Filtration process following thermochemical treatment of biomass

Abstract

Treatment of an aqueous fraction from a thermochemical treatment of biomass, e.g. process water from hydrothermal carbonisation, includes a membrane filtration step 30 after pre-treatment with an adsorption step (3, Fig. 2). The adsorbent may be a macroporous resin or activated carbon, and the membrane may be semi-permeable, e.g. a nanofiltration membrane (4, Fig. 2). Effluent from the thermochemical treatment, may be separated into a solids-rich fraction (10, Fig. 2) and an aqueous fraction (11, Fig. 2) by mechanical filtration (2, Fig. 2), sedimentation, or hydrocycloning. The method may also comprise a fermentation and / or phosphorus recovery step using the treated effluent. A device for performing the method is also claimed, with at least one filtration system 30, a particulate adsorption device 26 upstream of the filtration system(s), and at least one pump 38 either upstream of the adsorption device or between the adsorption device and the filtration system(s).

Description

Technical field The present invention relates to a method to treat the process water from a hydro-thermal (thermochemical) treatment of biomass, in particular its carbonisation. Background art The thermochemical treatment of biomass at temperatures of 175-275°C, <90 bar, for >2 hours, is known, e.g., from DE 10 2007 056 170 Al of one of the inventors. This treatment, sometimes termed “Hydrothermal carbonisation, HTC or wet torrefaction”, typically results in a slurry of largely carbonised solid matter dispersed in an aqueous phase. There may be a coarse separation of solid and liquid components, the latter still comprising substantial amounts of organic matter. It is further known from DE 10 2012 002 590 Al to employ nanofiltration to further treat the HTC effluent, often termed “process water” because part thereof actually stems from the processed biomass. Still further, it is known from DE 10 2009 053 867 Al to supply the product from an HTC process to a subsequent fermentation or anaerobic digestion process. Document EP 2 746 231 Al discloses another method and apparatus for the treatment of process water from a hydrothermal organic material conversion process, including a first separation step and a second, membrane filtering step by way of a reverse osmosis membrane filter. Similarly, document DE 10 2012 002 590 Al discloses treating process water obtained from a hydrothermal carbonization process by supplying the process water under pressure into a membrane. Document JP 2006-239617 A discloses passing water to be treated through a photocatalytic reaction column, an activated carbon column, an ion exchange resin column, and a reverse osmosis membrane device, in series. Document US 2012 / 0000851 Al discloses minimizing membrane fouling by circulating feed water past the membranes, while periodically varying the pressure differential across the membranes. Document DE 3543661 Al discloses continuously feeding water which is to be processed to an exchanger column with various types of ion exchangers, and then fed to the reverse osmosis device. Summary of the invention The filtration of HTC process water causes significant fouling and so it is sometimes not industrially viable to conduct (nano)filtration without some form of pre-treatment of the water. This has been shown in Crossley etal. (2020)* \ in which the treatment of HTC process water with nanofiltration was studied. It has been found that adsorption can remove the components in the water that cause the fouling, and so allow the flux through the filtering membrane to remain at a useful level. Adsorption onto activated carbon works but is not specific / selective to the fouling components; therefore, essentially all organic matter is adsorbed on activated carbon, removing also valuable substances from the liquid phase, and making it very cumbersome to regenerate the activated carbon once its capacity is exhausted. On the other hand, there are examples of commercially available macroporous resins that selectively remove only, or largely only, or mainly the hydrophobic fraction from water samples. It is this hydrophobic (and coloured) fraction that causes both reversible and irreversible fouling that prevents the efficient filtration of the water. This effect of the resin on nanofiltration permeate flux can be seen in the attached Fig. 1. Suitable resins are divinylbenzene (DVB )-crosslinked polystyrene (PS) resins or polymethylmethacrylate (PMMA) resins, in the shape of globular or spherical beads of, e.g., from 0.1 to 2 mm hydrodynamic (“Stokes”) diameter (i.e., the diameter of an idealised sphere having the same settling speed in water as the dispersed particle). The present invention solves the above mentioned problems by providing the methods of claim 1 and 17, the device of claim 13, and the use of claim 15. The adsorbing of hydrophobic substances before fine filtering the HTC effluent largely avoids membrane fouling, and thereby enables a continuous process. Advantageous modifications are defined in the dependent claims. In particular, the adsorbent is preferably a macro-porous adsorbent specific for hydrophobic matter. The membrane filtration may utilise a semi-permeable membrane. This membrane may provide for an observed solute ion rejection coefficient, Robs, of >90%, for the rejection of phosphorus, P. A second filtration step may then be performed that utilises a semi-permeable membrane with a smaller molecular weight cut-off, and hence higher observed rejection coefficient, than the first filtration step. In a preferred embodiment, the effluent from the HTC, after coarse solid / liquid separation, is acidic when undergoing the adsorption step. In particular, the pH is below 6.5, below 5.75, below 5.0, or between 4.0 and 4.75. According to one embodiment, mechanical filtration, sedimentation, or hydrocycloning is used to separate the solid-rich and aqueous fractions of the products of the thermochemical treatment in advance of the pre-treatment. The apparatus of present invention accordingly includes at least one or two (fine-) filtration devices, an adsorption device upstream thereof, and a pump positioned either upstream of the latter or between same and the (fine-) filtration device(s). Another (coarse-) solid / liquid separation device may be arranged upstream of the adsorption device, and preferably also upstream of the pump. Subsequent to the above HTC effluent treatment including an adsorption step and at least one filtering step, the present invention may comprise a separate step of phosphorus (P-) recovery; and / or a subsequent fermentation (anaerobic digestion). Preceding the above HTC effluent treatment including an adsorption step and at least one filtering step, the present invention may comprise a hydrothermal carbonisation at temperatures of 175°C to 275°C, a pressure of below 90 bar, and for more than 2 h. Brief description of drawings The present invention is exemplarily, and not limitatingly, explained referring to the attached drawings, of which Fig. 1 shows a diagram illustrating the dependence of flow resistance on cumulative filtered volume for various adsorbents, Fig. 2 shows a diagram illustrating an inventive post-treatment process, and Fig. 3 schematically shows a device for adsorption and filtering in sequence. Detailed description of the embodiments As is shown in Fig. 1, if untreated process water from an HTC process is passed through a given nanofilter, the flux rather rapidly drops from its initial value (see: control). PreAreated HTC process water, be it with activated carbon or with a resin adsorbent, maintains a high flux ratio of >85% for very large cumulative volumes. Incidentally, in the diagram of Fig. 1, the quantity Jo is the initial flux, and J is the flux after a cumulative volume (measured in L / m ) has passed through the filter. An embodiment of the inventive process is schematically shown in the diagram of Fig. 2: The mixture of biomass and water 8 enters into an HTC reactor 1, is processed at, e.g., 200°C and a pressure of 20 to 25 bar for five hours, and the effluent 9 (HTC slurry) is cooled or allowed to cool to about room temperature, and then enters into a mechanical filtration (or other separation) 2. The solid fraction 10 thereof (dry matter content of 30% or more) is the desired product of the HTC, and may be dried and pelletised, or otherwise prepared for further use, such as for burning it to produce energy. The aqueous fraction 11 (pH <6) is fed into the adsorption column 3 containing a DVB crosslinked PS resin to remove the hydrophobic fraction, and its effluent 12 is introduced into a filtration rig containing a semi-permeable membrane, such as a NF270 nanofiltration membrane 4, where a hydraulic pressure of 15 bar is applied. The permeate 14 from this process is sent to a second filtration rig containing another semi-permeable membrane with a smaller molecular weight cut-off (MWCO), such as a BW30 reverse osmosis membrane 5, where again a hydraulic pressure of 15 bar is applied. The permeate of the reverse osmosis 16, essentially clean water, has a much reduced organic and inorganic solute content and may be discharged, or returned to the preparation stage 8 of the HTC if needed. The concentrated retentate 13 that does not permeate through the membrane 4 is sent to a phosphorus recovery system 6, where the pH is increased, and any necessary reactants are added to allow the precipitation of a high-quality phosphorus product, which is separated and removed 17. The supernatant of this process 18 is mixed with the concentrated solution that does not permeate through the reverse osmosis membrane 15 and this mixture is treated in an anaerobic digestion stage 7 where organic material is digested to produce biogas 19 (predominantly methane), and the watery effluent 20 of which is also discharged. Although not shown in detail in Fig. 2, the resin contained in the adsorption column will become saturated after treatment of a given volume of inflow. At this point, the resin will be separated, rinsed with suitable fluids such as water, ethanol, isopropanol, or acetone, then optionally dilute NaOH solution, and then dilute HC1, and then will be put back online. The performance of the membranes in 4 and 5 will decrease over time due to membrane fouling. Cleaning-in-process (CIP) will be used periodically to return the permeation flux characteristics to close to that of the virgin membrane, such as whenever the flux characteristics decline to below 80% of that of the virgin membrane, under the prevalent conditions. In this specification, by “hydrophobic fraction” it is meant that portion of dissolved organic carbon (DOC) that sorbs to the resin at pH 2 with a capacity factor (kO) of 50, where kO is defined as the mass of a solute sorbed on the DAX-8 resin divided by the mass of the solute in column void volume (Aiken et al. 1992*2). The portion of DOC that passed through the DAX-8 resin at pH 2 is generally termed as the hydrophilic fraction. DAX-8 resin is commercially available from SigmaAldrich as Supelite™ An apparatus 21 for performing coarse separation, adsorption and fine-filtering in sequence is shown in Fig. 3. The effluent 22 of a preceding hydrothermal or thermo-chemical process is optionally subjected to some kind of coarse separation 24, such as sedimentation or mechanical filtration, such as by means of a filter press. The supernatant is then introduced into a column 26 partially filled with a suitable adsorbent 18, which may be in the form of globular beads; and then passes through a fine-filtering or nano-filtering membrane 30. From time to time, the adsorbent will have to be regenerated in place, or including exchanging the adsorbent altogether. Some particulate matter will nevertheless accumulate over time among and above the adsorbent, and it will be advisable to intermittently back-wash the adsorption column by introducing water through a valve from below (not shown) and discharge the washing water through another pipe 32. In an embodiment, this washing water is returned to the HTC process. The coarse separation is illustrated in Fig. 3 as a funnel with a discharge port at the bottom for drawing off the still moist sludge through a drain pipe 34, but may take any shape and use any principle; e.g., a hydrocyclone is equally suitable in some applications, or the filter press mentioned above. The filter column 26 has an outlet for the post-treated effluent 36, which may be employed for a subsequent treatment step, such as recovery of phosphorus and / or a step of anaerobic digestion (fermentation). In a variant, the adsorbent is occasionally replaced (not shown) with activated carbon or char from the preceding hydrothermal or thermochemical process. In some embodiments, it is preferred to use a specifically hydrophobic macro-porous resin as the adsorbent. This may be in the shape of generally globular or spherical beads having a mean diameter of, e.g., 0.2 to 1.2 mm. It may be noted that at rest, the resin globules preferably do not entirely fill the filter column housing 26 in order to allow for considerable expansion (25 to 75%) during back-washing. In an embodiment, a suitable pump 38 is positioned upstream of the filter column 26. In an alternative embodiment (not shown), the adsorbent and the filter membrane are spatially separated, and the pump is located therebetween. If there are (at least) two filtering membranes in succession, the first of which is immediately adjacent the adsorbent, the pump may alternatively be positioned between the filtering devices. Ideally, the adsorption step removes from about one third to about one half of the total organic carbon present in the effluent from the preceding process (HTC). More specifically, percentages of between 35% and 50% are suitable with resin adsorbents. More importantly, however, the hydrophobic fraction should be removed essentially completely, or at least to more than 90% by weight. As described above, following the adsorption step, there may be a nanofiltration step and then a reverse osmosis step of the nanofiltration permeate; but in some embodiments, only one filtration suffices. The nanofiltration is advantageous for the concentration of nutrients including, but not limited to phosphorus, into the nanofiltration retentate, for their efficient recovery. Thereafter, anaerobic digestion may take place on the concentrated retentate streams, if so desired. The permeate of the reverse osmosis is essentially clean water, and may be discharged or recycled in the preceding hydrothermal process. Naturally, however, other uses are also possible. *1 Non-patent literature: O. P. Crossley etal., “Phosphorus recovery from process waste water made by the hydrothermal carbonisation of spent coffee grounds”, Bioresource Technology 301 (2020)122664 *2 Non-patent literature: G.R. Aiken et al., “Isolation of hydrophilic organic acids from water using nonionic macroporous resins', Org. Geochem. 1992, Vol. 18(4), 567^573

Claims

1. A method for the treatment of an aqueous fraction derived from a thermochemical treatment of biomass, characterized in that membrane filtration is performed after pretreatment including an adsorption step of removing, with an adsorbent, such products of the thermo-chemical treatment in the aqueous fraction that cause the fouling.

2. The method according to claim 1, wherein the adsorbent is a resin.

3. The method according to claim 2, wherein the resin is a macroporous resin.

4. The method according to claim 1 or 2, wherein the adsorbent is activated carbon stemmingfrom the thermochemical treatment of the biomass.

5. The method according to one of claims 1 to 4, wherein the membrane filtration utilises a semi-permeable membrane.

6. The method according to claim 5, wherein the semi-permeable membrane provides an observed solute ion rejection coefficient, Robs, of >60%, preferably >70%, more preferably >80%, and most preferably >90% for phosphorus, P.

7. The method according to one of claims 1 to 6, wherein a second filtration step is performed on the permeate of the first filtration step that utilises a semi-permeable membrane with a smaller molecular weight cut-off, MWCO, than the first filtration step.

8. The method according to one of claims 1 to 7, wherein the thermochemical treatment of biomass occurs below 90 bar, at 175°C to 275°C, and / or for at least two hours.

9. The method according to one of claims 1 to 8, wherein mechanical filtration, sedimentation, or hydrocycloning is used to separate the solid-rich and aqueous fractions of the products of the thermochemical treatment.

10. The method according to one of claims 1 to 9, wherein the aqueous fraction undergoing the adsorption step is acidic.

11. The method according to one of claims 1 to 10, further comprising a fermentation step employing the concentrated fractions of the filtered effluent.

12. The method according to one of claims 1 to 11, further comprising a step of phosphorus recovery (P^recovery) employing the concentrated fractions of the filtered effluent.

13. A device for carrying out the process according to one of the preceding claims, including- at least one or two filtration systems (30)- a particulate adsorption device (26) upstream of the filtration systems, and- at least one pump (38) in one of the following positions:o upstream of the adsorption device (26), oro between the adsorption device (26) and the filtration systems (30).

14. The device of claim 13, wherein at least one of the filtration systems (30) is a membrane filtration system.

15. Use of the device according to claim 14, to treat the aqueous fraction derived from a thermochemical treatment of biomass by membrane filtration after a pre-treatment with an adsorption step.

16. The use of claim 15, wherein the thermochemical treatment of biomass occurred between 175°C and 275°C and / or below 90 bar.

17. A method of treatment of biomass, comprising a hydrothermal carbonisation process and then the method of one of claims 1 to 12.

18. The method of claim 17, wherein the pH of the treated biomass is reduced by at least 1 unit during the hydrothermal carbonisation process.

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