JPH0116560B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention aims to extract these compounds from wastewater containing ammonia and compounds containing nitrogen and sulfur in the same molecule (nitrogen-containing sulfur compounds, hereinafter abbreviated as NS compounds), such as denitrification and desulfurization wastewater from coal-fired power plants. It relates to a method of removal. Wastewater from coal-fired power plants contains NH ₃ (ammonia), imidodisulfonic acid (NH(SO ₃ ) ^2- ₂ ), and hydroxymonosulfonic acid (NH(OH)(SO ₃ ) ^- ), which are COD components. Contains NS compounds such as Since such NS compounds cannot be broken down by microorganisms, current practical treatment methods
Formula (1) NH(SO ₃ ) ^2- ₂ +H ₂ OH ⁺ −−→ NH ₂ SO ^- ₃ +H ₂ SO ₄ As shown in (1), for example, when imidodisulfonic acid is mixed at a pH of around 2 and around 80°C, After amidation, it further reacts with ^NO - 2 (nitrite ion) under acidic conditions as shown in formula ( ₂ ): NH ₂ SO ^- ₃ +NO ^- ₂ →N ₂ +SO ^2- ₄ +H ₂ O... (2) It is reacted and decomposed into _N2 gas and removed. The conventional wastewater treatment method for thermal power plants that employs NH ₃ nitrification, denitrification, and NS compound removal methods will be explained with reference to Figure 1. 2, the water temperature is 80℃, the pH is adjusted to 2 by adding acid 3, and the imide type NS compound is hydrolyzed into amide type NS compound. After the water is neutralized and the water temperature is lowered by heat exchange or injection of cooling water,
It flows into the nitrification step 6 under aerobic conditions together with the circulating denitrifying liquid 14, and the NH ₃ of the wastewater 1 is nitrified to NOx by the action of nitrifying bacteria. The purpose of circulating the circulating denitrification liquid 14 is to neutralize H ⁺ produced during nitrification with the alkaline content produced during denitrification, and an alkaline agent (for example, NaOH) 7 for neutralization is separately injected to compensate for the shortage. The nitrification process 6 is filled with a medium to which nitrifying bacteria are attached, and the nitrification reaction is operated on the premise that NO ^- ₂ is produced. The nitrifying solution then flows into the NS compound removal step 8, where the pH is adjusted to 3 or less using acid 9.
The amide type NS compound generated in the NS compound decomposition step 2 is decomposed into N ₂ gas at a pH of 3 or less in the presence of NO ^- ₂ , then flows into the neutralization tank 10 and is treated with an alkaline agent (for example, NaOH) 11. After being neutralized, it flows into the denitrification process 12 which is under anaerobic conditions and is filled with a medium to which denitrifying bacteria are attached, where a reducing agent 13 such as methanol is injected and NO ^- ₂ is decomposed and removed into N ₂ gas. be done. Since the alkalinity in the liquid increases due to the denitrification reaction, most of this alkalinity is recycled to the nitrification step 6 to be used as a neutralizing agent in the nitrification step 6, and the remainder is kept under aerobic conditions. After the reducing agent remaining in the denitrification liquid is oxidized and decomposed, SS is removed in a passing step 16, and the denitrification liquid is discharged or subjected to further advanced treatment. The reaction between NS compounds and NO ^- ₂ is usually NO ₂ -N/
The denitrification process is carried out with the NS compound-N≈1.5, but since the amount of the NS compound-N is much smaller than that of NO ₂ -N in the nitrification solution, most of the generated NO ₂ -N is removed in the denitrification step 12. However, the above conventional methods have the major drawback of high running costs due to the large amount of chemicals consumed for pH adjustment.
(3) Equation NH ⁺ ₄ 1.5O ₂ −−−→ NO ⁻ ₂ 0.5O ₂ −−−→ NO ^- ₃ ...As shown in (3), NH ₃ is nitrified to NO ^- ₃ (nitrate ion). Because NO ^- ₂ cannot be stably generated, the removal rate of NS compounds decreases, and there are cases where COD in treated water cannot be removed to the original target value. The present invention aims to eliminate the drawbacks of the above-mentioned conventional methods by saving chemicals for pH adjustment and performing more stable nitrite-type nitrification. That is, the present invention provides a method for treating wastewater containing ammonia and NS compounds, in which an NS compound decomposition and removal step, a neutralization step, a biological nitrification step divided into multiple stages, and a biological denitrification step are sequentially carried out downstream of the wastewater. A part of the nitrification solution from the front nitrification step of the divided nitrification step is circulated to the NS compound decomposition and removal step to remove NO ^- ₂ (nitrite ions) in the nitrification solution and NS in the wastewater. The NS compound is removed by reacting with the NS compound, and the remaining nitrification solution that is not circulated is passed through the post-nitrification process of the divided nitrification process together with the denitrification liquid that has been circulated from the denitrification process, and is submerged in the liquid. After nitrifying ammonia remaining in the liquid, NOx (NO ₂ and/or NO ₃ ) in the liquid is removed in the denitrification step. Next, one embodiment of the present invention will be described based on FIG. 2. Wastewater 1 flows into the NS compound decomposition and removal step 18 where the water temperature is 80°C and the pH is adjusted to 2 using acid 3. It reacts with NO ^- ₂ and is decomposed and removed into N ₂ gas or N ₂ O gas.
Both the above hydrolysis reaction and gasification reaction require high water temperature and low pH.
The more the reaction is accelerated. The liquid from which the NS compounds have been removed is neutralized to near neutrality in the neutralization tank 4, and the water temperature is lowered to approximately 40°C by heat exchange or injection of cold water, and then flows into the first nitrification step 6-1. , wastewater 1
A portion of the NH ₃ inside is nitrified, and a portion of the remaining NH ₃ is further nitrified in the next second nitrification step 6-2. In order to stop the nitrification reaction with NO ₂ in the nitrification step 6, it is best to inactivate the nitrate bacteria as seen from equation (3) above, and for that purpose, the water temperature must be maintained at a high temperature of about 40 to 42 degrees Celsius and/or Alternatively, it is effective to keep the NH ₃ concentration in the liquid high by setting the pH to around 7.5 or higher.
In the first nitrification step 6-1 and the second nitrification step 6-2
In order to allow NH ₃ to remain, the NH ₃ load in these nitrification processes can be increased excessively, or the equipment design and operation can be devised, such as by limiting the amount of oxygen supplied. Therefore, the nitrifying solution containing NO ^- ₂ and NH ₃ is used in the first nitrification step 6-1 and/or the second nitrification step 6-1.
2 to the NS compound decomposition and removal step 18 as the circulating nitrification liquid 17. An alkaline agent 7 for pH adjustment is injected into these nitrification processes,
Of course, injection is not necessary unless the PH decreases due to residual NH ₃ , and if the PH decreases in the third nitrification step 6-3 and the fourth nitrification step 6-4, both nitrification steps (not shown) occur. The alkaline agent 7 may be injected into either or both of steps 6-3 and 6-4. The nitrification process in which the effluent nitrification liquid is circulated is divided into two parts.
This is to promote the production of NO ^- ₂ by increasing the NH ₃ concentration.
Nitrification liquid is circulated at will from areas with high NO ^- ₂ concentration17
This is to take. In the third nitrification step 6-3 and the fourth nitrification step 6-4, the NH ₃ remaining in the previous nitrification step is completely nitrified, and the generated NOx is denitrified in the denitrification step 12.
A part of the denitrified water is circulated to the third nitrification process 6-3 and/or the fourth nitrification process 6-4, and the remainder flows into the aeration process 15, where the remaining reducing agent added in the denitrification process 12 is removed. After the fraction is oxidized and decomposed,
Alternatively, it can be released after undergoing further advanced treatment. The circulating denitrification liquid 14 is used in the subsequent nitrification process 6-3, 6-
4, which is recycled to the previous nitrification step 6-
In order to promote nitrite oxidation by keeping the residual NH ₃ of 1,6-2 at a high concentration without diluting it with the circulating denitrification solution 14, and to maintain the NO ₂ concentration in the circulating nitrification solution 17 at a high concentration. It will be held in The division in nitrification step 6 leaves NH ₃ in stages,
Because it is for promoting the production of NO ₂ ,
The number of divisions is not particularly limited, and can be divided from a minimum of two to several tenths, but the practical number of divisions is about 2 to 5. Said nitrification step 6, denitrification step 12, aeration step 1
5 is preferably filled with a medium to which microorganisms adhere, and the filling section may be a fluidized bed or a fixed bed. However, the suspended sludge method requires a solid-liquid separation step for each step before being returned, making the process complicated. The circulation of the denitrifying solution is used not only to neutralize the acid in nitrification step 6 with the alkaline content of the denitrifying solution as described above, but also to dilute the NOx in the nitrifying solution and keep its concentration low. Suppresses the PH rise in step 12 and increases the
This is to prevent the activity of denitrifying bacteria from decreasing due to pH and to prevent the pH of the treated water from exceeding the regulatory value range. The amount of circulating denitrification fluid depends on the NH ₃ concentration of wastewater 1, but the amount of NH ₃
- For wastewater with N content of about 200 to 500mg/10% of the amount of wastewater
You just need to circulate about twice the amount. On the other hand, if the NH ₃ -N concentration of the wastewater is low and the NOx flowing into the denitrification process 12 is low, or if the NOx concentration is reduced by injecting dilution water into the denitrification process influent, the denitrification process 12 Although it is possible to omit the circulation of the denitrifying liquid because the increase in pH at 100°C becomes smaller, it is preferable to circulate the denitrifying liquid. Further, even if wastewater has a strong PH buffering ability and the PH drop and rise are small during nitrification and denitrification, respectively, it is possible to omit the circulation of the denitrifying solution, but it is preferable to circulate it. Although a single tank may be used in the NS compound decomposition and removal step 18, it may be divided into two tanks so that the reactions of equations (1) and (2) described above proceed sequentially in each tank. Good too. In this case, it is preferable to add acid to the tank at the front stage and circulate the nitrification liquid to the tank at the rear stage. The amount of circulation should be such that NO ₂ -N in the nitrification solution is at least 1.5 times the NS compound in the wastewater. Steady nitrite type nitrification is the nitrification process 6-1, 6
-2 can be carried out by stably maintaining the water temperature at around 41℃, but it can be carried out discontinuously at 44 to 45℃.
Nitrite type nitrification can be maintained even at high temperatures. Ca as a neutralizing agent in nitrification steps 6-1 and 6-2
If (OH) ₂ is used, scales such as CaSO ₄ and CaCO ₃ will be generated in the process, so NaOH is recommended. As can be seen from the description of the embodiments above, in the present invention, there is no NS compound removal step 8 in the path that circulates a large amount of denitrification solution, so the acid and alkali agents for pH adjustment that are consumed in this step are not covered. This eliminates the need to use it, reducing running costs and simplifying the process.
By maintaining NH ₃ at a high concentration, nitrite-type nitrification can be easily carried out, resulting in stable nitrification.
Effects such as being able to remove NS compounds can be obtained. Next, examples of the present invention will be shown. Example Wastewater having the water quality shown in Table 2 was treated based on the treatment flow shown in FIG. The implementation conditions are shown in Table 1, and the treated water quality is shown in Table 2.

【table】

This is because the water was nitrified to [Table] and was diluted by the circulating denitrifying fluid.

[Brief explanation of drawings]

FIG. 1 is a flow sheet of a conventional method, and FIG. 2 is a flow sheet showing an embodiment of the present invention. 1...Wastewater, 2...NS compound decomposition process, 3...
...acid, 4...neutralization tank, 5...alkali agent, 6...
Nitrification step, 6-1... First nitrification step, 6-2...
Second nitrification step, 6-3...Third nitrification step, 6-4
...Fourth nitrification step, 7... Alkali agent, 8...
NS compound removal step, 9...acid, 10...neutralization tank,
11... Alkaline agent, 12... Denitrification process, 13...
...Reducing agent, 14... Circulating denitrifying liquid, 15... Aeration process, 16... Passing process, 17... Circulating nitrifying liquid,
18...NS compound decomposition and removal step.

Claims (1)

[Claims] 1. In the treatment of wastewater containing nitrogen-containing sulfur compounds and ammonia, the wastewater is subjected to a nitrogen-containing sulfur compound decomposition and removal step, a neutralization step, a biological nitrification step divided into multiple stages, and a biological nitrification step divided into multiple stages. At the same time, part of the nitrifying solution from the front nitrification step in the multi-stage nitrification step is sent to the nitrogen-containing sulfur compound decomposition and removal step, and part of the denitrifying solution from the biological denitrification step is passed through the biological denitrification step. A method for treating nitrogen-containing sulfur compounds and ammonia-containing wastewater, characterized in that a portion of the nitrogen-containing sulfur compound and ammonia-containing wastewater is recycled to a rear nitrification step in the multi-stage nitrification step. 2. The method according to claim 1, wherein the front nitrification step is at least one nitrification step other than the final stage in the multi-stage nitrification step. 3. The method according to claim 1, wherein the rear nitrification step is at least one nitrification step other than the first stage in the multi-stage nitrification step.