SUBMERGED WATER DESALINATION SYSTEM WITH REPLACEABLE DOCKABLE MEMBRANE MODULES

Abstract

A submersible water desalination apparatus includes an array of hot-swappable water separation membrane modules arrayed in an approximately polygonal configuration around a product water collection conduit that represents or is proximate to and generally aligned with a central axis of the array; a plurality of hot-swap product water valves connected to the conduit and detachably connected to generally radially-extending product water collection manifolds in fluid communication with a plurality of water separation membrane cartridges within the modules; the modules having in cross-section generally tapered module sides that converge towards the conduit and assist in underwater docking and attachment of a replacement module to a hot-swap product water valve.

Description

The present application is a continuation under 35 U.S.C. § 111(a) of International Patent Application No. PCT/US2020/058569, filed on Nov. 2, 2020, which claims priority to U.S. Provisional Patent Application No. 62/929,564, filed on Nov. 1, 2019, the disclosures of which are incorporated by reference herein.

TECHNICAL FIELD

This invention relates to water desalination.

BACKGROUND ART

The growth of saltwater (e.g., seawater) desalination has been limited by the relatively high cost of desalinated water. This high cost is due in part to energy and capital expenses associated with current desalination systems. Such systems typically employ an onshore facility containing reverse osmosis (RO) desalination membranes contained in high-pressure corrosion-resistant housings and supplied with seawater from a submerged offshore intake system. Very high pressures typically are required to drive water through the RO membranes. For example, the widely-used FILMTEC™ SW30 family of reverse osmosis membrane elements (from DuPont Water Solutions) require about an 800 psi (55 bar) pressure differential across the membrane to meet design requirements. In addition to such high pressures, onshore RO units suffer from high power demands, primarily for pressurizing the feedwater to membrane operating pressures, and for an onshore RO unit these power demands typically average about 13.5 kWh per thousand gallons of produced fresh water. The seawater and the concentrated brine stream produced by a typical onshore RO unit have high corrosion potential and consequently require expensive components and equipment, including pressure vessels and conduits made from specialized alloys. The highly-pressurized water flow also increases maintenance expenses. Onshore RO units typically also require significant amounts of expensive seaside real estate. Shore-based desalination has in addition been criticized for various environmental impacts, including entrainment of marine life in the intake water, greenhouse gas production associated with producing the energy required, discharge of a strong brine stream with the potential to harm marine life, the use of treatment chemicals that may enter the ocean, and onshore land use that is often expensive and may harm local ecosystems. RO units include those described in U.S. Pat. No. 4,334,992 (Bonin et al.), U.S. Pat. No. 5,192,434 (Moller), U.S. Pat. No. 5,620,605 (Moller et al.), U.S. Pat. No. 5,788,858 (Acernase et al. '858), U.S. Pat. No. 5,972,216 (Acernase et al. '216), U.S. Pat. No. 8,282,823 B2 (Acernase et al. '823) and U.S. Pat. No. 9,227,159 B2 (DuFresne et al.).

In the 50 years since the invention of semi-permeable RO membranes, various concepts for submerging such membranes and employing natural hydrostatic water pressure to help desalinate seawater have been proposed. Representative examples include the systems shown in U.S. Pat. No. 3,171,808 (Todd), U.S. Pat. No. 3,456,802 (Cole), U.S. Pat. No. 4,125,463 (Chenowith), U.S. Pat. No. 5,229,005 (Fok et al.), U.S. Pat. No. 5,366,635 (Watkins), U.S. Pat. No. 5,914,041 (Chancellor '041), U.S. Pat. No. 5,944,999 (Chancellor '999), U.S. Pat. No. 5,980,751 (Chancellor '751), U.S. Pat. No. 6,149,393 (Chancellor '393), U.S. Pat. No. 6,348,148 B1 (Bosley) and U.S. Pat. No. 8,685,252 B2 (Vuong et al.); US Patent Application Publication Nos. US 2008/0190849 A1 (Vuong), US 2010/0270236 A1 (Scialdone) US 2010/0276369 A1 (Haag) and US 2018/0001263 A1 (Johnson et al.); GB Patent No. 2 068 774 A (Mesple); International Application Nos. WO 00/41971 A1 (Gu), WO 2009/086587 A1 (Haag Family Trust), WO 2018/148528 A1 (Bergstrom et al.), WO 2018/148542 A1 (Bergstrom) and Pacenti et al., Submarine seawater reverse osmosis desalination system, Desalination 126, pp. 213-18 (November, 1999).

Other water desalination technologies have also been proposed, including systems employing microfiltration, nanofiltration, ultrafiltration and aquaporins. These likewise have various drawbacks. In general, submerged water desalination systems do not appear to have been placed in widespread use, due in part to factors such as the energy cost of pumping the desalinated water to the surface from great depth and the difficulty of maintaining component parts at depth.

From the foregoing, it will be appreciated that what remains needed in the art is an improved system for water desalination featuring one or more of lower energy cost, lower capital cost, lower operating or maintenance cost or reduced environmental impact. Such systems are disclosed and claimed herein.

SUMMARY

Compared to land-based water desalination, a submerged water desalination system can provide several important advantages. For example, submerged operation can significantly reduce pump power requirements, since hydrostatic pressure can provide much or all of the driving force required for desalination, and only desalinated water will need to be pumped onshore. Eventually however, whether operated onshore or submerged, water separation membranes may become fouled by scaling or debris or may otherwise lose their effectiveness, and will need to be replaced. Replacement can be difficult, especially when the membranes are submerged at significant depths, and may require shutting down an entire submerged desalination apparatus or in some cases bringing it to the surface so that membrane or cartridge replacement can be carried out. Although some submerged RO (SRO) systems have been said to allow membrane or cartridge replacement while the SRO apparatus is submerged, replacement remains difficult. This can be especially important when the submerged apparatus is located at a depth sufficient to require use of a remotely operated vehicle (ROV) for servicing, or when located in a region with moving subsea currents, such as certain tidal areas, narrow straits or some submarine canyons.

The disclosed invention provides in one aspect a submersible water desalination apparatus comprising an array of hot-swappable water separation membrane modules arrayed around and generally radially extending from a product water collection conduit; a plurality of hot-swap product water valves connected to the conduit and detachably connected to generally radially-extending product water collection manifolds in fluid communication with a plurality of water separation membrane cartridges within the modules; the modules having in cross-section generally tapered module sides that converge towards the conduit and assist in underwater docking and attachment of a replacement module to a hot-swap product water valve.

The disclosed invention provides in another aspect a method for maintaining a submerged water desalination apparatus, the method comprising the steps of:

• • i) undocking from a submerged water desalination apparatus a first hot-swappable water separation membrane module that is a member of an array of such modules that are arrayed around and generally radially extend from a product water collection conduit and are in fluid communication with the conduit via hot-swap product water valves connected to the conduit and detachably connected to generally radially-extending product water collection manifolds in fluid communication with a plurality of water separation membrane cartridges within the modules, the modules having in cross-section generally tapered module sides that converge towards the conduit and assist in underwater docking and reattachment of detached modules to a hot-swap product water valve; and
  • ii) docking a similarly sized and shaped replacement module and attaching it to the array while underwater by urging the converging tapered sides of the replacement module towards an available hot-swap product water valve.

The disclosed apparatus provides a submerged “Natural Ocean Well” that can provide desalinated water at reduced cost and with improved reliability compared to land-based RO systems, and with improved RO membrane maintenance and replacement compared to existing SRO systems, especially when replacement is accomplished using an ROV

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 and FIG. 2 are schematic side views of one embodiment of the disclosed apparatus;

FIG. 3 and FIG. 4 are respectively side and schematic views of Ocean Thermal Energy Conversion (“OTEC”) systems for supplying power to the disclosed apparatus;

FIG. 5 is a side view of alternate systems for supplying power to the disclosed apparatus, and for disposing of concentrate or brine with reduced greenhouse gas emissions;

FIG. 6 is a perspective view of a water farm formed by a connected array of the disclosed water desalination systems;

FIG. 7A, FIG. 7B and FIG. 7C are three perspective underside views of the disclosed apparatus with and without certain components;

FIGS. 8A through 8E are perspective topside or underside views, from several observation angles, of an individual hot-swappable water separation membrane module;

FIGS. 8F and 8G are cross-sectional views showing the use of an adhesive to bond and seal water separation cartridges in a water separation membrane module;

FIG. 9 is a top plan view of a generally polygonal array of the disclosed modules showing a module in an unattached condition;

FIG. 10A through FIG. 10C are perspective views of the disclosed modules arranged for shipment on a flatbed trailer; and

FIG. 11 and FIG. 12 are outlet and inlet side schematic views of a prefilter cleaning system.

Like reference symbols in the various figures of the drawing indicate like elements. The elements in the drawing are not to scale.

DETAILED DESCRIPTION

The recitation of a numerical range using endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

The terms “a,” “an,” “the,” “at least one,” and “one or more” are used interchangeably. Thus, for example, an apparatus that contains “a” reverse osmosis membrane includes “one or more” such membranes.

The term “airlift” when used with respect to a pump refers to a device or method for pumping a liquid or slurry by injecting air (and preferably only by injecting air) into the liquid or slurry.

The term “automatic” when used with respect to control of a submerged pump means that the control operates in the vicinity of and based on conditions in such pump, and without requiring the sending of signals to or the receipt of signals from a surface vessel, platform, or other non-submerged equipment.

The term “brine” refers to an aqueous solution containing a materially greater sodium chloride concentration than that found in typical saltwater, viz., salinity corresponding to greater than about 3.5% sodium chloride. It should be noted that different jurisdictions may apply differing definitions for the term “brine” or may set different limitations on saline discharges. For example, under current California regulations, discharges should not exceed a daily maximum of 2.0 parts per thousand (ppt) above natural background salinity measured no further than 100 meters horizontally from the discharge point. In other jurisdictions, salinity limits may for example be set at levels such as 1 ppt above ambient, 5% above ambient, or 40 ppt absolute.

The term “concentrate” refers to an RO apparatus discharge stream having an elevated salinity level compared to ambient surrounding seawater, but not necessarily containing sufficient salinity to qualify as brine in the applicable jurisdiction where such stream is produced.

The term “conduit” refers to a pipe or other hollow structure (e.g., a bore, channel, duct, hose, line, opening, passage, riser, tube or wellbore) through which a liquid flows during operation of an apparatus employing such conduit. A conduit may be but need not be circular in cross-section, and may for example have other cross-sectional shapes including oval or other round or rounded shapes, triangular, square, rectangular or other regular or irregular shapes. A conduit also may be but need not be linear or uniform along its length, and may for example have other shapes including tapered, coiled or branched (e.g., branches radiating outwardly from a central hub).

The term “depth” when used with respect to a submerged water desalination apparatus or a component thereof refers to the vertical distance, viz., to the height of a water column, from the free surface of a body of water in which the apparatus or component is submerged to the point of seawater introduction into the apparatus or to the location of the component.

The terms “desalinated water”, “fresh water” and “product water” refer to water containing less than 1000 parts per million (ppm), and more preferably less than 500 ppm, dissolved inorganic salts by weight. Exemplary such salts include sodium chloride, magnesium sulfate, potassium nitrate, and sodium bicarbonate.

The term “recovery ratio” when used with respect to an SRO system or SRO apparatus means the volumetric ratio of product water (permeate) produced by the system or apparatus to feedwater introduced to the system or apparatus.

The terms “remotely operated vehicle” and “ROV” refer to unoccupied submersible vehicles capable of underwater maneuvering and manipulation of submerged objects.

The terms “saltwater” and “seawater” refer to water containing more than 0.5 ppt dissolved inorganic salts by weight, and thus encompassing both brackish water (water containing 0.5 to 3.0 ppt dissolved organic salts by weight) as well as water containing more than 3.0 ppt dissolved organic salts by weight. In oceans, dissolved inorganic salts typically are measured based on Total Dissolved Solids (TDS), and typically average about 35 ppt TDS, though local conditions may result in higher or lower levels of salinity.

The term “submerged” means underwater.

The term “submersible” means suitable for use and primarily used while submerged.

The term “wide area” when used with respect to dispersal of a fluid (e.g., concentrate or brine) away from a conduit having a plurality of fluid outlets (e.g., concentrate or brine outlets) distributed along a length of the conduit, means dispersal into an outfall area, and typically into a volume, encompassing at least 5 meters of such length. The disclosed area or volume will also have other dimensions (e.g., a width, diameter or height) that will depend in part upon the direction and velocities of fluid streams passing through the fluid outlets. Because such other dimensions will be affected by variable factors including fluid flow rates inside and outside the conduit, and the overall shape of the dispersed fluid plume, the term “wide area” has been defined by focusing merely on the recited length along the recited conduit, as such length typically will represent a fixed quantity in a given dispersal system.

In the discussion that follows, emphasis will be placed on the use of RO membranes in a submerged RO (SRO) apparatus for carrying out water separation, it being understood that persons having ordinary skill in the art of desalination will after reading this disclosure be able to replace the disclosed RO membranes with other types of water separation membranes. Exemplary such other water separation membranes include those based on microfiltration, nanofiltration and ultrafiltration; aquaporins; and other water separation technologies that are now known or hereafter developed and which will be familiar to persons having ordinary skill in the desalination art.

Referring first to FIG. 1 and FIG. 2, SRO apparatus 100 is shown in schematic side view. Raw seawater 102 enters apparatus 100 via prefilter screens 104, and is separated by RO membrane modules 106 into product water permeate stream 108 and concentrate or brine discharge stream 110. Permeate stream 108 passes into permeate collector 112 and thence through permeate conduit 113, submerged pump 114 and delivery conduit 116 to a ship-borne or onshore collection point (not shown in FIG. 1 or FIG. 2) for post-treatment, conveyance or storage for later use. Such uses may include municipal, private or industrial purposes including potable water, bathing water, irrigation water, process water, water storage, water table replenishment, cooling or heat exchange, and a variety of other purposes that will be apparent to persons having ordinary skill in the desalination art. For example, potential cooling or heat exchange applications for such product water include providing or improving the efficiency of air conditioning systems including Sea Water Air Conditioning (SWAC) systems; operating or improving the efficiency of OTEC systems (in addition to those discussed herein); and operating or improving the efficiency of Rankine Cycle heat engines (again, in addition to those discussed herein).

In the disclosed apparatus, raw seawater, product water and concentrate or brine may each flow in a variety of directions, e.g., upwardly, downwardly, horizontally, obliquely or any combination thereof. In the embodiment shown in FIG. 2, reverse osmosis membranes within membrane modules 106 are oriented so that concentrate or brine 110 is discharged generally upwardly from the modules 106 and is captured and collected by hood 118. Axial pump 120 located at the lower end of riser 122 sends captured concentrate or brine 110 through riser 122 toward surface 124, for further use or dispersal.

In the embodiment shown in FIG. 2, concentrate or brine 110 exits riser 122, whereupon dispersion and dilution takes place in the surrounding seawater. In an additional embodiment (not shown in FIG. 2), concentrate or brine 110 is transported through a further conduit to undergo dispersal (and preferably wide area dispersal) at a significant distance (e.g., at least 50, at least 100, at least 200, at least 300, at least 400 or at least 500 meters) away from apparatus 100, or into a sustained underwater current 130, to be swept away from apparatus 100. In a further embodiment (also not shown in FIG. 2), concentrate or brine 110 is transported through a further conduit for an even greater distance (e.g., all the way to or nearly to surface 124) for further use or dispersal. If desired, the concentrate or brine may instead be discharged in another direction such as downwardly or horizontally, while preferably still undergoing wide area dispersal well away from apparatus 100.

The concentrate or brine may be used for a variety of purposes prior to discharge. In one embodiment, the concentrate or brine has desirable volumetric and thermal utility that may be used to operate an OTEC system and provide operating or surplus power, as discussed in more detail below and in copending International Application No. PCT/US2020/058567, filed on Nov. 2, 2020 and entitled OCEAN THERMAL ENERGY CONVERSION SUBMERGED REVERSE OSMOSIS DESALINATION SYSTEM, the disclosure of which is incorporated herein by reference.

In the embodiment shown in FIG. 2, buoyancy provided by a ring float 126 and a foam layer, e.g., an engineered syntactic foam layer (not shown in FIG. 2) located beneath the surface of hood 128, help maintain apparatus 100 at an appropriate depth D below surface 124. Catenary mooring lines 132 affixed to anchors 134 in seabed 136 help maintain apparatus 100 at an appropriate depth D below surface 124, an appropriate height H above seabed 136, and an appropriate height H′ above the inlet to pump 114. Depth D preferably is such that the hydrostatic pressure of seawater at depth D is sufficient to drive seawater 102 through membrane modules 106 and produce product water 108 and concentrate or brine 110 at a desired overall volume and recovery ratio without the need for additional pumps or other measures to pressurize seawater 102 on the inlet side of membrane modules 106. The chosen depth D will vary based on several factors including the pressure drop across the above-mentioned prefilter screens 104; the type, dimensions and arrangement of RO cartridges within the membrane modules 106; the type, sizing and operating conditions of permeate collector 112, permeate conduit 113, product water pump 114 and product water conduit 116; and the type, sizing and operating conditions of axial brine pump 120 and concentrate riser 122. For example, if operating the disclosed SRO apparatus using HYDRANAUTICS™ SWC cylindrical membrane cartridges from Nitto Hydranautics operated without pumps to pressurize the inlet seawater, then operation at a depth of at least about 350 m together with a pump to draw product water from the membrane elements is preferred in order to minimize or eliminate the need for a high pressure vessel surrounding the membrane elements. In some prior SRO designs, especially those that rely on a pressure pump to force seawater through the membranes, thick pressure-resistant vessels are employed to contain the high pressures needed for membrane separation. In preferred embodiments of the present SRO desalination apparatus, the prefilter elements and RO membranes will not require pressure-resistant vessels, as they will already be immersed at a sufficiently high pressure in the fluid to be purified. Desirably the disclosed SRO apparatus merely maintains a sufficiently low pressure on the membrane product discharge side, and a sufficient inlet side-outlet side pressure differential, so as to allow proper membrane operation without the use of a surrounding pressure-resistant vessel.

Greater depths than those needed for operation without a pressure vessel (e.g., at least about 400, at least about 450, at least about 500, at least about 550, at least about 600, at least about 650, at least about 700, at least about 750, at least about 800, at least about 900 or at least about 1,000 m) may be employed if desired, with operation at such greater depths increasing the pump suction head and inlet pressure, and enabling use of the same model pump as might be employed at lesser depths. Such lesser depths may for example be at least about 300, at least about 200 or at least about 100 m, with operation at such lesser depths typically requiring at least one pump to help push seawater through the RO membranes (or a suitable vacuum assist on the outlet side) in order to achieve efficient desalination, and possibly also requiring a pressure vessel surrounding and protecting the membrane elements. Near the surface, the hydrostatic pressure of the ocean will need to be augmented by mechanical pumping to provide the trans-membrane pressure differential needed for water separation. Excessive depths that may cause or accelerate pump failure should however be avoided. Overall exemplary depths for operation of the disclosed SRO desalination system are for example from just below the surface (e.g., from about 10 m), from about 100 m, from about 300 m, or from about 500 m, and up to about 2,000 m, up to about 1,500 m or up to about 1,000 m. Depending on the chosen pump and membranes, preferred depths are from just below the surface to as much as 1500 m depth. Near the surface, the hydrostatic pressure of the ocean typically will need to be augmented by mechanical pumping to provide the trans-membrane pressure differential needed for reverse osmosis.

Depth D may moreover be a fixed depth chosen at the time of installation, or an adjustable depth that may for example be changed following SRO apparatus startup or changed in response to changing conditions (e.g., changing wave, tidal, thermocline or halocline conditions, changing seawater salinity, sea level rise, or changes in the operating efficiency of the RO membranes). In a further embodiment, the disclosed SRO apparatus may include a pressure-seeking capability to enable the system to increase or decrease its depth in order to obtain desired hydrostatic pressures, to optimize or adjust RO operating conditions or to optimize or adjust product water and concentrate or brine delivery.

By way of example, if the disclosed apparatus is operated at a depth of about 700 m, hydrostatic pressure will provide approximately 68 bar on the high-pressure side of the semi-permeable RO membrane. When used with a presently preferred backpressure of 13 bar or less on the product discharge side of the membrane, this will result in a pressure differential across the membrane of 55 bar (approximately 800 psi) or more. In situations of higher- or lower-salinity waters, these depth and pressure values may vary. The inlet pressure will in any event normally be the ocean hydrostatic pressure at the chosen SRO operating depth.

The preferred depth and pressure values set out above may vary in systems that take advantage of future membrane developments enabling or requiring lower or higher differential pressures or higher or lower membrane backpressures. Adjustments to accommodate such developments may increase or decrease the preferred operating depth for the disclosed SRO apparatus. For many membranes, the pressure on the low-pressure side typically will not change appreciably with depth, and consequently changing the depth of operation may suffice to adjust the differential pressure across the membrane and achieve optimal operating conditions.

The heights H (the vertical spacing between the lowest inlets to prefilter screens 104 and seabed 136) and H′ (the vertical spacing between the membrane module 106 product water outlets and the inlet to pump 114) in FIG. 2 may for example each represent at least about 3, at least about 5, at least about 10, at least about 20, at least about 40 or at least about 50 m. Lesser heights H and H′ may be employed. For example, height H may be reduced to near zero or zero, so that the inlets to prefilter screens 104 are near or at the same depth as seabed 136. However, doing so typically will increase the turbidity of seawater 102 entering modules 106 and the possibility that foreign matter may be drawn through prefilter screens 104 and into modules 106. Also, height H′ between prefilter screens 104 and pump 114 may be reduced to near zero, zero or even less than zero (viz., by housing pump 114 inside the screened intake system between the prefilter screens 104 and the modules 106). In such reduced height H′ embodiments the pump 114 and prefilter screens 104 preferably will however remain elevated at a sufficient distance above seabed 136 to avoid turbidity that may be present near seabed 136.

The depth of the disclosed apparatus 100, height H′ and the diameter of the inlet to pump 114 are desirably sized to provide at least the net positive suction head (NPSH) or greater pressure (viz., the pressure caused by the height of the standing column of product water 108 in permeate conduit 113 and permeate collector 112 between membrane modules 106 and the inlet side of pump 114) sufficient to avoid inlet side cavitation upon startup and operation of pump 114. Further details regarding such cavitation avoidance during startup and operation may be found in copending International Application No. PCT/US2020/058573, filed on Nov. 2, 2020 and entitled SUBMERGED WATER DESALINATION SYSTEM WITH REMOTE PUMP, the disclosure of which is incorporated herein by reference.

As depicted in FIG. 2, pump 114 rests upon and if desired may be moored to seabed 136 or to other natural or artificial structures on the seabed. Pump 114 may if desired be suspended above the seabed, for example in locations where the seabed is uneven or inclined. In one embodiment, pump 114 is suspended beneath the remainder of apparatus 100 by underwater mooring lines affixed to the apparatus and pump. Pump 114 may if desired be located in other locations, for example affixed to an offshore oil or gas platform, offshore wind farm support, bridge pier or other partly or wholly submerged supporting structure.

Pump 114 and the other pumps referred to herein may be selected from a wide variety of submersible single stage or multistage pumps, including piston (e.g., axial piston), plunger, rotary (e.g., centrifugal impeller pumps and rim-driven shaftless thrusters) and screw pumps that may use a variety of flow schemes including positive displacement, centrifugal and axial-flow principles. Suitable pumps are available from a variety of sources that will be familiar to persons having ordinary skill in the desalination art, and may in appropriate instances be adapted from other fields such as subsea oil and gas exploration, and marine (including submarine) positioning and propulsion. Exemplary pump suppliers include Brunvoll, Cat Pumps, Copenhagen Subsea, Enitech, FMC Kongsberg Subsea AS, Fuglesang Subsea AS, Halliburton, Hayward Tyler, Ocean Yacht Systems, Parker, Rolls Royce, Schlumberger, Schottel, Silent Dynamics, Technical Supply & Logistics, Vetus and Voith. In some embodiments the disclosed pumps include hot-swap connectors to enable them to be removed from the disclosed apparatus while it is submerged, for replacement, repair or rebuilding.

In some embodiments, pump 114 includes one or more sensors, controls or a torque limiting coupling (e.g., a magnetic clutch, hydraulic torque converter, combination thereof or other such device) between the electrical motor powering the pump and the pump impeller so as to limit or avoid inlet side cavitation and accompanying stress or other disturbance of the RO membranes during pump operation. Further details regarding cavitation avoidance during such operation may be found in copending International Application No. PCT/US2020/058570, filed on Nov. 2, 2020 and entitled SUBMERGED WATER DESALINATION SYSTEM WITH PRODUCT WATER PUMP CAVITATION PROTECTION, the disclosure of which is incorporated herein by reference.

In one embodiment, pump 114 diverts at least a portion of the product water 108 for use as a lubricating or cooling fluid directed through one or more of the pump, pump motor or the coupling between the motor and pump. Doing so can improve the pump longevity, while avoiding the need to use seawater, hydraulic fluid or other potentially particulate-bearing, corrosive or toxic fluids for lubrication or cooling. Further details regarding the use of product water for such lubrication and cooling may be found in copending International Application No. PCT/US2020/058572, filed on Nov. 2, 2020 and entitled SUBMERGED WATER DESALINATION SYSTEM PUMP LUBRICATED WITH PRODUCT WATER, the disclosure of which is incorporated herein by reference.

Electrical power and appropriate control signals 138 may be supplied to pump 114 and other components of apparatus 100 through multi-conductor cable 140. The supplied electrical power operates pumps 114 and 120 and as needed other components in apparatus 100, such as a prefilter cleaning brush system. Further details regarding a desirable prefilter cleaning brush embodiment are discussed in more detail below.

When operated at sufficient depth, the RO membranes in apparatus 100 will not need to be encased in pressure vessels, and may instead be mounted in a lightweight supporting frame or other housing made from relatively inexpensive and suitably corrosion-resistant materials such as a corrosion-resistant metal skeleton or a housing made from a suitable plastic, fiber-reinforced (e.g., glass fiber- or carbon fiber-reinforced) plastic or other composite, or a variety of other unreinforced or engineered plastics the selection of which will be understood by persons having ordinary skill in the desalination art. Avoiding the need for a pressure vessel greatly reduces the required capital expenditure (CAPEX) for constructing apparatus 100 compared to the costs for constructing a shore-based RO unit. If the RO membranes are individual units (for example, cartridges containing spiral-wound membranes), then avoidance of a pressure vessel also enables modules 106 to be economically designed using a parallel array containing a significantly larger number of cartridges than might normally be employed in a shore-based RO unit, and operating the individual cartridges at a lower than normal individual throughput. For example, the number of cartridges may be at least 10% more, at least 15% more, at least 20% more or at least 25% more than might normally be employed in an onshore RO unit. Doing so can help extend the life of individual membrane cartridges while still providing a desired daily amount of product water. In the embodiment shown in FIG. 1 and FIG. 2, and as discussed in more detail below, modules 106 preferably contain a large array of parallel cylindrical RO cartridges operated not only at such low individual throughput, but also with a reduced recovery rate. Doing so can also provide reduced concentrate salinity, reduced fouling potential, and in preferred embodiments will result in a large volume of concentrate that does not qualify as brine in the applicable jurisdiction, and which has substantial cold thermal energy potential for cooling an OTEC system. For example, permeate stream 108 is depicted in FIG. 1 as having a substantially smaller volume than brine discharge stream 110, corresponding to a low recovery ratio. Exemplary recovery ratios may for example be no greater than 40%, no greater than 30%, no greater than 20%, no greater than 15%, no greater than 10%, no greater than 8% or no greater than 6%, and may for example be less than 3%, at least 3%, at least 4% or at least 5%. The chosen recovery ratio will depend upon factors including the selected RO membranes, and the depth and applicable jurisdiction in which the SRO apparatus operates. The chosen recovery ratio also influences pump sizing and energy costs. By way of example, for an SRO embodiment employing Dow FILMTEC membrane cartridges to treat seawater with an average 34,000 ppm salinity at an 8% recovery ratio, about 8% of the seawater inlet stream will be converted to product water having less than 500 ppm salinity, and about 92% of the seawater inlet stream will be converted to a low pressure or unpressurized brine stream having about 37,000 ppm salinity. By way of a further example, an SRO apparatus employing Nitto Hydranautics membrane cartridges operated at a depth of about 500 m and a 5% recovery ratio may be used to produce concentrate that does not qualify as brine under the current version of the California Water Quality Control Plan.

In one preferred embodiment, the disclosed SRO apparatus operates at a depth of at least about 350 m, does not employ seawater pumps on the RO membrane inlet side, and employs a product (fresh) water pump on the outlet side of the RO membranes to maintain at least a 27 bar and more preferably at least a 30 or 35 bar pressure differential across the membranes, to allow the ocean's hydrostatic pressure to force or to largely help force product water through such membranes. Advantages for such a configuration include a pump requiring much less energy when located at the membrane outlet rather than at the inlet, and the avoidance of, or much lower requirements for, any pressure vessels housing the membranes. Use of membranes with a low required pressure differential will enable operation at lesser depths or using smaller pumps. Currently preferred such membranes include Nitto Hydranautics SWC6-LD membranes (40 bar differential pressure) and LG Chem LG-SW-400-ES membranes (38 bar differential pressure).

The pumps in the disclosed apparatus may be supplied with power in a variety of ways. Referring now to FIG. 3, closed-cycle OTEC unit 300 employs a recirculating Rankine cycle heat exchange and generator system 302 mounted on or within a submerged platform 304 whose location below surface level 124 provides desirable protection from wind and wave action. Platform 304 may be affixed to seabed 136 using catenary mooring lines 306 affixed to anchors 308. Platform 304 may if desired be otherwise fixed in place, for example to an offshore oil or gas platform, offshore wind farm support, bridge pier or other partly or wholly submerged supporting structure. Cold liquid stream 310 is introduced into system 302 by upper concentrate pump 318 located at the upper end of riser 316. Stream 310 may if desired contain only concentrate or brine 110, preferably is diluted by cold seawater 102 that enters the exposed lower open end 320 of riser 316 beneath ring float 322. Doing so desirably decreases the salinity and increases the cold thermal energy potential of stream 310. Warm liquid stream 312 is obtained from shallow depth seawater near platform 304, and is pumped into system 302 by OTEC warm seawater pump 313. Warm liquid stream 312 may be screened, filtered or otherwise pretreated (e.g., with a biocide) to reduce entrainment of marine life and biofouling. In another embodiment that may be available at select locations, warm liquid stream 312 is obtained by capturing the output from an undersea thermal vent.

System 302 employs thermal energy extracted from the temperature differential between cold liquid stream 310 and warm liquid stream 312 to provide electrical power for operation of electrically-driven components in the disclosed SRO apparatus. Following utilization of such thermal energy, streams 310 and 312 may be separately discharged, but in the embodiment shown in FIG. 3 preferably are mixed to form wide area dispersal stream 314. Doing so helps further reduce the salinity of stream 314.

If desired, an open-cycle OTEC system may be used in place of a closed-cycle OTEC system. In an open-cycle system, the warm seawater is converted to vapor using a suitable evaporator. The vapor drives a turbine before being condensed to freshwater by the cold concentrate or brine stream. The condensed freshwater may be combined with product water from the ocean well to provide additional freshwater production. An open-cycle system avoids the need for a circulating working fluid and the associated circulation pump. Also, it generally is not necessary to employ screens, filtration or a biocide on the warm water side of the system, as the evaporation process normally is sufficient to prevent biofouling. A hybrid cycle OTEC system that combines features of both open and closed-cycle systems may also be employed. In general however, a closed-cycle OTEC system will be preferred for most applications.

FIG. 4 shows a schematic view of the disclosed closed-cycle OTEC system 302. Cold stream 310 and warm stream 312 are respectively used by system 302 to repeatedly liquefy (in condenser 402) and volatilize (in evaporator 404) a volatile circulating working fluid 406 such as ammonia or a hydrofluorocarbon circulated by OTEC working fluid pump 408. Warm stream 312 and cold stream 310 preferably have a temperature differential, as measured upon their arrival at system 302, of at least 18° C., at least 20° C., at least 22° C., at least 24° C. or at least 26° C. Based on current ocean temperature conditions, preferred such temperature differentials are generally found in offshore locations between the Tropic of Cancer to the north of the equator and the Tropic of Capricorn to the south. Somewhat reduced temperature differentials may be found at numerically higher northern or southern latitudes, as well as in the tropical latitude offshore regions lying just to the west of Africa and South America.

The differing thermal energy potentials of cold stream 310 and warm stream 312 enable working fluid 406 to repeatedly change from a vapor phase to a liquid phase and back to a vapor phase while circulating through condenser 402, working fluid pump 408 and evaporator 404, with the volume expansion caused by vaporization serving to drive turbine 410 and its coupled electrical generator 412. A portion of the electrical output 416 from generator 412 may be used to drive pump 408. Another portion of electrical output 416 may be used to drive pumps 114 and 120 using power respectively supplied via electrical cables 324 and 326 shown in FIG. 3. Another portion of electrical output 416 may be used to drive other submerged pumps such as pumps 313 and 318. Any remaining electrical output may be used for other purposes in connection with operating, monitoring or maintaining the SRO apparatus, or used externally (e.g., onshore).

Referring again to FIG. 3, pumps 120 and 318 promote inflow and mixing of cold seawater 102 into the exposed lower open end 320 of riser 316 and the formation of diluted cold concentrate or brine stream 310. Stream 310 has generally reduced salinity compared to concentrate or brine 110 and thus presents a reduced potential hazard to marine life. Stream 310 also has substantially greater volume and substantially greater cold thermal energy potential (e.g., for the above-mentioned SRO apparatus based on Dow FILMTEC membranes, about 11.5 times the volume and about 11.5 times the cold thermal energy potential) than would be available if an SRO product water stream was instead employed to cool the OTEC system. Accordingly, using a cold brine or concentrate stream (and especially using a low recovery ratio concentrate stream that is further diluted with cold seawater) for OTEC cooling can provide a significant improvement in overall OTEC energy efficiency compared to an RO system using cold product water for OTEC cooling. Doing so can also help reduce both CAPEX requirements and operating expenses. In particular, when the RO membranes are operated at deliberately lower recovery ratios than might be used in a shore-based system, the concentrate volume can become very large relative to the product water volume (e.g., up to 10-20 times the permeate volume at 5-10% recovery), thereby greatly facilitating the extraction of useful energy via OTEC system 302. In preferred embodiments the disclosed Ocean Well+OTEC system consequently can be optimized to provide both a 100% self-powered seawater desalination system as well as surplus power for a variety of uses. For example, system 302 may be sized to provide surplus electrical power to operate warning beacons, monitoring equipment, telemetry devices, ROVs and other electrical devices used in or with the disclosed SRO apparatus. In additional preferred embodiments, system 302 is sized to provide further additional electrical power for use in other nearby SRO units or for onshore delivery via suitable cabling for private or public onshore use.

Combining OTEC power generation with the use of and discharge of diluted concentrate accomplishes particularly important goals, including (1) transport of concentrate or brine 110 to an OTEC system located at or near the surface 124 moves the concentrate or brine 110 far away from both the disclosed SRO apparatus seawater intake and the ocean floor 136, where a buildup of salinity would be detrimental to the desalination process and the benthic environment, respectively; and (2) mixing cold seawater 102 and warm seawater 312 with concentrate or brine 110 can dilute the concentrate or brine 110 to negligible levels of elevated salinity, such that dispersal stream 314 may, depending on the jurisdiction, not be classified as “brine”. In any event, stream 314 may be controlled to pose little or no environmental threat. If desired, dispersal stream 314 may also or instead be pumped from system 302 using an airlift pump as discussed in the above-mentioned International Application WO 2018/148542 A1 (Bergstrom). Doing so can help oxygenate the dispersal stream, thereby promoting increased oxygenation of nearby seawater and a reduction in hypoxia.

As also depicted in FIG. 3, riser 316 preferably has a generally conical shape, with a narrow lower end proximate the top of riser 122 and a wide upper end proximate the base of platform 304. This shape helps maintain a sizable diameter water flow path through riser 316 despite bending or other constriction in riser 316 that may be caused by lateral or other movement of apparatus 100 and platform 304 relative to one another. The disclosed generally conical shape can also help reduce pipe friction near platform 304 (where there may be the most lateral motion) and can help reduce material stress distributions throughout riser 316 that may result from such motion. Riser 316 may be rigid or flexible and may be made from a variety of materials including insulated or uninsulated seawater-resistant fabric, textile, polymeric or composite material. Riser 316 preferably is both flexible and insulated and more preferably is made from an insulated and imperviously-coated flexible textile laminate. In one embodiment, riser 316 is also slidably mounted with respect to riser 122, thereby compensating for vertical movement (e.g., as may be caused by waves notwithstanding the retaining forces applied by mooring lines 306) of platform 304.

In an additional embodiment (not shown in FIG. 3), system 300 may employ one or more added devices to increase the warm-side stream 312 intake temperature, such as a suitably large floating or otherwise permanently or temporarily-exposed solar absorbing heat exchanger or solar pond whose color, texture, and geometry are tuned to absorb greater solar energy than would be absorbed by seawater occupying the same area.

As an alternative to the embodiment shown in FIG. 3, OTEC unit 300 may if desired be placed on a floating or fixed exposed platform located at or above surface 124. As shown for example in FIG. 5, sea level platform 500 may be anchored as described above or otherwise maintained at sea level. In some embodiments, the depth of platform 500 may be adjusted as needed to account for changing wave heights, tides or projected sea level changes. Moving or other replaceable parts on platform 500 desirably will be located mainly at depths readily reachable by human divers, e.g., at depths less than about 100 meters, less than about 60 meters, less than about 50 meters or less than about 30 meters. One or more auxiliary or alternative power sources on platform 500 such as underwater (viz., tidal) turbines 502, other conventional turbines (not shown in FIG. 5), wave energy generator 504, solar panels 506 or wind turbine generator 508 may be used to operate or help operate OTEC unit 300, or may be used to independently operate submerged pumps and other electrical components in the disclosed SRO apparatus. Suitable underwater turbines include those available from Nova Innovation. Suitable wave energy generators include those available from CalWave Power Technologies, Inc. Suitable solar panels include both platform-mounted and floating designs such as those available from Kyocera Corporation. Suitable wind turbine generators include those available from Principle Power, Inc.

Apparatus 100 may if desired be instead or in addition supplied with power from a conventional platform-mounted, ship-borne or onshore power source, for example an onshore power plant. Doing so may result in greater carbon emissions than when using the power supply systems disclosed in FIG. 3 through FIG. 5, but may be desirable in instances in which the disclosed OTEC system and the disclosed auxiliary or alternative power sources on platform 500 are not able to provide sufficient power or are not able to do so at all times or seasons when such power may be needed.

As also shown in FIG. 5, discharge conduit 510 may be used to transport concentrate or brine exiting the disclosed OTEC system away from the OTEC system for release into the surrounding water. The overall direction for such transport may be horizontal, upward, or (as shown in FIG. 5) downward with respect to the OTEC system. In a preferred embodiment, the effluent from conduit 510 is a reduced salinity blended effluent made by combining the disclosed cold concentrate or brine stream and warm water stream after these respective streams have passed through the OTEC system and their desired thermal energy potentials have been extracted. In a further preferred embodiment, a significant part (e.g., at least 20%, at least 30%, at least 40% or at least 50%) of the effluent in conduit 510 exits the conduit via a series of side orifices 512 arrayed along the terminal end of conduit 510, and the remaining effluent exits via end orifice 514. In a further preferred embodiment, the effluent from conduit 510 is discharged into a water column at a depth at which the surrounding seawater density exceeds that of the effluent (e.g., the blended effluent), thereby reducing atmospheric release of carbon dioxide compared to discharge at a lesser depth.

Referring to FIG. 6, a “water farm” containing an array of portable offshore desalination systems (“pods”) 600 is shown in perspective view. Product water flows downwardly from the modules 600 through conduits 602 and horizontally through pumps 604 to a centrally located hub 606, and is then pumped towards the surface through delivery conduit 608. Concentrate or brine is pumped upwardly through conduits 610 into ocean currents for dispersal away from the pods 600 or for use in an OTEC system like that discussed above. The conduits 610 may if desired be kept separate from one another, bundled together, or connected to a single larger diameter conduit, and may if desired by equipped with hot-swap water connectors (not shown in FIG. 6) to facilitate disconnection, maintenance or replacement of individual pods 600.

As depicted in FIG. 6, four pods 600 are employed. However, lesser or greater numbers of pods can be used if desired, for example 2, 3, 5, 6, 7, 8, 10, 20 or more pods. Using a plurality of connected pods provides redundancy and enables ready scaleup of the disclosed SRO apparatus to meet initial or growing water needs. Operation and maintenance of the disclosed apparatus can be facilitated by providing a plurality of hot-swap water connectors (not shown in FIG. 6) between each conduit 602 and its associated pump 604, or between each pump 604 and hub 606, or at both the inlet and outlet ends of each pump 604. Scaleup of the disclosed apparatus can be facilitated by providing one or more additional hot-swap water connectors (not shown in FIG. 6) on hub 606 or at another convenient location to enable connection of additional pods or water farm arrays to delivery conduit 608 at a later date. If for example the individual pods 600 shown in FIG. 6 each have a 5 million gallons per day product water capacity, and if five additional hot-swap connectors are included in hub 606, then the FIG. 6 water farm could provide 20 million gallons of product water per day as initially installed, and up to five additional similarly-sized pods 600 could be added in 5 million gallons per day increments to provide up to 45 million total gallons of product water per day. In another embodiment, a plurality of such arrays may be installed near one another to provide multiple instances of the 20 million gallon per day array shown in FIG. 6, thereby providing increased capacity, redundancy and multiplicity of scale for the individual components. In yet another embodiment, the pods are not grouped together as depicted in FIG. 6, and instead are spaced apart across the seafloor, for example to accommodate topographical changes in the seafloor landscape, mooring line locations or other subsea features.

FIG. 7A shows a perspective underside view of a polygonal array 700 of the disclosed submerged RO membrane modules 106 mounted beneath hood 118, with prefilter system 104 being removed and with four of the twelve modules 106 in the polygonal (viz., dodecagonal) array 700 being numbered in FIG. 7A (as modules 106A, 106B, 106K and 106L) and the remaining eight modules being unnumbered. Modules 106 have generally tapered module sides that converge towards centrally-located product water (viz., permeate) collector 112 and product water collection conduit 113. Modules 106 are in fluid communication with collector 112 and conduit 113 via hot-swap product water valves 706 mounted on collector 112, with three of the twelve valves connected to array 700 and conduit 113 being numbered in FIG. 7A (as valves 706A, 706B and 706K) and the remaining nine valves being unnumbered. Converging sides (two of which are numbered as side 702C and side 704C) on each module 106 assist in underwater docking and reattachment of a detached module 106 to a hot-swap product water valve 706. Central rails on modules 106 (one of which is numbered as rail 703C) provide further support for the modules 106. When the modules 106 are in operation, product water flows downwardly through permeate collector 112 and permeate conduit 113 and is carried away by a pump such as pump 114 in FIG. 2.

The disclosed hot-swap product water valves and associated components may utilize a variety of designs, including so-called “hot stab” check valves and receptacles like those used in undersea oil and gas equipment for handling hydraulic fluids. Suitable such valves and receptacles are available from a variety of suppliers including Blue Logic, FES Subsea Engineering Products, James Fisher Offshore, Oceaneering and Total Marine Technology and Unitech. By way of example, the M5 ROV Flyable Full Bore Connector from Oceaneering represents one useful such hot stab valve and receptacle combination. Because hot stab devices are typically designed for use in the undersea oil and gas industries and must tolerate the handling of hydrogen sulfide and other corrosive ingredients at significant pressures, they can be derated and their designs can be simplified and made less expensive when used to handle the noncorrosive or less corrosive fluids and much lower pressures present in the disclosed SRO apparatus.

FIG. 7B shows a perspective underside view of the disclosed SRO apparatus of FIG. 7A without array 700 and its modules 106. Hood 118 includes a supporting framework formed by inclined struts 708, crossbars 710A, 710B and 710C, lower circumferential rim supports 712, lower radial rails 714, lower inner anchoring ring 716 and an upper inner anchoring ring located (but not shown in FIG. 7B) at the junction of hood 118 and riser 122. The disclosed framework preferably also receives, captures and supports the disclosed modules and array. Each circumferential rim support 712 includes a slotted receiving aperture 718 that captures hangers atop each module 106, and which are discussed in more detail below. The disclosed framework supports an overlying, impervious protective hood cover 128 that may for example be made from an insulated or uninsulated seawater-resistant textile, plastic or metal covering material. In a preferred embodiment, the inner side 720 of hood cover 128 is made for example from a buoyancy-imparting material such as an engineered syntactic foam. Hood cover 128 preferably provides a protective cover that helps maintain a slight pressure differential (e.g., up to 50 psi or thereabouts) between the internal concentrate or brine collected by the hood and the external environment, while isolating the interior of the hood from penetrants. Indentations 722 and product water valve couplings 724 may be seen near the top of conduit 113, just below ring 716. Rails 714, apertures 718 and indentations 722 help guide, support and locate modules 106 when array 700 is installed, and couplings 724 assist in the hot-swap attachment and detachment of modules 106. If desired, corrosion resistant magnets or electromagnets may be used to guide, retain or both guide and retain modules 106 in place within array 700 and apparatus 100.

FIG. 7C shows a perspective underside view of the disclosed SRO apparatus of FIG. 7A with prefilters 104 installed. In the embodiment shown in FIG. 7C, each prefilter 104 has a generally triangular shape, and is periodically swept clean of debris by oscillating brush arms 726 mounted on pivot points 728 near lower mounting ring 730. Brush arms 726 repeatedly (e.g., intermittently, periodically or continuously, and based on predetermined times, signals from one or more sensors, or an externally-supplied control signal) sweep across the inlet face of each prefilter 104 toward central struts 732 and then stop, return to the positions shown in FIG. 7C, or initiate another sweep movement. Crossbars 734 help reinforce and support each prefilter 104 and can serve as guide rails supporting brush arms 726. In another embodiment, the assembled prefilters may have curved surfaces, a generally conical overall shape, and brush arms configured to sweep over or to revolve around the prefilters.

FIG. 8A and FIG. 8B are perspective topside views of a module 106 showing a plurality of RO membrane cartridges 802 suspended and sealed in apertures 804 in perforated, generally triangular divider plate 806. As depicted, cartridges 802 are generally cylindrical but may have other shapes if desired. As also depicted, module 106 contains 142 cartridges, but other greater or lesser numbers of cartridges may be employed in each module as desired, for example at least 40, at least 50, at least 60, at least 70, at least 80, at least 90 or at least 100 cartridges, and up to 200, up to 190, up to 180, up to 170, up to 160, up to 150 or up to 140 cartridges. Also, as depicted all the cartridges are generally parallel and in a single layer occupying a single plane, as doing so promotes efficient flow through the disclosed apparatus. However, if desired the cartridges in a module need not be generally parallel to one another, and also if desired multiple layers of cartridges could be employed in a module.

Using 140 of the above-mentioned Hydranautics cartridges in each module, the disclosed SRO apparatus may produce about 5 million gallons per day from a twelve such modules operated at a 5% recovery rate. Other RO membrane suppliers whose cartridges may be used will be apparent to persons having ordinary skill in the desalination art, and include Aquatech International, Axeon Water Technologies, DuPont Water Solutions (makers of the above-mentioned DOW FILMTEC cartridges), Evoqua Water Technologies, GE Water and Process Technologies, Koch Membrane Systems, Inc. and LG Chem. Customized cartridges having altered features (for example, wider gaps between layers, modified spacers, a looser membrane roll, a modified housing or modified ends) may be employed if desired.

As depicted, the cartridges 802 are substantially vertically aligned when module 106 is installed in array 700 and in use, with the concentrate or brine end outlets 804 in each cartridge 802 facing upwardly towards hood 118 and with the product water outlets (discussed below in connection with FIG. 8C) facing downwardly. However, other orientations and accompanying flow directions may be employed, for example with the outlets 804 facing downwardly, horizontally or obliquely, and with the product water outlets facing upwardly, horizontally or obliquely.

In certain embodiments, the cartridges 802 are mounted in the disclosed modules 106 by adhesively bonding and sealing the cartridges in holes in perforated divider plate 806. In the embodiment depicted in FIG. 8A and FIG. 8B, the adhesive bond may be at the upper end of the cartridges 802 near the concentrate or brine outlets 804. However, as depicted in FIG. 8G, more than one divider plate 806 may be employed, and the divider plate(s) and the adhesive 840 that bonds and seals the cartridges 802 into the perforations 842 in divider plate 806 may be located at either or both ends or anywhere along the length of the cartridges 802. FIG. 8G also illustrates the use of adhesive 840 to bond manifold 828 to permeate outlet 827 as discussed above. To provide greater buoyancy (and desirably neutral buoyancy at the intended operating depth) and as shown in FIG. 8F, the spaces between cartridges 802 are desirably filled or at least partially filled with a suitable buoyant medium 850 having a density less than that of seawater. A preferred such medium is engineered syntactic foam, available from suppliers including Engineered Syntactic Systems. Medium 850 may be provided in the form of shaped blocks that may be inserted into the modules 106 before or after the installation of the cartridges 802, as a premolded perforated slab that is inserted into the modules 106 before installation of the cartridges 802, or as an in-situ cured material that may be placed in spaces between the cartridges (e.g., by spraying or other form of injection) after the cartridges have been added to the modules 106. In an embodiment, beads of adhesive 840 may be omitted and medium 850 can instead serve as the adhesive that bonds and seals the cartridges 802 into the perforations 842 in divider plate 806. Use of medium 850 can significantly assist the underwater removal and installation of the modules 106, by reducing the cantilever effect of the mass of each module 106 as it is being removed from or flown into position in the disclosed array, and especially when such removal and installation are conducted using an ROV that grips the outer edge of a module 106.

Perforated divider plate 806 may have a variety of shapes, for example a generally polygonal perimeter such as generally triangular perimeter or a generally trapezoidal perimeter. Plate 806 and the remaining components in module 106 that support and envelop (viz., provide a frame for) the cartridges 802 may be made from a variety of materials, including corrosion-resistant metals such as stainless steel or titanium, fiber-reinforced polymers or filled composites. Preferably a mixture of such materials is employed, with lower density components being used in appropriate locations to reduce the overall module weight, and higher strength or higher durability materials being used in other appropriate locations within the module where such strength or durability may be required. Divider plate 806 and the remaining components in module 106 may if desired be surface-treated to resist biofouling, and may also be surface-treated to increase the associated surface area in locations that may require improved adhesion by adhesive 840.

A variety of adhesives may be used to bond and seal the cartridges in the modules. Exemplary adhesives include the above-mentioned engineered syntactic foams, as well as epoxy, polyurethane, polyester, acrylic, silicone and fluorinated resins. In one preferred embodiment, the adhesive is substantially free or completely free of bisphenol A, bisphenol F and their diglycidyl ethers. In another preferred embodiment, there are no gaskets, O-rings or other preformed seals between the cartridges and the divider plate and the adhesive is primarily or exclusively relied upon to hold the cartridges 802 in the modules 106. Suitable adhesives will include those classified as marine adhesives or sealants suitable for use below the waterline, and are available from a variety of suppliers including Dow Chemical Company, Loctite, Sika and 3M. In a further preferred embodiment, the cartridges are not encased in a pressure vessel.

When a module 106 is removed from the disclosed array for replacement of one or more of the cartridges 802, it may in some instances be desirable to remove and replace only certain of the cartridges, and in other instances it will be most economical to remove and replace all of them. Removal typically will require debonding the affected adhesive joints so that the associated cartridges 802 may be extracted from divider plate 806. Depending on the chosen adhesive, debonding may be performed using a variety of techniques. Exemplary techniques include chemical debonding (e.g., using solvents, hydrolysis, or other measures), cryogenic debonding (e.g., using liquid nitrogen), electrical debonding (e.g., using current from an arc welder or other power supply to heat a conductive filler in the adhesive) or thermal debonding (e.g., using a flame or other heat source) to dissolve, fracture, soften melt, or otherwise weaken or degrade the adhesive or its bond to the cartridges and divider plate. Once the adhesive bond has been sufficiently weakened or degraded, the cartridges may be pushed, pulled, twisted or a combination thereof to remove them from the module.

Use of the disclosed adhesive provides a number of advantages. Water separation membrane cartridges are normally sealed to other components in a water desalination apparatus using gaskets or O-rings. Gaskets and O-rings represent a potential array leakage point, especially if the gasket or O-ring undergoes significant compression set upon exposure to cold underwater temperatures. Adhesively bonding the membrane cartridges to a perforated divider plate eliminates this potential leakage point while meanwhile increasing the beam strength and rigidity of the assembled module.

Referring again to FIG. 7B and FIG. 8A, the lower edges of hood 118 preferably overlap with, have a gasketed connection to, or are otherwise sealingly engaged with the upper edges of the modules 106, thereby isolating the collected concentrate or brine from the salinated water surrounding module 100. Divider plate 806 is desirably fastened and sealed about its periphery to converging side plates 702 and 704, outer end plate 812 and inner end plate 814, thereby further isolating the collected concentrate or brine from the surrounding salinated water.

As depicted in FIG. 8A through FIG. 8C, suspending hooks 816 and 818 are fastened atop and near the outer edge of module 106, and point toward the inner edge of module 106. Hooks 816 and 818 mate with slotted receiving aperture 718 shown in FIG. 7B and help support and guide module 106 into a proper position when module 106 is pushed into an available open space in array 700. Guide rails 820 and 822 engage radial rails 714 beneath hood 118 during insertion of a module 106 into array 700. Generally wedge-shaped projecting tang 824 also helps to guide and properly affix module 106 into place in array 700, and helps ensure proper connection of hot-swappable product water valve 706 to permeate collector 112. Upon the completion of such connection, hot-swap valve 706 opens to permit the flow of product water into permeate collector 112 and permeate conduit 113.

FIG. 8C, FIG. 8D and FIG. 8E are perspective underside views of a module 106. The circular salinated water inlets 826 at the lower end of each cartridge 802 permit the entry of salinated water into the cartridges 802. Desalinated product water exits a typically centrally-located outlet 827 in each cartridge 802 via cartridge manifolds 828, branch manifolds 830A through 830I (for the nine depicted rows in the disclosed cartridge array that contain two or more cartridges each side of the array centerline) and a pair of radially-extending product water collection manifolds 832 located each side of the array centerline. Three single cartridges 802 located on each side of the array centerline near inner end plate 814 are directly connected by individual cartridge manifolds 828 to product water collection manifolds 832.

The prefilter screens 104 shown in FIG. 7C, sides 702, 704, 812 and 814 and the underside of perforated divider plate 806 cooperate to isolate the filtered water passing through prefilter screens 104 from the salinated water surrounding module 100 and ensure that the interior portion of module 106 surrounding the cartridges 802 will contain only filtered water that has passed through a screen 104.

In the embodiment depicted in FIG. 7A through FIG. 8E, the modules 106 have an approximately wedge-shaped or trapezoidal-shaped cross-section, with sides 702 and 704 that taper or converge towards permeate collector 112 and the vertical central axis of the disclosed SRO apparatus. The disclosed modules may be any desired size, and in a preferred embodiment may for example be about 5 to 8 meters long by about 4 to 5 m wide by about 0.5 to 1.5 m thick, and have shapes and dimensions that facilitate efficient packing of the modules in standard shipping containers as discussed in more detail below. The disclosed modules preferably have neutral buoyancy at the intended operating depth.

In the embodiment depicted in FIG. 9, twelve modules are shown, and in their assembled form the depicted modules provide an array with a dodecagonal perimeter in plan view. Other module shapes, numbers of modules (e.g., 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15 or 16) and array shapes (e.g., triangular, square, pentagonal, hexagonal, hexadecagonal, and other polygons made from the numbers of modules mentioned above, as well as circular or other curved shapes) can be employed if desired.

As illustrated in FIG. 9, maintenance of an array 900 containing a defective, outdated or otherwise ineffective module may be performed by withdrawing such module from the array, thereby leaving a gap formerly occupied by the withdrawn module, and replacing the withdrawn module with a new or rebuilt module 906L. Because such replacement will be carried out while the SRO apparatus is submerged, it is desirable that both the module removal and module replacement procedures proceed quickly and efficiently, with minimal disruption in product water output despite potential adverse conditions such as low visibility, underwater currents or difficulties in operating an ROV or other devices that might be used to carry out or assist in module replacement. Module maintenance may for example be scheduled or performed based on signals from one or more sensors that monitor the flow rate or salinity of product water, concentrate or brine flowing through the apparatus, or the flow of water into the apparatus. Such sensors may monitor individual cartridges, individual modules or an entire array. Module maintenance may in addition or instead be based on a preset or adjustable schedule, predictive algorithms, the availability of improved RO membranes or cartridges, and other measures that will be apparent to persons having ordinary skill in the desalination art upon reading this disclosure.

Removal of defective or ineffective modules can be facilitated while continuing to operate the remainder of the disclosed apparatus during module removal, and relying on the portion of the disclosed hot-swap valve 706 that remains connected to permeate collector 112 to close and seal off permeate collector 112 from the surrounding salinated water. Such valve closure may be initiated in a variety of ways, including in response to a suitable electrical command, mechanical switch, or in response to the outward motion of a module 106 away from permeate collector 112 and separation components in hot-swap valve 706. Hot-swap valve 706 accordingly desirably prevents the entry of salinated water into permeate collector 112 during module replacement. The portion of hot-swap valve 706 remaining on the removed module 106 may optionally also be closed upon removal in order to prevent entry of desalinated water into the product water outlet side of the removed module 106. However, doing so generally will not be needed, as the removed module 106 will normally be brought to the surface and flushed with fresh water as a part of a repair or rebuilding procedure.

Removal of a module 106 may cause unfiltered salinated water to enter the otherwise normally isolated chamber between the prefilters 104 and the modules 106 in the disclosed array 700. Typically however such unfiltered water entry would take place for a relatively brief time period, until such time as a replacement module or temporary blanking plate can be inserted into the array, and consequently will be unlikely to introduce significant detrimental quantities of debris or other solid matter into such chamber.

During insertion of replacement module 906L, converging sides 908 and 910 and hangers 916 and 918 assist in underwater docking and attachment of module 906L to array 900 by helping to align and guide module 906L into proper orientation and location in the three-dimensional volume between adjacent modules 906A and 906K, and by helping to align and guide hot-swap valve body 920 into proper alignment and engagement with the portion of valve body 706 that remains attached to permeate collector 112. Hangers 916 and 918 preferably have inwardly-pointing tapered ends (viz., ends that point towards the longitudinal central axis of the disclosed array and have a wedge-shaped profile in plan view, side view or both plan and side views). Such tapered ends will significantly assist in docking module 906L into the disclosed SRO apparatus. During insertion of module 906L into array 900, hangers 916 and 918 enter the slotted receiving aperture 718 shown in FIG. 7B, and rails like the rails 820 and 822 shown in FIG. 8A and FIG. 8B engage with the radial rails 714 shown in FIG. 7B. Upon reattachment of replacement module 906, the hot-swap valve formed by valve bodies 706 and 920 can then open to permit resumption of product water collection from the affected portion of the disclosed array. Such valve opening may be initiated in a variety of ways, including in response to a suitable electrical command, mechanical switch, physical manipulation by an ROV, or in response to the inward motion of module 906L and joinder of valve bodies 706 and 920. In this fashion, removal and inspection or replacement of individual modules 106 can be accomplished without having to shut down the disclosed SRO apparatus, thereby enabling continued production of product water and concentrate or brine from the remaining undisturbed modules 106.

In addition to the disclosed tapered sides, rails and hangers, the disclosed module reattachment procedure may be assisted by employing other guidance features or devices. Exemplary such other features or devices will be apparent to persons having ordinary skill in the desalination art upon reading this disclosure, and include appropriately-shaped (e.g., conical or tapered) mating or receiving surfaces, snubbers, guiderails or magnets on the sidewalls of the replacement module or adjacent modules, the upper or lower surfaces of the replacement module, adjacent portions of the framework receiving the replacement modules, or the hot-swap valve bodies 920. Such other guidance features or devices may for example contact the replacement module or adjacent modules during any or all of the start, middle, or end of the disclosed replacement procedure. If desired, one or more gaskets may also be employed on the modules 106, hood 118 or assembly of prefilters 104 to assist in sealing gaps between the modules 106 and the remainder of the disclosed SRO apparatus, and in some embodiments such gaskets may provide guidance features to assist during module insertion.

Maintenance requirements may be further simplified by employing suitable hot-swap connectors for other fluid or electrical connections between potentially replaceable components and the disclosed apparatus, and especially for components that may be subject to wear or premature failure. Such components may include wearable moving components such as pumps, valves, intake screen wipers and the like; electrical components such as motors, sensors, transformers, variable frequency drives (VFDs), and the like; and fouling-prone components such as intake screens. Desirably such connectors and the associated component(s) can be detached from and reinserted into the disclosed apparatus using a suitable ROV.

Shipment and storage of dry (viz., non-submerged) may be made more space-efficient by placing two generally triangular modules (or two generally triangular portions of a module) alongside one another and facing in opposite directions to provide a generally rectangular, dense-packed shipment or storage package. As shown in FIG. 10A, this may for example be performed by omitting parts near the major centerline of a module 106 (such as the hot-swap product water valve 706) that cannot conveniently be split in half, and splitting the remainder of the module 106 at or near its major centerline to provide modular halves 1000, 1002 and 1004. When modular halves 1002 and 1004 are placed alongside one another and facing in opposite directions, they form a generally rectangular pair 1006. As shown in FIG. 10B and FIG. 10C, a plurality (e.g., four) half-module pairs 1006 may be stacked together to provide a compact, readily transportable shipment 1008 containing eight half-module pairs 1006, which once the half-modules are combined with the omitted parts and reunited with one another, will correspond to four full modules 106. One or more such half-module pairs preferably fit in and substantially fill (e.g., to occupy 80% or more of the available floor space of and preferably the available interior volume of) a standard ocean shipping container for subsequent shipping and transport, or may be transported in enclosed or unenclosed fashion on a flatbed or other trailer 1010 by truck cab 1012. Representative standard shipping containers have typical widths of about 2.4 m (8 ft.), heights of about 2.6 m (8.5 ft.) and nominal lengths of about 3 m (10 ft.), 6.1 m (20 ft.) or 12.2 m (40 ft.).

Referring now to FIG. 11 and FIG. 12, the above-mentioned prefilter screens provide prefiltration for incoming salinated water that can reduce maintenance requirements in the disclosed SRO apparatus. The disclosed SRO apparatus typically will be operated at a significant depth, far from human hands, and it consequently will be desirable to employ stringent measures to avoid RO membrane fouling. Some prior SRO desalination proposals appear to assume that deep seawater is sufficiently clean to permit desalination without prefiltration. In some embodiments (e.g., in appropriately clean waters or when using membranes that are less susceptible to fouling), pretreatment may be omitted entirely. However, although seawater at depths of for example 700 meters is typically vastly cleaner than surface water, such seawater may nonetheless contain sufficient contaminants to require periodic replacement of the RO membranes to prevent or overcome clogging or fouling. The disclosed prefilter provides a one-stage and preferably a two-stage prefiltration treatment that can prolong RO membrane lifetime. By performing pretreatment at depth rather than near the shoreline, the overall likelihood of lethal marine life entrainment is reduced. The prefilter can be periodically or continuously back flushed or otherwise purged using air bubbles or an air/water stream to further prolong the prefilter lifetime while meanwhile further minimizing or avoiding harm to marine life. If desired, chlorine or ozone may be introduced near the prefilter, in order to disinfect the prefilter and discourage biofouling. By using the relatively cleaner waters available at the disclosed preferred depths and a prefilter cleaning procedure, the disclosed SRO apparatus can significantly reduce the need for prefilter replacement or RO membrane replacement that might otherwise be required for shore-based or shallow depth RO, while meanwhile reducing accompanying capital, operating, real estate and energy requirements.

FIG. 11 and FIG. 12 respectively show prefilter outlet side and prefilter inlet side schematic views of a system for cleaning prefilter screens that are similar but not identical to the prefilter screens 104 shown in FIG. 7C. Referring first to FIG. 11, prefilter 1100 includes a plurality (ten, as depicted in FIG. 11) of screens 1102 which are mounted on and surrounded by perimeter flange 1104, crosspieces 1106 and central flange 1108. Pivot bearings 1110 support movable cleaning arms 1112. When suitable electrical or hydraulic energy is supplied to a motor (not shown in FIG. 11 or FIG. 12) connected to the arms 1112, the arms 1112 sweep back and forth in an arc across the output face of the screens 1102. The arms 1112 may be moved in continuous fashion or at discontinuous, periodic, random or other intervals. Pressurized water or air supplied through the arms 1112 passes through water or air backflush jets 1114 that dislodge debris from the screens 1102.

Referring next to FIG. 12, arms 1212 are moved by the above-mentioned motor back and forth over the surfaces of screen 1102. The arms 1212 include suction ports 1214 and mechanical brushes 1216 that remove debris present on prefilter screen 1102. The suction ports 1214 also remove debris dislodged from the screens 1102 by the backflushing action of the jets 1114. In one embodiment, the brushes 1216 are moveable (e.g., rotatable) with respect to their respective arms 1212 as the arms traverse back and forth across the screens 1102, in order better to dislodge such debris.

The disclosed SRO desalination apparatus may be operated in a variety of locations. In one preferred embodiment, the apparatus is deployed in an ocean trench or dropoff (for example, the Monterey Submarine Canyon, Puerto Rico Trench, Ryukyu Trench, waters surrounding the Hawaiian Islands, and other accessible deep sea sites that will be familiar to persons having ordinary skill in the desalination art), near a populated area in need of desalinated water. The SRO inlet surfaces need not be placed at trench floor depth, and may instead be positioned along the trench wall at a depth sufficient to enable the use of hydrostatic pressure to drive seawater through the osmotic membranes.

Operation at appropriate depths can greatly reduce or eliminate the likelihood of algal bloom contamination, which can cause conventional shore-based plants with shallow water intakes to shut down in order to avoid toxins and clogging. Operation at such appropriate depths can also minimize or eliminate the loss of marine life, as most marine organisms are found within the photic zone (depending upon water clarity, corresponding to depths up to about 200 m) and thus at deeper depths will not be drawn into the SRO apparatus intake or against a prefilter screen.

The cold feedwater (e.g., 5-10° C. water) typically encountered at the above-mentioned recommended SRO operating depths can provide several useful advantages. For example, the feedwater is relatively free from critical organic and inorganic contaminants. It carries very little organic matter or chlorophyll and thus contains little bacteria, while still retaining valuable nutrients from the ionic minerals and trace elements present at the disclosed pressures and depths. A further advantage arises in connection with boron removal, which is important for irrigation water and health purposes. Boron is present in seawater, and at conventional RO operating temperatures such as are used in onshore RO units, enough boron may pass through the RO membrane to inhibit the growth of plants. Boron removal to agricultural standards of 0.5 mg/liter in a conventional RO facility may require double treatment of the water using a second RO pass, thus increasing capital and operating costs. Boron removal by reverse osmosis is however highly temperature-dependent, with lower amounts of boron and its salts passing through the membranes at colder temperatures. For example, borate passage may be reduced by several percentage points for every reduction of 10° C. in feedwater temperature. Placement of the disclosed SRO device in cold deep water consequently may help produce higher-quality desalinated water by improving the removal of boron and its salts while saving the energy, capital, and maintenance costs required for a double treatment system. Cold feedwater can also result in less overall salt passage through the membrane, allowing for remineralization of the product water for taste reasons while maintaining a low level of TDS to meet regulatory requirements. In addition, the use of cold feedwater can nearly eliminate the scaling of membranes by mineral deposition, as measured by the Langelier Index. Membrane scaling can be a problem with shore-based, shallow-intake RO units, and reduces system efficiency and lifetime. In the disclosed SRO apparatus, scaling is minimized because CO₂ will tend to be in equilibrium at the 5-10° C. temperatures at which the RO membranes may be operating. This can eliminate the need for the anti-scaling chemicals that often are employed in shore-based RO units. Biofilm growth, another form of membrane fouling, is also temperature-dependent, with more biofilm forming at warmer temperatures, and less at the low-temperature preferred operating environment of the disclosed SRO apparatus. Biological activity and hence biological fouling are thus reduced due to the use of water from a region having low light, low oxygen, and cold water temperatures.

The disclosed SRO apparatus can produce significantly lower concentrations of salt in the brine stream than will be the case for conventional RO, as the elimination of the requirement for pressure vessels permits the RO membranes to be arrayed in parallel rather than the typical seawater desalination industry practice of 5-7 membranes in a serial arrangement. A parallel array eliminates a common failure point in conventional RO systems, namely the o-ring interconnections between membranes. A parallel arrangement may also permit higher product water production per membrane. In addition, a parallel membrane arrangement creates much less salty concentrate or brine than a train of single membranes operating in series, and the salinity of such concentrate or brine can easily be adjusted by altering the membrane recovery ratio. The ability of the disclosed SRO apparatus to achieve low brine salinity is beneficial to sea life and allows easier dilution of the concentrate or brine. For example, when supplied with Southern California seawater containing about 34,250 ppm ambient TDS and operated at a 5% recovery ratio, the disclosed apparatus may provide concentrate containing only about 36,049 TDS versus the near-doubling in discharge stream salinity that may arise using conventional serially-configured onshore RO. A 36,049 ppm TDS discharge stream would be less than 1800 ppm above ambient, and thus well within the current brine discharge limit of 2,000 ppm above ambient TDS for California waters.

A principal benefit of the overall disclosed SRO apparatus is its significantly reduced energy requirements. The artificial pressurization of process water, the largest source of energy use in conventional RO desalination, can be reduced or eliminated. The energy consumption and associated greenhouse gas production to produce desalinated water using the disclosed SRO apparatus may consequently be significantly reduced. The associated capital expenditures and operating expenditures can also be significantly reduced, especially in comparison with those required for onshore RO desalination. These and other advantages of the disclosed SRO apparatus thus may include one or more of:

• • Greatly reduced power consumption.
  • Reduced greenhouse gas emissions to desalinate a given quantity of water.
  • Elimination of the artificial high-pressure environment used in conventional RO and the accompanying pressure vessels, high pressure piping, and fittings.
  • Reduced operation and maintenance requirements through elimination of parts, and especially the reduction of highly-pressurized connections.
  • Fewer precision parts requiring expensive alloys and other exotic materials resistant to seawater corrosion.
  • Reduced or eliminated pretreatment equipment and its associated operating capital and labor.
  • Reduced localized brine emission.
  • Parallel rather than series membrane configurations with even lower-salinity brine discharge.
  • Pipelines to shore that are over 50% smaller in diameter, as only product water is sent onshore, subject however to increased required length.
  • Reduced boron content in desalinated water, making it suitable for agriculture without further treatment.
  • Reduced bacterial content and bacterial fouling due to the use of deep-sea intake water that is relatively free of undesirable organic or inorganic contaminants.
  • Reduced susceptibility to desalination disruption caused by algal blooms.
  • Reduced visibility or invisibility from shore.
  • Reduced onshore noise pollution.
  • Reduced susceptibility to destruction due to adverse weather events, fires, terrorism or volcanic eruptions.
  • Reductions by as much as 90% in required onshore real estate.
  • Suitability for deployment as an “Ocean Well” that can provide a sustained freshwater supply without aquifer depletion.

Having thus described preferred embodiments of the present invention, those of skill in the art will readily appreciate that the teachings found herein may be applied to yet other embodiments within the scope of the claims hereto attached. The complete disclosure of all patents, patent documents, and publications are incorporated herein by reference as if individually incorporated.

Claims

1. A submersible water desalination apparatus comprising an array of hot-swappable water separation membrane modules arrayed around and generally radially extending from a product water collection conduit; a plurality of hot-swap product water valves connected to the conduit and detachably connected to generally radially-extending product water collection manifolds in fluid communication with a plurality of water separation membrane cartridges within the modules; the modules having in cross-section generally tapered module sides that converge towards the conduit and assist in underwater docking and attachment of a replacement module to a hot-swap product water valve.

2. An apparatus according to claim 1, wherein the water separation membranes are reverse osmosis membranes.

3. An apparatus according to claim 1, wherein the array has an approximately polygonal perimeter in plan view.

4. An apparatus according to claim 1, wherein the array has an approximately pentagonal to hexadecagonal perimeter in plan view.

5. An apparatus according to claim 1, wherein the array has an approximately dodecagonal perimeter in plan view.

6. An apparatus according to claim 1, wherein the modules have the same shapes and sizes and are interchangeable with one another.

7. An apparatus according to claim 1, wherein the modules are generally wedge-shaped in plan view.

8. An apparatus according to claim 1, wherein the modules have a generally trapezoidal perimeter in plan view.

9. An apparatus according to claim 1, wherein each module comprises a supporting frame or housing made from corrosion resistant metal and surrounding the water separation membranes in such module.

10. An apparatus according to claim 1, wherein each module comprises a supporting frame or housing made from fiber-reinforced polymer or other composite material and surrounding the water separation membranes in such module.

11. An apparatus according to claim 1, wherein the water separation membrane cartridges have generally cylindrical shapes.

12. An apparatus according to claim 1, wherein the product water collection conduit represents or is proximate to and generally aligned with a vertical central axis of the array, and the water separation membrane cartridges are generally parallel to the product water collection conduit.

13. An apparatus according to claim 1, wherein the modules and array are not encased in a pressure-resistant outer vessel.

14. An apparatus according to claim 1, wherein the apparatus further comprises a framework that is detachably connected to and receives, captures and supports the modules, and a hood that captures concentrate or brine from the water separation membrane modules.

15. An apparatus according to claim 1, wherein the apparatus further comprises one or more guidance features or devices comprising hangers, hooks, snubbers, guiderails or magnets that attract, contact or repel a replacement module or an adjacent module during underwater docking and attachment of a replacement module to a hot-swap product water valve.

16. An apparatus according to claim 1, wherein the modules have shapes and dimensions that enable a pair of modules or portions of modules to be placed alongside one another facing in opposite directions to provide one or more generally rectangular module pairs that will fit in and substantially fill the floor space of a standard ocean shipping container.

17. An apparatus according to claim 1, wherein the water separation membrane cartridges produce a volume of desalinated product water and a volume of concentrate, and the product water volume is sufficiently less than the concentrate volume so that the concentrate does not contain sufficient salinity to qualify as brine in the jurisdiction where such concentrate is produced.

18. An apparatus according to claim 1, further comprising one or more sensors that monitor the flow rate or salinity of desalinated water, concentrate or brine flowing through the apparatus.

19. An apparatus according to claim 1, further comprising a Remotely Operated Vehicle (ROV) for module replacement.

20. A method for maintaining a submerged water desalination apparatus, the method comprising the steps of:

a. undocking from a submerged water desalination apparatus a first hot-swappable water separation membrane module that is a member of a plurality of such modules arrayed around and generally radially extending from a product water collection conduit and in fluid communication with the conduit via hot-swap product water valves connected to the conduit and detachably connected to generally radially-extending product water collection manifolds in fluid communication with a plurality of water separation membrane cartridges within the modules, the modules having in cross-section generally tapered module sides that converge towards the conduit and assist in underwater docking and reattachment of detached modules to a hot-swap product water valve; and

b. docking a similarly sized and shaped replacement module and attaching it to the array while underwater by guiding the converging tapered sides of the replacement module and a detached product water collection manifold on such module towards a detached hot-swap product water valve connection on the product water collection conduit.