WO2020212980A1

WO2020212980A1 - A system and method for treating water

Info

Publication number: WO2020212980A1
Application number: PCT/IL2020/050439
Authority: WO
Inventors: Miriam Faigon; Elad DINAR
Original assignee: Hutchison Water Israel E.P.C Ltd
Priority date: 2019-04-17
Filing date: 2020-04-16
Publication date: 2020-10-22
Also published as: IL287029A

Abstract

The present disclosure provides a system and method for treating water to be treated and obtaining from same treated water that is suitable for distribution and human consumption. The system and method utilizes a lime water supply unit that provides lime water to water treated in at least one first treatment process, the lime water supply unit comprising at least (a) a mixing chamber configured to receive and admix calcium oxide (CaO) and/or calcium hydroxide (Ca(OH)2) powder with treated water to thereby form lime milk; (b) a lime water preparation unit comprising at least one water membrane filtration module and configured to pass said lime milk and treated water through the at least one water membrane filtration module and to discharge lime water; and (c) a communication line configured to communicate the lime water and admix the lime water with treated water discharged from the first water treatment section.

Description

A SYSTEM AND METHOD FOR TREATING WATER

TECHNOLOGICAL FIELD

The present invention relates to water treatment and specifically to the improvement of quality of water obtained from water treatment facilities, such as improving alkalinity, pH, turbidity etc.

BACKGROUND

The quality of water produced by different water treatment processes such as seawater reverse osmosis (SWRO), thermal processes such as multistage flash (MSF) or multi effect distillation (MED) can differ between the processes, but what is common to these processes is that all result in pure water from which salts & minerals that are considered good for humans’ health, have been removed.

In addition, to meet drinking water standards, the desalinated water should be non-aggressive (having a slightly positive Langelier saturation index (LSI)) and non- corrosive. The corrosiveness and instability of desalinated water is mainly to the absence of an absorption potential (buffer quality) of carbon-based molecules (alkalinity) of a type typically found in natural (mineral) water and in the absence of required alkalinity, desalinated water is often found to be aggressive towards distribution system components and incompatible with existing metal water distribution system infrastructure.

To provide water suitable for distribution and human consumption, desalinated water need to be subjected to post-treatment hardening processes to return to water a minimal calcium hardness, alkalinity and adjust the final water pH and thus make the received water permissible for human consumption. These post-treatment processes are referred to as hardening or re-mineralization processes.

Re-mineralization can be distinguished into the following treatment processes limestone dissolution by carbon dioxide (CO₂) and /or Acid application of sodium bicarbonate and calcium sulphate, application of hydrated lime and sodium carbonate,

application of carbon dioxide and hydrated lime,

The remineralization steps result in raising pH, alkalinity (HCO₃), and hardness of the water, thereby stabilizing the water.

In other words, the post-treatment phase generally includes two processes: Limestone with CO₂ or Acid followed by pH adjustment, e.g. in case the pH is still acidic, the majority of the facilities use chemical addition of Caustic Soda, which is also regarded as too expensive.

When re-mineralization includes the use of hydrated lime or limestone and carbon dioxide (and/or acid), the process can create fine insoluble particles (resulting in turbid water) and if excess of carbon dioxide still exists in the water (rendering the water acidic and thus is considered as aggressive CO₂), the pH is adjusted by addition of a base solution in order to achieve positive LSI (or non-corrosive water).

Alternatively, the method can also comprise of Lime and CO₂ only. Thus, there are 2 scenarios - where the lime water replaces the soda, and the second case, where the main process is based on lime and CO₂ reaction, to add all the alkalinity and hardness required (while still achieving the final pH required).

One should take into account that even if all the water passed through the post treatment reactors, there is a need to add some soda, as the dissolution reaction never reaches a positive LSI index, and if the requested quality of final product is to reach to a positive LSI, then Caustic Soda or Lime must be added to achieve the positive LSI value.

The techniques used for re-mineralization aim to add and achieve some minimum calcium hardness and alkalinity (HCO₃) of the desalinated water to make the re-mineralized water stable when it is released as potable water in distribution networks. There are various ways to re-mineralize desalinated water ensuring that it meets with drinking water standards required for public distribution. However, those used most often in practice are re-mineralization with either (a) calcium carbonate, carbon dioxide and/or acid; or, (b) calcium hydroxide, carbon dioxide. Another option will be to utilize direct mixing of chemicals (expensive to be used in high capacities), such as Calcium Chloride and Sodium Bicarbonate or Calcium Sulphate with Sodium Bicarbonate.

For the pH adjustment, the majority of the facilities use chemical addition of caustic soda (sodium hydroxide). More than one form of post-treatment was implemented with or without the need for by-pass or native source water blending, and dependent on the designer and the costs thereof.

The above described post treatment, although widely used, is not without drawbacks. The use of caustic soda (NaOH) is relatively expensive and thus, better cost- effective method for the remineralization and pH adjustment in the post treatment is still a long felt need.

Furthermore, Lime is used as additive to reach to alkalinity is required in combination with CO₂ , according to the following equation:

(unsoluble, small particles)

If excess CO₂ is added thus, the following process takes place:

soluble

The first reaction creates insoluble particles which can create turbidity, while the second reaction is in equilibrium such that the carbon dioxide still exists in the water (rendering the water acidic and thus is considered as aggressive CO₂), resulting in the requirements to adjust the water’s pH by the addition of a base solution [to eventually result positive LSI (or non-corrosive water)].

Thus, again, a cost-effective post treatment method is still a long felt need.

GENERAL DESCRIPTION

The present disclosure provides, in accordance with a first of its aspects a water treatment system comprising: (a) a first water treatment section configured to receive water to be treated and discharge treated water; and (b) a downstream second water treatment section configured to admix the treated water with lime water, to thereby convert the treated water from the first water treatment section into finished water, wherein said second water treatment section comprises a lime water supply unit comprising: - a mixing chamber configured to receive and admix at least one of calcium oxide (CaO) and calcium hydroxide (Ca(OH)₂) powder with treated water to thereby form lime milk;

- a lime water preparation unit comprising at least one water membrane filtration module and configured to pass said lime milk and treated water through the at least one water membrane filtration module and to discharge lime water;

- a communication line configured to communicate the lime water and admix the lime water with treated water discharged from the first water treatment section.

Further provided by the present disclosure is a method for treating water, the method comprises (a) subjecting water to be treated to at least one treatment process (that results in removal of one or more substances) thereby provide treated water; and (b) mixing the treated water obtained from the at least one treatment process with lime water in an amount that provides finished water; wherein said method further comprises producing lime water by

- mixing a portion of the treated water obtained from the at least one treatment process with at least one of calcium oxide (CaO) and calcium hydroxide (Ca(OH)₂) powder to obtain lime milk; and

- passing said lime milk together with a portion of the treated water through a lime water preparation unit comprising at least one water membrane filtration module to thereby obtain said lime water.

The system and method disclosed herein is of particular use in converting water permeate, received by desalination processes, into finished water.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figure 1A-1B are schematic block diagrams of a system including a lime water supply unit according to an embodiment of the present disclosure (Figure 1A), and specifically a lime water supply unit, illustrating the plurality of membranes within the membrane module (Figure IB).

Figure 2 is a schematic block diagram of a lime water supply unit including a module in backwash operation mode according to an embodiment of the present disclosure.

Figure 3 is a schematic block diagram of a water desalination system according to an embodiment of the present disclosure.

Figure 4 is a schematic block diagram illustrated the incorporation of an ion exchange module within the water treatment system in accordance with an embodiment of the present disclosure.

Figure 5 is a graph showing transmembrane pressure (TMP, mbar) and flux (L/m¾) when passing 5L of unsaturated lime water (1.38 g/L Ca(OH)2) through a silicon carbide (ceramic) membrane having an active filtration area of 0.008m², at three different fluxes (226 L/m¾, 450 L/m¾ and 900 L/m¾).

Figures 6A-6C are photographic images of lime water feed solution (“feed”, Figure 6A), treated water (“Permeate”, Figure 6B) and concentrate remaining solution (“Cone”, Figure 6C).

Figure 7 is a graph showing transmembrane pressure (TMP, mbar) when passing 5L of unsaturated lime water (1.38g/L Ca(OH)2) at a flux of 450L/m¾ through a silicon carbide (ceramic) membrane having an active filtration area of 0.008m² and subjecting the membrane to a backwash every hour with treated water at a backflush flux of 800 L/m¾.

Figure 8 is a graph showing transmembrane pressure (TMP, mbar) when passing 5L of unsaturated lime water (1.38g/L Ca(OH)2) at a flux of 675L/m¾ through a silicon carbide (ceramic) membrane having an active filtration area of 0.008m² and subjecting the membrane to a backwash every hour with treated water at a backflush flux of 800 L/m¾.

Figure 9 is a graph showing transmembrane pressure (TMP, mbar) reduction during a cleaning process of the ceramic membrane by citric acid solution, after membrane flushing by demineralized water. The cleaning solution demineralized water and l,000ppm citric acid (beginning at 60 minutes) and subsequently, backwashing again with demineralized water (beginning at 90 minutes).

DETAILED DESCRIPTION

The present disclosure is based on a long felt need to provide an improved technology for providing finished/potable water after being subjected to a water cleaning process, such as a desalination process.

Specifically, the present disclosure is based on the development of a sub-system that utilizes lime water in order to convert water that has been treated by any type of a water cleaning system (e.g. reverse osmosis (RO), nanofiltration (NF), and electrodialysis reversal (EDR) etc.) into finished/potable water.

As will be further discussed hereinbelow, one of the key limitations of using lime water or lime milk in water treatment processes is the risk in increasing turbidity of the finished water. The present disclosure has developed a system and method that overcomes at least this limitation while significantly decreasing any permeate water losses, reducing the need of expensive chemicals , and reduce wastes in the system.

Thus, in accordance with the present disclosure, there is provided a water treatment system comprising (a) a first water treatment section configured to receive water to be treated and discharge treated water; and (b) a second water treatment section, downstream to the first water treatment section, and configured to admix the treated water with lime water, to thereby convert the treated water from the first water treatment section into finished water; wherein, the second water treatment section comprises a lime water supply unit comprising

- a mixing chamber configured to receive and admix calcium oxide (CaO) and/or calcium hydroxide (Ca(OH)2) powder with treated water to thereby form lime milk;

- a lime water preparation unit comprising at least one water membrane filtration module and configured to pass said lime milk and treated water through the at least one water membrane filtration module and to discharge lime water; - a communication line configured to communicate the lime water and admix the lime water with treated water discharged from the first water treatment section.

Also provided by the present disclosure is a method for treating water, the method comprising:

— subjecting water to be treated to at least one treatment process that results in removal of one or more substances thereby provide treated water;

— mixing the treated water obtained from the at least one treatment process with lime water whereby finished water is obtained; wherein said method further comprises a lime water production process comprising:

- mixing, in a mixing chamber, a portion of the treated water obtained from the at least one treatment process with calcium oxide (CaO) and/or calcium hydroxide (Ca(OH)₂) powder to obtain lime milk; and

For simplicity, the following description provides various aspects that equally describe the system and the method of the present disclosure and the details of the various embodiments and examples should be considered, independently, embodiments and examples of the system and method disclosed herein, even if not explicitly referred to same or even if literally referred only to the system or to the method.

The system and method disclosed herein provide an improved finished water, this being achieved, inter alia, without the use of sodium hydroxide as a main pH adjusting agent of the treated re-mineralized water (after being discharged from water cleaning process/es and subjected to re-mineralization post treatment process(es)).

In the context of the present disclosure, the term“ water to be treated’ denotes any water comprising at least one substance, the reduction or removal of which is required. The substance can be any one or combination of a salt, a mineral, contaminating microorganism or any other substance considered undesired to those versed in the water treatment technologies.

In some examples, the water to be treated is salt water from which substances need to be removed which require RO thus most of calcium and alkalinity are removed.

The water to be treated can be of any source having high concentration of dissolved solids. In this context, "high salinity water" is to be understood to mean any water-based liquid having a salt content of at least about l,000ppm of total dissolved salts (TDS) and preferably above 10,000. According to some examples, the high salinity liquid is seawater from seas, salt lakes and ponds, high brackish water sources, brines, contaminated water from industrial or other source and other surface and subterranean sources of water having ionic contents which need to be removed from the water.

The water to be treated is subjected to at least one treatment process in the first water treatment section.

In accordance with some examples, the first water treatment section comprises any one or combination of multi-stage flash (MSF) distillation systems and/or reverse osmosis (RO) systems, as will be further discussed hereinbelow.

The water being discharged from the first water treatment process, namely the “ treated water'’ is water from which at least one substance has been removed as a result of treatment in the first treatment section.

In some examples, the treated water is de-mineralized water.

In some other examples, the treated water is desalinated water.

In some examples, the treated water existing the first treatment process is one requiring at least one process of remineralization and/or re-hardening in order to render the treated water suitable for distribution and consumption, i.e. to render them finished/potable water.

The term“ finished water” or“ potable water” is used herein to denote either (a) pure water that has been subjected to a post treatment so as to make them suitable for introduction into distribution systems and consumption, without any further treatment (except, perhaps, treatment necessary to maintain water quality in the distribution system (e.g. booster disinfection, addition of corrosion control chemicals, the addition of Chlorine etc.)); or, (b) water with basic pH and/or positive LSI. In this connection, it is noted that the system and method disclosed herein may include additional elements and method steps that turn the finished water obtained after the 2^nd treatment section into potable water, such as the addition of chlorine. Preferably such additional system elements and method steps do not significantly affect the parameters that characterize the finished water, as further detailed below.

The finished water are characterized by one of several acceptable parameters, such as, pH, Langelier saturation index (LSI), alkalinity, turbidity, CCPP and others, as known in the art.

In some examples, the finished water is characterized by a basic pH.

In some examples, the finished water has a pH between 7.0 and 8.5.

In some further examples, the finished water is characterized by a positive LSI value. LSI is an approximate indicator of the degree of saturation of calcium carbonate in water. The LSI is calculated using the pH, alkalinity, calcium concentration, total dissolved solids (TDS), and water temperature of the water sample used for its determination.

The LSI is equal to the measured pH minus the pH_s, where pH_s is the equilibrium pH value for the above equation.

A negative LSI indicates that the water is under saturated with calcium carbonate and will tend to be corrosive in the distribution system; a positive LSI indicates that the water is over saturated with calcium carbonate and will tend to deposit calcium carbonate forming scales in the distribution system; yet, a LSI that is close to zero, i.e. slightly positive, indicates that the water is just saturated with calcium carbonate and will neither be strongly corrosive or scale forming..

In accordance with the present disclosure, treated water that may be considered aggressive (e.g. desalinated water) is further processed, in what can be regarded at times, a’post treatment’ process, to obtain at least a slightly positive LSI so as to allow the production of a protective scale of calcium carbonate in pipelines conveying the finished water. In some examples, the finished water has a LSI between 0.1 and 0.5, preferably between 0.2 and 0.4.

The finished water obtained by the disclosed system and method can also be characterized by its calcium carbonate precipitation potential (CCPP) of the water. The CCPP value is the calculated mass of calcium carbonate expected to precipitate or be dissolved by a particular water. In the present disclosure, the CCPP of the finished water, can be characterized by a CCPP between 0 to 10 mg/Liter.

The finished water obtained by the disclosed system and method can also be characterized by its turbidity.

Turbidity is a measure of the clarity of any liquid, and in the context of the present disclosure, can characterize any one of: water to be treated, treated water, lime water, lime milk, and finished water. As appreciated, excessive turbidity (‘cloudiness’) of water indicates high concentrations of particulate matter, the latter potentially supporting the growth of pathogens in the water, potential for plugging of water systems and other health concerns. Therefore, one would desire to obtain finished water with as low as possible turbidity, the target being less than 0.5 NTU

The level of turbidity is of particular relevance to the present disclosure where lime water is used for re-mineralization of the treated water (to thereby convert them into finished water), as noted above, adjustments of water using calcite or calcium carbonate addition to the treated water is recognized as imposing a risk of increasing the turbidity of the finished water. This risk has been overcome by using the lime water supply unit disclosed herein, as an integral part of a water treatment system for converting water to be treated into finished water.

Yet further or alternatively, the finished water can be characterized by its alkalinity. Alkalinity is a measure of the capacity of water to neutralize acids or hydrogen ions. In other words, water with a high level of alkalinity is more tolerant ad stable in terms of its acidity (pH). Therefore, alkalinity measures how much acid can be added to a water body before a significant pH change occurs. In alkalinity tests the level of bicarbonates, carbonates, and hydroxides in water is measured and the value obtained is generally expressed as milliequivalent per liter (mEq/L) or parts per million (ppm) as calcium carbonate (CaCO₃).

In the context of the present disclosure, a desired alkalinity for the finished water is one having at minimum 40ppm, or at minimum 50, 60, 70, or even more than 80ppm CaCO₃. Other chemical factors which can influence the corrosive nature of water and thus need to adjusted in order to render the water suitable for distribution/consumption include dissolved oxygen, total dissolved solids (TDS), hydrogen sulfide (thS), poly and orthophosphates in on some cases, the quality of the finished water can be determined by each of these chemical factors, independently.

According to the present disclosure, treated water exiting the first water treatment section or process is converted into finished water by at least its mixing with lime water that is being supplied using dedicated filtration membranes forming an integral part of the whole water treatment system and operated as a closed circulating sub-system within the water treatment system. In fact, in accordance with the present disclosure, lime is used instead of Caustic Soda that is typically used in order to correct the pH of the treated water, notwithstanding the fact that lime is considered unfavorable due to its tendency to increase turbidity of the water to which it is introduced.

In the context of the present disclosure, the term lime water is to be understood as liquid, specifically water unsaturated with calcium hydroxide (Ca(OH)2). The concentration at which the water is unsaturated with Ca(OH)2 depends, inter alia, on temperature (solubility decreases with increasing temperature) and the solubility of calcium hydroxide in water as a function of temperature is well known in the art (e.g. P. J. de Moel et al.“ Assessment of calculation methods for calcium carbonate saturation in drinking water for DIN 38404-10 compliance" Drink. Water Eng. Sci., 6, 115-124, 2013, the content of which, in its entirety, is incorporated herein by reference]

In some examples, lime water has a calcium hydroxide concentration of at least 0.01%w/v (0. lmg/L), at times between 0.01% (0.lmg/L) and 0.153%w/v (0.01gr/L) when measured at room temperature, yet still unsaturated solution.

In accordance with the present disclosure, lime water is prepared from a powder of quick lime or a powder of hydrated lime that is mixed with water to initially obtain lime milk.

In some examples, the quick lime or hydrated lime powders are mixed with a portion of the water received from the first treatment section, i.e. a portion of the treated water.

In some other examples, the powder of quick lime or a powder of hydrated lime is mixed with water from a different source, yet that has undergone a cleaning process and thus is de-mineralized (pure) water.

In accordance with the present disclosure, the mixing of the powder with water first creates lime milk within a dedicated mixing chamber. The lime milk is a homogeneous suspension of excess calcium hydroxide in the water, and in other words, a supersaturated solution that comprises calcium hydroxide (Ca(OH)2) at a concentration higher than its saturation limits in the specific water temperature. In some examples, the supersaturated solution comprises at least 10%w/v (O.lgr/L), at times between 10% and 20%, when the concentration is measured at room temperature. A person versed in the art would know the concentration needed in order to provide a supersaturated solution at a given water temperature.

Yet, as appreciated by those versed in the art, in the presence of carbon oxide from air, some of the calcium hydroxide within the water is converted to insoluble particles of calcium carbonate, according to the following scheme:

(unsoluble, small particles)

Insoluble calcium carbonate particles (either particles that has no originally dissolved or particles formed due to the reaction of hydrated lime) can be removed from the water by adding excess CO₂ to the water, according to the following scheme:

soluble

As mentioned above, the first reaction creates insoluble particles which can create turbidity, while the second reaction is in equilibrium such that the carbon dioxide still exists in the water (rendering the water acidic and thus is considered as aggressive CO₂), resulting in the requirements to adjust the water’s pH by the addition of a base solution [to eventually result positive LSI (or non-corrosive water)].

Alternatively, the insoluble calcium carbonate can be removed from the water by subjecting the milky liquid to a settling and clarification process commonly used by means of Coagulation / Flocculation / Settling through a clarifier or a Lamella clarifier .

However, this procedure has its disadvantages.

For example, when clarifying the turbid water is by means of settling though Lamella clarifier (Coagulation / Flocculation / Settling) the level of turbidity it typically reduced to at most 2NTU and is in the range of 2-20NTU. In addition, chemicals are required such as polymers to promote coagulation and flocculation and to allow settling of the fine particles. Further, bulky and expensive constructions and equipment is required with part of the lime / calcium being lost in the process through settled matter.

It has now been found that when preparing lime water from quick lime powder or hydrated lime powder, and subjecting the lime milk and water to membrane filtration, under controlled conditions, it is possible to correct the parameters of the treated water from the first treatment section e.g. basic pH, slightly positive LSI), and convert the treated water into finished water suitable for distribution and consumption.

In some examples, the finished water obtained from lime water that has been subjected to at least one membrane filtration according to the present disclosure has a turbidity of less than 0.5 NTU, at times, less than 0.4NTU, at times less than 0.3NTU, at times, less than 0.2 NTU, at times between 0.1NTU and 0.5NTU, at times, between 0.1 and 0.4NTU, at times, between 0.1 and 0.3NTU, or even between 0.1 and 0.2NTU.

The formation of lime water suitable for treating the treated water from the first treatment section is prepared in a lime water preparation unit comprising a first stage at which lime milk is first prepared in a mixing chamber, as noted above, and a second stage where the lime milk mixed with water is filtered via at least one water membrane filtration module.

In the context of the present disclosure, a“ water membrane filtration module” is to be understood as a device comprising one or more individual membranes. Thus, a single water membrane filtration module can comprise, one, two, three, four or any number of such individual membranes, which may be of the same or different type.

In some examples, the lime water preparation unit comprises a plurality of water membrane filtration modules, each module comprising a single or a plurality of individual membranes.

In some examples, the lime water preparation unit comprises a plurality of water membrane filtration modules arranged in parallel.

In some examples, the lime water preparation unit comprises a plurality of water membrane filtration modules, each module comprises a plurality of membranes arranged in parallel within a specific module.

The membrane within a module can be of any type known in the water purification industry and it typically one providing either ultrafiltration (i.e. UF membrane) or microfiltration (i.e. MF membrane). It is to be noted that a single module can comprise the same or different types of membranes, and similarly, different modules within the unit can comprise the same or different types of membranes.

In some examples, the membranes are ceramic filtration membranes. Ceramic membranes are well known in the art and are a type of artificial membranes made from inorganic materials (such as alumina, titania, zirconia oxides, silicon carbide or some glassy materials) operated in a cross-flow or dead end filtration mode.

In some examples, the membranes are polymer membranes such as those including polyvinylidene fluoride (PVDF), polysulfone/polyethersulfone (PS/PES), polypiperazine and polyamide and any material which can withstand the high pH of the solution to be treated.

The operation of the water membrane filtration module depends on the type of membrane used, the temperature of the water (which affects solubility of the calcium hydroxide dissolved therein), the size of the pores in the membrane (MF, UF), the use (or no use) of chemicals, the backwash flux, the number of backwash stages, the number of filtration cycles, and the transmembrane pressure within a membrane of the module, etc.

In some examples, a membrane or an entire module is continuously operated until the transmembrane pressure (TMP) therein reaches a pre-determined threshold, at which point, the membrane or the entire module enters into a backwash operation mode.

In some other examples, a membrane or an entire module is operated with pre defined periodic backwashes, e.g. every hour, for several seconds.

In some examples, the washing of the membrane is in pulses. The duration of the pulses can vary from seconds to minutes or even more, depending on various criteria, including the time interval between each wash, the condition of the membrane, the type of the wash solution etc. In fact, during a water treatment process, the washing of the membrane can be a combination of different durations and/or time intervals and/or wash solution and/or backwash flux etc., used for cleaning the membrane(s), and the parameters of the backwash can be easily determined by those versed in the art.

The water membrane filtration module can be operated with different fluxes, an those versed in the art would know how to calibrate the optimum flux for the selected operation (or backwash operation) conditions (e.g. temperature of the water, type of membranes used, etc.).

The lime water discharged from the lime water preparation unit is communicated to the water line from the first treatment process and is mixed with the treated water whereby finished water is obtained.

Reference is now made to Figure 1A providing a schematic illustration of the system hitherto describes and to Figure IB more specifically illustrating the lime water preparation unit forming part of the system of Figure 1A.

Specifically, Figure 1A presents a water treatment system 100, comprising a first water treatment section 102 that receives water to be treated 104 and discharges treated water 106. The system 100 also comprises a downstream second water treatment section 108 that is designed to admix treated water 106 from the 1^st water treatment section 102, via communication line 110 with lime water transmitted in lime water communication line 112 from the second water treatment section 108, to thereby convert the treated water into finished water 114.

Second water treatment section 108 comprises a lime water preparation unit 116, that comprises a mixing chamber 120, that receives and admixes calcium oxide (CaO) and/or calcium hydroxide (Ca(OH)2) powder 122 with treated water communicated into the mixing chamber, via communication line 124, that communicated a portion of the treated water from the first water treatment section 102 or from another source, e.g. reservoir, to thereby form lime milk within the mixing chamber 120. The lime milk thus formed is then filtered through a water membrane filtration module 126 together with another portion of treated water, and discharge lime water into lime water communication line 112.

Turning now to Figure IB, there is illustrated, in more details, a lime water preparation unit 116 of Figure 1A, where a water membrane treated module is illustrated to encompass a plurality of membranes 130, which may be the same or different, each receiving lime milk from mixing chamber 120. At times, the lime water preparation unit comprises one or more membranes or modules that need regeneration, i.e. cleaning from excess of solids, particularly, calcium carbonate and/or lime water Ca(OH)2, disposed in the pores of the membrane, causing increase in water pressure or even blockage of the membrane. To overcome undesired increase in pressure or even blockage of a module, the system and method is configured to allow a backwash operation mode for a module or even for a single membrane within a module. To this end, each of the water membrane filtration modules has an inlet for receiving a wash solution and can be actuated to operate in backwash operation mode where the wash solution is introduced into the module or into a membrane within a module comprising a plurality of membranes, in a reversed flow mode and release from the module or membrane backwash water that comprises at least suspended calcium carbonate. This backwash water can be collected or returned into the mixing chamber (in which lime milk is formed) or directly into another module operated in a filtration mode.

In some examples, the wash solution is treated water, e.g. a portion of the treated water from the first treatment section.

In some examples, the wash solution is a strong acid diluted in the treated water. In the context of the present disclosure, the strong acid can be of any type compatible with the requirements of the finished water, namely, that is allowed for use in the water distribution system.

In some examples, the strong acid is HC1.

In some examples, the wash solution is the treated water in which gas is dispersed in a form of nano-bubbles. The gas can be any gas, for example, air, nitrogen, CO₂. In some examples, the wash solution comprises treated water with CO₂ nano-bubbles.

In some examples, the wash solution comprises a combination of HC1 and CO₂ nano-bubbles.

Figure 2 provides a schematic illustration of a backwash operation mode of the single membrane in a water membrane filtration module. For simplicity, like reference numerals to those used in Figure 1A, shifted by 100 are used to identify components having a similar function. For example, component 226 in Figure 2 is a module having the same function as module 126 in Figure 1A. Specifically, Figure 2 illustrates the introduction of wash solution 232, into a water membrane filtration module 226’ operated in a reverse flow direction, from which backwash water with lime is discharged via communication line 260, into a different water membrane filtration module 226 operated in filtration mode to produce the desired lime water and communicate the same into the treated water via lime water communication line 212.

The system disclosed herein is of particular use in water desalination plants, and in which the first treatment section acts as a water desalination treatment system, of a type comprising at least one sea water reverse osmosis (SWRO) module in liquid communication with at least one downstream Brackish Water Reverse Osmosis (BWRO) module. Liquid exiting the SWRO and/or the BWRO is then communicated into the second treatment section (comprising the lime water preparation unit) whereby it is mixed with lime water, passed through the membrane and finished water is obtained.

The incorporation of a second water treatment section of a type disclosed herein, within a desalination plant is schematically illustrated in Figure 3. For simplicity, like reference numerals to those used in Figure 1A, shifted by 200 are used to identify components having a similar function. For example, component 326 in Figure 3 is a module having the same function as module 126 in Figure 1A.

Figure 3 thus illustrated a water desalination system, comprising a first water treatment section 302 including a water desalination unit that receives sea water 304 and discharges permeate water 306. The permeate water undergoes a re -hardening process 334 of a type known in the art, and the discharged re -hardened water is then mixed with the lime water discharged from module 326, via communicated via lime water communication line 336. A portion of the permeate water is fed into the second water treatment section via communication line 324.

When the first treatment section is a desalination system, a portion of the water being treated either by the SWRO or by the BWRO can be communicated to the mixing chamber of the lime water preparation unit. In other words, for the preparation of lime water, it is not necessary that the treated water being introduced into the mixing chamber is one that has been completely desalinated. Similarly, a portion of the treated water (either that discharged from the SWRO or that discharged from the BWRO) can be used as a wash solution, i.e. to be introduced into the water membrane filtration module when operated in backwash mode.

When the first treatment section is a desalination system, the water treatment system of the present disclosure can also comprise one or more ion exchange (I/X) modules making use of the lime water for supply of NaOH to the water exiting the SWRO, and thereby adjust/increase the pH thereof, before entrance into the BWRO. Accordingly, the I/X module receives cleared lime water from the lime water preparation unit, and discharges a solution of NaOH that is communicated into the SWRO treated water. The I/X module can then be regenerated with sodium ions by periodically being backwashed with brine from the BWRO.

Figure 4 illustrates the incorporation of the line water preparation unit within the operation of the desalination process taking place in the first water treatment section of the herein disclosed system.

For simplicity, like reference numerals to those used in Figure 1A, shifted by 400 are used to identify components having a similar function. For example, component 426 in Figure 4 is a module having the same function as module 126 in Figure 1A.

Specifically, Figure 4 illustrates a first water treatment section 402 comprising a salt water reverse osmosis (SWRO) module 440, that receives sea water 404 and communicating SWRO treated water into a Brakish Water Reverse Osmosis (BWRO) module 442 via communication line 444. Also illustrated is an ion exchange module 446 that receives cleared lime water, i.e. the lime water from lime water preparation unit 416 and specifically from water treatment filtration module 426 and discharges from ion exchange module 446 a solution of NaOH 448 that is fed back into communication line 444.

From time to time, ion exchange module 446 needs regeneration and to this end, as also illustrated in Figure 4, a dedicated communication line 450 communicates a portion of brine from BWRO 442 into ion exchange module 446, when the later is operated in backflow mode, to thereby discharge from ion exchange module 446, excess calcium hydroxide, which is then communicated into mixing chamber 420.

Also illustrated in this Figure 4, is the backwash of the lime water preparation unit 416 (as described also in Figure 2) where backwash water with lime/lime milk is transferred into the module 426 via communication line 460.

It is appreciated that certain features of the present disclosure, which are, for clarity, described in the context of separate examples/embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

The invention will now be exemplified in the following experiments. It is to be understood that these experiments are intended to be in the nature of illustration rather than of limitation. Obviously, many modifications and variations of these examples are possible in light of the above teaching. It is therefore, to be understood that within the scope of the appended claims, the invention can be practiced otherwise, in a myriad of possible ways, than as specifically described hereinbelow.

DESCRITION OF NON-LIMITING EXAMPLES

The following examples aim at showing that it is possible to convert lime milk into lime water using a filtration membrane and that this technique can be employed in continuous processes such as those performed in water treatment plants.

Specifically, the following non-limiting examples and the accompanied Figures 5 to 9 show at least one of the following:

An indication that the membrane/s being used is/are not plugged immediately after the commencement of the process by particles (e.g., solids particularly of calcium carbonate) - thus, a continuous process is not prevented/is possible.

An indication that the filtrate is clean and that its quality is superior over filtrates obtained following clarification by standard processes such as lamella clarifier (based on turbidity reported in Lamella clarification processes of lime water). The results are compared to the turbidity typically reported in Lamella clarification of the Lime water. no chemicals are required during the filtration process. In other words, the quality of the filtrate is achieved without the use of chemicals.

there is no need for chemicals in the backwash process of the membrane and in fact the membrane can be regenerated with backwash using treated water, without any significant increase in transmembrane pressure (TMP). In this connection it is noted that the term “Transmembrane pressure” refers hereinafter to the difference in pressure between two sides of a membrane. It is a valuable measurement because it describes how much force is needed to push water (or any liquid to be filtered - referred to as the "feed") through a membrane. A low transmembrane pressure indicates a clean, well-functioning membrane. On the other hand, a high transmembrane pressure indicates a dirty or "fouled" membrane with reduced filtering abilities.

A blocked membrane can easily be restored (return to normal TMP values) using an acceptable acid. Thus, the conditions of filtration / backwash acid cycle as a continuous recycling / regeneration process is doable.

Example 1 - Batch filtration at different fluxes

A volume of concentrated lime solution was filtered at three sequential batches.

For each batch, the filtrate was collected, and its pH, Temperature, conductivity and turbidity were measured. Backwash was at a backflush flux of 800LMH:

Batch 1 : filtration at a flux of 226LMH and filter differentiation pressure, Ap of 5.5mbar/L (average differential pressure),

Batch 2: filtration at a flux of 450LMH and filter differentiation pressure, Ap of 5.0mbar/L (average differential pressure),

Batch 3: filtration at a flux of 900LMH and filter differentiation pressure, Ap of 7.0mbar/L (average differential pressure).

Figure 5 shows the transmembrane pressure (TMP) as a function of operation time of the filtration. The low levels of TMP, or more accurately, the lack of significant increase of TMP are indicative that the membrane is still clean (and not plugged).

In addition, Figure 5 shows very stable pressure values at each tested flux. Back flush rate used in all the three batches was the same (800 l/m²h). This test was conducted three times, as also shown in Table 1 below.

The total volume of each test was always the same: 5 Liters of solution were filtered each time (as flux was different the time to filter the same volume was shorter from batch to batch).

Table 1 shows the analytical results for the lime solution and filtrate parameters, which support the formation of lime water, with a turbidity of less than 0.5NTU, which is much lower than the turbidity of lime water obtained in the common process, typically in the range of 2 to 10 NTU. Table 1: Analytical Results

It should be noted that in all experiments, no chemicals were used to achieve the filtrate quality.

Figure 5 and Table 1 thus provide the indication that (a) the membrane is not plugged; (b) the resulted filtrate’s quality is characterized by lower levels of turbidity (0.5 NTU and less); and (c) no chemicals are required during the filtering process to reach the resulted filtrate.

Figures 6A-6C are images of the lime solution (“Feed”, Figure 6A), the filtrate (“Permeate, Figure 6B) and the concentrate (“Cone”, Figure 6C), further supporting the efficiency of the filtration in reducing turbidity of the lime solution.

It should be noted that no chemicals were used to clean the membrane while the resultant filtrate’s quality was superior. This indicates that the use of acid to clean the membrane is executable and would provide even further superior parameters for the filtrate. This also indicates it is possible to create the conditions of filtration / backwash acid cycle as a continuous recycling / regeneration process. Example 2 - Continuous filtration at flux 450LMH

A continuous filtration test was carried out to verify if a constant concentration also leads to a constant filtration pressure. The test was done at a flux of 450 l/m²h. Backwash was performed with permeate water every hour, and at a flux of 800 l/m²h and at temperature of 32°C. the whole process continued for 16 hours.

Samples of the filtrate were taken after about 3 hours and 14.5 hours, to test turbidity of the filtrate. As in Example 1, no chemicals were used during the filtration process.

Figure 7 shows the TMP during the continuous filtration. Inter alia, Figure 7 provides the indication that (a) the membrane is not plugged; (b) the resulted filtrate’s quality is characterized by lower levels of turbidity (0.17 & 0.09 NTU); (c) no chemicals are required during the filtration process to reach the desired parameters for the filtrate; and (d) backwash with treated water and without any chemicals can maintain the membrane clean over time without significant increase of TMP. Thus, the present invention allows the avoidance of using chemicals at all, while achieving the target turbidity and maintenance of the membrane operational without increase in the TMP (i.e., the membrane remains clean without being plugged).

Alternatively, one should point out that if chemicals are used, the quality of the filtrate would be even higher.

The analytical results of two samples taken during this test are shown in Table 2.

Table 2: Analytical Results

Example 3 - Continuous filtration at flux 675LMH

A continuous filtration test was carried out at a higher flux from that used in Example 2, to determine fouling potential rate in higher fluxes. The filtration involved backwash of the membrane every 1 hour, at a backflush flux of 800LMH and at temperature of 32°C. Samples if the filtrate were taken at several time points. The whole test continued, continuously, for 52 hours.

Figure 8 shows the TMP during the test and Table 3 provides analytical results of the samples taken during the test. Table 3: Analytical Results

As can be seen from Figure 8 and Table 3, the TMP did not increase significantly for the first 36 hours over time. However, the same has gradually increased thereafter. What is important to note is that there is a gradual increase in the TMP and no sudden increase. Optimizing the system, flux and the no. of membrane that will used in the system will prevent said gradual increase in TMP.

Nevertheless, despite the increase the Filtrate continues to be with a good quality of lower than 0.5 NTU during the filtration duration.

Also in this case, no chemicals were added to the filtration process.

As in Example 1 and 2, Example 3 provides the indication that (a) the membrane is not plugged immediately and even elevated fluxes can be sustained; (b) the resulted filtrate’s quality is characterized by lower levels of turbidity (0.5 NTU and less); (c) no chemicals are required during the filtering process to reach the resulted filtrate; and, (d) backwash with treated water and without any chemicals can maintain the membrane clean over time without significant increase of TMP. Thus, we can remove chemicals at all, while achieving the target turbidity and maintenance of the membrane operational without increase in the TMP (i.e., the membrane remains clean without being plugged).

Needless to say, also based on these results, it can be assumed that if chemicals are used, the quality of the filtrate would be even better.

Example 4 - Backwash

To test the ability to recover the membrane TMP, demineralized water was filtered (as a starting point reference), followed by l,000ppm citric acid (the wash solution).

Figure 9 shows that cleaning with citric acid (as an alternative to HC1) was very successful. A short contact time with the wash solution (20 minutes) led to a complete regeneration of the membrane which was verified by treated water filtration (after 90 minutes).

Claims

CLAIMS:

1. A water treatment system comprising:

(a) a first water treatment section configured to receive water to be treated and discharge treated water; and

(b) a downstream second water treatment section configured to admix at least a portion of treated water from the first water treatment section with lime water, to thereby convert the treated water into finished water;

wherein said second water treatment section comprises a lime water supply unit comprising:

2. The system of claim 1, wherein said first water treatment section configured to remove one or more chemical substances from the water to be treated to thereby obtain said treated water.

3. The system of claim 1 or 2, wherein said lime water supply unit comprises two or more water membrane filtration modules.

4. The system of claim 3, where said lime water supply unit comprise a plurality of water membrane filtration modules arranged in parallel.

5. The system of any one of claims 1 to 4, wherein said water membrane filtration module comprises a membrane selected from an ultrafiltration (UF) membrane or a microfiltration (MF) membrane.

6. The system of claim 5, wherein when said lime water supply unit comprises two or more filtration modules, each module can comprise, independently, a UF membrane and a MF membrane.

7. The system of any one of claims 1 to 6, wherein said at least one water membrane filtration module comprises a ceramic filtration membrane.

8. The system of any one of claims 1 to 7, wherein each of said water membrane filtration modules has an inlet for receiving a wash solution and is configured to independently operate in a backwash mode and discharge backwash water released therefrom once being washed with the wash solution.

9. The system of claim 8, comprising a communication line for communicating the backwash water into another water membrane filtration module.

10. The system of any claim 8 or 9, wherein said wash solution is said water to be treated, with or without a strong acid and/or gas nano bubbles.

11. The system of claim 10, wherein said strong acid is HC1.

12. The system of claim 10 or 11, wherein said gas is CO₂.

13. The system of any one of claims 1 to 12, wherein said first water treatment section comprises a water desalination treatment system.

14. The system of claim 13, wherein said water desalination treatment system comprises at least a Sea Water Reverse Osmosis (SWRO) module in liquid communication with at least one downstream Brackish Water Reverse Osmosis (BWRO) module, and a liquid communication line for communication water discharged from the BWRO into the second water treatment section.

15. The system of claim 14, comprising a liquid communication line for communication treated water discharged from the SWRO and/or the BWRO into said mixing chamber.

16. The system of any one of claims 13 to 15, comprising a liquid communication line for communication treated water into said at least one water membrane filtration module when said water membrane filtration module is operated in backwash mode.

17. The system of any one of claims 13 to 16, comprising a liquid communication line configured to communicate a mix of said treated water and wash solution comprising a strong acid and/or gas nano bubblies prior to being introduced into the water membrane filtration module that is operated in backwash mode.

18. The system of any one of claims 13 to 17, comprising at least one ion exchange module configured to receive lime water and discharge sodium hydroxide solution, and a sodium hydroxide liquid communication line configured to feed the discharged sodium hydroxide solution into liquid communicated into the BWRO.

19. The system of any one of claims 13 to 18, comprising a brine communication line communicating brine discharged from the BWRO into one of the at least one ion exchange module.

20. The system of any one of claims 1 to 19, comprising a re-hardening sub-system configured to receive treated water discharged from the first water treatment section and discharge re-hardened water for subsequent mixing lime water communicated from the second water treatment section.

21. The system of any one of claims 1 to 19, comprising a softening sub-system configured to soften the water to be treated prior to it being introduced into the first water treatment section.

22. A method for treating water, the method comprises

— mixing the treated water obtained from the at least one treatment process with lime water in an amount that provides finished water; wherein said method further comprises a lime water production process comprising:

- mixing a portion of the treated water obtained from the at least one treatment process with calcium oxide (CaO) and/or calcium hydroxide (Ca(OH)2) powder to obtain lime milk; and

23. The method of claim 22, comprising passing the lime milk together with the portion of treated water through at least one water membrane filtration module comprising an ultrafiltration (UF) membrane or a microfiltration (MF) membrane.

24. The method of claim 23, wherein said at least one water membrane filtration module comprises a ceramic membrane.

25. The method of any one of claims 22 to 24, comprising adding said lime water into the treated water in an amount to provide a basic pH and/or positive LSI value of the treated water.

26. The method of any one of claims 22 to 25, comprising subjecting said at least one water membrane filtration module to a backwash process with a wash solution comprising treated water with or without any one of a strong acid and gas nanobubbles.

27. The method of claim 26, wherein said strong acid is HC1.

28. The method of claim 26 or 27, wherein said gas is CO₂.

29. The method of any one of claims 26 to 28, wherein said backwash process is actuated when pressure in the at least one water membrane filtration module exceeds a predetermined pressure threshold.

30. The method of any one of claims 26 to 28, wherein said backwash process is actuated periodically.

31. The method of claim 29 or 30, wherein said backwash process comprises washing the at least one water membrane filtration module in one or more washing pulses.

32. The method of claim 31, wherein each of said one or more washing pulses have a pulse duration of between several seconds to several minutes.

33. The method of any one of claims 22 to 32, comprising passing said lime water through a plurality of water membrane filtration modules.

34. The method of claim 32, wherein said plurality of water membrane filtration modules are arranged in parallel.

35. The method of any one of claims 22 to 35, comprising operating said lime water supply unit under conditions that provide lime water with a turbidity of below 0.5NTU.

36. The method of any one of claims 22 to 35, wherein said at least one treatment process comprises a desalination process.

37. The method of any one of claims 22 to 36, wherein said treated water is desalinated water.

38. The method of any one of claims 22 to 37, comprising subjecting the treated water obtained from the at least one treatment process to a re-hardening process prior to mixing the same with lime water.

39. The method of any one of claims 22 to 37, comprising subjecting the water to be treated to at least one water softening process, prior to being subjected to the at least one treatment process.

40. The method of any one of claims 22 to 39, comprising discharging treated water having a pH of between 7.0 and 8.5.