FIGS. 1 and 2 are process diagrams showing an example of the wastewater treatment method according to the present invention, and FIG. 3 is a characteristic diagram showing changes over time in various treatment indicators in treatment test example 1.

Claims (1)

[Claims] 1 Excess sludge consisting of Ca or Mg carbonate and activated sludge is mixed with acid component-containing wastewater to produce Ca or Mg.
A dissolution step in which Mg is dissolved as a soluble salt, a solid-liquid separation step in which activated sludge is separated and removed after this dissolution, and this separated liquid is biologically treated in the presence of the carbonate and activated sludge, and the soluble salt is A biological treatment step in which the carbonate is produced as a by-product, a solid-liquid separation step in which the treated water after biological treatment is separated from the carbonate and activated sludge, and a return step in which the separated excess sludge is sent to the dissolution step. A wastewater treatment method. 2. The wastewater treatment method according to claim 1, wherein the acid component-containing wastewater is nitric acid-containing wastewater, and the biological treatment step includes biological denitrification treatment.