Field

The present invention relates to an apparatus, system and method for desalination of groundwater,

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Groundwater is an important resource for domestic, agricultural and industrial use. It is particularly important in arid areas where groundwater is often the only reliable source of water supply. The quality of the groundwater, however, may be poor, brackish, saline or otherwise contaminated from diffusely distributed sources.

A system for in. situ desalination of groundwater has been previously described in international Publication No. WOOS/068371. The system operates in a single borehole casing extending into an aquifer wherein the borehole casing has two open or screened portions separated into respective upper and lower portions by a sealing means, A reverse osmosis unit powered by a submersible pump is disposed in the upper portion of the borehole casing and screen from which the groundwater is drawn. The permeate from the reverse osmosis unit is delivered to the surface, while the concentrate from the reverse osmosis unit is reinjected into the lower screened portion of the borehole and thence the aquifer, all powered via the submersible pump.

The aquifer around the lower portion of the borehole (the reinjection zone) may be partially and hydraulically separated from the aquifer around the upper portion of the borehole (the feed zone) by natural stratification of the aquifer which limits vertical hydraulic conductivity. Additionally, the concentrate is denser than the natural groundwater. These two factors limit the upward flow of reinjected concentrates into the upper screen of the borehole during pumping of groundwater for reverse osmosis treatment, and thus limit concentrate returns into the feed stream, ideally, less than 20% concentrate returns (percentage by volume of returned concentrate flow to total flow to the treatment vessel) can be readily tolerated without significantly impacting treatment efficiency of this system. However, there are circumstances where increased concentrate return flow may occur, for example where the feed zone is of limited depth and the reinjection zone is similarly limited or of low hydraulic conductivity. In these cases, the desalination system as described above is not suitable.

There is therefore a need for alternative or improved methods and systems for desalination of groundwater, in particular where groundwater pumping and reinjection of concentrates are spatially separated in p!an view within the aquifer so as to limit the effects of concentrate returns into the feed stream, referred to hereafter as dipole flow. i0

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

According to a first aspect, there is provided a system for desalination of groundwater, the system com rising:

a production borehole casing disposed, in use, in a production borehole or well, 20 the production borehole casing being adapted for ingress of groundwater therein from at least a portion of the depth of the aquifer in which groundwater resides;

a water treatment system, the water treatment system being adapted to treat groundwater ingress and thereby produce a permeate having a lower concentration of dissolved solids than the groundwater and a concentrate having a higher concentration 25 of dissolved solids than the groundwater;

a permeate conduit configured to transport the permeate produced from the water treatment system to the surface for use above ground; and,

a concentrate conduit configured to transport the concentrate produced from the water treatment system to one or more reinjection boreholes, the one or more so reinjection boreholes being spaced apart from the production borehole at a distance from, and in a direction relative thereto, being adapted for ingress of groundwater and reinjection of concentrate into the aquifer through at least a portion of the depth of the aquifer, to minimise dipole flow of the concentrate from the one or more reinjection boreholes to the production borehole. !n one embodiment the water treatment system may be disposed in situ in the production borehole casing. For example, for downhoie treatment, the system may comprise a submersible pump disposed below groundwater level within the borehole for maintaining ambient groundwater quality of the feed stream. ingress of ambient groundwater into the production borehole casing may be through a screened section.

The water treatment system may comprise a vessel and one or more treatment elements housed therein, the vessel having an inlet to receive groundwater, an outlet for permeate, and an outlet for concentrate; and a pump disposed below the vessel for passing groundwater ingress from the production borehole casing to the vessel.

Generally, the pump may be disposed in situ in the production borehole casing.

The permeate outlet and the concentrate outlet of the vessel may be in fluid

communication, respectively, with the permeate conduit and the concentrate conduit.

The production borehole casing may comprise a wall including a screened portion to allow groundwater to ingress therethrough. Generally, the screened portion is disposed at a lower end of the casing. Advantageously, the screened portion may penetrate the full depth, or only a partial depth of the aquifer.

The system may further, optionally, comprise one or more seafs for sealing against the wall above and/or beiow the screened portion to inhibit mixing of water above and below the seal within the production borehole casing. The one or more seals may be positionabSe within the production borehole casing. in one embodiment, the vessel, associated pump and the one or more seals may be supported within the borehole casing by a frame member. The frame member may comprise a plurality of plugs connected to respective end flanges of the vessel and rigid strips interconnecting said flanges.

In a further embodiment, the one or more reinjection boreholes may be provided, respectively, with a reinjection borehole casing including a screened portion to allow reinjection of concentrate. The screened portion may be disposed vertically throughout the entire dept of the aquifer, or partially within the aquifer and at a lower level than the relative location from where groundwater is pumped in the production borehole to limit dipole flow between the reinjection borehole and the production borehole, in an alternative embodiment, the one or more reinjection boreholes may be provided, respectively, with a seal, positionable within the reinjection borehole, to limit air or oxygen ingress into the one or more reinjection boreholes. In this way, the

positionable seal limits the formation of iron floes which can cause borehole clogging during reinjection.

The reinjection borehole casing may comprise an upper portion and a lower portion having either the same or a smaller diameter than the upper portion, the lower portion being provided with a screened portion for egress of concentrate therethrough. The seal may be positioned to seal the lower portion above the screened portion to inhibit mixing of oxygenated water within the upper part of the casing which is exposed to air and concentrate within the reinjection borehole which is often anoxic. in a second aspect, there is provided a method for desalinating groundwater, the method com rising:

providing a production borehole casing in a production borehole or well, the production borehole casing being adapted for ingress of groundwater therein;

providing one or more reinjection boreholes;

passing groundwater from the production borehole to a water treatment system, the water treatment system being adapted to treat groundwater ingress and thereby produce a permeate having a lower concentration of dissolved solids than the groundwater and a concentrate having a higher concentration of dissolved solids than the groundwater;

treating the groundwater ingress in the water treatment system to produce the permeate and the concentrate;

transporting the permeate produced from the water treatment system to the surface for use above ground; and,

reinjecting the concentrate produced from the water treatment system in the one or more reinjection boreholes, wherein the one or more reinjection boreholes are spaced apart from the production borehole at a distance from, and in a direction relative thereto, to minimise dipole flow of the concentrate from the one or more reinjection boreholes to the production borehole. In a third aspect there is provided a reinjection borehole casing, for use in reinjection boreholes, adapted to limit air or oxygen ingress into the reinjection borehole when concentrate is reinjected into the reinjection borehole, the borehole casing comprising an upper portion and a lower portion having a smaller diameter than the upper portion, the lower portion being provided with a screened portion for egress of concentrate therethrough, and a sea! for sealing the lower portion above the screened portion to inhibit mixing of oxygenated water and concentrate within the reinjection borehole.

The upper portion of borehole casing may be of larger diameter relative to the lower portion to allow the ready insertion of a sealing means.

The seal for sealing the lower portion may have a smaller diameter than thai of the upper portion.

Brief Description of the Drawings

Notwithstanding any other forms which may fall within the scope of th system and method as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a system for desalination of groundwater in accordance with one embodiment;

Figure 2 is a schematic representation of a production borehole in the system shown in Figure 1 , depicting a borehole casing and water treatment system relative to depth of placement with respect to a confined groundwater system;

Figures 3a and 3b show schematic representations of two types of reinjection boreholes in the system with screened portions either the same diameter as the casing where the sealing means is an inflatable packer (Figure 3a), or where the screened portion is of smaller diameter, where the sealing means is a resiliency deformab!e material such as a polymeric material, natural or synthetic rubber (Figure 3b);

Figure 4 is a schematic representation of an upper portion of a vessel in the water treatment system shown in Figure 2; Figure 5 is a cross-sectional view of the upper portion of the vessel shown in Figure 2;

Figure 8 is a schematic representation of a lower portion of the vessel in the water treatment system shown in Figure 2;

Figure 7 is a schematic representation of a frame member that may be optionally employed to support the water treatment system shown in Figure 2 when disposed in situ in the production borehole casing;

Figure 8a is a FEFLOW numerical model simulation of horizontal (plan) distribution of salinity (TDS) of groundwater from continuous pumping for 5 years of brackish groundwater for RO treatment and reinjection using the same 6m deep aquifer as in Table 1 ;

Figure 8b is a FEFLOW simulation of a vertical section showing induced salinity (TDS) distribution from pumping and reinjection, following 5 years of continuous operation. Regional groundwater flow is left to right;

Figures 9a is a FEFLOW simulation of a horizontal (plan) distribution of salinity (TDS) from continuous groundwater pumping for downhole RO treatment for 5 years from an upper aquifer unit and reinjection from a lower aquifer unit each separated by a semi-confining layer as in Table 2: (i) salinity (TDS) of the upper layer (ii) TDS of the lower reinjection layer;

Figure 9b is a FEFLOW simulation of induced salinity (TDS) in a vertical section following 5 years of continuous operation of downhole desalination showing the pumping and the reinjection wells 200m apart in upper (pumping) and lower

(reinjection) aquifer units separated by a semi-confining layer. Note significant vertical movement of more saline water into the confining layer below the lower aquifer unit. Regional groundwater flow is left to right; and,

Figure 10 are modelled results for the system shown in Table 4 of the trial field site showing production bore (green flag), injection bore ~100m from production bore to the SSE (yellow cross), very limited dipoie flow towards the production bore and complex groundwater salinity close to a tidal river. B1-3 are monitoring bores.

Detailed Description in one aspect, the present application relates to a system for desalination of groundwater.

Desalination of groundwater

The term 'desalination' is used broadly to refer to any one of several processes that remove dissolved solids, in the form of salts and other mtnerais, from saline o brackish water to produce a permeate having a lower concentration of dissolved solids than the saline or brackish water and a concentrate having a higher concentration of dissolved solids than the saline or brackish water. Examples of membrane desalination processes include, but are not limited to, reverse osmosis, nanofiltratton, ultrafiltration, electrodialysis, and so forth.

The reference to 'groundwater' refers to water located beneath the earth's surface in solid pore spaces and in the fractures of rock formations. Groundwater may be extracted from an aquifer which is an underground layer of water-bearing permeable rock or unconsolidated materials (e.g. gravel, sand, silt, porous rock such as sandstone or limestone or variably fractured solid rock materials such as granites, basalt or metasedimentary rocks) that contain and transmit groundwater.

Aquifers may be ciassified as unconfined, semi-conftned and confined. An unconfined aquifer is one where water can flow directly between the surface and the saturated zone of the aquifer. Semi-confined and confined aquifers, respectively, are overlain and variously confined by a relatively impermeable layer of rock or substrate with low hydraulic conductively (i.e. an aquitard) or an impermeable rock or substrate (i.e. an aquiclude).

Groundwater may be obtained from aquifers by penetrating the aquifer with a well or borehole, these commonly being referred to as production boreholes or production wells if groundwater is to be extracted for water supplies. Water that flows from the surrounding permeable substrate of the aquifer into space formed by the production borehole can then be pumped to the surface, !t will be appreciated that the depth of the weil or the borehole drilled into the substrate will be dependent on the location and depth of the aquifer below the surface.

There is considerable variability in groundwater salinity (i.e. concentration of total dissolved solids (TDS)} between aquifers and within aquifers, both temporally and spatially, particularly where recharge is intermittent and spatially variable.

Groundwater is often considered brackish where groundwater contains between 1 ,000 mg/L to 15,000 mg/L as TDS although the definition of brackish is not precise. In comparison, fresh water contains under 1 ,000 mg/L TDS and seawater contains generafly 30,000 mg/L to 35,000 mg/L TDS. Saline groundwater is often considered to contain between 15,000 mg/L to 50,000 mg/L TDS, whilst water with TDS much higher than this is termed hypersaline, although again the definition of this type of salinity is not precise.

System for desalination of groundwater

Embodiments of the system for desalination of groundwater will now be described by way of example only, and with particular (though not exclusive) reference to in situ desalination of groundwater from a confined aquifer.

Referring to Figures 1 to 7, there is shown partial and schematic representations of a system 10 for desalination of groundwater from an aquifer 100 having a hydraulic gradient 110 disposed beneath an impermeable zone 120. As shown in Figure 1 , the direction of groundwater flo is from left to right in accordance with the hydraulic gradient 110. in Figure 1 a production borehole 130 extends through the impermeable zone 120 and into the aquifer 100. Groundwater proximal to the production borehole 130 flows into the production borehole 130 through a screened section 14 from which it is thence treated by a suitable desalination technique to produce a permeate having a lower concentration of TDS than the groundwater and a concentrate having a higher concentration of TDS than the groundwater. In the embodiment shown in Figure 1 , the screened section 14 partially penetrates the aquifer. In alternative embodiments (not shown), the screened section 14 may penetrate the entire aquifer, it will be appreciated that partial or entire penetration will depend on the aquifer depth and characteristics such as hydraulic conductivity. These factors are taken into

consideration in order to limit dipole flow between the production borehoie and one or more reinjection boreholes.

Figure 1 also shows a reinjection borehoie 140 disposed downstream with respect to the production borehole 130 and the direction of groundwater flow, although it will be appreciated that more than one reinjection boreholes 140 may be disposed

downstream with respect to the production borehole 130. The reinjection borehoie 140 extends through the impermeable zone 120 and deeper into the aquifer 100 than the production borehole 130 for reasons explained above and in later paragraphs. If wilt be appreciated that the screened portions 14 may either penetrate the entire aquifer o partially penetrate the aquifer as shown in Figure 1 , depending on the aquifer depth and hydraulic conductivity of the aquifer, to limit dipole flow between the production borehole and one or more reinjection boreholes.

The purpose of the one or more reinjection boreholes is to receive the concentrate produced by the desalination technique employed to treat the groundwater. The one or more reinjection boreholes 140 may be spaced apart from the production borehole 130 at a distance from, and in a direction relative thereto, to minimise dipole flow of the concentrate from the one or more reinjection boreholes 140 to the production borehole 130. The spacing between the one or more reinjection boreholes 140 and the production borehole 130 may be with regard to the lateral spatial distance and/or with regard to relative depth in the aquifer. Generally, a dipole flow of about 20% by volume or less is acceptable.

'Dipole flow' occurs when concentrate from the production borehole 130 is reinjected into one or more reinjection boreholes 140 which locally increases the hydraulic head or pressure adjacent to the point of reinjection at the reinjection borehole 140. In addition a local zone of low hydraulic head or pressure develops at the location of groundwater withdrawal at the production borehoie 130. The combination of localised zones of high and low hydraulic head induce a localised hydraulic gradient

superimposed on the regional hydraulic gradient 1 0, and some dipole flow of reinjected water takes place under this induced gradient generally between the reinjection boreholes 140 and the production borehole 130. It is desirable to minimise dipo!e flow of the concentrate from the one or more retnjection boreholes 140 to the production borehole 130 to reduce the potential for mixing (and therefore

contamination) to occur between the concentrate and groundwater in the aquifer proximal to the production borehole 130. Mixing of the concentrate with groundwater (thereby raising the effective TDS in the groundwater) in the aquifer proximal to the production borehole 130 would decrease the efficiency of the desalination system 10.

The relative placement of the one or more retnjection boreholes 140 with respect to the production borehole 130 to minimise dipole flow may be determined by various modelling techniques as will be well understood by those skilled in the art with reference to various characteristics of the aquifer including, but not limited to, the hydraulic conductivities, transmissivities, porosities and storativities of feed and retnjection zones, aquifer thicknesses, hydraulic heads and lithological variabilities (ie. stratifications) and groundwater and concentrate salinities (TDS) and so forth

Examples of suitabie groundwater flow and transport modelling software packages which simulate both groundwater flow and solute (salt) transport include, but are not limited to, FEFLGW, MOD FLOW, groundwater codes within MIKE-SHE, and so forth.

As shown in Figure 2, the system 10 includes a production borehoie casing 12 disposed in the production borehole 130. The production borehoie casing 12 is adapted for ingress of groundwater via a screened portion 14. The screened portion 14 may take the form of a slotted casing, a perforated casing, or a mesh such as a wire wound mesh. Preferably, perforations i the screened portion 14 are sized in the range of about .0 to about 0.7 mm or similar to reject sand particles and provide at least about 80% efficiency. It will be appreciated that at least part of the screened portion 14 of the production borehole casing 12 will be positioned so as to be located below the upper surface of the groundwater in the aquifer 100.

The production borehole casing 12 is generally tubular and may be of a range of dimensions having regard to the dimensions of the treatment system 16 emplaced within the borehole or well, the nature and depth of the aquifer, and the hydrogeoiogy and terrain in the area of the borehole 130. It will be appreciated that the production borehoie casing 12 will have a sufficient diameter for the water treatment system to be emplaced within the borehoie. Borehole casing nominal diameters would typically be of the order of 200mm or 250 mm for a preferred water treatment system, in some cases the aquifer 100 will be relatively near the ground surface and the relative length of the borehole casing 12 and screened portions 14 thereof can be short {e,g. of the order of several metres such as at least about 10-20 metres) whereas in other locations the hydrology and terrain may dictate relatively long borehole casing 12 from 20 metres up to 250 metres in length, although it is conceivable that the system would operate at greater depths. The production borehole casing 12 may be manufactured from impermeable materials which are preferably resistant to corrosion, such as stainless steel or more usually of polymeric materials (e.g. PVC).

The system 10 also includes a water treatment system 16. The water treatment system 16 is adapted to treat groundwater received in the production borehole casing 12 and thereby produce a permeate having a lower concentration of dissolved solids than the groundwater and a concentrate having a higher concentration of dissolved solids than the groundwater.

In the embodiment shown in Figure 2, the water treatment system 16 comprises a vessel 18 and one or more membrane treatment elements 20 housed therein.

Generally, two or more membrane treatment elements 20 may be arranged longitudinally in series in the vessel 8 as shown in Figure 2. The one or more membrane treatment elements 20 may take the form of spiral-wound elements selected from the group comprising reverse osmosis (RO) membranes, nanofiitration membranes, ultrafiltration membranes, electrodia!ysis reversal systems and/or combinations thereof. Reverse osmosis membranes are particularly preferred.

The vessel 18 may be a pressure vessel, preferably designed to withstand water pressures of up to 2000 kPa. Preferably the vessel 18 may be constructed from light but durable materials such as fibre reinforced plastic (FRP), as used in many current conventional RO desalination systems. The dimensions of the vessel 18 may be such that there may be a 10-12 mm gap between the outer surface of the vessel 18 and the production borehole casing 12 when the vessel 18 is disposed downhole in the production borehole 130.

The vessel 18 is provided with an inlet 22 to receive groundwater, an outlet 24 for permeate and an outlet 26 for concentrate. The water treatment system 18 may be provided with a pump 2β, in the form of a submersible pump. Generally, the pump 28 is disposed in situ in the production borehole casing 12 below the surface of the groundwater level received in the production borehole casing 12 and below the vessel 18. The pump 28 is configured for passing groundwater received in the production borehole casing 12 to the inlet 22 of the vessel 18, The pump is of suitable size to be located within the borehole, and to provide a groundwater flowrate determined from characteristic relationships given by manufacturers of membrane treatment systems between inlet f!owrates, water TDS and required permeate flowrates, which determine required operating pressures at the inlet and outlet to the system.

For in situ desalination, the submersible pump would be emplaced as low as possible below the groundwater level 110 within the borehole, to maintain ambient conditions of groundwater qualit at the pump intake. The pump provides the required flow rate of feed groundwater to the vessel 18, and also provides feed pressures for treatment through use of a flow control valve on the vessel outlet (not shown). Additional pressure is also required to lift permeate from the vessel 18 to the surface (the latter usually referred to as permeate backpressure) which increases overall feed pressures for treatment proportionally. Operating feed pressures also need to be adjusted for hydraulic heads imposed on the treatment system when the system is operating below the water table 1 10. Given the above regime of pressures and hydraulic heads within the bore, permeate backpressure would generally be close to zero if the vessel 18 was emplaced near the top of the borehole, or equal to the head required to lift permeate from the water table to the surface if the lower end of the vessel was immediately above, or below groundwater level.

Generally, there is also sufficient pressure at the concentrate outlet from the vessel to lift concentrates to the surface and in addition for these to be reinjected through reinjection boreholes by gravity. This would be assisted if the reinjection point was at a lower elevation than the submersible pump and if the vessel, concentrate tube and reinjection point form a closed loop, in which case suction pressure would assist with reinjection. In some cases, concentrates may need some additional pressurisation from an auxiliary pump at the surface for more effective reinjection, dependent on the characteristics of the receiving aquifer. Said membrane treatment elements 20 may comprise cylindrical spiral-wound membranes which are aligned to provide a central passage 30 inboard of the elements and an annular passage 32 outboard of the aligned elements. Each element 20 may be provided with a brine seal 34 disposed between respective annular passage 32 and the production borehole casing 12. The purpose of the brine seats 34 is to urge flow through successive membrane treatment elements 20.

The groundwater received in the vessel 18 may pass successivety through each of the elements on the feed side of the membrane, and permeate collects in the central passage 30 after passing through the membranes in each element, as in conventional reverse osmosis treatment systems. The permeate is then forced via permeate outlet 24 to the surface via permeate conduit 34. Permeate conduit 34 is configured to transport the permeate produced from the permeate outlet 24 of the water treatment system 16 to the surface for use above ground, such as for domestic, industrial or agricultural purposes.

The resutting concentrate collects on the feed side of the membrane in each element 20 and passes to the concentrate outlet 26 through a flow control valve (not shown in Figure 2). Concentrate conduit 38 is configured to transport the concentrate from concentrate outlet 26 of the water treatment system 6 to one or more reinjection boreholes 140. Preferably, the concentrate conduit 38 is closed to atmosphere

The system may further comprise respective reinjection borehole casings 40 disposed in the one or more reinjection boreholes 140 as shown in Figures 3a and 3b, wherein the reinjection borehole casing(s) 40 may be adapted to limit air or oxygen ingress into the one or more reinjection boreholes 140 through a sealing means 44.

In one embodiment, the reinjection borehole casing(s) 40 may be provided, with a screened portion 42 to allow egress of concentrate into the reinjection borehole 140 through a seal 44, The seal 44 may be positionable within the reinjection borehole casing 40 above the screened portion 42 to prevent mixing and reaction between the concentrate and oxygenated water in the reinjection borehole 140 above the screened portion 42.

!t is desirable to limit introduction of air or oxygen into the reinjection borehole 140 to reduce the likelihood of undesirable chemical reactions between chemically-reduced iron and manganese ions in the concentrate and dissolved oxygen, which would result in formation of suspended floes of iron and manganese oxyhydroxides and other colloidal materials. Such materials have the potential to clog the screened portion 42 and/or the aquifer media.

The seal 44 may be formed of any suitable material which wilt enable the seal 44 to abut the wall of the reinfection borehole casing 40 so as to inhibit the passage of water about the seal 44. In one embodiment, the seal 44 may take the form of an inflatable packer which can be selectively inflated to sea) the reinjection borehole casing 40 and subsequently deflated in order to remove or reposition the seal 44 in the reinjection borehoie casing 40 (Figure 3a). Alternatively, the seal 44 may be formed from a resiiiently deformable materia! such as a polymeric material, natural or synthetic rubber (Figure 3b).

The seal 44 may be provided with an aperture (not shown) therethrough to receive the concentrate conduit 38. Consequently, concentrate can be passed through the concentrate conduit 38 and reinjected into the reinjection borehoie casing 40 below the seal 44. Preferably, the sea! 44 and the screened portion 42 will be located deeper in the reinjection borehole casing 40 relative to the screened portion 14 in the production borehole casing 12 emplaced some distance hydraulically upgradient of the reinjection casing 40 as in Figure 1. In this way, the flow of the reinjected concentrate is not only influenced by the local hydraulic gradient but also the vertical transmissivity and hydraulic conductivity of the strata above where the reinjection bore 140 and its screened portion 14 are located. In view of the higher concentration of IDS in the concentrate, the concentrate will be denser than the local groundwater and therefore have a tendency to flow downwardly through the aquifer away from the production borehole 130.

The depth of the screened portion 42 is preferably determined to minimise dipole flow of the concentrate from the one or more reinjection boreholes 140 to the production borehoie 130. The depth of the screened portion 42 may be determined by

investigation and hydraulic testing of the aquifer 100 and using various modelling techniques to define dipole flow between the reinjection boreholes 140 and the production borehole 130. Groundwater and solute transport modelling will be well understood by those skilled in the art with reference to several characteristics of the aquife including, but not limited to, the hydraulic gradient 110, hydraulic heads in the aquifer, hydraulic conductivity, porosity, storativity and transmissivities of feed and reinjection zones, aquifer thickness and so forth, in some embodiments the reinjection borehole casing 40 may be further configured to limit mixing of the concentrate with dissolved oxygen or air. For example, the

reinjection borehole casing 40 may comprise at least two portions comprising an upper larger diameter portion 40a and a lower smaller diameter portion 40b which is attached to the larger diameter portion 40a. The upper portion may be attached to the lower portion with a reducer 40c which seals the join between said portions. Generally, the screened portion 42 will form part of the lower portion 40b of the reinjection borehole casing 40.

Referring more specifically to Figures 4 to 7, there is shown a series of more detailed views of one embodiment of the vessel 18 and a frame member 46 for supporting the vessel 18 in the production borehole casing 12.

An upper portion 48 of the vessel 18 is shown in Figures 4 and 5. The upper portion 48 is provided with an upper end flange 50 and a plug member 52 configured to engage with the upper end flange 50. The plug member 52 may be in the form of a cylindrical plug formed from stainless steel. A plurality of equi-angulariy spaced bolt holes 54 ma be provided in a cylindrical wail of the plug member 52 and the upper end flange 50. For example, in the embodiment shown in Figure 4, three bolt holes spaced at 120° from one another are respectively provided in the cylindrical wail of the plug member 52 and the upper end flange 50. A further three holes are similarly spaced in the upper part of the plug 52. in use, the plug member 52 is received and mounted in the upper end flange 50 by means of bolts extending through the respectively aligned bolt holes 54. The bolts also attach the frame member 46 to the end plug 52 and flange 50

The permeate outlet 20 of the vessel 18 is attached to a hollow cylindrical tube 58 that extends through a central longitudinal axis of the plug member 52. An upper end 58 of the hollow cylindrical tube 56 engages with the permeate conduit 34. A lower end 60 of the hollow cylindrical tube 56 engages with a central permeate line 20 from the uppermost treatment membrane unit.

Preferably the lower end 60 of the hollow cylindricai tube 56 is firmly attached to the plug member 52 because in some embodiments the treatment system 16 may be suspended on the central permeate line and thus needs to be well anchored to the piug member 52 and the vessel 18,

The concentrate outlet 26 of the vessel 18 is defined by a hollow cylindrical tube 62 eccentrically disposed in the piug member 52 extending into the vessel 18. An upper end 64 of the hollow cylindrical tube 62 engages with the concentrate conduit 28, which may be a flexible plastic hose or similar. A lower end 66 of the hoflow cylindrical tube 62 extends into the annular passage 32 of the vessel 18.

A lower portion 66 of the vessel 18 is shown in Figure 6. The lower portion 66 is provided with a lower end flange 68 and a plug member 70 configured to engage with the iower end flange 68, The plug member 70 may be in the form of a cylindrical plug formed from: stainless steel. A plurality of equi-anguiarly spaced bolt holes 72 may be provided in a cylindrical wail of the plug member 70 and the Iower end flange 68. For example, in the embodiment shown in Figure 6, three bolt holes spaced at 120° from one another are respectively provided in the cylindrical wall of the plug member 70 and the lower end flange 68. in use, the plug member 70 is received and mounted in the Iower end flange 68 by means of bolts extending through the respectively aligned bolt holes. A further three bolt holes are similarly spaced in tower part of the plug member, in use, bolts are inserted in the holes to attach the frame member 46 to the flange 68 and the end plug 70.

The inlet 24 of the vessel 18 is defined by a hollow cyiindrical tube 74 that extends into the plug member 70 and into the annular passage 32 of the vessel 8. In some embodiments, the inlet 24 and the hollow cylindrical tube 74 are firmly attached to the plug member 70.

Referring now to Figure 7 there is shown the frame member 46 for supporting the vessel 18 in the production borehole casing 12. In one embodiment the frame member 46 may comprise three or more rigid vertical strips 78 extending from and connected to each of the upper and Iower flanges 50, 68 of the vessel 18 and respective plug members 52, 70. The vertical strips may be approximately 20 mm wide and 5 mm in depth. Spaced annular members configured to encircle an outer surface of the vessel 18 may be connected to the strips 78 for added strength (not shown in Figure 8.

Generally the annular members and the vertical strips 78 will be formed from metals or alloys such as stainless steel.

Respective uppermost and lowermost plug members 52 (fig 4) and 70 (fig 6) and the end flanges 50 and 68 are connected to each of the verticai strips 78 by bolts 76 (fig 5) extending through the strips 78, and through the vessel flange through holes 54 (fig 4} and 72 (fig 6) extending into threaded holes in the end plugs 52 and 70. The strips 78 are also attached to the upper parts of the end plugs where these abut the ends of the vessel flanges by a second set of bolts 76 (Figure 8). in this way, the strips of frame member 46 and attached end plugs 52 and 70 support the weight of the vessel 18, its contents and the pump 28 when it is disposed downhole in the production borehole 130. In this way, the system may be suspended within the borehole casing so that pressure leakage from the inlets and outlets to the vessel 18 arising from flexure thereat is precluded.

The production borehole casing 12 may be optionally provided with a seal 82 for sealing against the production borehole casing 12 below the screened portion 14 to inhibit mixing of water above and below the seal 82 within the production borehole casing 12. The seal 82 may be positionable within the production borehole casing 12. The seal 82 may be formed of any suitable material which will enable the seal to abut the wail of the production borehole casing 12 so as to inhibit the passage of water about the seal 82. in one embodiment, the seal 82 may take the form of an inflatable packer which can be selectively inflated to seal 82 the production borehole casing 12 and subsequently deflated in order to remove or reposition the seal 82 in the production borehole casing 12. Alternatively, the seal 82 may be formed from a resiliently deformable material such as a polymeric material, natural or synthetic rubber.

The system 10 may further comprise one or more service lines extending from the water treatment system 16 to the surface. Such service lines may be in the form of pressure lines for measuring treatment pressures within the vessel 18, sampling lines for sampling groundwater, permeate and concentrate, and treatment lines for delivery 16

of membrane cleaning fluids, !n this way, the water treatment system 16 may remain in situ in the production borehole casing 12 for extended periods without the need to remove the water treatment system 16 for servicing. The pump 28 may be provided with a gate vaive in the form of a non-return valve when the pump 28 is not operational to hydrauiicaily isolate the vessel 18 during membrane cleaning operations.

!t will be appreciated that the system 10 may additionally be provided with a source of cleaning fluids which comprises at least one cleaning fluid selected from the group consisting of an alkaline (pH 10} solution of permeate, acidic (pH 2} solution of permeate, a sterilising solution of sodium meta bisul hite in permeate and permeate itself.

Non-limiting examples of desalination systems, methods and apparatus therefor will now be described.

Examples

Table 1 and Figures 8a and 8b show preliminary results of modelling using the groundwater flow and transport modelling package FEFLOW for pumping groundwater with TDS of 3500mg/L for downhole RO treatment and reinjection of concentrates of 5000mg/L TDS. Both groundwater extraction and reinjection is across the upper 3m and lower 3m of aquifer respectively, with pumping and reinjection bores being ~20Gm apart. The model input data is shown in Table 1.

A plan view of salinity (TDS) contours after 5 years of continuous pumping and reinjection are shown in Figure 8a, where higher TDS from reinjection of concentrates are just reaching the production borehole and contours have attained an approximation to steady state, i.e. the TDS of pumped groundwater will not increase substantially as pumping extends beyond 5 years. Modelling indicates that the TDS of groundwater feed in the production borehole does not exceed 3850mg/L after 5 years of continuous operation of the system, which is -10% increase in TDS from dipole flow. The latter is minor, and easily manageable.

A vertical section through the pumping and reinjection wells for the situation after 5 years of continuous operation is shown in Figure 8b. Here, the salinity plume is near steady state (ie it is not significantly expanding) and the front of the plume extends ~2G0m eastwards (in the regional direction of groundwater flow) to a point where TDS does not exceed more than 20% above background (ie 4200mg/L). There is significant dispersion of TDS into confining layers, particularly that below the plume, which is due to a density contrast between ambient groundwater and concentrates.

A similar modelled situation is shown in Table 2 and Figures 9a and 9b, although here a 3m thick upper aquifer unit being pumped is separated from a similar lower aquifer unit where reinjection takes place by a 3m thick fayer of low hydraulic conductivity (eg silt or clay}. The model input data is shown in Table 2, and the plan-view contours for TDS after 5 years of continuous operation are shown in Figure 9a. in the latter case, the lower aquifer unit retains most of the reinjected TDS, and the piume extends a similar distance downgradient (to the right) of the reinjection point. The average TDS of groundwater feed after 5 years of continuous operation is only 3650mg/L, less than 5% increase above background as a result of minor dipoie flow . There is significant dispersion of TDS into the confining layer below the injection point due to density contrast, as shown in the vertical section in Figure 9b for 5 years of continuous operation, which limits impacts of increased salinity downgradient.

Table 3 presents results of FEFLOW simulation modelling for two scenarios as presented in Tables 1 and 2, and for varying distances of separation of pumping well and reinjection well between 50m and 200m. The average groundwater TDS being pumped after 5 years of continuous operation of the downhote desalination system is shown in Table 3 for each of the separation distances and for each scenario. The predicted TDS of groundwater after 5 years for scenario 1 (model details in Table 1 ) increase as the separation distance decreases, although the TDS remains constant as the distance becomes less than 100m. This result indicates that reinjection close to the production bore (<1Q0m) achieves a steady state and the extent of dipoie flow remains steady at - 40% by volume, and the TDS of groundwater does not increase by more than 20%. The effect of this on RO treatment (TDS increasing from 3500mg/L to 4100mg/L) would be relatively small, and overail environmental impact on groundwater salinity would be similar to that shown in Figures 7a and 7b. !n scenario 2 (Table 2), the TDS of groundwater pumped after 5 years is less than in scenario 1 because the upper aquifer unit from which groundwater is pumped and the lower aquifer unit where reinjection takes place are separated by a semi-permeable layer as in Table 2, thus this stratification limits dipoie flow. In addition, as in scenario 1 , groundwater TDS after 5 years increases as the separation distance decreases, although the impacts of this on efficiency of RO treatment (ie increase in TDS from 3500 to 3720mg/L) would be very minor, and the environmental impact on groundwater salinity would be similar to that in Figures 9a and Sb,

Results from a field trial are shown in Table 4 and Figure 10, which is similar to the configuration of pumping and reinjection shown in Table 2 and Figures 9a-b, but with reinjection well being only -100m from the pumping well (Figure 0). FEFLOW model predictions for 5 years of continuous operation are given in Figure 10, with pumping rates being similar to those in Table 2, but groundwater feed and reinjection water salinities (TDS) being more than double those in Tabie 2. Background salinities are also very variable (as is often the case in natural brackish aquifers), being influenced by site dewatering operations near monitoring bore MB1 to the south of the reinjection bore, and hydraulic interaction of the aquifer with a tidal river which forms the southwestern boundary of the model domain. In this situation, the modelling predicts little impact of dipoie flow on system operation over a 5 year period, largely because of the relatively steep hydraulic gradients to the south of the reinjection point in Figure 10. in reality, feed groundwater TDS showed only minor increase as a result of dipoie flow over an approximate 2 month period.

!t will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

For example, it is generally preferred, for the sake of economics and to obtain a minimal environmental footprint, that the water treatment system 16 is disposed in situ in the production borehole casing 12 and that the permeate conduit 34 and concentrate conduit 38 transfer the permeate and concentrate, respectively, above ground.

Disposing the water treatment system 18 in situ in the production borehole casing 12 is regarded as advantageous because as the pump is an integral part of the treatment system and provides all the necessary flows and pressure, the hydrostatic pressure within the aquifer may be applied to assist passage of groundwater successively through the membrane treatment elements 20. This would not occur if the pump was simply used to pump groundwater to the surface for treatment as in conventional desalination plants. It will be appreciated that this benefit is achievable when the pump, vessel and permeate and concentrate lines comprise a single closed (i.e.

sealed) system. This is in contrast to conventional desalination systems where the pump is mostly used for delivery of groundwater to the surface for treatment, with pressures and flow being provided by an additional pump at the surface. it will be appreciated, however, that it is also possible to dispose the water treatment system 16, either entirely or in part, above ground, provided the pump 28 is maintained as an integral part of the water treatment system 16. For example, one or more membrane treatment elements 20 may be located above ground, if the water treatment system 16 or parts thereof, is/are located above ground then the system 10 may be configured with additional pumps and o conduits for lifting the

groundwater/treated groundwater from the production borehole casing 12 or membrane treatment element(s) 20 located therein. in the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Table 1. Summary of FEFLOW modei set-up for pumping and reinjection during downhoie O treatment over 5 years from a relatively thin aquifer of depth 6m, with pumping from the upper part of the aquifer and reinjection 200m from the injection point from the lower part of the aquifer, also showing predicted TDS in feed roundwater being pumped including dipole flow after 5 years of operation

Table 2. Summary of FEFLOW model set-up for pumping and reinjection during downhoie RO treatment over 5 years from a relatively thin upper confined aquifer of depth 3m, separated from a lower 3m thick aquifer unit by a 3m thick semi-confining layer, with reinjection 200m from the injection point from the lower aquifer unit Also shown is the predicted TDS in feed groundwater being pumped including dtpo!e flow after 5 years of operation.

Table 3. Results of FEFLGW simulations showing predicted TDS in pumped groundwater including dipoie flow from 5 years of continuous operation of a downhofe desalination system with reinjection of more saline residual fluids at varying distance from the pumping well, for an aquifer as described in Table 1 {scenario 1 ) and Table 2 {scenario 2).

Table 4. Trial site results and modelling, simitar to the model in Figure 6a illustrating pumping from a 3m thick gravel confined aquifer (17-21m bgl) with reinjection -100m from the pumping well into a lower 3m sandy gravel aquifer unit (24.5-28.5m), both aquifer units separated by a 3.5m thiek silty clay aquitard {semi-confining layer). Also shown is the approximate actual feed TDS of brackish groundwater at the pumping well after ~ 1 year of operation with stoppages of relatively minor duration.

Basement (thin

basalt overlying

mudstone)