CN114667269A

CN114667269A - Method for reducing phosphorus in effluent from pulping process

Info

Publication number: CN114667269A
Application number: CN202080074804.XA
Authority: CN
Inventors: E·德罗斯桑多斯; L·伊切瓦立亚; G·库巴斯
Original assignee: UPM Kymmene Oy
Current assignee: UPM Kymmene Oy
Priority date: 2019-08-23
Filing date: 2020-08-18
Publication date: 2022-06-24
Also published as: WO2021038126A1; EP4017835A1; FI20195695A1; FI129811B; UY38852A; BR112022003245A2; CL2022000408A1

Abstract

A method for reducing phosphorus in the effluent of a pulping process (1) is disclosed herein. The method comprises the following steps: combining a plurality of acidic effluents (2, 2') obtainable from a pulping process into an acidic effluent stream (3); combining a plurality of alkaline effluents (4, 4') obtainable from a pulping process into an alkaline effluent stream (5); at least a portion (6) of the acidic effluent stream is treated by: adjusting the pH of at least a portion (6) of the acidic effluent stream to basic with a basic agent (7) such that phosphorus in the at least a portion of the acidic effluent stream precipitates in the form of phosphate and the precipitate (8) is at least partially removed, thereby producing a purified effluent stream (9); and combining the purified effluent stream with the basic effluent stream and optionally with the untreated portion (10) of the acidic effluent stream.

Description

Method for reducing phosphorus in effluent from pulping process

Technical Field

The present disclosure relates to methods and apparatus for reducing phosphorus in pulp process effluents.

Background

Effluents produced by pulp or paper mills and other effluent-producing plants are typically treated in wastewater treatment plants to reduce the amount of compounds, such as phosphorus, that may be harmful to the environment. Wood materials used in pulping processes contain phosphorus compounds such as phosphates and organophosphorus compounds. Some phosphorus may end up in the effluent discharged from the pulping process.

Phosphorus should generally be removed or reduced from the effluent discharged from the plant into the environment (e.g., water systems) to prevent contamination. As environmental regulations limiting phosphorus emissions become more stringent, there is a need to reduce the phosphorus content of the effluent from the plant to the wastewater treatment facility and ultimately to the environment. Furthermore, as a non-process element, phosphorus may interfere with sub-processes of the chemical pulping process, especially when present in large quantities. For example, it may interfere with the operation of a lime kiln.

At the same time, reducing phosphorus may bring about huge investment and management costs.

Disclosure of Invention

A method for reducing phosphorus in a pulping process effluent is disclosed. The method can comprise the following steps: combining a plurality of acidic effluents obtainable from a pulping process into an acidic effluent stream; combining a plurality of alkaline effluents obtainable from a pulping process into an alkaline effluent stream; treating at least a portion of the acidic effluent stream by: adjusting the pH of at least a portion of the acidic effluent stream to basic with a basic agent such that phosphorus in at least a portion of the acidic effluent stream precipitates as phosphate and the precipitate is at least partially removed, thereby producing a purified effluent stream; the purified effluent stream is combined with the basic effluent stream and optionally with the untreated portion of the acidic effluent stream.

Drawings

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

FIG. 1 is a flowchart illustration of one embodiment of the method;

FIG. 2 is a schematic diagram of one embodiment of the method and apparatus; and

fig. 3 schematically shows an experimental setup.

Detailed Description

A method for reducing phosphorus in the effluent of a pulping process is disclosed.

The method can comprise the following steps:

combining a plurality of acidic effluents obtainable from a pulping process into an acidic effluent stream;

combining a plurality of alkaline effluents obtainable from a pulping process into an alkaline effluent stream;

treating at least a portion of the acidic effluent stream by: adjusting the pH of at least a portion of the acidic effluent stream to basic with a basic agent such that phosphorus in at least a portion of the acidic effluent stream precipitates as phosphate and the precipitate is at least partially removed, thereby producing a purified effluent stream; and

the purified effluent stream is combined with the basic effluent stream and optionally with the untreated portion of the acidic effluent stream.

By using this method, the required amount of phosphorus can be removed at reduced cost, or the phosphorus removal can be increased without significantly increasing the costs involved. Agents that can be used in any case to adjust the pH of the effluent, such as alkaline agents, can be used. The need for adjusting the pH of the effluent may also be reduced. Furthermore, the amount of sediment produced, for example, in the form of sludge, can be reduced, thereby reducing the management costs involved.

The process may also be operated as a real-time or on-line process so that a desired or predetermined amount of phosphorus may be removed even though the amount of phosphorus in the effluent may vary over time or, for example, depending on the wood or other lignocellulosic material used as feedstock in the pulping process.

Different acidic effluents from chemical pulping processes may have different pH values and different phosphorus contents. Therefore, measuring and controlling the amount of alkaline agent to be added to remove phosphorus can be challenging. When the multiple acidic effluents obtainable from the pulping process are combined into an acidic effluent stream, a more uniform acidic effluent stream (having a more uniform pH and phosphorus content) can be obtained. The relative uniformity of the acidic effluent stream may make it easier to operate and control the process in a more economical manner and/or may make it possible to remove a desired or predetermined amount of phosphorus. Thus, a more efficient way to control the amount of phosphorus to be precipitated and removed may be provided.

The effluent from the pulping process may comprise a plurality of acidic effluents obtainable from the pulping process; a plurality of alkaline effluents obtainable from a pulping process; and optionally other acidic effluents and/or other basic effluents. In at least some embodiments, the effluent from the pulping process may further comprise, for example, a neutral effluent. For example, in embodiments where the acidic effluent and/or the alkaline effluent comprise, or are, acidic and alkaline bleaching effluents, respectively, not all of the bleaching effluents (acidic or alkaline) need be combined into the acidic and/or alkaline effluent stream. Instead, they may be further treated or discharged separately, for example.

Thus, at least in some embodiments, "effluent from a pulping process" may be understood to refer to the total effluent obtainable from a pulping process. In one embodiment, they may comprise or be (total) bleaching effluents from the pulping process.

In the context of the present specification, the term "effluent" may be understood to mean an aqueous liquid containing phosphorus, which is to be at least partially or completely removed. For example, the effluent may be a filtrate or other waste liquid, which may be obtained from the pulping process and which is to be discharged from the pulping process. The effluent is typically treated by wastewater treatment, e.g. in a wastewater treatment plant, for (further) purification purposes prior to discharge. The wastewater treatment can be carried out at a wastewater treatment facility of a pulp mill or at an external wastewater treatment facility.

The acidic and basic effluent streams can be separately sent to, for example, a wastewater treatment facility where a phosphorus removal treatment (i.e., treatment of at least a portion of the acidic effluent stream by adjusting the pH of at least a portion of the acidic effluent stream to basic with a basic agent such that phosphorus in at least a portion of the acidic effluent stream precipitates as phosphate and at least partially removes the precipitate to produce a purified effluent stream) can be performed, as well as any portion or step of a process that can be performed after and/or in parallel with the phosphorus removal treatment. The wastewater treatment plant may be a pulp mill wastewater treatment plant or an external wastewater treatment plant.

The term "effluent" may be understood to refer, at least in some embodiments, to an effluent stream or streams.

In the context of the present specification, the term "plurality/plurality" may be understood to mean two, three or more, i.e. at least two, or in some embodiments at least three.

In the context of the present specification, the term "pulp" may be understood as referring to lignocellulosic fibre material prepared by chemically and/or mechanically separating cellulosic fibres from natural fibre based material, e.g. from lignocellulosic material. The pulping process may be used to produce pulp (paper pulp).

The pulping process may be a chemical pulping process. In chemical pulping processes, chemical pulp is produced using the so-called sulfate or kraft pulping process. Thus, the pulping process may be a kraft pulping process. However, other pulping processes, such as chemi-thermomechanical pulping processes, are contemplated.

In a chemical pulping process, multiple streams of acidic effluent and/or multiple streams of alkaline effluent may be obtained from a fiber line operation of the chemical pulping process. For example, they may be obtained from bleaching processes, from white liquor plants associated with pulping processes and/or from chemical plants associated with pulping processes.

In the context of the present specification, the term "associated with a pulping process" may be understood to mean an apparatus or process connected to the pulping process, for example an apparatus or process for treating a stream or product produced in the pulping process, or an apparatus or process for producing a product, chemical or stream used in or in the pulping process.

Thus, the term "pulping process" or in some embodiments "chemical pulping process" may be broadly understood in the context of this specification to also include or encompass related processes, such as, for example, one or more processes or equipment for recycling and/or producing chemicals used in a pulping process and/or one or more processes or equipment for treating products or chemicals produced by or produced in a pulping process.

Kraft pulp mills are generally divided into fiber production lines, chemical recovery systems and wastewater treatment. The white liquor plant may form part of a chemical recovery system of a pulp mill. Fiber line operations may generally include cooking, brown stock washing (to wash used cooking chemicals from the pulp), oxygen delignification, bleaching, and pulp drying. In many conventional kraft pulp mills, the pH of the cooking, brown stock washing, oxygen delignification and knotting and screening stages is alkaline. Any effluent or filtrate obtained from these stages is typically alkaline. Typically the pH of one or more bleaching stages is acidic. Thus, the effluent obtainable from the fiber line operations and bleaching stages may be acidic or alkaline, depending on the particular stage and chemicals used therein.

At least a portion (i.e., one or more) or all of the plurality of acidic effluents and/or the plurality of alkaline effluents may be obtained from a bleaching stage of a fiber line operation of the pulping process. Thus, at least a portion (i.e., one or more) or all of the plurality of acidic effluents and/or the plurality of alkaline effluents may comprise or be the acidic bleaching effluent or the alkaline bleaching effluent, respectively.

In the context of the present specification, the term "bleaching stage" may refer to any stage after the brown stock stage. It is understood to additionally comprise an oxygen delignification stage which selectively removes lignin, hexenuronic acids and/or metals. In the context of the present specification, the term "bleaching stage" may additionally or alternatively refer to any stage immediately after the oxygen stage. The first bleaching stage may be an alkaline or an acidic stage. Typically an acidic a stage followed by a D stage with no wash stage in between. This is the so-called a/D1 bleaching stage.

In the context of the present specification, a bleaching stage is understood to mean a bleaching stage and optionally a washing stage associated with or following the bleaching stage.

For example, the acidic effluent may include A, D, D1, A/D or A/D1 bleaching effluent or any other acidic bleaching effluent, or A, D, D1, A/D or A/D1 bleaching effluent or any other acidic bleaching effluent. "A" is understood to mean an acidic bleaching stage. "D" is understood to mean the chlorine dioxide bleaching stage. A. The D and D1 stages may be separate stages from each other; however, each of the A, D and D1 stages is understood herein as a bleaching stage.

Other examples of common bleaching sequences that may be used in pulping processes may include CEH, CEHH, CHEH, CEHEH, CCHEHH, CED, CEHD, CEHED, CEHDD, CEHDH, CEDED, CEHDED, CCHEDH, CDEODED, OCDEHD, CEHEDP, CEDPD, CEDEHD, (DC) (EPO) DED, D (EPO) DED, (D/Z) (EPO) DED, OD (EPO) D, OD (EPO) DD, OD (EPO) DP, OD (EPO) DZ, O (Z/D) (EPO) DP, (C + D) EODED, DEODED, OZEOD, and YP. The symbols applied to the chemical treatments and other commonly used chemical treatments in these bleaching sequences are for example as follows:

chlorination (C): elemental chlorine, acidic;

alkali extraction (E): dissolving the reaction product by NaOH;

chlorine dioxide (D): chlorine dioxide, acidic;

oxygen (O): molecular oxygen, basic, high pressure;

hypochlorite (H): hypochlorite, basic;

peroxide (P): peroxide, alkaline;

ozone (Z): ozone, acid;

chelate (Q): a chelation stage;

eo-extraction stage enhanced with oxygen;

eop-extraction phase enhanced with oxygen and hydrogen peroxide;

x ═ enzyme stage;

y ═ dithionite stage;

paa ═ peracetic acid stage;

a Pp-pressurized hydrogen peroxide stage;

q-half chelation stage.

One or more of the acidic effluents and/or one or more of the basic effluents may be obtained from these exemplary sequential acidic or basic stages, respectively.

One or more of the plurality of acidic effluents may also be obtained from the acidification treatment of unbleached pulp, which is carried out before bleaching, after the brown stock washing and the oxygen treatment of the oxygen delignification unit. This acidic effluent also typically contains dissolved phosphorus compounds.

One or more of the plurality of acidic effluents may also comprise an acidic effluent obtainable from or obtained from the unbleached pulp in an acidification treatment after washing of the brown stock, which acidification treatment is performed to dissolve acid-soluble phosphorus compounds contained in the pulp before bleaching.

At least a portion (i.e., one or more) or all of the plurality of acidic effluents and/or the plurality of alkaline effluents may be obtained from a bleaching stage of a fiber line operation of the pulping process, or from a white liquor plant or a chemical plant associated with the pulping process.

The acidic or neutral acid effluent may be obtained from a white liquor plant as part of a plurality of acidic effluents.

The chemical plant may be, for example, a plant that produces reagents for use in a pulping process. For example, chemical plants can be used to produce chlorate (e.g., sodium chlorate), chlorine dioxide (ClO)₂) Oxygen and/or hydrogen peroxide (H)₂O₂) For use in the pulping process.

The plurality of acidic or alkaline effluents, or any other acidic or alkaline effluent described herein, may be obtained from the pulping process using various suitable methods and apparatus. For example, the acidic or basic effluent may be separated from the bleaching stage using an extruder, a wash press, a drum displacement washer, or any other device capable of (or configured to) separate the effluent from the pulp.

Various suitable methods and apparatus may be used to combine the plurality of acidic effluents into an acidic effluent stream, or to combine any of the streams or streams described herein into another stream or stream. Likewise, the plurality of alkaline effluents may be combined into the alkaline effluent stream using various suitable methods and apparatus. For example, a plurality of such effluents may be mixed or otherwise combined in a mixer, tank, or pond; or the acidic or basic effluent may be injected into another acidic or basic effluent stream or into a pipeline (pipe) or sewer carrying another acidic or basic effluent.

It is not always necessary to combine all of the acidic effluent from the pulping process into an acidic effluent stream. Likewise, it is not always necessary to combine all of the alkaline effluent from the pulping process into the alkaline effluent stream. The ratio or amount of the plurality of acidic effluents and the ratio or amount of the plurality of alkaline effluents (relative to all acidic or alkaline effluents from the pulping process) may be selected based on various factors including, for example, the generation and input of phosphorus in the pulping process effluent.

For example, at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% (v/v) of all acidic effluents, e.g., all acidic bleaching effluents, from the pulping process and/or fiber line operations thereof may be combined into the acidic effluent stream. For example, all alkaline effluent from the pulping process and/or fiber line operation thereof, e.g., at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% (v/v) of all alkaline effluent may be combined into the alkaline effluent stream.

At least one of the plurality of acidic effluents may be an acidic bleaching effluent obtainable from the first acidic bleaching stage (i.e. the first acidic bleaching stage in the bleaching sequence). High phosphorus content is generally found in the acidic bleaching effluent of the first acidic bleaching stage. Thus, it may be useful to obtain an acidic bleaching filtrate from the first acidic bleaching stage. In this embodiment, the phosphorous content in the (total) effluent of the pulping process can be significantly reduced if the acidic bleaching effluent from the first acidic bleaching stage or a part thereof is treated as described in this specification.

In one embodiment, only at least a portion of the acidic effluent stream is treated by adjusting the pH of at least a portion of the acidic effluent stream to basic with a basic agent. In other words, in this embodiment, no portion of the alkaline effluent stream is treated by adjusting the pH to alkaline with an alkaline agent. Treating only the acidic effluent stream or a portion thereof may be sufficient to achieve the desired phosphorus (P) removal while minimizing the consumption of chemicals.

Additionally or alternatively, the entire acidic effluent stream, i.e., the entire volume of the acidic effluent stream, may be treated by adjusting its pH to basic with an alkaline agent.

However, the amount of at least a portion of the acidic effluent stream and/or the proportion (amount) of all basic effluent combined into the basic effluent stream may be selected according to various factors and may be different at different points in time. For example, they may be selected or adjusted on-line or at various points in time. These factors may include, for example, lignocellulosic material used as feedstock in the pulping process. For example, different wood materials may contain different amounts of phosphorus. Additionally or alternatively, these factors may include the amount of phosphorus in the combination of the purified effluent stream, the alkaline effluent stream, and optionally the untreated portion of the acidic effluent stream, or the amount of phosphorus in the (total) effluent from the pulping process.

At least a portion of the acidic effluent stream may be obtained by separating the acidic effluent stream into at least a portion of the acidic effluent stream and an untreated portion of the acidic effluent stream. This may be done, for example, using valves, suitable piping or sewers, or other suitable methods or devices for separating streams or streams.

In one embodiment, the amount of at least a portion of the acidic effluent stream is selected such that a predetermined amount of phosphorus is removed by treating at least a portion of the acidic effluent stream. A predetermined amount of phosphorus is removed from at least a portion of the acidic effluent stream by at least partially removing the precipitate (in the form of phosphate).

For example, a predetermined amount of phosphorus (target amount of phosphorus) may be set, for example as a specific value or threshold of KgP/day, and then the amount of at least a portion of the acidic effluent stream may be selected such that the predetermined amount of phosphorus is removed from the (total) effluent of the pulping process. The amount may be calculated and selected based on, for example, phosphorus input during pulping. This may depend, for example, on the amount of phosphorus contained in the lignocellulosic material used as starting material for the pulping process. Such a target amount may be imposed, for example, by environmental regulations, such as regulations regarding the phosphorus load ultimately discharged from the pulping process and/or any subsequent wastewater treatment stage. Thus, different pulping processes may have different goals and may select different amounts accordingly.

The process may be an on-line or continuous process. The process may be an in-line or continuous process, for example, in a sense that the amount of at least a portion of the acidic effluent stream may be selected at each separate point in time such that a predetermined amount of phosphorus is removed by treating at least a portion of the acidic effluent stream. Thus, the amount of at least a portion of the acidic effluent stream can be adjusted at various points in time to remove a predetermined amount of phosphorus by treating at least a portion of the acidic effluent stream.

In the context of the present specification, the term "amount" may refer to a volume and/or a flow rate, e.g. a volume and/or a flow rate of a flow. For example, the amount may be expressed in volume or cubic meters per air dry ton of pulp. Additionally or alternatively, it may refer to relative volume and/or relative flow.

The phosphorus content or concentration of the acidic effluent stream can be measured, and the amount of at least a portion of the acidic effluent stream can be selected based on the measured phosphorus content or concentration to remove a predetermined amount of phosphorus by treating at least a portion of the acidic effluent stream. The predetermined amount of phosphorus may be determined, for example, in units of KgP/day.

The phosphorus concentration of at least a portion of the acidic effluent stream can be, for example, to 5mg/l, or at least 10 mg/l. However, the phosphorus concentration may vary, for example, in different bleaching configurations, in different pulping processes, and/or depending on the wood or other lignocellulosic material used as feedstock in the pulping process.

The pH of the acidic effluent stream may be below 7, or below 6, or below 3.

For example, the pH of the acidic effluent stream may be from 1 to 7. Alternatively or additionally, the pH may be 2-7, or 1-6, or 2-6, or 1-3, or 2-3.

The pH of at least a portion of the acidic effluent stream can be adjusted, for example to a pH of at least 8; or at least 9; or 8 to 14; or 8 to 11; or 9-10.

In one embodiment, at least a portion of the basic effluent stream is combined into the acidic effluent stream prior to treating at least a portion of the acidic effluent stream by adjusting the pH of the at least a portion of the acidic effluent stream to basic with a basic agent. The amount of at least a portion of the basic effluent stream and the amount of at least a portion of the acidic effluent stream can be selected, for example, such that a predetermined amount of phosphorus is removed by treating at least a portion of the acidic effluent stream and at least a portion of the basic effluent stream. In this embodiment, any reference in this specification to "treating or adjusting the pH of at least a portion of the acidic effluent stream" may be understood to refer to treating or adjusting the pH of at least a portion of the basic effluent stream and the pH of at least a portion of the acidic effluent stream (i.e., combinations or mixtures thereof). In other words, in this embodiment, at least a portion of the acidic effluent stream may also comprise, for example, at least a portion of the basic effluent, e.g., as a mixture, in which case the pH of the combination or mixture is adjusted to basic with a basic agent. Furthermore, in this embodiment, any reference in this specification to a "basic effluent stream" that is combined with the purified effluent stream may be understood to refer to the untreated portion of the basic effluent stream.

When the pH of at least a portion of the acidic effluent stream (or a combination or mixture containing at least a portion of the acidic effluent stream) is adjusted to basic, at least a portion of the phosphorus contained therein may precipitate, typically as calcium phosphate or other phosphate salts. The calcium source in the precipitated calcium phosphate may be calcium originally contained in the effluent and/or calcium introduced into at least a portion of the acidic effluent stream as a component of the alkaline reagent. Thus, the precipitate usually contains calcium phosphate. The precipitate may also contain other phosphates, such as dibasic calcium phosphate; calcium carbonate; calcium sulfate; an organic phosphorus compound; and/or other precipitation/co-precipitation components.

The alkaline agent can be any agent that is capable of raising the pH to a pH at which phosphorus can precipitate. The alkaline agent may be a solution, such as an aqueous solution, a solid, a slurry, such as an aqueous slurry, or a mixture thereof. The solid alkaline agent may be in the form of, for example, a powder.

The alkaline agent may beIncluding or being, for example, an alkali metal hydroxide, such as NaOH; ca (OH)₂；CaO；NH₄OH; lime; an alkaline effluent; or any combination or mixture thereof. However, other alkaline agents are also contemplated.

In one embodiment, the alkaline agent is:

CaO；

alkali metal hydroxides, such as NaOH;

Ca(OH)₂；

NH₄OH；

lime;

an alkaline effluent or filtrate;

an alkaline bleaching effluent or filtrate; or any mixture or combination thereof.

In one embodiment, the alkaline agent is solid CaO.

In one embodiment, the alkaline agent comprises, or is, the following: aqueous slurries (i.e., suspensions) of CaO, aqueous slurries of alkali metal hydroxides such as NaOH, Ca (OH)₂Aqueous slurry of (3), NH₄An aqueous slurry or solution of OH, or any mixture or combination thereof.

The alkaline agent may also comprise a mixture of a solid and an aqueous slurry or solution, or a mixture of a solid and an aqueous slurry or solution.

If the alkaline agent comprises Ca (OH)₂CaO or any mixture or combination thereof, or Ca (OH)₂CaO, or any mixture or combination thereof, at least some of the calcium ions contained therein may precipitate with the phosphorus contained in at least a portion of the acidic effluent stream.

The alkaline agent may be an alkaline agent for adjusting the pH of the wastewater, such as Ca (OH)₂The aqueous slurry of (a).

The concentration of the alkaline agent can be varied or selected, for example, based on various factors. In the case of using a relatively strong alkaline agent or an aqueous slurry containing a relatively high concentration or amount of alkaline agent, a relatively small volume of alkaline agent may be requiredTo adjust the pH to alkaline. Thus, volume increase and dissolution of the precipitate can be avoided. For example, if Ca (OH) is used₂As an alkaline agent, Ca (OH)₂The concentration of the aqueous slurry of (a) may be, for example, at least 2g/l, or at least 20 g/l. Another very suitable alkaline agent is solid CaO or lime.

An alkaline agent can be mixed with at least a portion of the acidic effluent stream. This mixing generally accelerates the precipitation of phosphorus. The mixing can be achieved, for example, by stirring. The mixing can also be performed using equipment suitable for mixing an alkaline agent with at least a portion of the acidic effluent stream. Such a device may be, for example, a tank or a pipe comprising means for mixing. The means for mixing may be, for example, a mixer, a mixing arm, or a stirrer. Such means may also be, for example, a pipe equipped with one or more nozzles for injecting an alkaline agent into a pipe or sewer where at least a portion of the acidic effluent stream may flow.

The temperature at which the pH of at least a portion of the acidic effluent stream is adjusted to basic with the basic agent can be set, for example, between 25-90 ℃. Increasing the temperature may increase precipitation.

At least partial removal of the precipitate can be achieved, for example, by filtration, centrifugation, decantation, clarification, flotation or sedimentation.

After at least partial or in some embodiments complete removal of the precipitate from at least a portion of the acidic effluent stream, the purified effluent stream thus obtained has a significantly reduced phosphorus content or concentration. Thus, the at least partial removal of the precipitate results in the removal of phosphorus from at least a portion of the acidic effluent stream, thereby removing phosphorus from the (total) effluent of the pulping process.

Complete removal of phosphorus from the effluent of the pulping process may not always be desirable, especially if they are sent to further wastewater treatment stages. Many wastewater treatment plants and stages utilize bacterial processes for wastewater treatment, for which it may be necessary or even necessary to have a small amount of phosphorus in the effluent.

Furthermore, in order to significantly reduce the phosphorous content of the effluent, it may be sufficient to treat only at least a portion of the acidic effluent stream. For example, a predetermined amount of phosphorus may be removed from the effluent while minimizing the amount of reagents (e.g., alkaline reagents), associated costs, and/or the amount of precipitated sludge. At the same time, there is the possibility that (small amounts of) phosphorus remain in the effluent sent to the waste water treatment stage; this may be beneficial, for example, when the bacterial process is used in a subsequent wastewater treatment stage, which may require a certain amount of phosphorus in the effluent.

The purified effluent stream, the basic effluent stream and optionally the untreated portion of the acidic effluent stream may be passed to further wastewater treatment stages. This further wastewater treatment stage may be carried out in the same wastewater treatment plant as the one capable of carrying out the phosphorus removal treatment, or may be carried out in another (separate) wastewater treatment plant after the phosphorus removal treatment. The further wastewater treatment stage may comprise, for example, a biological wastewater treatment stage, or be, for example, a biological wastewater treatment stage.

The untreated portions of the purified effluent stream, the basic effluent stream, and the optional acidic effluent stream may be combined, for example, prior to passing them to the equalization tank (i.e., a combination thereof may be passed to the equalization tank), or they may be combined in the equalization tank. Alternatively or additionally, they may be combined at any stage prior to post-neutralization in a wastewater treatment facility, where post-neutralization is performed to adjust the pH suitable for aerobic treatment. Post-neutralization is usually carried out before the aeration tank. For example, post-neutralization may be performed in the fiber production line, prior to primary clarifier treatment, or at any other stage prior to aeration.

The amount of at least a portion of the acidic effluent stream and/or the proportion (amount) of all the basic effluent that is combined into the basic effluent stream and that can subsequently be treated can be selected, for example, based on the amount of phosphorus to be sent to a further wastewater treatment stage, for example, to a biological wastewater treatment stage (which may utilize a bacterial process), for example, to an aeration tank of the further wastewater treatment stage. Alternatively or additionally, they may be selected based on the amount of phosphorus to be delivered to further wastewater treatment stages (generally), for example to a wastewater treatment plant or effluent plant.

The process can further include adjusting the pH of the combination of the purified effluent stream, the basic effluent stream, and optionally the untreated portion of the acidic effluent stream. For example, the pH of the combination can be adjusted to a pH near neutral pH. The pH may be adjusted to a pH in the range of 5 to 8 or in the range of 5 to 6. Alternatively, the pH may be adjusted to a pH in the range of 6 to 8 or in the range of 6.5 to 7.5. Any suitable neutralizing agent, alkaline or acidic agent may be used to adjust the pH. E.g. H₂SO₄Or other suitable acids may be used as the acidic reagent. For example, any of the alkaline agents described in this specification may also be used herein.

This adjustment may be made to the pH of the combination, for example, before the combination reaches the equalization tank, in the equalization tank, or at any stage prior to post-neutralization of the wastewater treatment facility. However, it may not always be necessary to adjust the pH of the combination.

The combination of the purified effluent stream, the basic effluent stream and optionally the untreated portion of the acidic effluent stream may be further treated and/or discharged in a (further) wastewater treatment stage. For example, it may be transported from the equalization tank to a (further) wastewater treatment stage and/or discharged.

In some embodiments, the wastewater treatment or wastewater treatment stage may utilize a bacterial process.

An apparatus for reducing phosphorus in an effluent of a pulping process is also disclosed. The apparatus may include:

a pulp mill system for operating (or configured to operate) a pulping process;

a first pipe arrangement for combining (or configured to combine) a plurality of acidic effluents obtainable from a pulping process into an acidic effluent stream;

a second conduit arrangement for combining (or configured to combine) a plurality of alkaline effluents obtainable from the pulping process into an alkaline effluent stream;

a treatment facility for treating (or configured to treat) at least a portion of the acidic effluent stream by: adjusting the pH of at least a portion of the acidic effluent stream to basic with a basic agent such that phosphorus in at least a portion of the acidic effluent stream precipitates in the form of phosphate and the precipitate is at least partially removed, thereby producing a purified effluent stream;

wherein the first conduit means is arranged to convey at least a portion of said acidic effluent stream to a treatment facility; and

the apparatus also includes a third conduit means for combining (or configured to combine) the purified effluent stream with the basic effluent stream, and optionally with an untreated portion of the acidic effluent stream.

A pulp mill system may include a digestion facility for cooking (or configured to cook) pulp; and washing equipment for washing (or configured to wash) the pulp. The pulp mill system may also include a bleaching system comprising one or more acidic bleaching stages and one or more alkaline bleaching stages. The acidic and alkaline bleaching stages, or separate acidic or alkaline bleaching stages, may comprise equipment for separating acidic or alkaline effluent, respectively, from the pulp to obtain acidic or alkaline effluent, respectively. The bleaching system may further comprise additional acidic, alkaline or neutral bleaching stages.

The apparatus may also include pumps, suitable piping, tanks and/or mixers for the various fluids. The apparatus may also include, for example, a drying device for drying (or configured to dry) the bleached pulp, a chemical recovery system, and/or other components of a pulp mill system.

The plant and/or the pulp mill system may also comprise white liquor equipment or chemical equipment. The white liquor plant may be part of a chemical recovery system of a pulp mill. Alternatively or additionally, the white liquor plant and/or the chemical plant may be a separate plant from the pulp mill system.

The first conduit means is arranged to convey at least a portion of the acidic effluent stream to the treatment facility. The first pipe arrangement may be connected directly or indirectly to the treatment arrangement such that there is a flow connection to the treatment device.

The second conduit means may be arranged to convey the basic effluent stream to the third conduit means such that the purified effluent stream may be combined with the basic effluent stream and optionally with the untreated portion of the acidic effluent stream. The second pipe arrangement may be directly or indirectly connected to the third pipe arrangement such that there is a flow connection between them.

The first, second and/or third pipe means may comprise a conduit, such as a pipe and/or sewer, or any other means for establishing a flow connection. Thus, the term "conduit means" may be understood as referring alternatively or additionally to conduit means and/or sewer means. Such plumbing may further include, for example, one or more plumbing connections, valves, tanks or other containers, pumps, and the like.

The apparatus may also include a device for measuring (or configured to measure) the phosphorus content or concentration of the acidic effluent stream. The apparatus for measuring the phosphorus content or concentration of the acidic effluent stream may comprise, for example, an on-line phosphorus measuring device, or may be, for example, an on-line phosphorus measuring device. Such devices are commercially available. Alternatively or additionally, it may comprise or be a laboratory measurement system or device for measuring phosphorus content or concentration. Various devices, apparatuses, and systems for measuring the amount or concentration of phosphorus in an acidic effluent stream are contemplated and available to those skilled in the art.

The first conduit arrangement may further comprise means for selecting (or configured to select) and directing to the treatment means at least a portion of the acidic effluent stream, or an amount thereof, based on the measured phosphorus content or concentration, such that a predetermined amount of phosphorus is removed in the treatment means by treating at least a portion of the acidic effluent stream.

The treatment facility can include a facility for mixing (or configured to mix) an alkaline agent with at least a portion of the acidic effluent stream such that phosphorus in at least a portion of the acidic effluent stream precipitates in the form of phosphate, and the treatment facility further includes a facility for at least partially removing (or configured to remove) the precipitate to produce a purified effluent stream.

The components of the device may be connected directly or indirectly. Thus, there may be a flow connection between the connected components.

Examples

Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings.

The following description discloses some embodiments in such detail as to enable others skilled in the art to utilize the embodiments based on the disclosure. Not all steps or features of an embodiment are discussed in detail, as many steps or features will be apparent to those of ordinary skill in the art based on this description.

For simplicity, where components are repeated, item numbers will be maintained in the following exemplary embodiments.

Fig. 1 illustrates some embodiments of a method for reducing phosphorus in the effluent of a pulping process 1.

Pulping process 1 may be a chemical pulping process, such as a kraft pulping process. The pulp process 1 may be operated in a pulp mill system such as described in connection with fig. 2. For simplicity, various components that may be part of the pulping process are not included in this schematic. Furthermore, as one skilled in the art will appreciate, the pulping process 1 is merely an exemplary schematic of one embodiment of a pulping process. Lignocellulosic material 28, such as wood chips and/or various other types of lignocellulosic material, may be fed into the pulping process 1 and digested in a digestion stage 29, such as in a digestion unit 28 for cooking a pulp from the lignocellulosic material 28. Pulp (commonly referred to as "brown stock") is conveyed from the digestion stage 29 to the washing stage 30 for washing the pulp. The washing may be performed, for example, in a washing apparatus as described in connection with fig. 2. The fiber line operation 11 of the pulping process 1 may comprise a plurality of bleaching stages 14 in addition to the cooking stage 20 and the washing stage 30.

The washed pulp is transported from the washing stage 30 to the bleaching stage 14. The bleaching stage 14 may comprise, for example, one or more bleaching plants and washing plants for washing the pulp between the bleaching stages (not shown in detail in this schematic). The bleaching stage 14 is therefore understood to mean a bleaching stage and optionally a washing stage associated with or following the bleaching stage. Thus, at least in some embodiments, the term "bleaching effluent" may be understood to also include washing effluent from a washing stage associated with or immediately following a (actual) bleaching stage. The pulp may then pass from the bleaching stage 14 to a drying stage 31 for drying pulp 32 produced by the pulping process.

The individual bleaching stages 14 may be acidic or alkaline, depending on the stage and their order (bleaching sequence). Various bleaching sequences may be considered, for example, as described elsewhere in this specification, depending on the type and final quality of the pulp 32 to be produced, environmental issues, operating costs, raw lignocellulosic material 28, and the like; the processes described in this specification are not limited to any particular bleaching sequence. Different bleaching chemicals may be used in each bleaching stage 14, optionally with washing between the individual stages. The pH in each individual bleaching stage 14 may be alkaline, acidic or neutral.

In this figure, a plurality of

acidic effluents

2, 2' and 2 "are released from the bleaching stage 14 of the fibre line 11. From the bleaching stage 14 of the fibre line 11 also a plurality of

alkaline effluents

4, 4' and 4 "is released. Although the acidic effluent is described as coming from an earlier bleaching stage and the alkaline effluent from a later bleaching stage, this is for simplicity of the figure and it is not to be understood that the acidic and alkaline effluents will be obtained in and/or from a particular sequence of bleaching stages.

The plurality of

acidic effluents

2, 2', 2 "that may be obtained from the pulping process may be combined into an acidic effluent stream 3.

The plurality of

alkaline effluents

4, 4', 4 "that may be obtained from the pulping process may be combined into an alkaline effluent stream 5.

Alternatively or additionally, one or more of the plurality of acidic effluents may be obtained elsewhere. In this example, additional acidic effluent 2 "' may be obtained from chemical plant 13 and combined into acidic effluent stream 3. In additional or alternative embodiments, the acidic or neutral acid effluent may be obtained from the white liquor plant 12 and combined into the acidic effluent stream 3.

Alternatively or additionally, one or more of the plurality of alkaline effluents may be obtained from elsewhere. In this example, additional alkaline effluent 4 "' may be obtained from the white liquor plant 12 and combined into the acidic effluent stream 5. In additional or alternative embodiments, the alkaline effluent may be obtained from chemical plant 13.

The acidic effluent stream 3 may be treated at least partially or completely, i.e. a portion (portion) 6 of the acidic effluent stream 3 or the entire volume thereof may be treated. When only a portion of the acidic effluent stream 3 is treated, a portion 6 of the acidic effluent stream can be sent to a point of treatment, such as to a treatment facility 22. Thus, the acidic effluent stream 3 may be divided into a treated portion 6 and an untreated portion 10 of the acidic effluent stream 3. The amounts of the treated portion 6 and untreated portion 10 may be selected based on various factors, such as those described elsewhere in this specification.

The phosphorus content or concentration of the acidic effluent stream 3 can be measured using a suitable device 24. Then, in some embodiments, based on the measured phosphorus content or concentration, the amount of the at least a portion 6 of the acidic effluent stream may be selected, e.g., separated or diverted from the acidic effluent stream 3, such that a predetermined amount of phosphorus is removed by treating the at least a portion 6 of the acidic effluent stream 3.

The basic effluent stream 5 may be treated at least partially or completely, i.e. a portion 15 (part) of the basic effluent stream 5 or the entire volume thereof may be treated. When only a portion of the basic effluent stream 5 is treated, this portion 15 of the basic effluent stream 5 can be sent to a treatment device 22 for treatment. Thus, the basic effluent stream 5 may be separated into a portion to be treated 15 and an untreated portion 33 of the basic effluent stream 5. The amounts of the treated portion 15 and untreated portion 33 may be selected based on various factors, such as those described elsewhere in this specification. In some embodiments, the entire basic effluent stream 5 is untreated, i.e., no (no portion of) basic effluent stream 5 is sent to treatment facility 22.

The parts 6 and 15 may be combined before processing, or they may for example be transferred to a processing device 22, where they may be combined, for example by mixing.

During treatment in the treatment apparatus 22, the pH of at least a portion 6 of the acidic effluent stream and optionally at least a portion 15 of the basic effluent stream is adjusted to basic with the basic agent 7. For example, portion 6 and optional portion 15 can be sent to a reactor, tank, or mixer to contact the basic reagent 7 with them, or to other suitable equipment 26 to mix the basic reagent with at least a portion of the acidic effluent stream. The alkaline agent 7 may be, for example, any of the alkaline agents described in the present specification. When the pH of at least a part 6 and optionally at least a part 15 is adjusted to alkaline with alkaline agent 7, phosphorus in at least a part 6 and 15 or at least a part of the phosphorus may precipitate in the form of phosphate.

The precipitate 8 thus produced may then be at least partially removed to produce a purified effluent stream 9. The sediment 8 can be removed, for example, in the form of sludge. Thus, the treatment device 22 may comprise a device 27 for at least partially removing the precipitate 8 to produce a purified effluent stream 9.

For example, the sediment 8 (e.g., in the form of sludge) may be discharged (e.g., landfilled) or further processed. Because it may contain a significant amount of phosphorus and, for example, calcium, it can be used for various purposes, for example, as a fertilizer product or component thereof.

The purified effluent stream 9 can then be combined with the basic effluent stream 5, or optionally with the untreated portion 33 of the basic effluent stream, and optionally with the untreated portion 10 of the acidic effluent stream. This can be done, for example, with a suitable pipe arrangement 23. The combination 16 thus obtained can be further treated in a waste water treatment 17 and/or discharged 18.

The acidic and basic effluent streams 3, 5 can be separately sent, for example, to a wastewater treatment facility 35 where phosphorus removal treatment can be performed (i.e., at least a portion 6 of the acidic effluent stream is treated by adjusting the pH of at least a portion 6 of the acidic effluent stream to basic with an alkaline agent such that phosphorus in at least a portion of the acidic effluent stream precipitates as phosphate and the precipitate is at least partially removed, thereby producing a purified effluent stream), as well as any portion or stage of the process that can be performed after and/or in parallel with the phosphorus removal treatment. However, in other embodiments, the treatment may be performed, for example, on a fiber production line.

In this exemplary embodiment, the combination 16 may eventually be combined with additional acidic effluent 2 "" and/or additional basic effluent 4 "" that are not combined into the acidic effluent stream 3 or the basic effluent 5. These additional effluents may be obtained, for example, from the bleaching stage 14, but they may additionally or alternatively be obtained from other stages of the pulping process 1 or from other sources, for example from the white liquor plant 12 and/or the chemical plant 13.

FIG. 2 illustrates an embodiment of a method and apparatus 19 for reducing phosphorus in the effluent of a pulping process. The apparatus 19 described herein may be suitable, for example, for performing one or more embodiments of the processes described in the context of fig. 1 and elsewhere in this specification.

The arrangement 19 may comprise a pulp mill system 1 for operating a pulp making process. For example, the pulp mill system 1 may be a chemical pulp mill system, such as a kraft pulp mill system. For simplicity, various components that may be part of the pulp mill system 1 are not included in this schematic. Furthermore, as will be understood by a person skilled in the art, the pulp mill system 1 is only an exemplary schematic or embodiment of a pulp mill system. Lignocellulosic material 28, such as wood chips and/or various other types of lignocellulosic material, can be fed into the pulp mill system 1. The pulp mill system 1 may comprise a digestion plant 29 for cooking pulp from the lignocellulosic material 28. From the digesting unit 29, the pulp (often referred to as "brown stock") is conveyed to a washing unit 30 for washing the pulp. The washing apparatus 30 may comprise, for example, one or two brown stock washers, for example using countercurrent flow.

The fibre line or fibre line operation 11 of the pulp mill system 1 may comprise, in addition to the digestion plant 29 and the washing plant 30, also a number of bleaching stages 14. The washed pulp is transported from the washing apparatus 30 to the bleaching stage 14. The bleaching stage 14 may comprise, for example, one or more bleaching plants and washing plants for washing the pulp between the bleaching stages (not shown in detail in this schematic). The bleaching stage 14 is therefore understood to mean a bleaching stage and optionally a washing stage associated with or following the bleaching stage. The pulp mill system 1 may further comprise an apparatus 31 for drying pulp 32 to be produced.

The individual bleaching stages 14 may be acidic or alkaline, depending on the stage and their order (bleaching sequence), as described elsewhere herein. Different bleaching chemicals may be used in each bleaching stage 14, optionally with washing between stages. The pH in each bleaching stage 14 may be alkaline or acidic. Each bleaching stage 14 may also comprise equipment 34 for separating acidic or

alkaline effluents

2, 2', 2 ", 4', 4" from the pulp.

In this figure, a plurality of

acidic effluents

alkaline effluents

4, 4' and 4 "is released. Although the acidic effluent is described as coming from an earlier bleaching stage and the alkaline effluent from a later bleaching stage, this is for simplicity of the figure and it should not be understood that the acidic and alkaline effluents should be obtained in a particular order.

The plurality of

acidic effluents

2, 2', 2 "that may be obtained from the pulping process may be combined into an acidic effluent stream 3. Thus, the arrangement further comprises a first pipe arrangement 20 for combining the plurality of

acidic effluents

2, 2', 2 "obtained from the pulping process into an acidic effluent stream 3. The conduit means may comprise, for example, a conduit, such as a pipe (tube) and/or sewer for conveying the plurality of acidic effluents from the bleaching stage 14; these conduits are also understood to be denoted by the

reference numerals

2, 2' and 2 ". The first conduit means 20 may also comprise, for example, conduit connections, valves, tanks or other containers, etc., for transporting and combining the plurality of

acidic effluents

2, 2', 2 "into the acidic effluent stream 3.

The first conduit means 20 can be configured to convey at least a portion 6 of the acidic effluent stream to a treatment facility 22.

The plurality of

alkaline effluents

4, 4', 4 "that may be obtained from the pulping process may be combined into an alkaline effluent stream 5. Thus, the arrangement 19 may further comprise a second pipe arrangement 21 for combining the plurality of

alkaline effluents

4, 4', 4 "obtained from the pulping process (and from the pulp mill system 1) into an alkaline effluent stream 5. The second conduit means may comprise, for example, a conduit, such as a pipe (tube) and/or sewer for conveying the alkaline effluents from the bleaching stage 14; these ducts are also understood to be indicated by the

reference numerals

4, 4' and 4 ".

The acidic effluent stream 3 can be treated at least partially or completely, i.e., a portion 6 (portion) of the acidic effluent stream 3 or the entire volume thereof can be treated. Thus, the first conduit arrangement 20 may comprise a conduit for conveying at least a portion 6 of the acidic effluent stream to the treatment facility 22; the catheter is also understood to be indicated by reference numeral 6. The conduit 6 may be connected directly or indirectly to the treatment device 22. The apparatus 19 may also comprise means 25 for dividing the acidic effluent stream into at least a portion 6 to be treated and an untreated portion 10. The apparatus 25 may be, for example, any apparatus suitable for separating or diverting a stream or stream from another stream or stream, and may include, for example, valves, diverters, and/or control devices for controlling the amount of the treated portion 6 and untreated portion 10.

The apparatus may also include a device 24 for measuring the phosphorus content or concentration of the acidic effluent stream. In this embodiment, the first conduit means 20 may comprise means 25 for selecting and directing at least a portion 6 of the acidic effluent stream to the treatment means 22 based on the measured phosphorus content or concentration such that a predetermined amount of phosphorus is removed in the treatment means by treating at least a portion of the acidic effluent stream.

The means 25 for dividing the acidic effluent stream into at least a portion 6 to be treated and an untreated portion 10 may be arranged to select the amount of the at least a portion 6 of the acidic effluent stream based on the measured phosphorus content or concentration, e.g. to separate or divert it from the acidic effluent stream 3, thereby removing a predetermined amount of phosphorus by treating the at least a portion 6 of the acidic effluent stream 3.

The plant 19 may also comprise, for example, a chemical plant 13 and additional piping means, here indicated as 2' ", for obtaining additional acidic (or basic) effluent from the chemical plant 13 and conveying it to the first piping means 20 (or the second piping means 21) or to the treatment plant 22.

The plant 19 may also comprise, for example, a white liquor plant 12 and additional piping means, here indicated as 4' ", for taking further basic (or acidic) effluent from the white liquor plant and conveying it to a second piping means 21 (or first piping means 20) or to a treatment plant 22.

The treatment facility may include a facility 26 for mixing an alkaline agent 7 with at least a portion 6 of the acidic effluent stream such that phosphorus in the at least a portion of the acidic effluent stream precipitates in the form of phosphate.

The means 26 for mixing the alkaline agent 7 with the at least a portion 6 of the acidic effluent stream may comprise, for example, a tank for precipitating phosphorus. Such a tank may be, for example, a conical-bottomed tank. Alternatively or additionally, it may comprise, for example, a reactor, tank, cell, or mixer for contacting the alkaline agent 7 with at least a portion 6 of the acidic effluent stream. The tank, reactor or mixer may be equipped with one or more nozzles for injecting the alkaline agent therein. Thus, the basic agent may be introduced continuously by injecting the basic agent into the flow of at least a portion 6 of the acidic effluent stream. In one embodiment, the first portion 6 of the acidic effluent or filtrate may flow in a pipe or other conduit, and the alkaline agent 7 may be injected into the flow.

The treatment device 22 can also include a device 27 for at least partially removing the precipitate 8 to produce a purified effluent stream 9. The apparatus 27 for at least partially removing the precipitate 8 to produce a purified effluent 9 may be, for example, a filter, centrifuge, decanter centrifuge, clarifier, solids separator, or flocculation apparatus. However, other suitable devices are also contemplated.

The purified effluent stream 9 can then be combined with the basic effluent stream 5 or an untreated portion 33 of the basic effluent stream, and optionally with an untreated portion 10 of the acidic effluent stream. Thus, the apparatus may further comprise a third conduit means 23 for combining the purified effluent stream with the basic effluent stream or an untreated portion 33 of the basic effluent stream, and optionally with an untreated portion (10) of the acidic effluent stream. The third pipe arrangement 23 may comprise, for example, a conduit, such as a pipe (tube) and/or a sewer. It may also include, for example, an equalization pool. The equalization tank may be adapted for temporary storage of the combination 16 prior to discharge or further processing of the combination 16.

The apparatus 19 may also include a waste water treatment stage 17. For example, the wastewater treatment stage 17 may be a stage within a wastewater treatment plant associated with or forming part of the pulp mill system 1. However, in some embodiments, the combination 16 may be conveyed to a wastewater treatment stage 17 located elsewhere or not forming part of the apparatus 19.

The apparatus 19 may also comprise further pipes or conduits, also indicated by reference numeral 16 in the figure, for conveying the combination 16 to a waste water treatment stage 17 and/or a discharge 18.

Example 1

As shown in table 1, a jar test was performed to evaluate different phosphorus (P) alternatives.

Table 1. vial test for evaluating P removal alternatives. WAE ═ acid effluent stream; WAB ═ effluent stream (basic (EOP) or acidic, as shown herein).

The results of the jar test are shown in tables 2 and 3. When only acidic effluent streams (acidic)WAB or WAE), the flow rate of the dephosphorization treatment can be reduced, and less sludge is generated. However, at comparable lime/NaOH consumption and similar settleability, comparable P removal, and ultimately P, can be achieved<1 mg/l. Although H is used to adjust the pH of the purified effluent stream₂SO₄The consumption is higher but the overall chemical consumption is comparable or lower.

Example 2

Fig. 3 schematically shows the experimental setup of this embodiment. The acidic and alkaline bleaching effluents were collected from the chemical pulp mill (output: 3600ADt/d) into an acidic bleaching effluent stream (acidic WAB) and an alkaline bleaching effluent stream (alkaline WAB), respectively.

9.4m corresponding to 284kgP/d³The acidic bleaching effluent stream of/ADt contained 8.4mg/l phosphorus (P) and had a pH of 2.64. It was separated by 5.3m³Part of/ADt (160kgP/d), which was subsequently treated to remove phosphorus and was separated by 4.1m³Untreated fraction of/ADt (124 kgP/d). Then 5.3m of the acidic bleaching effluent stream was treated³fraction/ADt, phosphorus precipitated by raising its pH and the precipitate was removed as sludge, obtaining 5.3m containing only 10kgP/d³A purified effluent stream of/ADt.

4.0m corresponding to 35kgP/d³The alkaline bleaching effluent stream of/ADt contained 2.45mg/l of P and had a pH of 9.88. It is not processed.

The alkaline bleaching effluent stream and the purified effluent stream are combined into a stream of 169kgP/d (WAB to equilibrium) which is then sent to an equalization tank.

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. Thus, the embodiments are not limited to the examples described above; rather, they may vary within the scope of the claims.

The above-described embodiments may be used in any combination with each other. Several embodiments may be combined to form yet another embodiment. The methods or systems disclosed herein may comprise at least one of the embodiments described above. It is to be understood that the benefits and advantages described above may relate to one embodiment, or may relate to multiple embodiments. Embodiments are not limited to those embodiments that solve any or all of the problems or those embodiments that have any or all of the benefits and advantages described. It will be further understood that the recitation of "an" item refers to one or more of that item. The term "comprising" is used in this specification to specify the inclusion of a feature or action subsequent to that feature or action, but does not preclude the presence or addition of one or more other features or actions.

Claims

1. A method for reducing phosphorus in the effluent of a pulping process (1), wherein the method comprises:

combining a plurality of acidic effluents (2, 2', 2 ") obtained from a pulping process into an acidic effluent stream (3);

combining a plurality of alkaline effluents (4, 4', 4 ") obtained from the pulping process into an alkaline effluent stream (5);

at least a portion (6) of the acidic effluent stream is treated by: adjusting the pH of at least a portion (6) of the acidic effluent stream to basic with a basic agent (7) such that phosphorus in the at least a portion of the acidic effluent stream precipitates as phosphate and at least partially removes precipitate (8), thereby producing a purified effluent stream (9); and

the purified effluent stream is combined with a basic effluent stream and optionally with an untreated portion (10) of the acidic effluent stream.

2. The method according to claim 1, wherein the pulping process is a chemical pulping process, the plurality of acidic effluents and/or the plurality of alkaline effluents originating from a fiber line operation (11) of the chemical pulping process or from a white liquor plant (12) or a chemical plant (13) associated with the pulping process.

3. A method according to claim 2, wherein at least a part or all of the plurality of acidic effluents and/or the plurality of alkaline effluents comes from a bleaching stage (14) of a fibre line operation of a chemical pulping process or from a white liquor plant (12) or a chemical plant (13) associated with the pulping process.

4. A process according to any one of claims 1 to 3, wherein at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% (v/v) of all acidic effluents from the pulping process and/or its fibre line operation are combined into the acidic effluent stream and/or at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% (v/v) of all alkaline effluents from the pulping process and/or its fibre line operation are combined into the alkaline effluent stream.

5. The method of any of claims 1-4, wherein the amount of at least a portion of the acidic effluent stream is selected such that a predetermined amount of phosphorus is removed by treating the at least a portion of the acidic effluent stream.

6. The method of any of claims 1-5, wherein a phosphorus content or concentration of the acidic effluent stream is measured, and the amount of the at least a portion of the acidic effluent stream is selected based on the measured phosphorus content or concentration such that a predetermined amount of phosphorus is removed by treating the at least a portion of the acidic effluent stream.

7. The method of any of claims 1-6, wherein the pH of the acidic effluent stream is less than 7, or less than 6, or less than 3.

8. The method of any of claims 1-7, wherein the pH of at least a portion of the acidic effluent stream is adjusted to a pH of at least 8, or from 8 to 14, or from 8 to 11, or from 9 to 10.

9. The method of any of claims 1-8, wherein at least a portion (15) of the basic effluent stream is combined into the acidic effluent stream prior to treating at least a portion of the acidic effluent stream by adjusting its pH to basic with a basic agent; and wherein the amount of at least a portion of the basic effluent stream and the amount of at least a portion of the acidic effluent stream are selected such that a predetermined amount of phosphorus is removed by treating at least a portion of the acidic effluent stream.

10. The method of any one of claims 1 to 9, wherein the alkaline agent comprises, or is: alkali metal hydroxides, such as NaOH; ca (OH)₂；CaO；NH₄OH; lime; an alkaline effluent; or any combination or mixture thereof.

11. The method according to any one of claims 1-10, wherein the method further comprises adjusting the pH of the combination (16) of: a combination (16) of the purified effluent stream, the basic effluent stream, and optionally an untreated portion of the acidic effluent stream; optionally adjusting the pH to a range of 5 to 8, or a range of 5 to 6.

12. The method of any of claims 1-11, wherein the combination of the purified effluent stream, the basic effluent stream, and optionally the untreated portion of the acidic effluent stream is further treated and/or discharged (18) in a wastewater treatment plant (17).

13. An arrangement (19) for reducing phosphorus in a pulping process effluent, wherein the arrangement comprises:

a pulp mill system (1) for operating a pulp process;

a first pipe arrangement (20) for combining a plurality of acidic effluents (2, 2') obtained from a pulping process into an acidic effluent stream (3);

a second pipe arrangement (21) for combining a plurality of alkaline effluents (4, 4') obtained from the pulping process into an alkaline effluent stream (5);

a treatment device (22) for treating at least a portion (6) of the acidic effluent stream by: adjusting the pH of at least a portion (6) of the acidic effluent stream to basic with a basic agent (7) such that phosphorus in the at least a portion of the acidic effluent stream precipitates in the form of phosphate and the precipitate (8) is at least partially removed, thereby producing a purified effluent stream (9);

wherein the first conduit means is arranged to convey at least a portion of the acidic effluent stream to the treatment facility; and

the apparatus further comprises third conduit means (23) for combining the purified effluent stream with the basic effluent stream and optionally with the untreated portion (10) of the acidic effluent stream.

14. An apparatus according to claim 13, wherein the apparatus further comprises means (24) for measuring the phosphorus content or concentration of the acidic effluent stream, and the first conduit means comprises means (25) for selecting and directing at least a portion of the acidic effluent stream to the treatment means based on the measured phosphorus content or concentration such that a predetermined amount of phosphorus is removed in the treatment means by treating at least a portion of the acidic effluent stream.

15. An apparatus according to claim 13 or 14, wherein the treatment means (22) comprises means (26) for mixing an alkaline agent with at least a portion of the acidic effluent stream to cause phosphorus in at least a portion of the acidic effluent stream to precipitate in the form of phosphate, and the treatment means (22) further comprises means (27) for at least partially removing the precipitate to produce the purified effluent stream.