WO2021061402A1

WO2021061402A1 - Water treatment system with membrane bioreactor, electrodialysis and reverse osmosis

Info

Publication number: WO2021061402A1
Application number: PCT/US2020/049924
Authority: WO
Inventors: Neil Edwin Moe; Harikrishnan Ramanan; Seng Yong GOH; Girish GUNASHEELA; Ramakrishna Srinivasarao MALLAMPATI
Original assignee: Bl Technologies, Inc.
Priority date: 2019-09-24
Filing date: 2020-09-09
Publication date: 2021-04-01
Also published as: SG10201908838XA

Abstract

A water or wastewater treatment system includes an optional biological treatment system, a filter for example a membrane-based secondary or tertiary filtration system, an electrodialysis system, and a desalination system. The biological treatment system and a membrane-based secondary filtration system may be combined into a membrane bioreactor (MBR). The desalination system may be a reverse osmosis (RO) system. Wastewater is optionally treated biologically and filtered, optionally through a membrane. Filtrate is treated further by way of electrodialysis. Electrodialysis treated water is desalinated. Chemicals, for example a charged surfactant or polyelectrolyte are added to the water being desalinated. Desalination produces a concentrate that is recycled to an electrodialysis concentrate stream. The chemicals are conserved in the desalination concentrate and carried onto the electrodialysis concentrate chambers without passing through the electrodialysis membranes.

Description

WATER TREATMENT SYSTEM WITH MEMBRANE BIOREACTOR. ELECTRODIALYSIS

AND REVERSE OSMOSIS

RELATED APPLICATIONS

[0001] This application claims the benefit of Singaporean Patent Application Serial

No. 10201908838X, filed September 24, 2019, which is incorporated by reference.

FIELD

[0002] This specification relates to water (including wastewater) treatment and to membrane based treatment systems such as membrane bioreactors, electrodialysis and reverse osmosis.

BACKGROUND

[0003] Membrane bioreactors (MBR) are now commonly used to treat wastewater, particularly municipal wastewater. An MBR typically combines a biological process, such as an activated sludge process, with membrane filtration. For example, an MBR may have a tank with immersed membranes in place of the secondary clarifier of a conventional activated sludge plant. The effluent from an MBR is of higher quality than effluent from a conventional activated sludge plant. However, the effluent from an MBR is typically still not suitable for reuse as drinking water or industrial process water. The MBR permeate can be further treated with multi-stage reverse osmosis (RO). However, due to fouling of the RO membranes, water recovery is typically limited to 75-85% with moderate energy and chemical consumption rates.

SUMMARY

[0004] The following summary is intended to introduce the reader to the invention and the detailed description to follow but not to limit or define the claimed invention.

[0005] The specification describes a water treatment system and process. The system and process may be used, for example, to treat brackish water or municipal or industrial wastewater. Some of the treated water may be recovered for used or re-use, for example as irrigation water or industrial process water.

[0006] A system described herein includes an optional biological treatment system, a filtration system, optionally a membrane-based or other secondary or tertiary filtration system, an electrodialysis system, and a desalination system. The biological treatment system and a membrane-based secondary filtration system may be combined into a membrane bioreactor (MBR). Optionally, there may be an additional upstream desalination system between the filtration system and the electrodialysis system. The electrodialysis system may have a basic electrodialysis (ED) unit, or a variant such as an electrodialysis reversal (EDR) or supercapacitive desalination system. Either of the desalination systems may be a reverse osmosis (RO) system. A filtrate (i.e. permeate) outlet from the filtration system is connected to a diluent/feed inlet of the electrodialysis unit, optionally by way of the concentrate side of the optional upstream desalination system. A dilute outlet of the electrodialysis unit is connected to a feed inlet of the desalination unit. A concentrate outlet of the desalination unit is connected to a concentrate inlet of the electrodialysis unit. A chemical addition port is provided in communication with the feed inlet of the desalination unit to add a chemical, for example a charged surfactant or polyelectrolyte too large to pass through membranes of the ED unit, to the desalination unit.

[0007] In a method described herein, water is filtered, for example by way of a sand filter, media filter, surface filter, depth filter or membrane filter such as an ultrafiltration or microfiltration membrane filter. Optionally, wastewater is treated biologically and filtered through a membrane. Filtrate, optionally permeate from a membrane, is optionally pre concentrated and then treated further by way of electrodialysis (ED). ED treated water is desalinated, for example by reverse osmosis (RO). Chemicals, for example a charged surfactant or polyelectrolyte too large to pass through membranes of the ED unit, are added to the ED treated water being desalinated. Desalination produces a concentrate that is recycled to an electrodialysis concentrate stream.

[0008] The system and method described above allows for recovery of, for example, more than 90% of the filtrate. The membrane permeate is optionally concentrated and then treated first by the more resilient ED unit. RO unit operation is facilitated by the removal of divalent cations by the ED. Very high permeate quality is obtained by the RO unit since the ED treated water has significantly reduced the total dissolved solids (TDS). Chemical usage is moderated in part by removing multi-valent ions from the desalination stream and in part by recycling chemicals to the electrodialysis concentrate stream.

[0009] The ED-RO configuration described herein combines the strongest attributes of both technologies and is useful, for example, in treating water containing silica and organic contaminants. ED does not remove silica and many organic species effectively because they do not carry a charge while RO is very good at rejecting silica and many organic species on the basis of their size. On the other hand, ED is less susceptible to fouling than RO and enables the combined process to achieve higher recovery than RO alone. Chemicals such a charged surfactant or polyelectrolyte may be added to the RO feed in order to prevent fouling of the RO membranes. These chemicals are largely concentrated in the RO concentrate stream and are then beneficially sent to the concentrate side of the ED unit where they facilitate the smooth operation of the ED unit. These chemicals may include ones that are not acceptable to dose into the dilute side of an ED unit, for example because they are charged but too large to pass through membranes of the ED unit, but are acceptable to dose into the concentrate side of an ED unit. For example, certain biocides, dispersants or antisealants contain charged ionic surfactants or polyelectrolytes that may foul the ion exchange membranes if dosed on the dilute side of an ED unit. In one example, a silica dispersant or anti-foulant is used and allows for a recovery rate of 90% or more or 93% or more or 95% or more when treating wastewater containing silica.

BRIEF DESCRIPTION OF THE FIGURE

[0010] The Figure is a schematic process flow diagram of a wastewater treatment system.

DETAILED DESCRIPTION

[0011] The Figure shows a wastewater treatment system 10. The system 10 receives wastewater 12. The wastewater 12 may be, for example, municipal wastewater (i.e. sewage) or industrial wastewater. Optionally, the wastewater 12 may be pre-treated, for example by coarse screening to remove large objects and primary clarification to divert some of the suspended solids in a primary sludge stream. Primary sludge may be treated further, for example in an anaerobic digester, composting system or by land application.

[0012] The system 10 also includes an MBR 14. The MBR 14 may be configured, for example, according to an activated sludge process. In this case, the MBR 14 has one or more process tanks containing mixed liquor with microorganisms that digest one or more components of the wastewater. Activated sludge including microorganisms is in part retained in, or recycled to, the process tank or tanks by a filtration membrane. The filtration membrane may be, for example, a microfiltration or ultrafiltration membrane. Some of the activated sludge is wasted from the process but may be treated further, for example in an anaerobic digester, composting system or by land application. In other examples, a different biological process using, for example, an anaerobic digester, upflow anaerobic sludge blanket, sequencing batch reactor, granules or other reactor may be integrated with membrane filtration to produce a different type of MBR to be used in place of the MBR 14. In other examples, a tertiary membrane filter can be added to filter the effluent of a biological process without a membrane, for example a conventional activated sludge process. In other examples, there might be no biological process and the wastewater 12 might be replaced another type of feed water, for example seawater brackish water or well water. In other examples, the wastewater 12 or other feed water might be filtered with a non-membrane filter for example a sand filter, media filter, surface filter or depth filter.

[0013] The MBR 14 produces MBR permeate 16. Optionally, MBR permeate 16 is concentrated, for example by passing it though the feed/concentrate side of an upstream reverse osmosis unit. MBR permeate 16, optionally pre-concentrated, is treated further in electrodialysis unit 18. Electrodialysis unit 18 produces electrodialysis product 20. Electrodialysis product 20 is treated in desalination unit 22. The electrodialysis unit 18 removes, among other things, multivalent ions from the MBR permeate 16 such that electrodialysis product 20 can be treated at a higher recovery rate in desalination unit 22 without excessive scaling or fouling.

[0014] In some examples, desalination unit 22 includes one or more reverse osmosis

(RO) membranes. The RO membranes may be provided in a single-pass with or without concentrate recycle or, optionally, in other configurations.

[0015] Desalination unit 22 produces an effluent 24. Effluent 24 may be re-used, for example as irrigation water or industrial process water. Effluent 24 typically does not require any further treatment. However, some applications, for example industrial processes requiring ultra-pure water, may require further polishing of the effluent 24.

[0016] Chemicals 28 are added to the electrodialysis product 20, which becomes feed water to the desalination unit 22. The chemicals 28 thereby enter the desalination unit 22. The chemicals 28 can include, for example, one or more dispersants, one or more biocides, one or more anti-scaling compounds and/or other chemicals useful to improve the operation of the desalination unit 22. The chemicals 28 may include, for example, charged surfactants or polyelectrolytes that do not pass through the membranes in the electrodialysis unit 18. [0017] The desalination unit 22 also produces a desalination concentrate 26.

Desalination concentrate 26 is recycled to the electrodialysis unit 18. Because of the substantial rejection of nearly all solid and dissolved compounds in the desalination system 22, the chemicals 28 are substantially conserved in the desalination concentrate 26. The chemicals 28 enter the electrodialysis unit 18 with the desalination concentrate 26. .

[0018] The electrodialysis unit 18 has a plurality of membranes 30 separating the

MBR permeate 16 from electrodialysis concentrate 32. Although only a single membrane 30 is shown in the schematic Figure, an electrodialysis unit 18 typically has a stack with many anion exchange membranes and cation exchange membranes. MBR permeate 16 is split into many parallel sub-streams that each flow through a space between an anion exchange membrane and a cation exchange membrane. Negatively charged species in the MBR permeate 16 migrate through anion exchange membranes into a concentrate sub-stream and positively charged species in the MBR permeate 16 migrate through cation exchange membranes into a concentrate sub-stream. The EDR concentrate 32 is made up of the EDR concentrate sub-streams. The EDR product 20 is made up of the MBR permeate 16 sub streams that have had charged species removed from them. Although the simplified Figure shows the EDR concentrate 32 being wasted, a portion of the EDR concentrate may be recycled through the concentrate side of the electrodialysis unit 18. In this case, some or all of desalination concentrate 26 enters the electrodialysis unit 18 via a concentrate makeup inlet in a concentrate recycle loop.

[0019] In the electrodialysis unit 18, ions flow from the MBR filtrate 16, through the membranes 30, to the electrodialysis concentrate 32. In particular, multivalent cations are preferentially transported across the membranes 30 due to their larger charge and a lack of fouling of the cationic exchange membranes. Though not shown in the schematic Figure, some of the electrodialysis concentrate 32 is typically recycled through the concentrate side of the electrodialysis unit 18. The recycle of desalination concentrate 26 to the circulating electrodialysis concentrate 32 increases water recovery by avoiding the need to use some of the MBR filtrate 16 as a source of concentrate make up water. The desalination concentrate 26 increases the ionic strength of the electrodialysis concentrate 32. Electrodialysis concentrate 32 has high concentrations of monovalent and multivalent ions and may be discharged or, optionally, sent to a crystallizer and/or precipitation reactor to recover additional water. [0020] The chemicals 28 may be useful in reducing scaling or other issues in the electrodialysis concentrate 32. In at least some cases, the addition of a chemical 28 to the electrodialysis concentrate 32 is preferable to the additional of the chemical 28 to the MBR filtrate 16. For example, addition of the chemical 28 to the electrodialysis concentrate 32 may avoid passage of the chemical 28 through a membrane 30, avoid fouling of membranes 30 by the added chemical and/or provide a better environment for use of the chemical 28. [0021] Chemicals 28 may be, for example, a dispersant, biocide and/or antisealant that can help manage scaling and fouling in one or both of the electrodialysis unit 18 and the desalination unit 22. Dosing chemical 28 into electrodialysis product 20 helps achieve a high effluent 24 to MBR permeate 16 recovery rate with low chemical consumption. For example, MSI 410 (a HYPERSPERSE(TM) antisealant or antifoulant sold by Suez Water Technologies and Solutions) is efficient for inhibiting silica scaling but not compatible with some ED membranes. However, when dosed in the concentrate stream, the interactions with the ED membranes are hindered by high salt content and the organics in the water as well as the orientation of the electric field. Similarly, many other biocides, antifoulants or anti-sealants could not be used in ED feedwater. However, the process configuration described herein allows for the efficient use of these chemicals, which in turn enable the system to operate at high recoveries (i.e. 90% or more, 93% or more or 95% or more).

[0022] Without intending to be limited by theory, membrane fouling may be caused by organic substances (such as humic acid and biopolymers) and bivalent ions (such as Ca²⁺, Ba²⁺, HP04² and S04^2_). However, electrodialysis units are resistant to fouling in membrane filtered waste water. Electrodialysis removes most of the ions and low molecular weight charged organic substances that microfiltration and ultrafiltration membranes (as used for example in an MBR) cannot remove. Electrodialysis therefore reduces fouling or scaling in a downstream desalination process such as RO and allows the desalination process to operate at higher recovery rates. RO permeate quality can be higher than drinking water quality that can often be used for specific industry needs. Chemical consumption can be reduced by recycling chemicals from a desalination unit to the concentrate chambers of an electrodialysis unit.

[0023] Surprisingly, it appears that chemicals are not materially de-activated or depleted in the desalination process. One or more chemicals optionally pass only through the electrodialysis concentrate stream so that the membrane performance, efficiency and life time will not be affected, or be less affected, by these chemicals. Total system chemical consumption may also be reduced. In contrast, allowing one or more of these chemicals to pass through the feed/diluent streams could damage ion exchange membranes and deteriorate the overall product water quality. Chemical addition in only the concentrate stream reduces the chemical consumption and improved overall system performance simultaneously.

[0024] In some examples, the RO permeate or other final effluent may be of very high water quality (50 ppm TDS, 0.5 ppm S04² , 10 ppm HCO³ , 10 ppm Ch, 1 ppm Ca²⁺, 1 ppm Mg²⁺, 10 ppm Na⁺, 0.5 ppm silica, 0.1 ppm TOC) and can be used for make up water in the production of ultrapure water or various industrial needs. In some examples, adequate plant effluent quality can be achieved with the desalination unit 22 treating only some of the electrodialysis product 20, the plant effluent in this case being a blend of effluent 24 and electrodialysis product 20.

Claims

CLAIMS: We claim:

1. A water treatment system comprising a filtration system having a filtrate outlet; an electrodialysis system having a feed/diluent inlet, a dilute outlet, a concentrate inlet and a concentrate outlet; and, a desalination system having a feed inlet, an effluent outlet and a concentrate outlet, wherein the filtrate outlet is connected to the diluent/feed inlet of the electrodialysis unit; the dilute outlet of the electrodialysis unit is connected to a feed inlet of the desalination unit; and, a concentrate outlet of the desalination unit is connected to a concentrate inlet of the electrodialysis unit.

2. The system of claim 1 having a chemical addition port is in communication with the feed inlet of the desalination unit wherein the chemical addition port is connected to a supply of a charged surfactant or polyelectrolyte.

3. The system of claim 1 or 2 wherein the biological treatment system and the membrane-based secondary filtration system are combined into a membrane bioreactor.

4. The system of any of claims 1 to 3 wherein the electrodialysis system is an electrodialysis reversal unit.

5. The system of any of claims 1 to 4 wherein the desalination system is a reverse osmosis system.

6. The system of any of claims 1 to 5 further comprising a biological treatment system upstream of the filtration system.

7. The system of claim 6 wherein the filtration system is a membrane based secondary or tertiary filtration system.

8. The system of any of claims 1 to 7 further comprising a reverse osmosis unit with a feed inlet connected to the filtrate outlet and a concentrate outlet connected to the feed/diluent inlet of the electrodialysis unit.

9. A method of treating water comprising, filtering the water to produce a filtrate; treating the filtrate by way of electrodialysis; desalinating the electrodialysis treated water; and, recycling a desalination concentrate as an electrodialysis concentrate stream.

10. The method of claim 9 comprising adding one or more chemicals to the electrodialysis treated water being desalinated.

11. The method of claim 9 or 10 wherein the one or more chemicals comprise a charged surfactant or polyelectrolyte.

12. The method of any of claims 9 to 11 wherein the step of desalinating comprises reverse osmosis.

13. The method of any of claims 9 to 12 wherein 90% or more, or 95% or more, of the permeate is recovered as a desalination product.

14. The method of any of claims 9 to 13 further comprising concentrating the filtrate before treating the filtrate by way of electrodialysis.

15. The method of claim 14 wherein the filtrate is concentrated by reverse osmosis.