AU2017365413B2

AU2017365413B2 - Method for removing manganese from wastewater

Info

Publication number: AU2017365413B2
Application number: AU2017365413A
Authority: AU
Inventors: Satoshi Asano; Nobuyuki Kaji; Naoko Okibe; Kenji Takeda; Hiroshi Takenouchi
Original assignee: Sumitomo Metal Mining Co Ltd; Kyushu University NUC
Current assignee: Sumitomo Metal Mining Co Ltd; Kyushu University NUC
Priority date: 2016-11-22
Filing date: 2017-11-13
Publication date: 2019-10-03
Anticipated expiration: 2037-11-13
Also published as: PH12018501442A1; AU2017365413A1; JP6956971B2; JP2018086643A; PH12018501442B1

Abstract

Provided is a method for oxidizing manganese contained in wastewater and removing the same with the use of a manganese oxidizing bacterium. The method for removing manganese from manganese-containing wastewater using a manganese oxidizing bacterium comprises adding an amino acid having a molecular weight of 90 or greater and glucose. It is preferred that the amino acid is added in such an amount as to give an amino acid molar concentration which is 1.6-2.2 times as high as the molar concentration of manganese in the wastewater and glucose is added in such an amount as to give a glucose molar concentration which is 0.2-0.6 time as high as the molar concentration of manganese in the wastewater. The manganese oxidizing bacterium is proliferated and, at the same time, a carbon source as a cell constituent and an energy source are supplied so that manganese can be sufficiently oxidized by the manganese oxidizing bacterium and thus removed through precipitation without adding an alkali in a large amount.

Description

DESCRIPTION

Title of Invention: METHOD FOR REMOVING MANGANESE FROM

WASTEWATER

Technical Field [0001]

The present invention relates to a method for removing manganese from wastewater containing manganese. More specifically, the present invention relates to a method for treating wastewater containing manganese in a hydrometallurgical process using nickel oxide ore as a raw material.

Background Art [0002]

In a hydrometallurgical process using nickel oxide ore as a raw material, operations such as that of PTL 1 are performed. Specifically, an acid, such as sulfuric acid, is added to nickel oxide ore to leach out valuables, such as nickel, and an alkali is added to the obtained sulfuric acid-acidic leachate in order to separate most of the impurities contained therein, such as iron, aluminum, manganese, magnesium, and calcium, as neutralized products. Subsequently, a sulfurizing agent is added to the neutralized liquid in order to separate and collect valuables, such as nickel, as sulfides.

[0003]

When said process is used, although a part of the sulfuric acid-acidic wastewater after sulfurization (also referred to as sulfurized solution or simply as barren solution) is repeatedly used in the system, such as in the leaching step, most of the wastewater is sent to a wastewater treatment step during which an alkali is added to neutralize the remaining acid while removing impurities etc . contained therein, and discharge into the ocean etc. is performed thereafter.

[0004]

Impurities that need to be removed in the abovementioned wastewater treatment step include iron which is present as suspended particles, and aluminum and manganese which are dissolved as ions.

[0005]

Among the above impurities, iron can be removed from the wastewater by sending the wastewater to a thickener or the like to precipitate and separate the suspended particles, then allowing the water to pass through a tailing dam or the like, thereby practically precipitating the particles completely. Further, a filtering means, such as a filter press, is used in combination if necessary. Aluminum can be removed from the wastewater by adding an alkali to maintain the pH at a relatively low level, thereby neutralizing the aluminum and removing it from the wastewater as a hydroxide precipitate.

[0006]

However, manganese is present in a dissolved state in the wastewater; thus, a precipitate of manganese cannot be formed unless the pH, which is adjusted through the addition of an alkali etc. to the wastewater, is adjusted to an alkaline region with a pH exceeding 9. Accordingly, there was a problem in that the removal of manganese from the above wastewater required much time and cost.

Another problem is that, in cases where an alkali was added to increase the pH, if the pH exceeded 9, magnesium contained in the wastewater altogether with the impurities mentioned above would form hydroxide before manganese did.

[0007]

Since there are no significant effluent standards for magnesium and magnesium does not leave any impacts upon the environment, it is not necessary to remove magnesium from wastewater. However, the alkali added with the intention to remove manganese is consumed during the production of magnesium hydroxide; therefore, the alkali is required in an amount exceeding the amount required to hydroxylate the target manganese, thereby generating unnecessary cost. Furthermore, due to the precipitation of magnesium, the amount of generated materials increases, which is not preferable.

[0008]

Accordingly, the following operation has been carried out: without increasing the pH of the wastewater to 9 where both manganese and magnesium are precipitated, the pH of wastewater is adjusted to 8.6 or less where magnesium is not precipitated and a small amount of manganese remains as an aqueous solution, and the amount of alkali to be added is reduced in an amount corresponding to the pH difference (see PTL 2).

[0009]

According to this method, low-concentration manganese remaining in the wastewater is removed by actively oxidizing the wastewater, thereby effectively preventing the increase of the precipitate and reducing the cost for alkali. However, there had been cost issues regarding the amount of the oxidant involved and the facilities etc. required for the reaction. [0010]

In particular, in industrial wet processes of nickel oxide ore, an enormous amount of ore is often treated in order to separate and collect nickel which is contained by 1 to 2% at most, or even less. Therefore, an enormous amount of waste water is generated as well. Further, as described above, since the wastewater is treated after valuables, such as nickel, are deposited and collected in a reducing atmosphere, it is necessary to change the atmosphere from reducing atmosphere to oxidizing atmosphere to remove manganese by oxidation. In order to advance the treatment in a short period of time, it is necessary to add a large amount of powerful oxidants, such as sodium hypochlorite or ozone. Accordingly, there had been a problem in that the cost for the oxidant was required. [0011]

2017365413 27 Aug 2019

Accordingly, as a method for saving the cost and time required for oxidation, an attempt has been made for treatment methods using manganese-oxidizing bacteria.

To remove manganese using manganese-oxidizing bacteria, it is necessary to grow a certain amount of manganese-oxidizing bacteria, and enhance the removal ability of the grown manganese-oxidizing bacteria to oxidize manganese and precipitate the oxidized manganese to form precipitates.

[0012]

In particular, a large amount of wastewater may be handled in the hydrometallurgical method of nickel oxide ore, as described above. It is difficult to effectively grow manganese-oxidizing bacteria suitable for utilization in such an environment. There has been a demand for methods that can be more reliably applied to wastewater treatment.

Citation List

Patent Literature [0013]

PTL 1: JP-A No.2005-350766

PTL 2: JP-A No .2010-207674

Summary of Invention

Technical Problem [0014]

In consideration of the above circumstances, an aim of the present invention is to provide a method for removing manganese contained in wastewater by oxidation using manganese-oxidizing bacteria.

Solution to Problem [0015]

The method for removing manganese from wastewater as the first invention is a method for removing manganese from wastewater containing manganese using manganese-oxidizing

2017365413 27 Aug 2019 bacteria, the method comprising adding an amino acid having a molecular weight of 90 or more and glucose.

The method for removing manganese from wastewater as the second invention is the method of the first invention wherein the amino acid is added in an amount corresponding to a molar concentration that is 1.6 times to 2.2 times higher than the molar concentration of manganese in the wastewater.

The method for removing manganese from wastewater as the third invention is the method of the first invention wherein the glucose is added in an amount corresponding to a molar concentration that is 0.2 times to 0.6 times higher than the molar concentration of manganese in the wastewater.

The method for removing manganese from wastewater as the fourth invention is a method comprising, as a preliminary step, adding an alkali in advance to wastewater containing manganese in order to adjust the pH so that the manganese concentration is reduced to 65 mg/L or less, and then subjecting the wastewater obtained in the preliminary step to the treatment according to claim 1.

In a broad aspect, there is provided a method for removing manganese from wastewater having a manganese concentration of approximately 60 mg/L or less using manganese-oxidizing bacteria, the method comprising adding an amino acid having a molecular weight of 90 or more and glucose; wherein the amino acid is added in an amount corresponding to a molar concentration that is 1.6 times to 2.2 times higher than the molar concentration of manganese in the wastewater; and wherein the glucose is added in an amount corresponding to a molar concentration that is 0.2 times to 0.6 times higher than the molar concentration of manganese in the wastewater.

Advantageous Effects of Invention [0016]

According to the first invention, manganese-oxidizing bacteria are allowed to grow while a carbon source and an energy

2017365413 27 Aug 2019 source as cellular constituents are supplied, whereby manganese can be sufficiently oxidized by the manganese-oxidizing bacteria without supplying a large amount of alkali, and the oxidized manganese can be precipitated and removed.

According to the second invention, the molecular weight of the amino acid is within a suitable range; thus, manganese can be reduced without overly reducing the manganese oxidation ability, nor allowing sundry bacteria to grow due to the excess amino acid.

According to the third invention, the amount of glucose added is within a suitable range; thus, manganese can be reduced

5A without weakening the activity of manganese-oxidizing bacteria nor allowing sundry bacteria to grow due to the excess glucose.

According to the fourth invention, the manganese concentration is reduced in the preliminary step; thus, the addition of excess alkali is not required and the reaction time can be shortened, whereby providing advantages in terms of cost and facility.

Brief Description of Drawings [0017]

Fig. 1 is an explanatory diagram of the manganese removal method according to the present invention.

Fig. 2 is a graph showing the relationship between the molecular weight of amino acid added and the manganese removal concentration.

Fig.	3 is a	graph	showing	the	relationship	between	the
amount of	amino	acid	added	and	the manganese removal
concentration.
Fig.	4 is a	graph	showing	the	relationship	between	the

glucose amount and the manganese removal concentration.

Description of Embodiments [0018] (Technical principle of the present invention)

The method for removing manganese from wastewater according to the present invention is a method for removing manganese from wastewater containing manganese using manganese-oxidizing bacteria, and is performed by adding an amino acid having a molecular weight of 90 or more and glucose. The purpose of adding glucose is to supply a carbon source and an energy source as cellular constituents to the grown manganese-oxidizing bacteria. The purpose of adding an amino acid is to supply a carbon source and an energy source, as well as micronutrient elements, such as other nitrogen sources.

When manganese-oxidizing bacteria are allowed to grow while a carbon source and an energy source as cellular constituents are supplied, manganese can be sufficiently oxidized by the manganese-oxidizing bacteria without supplying a large amount of alkali, and the oxidized manganese can be precipitated and removed. Since this manganese removal effect is powerful, it is possible to remove manganese even from wastewater in a low pH range (i.e., a pH of less than 9).

[0019] (Manganese-oxidizing bacteria)

Manganese-oxidizing bacteria are often contained in wastewater; however, when they are not contained in wastewater or are contained only in a very small amount, manganese-oxidizing bacteria stored beforehand are added to the wastewater .

[0020]

Various types of manganese-oxidizing bacteria are known. Compared with oxidation using a single bacterial species, collectively handling bacteria obtained from places where manganese-oxidizing bacteria grow spontaneously, such as actual wastewater treatment sites, is more cost-effective and industrially practical. However, when various bacterial species obtained on site are handled collectively, various useless sundry bacteria are contained in addition to the useful manganese-oxidizing bacteria.

As described above, with regards to manganese-oxidizing bacteria which contain various bacterial species, it is generally difficult to precisely analyze and specify the species or to grow only specific bacterial species. Accordingly, in the present invention, bacterial species obtained and cultured under the same conditions are collectively handled, and various bacterial species are used as a whole to remove manganese. Therefore, in the present specification, bacteria capable of oxidizing manganese are collectively defined as manganese-oxidizing bacteria, and bacteria not involved in oxidation of manganese are defined as sundry bacteria.

Moreover, the manganese-oxidizing bacteria cultured and tested in the present invention were obtained from drainage piping of a wastewater treatment facility of a smelter operating in Palawan, Philippines, as described later; however, the place of production is not limited to specific places.

[0021] (Wastewater to which the present invention is applied)

The manganese removal according to the present invention is targeted for wastewater having a manganese concentration of approximately 60 mg/L, and is suitable for the reduction of the manganese concentration to 60 mg/L or less.

[0022]

As for the reasons to target the above manganese concentration, the following (1) and (2) can be stated.

(1) When a conventional manganese removal method where an alkali is added to increase the pH is used, the manganese concentration of wastewater can be reduced to approximately 60 mg/L even in a region having a pH of less than 9; however, in order to further reduce manganese, addition of an alkali in an excessive amount exceeding the equivalent amount and an excessive reaction time are required. Therefore, it is advantageous in terms of cost and facility to perform neutralization so that the manganese concentration is 65 mg/L or less, preferably about 60 mg/L, and then to treat the neutralized wastewater using manganese-oxidizing bacteria to reduce the manganese concentration to 1 mg/L or less.

[0023] (2) When an excessive amount of neutralizer is used, the pH of the wastewater increases excessively. This requires much time and cost to, for example, add an acid again to the wastewater which underwent manganese removal in order to adjust the pH to a range that is suitable for discharging.

[0024] (Conditions for maintaining the manganese removal ability)

The following explains the conditions for maintaining the manganese removal ability required to perform the manganese

removal method of the present

invention using

manganese-oxidizing bacteria.

[0025]

As a sample liquid, a culture	medium containing
manganese-oxidizing bacteria obtained	in the wastewater

treatment step on a site performing hydrometallurgy of nickel oxide ore was prepared. Since manganese is contained in the wastewater treatment step as described above, a proper amount of bacteria adapted thereto are often present.

[0026] (Amino acid)

The role of the amino acid added is to serve as nutrients, such as a carbon source, an energy source, and other nitrogen sources, to help the growth of manganese-oxidizing bacteria.

Low-concentration manganese and an amino acid having a molecular weight of 100 or more in a molar amount that was 0.01 times to 5 times higher than the amount of manganese were added to the above culture medium to prepare a sample aqueous solution .

[0027]

Types of amino acids include, for example, serine (molecular weight: 105.1), leucine (molecular weight: 131.2), aspartic acid (molecular weight: 133.1), and glutamic acid (molecular weight: 147.1) . These amino acids may be added in the form of sodium salts.

[0028]

Fig. 2 shows the molecular weight of

the amino acid added

to the aqueous solution and the amount of manganese that can be reduced, that is, the manganese oxidation capacity. In Fig.

2, the manganese reduction amount increases as the molecular weight of the amino acid increases.

It is found that, in order to remove manganese with a concentration difference of 60 mg/L, the amino acid to be added is required to have a molecular weight of approximately 90 or more. When an amino acid having a molecular weight of less than 90, such as glycine having a molecular weight of 75, is used, the manganese reduction amount only amounts to less than 60 mg/L .

[0029]

When the amount of amino acid added and the manganese reduction amount are compared using aspartic acid having a molecular weight of 133, it is necessary to add 3 mMol/L or more of amino acid in order to reduce the manganese concentration by 60 mg/L or more, as shown in Fig. 3.

[0030]

The addition of excess amino acid also contributes as nutrients for sundry bacteria and helps their growth. Accordingly, the growth of the target manganese-oxidizing bacteria is inhibited, which is not preferable.

Specifically, if more than approximately 4 mMol/L of amino acid is added to wastewater containing manganese at a concentration of 1.8 mMol/L (100 mg/L), the excess amino acid accelerates the growth of sundry bacteria. In addition, the required cost is equal to the cost for oxidation of manganese through the addition of sodium hypochlorite; thus, there is no advantage to use manganese-oxidizing bacteria. Therefore, it is preferable to add 3 to 4 mMol/L of amino acid, as shown in Fig. 3.

[0031]

That is, an amino acid may be added in an amount within a range of 1.6 times or more and 2.2 times or less relative to the number of moles of manganese.

When the original liquid has a different manganese concentration, an amino acid may be added in a concentration within the abovementioned range of multiple numbers. Specifically, for example, as for wastewater containing 50 mg/L (0.91 mMol/L) of manganese, an amino acid within a range of 1.4 mMol/L to 2 mMol/L is added.

[0032]

It also seems plausible to add amino acids having a higher molecular weight or proteins; however, if the molecular weight is overly high, it tends to be difficult for manganese-oxidizing bacteria to incorporate such amino acids as nutrients. In addition, such amino acids having a high molecular weight are generally expensive and using such amino acids for industrial wastewater treatment or the like costs too much, and therefore is not practical.

[0033] (Glucose)

In the culture medium containing manganese-oxidizing bacteria, glucose (molecular weight: 180) or corresponding hydrocarbon is preferably added to the manganese-oxidizing bacteria as a carbon source as a cellular constituent, and as an energy source as a cellular constituent for the manganese-oxidizing bacteria to work.

[0034]

As shown in Fig. 4, the amount of glucose added and the manganese reduction amount have a linear relationship. When 0.4 mMol/L of glucose is added, the manganese reduction amount is about 60 ΔΜη (mg/L), and when 1 mMol/L of glucose is added, the manganese reduction amount is about 75 ΔΜη (mg/L).

As is clear from the above, it is necessary to add 0.4 mMol/L or more of glucose. This is 0.2 times higher than the molar concentration of manganese contained, as with the case of the amino acid described above.

If the glucose concentration is overly low (e.g., less than 0.4 mMol/L) , the activity of manganese-oxidizing bacteria becomes weak, and the glucose does not contribute to their growth. Consequently, the manganese reduction amount is reduced.

[0035]

On the other hand, if the glucose concentration is overly high, the excess glucose serves as a nutrient for sundry bacteria, resulting in the sundry bacteria growing more than necessary. Consequently, the growth of manganese-oxidizing bacteria is inhibited, which is not preferable. In particular, if the amount of glucose added exceeds 1 mMol/L, the required cost exceeds the cost for oxidation using sodium hypochlorite. Therefore, the amount of glucose added is preferably 1 mMol/L or less. This is 0.6 times higher than the molar concentration of manganese.

That is, it is preferable to add glucose in an amount corresponding to a molar concentration that is 0.2 times to 0.6 times higher than the molar concentration of manganese. [0036] (Preliminary step)

It is also possible to oxidize manganese using manganese-oxidizing bacteria in high-concentration wastewater having a manganese concentration exceeding 60 mg/L. However, considering the size of a facility that can culture and secure the large amount of manganese-oxidizing bacteria required for the treatment and can also secure the reaction time required for the manganese-oxidizing bacteria to advance the treatment, an efficient method is such that the manganese concentration is reduced with an alkali to a concentration of 60 mg/L as a preliminary step, and then the present invention is applied to remove manganese using manganese-oxidizing bacteria.

Examples [0037] <Example 1>

About 1 g (including water) of sludge containing manganese-oxidizing bacteria obtained from drainage piping of a wastewater treatment facility of a nickel oxide ore hydrometallurgical plant owned by Coral Bay Nickel Corporation located in Rio Tuba, Bataraza, Palawan 5306, Philippines was collected and suspended in 250 ml of pure water.

[0038]

Next, 1 mMol/L (0.6 times higher than the amount of manganese) of glucose as a nutrient, and 4 mMol/L (2.2 times higher than the amount of manganese) of sodium salt of aspartic acid having a molecular weight of 133.1 as an amino acid were added, and the resulting solution was stirred.

[0039]

Subsequently, manganese sulfate in an amount corresponding to a manganese concentration of 1.8 mMol/L was added to the solution and was mixed to prepare an initial solution. The manganese concentration of the initial solution was 100 mg/L.

[0040]

The initial solution was maintained at a temperature of 30°C and was continuously stirred for 14 days.

The concentration of manganese contained in the initial solution showed an initial state where the concentration was rapidly reduced from 1.8 to 1.4 mMol/L (76 mg/L) during an initial time which is about an hour from the start of stirring. After that, the manganese concentration was reduced to reach the lower detection limit, i.e., virtually 0 mMol/L, over 14 days .

That is, 1.4 mMol/L of manganese was reduced by the manganese-oxidizing bacteria for 14 days.

[0041] (Evaluation of the activity of manganese-oxidizing bacteria based on Example 1)

The above results of Example 1 can be considered attributable to the added manganese-oxidizing bacteria growing and acting to oxidize and remove manganese in the solution. [0042] <Example 2>

Stirring was carried out for 14 days under the same conditions as those of Example 1, except that 4 mMol/L of sodium salt of glutamic acid having a molecular weight of 141 was added as the amino acid in place of the aspartic acid. The manganese concentration in this case was reduced from 1.8 to 1.5 mMol/L (84 mg/L) in the initial period, and then reached to the lower detection limit, i.e., virtually 0 mMol/L, 14 days thereafter. [0043]

CGomparative Example 1>

Stirring was carried out for 14 days under the same conditions as those of Example 1, except that 4 mMol/L of glycine having a molecular weight of 75 was added as the amino acid in place of the aspartic acid. The manganese concentration in this case was reduced from 1.8 to 1.3 mMol/L in the initial state, and reached 0.2 mMol/L 14 days thereafter . The amount of change in this case was 1.1 mMol/L (60 mg/L).

[0044] (Evaluation of the amino acid molecular weight based on Examples 1, 2 and Comparative Example 1)

A comparison between Examples 1, 2 and Comparative Example 1 reveals that amino acids which have too large a molecular weight are difficult to be produced and are not economically appropriate, and that it is desirable to use alanine, sodium glutamate and the like which have a molecular weight of approximately 90 or more as the amino acid.

[0045]

CGomparative Example 2>

Stirring was carried out for 14 days under the same conditions as those of Example 1, except that 1 mMol/L of aspartic acid was added as the amino acid. The manganese concentration in this case was reduced from 1.8 to 1.4 mMol/L in the initial state, and then reached 0.9 mMol/L 14 days thereafter. The manganese concentration was reduced only by 0.5 mMol/L (27 mg/L) , the activation of the manganese-oxidizing bacteria was insufficient, and the concentration was reduced more slowly than in Example 1.

[0046] (Evaluation of the amount of amino acid added based on Example 1 and Comparative Example 2)

The relationship with the amount of amino acid added shown in Fig. 3 demonstrates that it is desirable to add approximately 3 mMol/L or more of amino acid in order to ensure a manganese reduction amount (ΔΜη) of 60 mg/L or more.

Compared with the initial manganese concentration of 1.8 mMol/L, it is desirable to add the amino acid in an amount corresponding to a molar concentration that is 1.6 times or more higher than the manganese concentration.

[0047] cComparative Example 3>

Stirring was continued for 14 days under the same conditions as those of Example 1, except that glucose was not added as the nutrient. The manganese concentration in this case was reduced from 1.8 to 1.4 mMol/L in the initial state, and reached 0.4 mMol/L 14 days thereafter. The amount of change in this case was 0.9 mMol/L (concentration: 52 mg/L).

[0048] (Evaluation of glucose addition based on Example 1 and Comparative Example 3)

When Comparative Example 3 is compared with Example 1, a reduction in the manganese concentration can be recognized; however, the activation of the manganese-oxidizing bacteria was insufficient and the manganese concentration was reduced more slowly than in Example 1, thereby confirming the efficiency of adding glucose.

[0049] (Conclusion)

The relationship with the amount of glucose added shown in Fig. 4 demonstrates that it is necessary to add approximately 0.4 mMol/L of glucose in order to achieve a manganese reduction

2017365413 27 Aug 2019 amount (ΔΜη) of 60 mg/L or more . However, the addition of excess glucose results in adverse effects, such as the activation of the manganese-oxidizing bacteria slowing down due to the growth of sundry bacteria as described above; thus, the upper limit is preferably 1 mMol/L.

Since the manganese concentration begins to decrease at 1.8 mMol/L, it is effective to add glucose in an equivalent amount that is 0.2 times or more and 0.6 times or less higher than that of manganese.

Industrial Applicability [0050]

The manganese removal method of the present invention can be applied to the removal of manganese from various types of wastewater, and is suitable for removing manganese from a large amount of wastewater, such as wastewater produced in the hydrometallurgical process of nickel oxide ore.

[0051]

The reference in this specification to any prior publication (or information derived from it) , or to any matter which is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0052]

Throughout this specification and the claims which follow, unless the context requires otherwise, the wordcomprise, and variations such as comprises or comprising, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims

The claims defining the invention are as follows:

1. A method for removing manganese from wastewater having a manganese concentration of approximately 60 mg/L or less using manganese-oxidizing bacteria, the method comprising adding an amino acid having a molecular weight of 90 or more and glucose;

wherein the amino acid is added in an amount corresponding to a molar concentration that is 1.6 times to 2.2 times higher than the molar concentration of manganese in the wastewater; and wherein the glucose is added in an amount corresponding to a molar concentration that is 0.2 times to 0.6 times higher than the molar concentration of manganese in the wastewater.
2. A method for removing manganese from wastewater, the method comprising, as a preliminary step, adding an alkali in advance to wastewater containing manganese in order to adjust the pH so that the manganese concentration is reduced to 65 mg/L or less, and then subjecting the wastewater obtained in the preliminary step to the treatment according to claim 1.