WO2018146692A1

WO2018146692A1 - Method and system for treatment of water bodies

Info

Publication number: WO2018146692A1
Application number: PCT/IL2018/050168
Authority: WO
Inventors: Ronen-Itzhak Shechter; Noam Mordechai SIEGEL
Original assignee: Fluence Water Products And Innovation Ltd
Priority date: 2017-02-13
Filing date: 2018-02-13
Publication date: 2018-08-16
Also published as: CN208561869U; CN108423846A

Abstract

The present disclosure provides a method for treating a natural water body for removal of pollutants, comprising (i) submerging one or more water treatment units having a water-tight enclosure that comprises an oxygen-permeable, water impermeable membrane for releasing oxygen into the water by permeation therethrough; (ii) fixing the one or more water treatment units within the water body in a manner to permit polluted portions of the water body to come into contact with said one or more units; and (iii) feeding oxygen-containing gas into the enclosure of said one or more units. Also provided by the present disclosure is a system for treating a polluted natural water body, comprising (i) one or more water treatment units each having a water-tight enclosure that comprises an oxygen-permeable, water impermeable membrane for releasing oxygen into the surrounding medium by permeation therethrough; (ii) a fixing arrangement for fixing said units within the water body such that the unit is fully submerged therein; (iii) at least one gas inlet in said unit configured for receiving an oxygen-containing gas; and (iv) at least one gas outlet.

Description

METHOD AND SYSTEM FOR TREATMENT OF WATER BODIES

TECHNOLOGICAL FIELD

The present disclosure concerns a method and system for cleaning bodies of water, such as lakes, rivers, ponds, canals.

BACKGROUND ART

References considered to be relevant as background to the presently disclosed subject matter are listed below:

International patent application publication No. WO 2011/073977

International patent application publication No. WO 2016/038606

Acknowledgement of the above references herein is not to be inferred as meaning that these are in any way relevant to the patentability of the presently disclosed subject matter.

BACKGROUND

Nowadays more and more natural water bodies, such as streams, canals, rivers and, lakes and ponds are being polluted by men. Pollutants and wastewater, containing solid organic particles, flow to the natural water sources. The organic particles tend to settle, accumulate at the bottom of the stream, and form a layer of unstabilized sludge.

Means to deal with polluted water may include, for example, removal and disposal of sludge from the bottom of water bodies by mechanical excavation or treatment of such removed sludge at external sites.

Systems for treatment of polluted water are described, for example, in International patent application publication No. WO 2011/073977 or WO 2016/038606.

WO 2011/073977 describes a system for treating wastewater including a water- treatment pathway having an inlet, an oxygen-permeable, water-impermeable wall, separating an interior of the pathway from outside air, and a treated wastewater outlet and arranged for at least aerobic treatment of the wastewater as it flows from the wastewater inlet to the treated wastewater outlet, wastewater supply conduit, supplying the wastewater to the wastewater inlet of the water-treatment pathway and a treated wastewater conduit, supplying treated wastewater from the treated wastewater outlet of the water-treatment pathway.

WO 2016/038606 describes a water treatment module, a bioreactor comprising one or more of such modules, a receptive water treatment system and method of using the same. The water treatment module comprises (i) an elongated gas enclosure comprising a gas inlet and two vertical walls, at least one vertical wall comprising a water-impermeable and gas-permeable membrane having a water-facing side and a gas- facing side, the two vertical walls separating between water external to said enclosure and gas within said enclosure, the gas enclosure being in a rolled or folded configuration to thereby define a convoluted horizontal path and one or more water- treatment spaces formed between opposite water facing sides of the enclosure; and (ii) a diffuser arrangement comprising gas diffusers configured for introducing a stream of gas into the one or more water treatment spaces.

GENERAL DESCRIPTION

Provided by this disclosure is a method and system for treating a natural water body for removal of pollutants therefrom.

In the context of the present disclosure, when referring to a natural water body it is to be understood as encompassing any form of accumulation of water, including still or contained water bodies, e.g. pools, ponds, wetlands, puddles, as well as flowing water body forms (navigable waterways), e.g. rivers, streams, canals. Further, in the context of the present disclosure, while preferably being naturally occurring geographical features, in some embodiments, the water bodies are artificial, e.g. reservoirs created by engineering dams, artificial harbors (e.g. created through construction), natural bays, waterways

The method and system disclosed herein make use of one or more water treatment units, which in accordance with this disclosure, each have a water-tight enclosure with an oxygen-permeable, water-impermeable membrane that release oxygen into the polluted water by permeation from the enclosure's interior, through the membrane, into the surrounding water. As such, the water tight enclosure may be defined as having an interior facing side constituting the gas facing side of the membrane, and an exterior facing side constituting a water facing side of the membrane. The water-tight enclosure of the treatment unit may be formed by two elongated, essentially parallel oxygen membranes that are sealed to one another, along their longitudinal edges. The units may be also formed from a single elongated membrane sheet folded, with its two longitudinal edges sealed to one another, thereby defining an elongated sleeve. The water-tight enclosure may also be formed as an elongated tube, e.g. by extrusion of the oxygen-permeable, water-impermeable membrane.

The treatment unit also comprises at least one gas inlet linked to a source of an oxygen-containing gas, and at least one gas outlet. In use, oxygen-containing gas ingresses into the enclosure through the at least one gas inlet, flows through the water tight enclosure and at least a portion thereof, flows out of the enclosure through at least one gas outlet.

The at least one inlet and at least one outlet are typically, but not exclusively, located at extreme opposite ends of the enclosure.

By some embodiments, each water tight enclosure may have a single inlet and/or a single outlet. In other embodiments, there may be a single out let and two or more such inlets and outlets. In yet some other embodiments, there may be two or more inlets and a single outlet. In yet some other embodiments, there may be, independently, two or more inlets and two or more outlets.

In some embodiments, the oxygen-permeable, water-impermeable membrane comprises or is a fabric formed of a first polymer, extrusion coated with a second polymer. The coating may be applied to the water-facing side of the fabric, once sealed to form the water tight enclosure.

In some embodiments, the thickness of the coating is between 5 to 20 microns.

In some embodiments, the first polymer is a polyolefin, such as polyethylene or polypropylene or polyester. In some embodiments, the polyolefin is a dense polyolefin (medium to high density). In some embodiments, the water-impermeable, oxygen permeable membrane is formed of a microporous material.

In some embodiments, the microporous material is a non-woven polymer.

The membrane may be microporous by its structure and water-impermeable as a result of surface tension and hydrophobic features of the membrane material.

At a regular operation the material permits oxygen diffusion therethrough from air at a lower pressure than hydrostatic pressure of the surrounding liquid. During irregularities, its microporous structure permits releasing pulses of pressurized gas bubbles as a trouble shooting mechanism upon clogging or accumulation of solids on the water side surface of the membrane.

In some embodiments, the non-woven fabric is a flashspun high-density polyethylene fibers such as Tyvek®, commercially available from Dupont. Notably, a flashspun polymer is one formed from fine fibrillation of a film by the rapid evaporation of solvent and subsequent bonding during extrusion. In some embodiments the second polymer comprises or is an alkyl -aciylate.

Without being bound by theory, the coating by alkyl aciylate substantially seals the first polymer to passage of water without significantly depriving oxygen passage (by diffusion therethrough).

In some embodiments, the alkyl-acrylate is one compatible with the first polymer, specifically, with polyolefin fabrics and more specifically polyethylene fabrics, so as to facilitate coating by extrusion.

In some other embodiments, the first polymer comprises or is a polystyrene fabric.

In some embodiments, the second polymer is poly-methyl -pentene, that is compatible with polyester fabrics for coating by extrusion.

The enclosure may be made entirely of the oxygen-permeable and water- impermeable membrane or only portions thereof may be made of such membranes.

When only portions of the enclosure are made of an oxygen-permeable and water-impermeable membrane, the other portions are at least water impermeable. By some embodiments, the water tight enclosure comprises one or more spacer elements disposed within the interior of the enclosure for separating the wall sections from one another and maintaining a minimal distance between the oxygen facing sides of the membrane, and as a result an open oxygen flow from the at least one inlet to the at least one outlet thereof. The spacers are typically important when the intended gas pressure within the enclosure is below hydrostatic pressure; as then the spacer elements, in addition to imparting overall rigidity, also prevent the walls from collapsing one against the other and thereby blocking the path flow.

The one or more spacer elements can have the general form of a grid or net. While they can constitute independent elements, e.g. a netting element disposed within the interior of the enclosure, in some embodiments, the spacer element is integrally formed on at least a portion of one of the membrane. For example, such integral spacer elements can be configured as abutments on the gas facing side of the enclosure. The abutments can have the form of rails, dimples, corrugations, hook like protrusions or any combination thereof.

In some embodiments, the one or more spacer elements has a thickness or is otherwise configured to maintain the minimal distance between facing membranes of between about 1 to about 20 mm, at times, between 1 to 10mm, at times, between 2- 4mm. As the spacer elements are configured to withstand the hydrostatic pressure they are made of rigid materials, for example plastic materials including, without being limited thereto, polyethylene, polyethylene terephthalate (PET), polypropylene, polyamide, polyacetal and any combinations of same.

By some embodiments, the treatment unit comprises a single water-tight enclosure may be used, for example, arranged in a spiral path or having a zig-zag, winding or serpentine path within the natural water body.

In some other embodiments, the treatment unit comprise more than one watertight enclosure; for example, extending parallel to one another within the natural water body. The exact configuration may vary according to the intended use. In accordance with the method disclosed herein, the one or more treatment unit is submerged within the natural water body to be treated and fixed within the water body in a manner to permit polluted portions of the water body to come into contact with the one or more treatment units, and then, in operation, oxygen containing gas is fed into the water-tight enclosure, typically via the dedicated inlet(s) of the one or more enclosures. In some embodiments, the oxygen-containing gas is fed into the enclosure in a continuous mode, in some other embodiments, the oxygen containing gas is fed into the enclosure intermittently.

The oxygen containing gas can be air, oxygen enriched air, pure oxygen etc. In some embodiments, the oxygen containing gas is air. In some embodiments, the oxygen-containing gas, fed into the enclosure, is typically air coming from a fan, pump or blower.

Through maintaining the pressure of the oxygen, below that of the surrounding hydrostatic pressure, the oxygen will permeate through the membrane, by fine permeation or diffusion, without causing bubbles that could stir the polluted water and negatively affect the efficiency of treatment.

At times, e.g. for the purpose of un-clogging or preventing accumulated solids on the exterior of the enclosure, namely, the water-facing side of the enclosure, pulses of pressured air, typically above hydrostatic pressure, are introduced into the enclosure, and causes air bubbles to be released through the microporous structure of the membrane, removing clogging solids that would otherwise continue to accumulate on the external surface of the membrane.

By one embodiment, gas can be passively released from the enclosure out of one or more gas outlets. In other embodiments, the gas can be actively drawn or pumped out of at least one gas outlet. The use of oxygen containing air provides conditions for aerobic degradation of the sludge within the natural water body.

In the water, sludge can be degraded either in anaerobic or aerobic conditions, each produce different degradation products. The anaerobic degradation leads to products that are also considered as pollutants, commonly measured as Biochemical Oxygen Demand (BOD or Chemical Oxygen Demand (COD) as well as nitrogen compounds measured as Total Kjeldahl Nitrogen (TKN) or ammonium -nitrogen

Degradation of the aerobic path is environmental preferable since its products include carbon dioxide that can be released to the atmosphere leaving no pollutants in the water.

Conventional aeration means for aerobic degradation include diffusers that release bubbles of pressurized air in the depth of the water. The operation of such diffusers typically consumes high energy and cause suspension of the solid particles, thus unwanted turbidity of the water. As noted above, the one or more treatment units are fixed in the water body in a manner to permit polluted portions of the water body to come into contact therewith.

In some embodiments, the one or more treatment units are fixed at the bottom of the water body. Such fixing may be, without being limited thereto, by coupling weight- imparting elements to the enclosure, for example, metal chains, metal weights, etc. or by anchoring the units at the bottom by constructing a base structure, e.g. made of concrete, and connecting the enclosure thereto.

Provided by this disclosure, as noted above, is also a system useful in carrying out the method of this disclosure. The system can comprise one or more treatment units with water-tight enclosures of the type described above, an arrangement for fixing the unit within the water body, such that the units are essentially fully submerged therein (at times, e.g. due to water movement, some minor portions may be exposed to the atmosphere).

In some embodiments, the system comprises a source of an oxygen-containing gas as well as a gas-feeding conduit arrangement linking the source to the at least one gas inlet. As noted above, the source of gas can be any one of a fan, blower or pump. It should be noted that other than a fan, pump or blower, where the oxygen-containing gas is air, the gas may also come from a pressurized source, such as oxygen-containing air, enriched air, or oxygen container.

In some embodiments, the system comprises a gas releasing conduit arrangement linked to the at least one gas outlet ducting such gas out of the enclosure of the one or more treatment units. While this conduit arrangement may be a passive one, linking the enclosure's gas outlet towards an external port, for releasing this excess gas into the atmosphere, it may also comprise a gas-drawing unit (e.g. pump) for actively drawing or pumping gas out of the gas-releasing conduit arrangement.

In some embodiments, the fixing arrangement for fixing the unit(s) submerged in the water body, typically at the bottom portion of the water body can comprise a weight-imparting element, metal chains, concrete element, and the like, each constituting a separate embodiment of the present disclosure. In some embodiments the weight-imparting element can be part of the enclosure, either integral with the membrane or enclosed within the enclosure.. In other embodiments the weight- imparting element can be detachably attached to or coupled with the enclosure, typically to a portion of the water-facing surface of the enclosure.

The system may, by some embodiments, comprise also a floating platform supporting one or more of (i) a source of oxygen-containing gas, (ii) a gas releasing unit or port for releasing gas into the atmosphere and (iii) a system controller. The invention will now be described with reference to some exemplary embodiments depicted in the annexed drawings. These exemplary embodiments are meant to illustrate the method and system of this disclosure but not intended to be limiting in any way. In other words, the scope of this disclosure applies to the full contents of the above disclosure and is not limited in any way to these exemplary embodiments.

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:

Figs. 1A-1F are schematic illustrations of a system according to an embodiment of the present disclosure, comprising two treatment units deployed horizontally within a water body (e.g. river); where Fig. 1A is an isometric view of the river having submerged therein the system according to this embodiment; Fig. IB is an enlarged view of a transverse cross section of one of the water-tight enclosures of a treatment unit illustrated in Fig. 1A; Fig. 1C is an illustration of gas source connected to the inlet of a treatment unit of Fig. 1A via inlet conduit arrangement; Fig. ID is a top view of a portion of the system of Fig. 2A illustration of the two water-tight enclosures fixed to the bottom of the body of water via dedicated supporting elements; and Fig. IE is an illustration of a gas outlet arrangement connected to the enclosure of a the treatment units of Fig. 1A.

Figs. 2A-2E are schematic illustrations of a system disclosed herein in accordance with another embodiment of the present disclosure, with the water-tight enclosures having a tubular form, where Fig. 2A illustrates a river having submerged therein the system according to this embodiment; Fig. 2B provides a cross section of a portion of the water-tight enclosure of Fig. 2A; Fig. 2C is an illustration of gas source connected to the inlet of a treatment unit of Fig. 2A; Fig. 2D is a top view of a portion of the treatment unit of Fig. 2C; Fig. 2E is an illustration of a gas outlet arrangement connected to the enclosure of a the treatment units of Fig. 2 A.

Figs. 3A-3E are schematic illustrations of a system disclosed herein in accordance with yet another embodiment of the present disclosure, in this case, the water tight enclosure having a sleeve-like configuration, where Fig. 3A illustrates a river encompassing the system according to this embodiment; Fig. 3B provides a cross section of a portion of the water-tight enclosure of Fig 3 A; Fig. 3C is an illustration of gas source connected to the inlet of a treatment unit illustrated in Fig. 3A; Fig. 3D is a top view of a portion of the treatment unit of Fig. 3C; and Fig. 3E is an illustration of a gas outlet arrangement connected to the enclosure of a the treatment units of Fig. 3 A.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is made to Figs. 1A-1F which are schematic illustrations of an exemplary embodiment of a natural water body 100 illustrated as a river, including a system 101 for treating the water body 100 from pollutants therein. System 101 comprises, in accordance with this non-limiting embodiment, two treatment units 102 comprising each water tight enclosures 104 constructed from two elongated and essentially parallel sheets comprising, at in a portion thereof, oxygen permeable, water impermeable membranes 106 sealed to one another along their longitudinal edges in a manner to form an internal flow path for gas, as further described below. The water tight enclosures 104 are longitudinally and horizontally deployed along the water body 100, one beside the other such that one of the two membranes 106 faces the bottom of the river. It is noted that the water-tight enclosure may be similarly deployed from a vessel sailing along the river. In this case, the water tight enclosure may be rolled as spiral and deployed within the water by unfolding while the vessel moves along the water body.

The system also comprises oxygen-containing gas inlets 108 fixed to one end 107 of the elongated enclosure 104 and connected via gas-feeding conduit arrangement 110 to a gas source 112, in this embodiment in a form of an air blower placed on the river bank, as also illustrated in Fig. 1C.

In some embodiments, not illustrated, each water tight enclosure can be fed from a different source, via a separate gas feeding conduit arrangement.

System 101 also comprises two independent outlets 114, at opposite ends 109 of each elongated enclosure 104. The gas outlets 114 are linked to a gas-releasing conduit arrangement 116 for ducting gas out of the enclosures.

In some embodiments, outlet 114 can be coupled to outlet conduit arrangement 116 through a flexible connection segment 111, such as that shown in Fig. IF, to allow flexibility and free movements of the outlet conduit arrangement 116 e.g. due to water flow in the river 100. Further illustrated is gas releasing conduit arrangement 116 that links the gas outlets 114 to port 115, where gas flowing from within the enclosure 104 is released to the atmosphere. As illustrated in this figure, Port 115 may be retained in position by fixation to a platform 117, such as a concrete platform.

Air flow along the pathway from inlet 108 to outlet 114 is ensured by the incorporation of spacer elements within the interior space of the enclosure 104. Specifically, spacer elements 118 are configured to impart rigidity to the structure of the enclosures 104 such that under pressure, namely hydrostatic pressure, the two facing membranes 106 forming the enclosure 104 will not collapse. In operation, gas is introduced into enclosure 104 at a pressure lower than the hydrostatic pressure surrounding the enclosure, and will be permeate through the membrane to the surrounding water by diffusion, without causing any gas bubbles that may cause turbulence and/or carry pollutants to the water surface. Spacer elements may be in the form of a gridded net, such as the spacer element 118 illustrated in Fig. IB, or as dimples facing inwardly from the surface of the oxygen impermeable, gas permeable membrane, spacer elements may be made of a plastic material. For example, the spacer elements may be made of a polyolefin such as polyethylene, polypropylene, or nylon or any other material typically used for the production of drainage nettings.

Further illustrated in Fig. IB are flexible linkers 122 such as metal chains, coupled to enclosure 104 at edges 120 and used for anchoring treatment units 102 to a bottom portion of the water body 100. In this manner, environmental changes that may affect water flow within the water body, e.g. storms, will not affect or only mildly affect the position of the treatment unit 102 within the water body. Metal chains 102 can be coupled or fixed to the bottom of the water body, or to a weight-imparting element such as weight or a concrete platform (not illustrated), or it can have its own sufficient weight to provide fixation to the bottom, as illustrated in Fig. 1C or Fig. IE. In addition, treatment units 102 may be held in place by supporting elements

124, as illustrated in Figs. 1C and ID. These supporting elements may also function to retain the two enclosures 104 in a desired distance therebetween. Support elements 124 may be formed as single rigid platform connected to the two (or more) treatment units, such that they mutually move within the water body. Reference is now made to Figs. 2A-2E and 3A-3E providing schematic illustrations of some other alternative embodiments according to this disclosure. In these Figures, like elements to those of the embodiment shown in Figs. 1A-1E have been given like reference numerals, shifted by a hundred or two hundreds, respectively. Thus, by way of example, the water-tight enclosure 204 and its oxygen-permeable, water-impermeable membrane 206 are functionally equivalent to water-tight enclosure 104 and its oxygen-permeable, water-impermeable membrane 106.

Specifically, Fig. 2A-2E show a treatment unit 202, in which the enclosure 204 has a tubular form, e.g. tubular pipe. In this embodiment, there is illustrated a single enclosure 204 being windingly deployed along the elongated water body crossing from one bank 221 of the water body, to the other bank 223 thereof. The tubular configuration of enclosure 204 is maintained by the use of internal spacer element 218 that may be a coiled spacer within the enclosure as shown in Fig. 2B. Further illustrated in Fig. 2B as well as in Fig. 2C are peripheral rings 220 circulating the enclosure along its length, to which chains 222 are coupled so as to fixate the enclosure to the bottom portion of the water body.

(in figs 1 A and IE it is not floating but fixed to the bank) Floating platform 217, shown in Figs. 2A and 2E, is fixed to outlet conduit arrangement 216, thus maintaining port 215 above water level such that gas is released from the enclosure to the atmosphere.

Turning now to Figs. 3A-3E there is illustrated a treatment unit 302 according to another embodiment of the present disclosure. As noted above, in these Figures, like elements to those of the embodiment shown in Figs. 1A-1E have been given like reference numerals, shifted by two hundreds. Yet, in this embodiment, enclosure 304 is in a form of an elongated sleeve-like membrane deployed in a windingly manner as described above with respect to Figs 2A-2E. The sleeve-like configuration may be defined by a cross-section of the enclosure in one dimension that is greater than its cross section in a second dimension thereof, the second dimension being perpendicular to the first dimension.

Claims

CLAIMS:

1. A method for treating a natural water body for removal of pollutants, comprising: submerging one or more water treatment units having a water-tight enclosure that comprises an oxygen-permeable, water impermeable membrane for releasing oxygen into the water by permeation therethrough; fixing the one or more water treatment units within the water body in a manner to permit polluted portions of the water body to come into contact with said one or more units; and feeding oxygen-containing gas into the enclosure of said one or more units.

2. The method of claim 1, wherein each of the water treatment units comprises:

a water-tight enclosure with oxygen-permeable, water impermeable membranes permitting oxygen permeation from the enclosure to a surrounding medium,

at least one gas inlet linked to a source of an oxygen-containing gas, and at least one gas outlet; and wherein said feeding comprises introducing the oxygen-containing gas into said at least one gas inlet such that said gas flows from the at least one gas inlet to the at least one gas outlet through said enclosure.

3. The method of claim 1 or 2, comprising actively drawing or pumping gas out of the at least one gas outlet.

4. The method of claim 1 or 2, wherein said unit is fixed at a bottom portion of the water body.

5. The method of any one of claims 1 to 3, wherein said water body is a lake, stream (e.g. river or brook), pond or canal.

6. The method of any one of claims 1 to 5, wherein the water tight enclosure is formed by two elongated and essentially parallel oxygen-permeable, water impermeable membranes sealed along their longitudinal edges.

7. The method of any one of claims 1 to 5, wherein the water tight enclosure is formed by a single elongated membrane sheet with its two opposite long edges being sealed to one another.

8. The method of any one of claims 1 to 5, wherein the water tight enclosure is a tube formed by extrusion.

9. The method of any one of claims 1 to 6 wherein the water tight enclosure is formed of microporous material.

10. The method of any one of claims 1 to 9, comprising one or more spacer elements within the enclosure for separating opposite wall sections from one another.

11. The method of any one of claims 1 to 10, wherein said enclosure is elongated and has a gas inlet at one end a gas outlet at the other.

12. The method of any one of claims 1 to 11, wherein each enclosure has one or both of (i) two or more gas inlets, and (ii) two or more gas outlets.

13. The method of any one of claims 1 to 12, comprising continuously introducing the oxygen-containing gas to the at least one gas inlet of the enclosure.

14. The method of claim 13, comprising continuously drawing or pumping gas out of the one or more gas outlets of the enclosure.

15. The method of any one of claims 1 to 12, comprising intermittently introducing the oxygen containing gas into the enclosure.

16. The method of claim 15, comprising intermittently drawing or pumping gas out of the one or more gas outlets of the enclosure.

17. The method of any one of claims 1 to 16, comprising introducing the oxygen- containing gas at a pressure that is lower than the hydrostatic pressure of the surrounding medium.

18. The method of any one of claims 1 to 17, wherein said oxygen-containing gas is air.

19. The method of any one of claims 1 to 18, comprising introducing the oxygen- containing gas into said enclosure by means of a blower or a fan.

20. The method of any one of claims 1 to 19, comprising releasing gas exiting from the one or more outlets to the atmosphere.

21. The method of any one of claims 1 to 20, for treating a sludge-containing natural water body.

22. A system for treating polluted natural water body, for use in a method of any one of claims 1 to 19.

23. A system for treating a polluted natural water body, comprising: one or more water treatment units each having a water-tight enclosure that comprises an oxygen-permeable, water impermeable membrane for releasing oxygen into the surrounding medium by permeation therethrough; a fixing arrangement for fixing said units within the water body such that the unit is fully submerged therein; at least one gas inlet in said unit configured for receiving an oxygen-containing gas; and at least one gas outlet.

24. The system of claim 23, comprising a source of an oxygen-containing gas, and a gas-feeding conduit arrangement linking said source to the at least one gas inlet.

25. The system of claim 24, wherein said source is a blower or fan and said gas is air.

26. The system of any one of claims 23 to 25, comprising a gas-releasing conduit arrangement linked to the at least one gas outlet for ducting gas out of said enclosure.

27. The system of claim 26, comprising a gas-drawing unit for drawing or pumping gas out of the gas-releasing conduit arrangement.

28. The system of any one of claims 23 to 27, wherein said fixing arrangement is configured for fixing the unit at a bottom portion of the water body.

29. The system of any one of claims 23 to 28, configured for deployment in a lake, stream (e.g. river or brook), canal or pond.

30. The system of any one of claims 23 to 29, wherein the water tight enclosure is formed by two elongated and essentially parallel oxygen-permeable, water impermeable membranes sealed along their longitudinal edges.

31. The system of any one of claims 23 to 29, wherein the water tight enclosure is formed by a single elongated membrane sheet with its two opposite long edges being sealed to one another.

32. The system of any one of claims 23 to 29, wherein the water tight enclosure is a tube formed by extrusion.

33. The system of any one of claims 23 to 32 wherein said fixing arrangement comprises either one of: (i) a weight-imparting element external to said enclosure; and (ii) a weight-imparting element internal to said enclosure.

34. The system of any one of claims 23 to 33, wherein the water tight enclosure is formed of microporous material.

35. The system of any one of claims 23 to 34, wherein the enclosure comprises one or more spacer elements disposed within the enclosure for separating opposite wall sections from one another.

36. The system of any one of claims 23 to 35, wherein said water-tight enclosure is elongated and has a gas inlet at one end and a gas outlet at the other.

37. The system of any one of claims 21 to 36, wherein each enclosure - has one or both of (i) two or more gas inlets, and (ii) two or more gas outlets.

38. The system of any one of claims 23 to 37, comprising one or more floating platform supporting one or more of (i) a source of oxygen-containing gas, (ii) a gas releasing unit or port for releasing gas to the atmosphere and (iii) a system controller.