JPH0761473B2 - Wastewater coagulation treatment method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for coagulating boron in wastewater.

[Conventional technology]

Boron is contained in wastewater of coal-fired power plants, ceramics manufacturing plants, etc., and boron in such wastewater has an adverse effect on plants and the like. In recent years, with the expansion of coal-fired power plants, the release of boron into the environment has become a problem, and some local governments have regulated the concentration of boron in wastewater. The regulation value is quite strict. And this regulation may become stricter and wider in the future.

As a treatment technique corresponding to this, there are an aggregating treatment method and a chelate / ion exchange resin method.

〔Problems to be solved〕

Of these, the chelate / ion exchange resin method does not always present boron as simple ions in the waste water, and forms a complex compound or is in an associated state, so it is difficult to continuously exhibit sufficient performance. There are cases. Also,
Since the chelate / ion exchange resin method merely adsorbs and concentrates ions in water, it is essential to treat the regenerated waste liquid, and eventually, for example, it is necessary to stabilize the solid waste. Furthermore, in Ca-containing wastewater, chelate /
It is necessary to soften the water to be treated in advance before passing it through the ion exchange resin bed, and there is a problem that the construction cost and the operating cost are increased together with the necessity of the treatment of the waste liquid for regeneration. As described above, the chelate / ion exchange resin method is subject to many restrictions in its adoption.

On the other hand, the coagulation treatment method is characterized in that it is less affected by the existing form of boron in the waste water and does not require the treatment of the recycling waste liquid as compared with the chelate / ion exchange resin method. Therefore, the construction cost is low, and there are not many restrictions on the adoption, and it can be said that it is a very effective method for treating boron in wastewater. The economic efficiency of the coagulation treatment method largely depends on chemical costs, sludge treatment costs, operation management costs, etc., which vary depending on the type of chemicals used, the process to be adopted, the operating conditions, etc. There is a strong demand to reduce excessive utility costs.

[Means for Solving the Problems]

In the present invention, in the case of removing boron in wastewater by the coagulation method, the wastewater containing boron is subjected to the following steps a) to
The above problems are solved by sequentially treating the wastewater according to d) and adjusting the boron concentration in the wastewater after the treatment to less than 1 ppm: a) At least one of aluminum compounds in the wastewater and the step d) described later. The step of adding an acid at the same time as adding the solid slurry generated in step 3), adjusting the pH to 3.5 or less, and then adding calcium hydroxide, and aggregating at pH 7 to 11; Solid-liquid separation step; c) adding aluminum compound to the liquid fraction from step b) above, then adding calcium hydroxide, and performing coagulation treatment at pH 9-12 and a higher pH than step a); d) A step of performing solid-liquid separation of the treatment liquid of the above step c).

[Action]

Generally, an iron compound or an aluminum compound is used as the aggregating agent, but the aluminum compound has a higher effect of removing boron. Therefore, in the present invention, the aluminum compound is used. As an aluminum compound,
It can be generally used as long as it dissolves in water to release aluminum ions, and typical examples thereof include aluminum sulfate and aluminum chloride. These aluminum compounds may be added alone or in combination of a plurality of compounds.

The number of removal stages is increased to improve the removal performance of boron. However, it was found that if the number of stages is 4 or more, the degree of improvement of the removal performance becomes small, and that the number of stages is 2 because of the cost of the equipment, site area, and operability.

Further, in the coagulation treatment of boron, the higher the pH is, the higher the removal performance is, so that a large amount of alkali is consumed. Therefore, inexpensive calcium hydroxide is used because the ratio of the alkali cost to the total operating cost is high.
It was found that the effect is exhibited when the pH at which the flocculation treatment is carried out is 7 or more, preferably 9 or more, and when the pH is 12 or more, the removal performance is not improved so much and the alkali cost and the sludge amount are increased.

Further, aluminum is contained in the solid slurry generated from the solid-liquid separation in the coagulation treatment step, and sludge having a large aluminum / boron ratio is contained in the sludge in the case of treating a liquid containing a higher concentration of boron. It turns out that aluminum can be reused.

In addition, there is a considerable amount of coagulant (aluminum compound) in the sludge that is solid-liquid separated after the coagulation treatment, and since such aluminum compound exists mainly in the form of aluminum hydroxide, when the liquid to be treated is acidic, Can be used not only as a flocculating agent but also as a neutralizing agent.

In view of the above points, in the present invention, the coagulation treatment is divided into two stages, the first stage performs the coagulation treatment at a relatively low pH to remove most of the boron, and the second stage of the solid-liquid separation liquid. It is configured such that the residual concentration of boron is further reduced by performing agglutination treatment at a higher pH by eye. Then, the second stage sludge is returned to the first stage and reused as a flocculant and an alkaline agent. It is preferable to carry out the first stage coagulation treatment step at a pH as low as possible in order to suppress the amount of sludge generated,
High pH above pH 11 should not be adopted. In this way, the utilization efficiency of the coagulant and the alkaline agent is increased, and the economical efficiency of the entire apparatus is improved.

In addition, since a large amount of alkali is required in order to obtain a high pH in the second stage coagulation treatment step, it is particularly preferable to use low-priced calcium hydroxide. Since calcium hydroxide is a solid, the dissolution rate becomes extremely slow in the high pH range, and a large amount of unreacted solid is generated. Returning to the process can solve this problem.

In the coagulation treatment step, it is effective to add the alkali compound after the aluminum compound is once added to the waste water and dissolved, instead of simultaneously adding the aluminum compound and the alkaline agent, because the boron removal rate is high. The reason for this is not clear, but it is presumed that aluminum ions and boron have some effect and boron is incorporated when aluminum hydroxide precipitates. A polymer coagulant and an auxiliary agent can be added to promote coagulation.

For solid-liquid separation, commonly used methods such as liquid cyclone, filter press and precipitation separation can be used.

When the wastewater to be treated is strongly acidic like flue gas desulfurization wastewater, it is dissolved by adding the aluminum compound and the returned sludge from step d) in the first stage coagulation treatment step to release aluminum ions. However, if the wastewater to be treated is weakly acidic or alkaline, add acid to adjust the pH to 3.
It is necessary to adjust it so that it is 5 or less.

〔Example〕

Example 1 The waste water discharged from the simultaneous absorption oxidation type wet flue gas desulfurization apparatus for coal combustion exhaust gas was treated at a flow rate of 1 liter / hour according to the flow shown in FIG. The concentrations of boron, fluorine and cadmium in the wastewater to be treated and in the treated water were as follows.

Example 2 (Comparative Example) The same treatment as in Example 1 was performed except that iron chloride was used instead of aluminum sulfate as the coagulant. The boron concentration in the treated water was 112 ppm.

Example 3 (Comparative Example) The same treatment as in Example 1 was performed except that the slurry was not returned from the second solid-liquid separation tank to the dissolution tank. The boron concentration in the treated water was 5 ppm.

Example 4 (Comparative Example) The treatment was carried out by adding all of the aluminum sulfate added in the first and second flocculation treatment steps in Example 1 at once in the first treatment step, omitting the second treatment step. . The boron concentration in the treated water was 13 ppm.

Example 5 (Comparative Example) Treated water was treated as wastewater from an incinerator (pH 6.5, B: 52.0 ppm, F: 61.2 pp).
m, Cl: 39,000 ppm) and the pH in the first flocculation tank and the second flocculation tank was set to 7 and 9, respectively, and the same treatment as in Example 1 was performed. Boron concentration in treated water is 2.1p
It was pm.

Example 6 The same treatment as in Example 5 was carried out by setting the pHs in the first flocculation tank and the second flocculation tank to 9 and 10.5, respectively.
The boron concentration in the treated water was 0.9 ppm.

Example 7 (Comparative Example) The same treatment as in Example 6 was carried out with the pH values in both the first flocculation treatment tank and the second flocculation treatment tank being set to 7. The boron concentration in the treated water was 9.1 ppm.

〔effect〕

According to the present invention, boron in wastewater can be efficiently removed. In particular, stable continuous processing with reduced construction cost, utility cost and sludge generation amount can be performed.

The present invention is particularly suitable for removing boron in flue gas desulfurization wastewater. Of the substances that have to be removed from the flue gas desulfurization wastewater, dithioic acid, which is a COD component, cannot be efficiently removed by coagulation treatment. Therefore, dithionic acid is treated with an ion exchange resin or the like. However, in recent years, a so-called “simultaneous absorption oxidation” method in which an oxygen-containing gas is introduced into the absorption step has been increasing, and this method produces less dithionic acid which is difficult to coagulate. According to the present invention, it is possible to coagulate boron, fluorine, heavy metals and the like in flue gas desulfurization wastewater at the same time, and the flue gas desulfurization wastewater of the simultaneous absorption and oxidation system can be efficiently treated.

[Brief description of drawings]

The drawing is a flow sheet showing one embodiment of the method of the present invention.

Claims (2)

[Claims] 1. A drainage containing boron is a) to d) described below.
A method for coagulating wastewater, which is characterized in that the concentration of boron in the wastewater after treatment is 1 ppm or less according to the process of: Acid added, pH
After adjusting to 3.5 or less, add calcium hydroxide to adjust the pH to 7
B) a step of solid-liquid separation of the treatment liquid of the above step a); c) adding an aluminum compound to the liquid section from the above step b), and then adding calcium hydroxide; a step of aggregating at pH 9 to 12 and a higher pH than step a); d) a step of performing solid-liquid separation of the treatment liquid of step c) above. 2. The method according to claim 1, wherein the aluminum compound is at least one of aluminum sulfate and aluminum chloride.