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Title:
A PROCESS FOR REDUCING PHOSPHORUS IN EFFLUENTS FROM A PULPING PROCESS
Document Type and Number:
WIPO Patent Application WO/2021/038126
Kind Code:
A1
Abstract:
A process for reducing phosphorus in effluents from a pulping process (1) is disclosed. The process comprises combining a plurality of acidic effluents (2, 2', 2'') obtainable from the pulping process into an acidic effluent stream (3); combining a plurality of alkaline effluents (4, 4', 4'') obtainable from the pulping process into an alkaline effluent stream (5); treating at least a portion (6) of the acidic effluent stream by adjusting the pH thereof to alkaline with an alkaline agent (7), such that phosphorus in the at least the portion of the acidic effluent stream is precipitated in the form of a phosphate salt, and by removing the precipitate (8) at least partially to produce a purified effluent stream (9); and combining the purified effluent stream with the alkaline effluent stream and optionally with the untreated portion of the acidic effluent stream (10).

Inventors:
DE LOS SANTOS ERNESTO (UY)
ECHEVARRIA LORENA (UY)
CUBAS GASTON (UY)
Application Number:
PCT/FI2020/050537
Publication Date:
March 04, 2021
Filing Date:
August 18, 2020
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
UPM KYMMENE CORP (FI)
International Classes:
C02F1/52; C02F1/66; C02F101/10; C02F103/28; D21C11/00
Domestic Patent References:
WO2015001185A12015-01-08
Foreign References:
EP3126297A12017-02-08
CN105293660A2016-02-03
US5470480A1995-11-28
Attorney, Agent or Firm:
PAPULA OY (FI)
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Claims:
CLAIMS

1. A process for reducing phosphorus in ef- fluents from a pulping process (1), wherein the pro- cess comprises combining a plurality of acidic effluents (2, 2', 2'') obtainable from the pulping process into an acidic effluent stream (3); combining a plurality of alkaline effluents (4, 4', 4'') obtainable from the pulping process into an alkaline effluent stream (5); treating at least a portion (6) of the acidic effluent stream by adjusting the pH thereof to alka- line with an alkaline agent (7), such that phosphorus in the at least the portion of the acidic effluent stream is precipitated in the form of a phosphate salt, and by removing the precipitate (8) at least partially to produce a purified effluent stream (9); and combining the purified effluent stream with the alkaline effluent stream and optionally with the untreated portion of the acidic effluent stream (10).

2. The process according to claim 1, wherein the pulping process is a chemical pulping process, and the plurality of the acidic effluents and/or the plu- rality of the alkaline effluents are obtained from fi- ber line operations (11) of the chemical pulping pro- cess, or from a white liquor plant (12) or a chemical plant (13) associated with a pulping process.

3. The process according to claim 2, wherein at least a part of or all of the plurality of the acidic effluents and/or of the plurality of the alka- line effluents are obtained from bleaching stages (14) of the fiber line operations of the chemical pulping process, or from a white liquor plant (12) or a chemi- cal plant (13) associated with a pulping process.

4. The process according to any one of claims

1 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 from the fiber line operations thereof 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 obtained from the pulping process and/or from the fiber line operations thereof are combined into the alkaline effluent stream.

5. The process according to any one of claims

1 4, wherein the quantity of the at least the por- tion of the acidic effluent stream is selected such that a predetermined amount of phosphorus is removed by treating the at least the portion of the acidic ef- fluent stream.

6. The process according to any one of claims 1 - 5, wherein the phosphorus content or concentration of the acidic effluent stream is measured, and the quantity of the at least the portion of the acidic ef- fluent stream is selected on the basis of the measured phosphorus content or concentration such that a prede- termined amount of phosphorus is removed by treating the at least the portion of the acidic effluent stream.

7. The process according to any one of claims 1 - 6, wherein the pH of the acidic effluent stream is lower than 7, or lower than 6, or lower than 3.

8. The process according to any one of claims

1 - 7, the pH of the at least the portion of the acid- ic effluent stream is adjusted to a pH value of at least 8, or of 8 - 14, or of 8 - 11, or of 9 10.

9. The process according to any one of claims

1 8, wherein at least a portion (15) of the alkaline effluent stream is combined to the acidic effluent stream prior to treating the at least the portion of the acidic effluent stream by adjusting the pH thereof to alkaline with the alkaline agent; and wherein the quantity of the at least the portion of the alkaline effluent stream and the quantity of the at least the portion of the acidic effluent stream are selected such that a predetermined amount of phosphorus is re- moved by treating the at least the portion of the acidic effluent stream.

10. The process according to any one of claims 1 - 9, wherein the alkaline agent comprises or is an alkali metal hydroxide, such as NaOH; Ca(OH)2; CaO; NH4OH; lime; an alkaline effluent; or any combi- nation or mixture thereof.

11. The process according to any one of claims 1 10, wherein the process further comprises adjusting the pH of the combination (16) of the puri- fied effluent stream, the alkaline effluent stream and optionally the untreated portion of the acidic efflu- ent stream, optionally to a pH in the range of 5 to 8, or in the range of 5 to 6.

12. The process according to any one of claims 1 11, wherein the combination of the purified effluent stream, the alkaline effluent stream and op- tionally the untreated portion of the acidic effluent stream is treated further in a waste water treatment plant (17) and/or discharged (18).

13. An arrangement (19) for reducing phospho- rus in effluents from a pulping process, wherein the arrangement comprises a pulp mill system (1) for operating the pulping process; a first piping arrangement (20) for combining a plurality of acidic effluents (2, 2', 2'') obtaina- ble from the pulping process into an acidic effluent stream (3); a second piping arrangement (21) for combin- ing a plurality of alkaline effluents (4, 4', 4'') ob- tainable from the pulping process into an alkaline ef- fluent stream (5); a treatment apparatus (22) for treating at least a portion (6) of the acidic effluent stream by adjusting the pH thereof to alkaline with an alkaline agent (7), such that phosphorus in the at least the portion of the acidic effluent stream is precipitated in the form of a phosphate salt, and by removing the precipitate (8) at least partially to produce a puri- fied effluent stream (9); wherein the first piping arrangement is ar- ranged to convey the at least the portion of the acid- ic effluent stream to the treatment apparatus; and the arrangement further comprises a third piping arrangement (23) for combining the purified ef- fluent stream with the alkaline effluent stream and optionally with the untreated portion (10) of the acidic effluent stream.

14. The arrangement according to claim 13, wherein the arrangement further comprises an apparatus (24) for measuring the phosphorus content or concen- tration of the acidic effluent stream, and the first piping arrangement comprises an apparatus (25) for se- lecting the at least the portion of the acidic efflu- ent stream on the basis of the measured phosphorus content or concentration and directing it to the treatment apparatus, such that a predetermined amount of phosphorus is removed in the treatment apparatus by treating the at least the portion of the acidic efflu- ent stream.

15. The arrangement according to claim 13 or 14, wherein the treatment apparatus (22) comprises an apparatus (26) for mixing an alkaline agent with the at least the portion of the acidic effluent stream, such that phosphorus in the at least the portion of the acidic effluent stream is precipitated in the form of a phosphate salt, and an apparatus (27) for remov- ing the precipitate at least partially to produce the purified effluent stream.

Description:
A PROCESS FOR REDUCING PHOSPHORUS IN EFFLUENTS FROM A PULPING PROCESS

TECHNICAL FIELD

The present disclosure relates to a process and arrangement for reducing phosphorus in effluents from a pulping process.

BACKGROUND

Effluents produced in pulp or paper mills and other effluent-producing plants are typically treated in waste water treatment plants to reduce the amount of compounds which may be harmful to the environment, such as phosphorus. Wood material used in pulping pro- cesses contains phosphorous compounds, for example phosphates and organic phosphorous compounds. Some of the phosphorus may end up in effluents that are dis- charged from the pulping process.

Phosphorus should typically be removed from or reduced in effluents that are discharged from the mill to the environment, e.g. to a water system, to prevent pollution. As environmental regulations re- stricting phosphorus emissions tend to become strict- er, there is a need to reduce the amount of phosphorus in effluents that are conveyed from the mill to a waste water treatment plant and ultimately to the en- vironment. Furthermore, as a non-process element, phosphorus may interfere with subprocesses of a chemi- cal pulping process, in particular when present in high amounts. For example, it may disturb the func- tioning of lime kilns.

At the same time, the reduction of phosphorus may cause significant investment and management costs. SUMMARY

A process for reducing phosphorus in efflu- ents from a pulping process is disclosed. The process may comprise combining a plurality of acidic effluents obtainable from the pulping process into an acidic ef- fluent stream; combining a plurality of alkaline ef- fluents obtainable from the pulping process into an alkaline effluent stream; treating at least a portion of the acidic effluent stream by adjusting the pH thereof to alkaline with an alkaline agent, such that phosphorus in the at least the portion of the acidic effluent stream is precipitated in the form of a phos- phate salt, and by removing the precipitate at least partially to produce a purified effluent stream; and combining the purified effluent stream with the alka- line effluent stream and optionally with the untreated portion of the acidic effluent stream.

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, illus- trate various embodiments. In the drawings:

Fig. 1 is a flow chart illustration of an em- bodiment of the process;

Fig. 2 is a schematic illustration of an em- bodiment of the process and the arrangement; and

Fig. 3 shows schematically the experimental setup.

DETAILED DESCRIPTION

A process for reducing phosphorus in efflu- ents from a pulping process is disclosed.

The process may comprise combining a plurality of acidic effluents ob- tainable from the pulping process into an acidic ef- fluent stream; combining a plurality of alkaline effluents obtainable from the pulping process into an alkaline effluent stream; treating at least a portion of the acidic ef- fluent stream by adjusting the pH thereof to alkaline with an alkaline agent, such that phosphorus in the at least the portion of the acidic effluent stream is precipitated in the form of a phosphate salt, and by removing the precipitate at least partially to produce a purified effluent stream; and combining the purified effluent stream with the alkaline effluent stream and optionally with the untreated portion of the acidic effluent stream.

With the process, it may be possible to remove a desired amount of phosphorus with reduced costs, or increase phosphorus removal without significantly in- creasing costs involved. Reagents such as alkaline agents which would in any case be used to adjust the pH of the effluents may be utilized. The need to ad- just the pH of the effluents may also be reduced. Fur- thermore, the amount of the precipitate produced, for example in the form of a sludge, may be reduced, there- by reducing the management costs involved.

It may also be possible to operate the process as a real-time or online process, such that a desired or predetermined amount of phosphorus may be removed even if the amount of phosphorus in the effluents may vary in time or e.g. depending on the wood material or other lignocellulosic material used as a raw material in the pulping process.

Different acidic effluents from the chemical pulping process may have different pH and different phosphorus contents. Therefore it may be challenging to measure and control the amount of alkaline agent to be added in order to remove the phosphorus. When the plu- rality of acidic effluents obtainable from the pulping process is combined into the acidic effluent stream, a more uniform acidic effluent stream (with a more uni- form pH and phosphorus content) may be obtained. The relative uniformity of the acidic effluent stream may make it easier to operate and control the process in a more economic manner and/or such that a desired or predetermined amount of phosphorus may be removed. Therefore it may be possible to provide a more effi- cient way to control the amount of phosphorus to be precipitated and removed.

The effluents from the pulping process may in- clude the plurality of acidic effluents obtainable from the pulping process; the plurality of alkaline efflu- ents obtainable from the pulping process; and optional- ly also further acidic effluents and/or further alka- line effluents. The effluents from the pulping process may further comprise, at least in some embodiments, e.g. neutral effluents. For example, in embodiments in which the plurality of the acidic effluents and/or the plurality of the alkaline effluents comprise or are acidic and alkaline bleaching effluents, respectively, not all bleaching effluents (acidic or alkaline) are necessarily combined into the acidic and/or alkaline effluent streams. Instead, they may be e.g. treated further or discharged separately.

The "effluents from a pulping process" may thus be understood, at least in some embodiments, as referring to the total effluents obtainable from the pulping process. In an embodiment, they may comprise or be (total) bleaching effluents from the pulping process.

In the context of this specification, the term "effluent" may be understood as referring to an aqueous liquid that contains phosphorus, which is to be removed at least partially or completely. For exam- ple, the effluent may be a filtrate or other waste liquid, which is obtainable from the pulping process and which is to be discharged from the pulping pro- cess. The effluent is typically to be treated for (further) purifying purposes by waste water treatment, for example in a waste water treatment plant, before discharging. The waste water treatment may take place at a waste water treatment plant of a pulping mill or at an external waste water treatment plant.

The acidic and alkaline effluent streams may be conveyed separately e.g. to a waste water treatment plant, in which the treatment for removing phosphorus may be performed (i.e. treating the at least the por- tion of the acidic effluent stream by adjusting the pH thereof to alkaline with an alkaline agent, such that phosphorus in the at least the portion of the acidic effluent stream is precipitated in the form of a phos- phate salt, and by removing the precipitate at least partially to produce a purified effluent stream) as well as any parts or steps of the process that may take place subsequently and/or in parallel to the treatment for removing phosphorus. The waste water treatment plant may be a waste water treatment plant of a pulping mill or an external waste water treatment plant.

The term "effluent" may, at least in some em- bodiments, be understood as referring to an effluent flow or stream.

In the context of this specification, the term "plurality" may be understood as referring to two, three or more, i.e. at least two, or in some em- bodiments, at least three.

In the context of this specification, the term "pulp" may be understood as referring to a ligno- cellulosic fibrous material prepared by chemically and/or mechanically separating cellulose fibres from a natural fibre based material, for example from a lig- nocellulosic material. Pulping processes can be used for producing pulp.

The pulping process may be a chemical pulping process. In a chemical pulping process, chemical pulp is produced using the so-called sulphate or kraft pro- cess. Thus the pulping process may be a kraft pulping process. Other pulping processes may, however, be con- templated, for example a chemithermomechanical pulping process.

In a chemical pulping process, the plurality of the acidic effluents and/or the plurality of the al- kaline effluents may be obtained from fiber line opera- tions of the chemical pulping process. For example, they may be obtainable from bleaching processes, from a white liquor plant associated with the pulping process, and/or from a chemical plant associated with the pulp- ing process.

In the context of this specification, the term "associated with the pulping process" may be understood as referring to a plant or process connected to the pulping process, for example a plant or process for processing a stream or product produced in the pulping process or for producing a product, chemical or stream used or for use in the pulping process.

The term "pulping process" or, in some embodi- ments, "chemical pulping process", may thus be under- stood broadly in the context of this specification as also comprising or encompassing associated processes, for example one or more processes or plants for the circulation and/or production of chemicals used in the pulping process and/or one or more processes or plants for processing products or chemicals produced by or in the pulping process.

The operations of a kraft pulp mill are con- ventionally divided in a fiber line, a chemical recov- ery system and waste water treatment. The white liquor plant may form a part of the chemical recovery system of the pulp mill. The fiber line operations may typi- cally include cooking, brown stock washing (washing away spent cooking chemicals from the pulp), oxygen delignification, bleaching and pulp drying. In many conventional kraft pulp mills, the pH in the cooking, brown stock washing, oxygen delignification and knot- ting and screening stages is alkaline. Any effluents or filtrates obtained from such stages are typically alkaline. Often the pH in one or more bleaching stages is acidic. Thus effluents obtainable from fiberline operations and from bleaching stages may be acidic or alkaline, depending on the specific stage and the chemicals used therein.

At least a part of (i.e. one or more of) or all of the plurality of the acidic effluents and/or of the plurality of the alkaline effluents may obtained from bleaching stages of the fiber line operations of the pulping process. At least a part of (i.e. one or more of) or all of the plurality of the acidic efflu- ents and/or of the plurality of the alkaline effluents may thus comprise or be acidic bleaching effluents or alkaline bleaching effluents, respectively.

In the context of this specification, the term "bleaching stage" may refer to any stage follow- ing the brown stock stages. It may be understood as additionally including oxygen delignification stage (s), which selectively removes lignin, hexenuron- ic acid and/or metals. In the context of this specifi- cation, the term "bleaching stage" may, additionally or alternatively, refer to any stages immediately fol- lowing the oxygen stage. The first bleaching stage may be an alkaline or an acidic one. Commonly it is an acidic A stage followed by a D stage without a washing stage between them. This is the so-called A/Dl bleach- ing stage.

A bleaching stage may, in the context of this specification, be understood as referring to a bleach- ing stage and optionally a washing stage associated with or immediately following the bleaching stage.

For example, the acidic effluents may com- prise or be A, D, Dl, A/D or A/Dl bleaching effluents or any other acidic bleaching effluents. "A" should be understood as referring to an acidic bleaching stage. "D" should be understood as referring to a chlorine dioxide bleaching stage. A, D and Dl stages may be stages separate from each other; each of A, D and Dl stages should however in this context be understood as being bleaching stages.

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, CEDED, CEDEHD. (DC)(EPO)DED, D(EPO)DED, (D/Z)(EPO)DED,

OD (EPO)D, OD(EPO)DD, OD (EPO)DP, OD (EPO)DZ,

0 (Z/D)(EPO)DP, (C+D)BODED, DEODED, OZEOD, and YP. The symbols applied to the chemical treatments in these bleaching sequences and other commonly used chemical treatments are e.g. as follows:

Chlorination (C): Elemental chlorine, acidic; Alkaline extraction (E): Dissolution of reac- tion product with NaOH;

Chlorine Dioxide (D): Chlorine dioxide, acid- ic;

Oxygen (0): Molecular oxygen, alkaline, at high pressure;

Hypochlorite (H): Hypochlorite, alkaline; Peroxide (P): Peroxide, alkaline;

Ozone (Z): Ozone, acidic;

Chelats (Q): Chelating stage;

Eo - extraction stage reinforced with oxygen; Eop - extraction stage reinforced with oxygen and hydrogen peroxide;

X = enzymes stage;

Y = hydrosulfite stage; Paa = peracetic acid stage;

Pp - pressurized hydrogen peroxide stage; q - Semi-chelating stage.

One or more of the plurality of the acidic effluents and/or one or more of the plurality of the alkaline effluents may be obtainable from the acidic or alkaline stages of these exemplary sequences, re- spectively.

One or more of the plurality of the acidic effluents may also be obtainable from an acidification treatment of unbleached pulp, which is performed be- fore the bleaching, after brown stock washing and oxy- gen treatment of the oxygen delignification unit. This acidic effluent also typically contains dissolved phosphorus compounds.

One or more of the plurality of the acidic effluents may also comprise or be an acidic effluent obtainable from an acidification treatment of the un- bleached pulp after brown stock washing, which treat- ment is performed to dissolve acid-dissolving phospho- rus compounds contained in the pulp before bleaching.

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

Acidic or neutral-acid effluents may be ob- tained from a white liquor plant as a part of the plu- rality of the acidic effluents.

The chemical plant may be e.g. a plant for producing reagents for use in the pulping process. For example, the chemical plant may be for producing chlo- rate (for example, sodium chlorate), chlorine dioxide (CIO2), oxygen and/or hydrogen peroxide (H2O2) for use in the pulping process. The plurality of the acidic or alkaline efflu- ents, or any other acidic or alkaline effluent de- scribed in this specification, may be obtainable from the pulping process using various suitable methods and apparatuses. For example, from a bleaching stage, an acidic or alkaline 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.

Various suitable methods and apparatuses may be used to combine the plurality of the acidic efflu- ents into the acidic effluent stream, or to combine any stream or flow described in this specification to an- other stream or flow. Likewise, various suitable meth- ods and apparatuses may be used to combine the plurali- ty of the alkaline effluents into the alkaline effluent stream. For example, the plurality of such effluents may be mixed or otherwise combined in a mixer, tank or basin; or an acidic or alkaline effluent may be inject- ed to a flow of another acidic or alkaline effluent or to a piping (pipe) or sewer conveying another acidic or alkaline effluent.

It is not always necessary to combine all acidic effluents from the pulping process into the acidic effluent stream. Likewise, it is not always nec- essary to combine all alkaline effluents from the pulp- ing process into the alkaline effluent stream. The pro- portion or quantity of the plurality of the acidic ef- fluents and the proportion or quantity of the plurality of the alkaline effluents (relative to all acidic or alkaline effluents from the pulping process) may be se- lected based on various factors, including e.g. produc- tion and input of phosphorus to the effluents from the pulping process.

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, for ex- ample all acidic bleaching effluents, from the pulping process and/or from the fiber line operations thereof may be combined into the acidic effluent stream. 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 alkaline effluents, for example all alkaline effluents, obtained from the pulping process and/or from the fiber line operations thereof may be combined into the alkaline effluent stream.

At least one of the plurality of the acidic effluents may be an acidic bleaching effluent obtaina- ble from the first acidic bleaching stage (i.e. the first acidic bleaching stage in the bleaching se- quence). Often a high phosphorus content is found in the acidic bleaching effluent from the first acidic bleaching stage. Thus it may be useful to obtain the acidic bleaching filtrate from the first acidic bleaching stage. In this embodiment, phosphorus con- tent in the (total) effluents from the pulping process may significantly be reduced, if the acidic bleaching effluent from the first acidic bleaching stage, or a portion thereof, is treated as described in this spec- ification.

In an embodiment, only the at least the por- tion of the acidic effluent stream is treated by ad- justing the pH thereof to alkaline with the alkaline agent. In other words, in this embodiment, no portion of the alkaline effluent stream is treated by adjust- ing the pH thereof to alkaline with an alkaline agent. Treating only the acidic effluent stream or a portion thereof may be sufficient to achieve a desired phos- phorus (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 the pH thereof to alkaline with the alkaline agent.

The quantity of the at least the portion of the acidic effluent stream and/or the proportion (quan- tity) of all the alkaline effluents that are combined into the alkaline effluent stream may, however, be se- lected depending on various factors and may vary at different time points. They may, for example, be se- lected or adjusted online or at individual time points. These factors may include e.g. the lignocellulosic ma- terial used as a raw material in the pulping process. For example, different wood materials may contain dif- ferent amounts of phosphorus. These factors may in- clude, additionally or alternatively, 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 in the (total) effluents from the pulping process.

The at least the portion of the acidic efflu- ent stream may be obtainable by dividing the acidic ef- fluent stream into the at least the portion of the acidic effluent stream and to an untreated portion of the acidic effluent stream. This may be done e.g. using a valve, a suitable piping or sewer or other suitable method or apparatus for dividing a stream or flow.

In an embodiment, the quantity of the at least the portion of the acidic effluent stream is selected such that a predetermined amount of phosphorus is re- moved by treating the at least the portion of the acid- ic effluent stream. The predetermined amount of phos- phorus is removed from the at least the portion of the acidic effluent stream by the at least partial removal of the precipitate (in the form of a phosphate salt).

For example, a predetermined amount of phos- phorus (target amount of phosphorus) may be set, e.g. as a particular or threshold value of KgP/day, and the quantity of the at least the portion of the acidic ef- fluent stream may then be selected such that this pre- determined amount of phosphorus is removed from the (total) effluents of the pulping process. The quantity may be calculated and selected based on e.g. the phos- phorus input to the pulping process. This may depend e.g. the amount of phosphorus contained by a lignocel- lulosic material used as a starting material of the pulping process. Such target amounts may be imposed e.g. by environmental regulations, for example those regarding the phosphorus load to be eventually dis- charged from the pulping process and/or from any subse- quent waste water treatment stages. Thus different pulping processes may have different targets and may select different quantities accordingly.

The process may be an online or continuous process. It may be an online or continuous process e.g. in the sense that the quantity of the at least the por- tion of the acidic effluent stream may be selected at individual time points such that a predetermined amount of phosphorus is removed by treating the at least the portion of the acidic effluent stream. The quantity of the at least the portion of the acidic effluent stream may thus be adjusted at the individual time points such that a predetermined amount of phosphorus is removed by treating the at least the portion of the acidic efflu- ent stream.

In the context of this specification, the term "quantity" may refer to volume and/or flow, for example the volume and/or flow of a stream. For example, the quantity may be indicated as volume per air dry tonne of pulp or as cubic metres. It may, additionally or al- ternatively, refer to relative volume and/or relative flow.

The phosphorus content or concentration of the acidic effluent stream may be measured, and the quanti- ty of the at least the portion of the acidic effluent stream may be selected on the basis of the measured phosphorus content or concentration such that a prede- termined amount of phosphorus is removed by treating the at least the portion of the acidic effluent stream. The predetermined amount of phosphorus may be deter- mined e.g. as KgP/day.

The at least the portion of the acidic efflu- ent stream may have a phosphorus concentration of e.g. at least 5 mg/1; or at least 10 mg/1. However, the phosphorus concentration may vary e.g. in different bleaching configurations, in different pulping pro- cesses, and/or depending on the wood material or other lignocellulosic material used as a raw material in the pulping process.

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

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

The pH of the at least the portion of the acidic effluent stream may be adjusted e.g. to a pH value of at least 8; or at least 9; or of 8 - 14; or of

8 11; or of 9 - 10.

In an embodiment, at least a portion of the alkaline effluent stream is combined to the acidic ef- fluent stream prior to treating the at least the por- tion of the acidic effluent stream by adjusting the pH thereof to alkaline with the alkaline agent. The quan- tity of the at least the portion of the alkaline efflu- ent stream and the quantity of the at least the portion of the acidic effluent stream may be selected, for ex- ample, such that a predetermined amount of phosphorus is removed by treating the at least the portion of the acidic effluent stream and the at least the portion of the alkaline effluent stream. In this embodiment, any references in this specification to "treating or ad- justing the pH of the at least the portion of the acid- ic effluent stream" may be understood as referring to treating or adjusting the pH of the at least the por- tion of the alkaline effluent stream and the at least the portion of the acidic effluent stream (i.e. their combination or mixture). In other words, in this embod- iment, the at least the portion of the acidic effluent stream may then further comprise e.g. at least a por- tion of the alkaline effluent e.g. as a mixture, in which case the pH of the combination or mixture is ad- justed to alkaline with the alkaline agent. Further, in this embodiment, any references in this specification to "the alkaline effluent stream" being combined to the purified effluent stream may then be understood as re- ferring to an untreated portion of the alkaline efflu- ent stream.

When the pH of the at least the portion of the acidic effluent stream is adjusted to alkaline (or the combination or mixture containing the at least the por- tion of the acidic effluent stream), at least a part of the phosphorus contained therein may precipitate, typ- ically as calcium phosphate or other phosphate salts. The source of calcium in the calcium phosphate precip- itated may be calcium originally contained in the ef- fluent and/or calcium that is introduced to the at least the portion of the acidic effluent stream as a component of the alkaline agent. Thus the precipitate typically contains calcium phosphate. The precipitate may further comprise other phosphate salts, such as calcium hydrogen phosphate; calcium carbonate; calcium sulphate; organic phosphorus compounds; and/or other precipitated/coprecipitated components.

The alkaline agent may 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 e.g. in the form of a powder. The alkaline agent may comprise or be, for ex- ample, an alkali metal hydroxide, such as NaOH; Ca(OH)2; CaO; NH4OH; lime; an alkaline effluent; or any combina- tion or mixture thereof. However, other alkaline agents may also be contemplated.

In an embodiment, the alkaline agent is CaO; an alkali metal hydroxide, such as NaOH;

Ca (OH) 2 ;

NH 4 OH; lime; an alkaline effluent or filtrate; an alkaline bleaching effluent or filtrate; or any mixture or combination thereof.

In an embodiment, the alkaline agent is solid

CaO.

In an embodiment, the alkaline agent compris- es or is an aqueous slurry (i.e. suspension) of CaO, an aqueous slurry of an alkali metal hydroxide such as NaOH, an aqueous slurry of Ca(OH)2, an aqueous slurry or solution of NH4OH, or any mixture or combination thereof.

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

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

The alkaline agent may be an alkaline agent that is used for adjusting pH of waste water, such as an aqueous slurry of Ca(OH)2.

The concentration of the alkaline agent may vary or be selected e.g. based on various factors. In cases in which a relatively strong alkaline agent, or an aqueous slurry containing a relatively high concen- tration or amount of an alkaline agent, is used, a relatively small volume of the alkaline agent may be needed to adjust the pH to alkaline. Thus an increase in volume and dissolution of the precipitate may be avoided. For instance, if an aqueous slurry of Ca(OH)2 is used as the alkaline agent, the concentration of the aqueous slurry of Ca(OH)2 may be e.g. at least 2 g/1, or at least 20 g/1. Another well-suited alkaline agent is solid CaO or lime.

The alkaline agent may be mixed with the at least the portion of the acidic effluent stream. The mixing typically accelerates the precipitation of phos- phorus. The mixing may be effected e.g. by agitation. The mixing may also be effected using an apparatus suitable for mixing the alkaline agent with the at least the portion of the acidic effluent stream. Such apparatus may be e.g. a tank or a piping comprising means for mixing. The means for mixing may be e.g. a mixer, a mixing arm or a stirrer. Such apparatus may also be e.g. a piping fitted with one or more nozzles for injecting the alkaline agent into the piping or sewer in which the at least the portion of the acidic effluent stream may flow.

The temperature in which the pH of the at least the portion of the acidic effluent stream is ad- justed to alkaline with an alkaline agent may be set e.g. between 25 90 e C. Increasing the temperature may enhance precipitation.

The at least partial removal of the precipi- tate may be effected e.g. by filtration, centrifuga- tion, decantation, clarification, flotation or sedimen- tation.

After the precipitate is removed at least par- tially, or in some embodiments completely, from the at least the portion of the acidic effluent stream, the purified effluent stream thus obtained has a substan- tially reduced phosphorus content or concentration. The at least partial removal of the precipitate thus re- moves phosphorus from the at least the portion of the acidic effluent stream, and subsequently from the (to- tal) effluents from the pulping process.

It may not always be desirable to completely eliminate phosphorus from the effluents from the pulp- ing process, in particular in cases when they are con- veyed to a further waste water treatment stage. Many waste water treatment plants and stages utilize bacte- rial processes for waste water treatment, for which a low amount of phosphorus in the effluents may be de- sired or even necessary.

Further, it may be sufficient to treat only the at least the portion of the acidic effluent stream in order to significantly reduce phosphorus content in the effluents. For instance, it may be possible to re- move a predetermined amount of phosphorus from the ef- fluents while minimizing the amount of reagents (such as the amount of the alkaline agent), associated costs and/or the amount of the precipitate sludge. At the same time, a (small) amount of phosphorus may be re- tained in the effluents conveyed to a waste water treatment stage; this may be beneficial e.g. when the subsequent waste water treatment stage utilizes bacte- rial processes, for which an amount of phosphorus in the effluent may be desirable.

The purified effluent stream, the alkaline ef- fluent stream and optionally the untreated portion of the acidic effluent stream may be conveyed to a further waste water treatment stage. This further waste water treatment stage may take place in the same waste water treatment plant in which the treatment for removing phosphorus may be performed, or it may take place in another (separate) waste water treatment plant after the treatment for removing phosphorus. The further waste water treatment stage may comprise or be e.g. a biological waste water treatment stage. The purified effluent stream, the alkaline ef- fluent stream and optionally the untreated portion of the acidic effluent stream may be combined e.g. prior to conveying them into an equalization basin (i.e. their combination may be conveyed into an equalization basin), or they may be combined in an equalization ba- sin. Alternatively or additionally, they may be com- bined at any stage prior to post-neutralization in a waste water treatment plant, wherein the post- neutralization is performed for adjusting the pH suit- able for aerobic treatment. The post-neutralization is typically performed before the aeration basin. It may, for instance, be performed at the fiberline, before a preliminary clarifier treatment or at any other stage before aeration.

The quantity of the at least the portion of the acidic effluent stream and/or the proportion (quan- tity) of all the alkaline effluents that are combined into the alkaline effluent stream, and which may then subsequently be treated, may be selected e.g. based on the amount of phosphorus to be conveyed to the further waste water treatment stage, for example to a biologi- cal waste water treatment stage (which may utilize bacterial processes), e.g. to an aeration basin of the further waste water treatment stage. Alternatively or additionally, they may be selected based on the amount of phosphorus to be conveyed to a further waste water treatment stage (in general), for example to a waste water treatment plant or an effluent plant.

The process may further comprise adjusting the pH of the combination of the purified effluent stream, the alkaline effluent stream and optionally the un- treated portion of the acidic effluent stream. For ex- ample, the pH of the combination may 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. Alter- natively, 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. For example, H 2 SO 4 or other suitable acid may be used as the acidic agent. For ex- ample, any alkaline agent described in this specifica- tion may also be used in this context.

This adjustment of the pH of the combination may be made e.g. before the combination reaches an equalization basin, in the equalization basin, or at any stage prior to post-neutralization in a waste water treatment plant. However, adjusting the pH of the com- bination may not always be necessary.

The combination of the purified effluent stream, the alkaline effluent stream and optionally the untreated portion of the acidic effluent stream may be treated further in a (further) waste water treatment stage and/or discharged. For example, it may be con- veyed from the equalization basin to a (further) waste water treatment stage and/or discharged.

The waste water treatment or the waste water treatment stage may, in some embodiments, utilize bac- terial processes.

An arrangement for reducing phosphorus in ef- fluents from a pulping process is also disclosed. The arrangement may comprise a pulp mill system for operating (or config- ured to operate) the pulping process; a first piping arrangement for combining (or configured to combine) a plurality of acidic effluents obtainable from the pulping process into an acidic ef- fluent stream; a second piping arrangement for combining (or configured to combine) a plurality of alkaline efflu- ents obtainable from the pulping process into an alka- line effluent stream; a treatment apparatus for treating (or config- ured to treat) at least a portion of the acidic efflu- ent stream by adjusting the pH thereof to alkaline with an alkaline agent, such that phosphorus in the at least the portion of the acidic effluent stream is precipi- tated in the form of a phosphate salt, and by removing the precipitate at least partially to produce a puri- fied effluent stream; wherein the first piping arrangement is ar- ranged to convey the at least the portion of the acidic effluent stream to the treatment apparatus; and the arrangement further comprises a third pip- ing arrangement for combining (or configured to com- bine) the purified effluent stream with the alkaline effluent stream and optionally with the untreated por- tion of the acidic effluent stream.

The pulp mill system may comprise a digestion apparatus for cooking (or configured to cook) pulp; and a washing apparatus for washing (or configured to wash) pulp. The pulp mill system may further comprise a bleaching system comprising one or more acidic bleach- ing stages and one or more alkaline bleaching stages. The acidic and alkaline bleaching stages, or individu- al acidic or alkaline bleaching stages, may comprise an apparatus for separating acidic or alkaline efflu- ent, respectively, from pulp, for obtaining an acidic or alkaline effluent, respectively. The bleaching sys- tem may further comprise further acidic, alkaline or neutral bleaching stages.

The arrangement may also contain pumps for various fluids, appropriate piping, tanks, and/or mix- ers. It may further contain e.g. drying apparatus for drying (or configured to dry) bleached pulp, a chemical recovery system and/or other components of a pulp mill system.

The arrangement and/or the pulp mill system may further comprise a white liquor plant or chemical plant. The white liquor plant may be a part of a chemi- cal recovery system of the pulp mill system. Alterna- tively or additionally, the white liquor plant and/or the chemical plant may be a plant separate from the pulp mill system.

The first piping arrangement is arranged to convey the at least the portion of the acidic effluent stream to the treatment apparatus. The first piping arrangement may be directly or indirectly connected to the treatment apparatus, such that there is a flow connection to the treatment apparatus.

The second piping arrangement may be arranged to convey the alkaline effluent stream to the third piping arrangement, such that the purified effluent stream may be combined with the alkaline effluent stream and optionally with the untreated portion of the acidic effluent stream. The second piping arrange- ment may be directly or indirectly connected to the third piping arrangement, such that there is a flow connection between them.

The first, second and/or third piping arrange- ment may comprise conduits, for example piping and/or sewers, or any other means for establishing a flow connection. The term "piping arrangement" may thus be understood as referring, alternatively or additionally, to a conduit arrangement and/or sewer arrangement. Such a piping arrangement may further comprise e.g. one or more piping connections, valves, tanks or other con- tainers, pumps etc.

The arrangement may further comprise an appa- ratus for measuring (or configured to measure) the phosphorus content or concentration of the acidic ef- fluent stream. The apparatus for measuring the phospho- rus content or concentration of the acidic effluent stream may comprise or be e.g. an on-line phosphorus measuring device. Such devices are commercially availa- ble. Alternatively or additionally, it may comprise or be a laboratory measurement system or apparatus for measuring the phosphorus content or concentration. Var- ious different apparatuses, devices and systems for measuring the content or concentration of phosphorus in the acidic effluent stream may be contemplated and available to a skilled person.

The first piping arrangement may further com- prise an apparatus for selecting (or configured to se- lect) the at least the portion of the acidic effluent stream, or its quantity, on the basis of the measured phosphorus content or concentration and directing it to the treatment apparatus, such that a predetermined amount of phosphorus is removed in the treatment appa- ratus by treating the at least the portion of the acid- ic effluent stream.

The treatment apparatus may comprise an appa- ratus for mixing (or configured to mix) an alkaline agent with the at least the portion of the acidic ef- fluent stream, such that phosphorus in the at least the portion of the acidic effluent stream is precipitated in the form of a phosphate salt, and an apparatus for removing (or configured to remove) the precipitate at least partially to produce the purified effluent stream.

The components of the arrangement may be con- nected either directly or indirectly. There may thus be a flow connection between connected components.

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 process for reducing phosphorus in effluents from a pulping process 1.

The pulping process 1 may be a chemical pulp- ing process, such as a kraft pulping process. The pulping process 1 may be operated e.g. in a pulp mill system as described in connection with Fig. 2. Various components that may be a part of the pulping process have not been included in this schematic illustration for simplicity. Further, as a skilled person will un- derstand, the pulping process 1 is merely an exempla- ry, schematic illustration of an embodiment of a pulp- ing process. Lignocellulosic material 28, for example wood chips and/or various other types of lignocellulo- sic materials, may be fed into the pulping process 1 and digested at a digestion stage 29, e.g. in a diges- tion apparatus for cooking pulp from the lignocellulo- sic material 28. From the digestion stage 29, the pulp (often called "brown stock") is conveyed to a washing stage 30 for washing the pulp. The washing may be per- formed e.g. in a washing apparatus described in con- nection with Fig. 2. Fiberline operations 11 of the pulping process 1 may comprise, in addition to the di- gestion stage 20 and the washing stage 30, a number of bleaching stages 14.

The washed pulp is conveyed from the washing stage 30 to the bleaching stages 14. The bleaching stages 14 may comprise e.g. one or more bleaching ap- paratuses and washing apparatuses for washing the pulp between bleaching stages (not shown in detail in this schematic). A bleaching stage 14 may therefore be un- derstood as referring to a bleaching stage and option- ally a washing stage associated with or immediately following the bleaching stage. The term "bleaching ef- fluent" may thus be understood, at least in some em- bodiments, as encompassing also washing effluents from washing stages associated with or immediately follow- ing the (actual) bleaching stage. The pulp may then proceed from the bleaching stages 14 to a drying stage 31 for drying the pulp 32 produced by the pulping pro- cess.

Individual bleaching stages 14 may be acidic or alkaline, depending on the stage(s) and their order (bleaching sequence). Various bleaching sequences may be contemplated depending e.g. the category and final quality of the pulp 32 to be produced, environmental concerns, operating costs, the original lignocellulo- sic material 28, etc. as described elsewhere in this specification; the process described in this specifi- cation is not limited to any specific bleaching se- quence. Different bleaching chemicals may be used in each bleaching stage 14, optionally with a washing be- tween individual stages. The pH in individual bleach- ing stages 14 may be alkaline, acidic or neutral.

In this Fig •/ a plurality of acidic effluents

2, 2', and 2tt are issued from the bleaching stages 14 of the fiberline 11. A plurality of alkaline effluents 4, 4', and 4tt are also issued from the bleaching stages 14 of the fiberline 11. Although the acidic ef- fluents are depicted as being issued from earlier bleaching stages and alkaline effluents from later bleaching stages, this is merely to simplify the fig- ure and should not be understood that the acidic and alkaline effluents would be obtainable in a particular order and/or from bleaching stages in a particular or- der.

The plurality of the acidic effluents 2, 2’,

2·' obtainable from the pulping process may be com- bined into an acidic effluent stream 3. A plurality of alkaline effluents 4, 4', 499 obtainable from the pulping process may be combined into an alkaline effluent stream 5.

One or more acidic effluents of the plurality of the acidic effluents may, alternatively or addi- tionally, be obtainable from elsewhere. In this exam- ple, a further acidic effluent 2 999 is obtainable from a chemical plant 13 and combined into the acidic ef- fluent stream 3. An acidic or neutral-acid effluent could, in an additional or alternative embodiment, be obtainable from a white liquor plant 12, and combined into the acidic effluent stream 3.

One or more acidic effluents of the plurality of the alkaline effluents may, alternatively or addi- tionally, be obtainable from elsewhere. In this exam- ple, a further alkaline effluent 4 999 is obtainable from the white liquor plant 12 and combined into the acidic effluent stream 5. An alkaline effluent could, in an additional or alternative embodiment, be obtain- able from the chemical plant 13.

The acidic effluent stream 3 may be treated at least partially or completely, i.e. a portion (part) 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, the portion 6 of the acidic effluent stream may be conveyed to treatment e.g. in a treatment apparatus 22. The acidic effluent stream 3 may thus be divided into the portion 6 to be treated and to an untreated portion 10 of the acidic effluent stream 3. The quantities of the por- tion 6 to be treated and of the untreated portion 10 may be selected according to various factors, e.g. those described elsewhere in this specification.

The phosphorus content or concentration of the acidic effluent stream 3 may be measured using a suitable apparatus 24. The quantity of the at least the portion 6 of the acidic effluent stream may then, in some embodiments, be selected, e.g. divided or di- verted from the acidic effluent stream 3, on the basis of the measured phosphorus content or concentration such that a predetermined amount of phosphorus is re- moved by treating the at least the portion 6 of the acidic effluent stream 3.

The alkaline effluent stream 5 may also be treated at least partially or completely, i.e. a por- tion 15 (part) of the alkaline effluent stream 5 or the entire volume thereof may be treated. When only a portion of the alkaline effluent stream 5 is treated, the portion 15 of the alkaline effluent stream 5 may be conveyed to the treatment in the treatment appa- ratus 22. The alkaline effluent stream 5 may thus be divided into the portion 15 to be treated and to an untreated portion 33 of the alkaline effluent stream 5. The quantities of the portion 15 to be treated and of the untreated portion 33 may be selected according to various factors, e.g. those described elsewhere in this specification. In some embodiments, the entire alkaline effluent stream 5 is untreated, i.e. none (no portion) of the alkaline effluent stream 5 is conveyed to the treatment apparatus 22.

The portions 6 and 15 may be combined prior to the treatment, or they may e.g. be conveyed to the treatment apparatus 22, in which they may be combined e.g. by mixing.

During the treatment in the treatment appa- ratus 22, the pH of the at least the portion 6 of the acidic effluent stream and optionally of the at least the portion 15 of the alkaline effluent stream is ad- justed to alkaline with an alkaline agent 7. For exam- ple, the portion 6 and optionally the portion 15 may be conveyed to a reactor, tank or mixer for bringing the alkaline agent 7 in contact with them, or other suitable apparatus 26 for mixing an alkaline agent with the at least the portion of the acidic effluent stream. The alkaline agent 7 may be e.g. any alkaline agent described in this specification. When the pH of the at least the portion 6 and optionally of the at least the portion 15 is adjusted to alkaline with the alkaline agent 7, phosphorus, or at least a part of the phosphorus, in the at least the portions 6 and 15 may be precipitated in the form of a phosphate salt.

The precipitate 8 thereby produced may then be at least partially removed to produce a purified effluent stream 9. The precipitate 8 may be removed e.g. in the form of a sludge. The treatment apparatus 22 may thus comprise an apparatus 27 for removing the precipitate 8 at least partially to produce the puri- fied effluent stream 9.

The precipitate 8, e.g. in the form of a sludge, may be, for example, discharged (e.g. land- filled) or treated further. As it may contain signifi- cant amounts of phosphorus and e.g. calcium, it may be used for various purposes, for example as a fertilizer product or a component thereof.

The purified effluent stream 9 may then be combined with the alkaline effluent stream 5, or op- tionally with the untreated portion 33 of the alkaline effluent stream, and optionally with the untreated portion 10 of the acidic effluent stream. This may be done e.g. in a suitable piping arrangement 23. The combination 16 thus obtainable may be treated further in a waste water treatment 17 and/or discharged 18.

The acidic and alkaline effluent streams 3, 5 may be conveyed separately e.g. to a waste water treatment plant 35, in which the treatment for remov- ing phosphorus may be performed (i.e. treating the at least the portion 6 of the acidic effluent stream by adjusting the pH thereof to alkaline with an alkaline agent, such that phosphorus in the at least the por- tion of the acidic effluent stream is precipitated in the form of a phosphate salt, and by removing the precipitate at least partially to produce a purified effluent stream) as well as any parts or stages of the process that may take place subsequently and/or in parallel. However, in other embodiments, the treatment may be performed e.g. at the fiberline.

In this exemplary embodiment, the combination 16 may be eventually combined with a further acidic effluent 2 9999 and/or a further alkaline effluent

' ' ' , which have not been combined into the acidic effluent stream 3 or the alkaline effluent stream 5. These further effluents may be obtainable e.g. from the bleaching stages 14, but they may, additionally or alternatively, be obtainable from other stages of the pulping process 1 or from other sources, e.g. the white liquor plant 12 and/or the chemical plant 13.

Fig. 2 illustrates an embodiment of a process and an arrangement 19 for reducing phosphorus in ef- fluents from a pulping process. The arrangement 19 de- scribed herein may be suitable e.g. for performing one or more embodiments of the process described in the context of Fig. 1 and elsewhere in this specification.

The arrangement 19 may comprise a pulp mill system 1 for operating the pulping process. For exam- ple, the pulp mill system 1 may be a chemical pulp mill system, such as a kraft pulp mill system. Various components that may be a part of the pulp mill system 1 have not been included in this schematic illustra- tion for simplicity. Further, as a skilled person will understand, the pulp mill system 1 is merely an exem- plary, schematic illustration or embodiment of a pulp mill system. Lignocellulosic material 28, for example wood chips and/or various other types of lignocellulo- sic materials, may be input to the pulp mill system 1. The pulp mill system 1 may comprise a digestion appa- ratus 29 for cooking pulp from the lignocellulosic ma- terial 28. From the digestion apparatus 29, the pulp (often called "brownstock") is conveyed to a washing apparatus 30 for washing the pulp. The washing appa- ratus 30 may contain e.g. one or more brownstock wash- ers, for example using countercurrent flow.

The fiberline or fiberline operations 11 of the pulp mill system 1 may comprise, in addition to the digestion apparatus 29 and the washing apparatus 30, a number of bleaching stages 14. The washed pulp is conveyed from the washing apparatus 30 to the bleaching stages 14. The bleaching stages 14 may com- prise e.g. one or more bleaching apparatuses and wash- ing apparatuses for washing the pulp between bleaching stages (not shown in detail in this schematic). A bleaching stage 14 may therefore be understood as re- ferring to a bleaching stage and optionally a washing stage associated with or immediately following the bleaching stage. The pulp mill system 1 may further comprise an apparatus 31 for drying the pulp 32 to be produced.

Individual bleaching stages 14 may be acidic or alkaline, depending on the stages and their order (bleaching sequence), as described elsewhere in this specification . Different bleaching chemicals may be used in each bleaching stage 14, optionally with a washing between individual stages. The pH in individu- al bleaching stages 14 may be alkaline or acidic. Each bleaching stage 14 may further comprise an apparatus

34 for separating the acidic or alkaline effluent 2,

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

In this Fig •, a plurality of acidic effluents 2, 2', and 2 '' are issued from the bleaching stages 14 of the fiberline 11. A plurality of alkaline effluents 4, 4', and 4 '' are also issued from the bleaching stages 14 of the fiberline 11. Although the acidic ef- fluents are depicted as being issued from earlier bleaching stages and alkaline effluents from later bleaching stages, this is merely to simplify the Fig. and should not be understood that the acidic and alka- line effluents should be obtainable in a particular order.

A plurality of the acidic effluents 2, 2',

2'' obtainable from the pulping process may be com- bined into an acidic effluent stream 3. The arrange- ment thus further comprises a first piping arrangement

20 for combining a plurality of acidic effluents 2, 2 ' , 2 '' obtainable from the pulping process into an acidic effluent stream 3. The piping arrangement may comprise e.g. conduits, for example piping (pipes) and/or sewers for conveying the plurality of the acidic effluents from the bleaching stages 14; these conduits may also be understood as being denoted with the refer- ence numbers 2, 2', and 2''. The first piping arrange- ment 20 may further comprise e.g. piping connections, valves, tanks or other containers, etc. for conveying and combining the plurality of the acidic effluents 2, 2 ' , 2 '' into the acidic effluent stream 3.

The first piping arrangement 20 may be ar- ranged to convey the at least the portion 6 of the acidic effluent stream to the treatment apparatus 22.

A plurality of alkaline effluents 4, 4', 4 '' obtainable from the pulping process may be combined into an alkaline effluent stream 5. The arrangement 19 may thus further comprise a second piping arrangement 21 for combining a plurality of alkaline effluents 4,

4', 4 '' obtainable from the pulping process (and from the pulp mill system 1) into an alkaline effluent stream 5. The second piping arrangement may comprise e.g. conduits, for example piping (pipes) and/or sewers for conveying the plurality of the alkaline effluents from the bleaching stages 14; these conduits may also be understood as being denoted with the reference num- bers 4, 4', and 4 ''

The acidic effluent stream 3 may be treated at least partially or completely, i.e. a portion 6 (part) of the acidic effluent stream 3 or the entire volume thereof may be treated. The first piping ar- rangement 20 may thus comprise a conduit for conveying the at least the portion 6 of the acidic effluent stream into a treatment apparatus 22; this conduit may also be understood as being denoted with the reference number 6. The conduit 6 may be connected to the treat- ment apparatus 22 directly or indirectly. The arrange- ment 19 may further comprise an apparatus 25 for di- viding the acidic effluent stream into the at least the portion 6 to be treated and into an untreated por- tion 10. This apparatus 25 may be e.g. any apparatus suitable for dividing or diverting a flow or stream from another and may comprise e.g. a valve, a flow splitter, and/or a control device for controlling the quantities of the portion 6 to be treated and of the untreated portion 10.

The arrangement may further comprise an appa- ratus 24 for measuring the phosphorus content or con- centration of the acidic effluent stream. In this em- bodiment, the first piping arrangement 20 may comprise the apparatus 25 for selecting the at least the por- tion 6 of the acidic effluent stream on the basis of the measured phosphorus content or concentration and directing it to the treatment apparatus 22, such that a predetermined amount of phosphorus is removed in the treatment apparatus by treating the at least the por- tion of the acidic effluent stream.

The apparatus 25 for dividing the acidic ef- fluent stream into the at least the portion 6 to be treated and into an untreated portion 10 may be ar- ranged to select the quantity of the at least the por- tion 6 of the acidic effluent stream, e.g. to divide or divert it from the acidic effluent stream 3, on the basis of the measured phosphorus content or concentra- tion such that a predetermined amount of phosphorus is removed by treating the at least the portion 6 of the acidic effluent stream 3. The arrangement 19 may further comprise e.g. a chemical plant 13 and an additional piping arrange- ment denoted herein as 2 999 for obtaining a further acidic (or alkaline) effluent from the chemical plant 13 and conveying it into the first piping arrangement 20 (or second piping arrangement 21) or to the treat- ment apparatus 22.

The arrangement 19 may further comprise e.g. a white liquor plant 12 and an additional piping ar- rangement denoted herein as 4 999 for obtaining a fur- ther alkaline (or acidic) effluent from the white liq- uor plant and conveying it into the second piping ar- rangement 21 (or into the first piping arrangement 20) or to the treatment apparatus 22.

The treatment apparatus 22 may comprise an apparatus 26 for mixing an alkaline agent 7 with the at least the portion of the acidic effluent stream 6, such that phosphorus in the at least the portion of the acidic effluent stream is precipitated in the form of a phosphate salt.

The apparatus 26 for mixing the alkaline agent 7 with the at least the portion of the acidic effluent stream 6 may comprise e.g. a tank for allow- ing phosphorus to be precipitated. Such a tank may be e.g. a cone bottom tank. Alternatively or additional- ly, it may comprise e.g. a reactor, tank, basin, or mixer for bringing the alkaline agent 7 in contact with the at least the portion of the acidic effluent stream 6. The tank, reactor or mixer may be fitted with one or more nozzles for injecting the alkaline agent into it. The alkaline agent may thus be introduced con- tinuously by injecting it in a flow of the at least the portion 6 of the acidic effluent stream. In an embodi- ment, 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 apparatus 22 may further com- prise an apparatus 27 for removing the precipitate 8 at least partially to produce the purified effluent stream 9. The apparatus 27 for removing the precipi- tate 8 at least partially to produce the purified ef- fluent 9 may be e.g. filter, a centrifuge, a decanter centrifuge, a clarifier, a solids separator or a floc- culation apparatus. However, other suitable apparatuses may also be contemplated.

The purified effluent stream 9 may then be combined with the alkaline effluent stream 5, or the untreated portion 33 of the alkaline effluent stream, and optionally with the untreated portion 10 of the acidic effluent stream. The arrangement may thus fur- ther comprise a third piping arrangement 23 for com- bining the purified effluent stream with the alkaline effluent stream, or the untreated portion 33 of the alkaline effluent stream, and optionally with the un- treated portion (10) of the acidic effluent stream. The third piping arrangement 23 may comprise e.g. con- duits, for example piping (pipes) and/or sewers. It may further comprise, for instance, an equalization basin. The equalization basin may be suitable for temporary storage of the combination 16 prior to its discharge or further treatment.

The arrangement 19 may further comprise a waste water treatment stage 17. For example, the waste water treatment stage 17 may be a stage within a waste water treatment plant associated with or forming a part of the pulp mill system 1. However, the combina- tion 16 may, in some embodiments, be conveyed to a waste water treatment stage 17 located elsewhere or otherwise not forming a part of the arrangement 19.

The arrangement 19 may further comprise a further piping or conduit, also denoted in this Fig. with the reference number 16, for conveying the combi- nation 16 to the waste water treatment stage 17 and/or to discharge 18.

EXAMPLE 1

Jar tests were made to evaluate different al- ternatives for phosphorus (P) removal as shown in Ta- ble 1.

Table 1. Jar tests for evaluating P removal alterna- tives. WAE = acidic effluent stream; WAB = effluent stream (alkaline EOP) or acidic, as indicated).

The results of the jar tests are shown in Ta- bles 2 and 3. When only the acidic effluent stream (acid WAB or WAE) was treated, the flow to the phos- phorus removal treatment could be reduced, and less sludge could be generated. However, a comparable P re- moval with a final P <1 mg/1 could be achieved with a comparable lime/NaOH consumption and a similar sedi- mentability. While the consumption of H2SO4 for adjust- ing the pH of the purified effluent stream was higher, the overall chemical consumption was comparable or lower.

EXAMPLE 2

The experimental setup of this example is de- picted schematically in Fig. 3. Acidic and alkaline bleaching effluents were collected separately from a chemical pulping mill (production: 3600 ADt/d) into an acidic bleaching effluent stream (acid WAB) and an al- kaline bleaching effluent stream (alkaline WAB).

The acidic bleaching effluent stream of 9.4 m 3 /ADt corresponding to 284 kgP/d contained 8.4 mg/1 of phosphorus (P) and had a pH of 2.64. It was divided into a portion of 5.3 m 3 /ADt (160 kgP/d), which was subsequently treated to remove phosphorus, and to an untreated portion of 4.1 m 3 /ADt (124 kgP/d). The por- tion of 5.3 m 3 /ADt of the acidic bleaching effluent stream was then treated to precipitate phosphorus by raising its pH and by removing the precipitate as a sludge to obtain a purified effluent stream of 5.3 m 3 /ADt containing only 10 kgP/d.

The alkaline bleaching effluent stream of 4.0 m 3 /ADt corresponding to 35 kgP/d contained 2.45 mg/1 of P and had a pH of 9.88. It was not treated.

The alkaline bleaching effluent stream and the purified effluent stream were combined into a stream (WAB to equalization) with 169 kgP/d before conveying it to an equalization basin.

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 or a system, 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.