Process for treating dyeing effluent stream with sodium sulfate recovery

Abstract

After an initial treatment to reduce COD and color, dyeing effluent is treated by membrane filtration, preferably with nanofiltration membranes. The membrane permeate is suitable for reuse for various industrial processes, for example in a textile plant. The membrane reject is rich in sodium sulfate. After optional hardness removal, the membrane reject may be used in a dyeing process. Use of the membrane reject reduces the amount of Glauber's salt required in a textile dyeing process.

Description

PROCESS FOR TREATING DYEING EFFLUENT STREAM WITH SODIUM SULFATE RECOVERYRELATED APPLICATIONS

[0001] This application claims the benefit of, and priority from, Singapore Patent Application No. 10202401623W, filed on June 7, 2024, which is incorporated by reference.FIELD

[0002] This specification relates to systems and methods of treating wastewater, for example dyeing effluent, and to the recovery of sodium sulfate.INTRODUCTION

[0003] Glauber’s salt, also known as mirabilite or sodium sulfate decahydrate, is a hydrous mineral form of sodium sulfate (Na2SO ). Dehydrating Glauber’s salt produces thernadite, an anhydrous sodium sulfate powder. Dissolving Glauber’s salt in water produces a sodium sulfate solution.

[0004] Glauber’s salt is used in textile wet processing during the dyeing process, typically in combination with acid dyes or reactive dyes. Glauber’s salt improves the affinity of the dye towards the fiber and increases the exhaustion rate of reactive dyes.

[0005] Due to the addition of Glauber’s salt, sodium sulfate is a major inorganic component in dyeing effluent, along with color and chemical oxygen demand (COD). Dyeing effluent typically exceeds regulatory limits for COD, color or total dissolved solids (TDS) and cannot be discharged to surface water or groundwater.

[0006] In a dyeing effluent treatment process used in some textile plants, dyeing effluent is dosed with a cationic polymer and treated in a membrane bioreactor (MBR). The cationic polymer assists with removing color and the MBR reduces COD. However, the MBR filtrate still has unacceptable concentrations of total dissolved solids (TDS), most of which is sodium sulfate. At some sites, the MBR filtrate is treated by reverse osmosis (RO). RO permeate may be discharged or reused in the textile plant. RO concentrate is also discharged in some jurisdictions. However, the RO concentrate may have over 10,000 ppm TDS, which far exceeds the discharge limits in many jurisdictions, typically around 2100 ppm TDS. In jurisdictions that regulate TDS concentration limits, the RO concentrate may be furthertreated in a zero liquid discharge (ZLD) system. For example, the RO concentrate may be treated by multi-stage RO followed by evaporation and crystallization. The TDS in the RO concentrate is converted into a solid, which is then sent to landfill.SUMMARY

[0007] This specification describes a system and method for treating dyeing effluent. The system and method recover sodium sulfate from the dyeing effluent. The sodium sulfate can be recycled back to the dyeing process. Recycling the sodium sulfate can reduce the amount of Glauber’s salt consumed by the dyeing process.

[0008] In a process, the dyeing effluent is initially treated to produce an intermediate effluent with reduced color and COD. Optionally, the initial treatment may be in a membrane bioreactor (MBR), optionally with color removing chemical or color removing bacteria dosing. Optionally, the initial treatment may be an ion exchange resin treatment. Optionally, the initial treatment may be an ozone treatment. Optionally, the initial treatment comprises both MBR, and an ion exchange resin treatment or an ozone treatment. The intermediate effluent, optionally after softening, is treated by membrane separation, for example nanofiltration, to produce a membrane permeate stream and a membrane reject stream. The membrane reject stream contains sodium sulfate which can be reused in a dyeing process, optionally after softening, to reduce the need for Glauber’s salt. The membrane permeate stream may also be reused, for example in the dyeing process, or in any other application that is tolerant of some monovalent dissolved solids.

[0009] A dyeing effluent treatment system has an initial treatment system and a membrane separation system. The initial treatment system receives dyeing effluent and produces an intermediate effluent, for example by one or more chemical, physical or biological treatment processes. The membrane separation system receives the intermediate effluent and produces a membrane reject. The membrane separation unit may have nanofiltration membranes. The membrane reject may be sent to a textile dyeing unit. The system optionally has a softening unit, optionally between the initial treatment and the membrane separation, or optionally between the membrane separation system and the dyeing unit. In some examples, the initial treatment system includes a membrane bioreactor. In some examples, the initial treatment system includes an ion exchange resin treatment unit, or anozone treatment unit. The membrane bioreactor may include a color removing chemical addition system and / or a color removing bacteria addition system.BRIEF DESCRIPTION OF THE FIGURES

[0010] Figure 1 is a schematic drawing of a system and process for treating dyeing effluent having an MBR and membrane separation, wherein a color removal chemical is used in combination with the MBR.

[0011] Figure 2 is a schematic drawing of a system and process for treating dyeing effluent having an MBR and membrane separation, wherein a color removal chemical and color removing bacteria are used in combination with the MBR.

[0012] Figure 3 is a schematic drawing of an exemplary system and process for treating dyeing effluent.

[0013] Figure 4 is a schematic drawing of a system and process for treating dyeing effluent having an ion exchange resin treatment or an ozone treatment and membrane separation.

[0014] Figure 5 is a schematic drawing of a system and process for treating dyeing effluent having an MBR and ion exchange resin treatment or an ozone treatment, and membrane separation.

[0015] Figure 6 is a schematic drawing of a system and process for treating dyeing effluent having an initial treatment, and hardness removal before membrane separation.DETAILED DESCRIPTION

[0016] In an exemplary system and method, dyeing effluent is collected and treated to reduce COD and color. Optionally, an initial treatment may include adding a color removing chemical, for example a cationic polymer, or a color removing bioculture, in combination with membrane bioreactor (MBR) treatment. Biodegradable COD is removed in the MBR. Optionally, the initial treatment may include an ion exchange resin treatment or an ozone treatment, optionally in combination with membrane bioreactor (MBR) treatment. Accordingly, an interim effluent is provided from the initial treatment. The interim effluent is subjected to further treatment by membrane filters, for example nanofiltration (NF) membranes.

[0017] NF membranes concentrate the divalent salts in the MBR filtrate. The NF permeate may be a low TDS stream of soft water containing monovalent salts. The NF permeate may be reused, for example recycled back to the dyeing process. NF concentrate contains Na2SO4 and may be reused in the dyeing process, optionally after removing hardness (Ca2+& Mg2+) and optionally after adding freshwater and / or top up Glauber’s salt. NF permeate may be used in place of some of the freshwater.

[0018] The system and method allow for resource recovery and recycling of Na2SO4. This results in decreased consumption of fresh Glauber salt in a dyeing process. Furthermore, the system and method provide softened NF permeate for reuse back in the dyeing process or in other applications, optionally without any further treatment.

[0019] In some examples, the process can provide nearly complete color removal in the initial treatment. Subjecting the interim effluent to NF helps to recover optionally up to 95% or more of the sodium sulfate in the dyeing effluent. After optional softening before or after NF, the NF concentrate is reduced in monovalent ions and multivalent cations and is suitable to recycle back to the dyeing process. The process also provides NF permeate, which is soft water, for reuse back in the process, optionally without any further treatment.

[0020] Figure 1 shows a dyeing effluent treatment process 100. The process 100 includes initial treatment step 102. The dyeing effluent is treated first in the initial treatment step 102, which removes color and COD. Any one or more process steps or units known in the art may be used in the initial treatment step 102. In the example shown, the initial treatment step 102 includes treating the dyeing effluent by way of a combination of chemical, biological and physical treatments in a color removal membrane bioreactor (CR-MBR). In the CR- MBR, dyeing effluent is held in an equalization tank before being transferred to an MBR. The MBR includes a biological reactor, typically made up of one or more tanks, and a membrane filtration unit. The membrane filtration unit may be immersed in the biological reactor or provided in a separate membrane tank with a partial recycle of activated sludge to the biological reactor. A color removal chemical is mixed with the dyeing effluent as it travels in a pipe between the equalization tank and the bioreactor. Alternatively, the color removal chemical may be dosed into the equalization tank or the bioreactor. The color removal chemical may be a cationic polymer, for example a polydicyandiamide (PDCDA), optionallycombined with DADMAC. The color removal chemical agglomerates with dye molecules that were released from the dyeing process. In the biological reactor, the agglomerates are degraded by bacteria. The membrane filtration unit may have microfiltration or ultrafiltration membranes. The color removal chemical may be dosed at, for example, 100-500 ppm of PDCDA.

[0021] In experimental examples, the addition of 100-500 ppm PDCDA in an MBR with ultrafiltration membranes (ZeeWeed™ modules by Veolia) had no material effects on membrane fouling, COD or MLSS of the activated sludge. The color removal chemical was below detection limits (2 mg / L) in the MBR filtrate. COD and color in the MBR filtrate were both reduced by the addition of the color removal chemical.

[0022] While not shown, the CR-MBR typically produces a waste activated sludge (WAS) stream. The WAS stream may be treated further, for example by way of one or more of anaerobic digestion, dewatering and drying. Since the process 100 includes a recycle to the dyeing vats (or other dyeing equipment), in addition to removing excess solids the WAS removes minor dyeing effluent contaminants that might otherwise accumulate in the process 100. Removal of minor dyeing effluent contaminants may also occur in a softening step described further below.

[0023] Permeate from the CR-MBR is treated in a membrane separation step 104. Depending on the dyeing effluent, the MBR filtrate may already meet COD and color standards for reuse. However, in some cases involving dyeing effluent with very high color, the MBR filtrate will not be colorless (i.e. with < 150 PtCo). The membrane separation step 104 produces permeate 106 which meets COD and color specifications for reuse. The permeate 106 may be recycled to the textile plant, for example for reuse in dyeing or other textile processing steps.

[0024] The membrane separation step preferably uses nanofiltration (NF) membranes. Alternatively, reverse osmosis (RO) membranes may be used. However, RO membranes are more energy intensive and typically require more cleaning chemicals than NF membranes. RO membranes typically separate more monovalent ions, but in most cases NF permeate has sufficiently low TDS for reuse. In another alternative, a low molecular weight cut off ultrafiltration (LMWCO-UF) membrane is used. In experimental examples,LMWCO-UF membranes produced colorless permeate (i.e. less 150 PtCo), typically less than 50 PtCo, even with dyeing effluents having very high color. However, the rejection of sodium sulfate from LMWCO-UF membranes is less than NF membranes.

[0025] The membrane separation step 104 also produces a reject 108 (alternatively call retentate, concentrate or brine). The reject 108 preferably includes a material part, for example at least 40%, at least 50% at least 60%, or at least 80% of the sodium sulfate that was present in the dyeing effluent.

[0026] The reject stream 108 is optionally treated in a hardness removal step 110, shown downstream to the NF membrane. In an alternate embodiment, the hardness removal step may be located upstream of the NF membranes, at the NF feed stream, not shown. Hardness may be removed, for example, by chemical precipitation (e.g. lime softening) or ion exchange. After hardness removal, the treated water may be reused as a source of sodium sulfate for the dyeing process.

[0027] Figure 2 shows a second dyeing effluent treatment process 112. The second process 112 includes similar steps as described for the dyeing effluent treatment process 100 of Figure 1 . Process steps with the same reference number in Figure 2 are similar to the steps with the same reference number in Figure 1 .

[0028] A second initial treatment step 114 in Figure 2 is similar to the initial treatment step 102 of Figure 1 but with one notable change. The second initial treatment step 114 also involves a CR-MBR. However, the biological reactor of the MBR includes color removing bacteria. The color removing bacteria are one or more species or strains of aerobic or anaerobic bacteria that degrade organic compounds. For color removal, the color removing bacteria typically neutralize the chromophore of the dye. Color removing bacteria may be found in nature or genetically modified. However, since the color removing bacteria are not typically present in the environment in large populations, they are cultured in separate bioreactors and added to the CR-MBR. For example, a broth containing one or more species or strains of color removing bacteria may be added at 1-10 ppm of the CR-MBR biological reactor volume. The color removing bacteria may be added in an initial seed dosage, and optionally in supplemental doses thereafter as required to maintain an effective population.

[0029] In Figures 1 and 2, color is removed in combination with the MBR by way of adding polymers and / or organisms that remove color. Alternatively, the MBR may be operated without enhancements to remove color. Most of the color in the dyeing effluent will be carried over into the MBR filtrate. The NF membrane rejects most of the color such that the NF permeate is still suitable for reuse. In some cases, the NF concentrate can be used with color. Optionally, the color can be removed from the NF concentrate, for example by way of polymers, prior to reusing the NF reject as a source of sodium sulfate.

[0030] Figure 3 shows an exemplary dyeing effluent treatment process 200. The exemplary process 200 is similar to the processes shown in Figures 1 and 2, and the reference numbers have the same meaning as in Figures 1 and 2. The values in Figure 3 are obtained from a combination of measurements, experiments and design calculations.

[0031] The exemplary process 200 as shown is used to treat a typical dyeing effluent 202. The dyeing effluent 202 has about 1500 - 5000 ppm total dissolved solids (TDS) including about 1000 - 3200 ppm sodium sulfate, and a COD of about 500-1500 ppm. In a typical textile plant, the dyeing effluent 202 may have a flow rate of 4.8 to 6 million liters per day (MLD). The dyeing effluent 202 may have 600-700 mg / L biological oxygen demand (BOD); 400-500 mg / L total suspended solids (TSS); pH of 5-10; total chlorides less than 200 mg / L; hardness of 50-150 ppm; and, total silica of 25-75 mg / L.

[0032] The dyeing effluent 202 is collected in an equalization tank. The dyeing effluent 202 is mixed with one or more additives 204. For example, the additives 204 may be added to the equalization tank, to the biological reactor of the CR-MBR, or in a pipe between the equalization tank and the CR-MBR. The additives 204 may include, for example, one or more of a pH adjusting agent, a color removing chemical and color removing bacteria. In the examples shown, the additives 204 include PDCDA.

[0033] The dyeing effluent 202 is treated in an initial treatment step 102, 114 as described above, for example in a CR-MBR. The initial treatment step 102, 114 produces an intermediate effluent 206. In the example shown, the intermediate effluent 206 is an MBR filtrate. The intermediate effluent 206 has a TDS of about 1500-5000 ppm, including about 1000-3200 ppm of sodium sulfate. COD in the intermediate effluent 206 has been reduced to less than 100 ppm. The intermediate effluent 206 is colorless (i.e. < 150 PtCo).

[0034] The intermediate effluent 206 is treated by membrane separation 104 to produce permeate 106 and reject 108. Permeate 106 has a TDS of about 1000-2000 ppm. The permeate 106 is soft water with essentially no color and may be reused in a textile making or dyeing process.

[0035] In the example shown, the membrane separation step 104 uses NF membranes operated at about a 85-90% recovery rate. The reject 108 has a TDS of about 12000-37000 ppm including about 10000-32000 ppm of sodium sulfate.

[0036] The reject 108 is treated by hardness removal 110 to produce recycle stream 208, although the hardness removal may be alternatively conducted on intermediate effluent 206 before NF (not shown). The recycle stream 208 is rich in sodium sulfate and may be reused in the dyeing process. Reusing the recycle stream 208 reduces the amount of Glauber’s salt that is required for dyeing textiles. Optionally, the recycle stream 208 may be mixed with freshwater, freshwater to recycle stream 208, topped up with Glauber’s salt, and used as the primary supply of water for a dyeing process, for example the dyeing process that produced dyeing effluent 202.

[0037] Figure 4 shows another exemplary embodiment of a dyeing effluent treatment process 400, where the initial treatment step 402 is an ion exchange resin treatment or an ozone treatment. As previously explained above, the dyeing effluent is treated first in the initial treatment step 402, which removes color and COD. In the example shown, the initial treatment step 402 includes treating the dyeing effluent by way of an ion exchange resin. In some embodiment, the ion exchange resin treatment may include an anion exchange resin such as a basic anion exchange resin. Any suitable resin may be used, but for example a macroporous, monodisperse, strongly basic (type I) anion exchange resin based on a styrene-divinylbenzene copolymer, such as Lewatit™ 6368, may be used. The ion exchange resin treatment may include a pretreatment step with an oxidizing agent, such as ozone. NF membrane 104, permeate 106, reject 108 and hardness removal 110 are as previously described.

[0038] Figure 5 shows another exemplary embodiment of a dyeing effluent treatment process 500, where the initial treatment step 502 includes a MBR, with or without enhancements (polymers and / or organisms) as previously described, and an ion exchange resin treatmentor an ozone treatment downstream of the MBR. NF membrane 104, permeate 106, reject 108 and hardness removal 110 are as previously described.

[0039] Figure 6 shows another exemplary embodiment of a dyeing effluent treatment process 600, where the initial treatment step 602 includes any one or more of a MBR, with or without enhancements (polymers and / or organisms), an ion exchange resin treatment or an ozone treatment as previously described, and. NF membrane 104, permeate 106, reject 108 and hardness removal 110 are as previously described, but in this embodiments, the hardness removal 110 is upstream of the NF membrane 104.

[0040] In an experimental example, a yarn dyeing effluent has 580 ppm COD and 2370 PtCo. The yarn dyeing effluent is treated in an MBR. The MBR filtrate had 110 ppm COD and 440 PtCo. A color removing chemical addition system was added to the MBR to produce a CR- MBR. The CR-MBR filtrate had 90 ppm COD and 115 PtCo. Filtering the CR-MBR filtrate with a LMWCO-UF membrane (GK8040F membranes by Veolia) produces permeate with 55-60 ppm COD and 20-30 PtCo. The LMWCO-UF permeate meets discharge requirements and meets standards for reuse in the yarn dyeing facility.

[0041] In another experimental example, samples of RO reject were obtained from a textile plant and filtered with an NF membrane to examine the recovery of sodium sulfate. The textile plant treats dyeing effluent by way of a CR-MBR using a color removal chemical. The MBR filtrate is then filtered by RO modules. The RO reject has TDS of about 70,000 mg / L including about 26,000 mg / L of sodium and about 10,000 mg / L of sulfate. COD of the RO reject was about 250 mg / L.

[0042] The RO reject was filtered with a DL1812 NF membrane from Veolia. The feed pressure ranged from 300-340 psi. The recovery rate was about 80%. Flux varied from about 18 LMH to about 10 LMH over a 120 minute period. Sulfate rejection was more than 99%. While NF membranes are typically described as rejecting multivalent ions, sodium rejection was about 50%. Accordingly, a substantial amount of the sodium sulfate is recovered in the NF concentrate.

[0043] In another experimental example, color removal by ozone treatment shows that at 40- 50 ppm of ozone dosage, color was reduced from 1300 pt.co to 55 pt.co, which corresponds to > 95 % color reduction.

[0044] The systems and methods described above are non-limiting examples intended to help provide an enabling description of the invention, which is defined in the following claims.

Claims

CLAIMS:

1. A process for treating dyeing effluent comprising, initial treatment to produce an interim effluent with reduced COD; and, membrane separation of the interim effluent to produce a membrane permeate and a membrane concentrate, wherein the membrane concentrate is reused in a dyeing process and provides a source of sodium sulfate.

2. The process of claim 1 wherein the initial treatment removes some color from the dyeing effluent.

3. The process of claim 1 or 2 wherein the initial treatment includes treating the dyeing effluent in a membrane bioreactor.

4. The process of claim 3 wherein the initial treatment further includes adding a color removing chemical to the dyeing effluent.

5. The process of claim 4 wherein the color removing chemical is a cationic polymer such as PDCDA.

6. The process of any of claims 3 to 5 wherein the initial treatment further includes adding color removing bacteria to the dyeing effluent.

7. The process of any one of claims 1 to 6 wherein the initial treatment includes an ion exchange resin treatment.

8. The process of any one of claims 1 to 6 wherein the initial treatment includes an ozone treatment.

9. The process of any of claims 1 to 8 wherein the membrane separation includes nanofiltration.

10. The process of any of claims 1 to 9 further comprising softening the interim effluent.

11. The process of any of claims 1 to 9 further comprising softening the membrane concentrate.

12. A dyeing effluent treatment system comprising, an initial treatment system and a membrane separation system, wherein the initial treatment system receives dyeing effluent and produces an intermediate effluent, wherein the membrane separation system receives the intermediate effluent and produces a membrane reject, and wherein the membrane reject is transferred to a dyeing unit.

13. The system of claim 12 further comprising a softening unit between the initial treatment and the membrane separation system.

14. The system of claim 12 further comprising a softening unit between the membrane separation system and the dyeing unit.

15. The system of any of claims 12 to 1 wherein the membrane separation unit includes nanofiltration membranes.

16. The system of any of claims 12 to 15 wherein the initial treatment system includes a membrane bioreactor.

17. The system of claim 16 wherein the membrane bioreactor includes a color removing chemical addition system.

18. The system of claims 16 or 17 wherein the membrane bioreactor includes a color removing bacteria addition system.

19. The system of any of claims 12 to 18 wherein the initial treatment system includes an ion exchange treatment unit.

20. The system of any of claims 12 to 18 wherein the initial treatment system includes an ozone treatment unit.

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