JPS6349554B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention applies to non-decomposable materials such as flue gas desulfurization wastewater.
It concerns a biological treatment method for wastewater containing COD and nitrogen compounds. Flue gas emitted from boilers and other combustion equipment using fossil fuels contains high concentrations of sulfur oxides and nitrogen oxides. If these are discharged into the atmosphere without treatment, they will cause many problems in terms of environmental conservation, both because of their own toxicity and because they change into even more harmful substances through photochemical reactions in the atmosphere. Therefore, in accordance with legal regulations, flue gas containing these substances is subjected to denitrification and desulfurization treatment before being discharged into the atmosphere. One of the methods most commonly used today for denitrification and desulfurization of flue gas is a method in which sulfur oxides and nitrogen oxides in flue gas are absorbed and removed by an alkaline solution. In this method, most of the sulfur oxides are removed as sulfates such as calcium sulfate and magnesium sulfate, but compounds that are difficult to be oxidized and decomposed such as dithionates are also produced as by-products. Among sulfur oxides that are difficult to decompose by oxidation, especially dithionic acid, decomposition by chlorine or ozone does not effectively decompose it, and physicochemical methods are mostly limited to methods using wet combustion. In view of the current situation, the present invention decomposes the above-mentioned sulfur oxide, a persistent COD component, by utilizing the biodegradation ability of microorganisms, and also decomposes it into nitrogen compounds (hereinafter abbreviated as "NH ₃ "). The purpose is to provide an effective and rational biological treatment method for The present invention involves adding organic matter to wastewater containing at least dithionic acid or its salt as nitrogen compounds and sulfur oxides, performing anaerobic biological treatment while absorbing and removing generated hydrogen sulfide, and then adding organic matter to the anaerobic biological treatment liquid. This is a biological treatment method for wastewater containing nitrogen compounds and dithionic acid, which is characterized in that biological nitrification and denitrification treatment is performed by adding an absorbent that has absorbed hydrogen sulfide. The basic structure of the present invention will be explained in detail below based on the research history that led to the present invention. It is well known that sulfur-oxidizing bacteria (such as the genus Thiobacillus) and sulfate-reducing bacteria (such as the genus Desulfoviburi) biodegrade dithionic acid and the like. This research also began with a simple continuous fermentation treatment method using anaerobic bacteria as an energy-saving treatment method for decomposing persistent COD components of sulfur oxides containing dithionic acid instead of the wet combustion method. Ta. The sample flue gas desulfurization wastewater is concentrated by dilution or physicochemical treatment such as ion exchange treatment.
Dithionic acid concentration is 100 mg/asS ₂ O ^2- ₆ , 300 mg/
_asS2O2-6 , _500mg / _asS2O2-6 ^, _1000mg ^/
Adjusted to 4 levels of asS ₂ O ^2- ₆ , and further added 1000 mg/(lactic acid) using lactic acid as a BOD source.
It was added and used so that it became. The inoculum used was a mixed bacteria cultured in an enrichment medium containing sodium dithionate from digested sludge obtained from an anaerobic digestion tank of sewage sludge, and the concentration in the experimental reaction tank was
Seeds were planted at 3000mg/・MLSS. As a result, the decomposition and removal of dithionic acid was almost completely achieved in the case of the sample wastewater with a dithionic acid concentration of 100 mg/day, but the dithionate concentration in the sample wastewater was
At 300 mg/dithionic acid, the removal rate of dithionic acid decreased to about 60%, and at dithionic acid concentrations of 500 mg/d or more, dithionic acid was hardly removed as days passed. As a result of investigating factors that inhibit the decomposition of dithionic acid, it was found that the hydrogen sulfide concentration in the treated water at the time when the inhibition phenomenon was observed changes depending on the dithionic acid concentration in the sample wastewater, as shown in Table 1. It was done.

[Table] The above experiment revealed that anaerobic biodegradation of dithionic acid cannot be continued by a simple anaerobic biodegradation method when the concentration of dithionic acid in the sample wastewater exceeds 100mg/ It is known that when this phenomenon occurs, the concentration of hydrogen sulfide in the reaction solution is relatively high. Next, dithionic acid concentration 300 mg/, 500
A similar biological treatment experiment was carried out on wastewater with concentrations of 1,000 mg/mg/mg/mg/mg/, and 1,000 mg/ml while desulfurizing the hydrogen sulfide concentration in the reaction solution to below 8 mg/ml. The decomposition treatment of dithionic acid was achieved. From the above results, it is known that in the anaerobic biodegradation treatment of dithionic acid, a high concentration of hydrogen sulfide is produced, which inhibits the decomposition of dithionic acid. The effectiveness of decomposition treatment while removing hydrogen has become clear. The inventors have already completed the process conceived based on the above knowledge (patent application
57-126811). In order to make further improvements, the present inventors investigated an effective treatment method for NH ₃ that cannot be removed by anaerobic biological treatment (a portion of NOx is removed in anaerobic biological treatment). In addition, considering the case where dithionic acid leaks from the anaerobic reaction tank due to fluctuations in water quality, we also investigated an aerobic dithionic acid decomposition method in the biological nitrification and denitrification treatment process. As a result of these studies, we found that the remainder of the BOD source used for anaerobic biological treatment can be used advantageously for biological denitrification, and that the absorption liquid or desulfurization agent that absorbed hydrogen sulfide generated from the anaerobic reaction liquid can be used for denitrification. The present invention was completed by discovering that it can be used as a nitrogen reducing agent and that by adding a hydrogen sulfide absorbing solution or a desulfurizing agent to the biological nitrification and denitrification process, the process has dithionate oxidation ability. It is. Next, an example of an embodiment of the present invention will be described in detail based on the drawings. Wastewater containing NH ₃ and dithionic acid discharged from a flue gas denitrification and desulfurization process, etc. is input as an inflow liquid 1 into an anaerobic biodecomposition tank 2 (hereinafter abbreviated as "decomposition tank"). The residence time in the decomposition tank 2 at this time is usually set in the range of 0.5 to 10 days, and the residence time of this wastewater can be determined depending on the type of organic matter 3 added to the decomposition tank 2. . When using methanol as the additive organic substance, the residence time can be particularly shortened to a range of 0.3 to 2.0 days, while when using sewage sludge, industrial wastewater sludge, human waste, etc., the residence time can be reduced to 4 days. It is necessary to take a slightly longer period of 1-10 days. In addition to these organic wastewaters that have an additive effect on the decomposition of dithionic acid, there are municipal sewage, pit garbage juice at garbage treatment plants, sewage sludge or leachate discharged from municipal garbage composting plants, and garbage landfills. leachate, leachate from manure including livestock manure from pig pens and cow sheds, and wastewater from barn cleaning, molasses from sugar mills, steam drains from pulp mills, brewing and distillation wastewater from breweries, food. Examples include factory wastewater from processing plants, and when these are used, the residence time in the reactor is 0.8
It is necessary to take a slightly longer period of 5.0 days to 5.0 days than when using methanol as the added organic substance. In addition, commercially available organic substances other than methanol that have an additive effect are alcohols such as propanol,
Butanol, glycerol, and ethanol can be mentioned, but among these, ethanol had a rather low addition effect. Among organic acids, additive effects were seen for formic acid, pyruvic acid, lactic acid, propionic acid, butyric acid, succinic acid, citric acid, and acetic acid. Particularly in the case of acetic acid, the addition effect was high when mixed with other organic acids. The reactor residence time for the addition of these organic acids is
Although it was necessary to make it about 0.6 to 4.0 days, which was slightly longer than in the case of methanol addition, it was found that it was shorter than in the case of adding the various waste sludge and wastewater. Also maltose,
Oligosaccharides such as cellobiose and glucose, and reducing sugars also required approximately the same residence time. The amount of organic matter to be added varies depending on the concentration of dithionic acid (salt) and other persistent COD substances in the wastewater to be treated, and the amount of organic matter to be added ranges from equivalent to the concentration of dithionic acid to 7 times the concentration of dithionic acid in terms of BOD. It is preferable to add it so that If the amount of added organic matter is less than this, dithionic acid will not be fully decomposed and undecomposed dithionic acid will remain in the treated water, and if more organic matter is added, the residual BOD concentration in the treated water will increase. However, since it can be used as a reducing agent for denitrification, it is preferable to add an excessive amount of organic matter from the standpoint of decomposing dithionic acid. In addition, in the present invention, the above-mentioned organic wastewater, organic sludge, alcohols,
It goes without saying that organic acids and saccharides can be used in appropriate combinations. Under the above-mentioned conditions of added nutrients and residence time, dithionic acid in the input wastewater is almost completely reduced and decomposed to hydrogen sulfide, and some remains as dissolved hydrogen sulfide in the tank liquid, but the rest remains. Transfers into digestive gas 4. The dithionic acid decomposition tank 2 of the present invention is operated under the temperature conditions adopted for normal methane fermentation, that is, 20°C to 40°C for decomposition using mesophilic bacteria, and 45°C for decomposition using thermophilic bacteria. Its efficiency can be significantly increased by being kept at ~70°C, especially operation at 45°C to 70°C is more effective in reducing the dissolved _H2S concentration, but it also reduces the decomposition efficiency. If there is no problem, operation without temperature control may be used. Various methods can be used to remove hydrogen sulfide from the liquid in the decomposition tank 2. For example, providing a gas circulation path for hydrogen sulfide accumulated in the digestion gas 4 and providing an absorption device 5 therein creates a low-concentration H ₂ S atmosphere in the gas phase of the head space of the decomposition tank, which allows hydrogen sulfide to remain in the liquid in the decomposition tank. This is done with the purpose of expelling H ₂ S. This H ₂ S absorption remover can be used with caustic soda solution, ferrous solution, calcium carbonate solution, lime solution, ammonia solution,
Liquid absorbents such as monoethanolamine may also be used.
It is also possible to use a solid absorbent such as iron oxide. In addition, the gas circulation route for hydrogen sulfide absorption is
For example, as shown in Fig. 1, by placing the air supply pipe from the gas circulation pump 6 below the liquid level in the decomposition tank and also using it for gas agitation, the concentration of hydrogen sulfide dissolved in the liquid can be reduced to the gas in the head space only. This is preferable because it can be further reduced by the stripping effect rather than by circulation. In addition to the above-mentioned gas absorption method, hydrogen sulfide can be removed by adding metal salts such as iron salts to the decomposition tank liquid to remove the hydrogen sulfide as insoluble sulfide. In any of these cases, the concentration of hydrogen sulfide in the decomposition tank liquid is preferably 20 mg/or less, and particularly preferably within the range of 0 to 8 mg/. If the pH in the decomposition tank shows extreme fluctuations, it is preferable to adjust the pH to a range of 5.5 to 7.5. This pH adjustment is effective not only in promoting the anaerobic biodecomposition reaction of dithionic acid, but also in performing gas circulation absorption and removal of hydrogen sulfide. This is especially true when the pH in the digester liquid is
It becomes important when it rises above 7.5. That is,
This is because at a pH higher than 7.5, the amount of dissolved hydrogen sulfide becomes significantly high, and hydrogen sulfide is not sufficiently gasified, making it impossible to keep the residual concentration of hydrogen sulfide in the liquid low. In addition to the embodiments described above, the present invention includes a semi-batch contact digestion method, which is a conventionally known anaerobic digestion device, and an upflow anaerobic filter bed as a form of an anaerobic biological treatment device for flue gas denitrification and desulfurization wastewater. The fact that most of the equipment belonging to the method, fluidized bed anaerobic reactor method, upflow anaerobic sludge blanket method, etc. This can be cited as an advantage of widening the range of selection when constructing a highly efficient device. Next, the decomposition liquid 7 flows into the first denitrification tank 10 under anaerobic conditions together with the return sludge 8 and the circulating nitrification liquid 9. The absorption liquid 11 or the organic matter 3 added from the outside is used as a reducing agent to be reduced and decomposed into _N2 gas by denitrifying bacteria. There are many types of denitrifying bacteria that use residual BOD and organic matter3, but there are bacteria that denitrify using hydrogen sulfide.
A limited number of species, such as Thiobacillus and Denitrificans, oxidize hydrogen sulfide to sulfuric acid using bound oxygen in NOx. As long as the amount of reducing agent added to the first denitrification tank 10 is sufficient for the residual BOD component, there is no need to intentionally inject the organic matter 3, which is normally valuable. After the NOx in the circulating nitrification solution 9 is denitrified, the NH ₃ in the decomposition solution 7 is oxidized to NOx in the next step, the nitrification tank 13 maintained under aerobic conditions, and most of it is in the first denitrification tank. 10, and the remainder flows into the second denitrification tank 14, where it is reduced and decomposed to _N2 using the organic matter 3 or hydrogen sulfide in the hydrogen sulfide absorption liquid 11 as a reducing agent. If the reducing agent used in the second denitrification tank 14 contains NH ₃ , NH ₃ will remain in the treated water 16' and the nitrogen removal rate will decrease. Preferably, it does not contain _NH3 . Therefore, when using NH ₃ -containing organic matter as the organic matter in the decomposition tank 2 , it can only be used as a reducing agent for denitrification in the first denitrification tank 10 , so the NH ₃ -containing organic matter can be used in the second denitrification tank 14 . It is preferable to add organic substances separately. The same applies to the hydrogen sulfide absorption liquid 11, and the hydrogen sulfide absorption liquid 11 containing NH ₃ is contained only in the first denitrification tank 10,
The hydrogen sulfide absorption liquid 11 that does not contain NH ₃ can be used in both the first denitrification tank 10 and the second denitrification tank 14. The effluent from the second denitrification tank 14 is transferred to the next re-aeration tank 15.
After the residual organic matter is oxidized and the _N2 gas in the liquid is diffused, it flows into the settling tank 16 where it is separated into solid and liquid, and the supernatant water is discharged as treated water 16'.
Most of the sludge is returned to the first denitrification tank 10 as return sludge 8, and the remainder flows into the decomposition tank 2 as surplus sludge 12 and is used as organic matter to decompose dithionic acid, or is treated in the surplus sludge treatment step 17.・It will be disposed of. As described above, in the present invention, even if dithionic acid remains in the treatment liquid 7 due to fluctuations in water quality, the dithionic acid can be quickly oxidized in the nitrification tank 13 under mainly aerobic conditions. Bacteria that originally have the ability to oxidize dithionate cannot obtain sufficient growth energy by oxidizing dithionate alone, so dithionate is used in the biological nitrification and denitrification process due to water quality fluctuations or constant residual activity. Even if dithionic acid flows in, dithionic acid-degrading bacteria will not be generated in this step alone, and therefore dithionic acid will not be removed. On the other hand, the energy generated by the oxidation of sulfur in the absorption liquid is fully utilized as energy for growth of dithionate-oxidizing bacteria. When oxidizing sulfur, dithionate oxidizing bacteria can use both molecular oxygen and combined oxygen of NOx, so the absorption liquid containing sulfur is injected into a denitrification tank and used as a reducing agent for denitrification, dithion. It is designed to double as an energy source for the growth of acid-oxidizing bacteria. Of course, even if the hydrogen sulfide absorption liquid 11 is injected into the nitrification tank 13, the dithionate oxidizing bacteria can be sufficiently grown. Although Fig. 1 shows the activated sludge method, there is a nitrification tank 13, a first denitrification tank 10, a second denitrification tank 14, and a re-aeration tank 15.
A granular or plate-shaped biofouling medium may also be used. In FIG. 1, 18 is an organic matter storage tank, and 19 is surplus gas. The present invention can be particularly effectively applied to denitrification and desulfurization wastewater since it contains a large amount of NH ₃ , but even in dithionic acid-containing wastewater with a low nitrogen content, NH ₃ is produced as organic matter in the decomposition tank 2. a lot of human urine,
Alternatively, using organic sludge that elutes NH ₃ during anaerobic decomposition is also effective in reducing the nitrogen concentration in effluent water. Furthermore, it is recommended to use an alkaline absorption liquid as the hydrogen sulfide absorption liquid. This is because it can be used as part of an alkaline agent that prevents the pH from decreasing due to sulfuric acid produced during the oxidation of sulfur. Next, the results of carrying out an embodiment of the present invention based on the flow shown in FIG. 1 will be described. The anaerobic reaction apparatus (corresponding to number 2 in FIG. 1) shown in FIG. 2 was used. In addition, in FIG. 2, 20 is a sedimentation tank for the anaerobic biological treatment liquid, 21 is a return sludge, and 22 is a water seal tank. The implementation conditions are as shown in Tables 2 and 3.

【table】

[Table] Comparing implementation numbers C and D in Table 3,
It can be seen that when the concentration of S ₂ O ^2- ₆ becomes high, a large amount of S ₂ O ^2- ₆ remains in D without the addition of a hydrogen sulfide absorption liquid. In addition, using the treatment equipment shown in the implementation conditions in Table 2, wastewater containing 1000 _mg of S ₂ O ^2-6 and 300 mg of NH ₃ -N per day is processed to treat human waste and sewage sludge as organic matter. When the excess sludge was treated with methanol, it was possible to remove dithionic acid and nitrogen at a high rate. However, methanol was used as the organic substance in the second denitrification tank. As described above, the present invention can bring about many effects listed below. Even if excess organic matter is added to the anaerobic biological treatment tank to accommodate fluctuations in the amount of dithionate inflow,
It can be effectively used for the next stage of biological denitrification. Since the absorbent that has absorbed hydrogen sulfide generated in the anaerobic biological treatment tank is used as a reducing agent for denitrification, the amount of organic matter added from the outside can be reduced and the absorbent can be treated at the same time. By injecting sulfur into the nitrification and denitrification process, microorganisms capable of oxidizing dithionic acid can be grown in the process, so when combined with the preceding anaerobic biological treatment process, extremely stable biological treatment of dithionic acid can be achieved. be able to. Since the excess sludge generated in the nitrification and denitrification process can be used for the reductive decomposition of dithionic acid, the amount of sludge generated in the entire process is reduced, and sludge treatment costs are reduced.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing one embodiment of the present invention, and FIG. 2 is a flow sheet showing an anaerobic reactor used in an example of the present invention. 1... Inflow liquid, 2... Decomposition tank, 3... Organic matter, 4... Digestion gas, 5... Absorption device, 6... Gas circulation pump, 7
... Decomposition liquid, 8 ... Returned sludge, 9 ... Circulating nitrification liquid, 10
...First denitrification tank, 11...Hydrogen sulfide absorption liquid, 12...Excess sludge, 13...Nitrification tank, 14...Second denitrification tank, 15
... Re-aeration tank, 16... Sedimentation tank, 16'... Treated water,
17... Surplus sludge treatment process, 18... Organic matter storage tank, 1
9... Surplus gas, 20... Sedimentation tank, 21... Return sludge,
22... Water seal tank.

Claims (1)

[Claims] 1. Organic matter is added to wastewater containing at least dithionic acid or its salt as a nitrogen compound and sulfur oxide, and after anaerobic biological treatment while absorbing and removing generated hydrogen sulfide, the anaerobic organism A biological treatment method for wastewater containing nitrogen compounds and dithionic acid, characterized in that biological nitrification and denitrification treatment is performed by adding an absorbent that has absorbed the hydrogen sulfide to the treatment liquid. 2. The method according to claim 1, wherein the anaerobic biological treatment is carried out while maintaining the hydrogen sulfide concentration of the biological reaction liquid at 20 mg/or less, preferably 8 mg/or less. 3. The method according to claim 1 or 2, wherein the absorbent that has absorbed hydrogen sulfide is added to the denitrification step of the biological nitrification and denitrification step. 4. The method according to claim 1 or 2, wherein the absorbent that has absorbed hydrogen sulfide is added to the nitrification step of the biological nitrification and denitrification step. 5 Methanol, ethanol,
2. A method according to claim 1, wherein a nitrogen-free substance such as acetic acid is used. 6. The method according to claim 1, wherein excess sludge obtained in the biological nitrification and denitrification step is used as the organic matter.