WO2019000160A1

WO2019000160A1 - Method for cleaning filtration membrane contained in water treatment system and water treatment system

Info

Publication number: WO2019000160A1
Application number: PCT/CN2017/089978
Authority: WO
Inventors: Bo Yan; Zijun Xia; Hua Wang; Asal AMIRI; Jose Luis PLASENCIA CABANILLAS; Marcelo ANDREOTTI; Lei Cao; Wujun Rong
Original assignee: General Electric Company
Priority date: 2017-06-26
Filing date: 2017-06-26
Publication date: 2019-01-03

Abstract

Provided is a method for cleaning a filtration membrane contained in a water treatment system and the water treatment system. The method comprises a first cleaning mode. The first cleaning mode comprises: adding a first cleaning solution to the feed water stream; pre-filtering the feed water stream containing the first cleaning solution to obtain a first pre-treated water stream; filtering the first pre-treated water stream with the filtration membrane to obtain a first product water stream; and neutralizing the first product water stream to obtain a first neutralized product water.

Description

METHOD FOR CLEANING FILTRATION MEMBRANE CONTAINED IN WATER TREATMENT SYSTEM AND WATER TREATMENT SYSTEM

BACKGROUND

The present disclosure generally relates to a method for cleaning a filtration membrane contained in a water treatment system and the water treatment system.

Membrane separation is a technology which selectively separates (fractionates) materials via pores and/or minute gaps in the molecular arrangement of a continuous structure. Membrane separations are classified by pore size and by the separation driving force. These classifications are: microfiltration (MF) , ultrafiltration (UF) , nanofiltration (NF) , and reverse Osmosis (RO) .

Nowadays, membrane separation has been widely used in water treatment, food, biotechnology, and pharmaceutical industries. In water treatment, membrane technology is becoming increasingly important. For example, with the help of MF and UF, it is possible to remove particles, colloids and macromolecules, so that water can be disinfected in this way. With the help of NF and RO, it is possible to remove nano size molecules and ions to further purified the water and realize desalination. However, during membrane separation operation, the membrane is gradually contaminated by the contaminates in the water, causing a significant deterioration in permeation performance of the membrane. Therefore, there exists a need for various methods for cleaning contaminated membrane in different manners. Usually, a conventional chemical cleaning of a filtration membrane is performed periodically, such as cleaning-in-place (CIP) . The cleaning solution used comprises an acid solution, such as hydrochloric acid (HCl) , citric acid or the like, and/or a alkaline solution, such as sodium hydroxide (NaOH) , EDTA-Na, or the like. However, the conventional chemical cleaning method is performed during a temporary stoppage of the filtration membrane. Thus, in the water treatment, the water production efficiency is reduced due to the membrane cleaning. And when the water filtration using the filtration membrane is stopped, a series of stops may happen in downstream of the water filtration where the product water is needed in a continuous mode. Moreover, in some operation environments, such as underwater, the conventional membrane cleaning process which needs to stop the water production, may need significant capital investment since all the devices need high pressure sealing.

Therefore, a continuous cleaning method for filtration membrane which will not affect the normal operation of the membrane filtration is -desired for water treatment systems.

BRIEF DESCRIPTION

One aspect of the present disclosure provides a method for cleaning a filtration membrane contained in a water treatment system. The method comprises performing a first cleaning mode. Performing the first cleaning mode comprises: adding a first cleaning solution to a feed water stream； pre-filtering the feed water stream containing the first cleaning solution to obtain a first pre-treated water stream； filtering the first pre-treated water stream with the filtration membrane to obtain a first product water stream； and neutralizing the first product water stream to obtain a first neutralized product water stream.

Another aspect of the present disclosure provides a water treatment system. The water treatment system comprises: a pre-filtration unit having an inlet for receiving a feed water stream and an outlet for discharging a pre-treated water stream； a filtration unit comprising a filtration membrane, an inlet for receiving the pre-treated water stream and an outlet for discharging a product water stream； a cleaning solution source comprising a first cleaning solution and a second cleaning solution； and a cleaning solution supply unit configured to supply the first cleaning solution to the feed water stream and supply the second cleaning solution to the product water stream to obtain a first neutralized product water stream.

The present disclosure provides a continuous cleaning method for a filtration membrane contained in a water treatment system. With this method, the contaminated filtration membrane could be cleaned during its normal operation, so it is not necessary to stop the water production with the filtration membrane.

Furthermore, during the cleaning process a pre-filtration is performed before the filtration to remove particles and precipitates resulted from chemical reactions of the first or the second cleaning solution, as well as matters contained in the feed water stream. Therefore, the fouling or scaling of the filtration membrane during the cleaning process may be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the subsequent detailed description when taken in conjunction with the accompanying drawings in which:

Fig. 1 is a block diagram of a water treatment system in accordance with one embodiment of the present disclosure；

Fig. 2 is a block diagram of a water treatment system in accordance with another embodiment of the present disclosure；

Fig. 3 is flow chart of a method for cleaning a filtration membrane in accordance with one embodiment of the present disclosure；

Fig. 4 is a flow chart of a method for cleaning a filtration membrane in accordance with another embodiment of the present disclosure；

Fig. 5 is a diagram of an experiment result in accordance with Example 1 of the present disclosure；

Fig. 6 is a diagram of another experiment result in accordance with Example 2 of the present disclosure； and

Fig. 7 is a bar chart of a relationship between the turbidity and the pH in accordance with Example 3 of the present disclosure.

DETAILED DESCRIPTION

Embodiments of the present disclosure relate to a method for cleaning a filtration membrane contained in a water treatment system and the water treatment system. The present invention is not limited by the embodiments.

Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the alkaline function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially” , are not to be limited to the precise value specified. Additionally, when using an expression of “about a first value -a second value, ” the about is intended to modify both values. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here, and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. Moreover, the suffix “ (s) ” as used herein is usually intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term.

Any numerical values recited herein include all values from the lower value to the upper value in increments of one unit provided that there is a separation of at least 2 units between any lower value and any higher value. As an example, if it is stated that the amount of a component or a value of a process variable such as, for example, temperature, pressure, time and the like is, for example, from 1 to 90, it is intended that values such as 15 to 85, 22 to 68, 43 to 51, 30 to 32 etc. are expressly enumerated in this specification. For values which are less than one, one unit is considered to be 0.0001, 0.001, 0.01 or 0.1 as appropriate. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

Fig. 1 is a block diagram of a water treatment system 100 according to one embodiment of the present disclosure. As shown in Fig. 1, the water treatment system 100 comprises a pre-filtration unit 101, a filtration unit 103 comprising a filtration membrane 104, a cleaning solution source 105, and a cleaning solution supply unit.

The pre-filtration unit 101 has an inlet for receiving a feed water stream 110 and an outlet for discharging a pre-treated water stream 111. The pre-filtration unit 101 may comprise a coarse filter, a MF membrane, a UF membrane, or a combination thereof. With the pre-filtration unit 101 placed upstream the filtration unit 103, large particles in the feed water stream 110 including some precipitates generated during cleaning process may be removed, so that the fouling or scaling of the filtration membrane 104 to be cleaned can be prevented.

The filtration unit 103 has an inlet for receiving the pre-treated water stream 111, a first outlet for discharging a product water stream 113 and a second outlet for discharging a concentrate water stream 114. The filtration membrane 104 contained in the filtration unit 103 may be a RO membrane, a NF membrane or a UF membrane. In some embodiments, the pre-filtration unit 101 and the filtration unit 103 in the water treatment system 100 are respectively coarse filter and UF, UF and NF, or UF and RO.

The cleaning solution source 105 comprises an acid solution 121 and an alkaline solution 123, which are stored separately in the cleaning solution source 105. Specifically, the acid solution 121 may comprise HCl, citric acid or the like, and the alkaline solution 123 may comprise NaOH, EDTA-Na, or the like.

The cleaning solution supply unit in the water treatment system 100 comprises a set of conduits for supplying the acid solution 121 and/or the alkaline solution 123 for cleaning purpose. Specifically, during an acid cleaning mode, the cleaning solution supply unit may comprise conduits 131, 133 for supplying the acid solution 121 to the feed water stream 110 and

conduits

135, 137 for supplying the alkaline solution 123 to the product water stream 113 to neutralize a residue of the acid solution 121 in the product water stream 113, to obtain a neutralized product water stream 115. While during an alkaline cleaning mode, the cleaning solution supply unit may comprise

conduits

132, 133 for supplying the alkaline solution 123 to the feed water stream 110 and

conduits

136, 137 for supplying the acid solution 121 to the product water stream 113 to neutralize a residue of the alkaline solution 123 in the product water stream 113, to obtain a neutralized product water stream 115. Each of the

conduits

131, 132, 135, 136 may comprise a valve (not shown in Fig. 1) configured to open and close its corresponding conduit. The cleaning solution supply unit may further comprise a control unit (not shown in Fig. 1) to control the valves in the

conduits

131, 132, 135, 136, so that the switch of the acid cleaning mode and the alkaline cleaning mode can be controlled by the control unit.

To achieve better cleaning performance and avoid large-scale precipitation, the pH value of the feed water stream 110 can be controlled. During the acid cleaning mode, the pH of the feed water stream 110 containing the acid solution 121 is controlled in a range of 1.0 to 5.0. During the alkaline cleaning mode, the pH of the feed water stream containing the alkaline solution 123 is controlled in a range of 9.0 to 10.4.

During water production with the filtration unit 103, when an indicator, such as flux drop, pressure drop, quality of the

product water stream

113 or 115, shows that the filtration membrane 104 needs to be cleaned, an acid cleaning mode or an alkaline cleaning mode may be performed. In some embodiments, the acid cleaning mode and the alkaline cleaning mode may be carried out alternately, to remove both acid soluble contaminates and alkali soluble contaminates from the filtration membrane 104. A whole cleaning process may last a couple of hours including several cycles of acid cleaning mode and alkaline cleaning mode. Preferably, the cleaning process end up with the acid cleaning mode. Since the water production is continuous during membrane cleaning, the cleaning method of the present disclosure is called a continuous cleaning method.

Fig. 2 shows a block diagram of a water treatment system 200 according to another embodiment of the present disclosure. As shown in Fig. 2, the water treatment system 200 comprises: a pre-filtration unit (e.g. UF unit) 201, a filtration unit (e.g. NF unit) 203 comprising a filtration membrane (e.g. NF membrane) 204, and a bipolar membrane electro-dialyzer (BPED) unit 205. During normal operation of the water treatment system 200, the UF unit 201 receives a feed water stream 210 and discharges a pre-treated water stream 211. The pre-treated water stream 211 is introduced to the NF unit 203 to obtain a product water stream 213 and a NF concentrate stream 214. The water treatment system 200 may further comprise a boosting pump 202 to increase the pressure of the pre-treated water stream 211.

The BPED unit 205 is configured to receive a salt water stream 217 and produce an acid solution 221 and an alkaline solution 223. Electrodialysis (ED) is an electrochemical process mainly used in industry for solution demineralization. The BPED unit 205 employs a new type of membrane, the bipolar membrane, to allow the electrodissociation of water. So the BPED unit 205 could recover acids and alkali from a salt water stream. In the water treatment system 200, the salt water stream 217 may comprise an aqueous solution containing soluble salts, such as sodium chloride (NaCl) , sodium sulfate (Na₂SO₄) . For example, the salt water stream 217 may be seawater, a wastewater stream produced from an industrial process, a reverse osmosis (RO) concentrate stream, or an underground brine. In some embodiments, the salt water stream 217 comprises at least part of a neutralized product water stream 215 discharged from the NF unit 203. Usually the neutralized product water stream 215 may contain NaCl. When the salt water stream 217 containing NaCl is introduced to the BPED unit 205, the acid solution 221 and the alkaline solution 223 comprise HCl and NaOH respectively.

When the NF membrane 204 of the NF unit 203 needs to be cleaned, the normal water production of the water treatment system 200 can be kept and an acid cleaning mode or an alkaline cleaning mode is then opened. In the acid cleaning mode (as shown in dotted lines in Fig. 2) , the acid solution 221 is introduced to the feed water stream 210 through a conduit 231. The feed water stream 210 containing the acid solution 221 is filtered by the UF unit 201 and the NF unit 203, to produce the product water stream 213. The alkaline solution 223 is introduced to the product water stream 213 through a conduit 233, to neutralize a residual of the acid solution 221 to obtain the neutralized product water stream 215. In the alkaline cleaning mode (as shown in dashed lines in Fig. 2) , the alkaline solution 223 is introduced to the feed water stream 210 through a conduit 232, and the acid solution 221 is introduced to the product water stream 213 through a conduit 234 to neutralize a residual of the alkaline solution 223 to obtain the neutralized product water stream 215. Each of the

conduits

231, 232, 233, 234 may comprise a valve (not shown in Fig. 2) to control the supply of the acid solution 221 and/or the alkaline solution 223.

Embodiments of the present disclosure also provide a method for cleaning a filtration membrane contained in a water treatment system. Fig. 3 shows a method 300 for cleaning a filtration membrane according to one embodiment of the present disclosure. The method 300 comprises a first cleaning mode, and during performing the first cleaning mode, the method 300 may include the steps as follows.

As shown in Fig. 3, in step 301, a first cleaning solution is added to a feed water stream. The first cleaning solution in step 301 may comprise an acid solution or an alkaline solution.

In step 303, the feed water stream containing the first cleaning solution is pre-filtered to obtain a first pre-treated water stream.

In step 305, the first pre-treated water stream is filtered with the filtration membrane to obtain a first product water stream.

In step 307, the first product water stream is neutralized to obtain a first neutralized product water stream.

The feed water stream in step 301 may be an aqueous solution containing soluble salts (both inorganic salts and organic salts are included) , such as NaCl and Na₂SO₄. For example, the feed water stream may be seawater, a produced water from oil and gas recovery process (both on shore and subsea are included) , a wastewater stream produced from an industrial process, an underground brine, a seawater desalination waste brine, a RO concentrate stream, or a NF permeate stream.

Fig. 4 shows a flow chart of a method 400 for cleaning a filtration membrane contained in a water treatment system according to another embodiment of the present disclosure. The method 400 comprises an acid cleaning mode and an alkaline cleaning mode which are performed alternately. The method 400 may include the steps as follows.

In step 401, an indicator identifies that the filtration membrane needs to be cleaned. For example, the indicator may include a flux drop through the filtration membrane, such as 30％drop of an original flux of the filtration membrane.

In step 403, an acid cleaning mode is performed.

In step 405, the flux of the filtration membrane is compared to the predetermined threshold value. If the flux of the filtration membrane is larger than or equal to the predetermined threshold value, the execution continues to a final step 410 where the cleaning ends. If the flux of the filtration membrane is less than the predetermined threshold value, the step 407 will start.

In step 407, an alkaline cleaning mode is performed.

In step 409, the flux of the filtration membrane is compared to the predetermined threshold value again. If the flux of the filtration membrane is larger than or equal to the predetermined threshold value, the execution continues to a final step 410 where the cleaning ends. If the flux of the filtration membrane is less than the predetermined threshold value, the step 403 will start again.

With the method 400, the acid cleaning mode and the alkaline cleaning mode are carried out alternately, until the flux of the filtration membrane is larger than or equal to the predetermined threshold value. As shown in Fig. 4, the cleaning method 400 is started with the acid cleaning mode. While in some other embodiments, the cleaning method may start with the alkaline cleaning mode.

In addition, the indicator that triggers the cleaning process 400 and judging criteria for whether to stop the cleaning or not may be not limited to the flux of the filtration membrane, but may also include other characteristics of the filtration membrane, such as pressure drop, quality of a product water stream.

With the method of the present disclosure, the filtration membrane could be cleaned during normal working of the water treatment system without stopping the membrane filtration. The production amount and properties of the product water stream are unchanged, for example, the product water stream is always kept neutral.

Example 1

The first example was carried out with a lab scale NF system. The feed water stream of the NF system was set as synthetic seawater (conductivity 43 mS/cm) at 4 ℃ to mimic a feed water stream under deep sea. The lab scale NF system comprised a 30.48 cm long NF membrane module. As shown in Fig. 5, the test on the NF system comprised: membrane stabilization, a first scaling formation, a first acid cleaning, a second scaling formation, and a second acid cleaning.

At first, the NF membrane module was stabilized and then operated in a high hardness (2000 ppm) and high alkalinity (1280 ppm) environment containing CaCO₃ until the permeate flux dropped by over 30％. Then a first acid cleaning of the NF module was performed. During the first acid cleaning, a HCl solution was added to the feed water stream to adjust its pH between 2-2.5 and further supplemented when the HCl acid was consumed in the system. After 1-hour acid cleaning, a performance test was performed to check the cleaning efficiency, followed by a first 2-hour acid cleaning to further remove CaCO₃ scales on the NF membrane as indicated by a further flux increase. After the first 2-hour acid cleaning, the flux of the NF membrane module was recovered back to initial performance. Next, a second cycle of scaling forming and a second 2-hour acid cleaning was performed as a repeat and reached to the same result as the first testing cycle.

The flow velocity on the membrane surface was set at 6.4 cm/s, which equaled to the surface velocity at the outlet of the membrane skid in real subsea application. The surface velocity in the continuous acid cleaning process was critically different from conventional process. It was about half of the velocity at inlet since 50％of the feed water stream was changed into product water stream during continuous cleaning.

Example 2

The second example was also carried out with a lab scale NF system. The lab scale NF system comprised a 12-inch long NF membrane module. As shown in Fig. 6, the test on the NF system comprised: membrane stabilization, membrane scaling formation, continuous cleaning with two cycles of alkaline cleaning and acid cleaning.

In this test, the membrane was firstly stabilized and then fouled until the membrane flux decreased by 20％. The continuous cleaning process started with flushing membrane with an alkaline solution to adjust the pH of the feed water stream to 9.6. The pH of the cleaning solution was critically controlled to minimize mass precipitation in the seawater application. After an 8-hour alkaline cleaning, the membrane was tested and switched into an acid cleaning mode. After a certain period of the acid cleaning, the membrane performance was checked to verify the performance recovery. Since the membrane flux did not reach the targeted performance (95％of initial flux in this example) , a second cycle of alkaline/acid cleaning was carried out.

In the whole cleaning process, the NF membrane was operated at a normal production pressure to continuously generate a permeate water. The surface velocity during the continuous cleaning was controlled to be at 6.4 cm/sto mimic the velocity at outlet of the membrane skid in subsea application. The membrane flux was cleaned back to 97％of the initial flux after two cycles of continuous alkaline/acid cleaning.

Example 3

As for seawater application, the feeding water stream contains significant amounts of Mg²⁺ and Ca²⁺, which may generate Mg (OH) ₂ and CaCO₃ in high pH solution. Those precipitates will cause blocking of downstream filtration assets, such as UF membranes, NF membranes. Therefore, the pH of alkaline cleaning mode should be controlled to avoid mass formation of those precipitates.

A lab test was performed to verify the pH upper limit of the alkaline cleaning mode. A series of synthetic seawater samples were prepared according to the recipe shown in Table 1. The solutions were adjusted to pH 9.48-10.89 and cooled down to 3-4 ℃ through ice bath. The precipitation of the solution was measured through turbidity test which indicated the transparency of the sample. Higher turbidity was a result from greater amount of particles precipitated in solution. With pH of the solution raised above 9.9, turbidity of the solution increased to 0.6 Nephelometric Turbidity Unit (NTU) which was higher than the turbidity of the prepared synthetic seawater samples (around 0.3 NTU) . As shown in Fig. 7, when pH was increased above 10.4, turbidity raised over 1 NTU and continued to increase To avoid accumulation of fine particles during long term operation, the pH of the alkaline solution would be controlled to less than 10.5 or more stringently less than 10.

Table 1

Ions	Concentration (mg/L)
Calcium	408.00
Magnesium	1298.00
Sodium	10768.00
Potassium	396.00
Sulfate	2702.00
Chloride	19363.90
Bicarbonate	145.40

Example 4

The water treatment system 200 shown in Fig. 2 was employed in Example 4. Seawater 210 was introduced to the UF unit 201 as a feed water stream to remove fine particles (50～100 nm in diameter) therein. The pre-filtered seawater 211 was boosted by the boost pump 202 and fed into the NF unit 203 to produce the product water stream 213 and the concentrate stream 214. The concentrate stream 214 with rich amount of divalent ions (such as Ca, Mg, SO₄ ^2-etc. ) was discharged at an assigned brine region. One part of the product water stream 213 was sent to an injection pump (not shown in Fig. 2) for downstream application. The other part of the product water stream 213 was introduced into the BPED unit 205 to generate the acid solution 221 and the alkaline solution 223.

During a first cleaning mode, the alkaline solution 223 from the BPED unit 205 was introduced to the seawater 210 and the pH of the seawater 210 was controlled at 9.5～9.8 to minimize the precipitation of ions contained in seawater 210. The UF unit 201 was deployed to block any potential existing fine particles to secure the downside NF unit. The acid solution 221 from the BPED unit 205 was introduced to the product water stream 213 from the NF unit 203 to neutralize the product water stream 213. In this case no significant impact was generated with the usage of NF permeate water in downstream application (e.g. subsea injection) . The above described first cleaning mode is an alkaline cleaning mode.

Once the NF membrane 204 was cleaned at high pH for a certain period to have the productivity recovered, the cleaning was switched to a second cleaning mode: acid cleaning mode. In the acid cleaning mode, the acid solution 221 from the BPED unit 205 was introduced to the seawater 210, and the alkaline solution 223 from the BPED unit 205 was introduced to the product water stream 213 for neutralization.

Considering the acid or alkaline residual could be in the backwash stream of the UF membranes and concentrate stream 214 of the NF unit 203, additional neutralization stream was sent from the BPED unit 205 to mix with those solutions. Overall the BPED generated relatively equal amount of acid and alkaline solutions, and both chemicals were discharged to neutral or seawater pH (～8) when checking at a system level.

As a calculation, about 58.5 ppm (1mM) of additional NaCl was introduced from the acid/alkaline neutralization. This amount of salt was added into the final permeate water for injection. Compared to the averagely 3.5％ (35000 ppm) total dissolved solids (TDS) in seawater, this increase in product water TDS was negligible.

This written description uses examples to describe the disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims

A method for cleaning a filtration membrane contained in a water treatment system, comprising performing a first cleaning mode, wherein performing the first cleaning mode comprises:

adding a first cleaning solution to a feed water stream；

pre-filtering the feed water stream containing the first cleaning solution to obtain a first pre-treated water stream；

filtering the first pre-treated water stream with the filtration membrane to obtain a first product water stream； and

neutralizing the first product water stream to obtain a first neutralized product water stream.
The method of claim 1, wherein the filtration membrane comprises a reverse osmosis membrane, a nanofiltration membrane or an ultrafiltration membrane.
The method of claim 1, wherein the pre-filtering comprises using a coarse filter, a microfiltration membrane, an ultrafiltration membrane, or a combination thereof for pre-filtering.
The method of claim 1, wherein the first cleaning solution is an acid solution.
The method of claim 4, wherein the acid solution comprises hydrochloric acid.
The method of claim 4, wherein the pH of the feed water stream containing the first cleaning solution is controlled in a range of 1.0 to 5.0.
The method of claim 1, wherein the first cleaning solution is an alkaline solution.
The method of claim 7, wherein the alkaline solution comprises sodium hydroxide.
The method of claim 7, wherein the pH of the feed water stream containing the first cleaning solution is controlled in a range of 9.0 to 10.4.
The method of claim 1, further comprising performing a second cleaning mode, wherein the first cleaning mode and the second cleaning mode are performed alternately, and performing the second cleaning mode comprises:

adding a second cleaning solution to the feed water stream；

pre-filtering the feed water stream containing the second cleaning solution to obtain a second pre-treated water stream；

filtering the second pre-treated water stream with the filtration membrane to obtain a second product water stream； and

neutralizing the second product water stream to obtain a second neutralized product water stream.
The method of claim 1, further comprising generating the first cleaning solution using a bipolar electro-dialyzer.
The method of claim 11, further comprising introducing at least part of the first neutralized product water to the bipolar electro-dialyzer.
A water treatment system comprising:

a pre-filtration unit having an inlet for receiving a feed water stream and an outlet for discharging a pre-treated water stream；

a filtration unit comprising a filtration membrane, an inlet for receiving the pre-treated water stream and an outlet for discharging a product water stream；

a cleaning solution source comprising a first cleaning solution and a second cleaning solution； and

a cleaning solution supply unit configured to supply the first cleaning solution to the feed water stream and supply the second cleaning solution to the product water stream to obtain a first neutralized product water stream.
The system of claim 13, wherein the filtration membrane comprises a reverse osmosis membrane, a nanofiltration membrane or an ultrafiltration membrane.
The system of claim 13, wherein the pre-filtration unit comprises a coarse filter, a microfiltration membrane, an ultrafiltration membrane, or a combination thereof.
The system of claim 13, wherein the cleaning solution supply unit is also configured to supply the second cleaning solution to the feed water stream and supply the first cleaning solution to the product water stream to obtain a second neutralized product water stream.
The system of claim 13, wherein the cleaning solution source comprises a bipolar electro-dialyzer configured to generate the first cleaning solution and the second cleaning solution.
The system of claim 16, wherein the bipolar electro-dialyzer has an inlet connected to the outlet of the filtration unit and is configured for receiving at least part of the product water stream.