EP4313879A1

EP4313879A1 - Method for reducing the amount of chlorate in wastewater

Info

Publication number: EP4313879A1
Application number: EP22712956.6A
Authority: EP
Inventors: Ernesto De Los Santos; Miguel PAOLINO; Lorena ECHEVARRIA; María GUBITOSI; Darío CAME; Dayana TRAVERS; Gervasio GONZÁLEZ; Valentina VIGNOLA; Maite MENDEZ; Jonathan RAGGHIANTI
Original assignee: UPM Kymmene Oy
Current assignee: UPM Kymmene Oy
Priority date: 2021-03-26
Filing date: 2022-03-21
Publication date: 2024-02-07
Also published as: WO2022200683A1; BR112023019470A2; FI130296B; FI20215350A1; UY39700A; CL2023002819A1

Abstract

A method for reducing the amount of chlorate in wastewater treated in a wastewater treatment process (1) is disclosed. The wastewater treatment process is operated using a system comprising an equalization basin (5, 6, 7) to which at least a part of the wastewater (8, 9) is fed during the wastewater treatment process, and the method comprises adjusting the conditions in the equalization basin (5, 6) to which the at least the part of the wastewater is fed so as to maintain conditions suitable for the reduction of chlorate in the at least the part of the wastewater, such that the chlorate is at least partially reduced in the equalization basin.

Description

METHOD FOR REDUCING THE AMOUNT OF CHLORATE IN WASTEWATER

TECHNICAL FIELD

The present disclosure relates to a method for reducing the amount of chlorate in wastewater.

BACKGROUND

Chlorate, when present in wastewaters or ef fluents from industrial processes, may have undesirable effects when it ends up in natural environments. Chlo rate may be found e.g. in pulp mill effluents and wastewaters due to bleaching stages utilizing chlorine compounds .

There may therefore be a need to remove or reduce the amount of chlorate in wastewater prior to discharging the wastewater.

SUMMARY

A method for reducing the amount of chlorate in wastewater treated in a wastewater treatment process is disclosed. The wastewater treatment process may be operated using a system comprising an equalization basin to which at least a part of the wastewater is fed during the wastewater treatment process. The method may com prise adjusting the conditions in the equalization basin to which the at least the part of the wastewater is fed so as to maintain conditions suitable for the reduction of chlorate in the at least the part of the wastewater, such that the chlorate is at least partially reduced in the equalization basin.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the embodiments and constitute a part of this specification, illustrate various embodiments. In the drawings:

Figure 1 illustrates an embodiment of the method for reducing the amount of chlorate in wastewater treated in a wastewater treatment process.

DETAILED DESCRIPTION

With the method, it is possible to create con ditions in which chlorate may be reduced in the wastewater. It may thus be possible to reduce the amount of chlorate in the wastewater significantly. At the same time, it may, at least in some embodiments, be possible to avoid odour issues caused by anoxic conditions in the equalization basin (s). The process is simple to operate and does not necessarily require significant investments e.g. in equipment, energy and/or reagents.

In the context of this specification, the term "chlorate" may be understood as referring to ClC-, i.e. the anion having the formula ClC- as well as to compounds containing the anion. In other words, chlorate may be at least partially present as a salt. In chlorate, the chlorine atom is in the +5 oxi dation state, i.e. chlorate (V).

The wastewater treated in the wastewater treat ment process may be obtainable or obtained from a chemical process, for example a process for the produc tion of chlorine-containing chemicals or a textile in dustry process. The chemical process may generate chlo rate, which then may be conveyed to the wastewater. For example, the chemical plant may be for producing chlorate (for example, sodium chlorate) or chlorine dioxide (CIO₂), e.g. for use in pulping processes.

The wastewater may comprise or be e.g. an ef fluent, filtrate and/or other waste liquid that is to be discharged. The wastewater is to be treated for (fur ther) purifying purposes by the wastewater treatment process, for example in a wastewater treatment plant, before discharging. The wastewater treatment process may take place e.g. at a wastewater treatment plant of a pulping mill or other chemical process from which the wastewater is obtained or obtained, or at an external wastewater treatment plant.

The wastewater treated in the wastewater treat ment process may be obtainable or obtained from a pulp ing process. The pulping process may be e.g. a chemical pulping process. The process may be a kraft pulping process (the so-called sulphate process). For example, the wastewater may be at least partially issued from fiber line operations of a chemical pulp mill, such as a kraft pulp mill. The pulping process may be, addi tionally or alternatively, a chemithermomechanical pulping process.

The pulping process may include a chlorine based bleaching stage. Such a chlorine based bleaching stage may produce chlorate, which may then end up in the wastewater obtained or obtainable from the pulping pro cess. In other words, the wastewater may comprise or be wastewater obtainable or obtained from one or more bleaching stages, for example a chlorine based bleaching stage and/or associated washing stage(s), of a chemical pulping process. In the context of this specification, the term "pulp" may be understood as referring to a lignocellu- losic fibrous material prepared by chemically and/or mechanically separating cellulose fibres from a natural fibre based material, for example from a lignocellulosic material. Pulping processes can be used for producing pulp.

The wastewater may be obtainable from the chem ical process, such as a pulping process, using various suitable methods and apparatuses. For example, from a bleaching stage, a chlorate-containing effluent may be separated using a press, a wash press, a drum displacer washer or any other apparatus which is capable of (or configured to) separating the effluent from pulp.

In the wastewater treatment process, the rate at which the wastewater arrives at the wastewater treat ment process might vary, even dramatically, during the day. Therefore it may be convenient to equalize the flow before feeding it to the various treatment steps. The wastewater treatment process may thus be operated using a system comprising one or more equalization basins to which at least a part of the wastewater is fed during the wastewater treatment process.

The method may comprise adjusting the condi tions in the equalization basin to which the at least the part of the wastewater is fed so as to maintain conditions suitable for the reduction of chlorate in the at least the part of the wastewater, such that the chlo rate is at least partially reduced in the equalization basin.

In an embodiment, the wastewater treatment pro cess is operated using a system comprising a first equalization basin and a second equalization basin, which are operated such that the first equalization ba sin is fed with a first part of the wastewater while emptying the second equalization basin from a second part of the wastewater contained therein. The reductive conditions may be maintained in the first equalization basin when it is being fed with the first part of the wastewater, such that the chlorate in the first part of the wastewater is at least partially reduced in the first equalization basin.

Subsequently, when the second equalization ba sin has been emptied, it may further be fed with a third part of the wastewater, while the first equalization basin is emptied of the first part of the wastewater. This cycle may then be repeated, for example one or more times.

In such embodiments, the reductive conditions may be maintained in the first equalization basin when it is being fed with the first part of the wastewater. The reductive conditions may, in some embodiments, not be maintained in the second equalization basin when it is being emptied. However, the conditions in the second equalization basin may nonetheless be reductive to some extent when it is being emptied, although possibly to a lesser extent.

In suitable conditions such as in equalization basins, chlorate may be reduced to chloride (Cl-). Re ducing aeration in equalization basins may unexpectedly result in such suitable conditions for the reduction of chlorate in the wastewater contained in the equalization basins. Thus the absence of aeration may assist in providing conditions suitable for the reduction of chlo rate.

However, in the absence of aeration, solids may settle in equalization basins, leading to undesired un pleasant odours, possibly because of decomposition of the sludge resulting from the settling of solids pro ducing malodourous compounds, such as ¾S, mercaptans, etc. Suitable aeration equipment for equalization ba sins is available to a skilled person.

In an embodiment, the conditions suitable for the reduction of chlorate are maintained by not aerating the at least the part of the wastewater in the equali zation basin or the first part of the wastewater in the first equalization basin.

In an embodiment, the wastewater treatment pro cess is operated using a system comprising a first equalization basin and a second equalization basin, which are operated such that the first equalization ba sin is fed with a first part of the wastewater while the second equalization basin is emptied from a second part of the wastewater contained therein. In other words, the first equalization basin is fed with the first part of the wastewater at the same time the second equalization basin is emptied from the second part of the wastewater contained therein. The second part of the wastewater in the second equalization basin may be aerated when the second equalization basin is being emptied, and the first part of the wastewater in the first equalization basin is not aerated when the first equalization basin is being fed with the first part of the wastewater, such that the chlorate in the first part of the wastewater is at least partially reduced in the first equalization basin.

In an embodiment, the first equalization basin and the second equalization basin are operated such that the first equalization basin is fed with a first part of the wastewater while the second equalization basin is emptied from a second part of the wastewater con tained therein. The second part of the wastewater in the second equalization basin may be aerated when the second equalization basin is being emptied, and the first part of the wastewater in the first equalization basin is not aerated when the first equalization basin is being fed with the first part of the wastewater until at least about 70 %, or at least about 80 %, of the volume of the first equalization basin is filled with the first part of the wastewater. In other words, the aeration of the first equalization basin is started when at least about 70 %, or at least about 80 %, of the volume of the first equalization basin has been filled. In such an embodi ment, the chlorate in the first part of the wastewater is at least partially reduced in the first equalization basin.

In an embodiment, the first equalization basin and the second equalization basin are operated such that the first equalization basin is fed with a first part of the wastewater while emptying the second equalization basin from a second part of the wastewater contained therein, such that the second part of the wastewater in the second equalization basin is aerated when the second equalization basin is being emptied, and the first part of the wastewater in the first equalization basin is not aerated when the first equalization basin is being fed with the first part of the wastewater until at least about 70 %, or at least about 80 %, of the volume of the first equalization basin is filled with the first part of the wastewater. When the second equalization basin is subsequently fed with a third part of the wastewater, the first equalization basin is emptied from the first part of the wastewater, and the first equalization basin is aerated when it is emptied from the first part of the wastewater. The aeration of the first equalization basin may be started when at least about 70 %, or at least about 80 %, of the volume of the first equalization basin has been filled.

The first equalization basin and the second equalization basin of the system may be operated such that when the first equalization basin is fed with a first part of the wastewater, the second equalization basin is emptied from a second part of the wastewater contained therein, and when the second equalization ba sin is subsequently fed with a third part of the wastewater, the first equalization basin is emptied from the first part of the wastewater. In such an embodiment, the reductive conditions may be maintained in the equalization basin that is being fed. The reductive con ditions may or may not be maintained in the equalization basin that is being emptied.

The first equalization basin and the second equalization basin of the system may be operated dis- continuously, e.g. such that when the first equalization basin is fed with a first part of the wastewater, the second equalization basin is emptied from a second part of the wastewater contained therein, and when the second equalization basin is subsequently fed with a third part of the wastewater, the first equalization basin is emp tied from the first part of the wastewater. In such an embodiment, the reductive conditions may be maintained in the equalization basin that is being fed. The reduc tive conditions may or may not be maintained in the equalization basin that is being emptied.

The first equalization basin and the second equalization basin may be operated such that when the first equalization basin is fed with a first part of the wastewater, the second equalization basin is emptied from a second part of the wastewater contained therein, and when the second equalization basin is subsequently fed with a third part of the wastewater, the first equalization basin is emptied from the first part of the wastewater. In such an embodiment, the reductive condi tions are maintained in the equalization basin that is being fed by not aerating said equalization basin, and the equalization basin that is being emptied is aerated. In other words, in such an embodiment, the equalization basin that is being fed is not aerated, and the equal ization basin that is being emptied is aerated.

The first equalization basin and the second equalization basin may be operated discontinuously such that when the first equalization basin is fed with a first part of the wastewater, the second equalization basin is emptied from a second part of the wastewater contained therein, and when the second equalization basin is subsequently fed with a third part of the wastewater, the first equalization basin is emptied from the first part of the wastewater. In such an embodiment, the reductive conditions are maintained in the equali zation basin that is being fed by not aerating said equalization basin (i.e. not aerated when it is being fed), and the equalization basin that is being emptied is aerated (i.e. aerated when it is being emptied). In other words, in such an embodiment, the equalization basin that is being fed is not aerated (i.e. not aerated when it is being fed), and the equalization basin that is being emptied is aerated (i.e. aerated when it is being emptied).

The aeration of the equalization basin that is being emptied may reduce the settling of solids in the equalization basin. The settling and accumulation of solids in the equalization basin and at the bottom thereof may cause other issues, such as an unpleasant smell. However, the aeration of the equalization basin e.g. continuously may reduce the reduction of chlorate in the part of the wastewater contained in the equali zation basin. Thus there may be a need to ensure that the settling of solids contained in the part of the wastewater in the equalization basin and any resulting smell remains at an acceptable level, while achieving a desired reduction of chlorate.

The system may further comprise e.g. a third equalization basin. For example, such a third equaliza tion basin may operate as an emergency basin, to which wastewater may be directed when it cannot be directed to the first and/or the second equalization basin.

The wastewater treatment process and/or the system may further comprise e.g. one or more of or all of the following: a screening stage, a primary clari fier, one or more cooling towers, a postneutralization stage, an aeration basin, or a secondary clarifier. Other possible stages and/or apparatuses may be contem plated.

The conditions suitable for the reduction of chlorate in the equalization basin (e.g. the first equalization basin, or the equalization basin that is being fed) may be maintained for a time period of at least 2 hours. In some embodiments, they may be main tained for a time period of at least 4 hours, of at least 6 hours, or of about 6 - 8 hours, or of about 2 - 8 hours.

The equalization basin (s) may be industrial scale, such that their retention time and/or the time for which the conditions suitable for the reduction of chlorate in the equalization basin may be maintained may depend e.g. on the volume and the flow of the wastewater.

The temperature in the conditions suitable for the reduction of chlorate in the equalization basin (e.g. the first equalization basin, or the equalization basin that is being fed) may be maintained e.g. at a temperature of at least 50 °C, or at least 55 °C, or of about 55 - 63 °C. There may not be a need to heat the wastewater, if it already has a suitable temperature when entering the equalization basin (s).

The conditions suitable for the reduction of chlorate in the equalization basin (e.g. the first equalization basin, or the equalization basin that is being fed) may be maintained by maintaining the level of dissolved Cy in the at least the part of the wastewater or in the first part of the wastewater at up to 1 ppm.

The pH of the wastewater may be, or it may be adjusted to, e.g. to a pH of at least 4, or at least 5, or at least 6. While the pH may not be critical for the conditions suitable for the reduction of chlorate, it may affect potential anoxic conditions in the equaliza tion basin (s) and thereby affect the issue of odour being generated therein due to settling of solids. The part of the wastewater being emptied may then be conveyed e.g. to subsequent wastewater treatment stages. For example, it may be conveyed to an aeration basin, optionally via one or more cooling towers and/or a postneutralisation stage. Chlorate may also be reduced to some extent in one or more subsequent stages, e.g. in the aeration basin. However, adjusting the conditions e.g. in the aeration basin may be more challenging with out adversely affecting its function.

EXAMPLES

Reference will now be made in detail to various embodiments, an example of which is illustrated in the accompanying drawings.

The description below discloses some embodiments in such a detail that a person skilled in the art is able to utilize the embodiments based on the disclosure. Not all steps or features of the embodiments are discussed in detail, as many of the steps or features will be obvious for the person skilled in the art based on this specification.

For reasons of simplicity, item numbers will be maintained in the following exemplary embodiments in the case of repeating components.

Figure 1 illustrates some embodiments of a method for reducing the amount of chlorate in wastewater treated in a wastewater treatment process 1.

The wastewater to be treated is, in this em bodiment, obtained from a pulping process 2. The flow of the wastewater is illustrated by arrows. The pulping process 2 may be a chemical pulping process, such as a kraft pulping process. The pulping process 2 may be operated e.g. in a pulp mill system. Various components that may be a part of the pulping process have not been included in this schematic illustration for simplicity. In this exemplary embodiment, the wastewater from the pulping process 2 first enters a screening stage 3 and subsequently a primary clarifier 4.

The wastewater treatment process 1 further com prises a first equalization basin 5, a second equaliza tion basin 6, and a third equalization basin 7. Overflow from the primary clarifier 4 may be directed to these equalization basins 5, 6, and 7. The equalization basins are operated such that the first equalization basin 5 is fed with a first part of the wastewater 8 while emptying the second equalization basin 6 from a second part of the wastewater 9 contained therein. Subse quently, the second equalization basin 6 may be fed with a third part of the wastewater, while the first part of the wastewater 8 is emptied from the first equalization basin 5. This cycle of emptying and filling the first equalization basin 5 and the second equalization basin

6 then may be repeated one or more times. In other words, the first and the second equalization basin 5, 6 may be fed and emptied in cycles. The third equalization basin

7 may be used e.g. as an emergency basin, to which wastewater may be directed when it cannot be directed to the first and the second equalization basins. Alter natively or additionally, the three equalization basins may be operated discontinuously (in cycles) in a similar manner as described above for the first and second equalization basins.

In an embodiment, the second part of the wastewater 9 in the second equalization basin 6 may be aerated when the second equalization basin 6 is being emptied, and the first part of the wastewater 8 in the first equalization basin 5 is not aerated when the first equalization basin 5 is being fed with the first part of the wastewater 8. When the first part of the wastewater 8 in the first equalization basin 5 is not aerated when the first equalization basin 5 is being fed, the conditions in the first equalization basin 5 may be favourable for the reduction of chlorate, such that the chlorate contained in the first part of the wastewater 8 is at least partially reduced, e.g. to chloride ions (Cl-), in the first equalization basin 5. For example, the level of dissolved Cy in the first part of the wastewater 8 in the first equalization basin 5 may be at up to 1 ppm when it is being fed. Depending on the flow of the wastewater, the volume of the first equalization basin 5 and other considerations, the first part of the wastewater 8 may be retained in the first equalization basin 5 for e.g. at least 2 hours, or for at least 4 hours, or for about 6 - 8 hours.

In another embodiment, the second part of the wastewater 9 in the second equalization basin 6 may be aerated when the second equalization basin 6 is being emptied, and the first part of the wastewater 8 in the first equalization basin 5 is not aerated when the first equalization basin is being fed with the first part of the wastewater until at least about 70 %, or at least about 80 %, of the volume of the first equalization basin 5 is filled with the first part of the wastewater 8. In other words, the aeration of the first equaliza tion basin 5 is started when at least about 70 %, or at least about 80 %, of the (entire) volume of the first equalization basin 5 has been filled.

The wastewater (such as the first and second parts 8, 9) may subsequently be conveyed to further stages of the wastewater treatment process 1. For exam ple, in this embodiment, the wastewater may be conveyed to one or more cooling towers 10. From the cooling tower (s) 10, the wastewater may be conveyed e.g. to a postneutralisation stage 11, to an aeration basin 12 and to a secondary clarifier 13. In the aeration basin 12, the wastewater may be aerated in the presence of aerobic microorganisms. The organic material in the wastewater may be subjected to biological treatment breaking down the organic material, forming biosludge. From the secondary clarifier 13, biosludge may be conveyed else where and treated as desired; this has not been illus trated in Fig. 1 for reasons of simplicity. From the secondary clarifier 13, the purified wastewater may be conveyed e.g. to discharge 14.

However, as a skilled person will understand, the wastewater treatment process 1 may include addi tional and/or alternative stages and components.

EXAMPLE 1

Wastewater from a pulping mill was treated in a biological wastewater treatment process and subse quently discharged. The wastewater treatment process included two equalization basins that were filled (fed) and emptied alternately. Typically, the concentration of [CIO₃-] in the wastewater had been found to be close to zero in the discharge.

Due to odour issues that were found to be caused by settling solids in the two equalization basins of the wastewater treatment process, aerators were in stalled in the equalization basins. However, subse quently the amount of chlorate in the wastewater dis charged was found to significantly increase.

It was unexpectedly found that the aerators installed in the equalization basins were affecting the conditions in the equalization basins. Upon closer in spection, it was found out that the aerators decreased the reductive power in the equalization basins, as ox ygen was being introduced.

An aeration scheme was introduced in the equal ization basins, such that the equalization basins were aerated only while in the emptying cycle. A dramatic reduction in the amount of chlorate in the wastewater being discharged was then observed. Further, the odour issues were not observed again. Cy levels in the wastewater in the equalization basins was found to be in the range of 0.29 - 0.36 ppm when the equalization basins were being filled (and not aerated until the equalization basin being filled was at least 80 % full), and in the range of 0.37 - 0.56 ppm when being emptied (and simultaneously aerated). The pH of the wastewater fed to the equalization basins was within the range of about 6.3 - 6.7.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implemented in various ways. The embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

The embodiments described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment. A process disclosed herein, may comprise at least one of the embodiments described hereinbefore. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items. The term "comprising" is used in this specification to mean including the feature (s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts.

Claims

1. A method for reducing the amount of chlorate in wastewater treated in a wastewater treatment process (1), wherein the wastewater treatment process is oper ated using a system comprising an equalization basin (5, 6, 7) to which at least a part of the wastewater (8, 9) is fed during the wastewater treatment process, and wherein the method comprises adjusting the conditions in the equalization basin (5, 6) to which the at least the part of the wastewater is fed so as to maintain conditions suitable for the reduction of chlorate in the at least the part of the wastewater, such that the chlo rate is at least partially reduced in the equalization basin.

2. The method according to claim 1, wherein the wastewater treated in the wastewater treatment process is obtainable or obtained from a pulping process (2).

3. The method according to claim 1 or 2, wherein the wastewater treatment process is operated using a system comprising a first equalization basin (5) and a second equalization basin (6), which are operated such that the first equalization basin is fed with a first part of the wastewater (8) while emptying the second equalization basin from a second part of the wastewater (9) contained therein; wherein the reductive conditions are maintained in the first equalization ba sin when it is being fed with the first part of the wastewater, such that the chlorate in the first part of the wastewater is at least partially reduced in the first equalization basin.

4. The method according to claim 3, wherein the reductive conditions are maintained in the first equal ization basin when it is being fed with the first part of the wastewater, and wherein the reductive conditions are optionally not maintained in the second equalization basin when it is being emptied.

5. The method according to any one of claims 1

- 4, wherein the wastewater treatment process is oper ated using a system comprising a first equalization ba sin and a second equalization basin which are operated such that the first equalization basin is fed with a first part of the wastewater while emptying the second equalization basin from a second part of the wastewater contained therein; wherein the second part of the wastewater in the second equalization basin is aerated when the second equalization basin is being emptied, and the first part of the wastewater in the first equaliza tion basin is not aerated when the first equalization basin is being fed with the first part of the wastewater, such that the chlorate in the first part of the wastewater is at least partially reduced in the first equalization basin.

6. The method according to any one of claims 3

- 5, wherein the first equalization basin and the second equalization basin are operated discontinuously such that when the first equalization basin is fed with a first part of the wastewater, the second equalization basin is emptied from a second part of the wastewater contained therein, and when the second equalization ba sin is subsequently fed with a third part of the wastewater, the first equalization basin is emptied from the first part of the wastewater; wherein the reductive conditions are maintained in the equalization basin that is being fed.

7. The method according to any one of claims 3

- 6, wherein the equalization basin that is being fed is not aerated, and the equalization basin that is being emptied is aerated.

8. The method according to any one of claims 1

- 7, wherein the first equalization basin and the second equalization basin are operated such that the first equalization basin is fed with a first part of the wastewater while emptying the second equalization basin from a second part of the wastewater contained therein, such that the second part of the wastewater in the second equalization basin is aerated when the second equaliza tion basin is being emptied, and the first part of the wastewater in the first equalization basin is not aer ated when the first equalization basin is being fed with the first part of the wastewater until at least about 70 %, or at least about 80 %, of the volume of the first equalization basin is filled with the first part of the wastewater; and when the second equalization basin is subsequently fed with a third part of the wastewater, the first equalization basin is emptied from the first part of the wastewater, and the first equalization basin is aerated when it is emptied from the first part of the wastewater.

9. The method according to claim 8, wherein the aeration of the first equalization basin is started when at least about 70 %, or at least about 80 %, of the volume of the first equalization basin has been filled.

10. The method according to any one of claims 1- 9, wherein the conditions suitable for the reduction of chlorate are maintained for a time period of at least 2 hours, or of at least 4 hours, or of about 6- 8 hours.

11. The method according to any one of claims 1 - 10, wherein the conditions suitable for the reduc tion of chlorate are maintained by not aerating the at least the part of the wastewater in the equalization basin or the first part of the wastewater in the first equalization basin.

12. The method according to any one of claims 1 - 11, wherein the temperature in the conditions suit able for the reduction of chlorate is maintained at a temperature of at least 50 °C, or at least 55 °C.

13. The method according to any one of claims 1 - 12, wherein the conditions suitable for the reduc tion of chlorate are maintained by maintaining the level of dissolved Cy in the at least the part of the wastewater or in the first part of the wastewater at up to 1 ppm.