JPS6260956B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing dithionic acid from waste liquid, and in particular, to a method for removing dithionic acid from waste liquid, particularly when treating waste liquid discharged when flue gas is subjected to wet desulfurization treatment, and when flue gas is subjected to dry denitrification treatment and wet desulfurization treatment. The present invention relates to a method for removing dithionic acid contained in a processing liquid discharged into a process.

Currently, many processes are in practical use as flue gas desulfurization treatment, and sulfur oxides (SOx) in flue gas are recovered mainly as calcium sulfate or sodium sulfate using an absorption liquid.
In this case, SOx first becomes acidic sulfite ion (HSO ^- ₃ )
The calcium or sodium salt is then recovered as the calcium or sodium salt by air oxidation. By the way, dithionic acid is produced as a by-product during the air oxidation of HSO ^- ₃ . Dithionic acid is a very stable substance, so the COD analysis method using KMnO ₄ (JISK0102) shows a difference of 20 to 30% of the theoretical COD value.
The COD value of this substance is said to vary depending on the oxidation time. Therefore, dithionic acid, which is one of the COD components of waste liquid, can be removed using the coagulation-precipitation method, which is a common COD removal method.
Ozone oxidation method, activated carbon adsorption method, ultraviolet irradiation decomposition method, etc. are not able to remove the water sufficiently.
Decomposition into SO ^2-4 , concentration separation using a reverse osmosis membrane, and evaporation of the concentrated liquid to _dryness are being put into practical use. However, these methods require treatment of the concentrated or regenerated waste liquid, and methods for this include evaporative concentration or wet combustion, precipitation in the liquid by mixing with a hydrophilic solvent, heating in the presence of oxygen, and treatment of particulate matter. Methods such as heating in a rotary kiln after mixing are being considered. As described above, the removal of dithionic acid from the wet desulfurization waste liquid by the above-mentioned method requires large-scale equipment, which leads to high costs.

Recently, dithionate ions have been converted into Al ³⁺ or SO ²⁻ ₄
A method of removal has been proposed by forming a complex compound with. This method is significantly more advantageous than the conventional method in terms of equipment costs and operating costs, but in this case _, the required amount of PAC is S ₂ O ^2-6 as Al ₂ O ₃ ,
Since this requires an extremely large amount, which is half of the sum of the SO ^2-4 concentrations, and the pH must be 11 or higher, there was also a problem _in terms of the amount of sludge produced.

In view of the above-mentioned circumstances, the present inventors investigated the production and decomposition of dithionic acid in waste liquid using sewage treatment sludge and human waste treatment sludge containing sulfur-oxidizing bacteria and nitrifying bacteria. As a result _, it was found that thiosulfate ions (S ₂ O _2-3 ) are essential for decomposing dithionate ions (S ₂ O ^2-6 ) in the waste liquid in ^the aerobic treatment method. However, in this case, the amount of S ₂ ^O- _{3 required} to almost completely decompose S ₂ O ^2-6 is
[S ₂ O ^2- ₃ ]/[S ₂ O ^2- ₆ ] molar ratio is extremely high at 0.7 or more.
Since neutralization _of SO ^2-4 also requires high costs, this method has not yet been satisfactory. Therefore, as a result of repeated research on a method _for reducing the required amount of S ₂ ^{O 2-3} _without reducing the decomposition rate of S ₂ O ^2-6 , the present inventor arranged multiple decomposition tanks in series, By separately supplying S ₂ O ^2-3 to each tank _,
The required amount of S ₂ O ^2-3 is determined by the molar ratio of [S ₂ O ^2- ₃ ]/[S ₂ O ^2- ₆ ] = approx.
This invention was completed based on the knowledge that it can be reduced to 0.2.

That is, in the method for removing dithionic acid according to the present invention, in biologically decomposing dithionic acid in a waste liquid, a plurality of decomposition tanks are arranged in series, and the waste liquid containing S ₂ O ^2-6 is decomposed in each decomposition tank _. As well as being distributed sequentially to the tanks,
_This method is characterized in that S ₂ O ^2-3 is separately supplied to each decomposition tank.

As shown in Comparative Example 2 described later ^, [ _{S2O2-3 ]} /
[S ₂ O ^2-6 _] S ₂ O ^2-6 decomposition rate = by _molar ratio = 0.1
about
Since 60% is achieved, the amount of S ₂ O ^2-3 added to each decomposition tank is set to the above molar ratio of 0.1 to the S _{2 O} ^2-6 concentration at the tank inlet _, and the _S2O2-6 ^_ _{_}
The decomposition rate and _the cumulative addition amount of S ₂ O ^2-3 are calculated as follows.

S ₂ O ^2-6 decomposition rate = X ₀ -X _o /X _{0 =} 1-0.4 ⁿ Here, if n is large (for example, in Examples 1 and 2, four decomposition tanks are used, so n
= 4), 0.4 ⁿ is a negligibly small value, so the above cumulative addition amount can be approximated as 0.1X ₀ 1/1 - 0.4 = 0.167X ₀ .

(In the formula, X ₀ is _the molar concentration of S ₂ O ^2-6 in the wastewater _,
is _the molar concentration of S ₂ O ^2-6 in the n-th decomposition tank, Yn
respectively mean the molar amount of S ₂ O ^2-3 _added in the n-th decomposition tank. ) Next, examples of the present invention and comparative examples showing the prior art will be given.

Comparative Example 1 In the aerobic biodegradation device for dithionic acid using the submerged bed method shown in Fig. 1, 350 mg/ml was added to the decomposition tank 1 with a volume of 1 filled with sewage treatment sludge containing sulfur-oxidizing bacteria and nitrifying bacteria as seed sludge. A test waste liquid containing S ₂ O ^2-6 was supplied at a flow rate _of 0.55/day, and separately _{, sodium thiosulfate was converted into S 2 O 2-3} ^and _S ² _{O 2-6}
260 ml/was added. In this way, S ₂ O ^2-6 was biodegraded at pH=7 and temperature of 30° C., and the _{concentration of} S ₂ O ^2-6 in the treated solution was measured.

Furthermore, in order to investigate the influence of S ₂ O ^2-3 on the decomposition of S ₂ O ^2-6 , the addition of S ₂ O _2-3 was stopped and ^the concentration was measured _in the same manner as _above .

The results of these measurements are shown in FIG. As can be seen from the figure, when S ₂ O ^2-3 is not added, S ₂ O ^2-6 is not decomposed at _all , but with the addition of S ₂ O ^2-3 , after about 12 _days of _operation , , S ₂ O ^2-6 in _the waste liquid was completely decomposed. Therefore _, S ₂ O _2-3 is essential for the decomposition ^of S ₂ O ^2-6 .

Comparative example 2 Same operation as comparative example 1, decomposition tank volume load 1.7Kg
Effect of S ₂ O ^2- ₆ on the decomposition of S ₂ O ^2- ₆ at S ₂ O 2- ⁶ /m ³ ·day
We investigated the influence of ₃ . The results are shown in Figure 3. As can be seen from the figure, when the [S ₂ O ^2- ₃ ]/[S ₂ O ^2- ₆ ] molar ratio = 0.1, the S ₂ O ^2- ₆ decomposition rate was 60%, but when the molar ratio was increased, The decomposition rate improved, and at a molar ratio of 0.7 or higher, a decomposition rate of 100% was achieved.

Example 1 Four aerobic biodecomposition tanks for dithionic acid shown in FIG. 1 were prepared and arranged in series to construct a multistage series dithionic acid decomposition apparatus shown in FIG. 4. A test waste liquid containing 350 mg/ _day of S ₂ O ^2-6 was supplied to the first stage decomposition tank 11 of the same equipment at a flow rate of 8 days, and was distributed sequentially to each tank. Decomposition tank 11,1
25 mg/, 10 mg/, 5 mg/ _, and 2 mg/of sodium thiosulfate were added as S ₂ O ^2-3 to Nos. 2, 13, and 14, respectively. Thus PH=7, temperature 30℃
When S ₂ O ^2-6 was biodegraded in the 4th stage decomposition tank 14, _the concentration of S ₂ O ^2-6 in the treated liquid from _the fourth stage decomposition tank 14 was approximately 20
mg/, and 42 mg as S ₂ O ^2-3 _, that is, about 9 at [S ₂ O ^2- ₃ ]/[S ₂ O ^2- ₆ ] molar ratio = 0.2.
Five
% dithionic acid decomposition rate was achieved.

Example 2 As shown in Figure 5, volumes 5.7
A waste liquid treatment device was constructed which consisted of a plurality of decomposition tanks 21, 22, 23, 24, a denitrification tank 25 disposed on the downstream side thereof, and a re-aeration tank 26 disposed further on the downstream side. As sample waste liquid, 200mg/
The flue gas denitrification and desulfurization waste liquid containing S ₂ O ^2-6 and 330 _mg /N of NH ⁺ ₄ -N is supplied to the first-stage decomposition tank 21 at a flow rate of 55/day, and is sequentially distributed to each tank. , 15 mg/, 10 mg/, ₃ of sodium thiosulfate as S ₂ O ^2-3 in 21, 22, 23, 24 in each stage of the decomposition tank.
mg/ and 1 mg/ were added. And decomposition tank 2
At 1, 22, 23, 24, temperature = 30℃, PH =
In 7.7, nitrification of NH ⁺ ₄ -N and decomposition of S ₂ O ^2- ₆ were carried out simultaneously, and the influence of the amount of S ₂ O ^2- ₃ added was investigated.
As a result, _the S ₂ O ^2-6 concentration of the treated liquid discharged from the reaeration tank 26 was 10 mg/and the NH ⁺ ₄ concentration was 0.2 mg/ or less. In this way, [S ₂ O ^2- ₃ ]/[S ₂ O ^2- ₆ ] molar ratio =
A dithionic acid decomposition rate of 95% was achieved with _an addition of about 0.21 S ₂ O ^2-3 . In addition, in Comparative Example 2, in order to obtain a dithionic acid decomposition rate of 100% ^, [ _{S2O2-3 ]} / _{[ S2O2-6} ^]
]
It was necessary to add S ₂ O ^2-3 so that the molar ratio was 0.7 or _more .

In some cases, a removal tank for removing nitrogen compounds containing sulfur was installed between the decomposition tank 24 and the denitrification tank 25, and in this case, the decomposition rate of dithionic acid was not affected at all.

According to the above, in the method for removing dithionic acid according to the present invention, a plurality of decomposition tanks are arranged in series,
_Since the waste liquid containing S ₂ O ^2-6 is sequentially distributed to each decomposition tank, and S ₂ O ^2-3 is separately supplied to each decomposition tank, the _{S 2} necessary _for decomposing S ₂ O ^2-6 is The amount _of O ^2-3 added can be greatly reduced, _and S ₂ O ^2-6 can be removed cost-effectively.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view showing the dithionic acid decomposition equipment used in _the comparative example, Fig. 2 is a graph showing the relationship between treatment time and S ₂ O ^2-6 , and Fig. ₃ is a graph showing the relationship between [S ₂ O ^2-3 ]/ [S ₂ O ^2-6 _] Graph showing _the relationship between molar ratio and S ₂ O ^2-6 decomposition rate, 4th
FIG. 5 is a process diagram showing an embodiment of the present invention.

Claims (1)

[Scope of Claims] 1. In biologically decomposing dithionic acid in a waste liquid, a plurality of decomposition tanks are arranged in series, and the waste liquid containing dithionate ions is sequentially distributed to each decomposition tank. A method for removing dithionic acid from waste liquid, characterized by supplying sulfate ions to each decomposition tank separately. 2. The method according to claim 1, wherein the waste liquid is a flue gas wet desulfurization treatment waste liquid. 3. The method according to claim 1, wherein the waste liquid is a flue gas denitrification and desulfurization treatment waste liquid containing ammonia nitrogen and dithionate ions.