Fractionation of effluent from a pulp mill

Abstract

The invention pertains to a process for fractionation of an effluent stream from a pulp mill, being a continuous combination treatment comprising the steps of treating a mainstream of the effluent with electrocoagulation (EC), producing a primary effluent and a floe fraction which is separated from the primary effluent, biologically treating the primary effluent in one or several biofilm reactors, and producing a secondary effluent and a biosludge fraction which is separated from the secondary effluent, thereby the effluent stream is fractionated into a water fraction, comprising said secondary effluent, a compact solids fraction, comprising the separated floe fraction, and a biosludge fraction, comprising said separated sludge fraction.

Description

This invention aims to fractionate the effluent from a pulp mill into different reusable fractions, which could ultimately result in a completely closed mill in terms of wastewater. More particularly the invention relates to a process for fractionation of an effluent stream from a pulp mill, being a continuous combination treatment, whereby the effluent stream is fractionated into at least one water fraction, one compact solids fraction, and one biosludge fraction.Background of the InventionIt is known that many industrial processes use water in one way or another and most of these also give rise to contaminated wastewater. The contaminants must or should be removed before the wastewater is sent on to the recipient. Using a traditional approach to this challenge, solutions are sought to treat the water in question to reach a degree satisfactory to comply with environmental standards put up e.g. by the authorities. In this process, the wastewater is treated so that the treated water could be sent to the recipient while the remainder is separated in the form of e.g. a concentrate or sludge of some kind. This residue (e.g. concentrate or sludge) will need to be taken care of and / or handled in some way. Using this approach, as long as the wastewater treatment works, the challenge is solved.One industrial process that has undergone a transformative shift towards greater efficiency is kraft pulping. Traditionally, pulp production was notorious for its heavy consumption of resources and waste generation. However, innovative approaches have emerged with a notable emphasis on water reuse.Water, a critical component in pulp production, is being increasingly reclaimed and used more efficiently throughout the process. Improved treatment technologies have enabled mills to treat and reuse water multiple times, significantly reducing freshwater intake and wastewater discharge. Chemical recovery systems, such as the kraft recovery cycle, capture and regenerate spent pulping chemicals, notably pulping liquors and cooking chemicals like sodium hydroxide and sodium sulfide. Although water is being reclaimed to a greater extent, full recirculation of water is not yet possible, since it would lead to scaling, corrosion and quality problems in the final product as a result of gradual build-up of contaminants and salt. Thus, the mill still generally lets an effluent out to the recipient.The effluent generated during the pulping process contains various contaminants, including suspended solids, organic compounds e.g. lignin residues, and potentially harmful substances. Commonly, the effluent is led to an aerated pond or lagoon for settling of suspended solids or bioremediation. Such treatment removes a great deal of suspended solids, organic matter, and other contaminants through sedimentation and microbial activity. However, the wood extractives will largely remain (either in the suspended matter or the treated water), and is therefore still problematic to discharge to the recipient, where in that case a negative effect in terms of toxicological or ecotoxicological impact should be expected.In fact, the fully closed- loop and effluent- free pulp mill is still a looming goal, as Vidal et al. (2021) states in a recent review article. Vidal underlines the importance of recirculation of water in a pulp mill in a closed system, as well as disposal of organic substances but also of salts in the water to avoid scaling, corrosion and quality problems in the final product as a result of gradual build-up of levels. Vidal et al. reaches the conclusion that the way forward is anything but obvious in concrete terms if it is also to be sustainable over time, but speculates that the interconnection of various biological, physical and chemical technologies may play a role on the way forward.Therefore, there is still a need for further development of effluent treatment techniques in relation to kraft pulping.Summary of the InventionAccordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above mentioned problems by providing a process for fractionation of an effluent stream from a pulp mill, being a continuous combination treatment comprising the steps of treating a mainstream of the effluent with electrocoagulation (EC), producing a primary effluent and a floe fraction, which is separated from the primary effluent, biologically treating the primary effluent in one or several biofilm reactors, and producing a secondary effluent and a biosludge fraction which is separated from the secondary effluent, thereby the effluent stream is fractionated into a water fraction, comprising said secondary effluent, a compact solids fraction, comprising the separated floe fraction, and a biosludge fraction, comprising said separated sludge fraction.In one aspect of the invention, the effluent stream from a pulp mill is or comprises the wastewater from the debarking area and / or bleaching effluent.In one further aspect, the compact solids fraction comprises the greater part of any transition metal ions removed from the effluent stream in the process, and the greater part of any sterol, sterylester, phenol and / or terpenoide removed in the purification process.In one aspect, the compact solids fraction is recirculated to the pulp mill, and added to a black liquor flow, or preferably to a medium-thick liquor flow, in the evaporation process for co-purification. In one further aspect, any phosphorus in the recirculated compact solids fraction will reach the lime kiln, where it can be ejected and recovered as a phosphorus rich lime with low Cd / P quote.In one aspect, the EC treatment uses of iron (Fe) electrodes.In one aspect, the biological treatment is anaerobic treatment, aerobic treatment or a combination of both anaerobic and aerobic treatment steps.In one further aspect, the biological treatment biofilm reactors are selected from Membrane Biofilm Reactors (MBR), Moving Bed Biofilm Reactors (MBBR), Rotating Biological Contactors (RBC) or Sequential Batch Reactors (SBRs).In one further aspect, the biological treatment biofilm reactors are Membrane Bio film reactors (MBR).In one further aspect, the biosludge is suitable for use as a soil improvement product.According to one aspect, said water fraction comprises, if any, at least 90% less of any cadmium content than the effluent stream, at least 90% less of any zinc content than the effluent stream, at least 90% less of any lead content than the effluent stream, at least 50% less of any sterol and / or sterylester content than the effluent stream, at least 50% less of any phenols and / or terpenoide content than the effluent stream, and at least 50% less of any resin acid content than the effluent stream.According to one aspect, the process comprises a step of treating the water fraction (secondary effluent stream) using reverse osmosis (RO), further resulting in an additional brine fraction comprising a concentrate of any salt ions present in the water fraction, such as any sodium, calcium, potassium, sulfate, carbonate and chloride ions -and a pure water fraction (tertiary effluent stream).In one further aspect, the pure water fraction is recirculated to the pulp mill to be used as fresh water.In one further aspect, the brine fraction is recirculated to the pulp mill, and used as a liquid for leaching and dissolving boiler fly ash from the recovery boiler, resulting in concentrate.In one aspect, the resulting concentrate is treated using with electrocoagulation (EC), producing a solid floe and second brine fraction.In one further aspect, the resulting second brine fraction is treated using electrolysis, resulting in a gas phase comprising chloride gas and a salt solution.In one further aspect, the resulting salt solution is used as make-up chemicals during the evaporation step in the kraft pulp mill.Brief Description of the DrawingsThese and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in whichFig. 1 is a schematic of the process for fractionation of an effluent stream from a pulp mill according to the invention, resulting in a compact solids fraction (floe fraction), a biosludge fraction and a water fraction;Fig. 2 is a schematic of a further embodiment for fractionation of an effluent stream from a pulp mill, resulting in a compact solids fraction (floe fraction), a biosludge fraction, a brine fraction, and a water fraction;Fig. 3 is a schematic of a further embodiment for fractionation of an effluent stream from a pulp mill, resulting in a compact solids fraction (floe fraction), a biosludge fraction, a brine fraction, and a water fraction, wherein the compact solids fraction is further processed in the recovery line of the kraft pulp mill, providing heat and optionally resulting in a phosphorus-rich calcium fraction;Fig. 4 is a schematic of a further embodiment for fractionation of an effluent stream from a pulp mill, resulting in a compact solids fraction (floe fraction), a biosludge fraction, a brine fraction, and a water fraction, wherein the brine fraction is further used in the kraft pulp mill as leaching chemicals for the recovery boiler fly ash, and the resulting solution is treated with electrocoagulation, resulting in a solid floe (metal ions), and electrolysis, resulting in chlorine gas and a salt solution, where the salt solution can be used as make-up chemicals in the recovery line of the kraft pulp mill; Fig. 5 is a graph showing the treatment effect of EC+Biotreament effect on susp and organics;Fig. 6 is a graph showing the treatment effect of Biotreament+EC effect on susp and organics;Fig 7 is a graph showing the treatment effect of EC+ Biotreament effect on extractives; andFig. 8 is a graph showing the treatment effect of Biotreament+EC effect on extractives.Description of embodimentsThe following description focuses on an embodiment of the present invention applicable to a process for fractionation of pulp mill effluent.In the invention, it was found that useful fractions could be created from the effluent if focus is removed from wastewater treatment.It was found that by combining electrocoagulation (EC) with biological treatment, sufficient separation of the different constituents was achieved in order to create useful fractions.Further, it was found that the obtained fractions could be fed back to the pulp mill as reuseable streams, thus transforming the previous external wastewater treatment into an integral part of a modem biocombine.In the invention is shown a process for fractionation of an effluent stream from a pulp mill, being a continuous combination treatment comprising the steps of: Treating a mainstream of the effluent with electrocoagulation (EC), producing a primary effluent and a floe fraction which is separated from the primary effluent. Biologically treating the primary effluent in one or several biofilm reactors, and producing a secondary effluent and a biosludge fraction which is separated from the secondary effluent.Thereby the effluent stream is fractionated into a water fraction, comprising said secondary effluent, a compact solids fraction, comprising the separated floe fraction, and a biosludge fraction, comprising said separated sludge fraction.The effluent stream from the pulp mill may be or include the wastewater from the debarking area and / or bleaching effluent.Today, pulp mills are usually equipped with an external wastewater treatment of some kind. This is normally not capable of separating the wastewater into reusable fractions. An important exception is, however, the use of filters which capture fibers in the wastewater and return these to the pulp production line. After that, there is often a biological treatment and / or a sedimentation pond. Sometimes these are combined in a larger aerated pond that also functions as a sedimentation pond.In a sedimentation pond, much of the suspended matter settles together with a significant portion of the transition metal ions, a smaller portion of phosphorus and some extractives. The wastewater does not become clean this way - rather less dirty retaining the rest of the contaminants. Thus, the aqueous phase cannot be reused in the mill as extractives could clog nozzles and filters, the calcium content would build up over time and create precipitates and the chloride content would also build up over time and cause corrosion etc. Sedimentation alone does not normally lead to the wastewater being sufficiently clean to be approved by the authorities.If an aerobic bioremediation is used, much of the suspended matter settles together with a significant part of the transition metal ions and a lot of phosphorus. Organic substances are largely degraded into carbon dioxide and water, and some nitrogen compounds are degraded into water and gaseous nitrogen going to the atmosphere. Extractives are to a smaller or greater degree removed from the aqueous phase. However, this is not the same as the substances are degraded in the process.Kostamo et al. (2004) suggest that 1.1-64% of the substances were adsorbed rather than degraded. Later studies (Mahmood-Khan and Hall 2013 and Khan et al. 2015 respectively) have found that the adsorption mechanism is significantly more important than that, especially for those sterols that are in themselves particularly problematic from a toxicity perspective vis-å-vi the recipient. Mahmood-Khan and Hall (2013) also reported that the biodegradation of sterols that nevertheless occurred was very sensitive to small variations in the process conditions. Other metal ions and anions remain in the aqueous phase, but will also form a significant part of the sludge phase since the sludge is strongly water-holding and difficult to dewater. The aqueous phase cannot be reused in the mill as the calcium content would build up over time and cause scaling and the chloride content would also build up over time and cause corrosion etc. In addition, there is an uncertainty about how effective the removal of the extractives really is and how sensitive the process in general is for disturbances of various kinds, where there is a risk that extractives remain to a greater or lesser degree and then risk clogging nozzles and filters.In anaerobic bioremediation, many of the suspended substances settle. Organic substances are largely degraded to methane, carbon dioxide and water and some nitrogen compounds are reduced to ammonia / ammonium ion. The methane can be taken care of and put to good use, e.g. for electricity or heat production or after upgrading as vehicle fuel. Extractives are likely to be largely removed from the aqueous phase, but are adsorbed in the sludge rather than being degraded similar to what is discussed above for aerobic treatment. Sulfate is reduced to sulfide and effectively precipitates transition metal ions. Other metal ions and anions remain in the aqueous phase, but will also form an essential part of the sludge phase because the sludge is strongly water-retaining and difficult to dewater.Thus, using aerobic or anaerobic bioremediation, generally the aqueous phase cannot be reused in the mill as the calcium content would build up over time and cause scaling and the chloride content would also build up over time and cause corrosion etc.Chemical coagulation is another commonly used wastewater treatment technique. In such purification, ferric chloride or aluminum chloride is typically charged as precipitation chemicals. In the process, the suspended solids are coagulated and the transition metal ions are precipitated together with phosphorus. Organic substances are reduced to a limited extent, as is nitrogen. Extractives are probably not removed from the aqueous phase, but may possibly be partially adsorbed in the sludge. Other metal ions and anions remain in the aqueous phase, but will also form a significant part of the sludge phase because the sludge is strongly water-retaining and difficult to dewater. Here, generally the water cannot be reused in the mill as the calcium content would build up over time and cause scaling and the chloride content would also build up over time and cause corrosion etc. Extractives are probably still present to a significant extent and risk clogging nozzles and filters.In all the conventional cases above, a treated water is thus obtained that cannot be reused and a sludge containing phosphorus, transition metal ions and extractives mixed with other substances and which, given this, cannot be used as a soil improver. In addition, the sludge is water-retaining and difficult to dewater. The combustion, which is then closest to hand, is less attractive as the high water content requires support fuel and the chloride content creates corrosion problems in the boiler over time. In addition, the ash after incineration will not be usable in any obvious way since it contains a combination of phosphorus and transition metal ions. The sewage treatment has thus separated the sewage into two parts, neither of which can be reused in any obvious or beneficial way.The fully closed-loop and effluent-free pulp mill is still a looming goal, as Vidal et al. (2021) states in a recent review article. Vidal underlines the importance of recirculation of water in a pulp mill in a closed system, as well as disposal of organic substances but also of salts in the water to avoid scaling, corrosion and quality problems in the final product as a result of gradual build-up of levels. Vidal et al. reaches the conclusion that the way forward is anything but obvious in concrete terms if it is also to be sustainable over time, but speculates that the interconnection of various biological, physical and chemical technologies may play a role on the way forward.Thus, in the invention is presented a way forward through a new strategy for the management of the sewage from a traditional sewage treatment to a sewage fractionation aimed at recycling.In the invention it is was realized that while focusing on treating the wastewater seems logical, it often results in a less contaminated water stream, treated to meet the local effluent (or water quality) standards, leaving a mixed sludge that requires further processing or disposal. If one instead focuses on fractionation of the effluent stream into fractions that are possible to reuse, then one might sort the effluent constituents into different fractions that may be reusable.This in turn requires that compounds are grouped together in such a way that they are possible to further process or reuse, without having to first separate out any specific compounds (and without ending up with one useful fraction and one fraction with negative value) - A far from trivial task. However, it was realized that for kraft pulping effluent, one could make use of the existing infrastructure in the kraft pulping process, which gives further opportunities to make such fractions usable.In the process of the invention, a main effluent stream is first treated with electrocoagulation (EC), producing a primary effluent and a floe fraction which is separated from the primary effluent.Electrocoagulation (EC) is well known to be an alternative technique to chemical coagulation with high efficiency in terms of removal of e.g. suspended solids, phosphorus, transition metal ions and others. In electrocoagulation, a dirty water is treated in an electrochemical reactor whereby sacrificial electrodes of iron or aluminium are gradually dissolved over time so that ferrous ion or aluminium ion is released and acts as a coagulant and nucleation nucleus for the contaminants in the sewage. The electric field that exists between the electrodes also contributes to the efficiency of this wastewater purification process. After passing through the reactor, the effluent flow can be separated into an aqueous phase and a floe in a flotation unit. The process also affects the physical properties of the floe such that it easily drains both spontaneously and even more in connection with an externally applied pressure.In the EC process, the suspended solids are coagulated and the transition metal ions are precipitated together with phosphorus. Emulsified droplets of oils and fats that are part of the organic substances coalesce effectively. Other organic substances are reduced to a certain but limited extent, as is nitrogen. Extractives are removed (more on this below). Other metal ions and anions largely remain in the aqueous phase, but here they will NOT form a significant part of the floe phase since the floe is easily dewatered to a high dry solids content.It is known that the water cannot be reused in the mill as the calcium content would build up over time and cause scaling and the chloride content would also build up over time and cause corrosion etc. The floe can in this case be incinerated as it has a positive calorific value, but equally an ash is obtained which cannot be further used as it contains a combination of phosphorus and transition metal ions. Although this is an important step in a fractionation strategy, there still remains an aqueous phase that cannot be used.Thus, in the method of the invention, the electrocoagulation is followed by biologically treating the primary effluent in one or several biofilm reactors, and producing a secondary effluent and a sludge fraction which is separated from the secondary effluent.In order to study more closely the effect of this combination fractionation or pulp mill effluent, several experiments were carried out for the combination of electrocoagulation and biological treatment.In the invention, iron was preferred as the electrode material during the electrocoagulation process. It not only provides very good purification results, as can be seen in tables 1 and 2, it was also found to be the most cost-effective electrocoagulation set-up option. Also, it does not conflict with the subsequent biological treatment, since any leftover ferrous or ferric ions will serve as a micronutrient for microorganisms, making it beneficial for the biological treatment in the co-treatment processes.Thus, the EC treatment may use iron (Fe) electrodes.The voltage used in the electrocoagulation process is 100 V or less, such as 50 V or less, such as 25 V or less, such as 10 V or less, preferable between 100 to 5 V, such as between 50 to 10 V.The iron concentration may be monitored in the primary effluent fraction, to ensure that not too much iron is released in the electrocoagulation process. Too high iron concentration in the secondary effluent is indicative of a too high voltage, whereby the released iron is not all used up, which causes unnecessary electrode wear. If so, the voltage may be lowered, and the iron release adjusted to match effluent properties, resulting in less excess iron in the primary effluent.Untreated wastewater from a kraft pulp mill was sampled, before and after purification with electrocoagulation. The result of a conventional chemical analysis is given in Table 1 and the result of a wood chemical analysis focusing on wood chemicals at group level is presented in Table 2.Table 1. Chemical analysis of the composition of an effluent from a pulp mill with bleached production before and after purification with electrocoagulation.The results clearly show that the removal of suspended substances and phosphorus is very effective. Thus, no build-up of large-scale fiber sludge deposits in the recipient or significant contribution to eutrophication of the recipient can be avoided using the method of the invention.Similarly, it is noted that the removal of transition metal ions is almost complete in all cases presented in Table 1. This is important, since it means that the recipient will not be fed with more than marginal amounts of these environmentally harmful ions.Further is noted that COD and TOC are reduced by about 50%, BOD is only reduced by about 25%.Table 2. Wood chemical analysis at group level of the same pulp mill wastewater as in Table 1, before and after electrocoagulation treatment.In Table 2, the effluent has been analysed at group level with respect to their content of different types of wood extractives. Electrocoagulation thus proves to be very efficient for the removal of phosphorus (contrary to a biological treatment process) as well as transition metal ions. It also proves to be very efficient at removing toxic and ecotoxic substances (wood extractives).Thus, almost any transition metal ion coming with the wastewater will be removed by the elctrocoagulation and thus end up in the compact solids fraction, and so will almost any sterol, sterylester, phenol, terpenoide removed in the purification process.Organic substances smaller in size and those with high polarity, such as acetic acid, glucose, methanol, etc. remains in the water after the electrocoagulation treatment and is responsible for the remainder of organic substances measured as BOD, COD or TOC. The same applies to e.g. sodium, potassium, calcium, sulfate, carbonate and chloride. Thus, EC alone is not enough to result in a useful water fraction and a floe fraction.In the invention, it was found that a combination of EC and biological treatment may be used to further fractionate the kraft pulp effluent. This combination of treatments could potentially be carried out either so that EC precedes the biological treatment - or the other way around. Attempts were made to investigate the combined effect of the two techniques, where the results are shown in Figures 1-4.Several advantages could be found for either combination, such as that a carbon rich effluent provides a lot of bioenergy for a biological treatment step preceding an EC-treatment step. However, it was found that when the EC treatment precedes the biological treatment, the amount of pollutants remaining for the biological treatment to clean is radically reduced, which reduces the overall footprint and space requirement of the system.The biological treatment may be and anaerobic treatment, aerobic treatment or a combination of both anaerobic and aerobic treatment steps. It was found that preferably the biological treatment is aerobic treatment (or a combination of both anaerobic and aerobic treatment steps, where the anaerobic treatment precedes the aerobic treatment).The biological treatment biofilm reactors may be any suitable technique, such as Membrane Biofilm Reactors (MBR), Moving Bed Biofilm Reactors (MBBR), Rotating Biological Contactors (RBC) or Sequential Batch Reactors (SBRs).Preferably, the biological treatment biofilm reactors are Membrane Biofilm Reactors (MBR) or Moving Bed Biofilm Reactors (MBBR), which was found both effective and space saving.It was found that the footprint required for the purification process was less than 5 m2 per m3 of treated flow per hour, it may be as small as less than 2 m2 per m3 of treated flow per hour, or even less than 1 m2 per m3 of treated flow per hour.Membrane Biofilm Reactors (MBR) have a small footprint because MBR combines the biological treatment and solid-liquid separation into a single step, and results in a very compact system together with the electrocoagulation (EC) treatment. The combination of EC and MBR also produces high quality effluent due to the MBR membrane barrier that physically removes any final fine particles, pathogens, or microorganisms.Most space efficient was the combination of electrocoagulation and Membrane Biofilm Reactors (MBR), where the footprint required for the purification process was less than 1 m2 per m3 of treated flow per hour during trials.As can be seen in figure 1 to 4, it was found that not only does the overall purification efficiency for BOD, COD, TOC and suspended solids was far better if EC preceded the aerobic biological treatment than if the opposite order of treatments was instead employed.The experiments also showed that the overall purification of extractives linked to toxicological and ecotoxicological effects (i.e. primarily sterols, steryl esters, resin acids and phenols and terpenoids) was better when EC preceded the biological treatment than vice versa. Compound classes, such as resin acids, that were not purified efficiently during the electrocoagulation step (see Table 2) were efficiently removed during the subsequent biological treatment step, as can be seen in Figure 3.Furthermore, the EC purification process is a continuous, fast and efficient process. Thereby, by having the electrocoagulation first, a lot of the heat of the kraft pulp effluent may be retained, which was found to further speed up the subsequent bioremediation step.It was found that another advantage of having the electrocoagulation step before the biological treatment is that degree of contaminants in the water (such as BOD, COD, TOC and suspended solids) will be decreased before the biological treatment step. This is advantageous, since several biological treatments require may be made smaller and more energy-effective (less need for aeration in MBBR processes, or for pumps etc in Membrane Biofilm Reactors (MBR)). As such, by having the electrocoagulation step first, the total energy usage for the system could be decreased.From the standpoint of fractionation and effluent recovery, it was found that the sludge from the aerobic biological treatment ("Bio-sludge"), when EC has been used as pre -treatment, consists mainly of organic substances, phosphorus and, given the high water content, also some sodium, calcium, sulfate, carbonate and chloride - but, on the other hand, only trace amounts of transition metal ions and extractives. This is important, since it means that this biosludge fraction is well suited for use as soil improvement. Thus, instead of a sludge in need of further processing (as is the case if biological treatment is the first stage), it becomes a useful fraction with value (when biological treatment is used as the second stage).Since the EC treatment to a large extent removed the phosphorus from the effluent stream, additional phosphorus may be added to the primary effluent before the biological treatment for nutrient balance or if needed to enhance the biological activity and organic substance removal.Thus, in the invention, the kraft pulp effluent stream is first processed by EC followed by a biological treatment step. Thereby, the effluent stream can be fractionated into a water fraction, a compact solids fraction, comprising the separated floe fraction from the EC-treatment, and a biosludge fraction, comprising said separated sludge fraction from the bio-treatment.EC pre-treatment has in part been proposed before. Vepsäläinen (2012) has studied the effect of EC on industrial wastewater, including bark water from a pulp mill. In experiments where he actively added a resin acid to the water, depending on the pH, this resin acid could be removed by simple filtration up to 74%, while the use of EC could remove it almost completely (97%). However, this reference is silent as to any kind of continuous fractionation of the kraft pulp effluent flow into useful fractions.However, to fully make use of the fractions, it was realized that one may use the advantage of having the infrastructure of the kraft pulp close at hand. Thus, the compact solids fraction may be recirculated to the pulp mill. It may be added to a black liquor flow, or preferably to a medium-thick liquor flow, in the evaporation process for co-purification.Since the compact solids fraction will (preferably) have a positive calorific value (CV), it is suitable for incineration. Thereby, when further refining the compact solids fraction at the pulp mill, it will provide (re-circulate) energy to the recovery cycle, when incinerated in the recovery boiler. Any transition metal ion brought to the recovery boiler will face the same destiny as transition metal ions from the black liquor itself would do. This means that the particularly toxic transition metal ions (e.g. Cd, As, Hg, Pb, Zn) will mainly end up in the boiler fly ash, whereas the less toxic transition metal ions (e.g. Fe, Cu, Mn, Al, Mg) will end up in the bottom smelt and eventually ejected in the green liquor dregs.The positive calorific value (CV) may be higher heating value (HHV) measured using a bomb calorimeter or a lower heating value (LHV) estimated using Tanner diagram.Any phosphorus in the recirculated compact solids fraction will reach the lime kiln. Here, it has been shown in patent application SE545642 that the phosphorus may be removed as a phosphorus rich lime. Thus, the phosphorus of the compact solids fraction may potentially be used for soil improvement, possibly together with the biosludge fraction (see below).The biosludge fraction comprises primarily organics, water and phosphorus, resin acids, making it suitable for use as a soil improvement product.Toxic suspended metals and metal ions will primarily end up in the compact solids fraction. Therefore, the biosludge will comprise a low level of toxic metal ions, such as cadmium.Thus, there is no need for using the kraft pulp infrastructure for further processing of the biosludge fraction and for its direct use e.g. as a soil improver whereby also the biocarbon will be directly reused.With regards to the obtained water fraction, the resulting water fraction comprises, if any, at least 90% less any cadmium content than the effluent stream, at least 90% less any zinc content than the effluent stream, at least 90% less any lead content than the effluent stream, at least 50% less any sterol and / or sterylester content than the effluent stream, at least 50% less any phenols and / or terpenoide content than the effluent stream, and at least 50% less any resin acid content than the effluent stream.Thus, the water fraction will in most cases be possible to release into nature without further processing.Alternatively, the water fraction may be recirculated to the pulp mill.In fact, there are several advantages for recirculating the water fraction to the kraft pulp mill. Due to efficiency of the EC treatment, the residence time of the wastewater flow in the fractionation process may be very short, such as maximum 3 hours, such as maximum 2 hours, such as maximum 1 hour (or from 30 minutes to 3 hours, such as between 1 hour to 2 hours). If so, the temperature of the recirculated water fraction after fractionation may keep a temperature of at least 40 °C, or at least 30 °C or at least 20 °C, or at least 10 °C, preferably more than 4 °C. Thus, the heat energy of the effluent will also be recovered when the water fraction is recirculated to the kraft pulping process.However, it is likely that the water has a salt concentration that is not optimal for recirculating the water fraction to the kraft pulp mill as fresh water, especially if the water contains a high concentration of chloride ions.Thus, the process may further comprise a step of treating the water fraction (secondary effluent stream) using reverse osmosis (RO). This results in an additional brine fraction comprising a concentrate of any salt ions present in the water fraction, such as any sodium, calcium, potassium, sulfate, carbonate and chloride ions.The water fraction may then be recirculated to the pulp mill, to be used as fresh water.The brine fraction may be evaporated to become road salt, or released to a recipient, such as to the sea.Alternatively, the brine fraction may be recirculated to the pulp mill, and used as a liquid for leaching and dissolving boiler fly ash from the recovery boiler, resulting in a concentrate mainly consisting of non-toxic salts but also small amounts of toxic transition metal ions e.g. cadmium, arsenic, mercury, lead and zinc.The concentrate can then be purified in a new, second EC process. If so, the transition metal ions are separated as a new floe ("Solid floe") while a stronger liquor of sodium, calcium, potassium, sulfate, carbonate and chloride is obtained as a residue (“lye”).It was found that the second EC could have either aluminium or iron as the electrode material during the electrocoagulation process. One could also consider using iron or aluminium together.The voltage used in the electrocoagulation process is 100 V or less, such as 50 V or less, such as 25 V or less, such as 10 V or less, preferable between 100 to 5 V, such as between 50 to 10 V.The resulting stronger liquor can in turn be electrolysed, whereby the chloride ions are removed from the water and converted to chlorine gas. The gas phase can thus be separated and made use of e.g. for preparation of chlorine dioxide used for bleaching in the mill or sold externally.The remaining salt solution ("Salt solution") can then be added to the black liquor going to the evaporation in order to reduce the need for make-up chemicals as both sodium and sulfur are needed for this purpose. Calcium and carbonate are taken care of in the causticization process and do not pose a problem in the mill. The potassium content will build up over time but is not a problem as potassium-based kraft pulping is similar to sodium-based and if anything even more efficient.Should the method of the invention encompass all of these steps (EC and recirculation of solids fraction, biological treatment, reverse osmosis and recirculation of water and brine fraction, second EC and electrolysis, the kraft pulp mill effluent would be fractionated into the following fractions:1. Phosphorus-rich lime as fertilizer (in accordance with SE545642) by refining the EC floe.2. Energy from combustion of EC floe as part of the refining3. Biosludge for soil improvement purposes4. Raw water for the mill5. Energy provided by the warm raw water recirculation6. Sodium sulfate as a make-up chemical for the process via the use of the RO concentrate as water for dissolving the boiler fly ash and the later EC treatment of this leachate7. Chlorine gas as a raw material for chlorine dioxide preparation and / or hydrochloric acid productionThereby, the method brings the pulp mill closer to a closed mill in terms of wastewater.In the claims, the term "comprises / comprising" does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and / or advantageous. In addition, singular references do not exclude a plurality. The terms ”a “, ”an“, “first”, “second” etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.ReferencesKhan, Z. M.; Hall, E. R.; Khan, M.; Role of secondary sludge in the removal of phytosterols during secondary wastewater treatment, J. Environ. Eng. 2015, 141 Kostamo, A.; Holmbom, B.; Kukkonen, J. V. K.; Fate of wood extractives in wastewater treatment plants at kraft pulp mills and mechanical pulp mills, Wat. Res. 2004, 38, 972-982Mahmood-Khan, Z.; Hall, E. R.; Biological removal of phyto-sterols in pulp mill effluentsZahid Mahmood-Khan, J. Env. Management 2013, 131, 407—414 Vepsäläinen, M.; Electrocoagulation in the treatment of industrial waters and wastewaters, VTT Science Dissertation 19, University of Jyväskylä 2012Vidal, G.; Gonzålez, Y.; Pina, B.; Jarpa, M.; Gomez, G.; Minimization of environmental im-pact of kraft pulp mill effluents: Current practices and future perspectives towards sustainability, Sustainability 2021, 13, 9288

Claims

1. A process for fractionation of an effluent stream from a pulp mill, being a continuous combination treatment comprising the steps oftreating a mainstream of the effluent with electrocoagulation (EC), producing a primary effluent and a floe fraction which is separated from the primary effluent, biologically treating the primary effluent in one or several biofilm reactors, and producing a secondary effluent and a biosludge fraction which is separated from the secondary effluent,thereby the effluent stream is fractionated intoa water fraction, comprising said secondary effluent,a compact solids fraction, comprising the separated floe fraction, and a biosludge fraction, comprising said separated sludge fraction.

2. The process according to claims 1, wherein the effluent stream from a pulp mill is or comprises the wastewater from the debarking area and / or bleaching effluent.

3. The process according to any one of claims 1 to 2, wherein the compact solids fraction comprises the greater part ofany transition metals removed from the effluent stream in the process, and any sterol, sterylester, phenol, terpenoide removed in the purification process.

4. The process according to any one of claims 1 to 3, compact solids fraction with a positive calorific value (CV), thereby making it suitable for incineration.

5. The process according to any one of claims 1 to 4, wherein the compact solids fraction is recirculated to the pulp mill, and added to a black liquor flow, or preferably to a medium-thick liquor flow, in the evaporation process for co-purification.

6. The process according to claim 5, wherein any phosphorus in the recirculated compact solids fraction will reach the lime kiln, where it can be ejected and recovered as a phosphorus rich lime with low Cd / P quote.

7. The process according to claim 4, wherein the positive calorific value (CV) is higher heating value (HHV) measured using a bomb calorimeter or a lower heating value (LHV) estimated using Tanner diagram.

8. The process according to any one of claims 1 to 7, wherein the EC treatment uses of iron (Fe) electrodes.

9. The process according to any one of claims 1 to 8, wherein the voltage used in the electrocoagulation process is 100 V or less, such as 50 V or less, such as 25 V or less, such as 10 V or less, preferable between 100 to 5 V, such as between 50 to 10 V.

10. The process according to any one of claims 1 to 9, wherein the biological treatment is anaerobic treatment, aerobic treatment or a combination of both anaerobic and aerobic treatment steps.

11. The process according to any one of claims 1 to 10, wherein the biological treatment biofilm reactors are selected from Membrane Biofilm Reactors (MBR), Moving Bed Biofilm Reactors (MBBR), Rotating Biological Contactors (RBC) or Sequential Batch Reactors (SBRs).

12. The process according to any one of claims 1 to 11, wherein the biological treatment biofilm reactors are Membrane Biofilm Reactors (MBR).

13. The process according to any one of claims 1 to 12, wherein the biosludge fraction comprises primarily organics, water and phosphorus, resin acids, making it suitable for use as a soil improvement product.

14. The process according to any one of claims 1 to 13, wherein phosphorus is added to the primary effluent during biological treatment.

15. The process according to claim 1 or 14, wherein said water fraction comprises, if any,at least 90% less of any cadmium content than the effluent stream, at least 90% less of any zinc content than the effluent stream, at least 90% less of any lead content than the effluent stream, at least 50% less of any sterol and / or sterylester content than the effluent stream,at least 50% less of any phenols and / or terpenoide content than the effluent stream, andat least 50% less of any resin acid content than the effluent stream.

16. The process according to any one of claims 1 to 15, wherein the water fraction is recirculated to the pulp mill.

17. The process according to any one of claims 1 to 16, wherein the residence time of the wastewater flow in the fractionation process is maximum 3 hours, such as maximum 2 hours, such as maximum 1 hour, preferably from 30 minutes to 3 hours, such as between 1 hour to 2 hours.

18. The process according to any one of claims 1 to 17, wherein the temperature of the recirculated water fraction is at least 40 °C, or at least 20 °C or at least 10 °C.

19. The process according to any one of claims 1 to 18, further comprising a step of treating the water fraction (secondary effluent stream) using reverse osmosis (RO), further resulting in a brine fraction comprising a concentrate of any salt ions present in the water fraction, such as any sodium, calcium, potassium, sulfate, carbonate and chloride ions and a pure water fraction (tertiary effluent stream).

20. The process according to claim 19, wherein the brine fraction is evaporated to become road salt, released to a recipient, such as to the sea.

21. The process according to claim 19, wherein the pure water fraction is recirculated to the pulp mill, to be used as fresh water.

22. The process according to claim 21, wherein the brine fraction is recirculated to the pulp mill, and used as a liquid for leaching and dissolving boiler fly ash from the recovery boiler, resulting in concentrate.

23. The process according to claim 22, wherein the resulting concentrate is treated using with electrocoagulation (EC), producing a solid floe and second brine fraction.

24. The process according to claim 23, wherein the resulting second brine fraction is treated using electrolysis, resulting in a gas phase comprising chloride gas and a salt solution.

25. The process according to claim 24, wherein the resulting salt solution is used as make-up chemicals during the evaporation step in the kraft pulp mill.

26. The process according to any one of claims 1 to 5, wherein the footprint required for the purification process is less than 2 m2 per m3 of treated flow per hour, such as less than 1 m2 per m3 of treated flow per hour.