Passive hydrodynamic apparatus and method for surface interaction mediated harvesting of marine biomass

Abstract

A passive hydrodynamic apparatus and method for harvesting marine biomass from ambient currents. The apparatus includes a non-sieving retention object comprising a porous 3D network of fibres having a fibre volume fraction (solidity) of less than 5% relative to an envelope volume defined by outer boundaries of the network inside the current. The fibres comprise surface functionalisation and have a fibre diameter of between about 1 micron and about 3000 microns, defining effective void spaces significantly larger than the target biomass particles. Retention is mediated by surface interactions rather than mechanical obstruction, allowing for high capture efficiency of microscopic particles (e.g., phytoplankton) with minimal drag (e.g., drag coefficient is less than 300 at 6 cm / s). A mechanical harvester featuring a sacrificial agitating element may be included to continuously recover biomass while preserving the retention fibres.

Description

PASSIVE HYDRODYNAMIC APPARATUS AND METHOD FOR SURFACE INTERACTION MEDIATED HARVESTING OF MARINE BIOMASSTECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates generally to the field of aquaculture and biomass harvesting. More specifically, the present disclosure relates to a passive hydrodynamic apparatus and method for harvesting marine biomass, such as phytoplankton, from ambient currents using a high-porosity, surface-functionalised three-dimensional fibre network that relies on surface interactions for retention rather than mechanical obstruction.BACKGROUND

[0002] Microbial phytoplankton constitutes the very basis of the marine food chain. Although it represents a relatively small portion of the planet’s biomass compared to terrestrial flora, it is responsible for approximately 50% of global photosynthetic activity. The main groups of phytoplankton include, but are not limited to, cyanobacteria, diatoms, coccolithophores, and dinoflagellates. These unicellular organisms feature exceptionally high reproductive rates, far exceedingthose of nekton or other complex life forms.

[0003] In recent years, traditional fisheries have experienced a constant decline due to over-fishing, while the global demand for food continues to rise. Consequently, scarcity in the food market leads to rising prices and supply disruptions. In light of this, microbial phytoplankton represents a promising, inexhaustible food source that could potentially address unfolding food scarcity. The open oceans contain a standing stock of microbial plankton measuring billions of tons. Unlike terrestrial agriculture or intensive closed-system aquaculture, which require significant land, water, fertilizers, and energy, open-ocean phytoplankton is a naturally abundant resource that is resilient to large- scale harvesting.

[0004] However, this vast resource remains largely unexploited due to the significant economic and technical challenges involved in harvesting it from the wild. The primary challenge stems from a set of conflicting physical requirements, often referred to as the “filtration paradox”. First, the target organisms are microscopic (typically 0.5 pm to 100pm), requiring filtration elements with comparably small pore sizes to separate them from the medium. Second, the biomass is extremely dilute in open waters, often with concentrations on the order of 1 part per million (ppm). This necessitates the processing of extremely large volumes of water to obtain commercially significant amounts of biomass.

[0005] At these microscopic scales, low Reynolds numbers prevail, meaning that water viscosity becomes a dominant factor causing significant friction. Conventional harvesting methods cannot overcome this friction without expending more energy than is viable to expend on the harvested biomass. For example, standard plankton tow nets or sieves rely on mechanical obstruction (pores smaller than the particle). To filter such tiny organisms, significant pressure gradients or large tow forces are required to overcome the hydrodynamic drag of the fine mesh. This renders the process energy- intensive and difficult to scale. Another example is centrifugation and tangential flow filtration (TFF). While effective for harvesting concentrated cultured stocks in closed systems, these methods are energetically demanding and limited to relatively small volumes. They are impractical for the massive throughputs required to harvest dilute biomass from open waters.

[0006] Therefore, there is a long-felt need for a harvesting solution that can effectively retain microscopic biomass from dilute ambient currents without the prohibitive energy costs associated with high-pressure pumping, centrifugation, or high-drag sieving.SUMMARY OF THE INVENTION

[0007] The present invention provides an apparatus and method for harvesting marine biomass from an ambient current by utilising surface interactions within a high-porosity three-dimensional structure rather than mechanical obstruction.

[0008] The present invention provides an apparatus and method for harvesting marine biomass from an ambient current by utilising surface interactions within a high-porosity three-dimensional (3D) structure rather than mechanical obstruction.

[0009] In one aspect, the present invention provides an apparatus for harvesting marine biomass from an ambient current, comprising a non-sieving retention object comprising a plurality of fibres forming a porous 3D network. The porous 3D network has a fibre volume fraction (solidity) of less than 0.05 (5%) relative to an envelope volumedefined by outer boundaries of the porous 3D network inside the ambient current. The fibres comprise surface functionalisation configured to promote adhesion of marine biomass and have a diameter of between about 1 micron and about 3000 microns. Crucially, the porous 3D network is configured to exhibit a drag coefficient (Cd) of less than 300 at an ambient current velocity (v) of 6 cm / second. The drag coefficient is defined as:APCd= 2 — pv wherein AP is a dynamic pressure drop across the retention object, defined as a drag force divided by an element projected area normal to a flow direction, and p is a fluid density. This configuration ensures the retention object retains said marine biomass via surface interactions rather than mechanical sieving.

[0010] In one embodiment, the fibres are arranged as a non-woven, randomly oriented fibre 3D network. In another embodiment, the fibres are arranged as a periodic array of fibres oriented substantially parallel to a direction of the ambient current. In certain embodiments, the retention object comprises a flow depth along a direction of the ambient current that is at least WO times a mean diameter of the fibres.

[0011] In a specific embodiment dependent thereon, the retention object comprises a flow depth of at least 5 mm along a direction of the ambient current. In still another embodiment, the porous 3D network is configured to generate a pressure gradient of less than 100 Pascals at an ambient current velocity of 6 cm / second.

[0012] In various embodiments, the fibres comprise silica, quartz, glass, carbon, polymer, or natural fibres. The fibres may have a surface functionalisation selected from the group consisting of: hydrophilic groups, hydrophobic groups, charged groups, epitaxially functionalising groups, sterically functionalising groups, amino acids, and sugars. The fibres may carry a positive or negative surface charge, or comprise amide groups. In a specific embodiment, the fibre volume fraction is less than 0.02 (2%) relative to an envelope volume. The apparatus is configured to operate within an ambient current velocity of between 1 cm / sec and 200 cm / sec without external pumping. In some embodiments, the fibres comprise a non-cylindrical cross-section geometry, selected from the group consisting of: oval, elliptical, and ribbon-like geometries.

[0013] In further embodiments, the apparatus comprises a mechanical harvester comprising an agitating assembly configured to apply mechanical agitation to said retention objectto detach biomass therefrom. The mechanical agitation may be selected from the group consisting of: rotating, contact scrubbing, fluid jets, bubbling, ultrasound, and vibration. In a specific embodiment, the agitating assembly comprises an element positioned to contact said retention object, wherein said element comprises material configured to undergo sacrificial wear relative to the fibres of the retention object. The mechanical harvester may further comprise a collection nozzle positioned adjacent to the agitating assembly and may be configured to continuously traverse a surface of the retention object during operation.

[0014] In yet further embodiment, the apparatus also includes a collection reservoir connected to the mechanical harvester, wherein the apparatus is configured to reduce a water content of harvested biomass prior to storage.

[0015] In another aspect, the present invention provides a non-sieving method for harvesting marine biomass using the apparatus described herein. The method of the present invention comprises: (i) deploying the retention object in an ambient marine current; (ii) allowing water to flow through the retention object at a velocity driven by the ambient marine current; (iii) retaining biomass particles on said fibres, resulting in said retaining being mediated by attractive surface interactions rather than mechanical obstruction; and (iv) mechanically detaching the retained biomass from the fibres via mechanical agitation.

[0016] In one embodiment of the method, the step of mechanically detaching comprises agitating the fibres with the agitating assembly described above. In another embodiment, the step of retaining achieves capture of said biomass particles via statistical encounter. In still another embodiment of the method, the retention object comprises fibres treated with acid. The method may further comprise concentrating the detached biomass using tangential flow filtration or centrifugation after mechanical detachment.

[0017] In specific embodiments of the method of the present invention, the retention object is configured to exhibit a drag coefficient (Cd) of less than 300 at an ambient current velocity (v) of 6 cm / second, defined as:wherein AP is a dynamic pressure drop across the retention object, defined as a drag force divided by an element projected area normal to a flow direction, and p is a fluid density.

[0018] Alternatively, or additionally, the retention object is configured to generate a pressure gradient of less than 100 Pascals at an ambient current velocity of 6 cm / second. In a further embodiment of the method, a geometrical cross-section for encounter between said biomass particles and said fibres within a slice of the retention object having a thickness of three times a mean diameter of the biomass particles is substantially less than unity, calculated assuming said biomass particles follow straight paths perpendicular to a plane of the retention object.

[0019] In an additional aspect, the present invention provides for the use of the apparatus described herein as a passive hydrodynamic trap for marine phytoplankton.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The foregoing and other objects, features, and advantages of the disclosed embodiments will be apparent from the following detailed description taken in conjunction with the accompanying drawings:FIG. 1 is a schematic diagram of a retention object accordingto an embodiment.FIG. 2 is a microscopic image of a cross-section of a retention object according to an example embodiment.FIG. 3 is an image demonstrating precipitation of biomass using positively charged silica accordingto an example embodiment.FIG. 4 is a diagram showing expected amounts of biomass to be collected over one year accordingto example embodiments.FIG. 5 is a schematic diagram of a mechanical apparatus for continuous agitation of a retention object according to an embodiment.DETAILED DESCRIPTION OF THE INVENTION

[0021] In the following description, various aspects of the present invention will be described. For purposes of explanation, specific details are set forth in orderto provide athorough understanding of the present invention. However, it will also be apparent to one skilled in the art that the present invention may be practiced without the specific details presented herein. Furthermore, well-known features may be omitted or simplified in order not to obscure the present invention.

[0022] The term “comprising”, used in the claims, is “open ended” and means the elements recited, or their equivalent in structure or function, plus any other element or elements which are not recited. It should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “an apparatus comprisingx and z” should not be limited to devices consisting only of components x and z. Also, the scope of the expression “a method comprising the steps x and z” should not be limited to methods consisting only of these steps.

[0023] Unless specifically stated, as used herein, the terms “about” and “approximately” are understood as within a range of normal tolerance in the art, for example within two standard deviations of the mean. In one embodiment, the term “about” means within 10% of the reported numerical value of the number with which it is being used, preferably within 5% of the reported numerical value. For example, the term “about” can be immediately understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1 %, 0.5%, 0.1%, 0.05%, or 0.01 % of the stated value. In other embodiments, the term “about” can mean a higher tolerance of variation depending on for instance the experimental technique used. Said variations of a specified value are understood by the skilled person and are within the context of the present invention. As an illustration, a numerical range of “about 1 to about 5” should be interpreted to include not only the explicitly recited values of about 1 to about 5, but also include individual values and subranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3, and 4 and sub-ranges, for example from 1-3, from 2-4, and from 3-5, as well as 1 , 2, 3, 4, 5, or 6, individually. This same principle applies to ranges reciting only one numerical value as a minimum or a maximum. Unless otherwise clearfrom context, all numerical values provided herein are modified by the term "about". Other similar terms, such as “substantially”, “generally”, “up to” and the like are to be construed asmodifying a term or value such that it is not an absolute. Such terms will be defined by the circumstances and the terms that they modify as those terms are understood by those skilled in the art. This includes, at very least, the degree of expected experimental error, technical error and instrumental error for a given experiment, technique or an instrument used to measure a value.

[0024] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealised or overly formal sense unless expressly so defined herein. Well-known functions or constructions may not be described in detail for brevity and / or clarity.

[0025] As used herein, the term “retention object” refers to the passive capturing component of an apparatus of the present invention. It acts as a hydrodynamic trap for biomass. It is not limited to a specific shape and may take the form of a curtain, a net, a mesh, a block of wool, a brush-like structure, or any volumetric arrangement of fibres. Unlike a two-dimensional (2D) sieve or screen which relies on a single plane of filtration, the retention object comprises a “porous 3D network” having a defined flow depth (z- dimension) alongthe axis of the current.

[0026] It is noted that while this depth is “significant” relative to a thin 2D screen, it represents a critical design trade-off with hydrodynamic drag. Excessive depth increases drag, which bears heavily on mooring requirements and represents substantial capital expenditure. Therefore, in some embodiments, a “significant” flow depth may be as small as approximately 10 mm (1 cm), which is sufficient to establish a 3D capture zone without incurring prohibitive drag forces.

[0027] The term “porous 3D network” refers to a volumetric arrangement of fibres that allows fluid to flow through it in three dimensions. The network is characterised by a “flow depth”, which is defined as a distance the ambient current of water travels through the network, and a “void volume”, which is the empty space available for water flow. Thenetwork may be “non-woven” (e.g., random orientation, like quartz wool or felt) or “ordered” (e.g., a periodic array of parallel fibres or a 3D printed lattice).

[0028] The term “drag coefficient” (Cd) as used herein is a dimensionless quantity that is used to quantify the drag or resistance of the porous 3D network in a fluid environment. In the context of the present claims, the drag coefficient is defined by the following equation:APCd= 2 — pv wherein AP is the dynamic pressure drop across the retention object. This is specifically defined as the total drag force exerted by the fibres on the fluid divided by the element projected area normalto the flow direction (force / area). The value of p is the fluid density (typically approximately 1025 kg / m3for seawater); and v is the velocity of the ambient current incident on the retention object (specifically defined as 0.06 m / s for the purpose of the threshold test in the calculations.

[0029] This expression relates the pressure drop AP to the dynamic pressure (0.5pv2), and thus ensures that the drag coefficient (Cd) value remains a dimensionless constant characteristic of the geometry, while the drag force itself scales with the square of the velocity (v2). A drag coefficient of less than 300 indicates a highly permeable structure where the pressure drop is minimal relative to the dynamic energy of the flow, thereby confirming the non-sieving nature of the trap.

[0030] The term “envelope volume” is used to define a total volume enclosed by external geometric boundaries of the porous 3D network when deployed in its operational state inside the ambient current. It is critical for calculating solidity of loose orflexible 3D networks. For a constrained 3D network (e.g., fibres held inside a rigid cage or frame), the envelope volume is the internal volume of that cage / frame. For an unconstrained or flexible network (e.g., a loose curtain of fibres flowing in the ambient current of water), the envelope volume is defined by the outer boundaries of the fibre mass as expanded by the hydrodynamic drag of the ambient current. Mathematically, it is the volume of the smallest convex hull or simple geometric prism (e.g., cylinder, rectangular box) that fully encloses the fibres during operation.

[0031] In this context, “solidity” or “fibre volume fraction” refers to the ratio of the volume of the solid fibre material (Vfibres) itself to the “envelope volume” Venveiope) defined above:Solidity = flbres''envelope

[0032] In the present invention, the solidity parameter is exceptionally low to prevent pressure buildup, specifically less than 0.05 (5%), and preferably less than 0.02 (2%).

[0033] The term “ambient current” (without external pumping) refers to the natural flow of water in the marine environment (e.g., ocean currents, tidal flows, river flows) or induced flows that are not driven by high-pressure pumps directly coupled to the filtration media. The apparatus is configured to operate effectively at low velocities typical of open water, specifically between 1 cm / sec and 200 cm / sec, and even more specifically at passive drift velocities of <10 cm / sec.

[0034] The term “fibre diameter” refers to the average thickness of the fibres forming the network. In the present invention, the fibre diameter is between about 1 micron and about 3000 microns. The porous 3D network is configured such that the effective void spaces between fibres are significantly larger than the target particles, ensuring that capture is mediated by surface interactions rather than sieving.

[0035] The terms “marine biomass” and “target particles” refer to the biological matter intended for harvesting. Non-limiting examples are phytoplankton, microalgae, cyanobacteria, bacteria, and suspended organic particulate matter. While the invention utilises a “relative” definition (particle < gap), specific embodiments target particles having a diameter of about 0.2 micron to about 200 microns. In more specific embodiments, the target particles are between about 0.5 micron to about 100 microns.

[0036] The biomass typically carries a natural surface charge (often negative, e.g., cell wall carboxyl groups) or an induced potential difference essential for the operation of various cell membrane associated machinery, which allows for electrostatic manipulation. The term “surface charge” refers to the electrostatic potential relative to a point in the bulk fluid. The potential greater than 0 mV, preferably greater than +10 mV in seawater is considered a positive electrostatic potential. It is achieved via functionalisation with cationic groups, for example, quaternary amides or tertiaryamides, or via “acid-mediated protonation”. The potential less than 0 mV is a negative electrostatic potential. It is achieved via anionic groups (e.g., carboxyl, sulfonic, phosphonates).

[0037] The term “geometrical cross-section” refers to the theoretical area blocked by fibres within a given slice of the network, calculated assumingthatthe biomass particles follow straight, laminar paths perpendicular to the plane of the retention object. If this cross-section is substantially less than unity (1 ), it indicates that a sieving mechanism (which would require a blocked cross-section approaching unity) is not the primary mode of capture.

[0038] “Surface interactions” lead to retention mediated by short-range forces, such as electrostatic (Coulombic) attraction, but may also include Van der Waals forces, hydrogen bonding, or hydrophobic interactions. This occurs when a particle adheres to a fibre upon contact or sticks to it. In stark contrast, “mechanical obstruction” (sieving) leads to retention that occurs because the particle is physically larger than the pore or gap it is trying to pass through. Interestingly, such obstruction is a consequence of repulsion, in striking contrast to the mechanism proposed herein, which is mediated by attraction. The claimed invention expressly excludes mechanical obstruction as the retention mechanism, as defined by the requirement that the target particle diameter is smaller than the effective void spaces defined by the porous 3D network.

[0039] The term “sacrificial” or “mechanically weaker” refers to a material property relationship between the “agitating element” (e.g., brush), and the retention object (trap). The agitating element is composed of a material that has a lower yield strength, hardness, or abrasion resistance than the fibres of the retention object, such that any wear resulting from their contact occurs preferentially on the agitating element. For example, the retention fibres may comprise silica or quartz, while the agitating fibres comprise a polymer such as nylon, polypropylene, or polyurethane.

[0040] The term “cumulative statistical probability” refers to the likelihood that a particle traveling along a streamline through the porous 3D network will collide with a fibre surface at least once before exiting the network. In a single thin slice of the network, the probability of encounter might be low (e.g., 25%). However, over a sufficient flow depth or path length (e.g., more than 20 mm), the cumulative probability of evading all fibres drops exponentially:1p total_miss —1P sNlicejnissThis results in a capture probability approaching 100% (e.g., > 95%).

[0041] The core of the invention capitalises on a surprisingand counterintuitive insight regarding the hydrodynamics of filtration at the micro-scale. Traditional wisdom suggests that to catch a small particle, one requires a sieve with holes smaller than that particle. However, the present invention demonstrates that the retention of target particles requires neither long-range attraction, as seen in high-voltage electrostatic precipitators, nor physical obstruction, as used in sieves or TFF apparatuses. Rather, it can be theoretically demonstrated that a combination of short-range attraction to target particles and a volumetric arrangement of very sparse obstacles is sufficient to facilitate highly efficient retention.

[0042] To understand the mechanics of the retention object, it is instructive to consider the scenario of dilute target phytoplankton creatures carried by a slow, laminar flow through a mesh of randomly oriented fibres. The inventor employed a set of assumptions that significantly simplify the calculations while remaining physically realistic to the marine environment. For the purposes of this theoretical model, he assumed the fibres are cylinders with a diameter of ten microns and a correlation length of approximately 1 mm. The inventor further assumed the target microbes are spherical, the flow is laminar, and the density of the fibres by volume is 2.5%.

[0043] Under these assumptions, if one were to distribute the fibres in a perfectly ordered lattice, the distance between them would be approximately 56 microns. A more realistic picture can be approached by considering a random orientation where statistically one-third of the fibres are oriented in the Z-direction, another third along the X-direction, and thefinalthird alongthe Y-direction. In this distributed three-dimensional scenario, the mean distance between fibres grows by a factor of the square root of 3 (V3) to approximately 100 microns. This increase reflects the geometric dilution of the fibres when distributed across three axes as opposed to a single packed array.

[0044] Therefore, the trap can be modelled as a series of “slices”, each 100 microns thick. For a particle following a laminar trajectory through a single 100-micrometer slice, there is a distinct statistical chance to encounter a fibre. However, due to the correlation length being approximately 1 mm, the particle has little chance to encounter this samefibre again within the same slice. Furthermore, because the mean distance between adjacent fibres is approximately 100 microns, the particle has little chance of subsequently encountering another fibre on the same trajectory within that single slice; to meet another fibre, the particle must essentially travel to the next slice.

[0045] Analysing the cross-section within this 100-micron slice reveals the capture probability. This is calculated for a length of fibre that occupies, for example, 2.5% of the volume. Over a slice of area A and thickness 100 microns, the accommodated fibres will have an overall length I calculated by volume: 0.025 ■ 100 [ / zm] ■ A

[0046] Considering that they are randomly oriented, their weighted projected crosssection a can be calculated as follows:where 10 pm is a fibre thickness, I is the aforementioned fibre length, and the integral term is a weighted projection:

[0047] This projection covers approximately 25% of the slice area. This result is notably close to the projected cross-section of a regular ordered grid of fibres aligned alongthe X, Y and Z axes, which is 19%, strengtheningthevalidity of this simplified model and indicating that reasonable simplifying assumptions do not sway the outcome significantly.

[0048] According to this result, the probability for an encounter (a “hit”) within a slice of 100-micron thickness is 25%, and conversely, the probability for not encountering anything (a “miss”) is 75%. Accordingly, the cumulative probability for non-encounterfor consecutive slices decreases exponentially as a function of the distance travelled along the fibre-containing column. This probability follows the following function:where x is the distance measured in millimetres (mm). This mathematical derivation implies that within a flow path of just a few millimetres, the probability for non-encounter drops practically to zero, meaning that statistically, all particles are bound to be encountered by a fibre surface.

[0049] This “encounter” model contrasts sharply with the probability for mechanical “obstruction” (sieving). For obstruction to occur in such a loose net, two fibres must intersect to form a pinch point. The probability for the intersection of their projections on the slice plane is low: (0.25)2= 1 / 16. Moreover, it is not sufficient that the projections intersect; the fibres themselves must be near one another to within a distance comparable to the size of the target particle, which may reasonably be considered to be 10 microns. This implies that only fibres that intersect within 10 microns of each other, out of the full 100-micron thickness of the slice, are relevant for obstruction, reducing the number of relevant intersections by a factor of ten.

[0050] In addition, the intersection itself occupies an area of / 16 but capture can occur at a circle of about 20 pm around its centre, enlargingthe cross section by a factor of 3 (the area of the 20 pm radius circle is offset by an area of the 10 pm circle in the middle considered to occupy the intersecting fibres and be inactive in capturing).

[0051] Furthermore, one must consider that only intersections at a substantially acute angle are capable of trapping a particle. A reasonable cutoff might be 45 degrees, and the weighted portion of the circular ring area attributed to these angles further reduces the probability. The weighted portion of the circular ring area attributed to these angles can be calculated according to the following formula:45° r de e > i J 90° 180°—8 o where dO / 90° represents the probability of getting an acute angle among all acute angles and 0 / 180° represents the fraction of the area of the capture circle occupied by a given angle.

[0052] Additionally, one must filter for relatively in-plane intersections, as intersections that are too steep relative to the flow will cause the target particles to simply roll off. An appropriate angle of steepness may be 45 degrees. The portion of all intersections satisfying this condition is calculated by integrating the orientationprobability (2TT sin 6) over the valid range (0 to TT / 4) and normalising by the hemisphere (2TT): 0.29

[0053] Given all these considerations, the effective cross-section for capture via mechanical obstruction per slice is:1 1 1A — ■ ■ 3 ■ - ■ 0.29 « 6.8 X 10-4416 10 8It is dramatically lower than that for surface encounter, and the chance for nonencountervia obstruction drops off much slower, as follows:

[0054] According to this analysis, the characteristic capture distance via obstruction is calculated to be 2.8 / 0.0068 = 412, which is orders of magnitude larger than that for the impaction / adhesion mechanism. This fits well with experimental observations, which show high capture efficiency in short columns where mechanical sieving would predict almost zero capture.

[0055] Based on the theoretical model above, the retention object (or trap) is constructed as a loose fibre network where fibres are dispersed at densities below a predetermined threshold, for example, less than 2% by volume. This loose architecture ensures a lower pressure gradient across the retention object, thereby mitigating viscosity and friction issues that normally plague micro-filtration. The retention object may be formed as a curtain, a net, or a thick block of fibres, and may be placed in open waters to harvest target particles like phytoplankton.

[0056] Reference is now made to FIG. 1 showing an example schematic diagram of a retention object 100 according to an embodiment. The retention object 100, which may be referred to as a net or trap, includes a plurality of fibres 110 arranged in a network to retain biomass target particles. The retention object is shown as a two-dimensional (2D) rectangular plane for illustration, but implies a three-dimensional (3D) structure.

[0057] In the example shown in FIG. 1, the fibres form a streamline network where the fibres are arranged in curved formations. Such an arrangement allows little resistance tocurrent flow in a current direction 120 through the retention object 100. In other embodiments, the fibres may be arranged randomly. Regardless of the arrangement, the plurality of fibres 110 are dispersed sparsely. That is, unlike commonly found non-woven filtering elements that include a tightly packed meshwork, the fibre network includes substantial distances between the fibres. These substantial distances are apparent in the cross-sectional planes perpendicular to the current flow. In an embodiment, the plurality of fibres occupies equal to or less than 2% by volume in a cross-section of the retention object 100.

[0058] Reference is made to FIG. 2 showing an example microscopic image 200 of fibres comprising a retention object according to an embodiment. The example image 200 shows the plurality of fibres 210 occupying close to 2% by volume of the example cross-sectional plane 200. It should be noted that only a few out of the plurality of fibres 210 are labelled in the example image, but the white fibres throughout the image represent the network. The large voids relative to the fibre diameter illustrate the high porosity of the structure.

[0059] In addition to the arrangement, fibre dimensions and spacings may be modified. In an example embodiment, the retention object 100 may be composed of 20 pm diameter fibres with lengths between 0.5 mm to 100 mm and spacings between 0.13 mm to 0.5 mm. In another embodiment, the density is determined by requiring that the pressure gradient across the retention object does not exceed a threshold value, for example 100 Pascals, at operating water speeds (1 -200 cm / sec).

[0060] The fibres may be composed of various materials, including carbon fibres, mineral fibres such as fibre glass or fibre quartz, metal fibres such as amorphous iron fibres, or polymer-based fibres such as polyamide, polyethylene, nylon, Teflon®, Kapton®, polystyrene, cellulose, lignin, or chitin. In some embodiments, the fibres comprise a non-cylindrical cross-section geometry, selected from the group consisting of: oval, elliptical, and ribbon-like geometries.

[0061] To ensure the “encounter” turns into a “capture”, the surfaces are functionalised. This functionalisation may be hydrophobic, hydrophilic, charged, epitaxially functionalised, or sterically functionalised. The fibres may possess multiple types of functionalisation, distributed evenly or partitioned into domains. In a preferred embodiment, the fibres are functionalised to display a positive surface charge. This isparticularly effective for capturing marine phytoplankton, which typically carry a negative surface charge. One method of achieving this is by functionalising the fibres with chemical moieties containing a quaternary amide. Alternatively, the fibres may be treated with acid. This acid treatment serves to clean the fibre surface, removing impurities and exposing the surface structure to facilitate efficient interaction with the target biomass.

[0062] In an example embodiment, aminopropyl functionalised silica particles were placed in a culture media (HEPES, pH 7.5) containing cyanobacteria (of the type Prochlorococcus) ranging in size between 0.2 pm to 0.6 pm at a concentration of 108cells / mL. At a neutral pH of 7.5, no notable effect was observed. However, when the media was made slightly acidic by the addition of HCl (e.g., 2 drops of HCl 1 N), massive clearance of cyanobacteria from the medium was observed as the bacteria precipitated with the silica.

[0063] FIG. 3 shows an example image 300 of an Eppendorf demonstratingthis effect. The image shows precipitated silica / bacteria flocculates 310 (the darker grey area) that have separated from the medium 320. While this effect in the example was partly due to protonating the amine groups, a similar effect is observed with bare silica or quartz treated with acid. This acid treatment serves to clean the fibre surface, removing impurities and exposing the surface structure to facilitate efficient interaction with the target biomass, resulting in the massive adherence between the negatively charged cyanobacteria and the silica fibres.

[0064] Reference is now made to FIG. 5 showing an example schematic diagram of a mechanical harvester apparatus 500 for continuous mechanical agitation of a retention object. The mechanical apparatus 500 allows for uninterrupted collection of biomass. The apparatus includes an agitating assembly 520 (e.g., a rotating brush), driven by a motor 530, which continuously agitates the retention object fibres 510. It is understood that the agitating assembly 520 is not limited to rotating brushes but may include any mechanism configured to apply mechanical agitation such as contact scrubbing, fluid jets, bubbling, ultrasound, and vibration.

[0065] The invention employs a “sacrificial” maintenance strategy here. The agitating assembly 520 is composed of material configured to undergo sacrificial wear relative to the fibres 510 of the retention object. Tensile strength is a key factor in this configuration.For example, the agitating element may be made of a soft polymer with a lowtensile yield strength, while the retention trap is made of brittle but chemically robust quartz or silica. This ensures that the inevitable degradation from friction is inflicted on the cheap, replaceable agitator rather than the complex, expensive retention network. Detached particles are then suctioned up by a collection nozzle 540, which is located close to the agitating assembly 520 to transfer the detached targeted particles to a connected collection reservoir (not shown).

[0066] In some embodiments, the material drawn by the collection nozzle 540 can be further treated to reduce its water content before reaching the collection reservoir by means of, for example, but not limited to, filtration, tangential flow filtration (TFF), centrifugation, and the like. In a certain embodiment, the collection reservoir may be located at a separate location away from the mechanical apparatus. In another embodiment, the collection reservoir may be part of the mechanical apparatus 500 of the present invention. It should be noted that the shape of the mechanical apparatus 500 here is shown for illustrative purposes and does not limit the scope of the present disclosure.EXAMPLES

[0067] The following non-limiting examples are provided to further illustrate the invention. The theoretical model was validated through rigorous experimentation resembling real seawater conditions.Example 1: Functionalisation of the Quartz Wool Fibres

[0068] Functionalisation of the quartz wool to result in positively charged fibre networks, and thus, positively charged retention object, may be performed using 3- (trimethoxysilyl)propyl-N,N,N-trimethylammonium. A portion of dry quartz wool was placed in a reaction medium of distilled water, ethanol and 3-(trimethoxysilyl)propyl- N,N,N-trimethylammonium for about 24 hours. The quartz wool sample is rinsed and dried on a sieve to result in dry functionalised quartz wool, with the capacity to assume a positive surface charge when immersed in water.

[0069] In this example, 291 mg of pretreated dry quartz wool is placed in a reaction medium of 9.5mL of distilled water, 8.5mL of ethanol, and 2mL of 3-(trimethoxysilyl)propyl-N,N,N-trimethylammonium. In the same example, rinsing of the quartz wool is performed in multiple steps: 3 times with a wash bottle of ethanolfollowed by squeezing of the wool against the tube wall, 3 times with a wash bottle of ethanol in a spherical tea sieve followed by pressing down with a gloved finger, under running distilled water in the spherical tea sieve followed by pressing down with a gloved finger, and another rinse with ethanol with a finger press at the end. In each step of rinsing, excess liquid is removed by absorbing with a paper towel.Example 2: Comparative Harvesting of Synechococcus WH8109

[0070] In this example, harvesting of cyanobacteria Synechococcus WH8109 was performed using quartz wool fibres. Two separate harvesting runs were conducted: one with unfunctionalised quartz fibres and one with positively functionalised quartz fibres. A total volume of 1 litre of Instant Ocean™ seawater was circulated through 40 mg of fibre network for 1 hour at a slow flow velocity of less than 6 cm / sec.

[0071] The results were compelling. The harvested cyanobacteria yielded 0.016mg from the unfunctionalised fibres and 0.0213 mg from the functionalised fibres. This represents an approximately 33% improvement in harvesting efficiency solely due to the surface charge. While 33% might seem modest in some contexts, in the economics of massive open-ocean harvesting, this improvement is significantfor commercial viability.Example 3: Extrapolated Annual Yields Across Different Strains

[0072] FIG. 4 is a diagram showing the expected amounts of biomass to be collected over one year according to example experiments. The amounts are extrapolated from five separate results collected during a specific time period. The x-axis shows five distinct experiments (numbered 1 to 5) all performed using various fibre types for a duration of 1 hour at a flow velocity of less than 6 cm / second and a dry target particle concentration of 160 ppb. The biomass target particles were less than 0.5 pm in size, including different strains of cyanobacteria selected from:

[0073] The y-axis shows the expected annual yields in kg / (year x kg of retention object). The resulting expected amounts of biomass to be harvested over one year using a retention object weighing 1 kg were 11 kg, 19 kg, 6 kg, 9 kg, and 5 kg for Experiments 1 through 5, respectively. These annual yield projections were extrapolated from these specific experiments.Example 4: Quantitative Capture Efficiency of Four Phytoplankton Species

[0074] In a further quantitative example, four different phytoplankton species were tested: Isochrysis gaibana, Phaeodactyium tricornutum, Thaiassiosira weissfiogii, and Dunaiieiia salina. The cultures were diluted into 20 litres of sand-filtered seawater to mimic realistic eutrophic ocean concentrations. The water was pumped through 30 cm long Pyrex tubes packed with quartz wool at a flow velocity of about 1.3 cm / sec. The experiment ran for 60min with samples taken at 15 min intervals from the exit of the columns and from the reservoir vessel at the beginning and the end of the experimental run (t = 0, t = 60 min). Samples were filtered and evaluated for chlorophyll content, giving a measure of the capture efficiency of the fibres.

[0075] The results demonstrated the ‘paradox’ perfectly. The fibres were packed loosely, occupying 5-15 cm of the column length. Despite the large gaps in the wool, the capture efficiency was remarkable: approximately 100% for P. tricornutum, 70% for D. salina, 30% for T. weissfiogii, and 15% for I. gaibana. No significant difference was observed between the acid treated and untreated quartz wool. Most strikingly, it was observed that most of the capture occurred within the first 10 mm to 20 mm of the fibre column. This corresponds very well with the theoretical prediction described above, which stated that within 2-3 mm, almost all capturable particles should indeed be encountered and captured. The fact that the capture occurred over such a short travel distance (depth), and varied wildly by species despite the species having comparable physical sizes, confirms that the mechanism is attraction-based encounter rather than mechanical obstruction.

[0076] To summarise, the experimental results clearly demonstrate the viability of capturing phytoplankton at ambient concentrations using ambient ocean currents using attraction capture rather than mechanical obstruction. This is clearly seen primarily by the fact that the gaps between the fibres are significantly larger than the dimensions of the cells themselves and also by the great variability in capture efficiencies between different species which are very comparable in physical size.

Claims

CLAIMS1. An apparatus for harvesting marine biomass from an ambient current, comprising a non-sieving retention object comprising a plurality of fibres forming a porous 3D network; wherein the porous 3D network has a fibre volume fraction (solidity) of less than 0.05 (5%) relative to an envelope volume defined by outer boundaries of the porous 3D network inside the ambient current; wherein said fibres comprise surface functionalisation configured to promote adhesion of marine biomass; wherein the fibres have a diameter of between about 1 micron and about 3000 microns; and wherein the porous 3D network is configured to exhibit a drag coefficient (Cd) of less than 300 at an ambient current velocity (v), wherein said drag coefficient is defined as:APCd= 2 — pv wherein AP is a dynamic pressure drop across the retention object, defined as a drag force per element projected area normal to a flow direction, and p is a fluid density, thereby retaining said marine biomass via surface interactions.

2. The apparatus of claim 1 , wherein the plurality of fibres are arranged as a non-woven, randomly oriented fibre 3D network.

3. The apparatus of claim 1 or 2, wherein the plurality of fibres are arranged as a periodic array of fibres oriented substantially parallel to a direction of the ambient current.

4. The apparatus of any one claims 1 to 3, wherein the retention object comprises a flow depth along a direction of the ambient current that is at least 100 times a mean diameter of the fibres.

5. The apparatus of any one of claims 1 to 4, wherein the porous 3D network is configured to generate a pressure gradient of less than 100 Pascals at an ambient current velocity of 6 cm / second.

6. The apparatus of any one of claims 1 to 5, wherein said fibres comprise silica, quartz, glass, carbon, polymer, or natural fibres.

7. The apparatus of claim 6, wherein said fibres have a surface functionalisation selected from the group consisting of: hydrophilic groups, hydrophobic groups, charged groups, epitaxially functionalising groups, sterically functionalising groups, amino acids, and sugars.

8. The apparatus of Claim 1 , wherein said fibres carry a positive or negative surface charge.

9. The apparatus of Claim 1 , wherein said fibres comprise amide groups.

10. The apparatus of any one claims 1 to 9, wherein the fibre volume fraction is less than 0.02 (2%) relative to an envelope volume defined by outer boundaries of the porous 3D network inside the ambient current.

11. The apparatus of any one claims 1 to 10, configured to operate within an ambient current velocity of between 1 cm / sec and 200 cm / sec without external pumping.

12. The apparatus of any one of claims 1 to 11 , wherein said fibres comprise a non- cylindrical cross-section geometry, selected from the group consisting of: oval, elliptical, and ribbon-like geometries.

13. The apparatus of claim 4, wherein the retention object comprises a flow depth of at least 5 mm along a direction of the ambient current.

14. The apparatus of any one of claims 1 to 13, further comprising a mechanical harvester comprising an agitating assembly configured to apply mechanical agitation to said retention object to detach biomass therefrom.

15. The apparatus of claim 14, wherein said mechanical agitation is selected from the group consisting of: rotating, contact scrubbing, fluid jets, bubbling, ultrasound, and vibration.

16. The apparatus of claim 14, wherein said agitating assembly comprises an element positioned to contact said retention object; and wherein said element comprises material configured to undergo sacrificial wear relative to the fibres of the retention object.

17. The mechanical harvester of claim 14, further comprising a collection nozzle positioned adjacent to the agitating assembly, configured to collect biomass detached from the retention object.

18. The apparatus of any one of claims 14 to 17, wherein the mechanical harvester is configured to continuously traverse a surface of the retention object during operation.

19. The apparatus of any one of claims 14 to 18, further comprising a collection reservoir connected to the mechanical harvester, wherein the apparatus is configured to reduce a water content of harvested biomass prior to storage in said collection reservoir.

20. A method for harvesting marine biomass using the apparatus of any one of claims 1 to 19, said method comprising:(ii) deploying the retention object in an ambient marine current;(iii) allowing water to flow through the retention object at a velocity driven by the ambient marine current;(iv) retaining biomass particles on said fibres, resulting in said retaining being mediated by attractive surface interactions rather than mechanical obstruction; and(v) mechanically detaching the retained biomass from the fibres via mechanical agitation.

21. The method of claim 20, wherein Step (iv) of mechanically detaching comprises agitating the fibres with the agitating assembly of claim 14.

22. The method of claim 20, wherein Step (iii) of retaining achieves capture of said biomass particles via statistical encounter.

23. The method of any one of claims 20 to 22, wherein the retention object comprises fibres treated with acid.

24. The method of any one of claims 20 to 23, further comprising concentrating the detached biomass using tangential flow filtration or centrifugation after mechanical detachment.

25. The method of any one of claims 20 to 24, wherein the retention object is configured to exhibit a drag coefficient (Cd) of less than 300 at an ambient current velocity (v), defined as:AP Cd= 2 — pv wherein AP is a dynamic pressure drop across the retention object, defined as a drag force per element projected area normal to a flow direction, and p is a fluid density, thereby retaining said marine biomass via surface interactions.

26. The method of any one of claims 20 to 25, wherein the retention object is configured to generate a pressure gradient of less than 100 Pascals at an ambient current velocity of 6 cm / second.

27. The method of any one of claims 20 to 26, wherein a geometrical cross-section for encounter between said biomass particles and said fibres within a slice of the retention object having a thickness of three times a mean diameter of the biomass particles is substantially less than unity, calculated assuming said biomass particles follow straight paths perpendicular to a plane of the retention object.

28. The apparatus of any one of claims 1 to 19 used as a passive hydrodynamic trap for marine phytoplankton.

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