FI125025B - Method for the reduction of phosphorus in effluents and process waters - Google Patents

Description

Method for reducing phosphorus in effluents and process waters

Field of the Invention

The invention relates to a method for reducing phosphorus in effluents and process waters of a pulp or paper mill. The invention relates in particular to treating effluents and process waters of a chemical pulp mill in such a way that the content of the phosphorus therein can be reduced.

background

Effluents of pulp or paper Mills are typically treated in waste water treatment plants in order to reduce the amount of compounds that may be harmful to the environment. The effluents of the chemical pulp The mills can add to the substance which can be valuable to the chemical pulp making process. Environmental Trends And Sustainable Development Are The Driving Factors To Find New Ways To Enhance Recyclability Of These Substances.

Nowadays, chemical pulp mills are often built in areas and environments with very strict environmental regulations. In many areas, a cleaner environment is desired in such a way that the Mills produce substances that are less detrimental to the environment. Therefore, it is important to look for new solutions for pulp Mills.

In chemical pulp Mills, NPEs (non-process elements) tend to accumulate in process waters and will cause disturbances in the processes if they are not removed. For example, calcium, the origin of which is part of the wood raw material of the chemical pulp mill, is known to cause problems. Conventional methods for removing calcium are found to be insufficient and the level of calcium may still remain high in the process waters that are reused in various processes.

Another non-process element that exists in chemical pulp mills is phosphorus. Wood material that is used for pulp making up phosphorous compounds. A portion of these compounds end up in a liquid stream of a chemical pulp mill and are therefore an element of concern in a chemical pulp mill. The phosphorous compounds may comprise, for example, Phosphates and organic phosphorous compounds. Therefore, phosphorus must be removed from the waters that are reused as recyclable waters in the chemical pulp mill to avoid accumulation of phosphorus. As a non-process element, the phosphorus will interfere with the processes of the chemical pulp mill if its content rises too high, for example it disturbs the function of the lime kiln.

On the other hand, phosphorus must be removed from the effluents that are being discharged from the mill to the environment, conventionally to the water system (river, lake, or sea) to prevent pollution. The aim is therefore to eliminate phosphorus compounds as efficiently as possible from the effluents that end up outside the mill. The phosphorus load at the waste water plant of a chemical pulp mill becomes high for the reasons that the phosphorus is continuously removed from the chemical pulp mill and discharged. Environmental regulations are expected to become stricter with regard to phosphorus emissions.

Summary

The object of the invention is to provide a method for treating effluents and process waters in a pulp or paper mill to remove phosphorus. An object of the invention is to provide a method for coprecipitating the phosphorus together with other non-process elements, especially calcium. The coprecipitation is performed in an effluent or process water in a pulp or paper mill, especially in a chemical pulp mill. In the following, these liquids that have phosphorus content which are reduced or eliminated are commonly designated "water to be treated".

The method is performed by adjusting the pH of the water to be treated with an alkaline agent, precipitating calcium carbonate from the water by effecting the water, and coprecipitating the phosphorus together with the Precipitated calcium carbonate.

The source of calcium in the calcium carbonate Precipitation process can be the calcium Originally contained in the water to be treated and / or the calcium that is introduced to the water with the alkaline agent used for the pH adjustment.

The alkaline agent is an agent that is capable of raising the pH of the water to be treated to a level where the calcium can precipitate as the effect of carbonate ions while simultaneously coprecipitating the phosphorus. The range to which the pH of the water is adjusted is 8-12, preferably 9-10. The alkaline agent can be calcium oxide, calcium hydroxide (milk of lime), alkali metal hydroxide, especially sodium hydroxide, or white liquor from the chemical recovery cycle of the pulping process. If the alkaline agent is calcium oxide or calcium hydroxide, at least part of the calcium ions that will form the calcium carbonate precipitate with the coprecipitated phosphorus comes with the alkaline agent. If the alkaline agent is an alkali metal hydroxide, the calcium contained in the water to be treated will form the calcium carbonate precipitate. If the alkaline agent is a white liquor from a chemical pulping process, at least part of the calcium ions that will form the calcium carbonate precipitate with the coprecipitated phosphorus may come with the alkaline agent. The pH of the white liquor, the substance available at the chemical pulp mill as a result of causticizing the green liquor, is alkaline by nature.

The carbonate ions can be supplied to the water to be treated by introducing carbon dioxide rich gas to the water. This is done preferably after adjusting the pH of the water to be treated by the alkaline agent. The carbon dioxide rich gas can be pure carbon dioxide, but preferably it is exhaust gas containing carbon dioxide. One preferred source of exhaust gas is the lime kiln flue gases. In lime kiln the lime reburning process, part of the "lime circulation" of the chemical pulp mill, is performed. The lime reburning process yields exhaust gases where the concentration of carbon dioxide is high as a result of the decomposition of the lime mud to calcium oxide and carbon dioxide.

The operations can be performed on the water to be treated in any order. The carbon dioxide can even be introduced first to the water and right thereafter the pH can be raised by introducing the alkaline agent which causes the instantaneous formation of carbonate ions which causes the Precipitation. However, if the water is to be treated with acidic, such as is the case with example acidic bleaching effluents, it is preferable to raise the pH before introducing carbon dioxide, which causes fast Precipitation and is more efficient with respect to utilization of CO2. The carbonate ions can be introduced also together with the alkaline agent by which the pH is adjusted.

This can be done by introducing carbonated white liquor to the water to be treated. The carbonated white liquor has been obtained by bringing white liquor in contact with carbon dioxide to form carbonate ions in the white liquor. The alkaline pH of the white liquor favors the formation of carbonate ions.

The water to be treated by coprecipitating the phosphorus together with the calcium carbonate is preferably a filtrate, that is, a liquid from which suspended solid matter has been removed.

The water to be treated is preferably acidic filtrate from fiber line operations containing dissolved phosphorus compounds. Especially the acidic filtrate is water from the acidification treatment of the unbleached pulp after brown stock washing, which treatment is performed to dissolve the acid-dissolving phosphorus compounds contained in the pulp before bleaching. The acidic filtrate can also be bleaching filtrate such as A, D and A / D bleaching filtrates. "A" refers to an acid stage of the bleaching unit and "D" refers to a chlorine dioxide stage of the bleaching unit.

Treatment of water to be treated according to the method causes Precipitation of calcium in the form of calcium carbonate and simultaneous removal of the phosphorus in the form of calcium carbonate precipitate, especially coprecipitation of the phosphorus in the form of Phosphates. The precipitate contains at least calcium phosphate, but it may also contain calcium hydrogen phosphate. The existence of various phosphate species in the coprecipitate is also dependent on pH. Some organic phosphorus compounds may also be present in solid matter impurities that will be removed with calcium carbonate precipitate. The precipitate can be separated from the effluent or the process water, and the result is the effluent or the process water with the severely restricted phosphorus content, below a level that does not interfere with the processes if the process waters or the effluents are used later in the processes of the pulp mill or paper mill. The removal of the phosphorus is so efficient that the effluent or process water resulting from the treatment can be obtained to be largely phosphorus-free.

Removal of the phosphorus in the process waters that are recycled in the pulp mill or paper mill will prevent the accumulation of the phosphorus that is a non-process element. For example, inert load caused by phosphorus compounds to the lime kiln in a chemical pulp mill can be reduced. Likewise, the phosphorus load to the waste water treatment plant can be reduced by removing the phosphorus from an effluent.

The treatment to precipitate calcium carbonate by the method can be performed at a waste water treatment plant of a pulp or paper mill, or in any other location where the waters or effluents are suitable for the treatment are formed and / or where there is space for the operation. Effluents or process waters can be treated. If effluents are treated, the treatment is most conveniently performed at the waste water treatment plant. If the process waters are treated, the treatment is most conveniently performed at the pulp or paper mill. The system needed the means to supply the alkaline agent and the carbonate ions to the water to be treated. The means for supplying the alkaline agent to the water and the means for supplying the carbonate ions to the water can be separate means. In this case, the means for supplying carbonate ions to the water can be the means for injecting carbon dioxide rich gas to the water. If the alkaline agent which is supplied to the water to be treated is carbonated white liquor, the means for the supply of the alkaline agent and the means for supply of carbonate ions to the water are treated can consist of the same means, because the carbonate ions enter the water together with the alkaline agent.

In a chemical pulp mill, the Precipitated calcium carbonate and coprecipitated substances are preferably removed from the chemical pulp mill instead of adding it to the lime kiln feed as makeup calcium carbonate for the calcium circuit. The calcium carbonate containing coprecipitated Phosphates can be used for example as a raw material for a fertilizer.

Description of the Drawings

The method will be described in the following with reference to the appended drawing, where

Figure 1 is a schematic representation of the processes of a chemical pulp mill where the method can be used.

Detailed Description

The term "effluent" refers to the waste water discharged from a pulp or paper mill which is treated for purifying purposes at a waste water treatment plant of the mill. Effluents may comprise, for example, bleaching effluents and ash leaching purge. In addition, the effluents may contain wood handling effluents.

The term "bleaching effluents" refers to effluents from bleaching unit and washing steps therein. Advantageously bleaching effluents contain bleaching filtrates. Most advantageously bleaching effluents consist of bleaching filtrates. The bleaching filtrates contain acidic and / or alkaline flows. Advantageously bleaching effluents do not contain fibers.

The term "process water" refers to the waters that originate from an operation in the pulp mill and are reused in the process, after possible tratment such as filtration, etc .. Mostly the process water is water that originates from the operation in the fiber line and is reused. Thus, water issuing from a bleaching step can be considered as a process water if it is reused. Process water also refers to the effluent that has been purified and is reused in the process after the purification.

The pulp mill also has cooling waters, sealing waters, reject flows, channel waters, washing waters of the plant, and rain waters, as well as wood processing water. Said waters have typically not been in contact with the pulping process with the exception of wood processing water and some channel waters that originate from the process overflows and are therefore in contact with the pulping process. Thus, emissions accumulated therein are mainly leakages and overflows, occasional emissions caused by apparatus breakages, washing waters of devices, textiles (wires and felts) or containers from continuous or batch washings, and leakages from the reject system. These waters can be effluents or process waters depending on whether they are discharged or reused.

The term "filtrate" refers to an effluent from which solid suspended matter is removed by a filtration treatment or a flotation treatment or any other treatment. Thus, the term "filtrate" does not necessarily imply that the solid suspended matter, such as fibers, has been removed by filtration.

The term "recyclable purified water" refers to effluent or process water that is purified and is clean enough to be reused as process waters. In this context, the recyclable purified water is especially water from where the phosphorus is removed by the method.

The term "water to be treated" refers to an effluent or a process water that is produced by a pulp or paper mill in the course of its processes, especially by a chemical pulp mill. These liquids are aqueous liquids a phosphorus content that is to be reduced or eliminated.

The term "exhaust gas" refers to a flue gas comprising carbon dioxide. Flue gas is a combustion exhaust gas produced by a furnace or a power plant. An example of a flue gas is a recovery boiler exhaust gas or lime kiln exhaust gas.

The method can be used especially in a chemical pulp mill where a chemical pulp is produced using a so-called sulphate or kraft process, that is, in a kraft pulp mill. The operations in a kraft pulp mill are conventionally divided into fiber line, chemical recovery system and wastewater treatment. The chemical recovery system includes an "alkali circuit" or an alkali cycle and a lime circulation. In the alkali circuit, spent alkaline cooking liquor (black liquor) is concentrated by evaporation, burnt in a recovery boiler, the resulting smelt from the recovery boiler is dissolved in water to form green liquor, and the green liquor is made to cooking liquor, white liquor, in causticizing. In the lime circulation, the green liquor is causticized by CaO which converts sodium carbonate into white liquor to sodium hydroxide and precipitates calcium carbonate in the form of lime mud which is separated from the white liquor which can thereafter be used for cooking the raw material (for example wood chips) in a digester. The lime mud separated from the white liquor is burnt in the lime kiln to convert calcium carbonate back to calcium oxide, with gaseous carbon dioxide as the other product.

The fiber line operations of the kraft pulp mill include typically cooking, brown-stock washing (washing away spent cooking chemicals from the pulp), oxygen delignification, bleaching and pulp drying.

To illustrate the processes of the kraft pulp mill in more detail, reference is made to Fig. 1, in which the following reference numerals are used: 21 cooking, 22 brown stock washing, 25 oxygen delignification stage comprising, among other things, an oxygen treatment stage, 30 bleaching, comprising bleaching stages and washing therein, 40 recovery island, 50 waste water treatment plant, and 80 drying machine for drying pulp A introduction of alkaline agent to a phosphorus reduction treatment unit, and G introduction of gas that contains carbon dioxide to a phosphorus reduction treatment unit.

The symbol of the phosphorus reduction treatment unit is a rectangle to which the Arrows A and G are pointing.

Returning the recyclable purified water obtained from the phosphorus reduction treatment unit back to the process of the kraft pulp mill is denoted by dashed line Arrows.

Phosphorus is coprecipitated from water to be treated by introducing alkaline agent and carbonate ions to water to be treated. The alkaline agent is used to raise the pH of the be treated to a level which favors the formation of carbonate ions and precipitation of calcium ions as calcium carbonate and coprecipitation of the phosphorus together with the calcium carbonate when introduced into the process water or effluent.

The pH of the process water or effluent is raised to 8-12 by the alkaline agent which can be calcium oxide, calcium hydroxide (milk of lime), alkali metal hydroxide, especially sodium hydroxide, or white liquor, which is obtainable from the kraft Pulping process. If the alkaline agent is calcium oxide or calcium hydroxide, at least part of the calcium ions that will precipitate as calcium carbonate with coprecipitated phosphorus comes with the alkaline agent. The rest of the Precipitated calcium carbonate may contain calcium ions Originally present in water to be treated. If the alkaline agent is an alkali metal hydroxide, the calcium contained in the water to be treated will form the calcium carbonate precipitate. If the alkaline agent is white liquor from the kraft pulping process, at least part of the calcium ions that will form the calcium carbonate precipitate with the coprecipitated phosphorus may come with the alkaline agent. The pH of the white liquor, the substance available at the kraft pulp mill as a result of causticizing the green liquor, is alkaline by nature and also contains calcium.

The white liquor used as the alkaline agent is preferably oxidized white liquor. The oxidized white liquor has been oxidized for converting the sulphides to a less harmful form, and it is usually readily available in the kraft pulp mill because the oxidized white liquor is used normally in fiber line operations for oxygen pulping in alkaline conditions, which is done after the brown stock washing. When the oxidized white liquor is used to raise the pH of the acidic effluents or process waters, no volatile hydrogen sulphides are formed.

The carbon dioxide rich gas introduced to the water to be treated after raising the pH by the alkaline agent can be pure carbon dioxide or flue gas or exhaust gas that contains carbon dioxide. In a kraft pulp mill flue gas that is rich in carbon dioxide can be obtained from the recovery boiler, power boiler or the lime kiln. The carbon dioxide rich gas is introduced to the water to be treated by injecting the gas to the liquid so that the gas bubbles through the liquid. The precipitation reaction where the calcium carbonate precipitates with the simultaneous coprecipitation of the phosphorus is instantaneous, and as a result of the rapid reaction, the calcium carbonate does not form pure crystals but also the other inorganic compounds, especially the phosphorus, from the water. At the same time, COD (Chemical Oxygen Demand) decreases due to the entrapment of relevant impurities by calcium carbonate.

The white liquor that is used to adjust the pH can also be carbonated white liquor. By using carbonated white liquor the carbonate ions are introduced to the effluent or process water simultaneously with raising the pH. The Precipitation reaction is instantaneous with the same results as explained above. The carbonated white liquor can be obtained by bringing the white liquor in contact with the carbon dioxide rich gas such as pure carbon dioxide or flue gas which can be from the recovery boiler, power boiler or lime kiln. The carbon dioxide rich gas is bubbled through the white liquor. The white liquor is preferably oxidized white liquor, in which case the alkaline agent used will be carbonated oxidized white liquor.

Still one alkaline agent that can be used is carbonated sodium hydroxide solution. The preparation and function of the carbonated sodium hydroxide is analogous to the carbonated white liquor, with the difference that the NaOH solution is significantly higher in purity with respect to the white liquor. The carbonation can take place with pure CO2 or flue gases as mentioned above for white liquor and the result is aqueous sodium carbonate solution. By using carbonated sodium hydroxide the alkaline agent contains carbonate ions that are introduced simultaneously with the alkaline agent to the water to be treated. Sodium hydroxide is one of the chemicals regularly supplied to the pulp mill and is always available for making carbonated sodium hydroxide.

If flue gas is used for carbonation of white liquor or NaOH, it can be purified before carbonation to remove such constituents that could form non-process elements in the water to be treated.

The temperature in the Precipitation reaction can be set between 25-70 ° C. The effluent or process water can Originally be at a temperature which will set the temperature to the required level when the carbonate ions are introduced, or the effluent or process water can be heated before the carbonate ions are introduced. At least part of the heating can be performed by using the heat from the exhaust gas introduced to the effluent or process water.

The Precipitation of calcium carbonate can be accelerated by agitating the water to be treated to effect its vigorous mixing. At least part of the agitation can be accomplished by injection of carbon dioxide, flue gas or the alkaline agent.

In the final stage, the precipitated calcium carbonate which contains the coprecipitated other inorganic compounds, especially the phosphorus compounds, is separated from the water to be treated using sedimentation, DAF (dissolved air flotation) or any other suitable separation method. After this, a purified phosphorus-free water is obtained, which can be used further in a chemical pulp mill, for example as washing or dilution water.

The precipitated calcium carbonate containing coprecipitated phosphorus compounds and possibly coprecipitated other inorganic compounds can be reprocessed further for end use, such as use as a fertilizer.

The method can be used to treat such effluents and process waters in a chemical pulp mill containing phosphorus. The source of carbon dioxide needed for the Precipitation reaction is preferably located in the same mill, for example a recovery boiler or lime kiln. If white liquor is used as an alkaline agent, it is also available in the same mill. The method can be used also in a paper mill where paper or paperboard is made from fibrous raw material to treat process waters and effluents of the paper mill. The method can be used especially in a paper mill where flue gases are used for a short distance, for example when a chemical pulp mill and a paper mill using a pulp are integrated.

The effluents and process waters treated by the method are especially acidic effluents and process waters that contain phosphorus compounds in dissolved form. The pH of the acidic effluents and process waters is preferably below 7, more preferably below 6. By introducing the alkaline agent the pH can be raised to the optimum range of 8-12.

The above-mentioned acidic effluents and process waters are especially acidic filtrates from fiber line operations, especially acidic bleaching filtrates such as A, D1 and A / D1 bleaching filtrates. "A" refers to an acid stage of the bleaching unit and "D" refers to a chlorine dioxide stage of the bleaching unit. The acidic effluent treated can also be from the acidification treatment of the unbleached pulp, which is performed before the bleaching, after brown stock washing and oxygen treatment of the oxygen delignification unit. This acidic effluent also contains dissolved phosphorus compounds.

The acid effluent from the bleaching unit 30 is shown in Figure 1, and the phosphorus reduction treatment unit is indicated with a rectangle to indicate the introduction of the alkaline agent A and the gas containing carbon dioxide G are pointing.

The phosphorus reduction treatment unit contains means suitable for treating water to be treated. The treatment unit may contain pumps for various fluids, appropriate piping, tanks, conduit for introducing the alkaline agent (arrow A) and conduit for introducing gas containing carbon dioxide (arrow G), or alternatively conduit for introducing carbonated white liquor or carbonated NaOH to water to be treated. The treatment unit contains a Reactor for bringing in the alkaline agent and the gas containing carbon dioxide, or alternatively the carbonated white liquor or carbonated NaOH in contact with the water to be treated to effect the Precipitation of calcium carbonate and removal of phosphorus impurities with calcium carbonate. The phosphorus reduction treatment unit may also comprise a carbonation device for making carbonated white liquor or carbonated sodium hydroxide on the spot, if it is not carbonated elsewhere. The treatment unit may also contain separation equipment for separating the precipitated calcium carbonate and other impurities contained in the precipitate from the treated water, and piping for passing the treated water from which the precipitate has been removed, further outside the treatment unit and further as purified water to be discharged, or as recyclable purified water to be returned to the process in the mill.

From the foregoing description, it will also clear that connection with the symbol of the phosphorus reduction treatment unit, arrow A can indicate the conduit for introducing the alkaline agent and carbonate ions together with the alkaline agent to the water to be treated, as is the case when carbonated white liquor or carbonated NaOH is used, and arrow G indicates introduction of gas containing carbon dioxide to water treated can be omitted.

The purified water obtained after the treatment can be recycled back to the mill as process water (as denoted by the dashed line arrow R), preferably to be used in fiber line operations. The recyclable purified water can be used as dilution and washing waters in fiber line operations such as in bleaching. The purified water can be passed further to the purification process of the waste water treatment plant 50, as indicated by the continuous line arrow.

The treatment can take place at the mill or at the waste water treatment plant 50. As shown in Fig. 1, the treatment of acid effluent from bleaching can be performed already at the fiber line. Alternatively, the treatment of acid effluent from bleaching can be performed in a waste water treatment plant 50.

The effluents and process waters can also be water from log washing, brown stock washing water, or clarified water at waste water treatment plant, especially water after the secondary clarifier. The treatment of clarified water at the waste water treatment plant 50 to remove the phosphorus is denoted with a rectangle inside the waste-water treatment plant 50 in Figure 1, and the recycling of at least part of the water (recyclable purified water) back to the process with the dashed line arrow R. At least part of the water can be discharged as purified water to the water system (continuous line arrow). The recyclable purified water can be used especially as various washing and dilution waters in the process.

The treatment of brown stock washing water is also indicated in Figure 1 (arrow from brown stock washing 22). The recyclable purified water can be used as dilution and washing water in fiber line operations as indicated by dashed line Arrows, for example in bleaching.

The treatment by alkaline agent and carbonate ions can be performed in a reactor where the calcium carbonate is Precipitated in a continuous process. The alkaline agent and gas containing carbon dioxide, or carbonated white liquor or carbonated NaOH, are introduced continuously by injecting them in a stream of water to be treated. In the Reactor, the water flows in a pipe and the above-mentioned reactants are injected into the process water during its flow in the pipe. The reaction is instantaneous after all the reactants necessary for the formation of calcum carbonate have been introduced.

After the precipitation and separation of calcium carbonate and phosphorus coprecipitant, the pH of the purified, phosphorus-free recyclable water can again be adjusted to the desired level for further use of the water.

In the following example, the reduction of the phosphorus by the method using carbonated oxidized white liquor as an alkaline agent is shown.

EXAMPLE

Table 1. Reduction of phosphorus in effluent using alkaline agent and carbonate. The effluent used was the filtrate obtained after the secondary clarifier of a waste water plant of a kraft pulp mill.

5 liters of effluent was taken and its total phosphorus content was determined, giving 1.73 mg / L (analysis method: "ICP (Inductive Coupled Plasma) Metals with Microwave Digestion" SFS-EN ISO 11885).

Next, the sample was filtered through a 0.45 pm filter and the concentration of dissolved phosphorus was determined, giving 1.64 mg / l. Carbonated oxidized white liquor was added stepwise to the sample and after each addition, a 100 mL sample was taken for analysis. In test point 4, 4.4 liters of Precipitated effluent was removed. From the remaining sample, 225 mL was taken and the carbonated oxidized white liquor was added to it up to a pH of 10.57. Thereafter, a 100 mL sample was taken for analysis. All samples were analyzed according to ICP metals (dissolved) for water sample SFS-EN ISO 11885.

As the gas that contains carbon dioxide, the lime kiln is the preferred source of flue gases because of their high carbon content and easy availability at the kraft pulp mill. The recommended composition of the lime is a flue gas (emissions in the acceptable limits) is given as an example (calculated from dry gas at 273 K, 101.3 kPa, corresponding to an oxygen content of 6 vol-%). The percentage values given are vol-%.

Besides removing phosphorus, the method can also be used for removing calcium from effluents and process waters in a pulp mill or a paper mill.

Claims (18)

1. A process for reducing phosphorus in wastewater and process water from a pulp or paper mill, comprising: - adjusting the pH of the water to be treated with an alkaline agent, - introducing carbonate ions into the water, and - extracting the phosphorus together with the precipitated calcium carbonate from the waste water or process water. The method of claim 1, wherein the pH of the water to be treated is adjusted to a range of 8-12. The method of claim 1, wherein the pH of the water to be treated is adjusted to a range of 9-10. The process according to claim 2 or 3, wherein the water to be treated is acidic. The method of any one of claims 1 to 4, wherein the water to be treated comes from a fiber line of a pulp mill. The method of claim 5, wherein the water to be treated is acidic filtrate from the pulp mill fiber line operations. The method of claim 6, wherein the acidic filtrate is a bleaching filtrate. The method according to any one of claims 1 to 3, wherein the water to be treated is log washing water, brown pulp washing water or purified water from a sewage treatment plant. A process according to any one of the preceding claims, wherein carbonate ions are fed to the water to be treated by introducing a gas rich in carbon dioxide into the water. The method of claim 9, wherein the carbon dioxide-rich gas is introduced into the water to be treated prior to adjusting the pH with an alkaline agent. The process of claim 9, wherein the carbon dioxide-rich gas is introduced into the water to be treated at the same time as or after the pH is adjusted with an alkaline agent. The process according to any one of claims 9 to 11, wherein the carbon dioxide-rich gas is a flue gas. The method of claim 12, wherein the flue gas is flue gas from a lime kiln. The process according to any one of claims 9 to 13, wherein the alkaline agent is calcium oxide, calcium hydroxide, alkali metal hydroxides or white liquor. A process according to any one of claims 1 to 8, wherein the carbonate ions are fed to the water to be treated with carbonated white liquor or carbonated sodium hydroxide solution used as an alkaline agent. The process of claim 15, wherein the carbonate ions are formed in the alkaline agent by carbonation of the white liquor or sodium hydroxide solution with a gas containing carbon dioxide. The process according to any one of the preceding claims, wherein the phosphorus is co-precipitated, at least in part, with calcium carbonate as phosphates. The method of any preceding claim, wherein the pulp mill is a sulphate pulp mill.