Patent Publication Number: US-2021169005-A1

Title: Biofiltration System for Harvesting Microalgae and Related Methods

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     Priority is claimed to U.S. Provisional Application No. 62/944,088 (filed Dec. 5, 2019), which is incorporated herein by reference in its entirety. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     None. 
     BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The disclosure relates to a filtration system for harvesting microalgae. The filtration system includes a filter housing with modular filtration units containing a fungal filter. The fungal filter includes a perforated support with a capture fungus on the support. The fungal filter is adapted to be removed and replaced from the modular filtration unit. The capture fungus is adapted to capture one or more types of microalgae upon contact of the capture fungus with the microalgae, for example via fungal mycelium. 
     Brief Description of Related Technology 
     Algae are photosynthetic aquatic organisms that are diverse and possess little to no tissue differentiation. Microalgae are a subset of algae which range from several to a few hundred micrometers. There are species adapted to living in freshwater, and others adapted to brackish or marine environments. Microalgae can grow in extreme environments such as deserts and polar regions, and often show greater efficiency in synthesizing bioproducts compared to land plants. Moreover, microalgae produce oxygen and sequester the greenhouse gas carbon dioxide at global scales, accounting for over half of the carbon fixation that occurs in oceans. In addition to having fast growth, they can produce high-value biomass composed of proteins (40%-60%), lipids (5%-60%), and carbohydrates (8%-30%), making them ideal supplies for food and nutrition. Algal protein is complete with all the essential amino acids. 
     The unicellular structure of microalgae allows them to easily convert sunlight into chemical energy. Many microalgae have the ability to survive in the most extreme environments, allowing microalgae incubation in harsh environments such as deserts, coastal areas, islands, and urban centers. However, in contrast to the simple growth system, harvesting microalgae is challenging due to their small cell size (typically 2-20 μm) and relatively low density (0.3-5 g/L). Harvesting microalgae is a major cost of derived products. For example, harvesting raceway pond systems with chemical flocculation or the use of industrial-scale centrifuge instruments are both very expensive. Current methods for harvesting microalgae are limited by high cost, introducing contamination and pollution, and high demand for instrument and maintenance. Thus, an economical and environment-friendly method to harvest microalgae is needed to meet the demand for increasing the algae market. 
     Several species of  Mortierella  fungi are able to capture  Nannochloropsis oceanica  with high-efficiency. This marine alga has a high oil yield and makes valuable omega-3 fatty acid EPA.  Mortierella  fungi are common and widespread in soils, but they are also industrial fungi used in human supplements owing to their high lipid content.  Mortierella  are non-pathogenic to plants, animals, and humans.  Mortierella  fungi are easy to cultivate as they utilize simple sugars and grow rapidly. They can even be grown utilizing food or brewery wastewater effluents.  Mortierella  fungi can form thick biofilms that can float in liquid culture.  Mortierella  fungi make high amounts of oil that are enriched in ARA, an omega-6 polyunsaturated fatty acid that is beneficial for heart health and systemic inflammation. 
     SUMMARY 
     In one aspect, the disclosure relates to a microalgae filtration system comprising: a filter housing having an inlet and an outlet, and comprising a plurality of modular filtration units defining a continuous fluid flow path from the inlet, through the modular filtration units, and to the outlet. Each modular filtration unit comprises a fungal filter comprising (i) a perforated support (e.g., mesh plate) having a solid surface and defining an open area, and (ii) a (living) capture fungus on the solid surface of the perforated support (e.g., adhered thereto). The fungal filter is adapted to be removed and replaced from the modular filtration unit, for example during operation of the filtration system or during an intermittent shutdown of the filtration system. The continuous fluid flow path passes through the fungal filter, for example the perforated support open area. Thus, the inlet and outlet are in fluid communication through the fungal filters. The capture fungus is adapted to capture one or more types of microalgae upon contact of the capture fungus with the microalgae, for example via fungal mycelium. 
     Various refinements of the disclosed microalgae filtration system are possible. 
     In a refinement, the fungal filter is removably engaged (e.g., slidably engaged) with the modular filtration unit from an external region of the filter housing. For example, a filter housing can include a plurality of externally accessible slots or other orifices for insertion/installation of a new/fresh fungal filter and removal of a used/coated fungal filter after accumulation of microalgae. 
     In a refinement, the modular filtration unit comprises a modular housing unit defining a portion of the filter housing and containing the fungal filter. The modular filtration units can be removably connectable and disconnectable to each other by any suitable means, for example threaded connectors, latch connectors, pin or screw connectors, etc. Such removable connections allow for temporary disassembly of a portion of the filter housing to permit removal and replacement of the fungal filter. The removable connections also allow the filtration system to be assembled with any desired number of modular filtration units, for example where a higher or lower number of filtration stages are desired based on the nature of the filter feed stream (e.g., concentration and/or type of microalgae in feed). 
     In a refinement, the perforated support comprises a mesh plate. The perforated support generally can include any solid material with a series of holes, orifices, gaps, etc. to provide fluid flow therethough. Such open areas can be sized to provide relatively little or some resistance to fluid flow. The open areas are preferably large enough to prevent filter clogging during operation, but they can be sized to control the retention time of fluid in filtration system. 
     In a refinement, the perforated support has a ratio of open area to closed (solid surface) area in a range of 20:80 to 80:20. The area is generally defined in a plane perpendicular to the direction of fluid flow through the perforated support, for example in a direction normal to the plate or other major surface of the support. Thus, about 20-80% (e.g., at least 20, 30, 40, or 50% and/or up to 50, 60, 70, or 80%) of the surface area of the perforated support is open and available for fluid flow therethrough. Correspondingly, about 20-80% (e.g., at least 20, 30, 40, or 50% and/or up to 50, 60, 70, or 80%) of the surface area of the perforated support is the solid surface upon which the capture fungus can grow and adhere. Thus, the fractional solid surface area of the support is desirably high to provide a higher capture capacity for microalgae, and it is balanced with open area to allow sufficient fluid throughput through the fungal filter. 
     As a non-limiting example, perforated supports or mesh plates can be formed with circular holes having a diameter of about 6.35 mm (or area of about 32 mm 2 ) with a nominal spacing of about 12.7 mm between holes to provide good balance between fungal growth/capture surface and fluid flow area through the filter. The overall area/diameter of the support/plate can be scaled to the filtration unit and desired throughput. The thickness of the support/plate is suitably large enough to support the load in the filtration unit and to prevent deformation or breaking of the support/plate during heat treatment, for example after filter removal to harvest algae and before being placed back into the system. 
     In a refinement, the perforated support comprises a material selected from the group consisting of plastics, metals, and combinations thereof. Any solid materials are generally suitable, as the capture fungus is generally capable of attaching/adhering to most solid surfaces during fungal filter formation/growth. Plastics such as polycarbonate, polylactic acid, polyvinyl chloride, polyolefins, and other thermoplastic or thermoset materials can be desirable for cost and ease of manufacturing considerations (e.g., via 3D printing). Metals such as stainless steel can be desirable for strength, longevity, and reusability considerations. 
     In a refinement, the capture fungus comprises a  Mortierella  fungus.  Mortierella  species are widespread soil fungi and they are safe to plants or animals and humans. Many  Mortierella  strains are used for human nutraceuticals because of their outstanding productivity in polyunsaturated fatty acids.  Mortierella  fungi form dense biofilms along culture surfaces, and being indeterminate in growth form, can grow into the size and shape of their incubation container to make standard size fungal filters. 
     In a refinement, the microalgae capable of being captured upon contact with the capture fungus are selected from the group consisting of  Nannochloropsis, Chlorella, Chlamydomonas , and combinations thereof. 
     In another aspect, the disclosure relates to a method for harvesting microalgae, the method comprising: providing a filtration system according to any of the various disclosed embodiments; feeding an influent comprising microalgae to the inlet of the filter housing; capturing microalgae on one or more fungal filters in the filter housing, thereby forming one or more coated fungal filters comprising (i) the perforated support, (ii) the capture fungus thereon, and (iii) microalgae bound to the capture fungus; withdrawing an effluent from the outlet of the filter housing, the effluent containing less microalgae than the influent; and removing one or more coated fungal filters from the filtration system and harvesting the microalgae therefrom. For example, the collective biomass on the coated filter can be scraped or otherwise mechanically removed from the support. The harvested biomass can be used as mixture of fungus and algae. For example, combined with oil-producing microalgae, the algae-fungi feedstocks harvested from the fungal filters could be processed simultaneously for food, nutraceutical, and animal feed markets. 
     Various refinements of the disclosed method for harvesting microalgae are possible. 
     In a refinement, the method further comprises replacing the removed coated fungal filters with fresh fungal filters, for example with a new/different support having a newly grown capture fungus thereon which is suitably free from microalgae. Removing and replacing can be performed while the filtration system is in operation or while the system is temporarily halted (e.g., valve shut-off to halt influent flow when changing filters). 
     In a refinement, the method comprises performing feeding the influent, capturing the microalgae, and withdrawing the effluent as continuous process operations. 
     In a refinement, at least 70% of the microalgae in the influent is captured on the fungal filters. For example, at least 70, 80, 90, or 95% and/or up to 90, 95, 98, 99, or 100% of the microalgae is captured. Alternatively or additionally, the concentration of microalgae in the effluent can be 30% or less than that in the influent (e.g., at least 1, 2, 5, or 10% and/or up to 5, 10, 20, or 30%). 
     In a refinement, the influent is a bioreactor product stream, for example a bioreactor operated to grow microalgae specifically for subsequent harvest. 
     In a refinement, the influent is from a natural body of water, for example a pond or lake where microalgae are present. 
     In a refinement, the method comprises feeding the influent and withdrawing the effluent via gravity. Gravity can be a sufficient driving force for flow through the filter housing (i.e., with the inlet at a higher relative elevation than the outlet), in particular because large velocities are not required for effective operation of the filtration system. Lower velocities can promote extended contact time and capture efficiency between the influent and the capture fungus, and they also limit fluid shearing forces that can detach or otherwise damage a capture fungus attached to its support. Nonetheless, in some embodiments, positive pressure or a suction pressure (e.g., via a pump or otherwise) can be used to assist flow through the filtration system, in particular if care is taken to limit or prevent damage to the capture fungus. 
     While the disclosed apparatus, systems, compositions, articles, and methods, are susceptible of embodiments in various forms, specific embodiments of the disclosure are illustrated (and will hereafter be described) with the understanding that the disclosure is intended to be illustrative, and is not intended to limit the claims to the specific embodiments described and illustrated herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein: 
         FIG. 1  illustrates an embodiment of a modular filtration unit according to the disclosure. 
         FIG. 2  illustrates another embodiment of a modular filtration unit according to the disclosure. 
         FIG. 3  illustrates an embodiment of a filtration system according to the disclosure, including (A) a filtration housing and (B) a modular filtration unit/fungal filter. 
         FIG. 4  illustrates another embodiment of a modular filtration unit according to the disclosure. 
         FIG. 5  illustrates another embodiment of a modular filtration unit according to the disclosure. 
         FIG. 6  illustrates another embodiment of a modular filtration unit according to the disclosure. 
         FIG. 7  illustrates an embodiment of a filtration system according to the disclosure. 
         FIG. 8  illustrates side cross sectional views of perforated supports according to the disclosure, including (A) a perforated support prior to binding a capture fungus thereto and/or after harvesting microalgae therefrom, (B) a perforated support with capture fungus thereon in the form of a fungal filter, and (C) a perforated support with capture fungus and microalgae thereon in the form of a coated fungal filter. 
         FIG. 9  illustrates a process for growing/binding a capture fungus on a perforated support to form a corresponding fungal filter according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure relates to microalgae filtration system using fungal filters. The system components can be formed from safe and durable plastic materials, for example polycarbonate, silicon-based materials, polylactic acid, and other thermoplastic or thermoset materials. The system can be used to harvest microalgae from any suitable source in a continuous manner. The system incorporates a pod-type or modular arrangement which allows for any desired number of filtration units as well as easy removal and replacement of fungal filters in a filtration unit after the algae have been collected on the surface of the fungi/fungal filter, for example while maintaining continuous operation of the system. 
     The fungal filter can be formed by placing a perforated support (e.g., mesh plate) in a liquid or other culture medium containing a capture fungus of interest, for example  Mortierella  fungi. The fungal filters typically take about 2-7 days to grow, for example based on the size of the filter and/or the fungal strain used, during which time the capture fungus grows in the form of a living fungal biofilm adhered to the perforated support. Once formed, the fungal filters can remain active and alive several weeks (e.g., at least 10, 15, or 20 days and/or up to 20, 30, 40, or 50 days), during which time the fungal biofilm remains living and is able to effect specific capture interactions between its fungal mycelium and various target microalgae. The fungal filters are suitably used in the filtration system during this activity period of the living fungal biofilm, for example being inserted as a fresh fungal filter (e.g., a microalgae-free filter as originally formed) and/or being withdrawn as a coated fungal filter (e.g., a filter with microalgae adsorbed or otherwise captured on the fungal biofilm) while the fungal biofilm is alive. 
     As discussed above, harvesting microalgae is very difficult and expensive because they are so small, typically about 2-20 microns in size. The microalgae filtration system facilitates harvesting of microalgae with a simple capture and recovery process. Fungal filters that have been operating in the filtration system for a period accumulate captured microalgae thereon. Such coated fungal filters can be easily removed from filtration system due to its pod-type or modular design, and a new, fresh fungal filter can be inserted to maintain essentially continuous operation. Biomass on the coated fungal filters includes both the capture fungus coated thereon as well as the microalgae captured by the fungus during operation. This biomass can be recovered, for example by scraping or other mechanical means. The perforated support can be reused, for example by sterilization (e.g., in an autoclave) followed by growing a new living fungal biofilm thereon. Thus the filtration system can be used to harvest microalgae in a very safe, cheap, and environmentally friendly way. The microalgae that are harvested can be used for any of a variety of downstream uses, for example as an animal food and/or a source of renewable energy, due to their high content of protein, oil, and carbohydrates. 
       FIGS. 1-9  generally illustrate apparatus and methods according to the disclosure, including a microalgae filtration system  10  and related methods for harvesting microalgae using the filtration system  10 . The microalgae filtration system  10  includes a filter housing  100  having an inlet  110  and an outlet  120 . The filter housing  100  defines an internal volume V through which microalgae-containing fluid (e.g., an aqueous medium) flows during use of the filtration system  10 . The filtration system  10  further includes a plurality of modular filtration units  200  to bind, capture, and remove microalgae from an influent feed stream  20  introduced via the inlet  110  and passing through the internal volume V to provide an effluent product stream  30  with reduced microalgae content (e.g., relative to the influent microalgae content) via the outlet  120 . The filtration system  10  and modular filtration units  200  together collectively define a continuous fluid flow path  130  from the inlet  110 , through the modular filtration units  200 , and to the outlet  130 . The continuous fluid flow path  130  generally passes through the internal volume V of the filter housing  100  and across individual filtration units  200  (e.g., via open areas  224  in individual fungal filters  210  as described below). Thus, the inlet  110  and outlet  120  are in fluid communication via the internal volume V of the filter housing  100  and the filtration units  200  (or fungal filters  210  thereof). The filter housing  100  can have any suitable structure to permit fluid flow therethrough and support the modular filtration units  200 . For example, in some embodiments, the filter housing  100  can have a cabinet or box structure that permits individual filtration units  200  to be slidably or otherwise easily inserted and removed from the filter housing  100  (see  FIGS. 3 and 7 ). In other embodiments, the filter housing  100  can be defined by the plurality of filtration units  200  such that individual filtration units  200  can be added or removed to increase or decrease the internal volume V, or replaced to provide a fresh fungal filter  210  when desired (see  FIG. 4 ). 
     Each modular filtration unit  200  includes a fungal filter  210 . The fungal filter  210  includes a perforated support  220  (e.g., a mesh plate) having a solid surface  222  that defines at least one open area  224 , but more typically a plurality of open areas  224  that can have a regular or consistent shape and spacing on the perforated support  220 . The perforated support  220  generally can include any solid material with a series of holes, orifices, gaps, etc. to provide fluid flow therethough, for example a mesh plate. Such open areas  224  can be sized to provide relatively little or some resistance to fluid flow. The open areas  224  are preferably large enough to prevent filter  210  clogging during operation, but they can be sized to control the retention time of fluid in filtration system  10 . The overall area or diameter of the support  220  can be scaled to the filtration unit  10  and desired throughput. The thickness of the support  220  is suitably large enough to support the load in the filtration unit  10  and to prevent deformation or breaking of the support  220  during heat treatment, for example after filter  210  removal to harvest algae and before being placed back into the system. Any solid materials are generally suitable for forming the perforated support  220 , as the capture fungus  230  is generally capable of attaching/adhering to most solid surfaces during fungal filter  210  formation/growth. In various embodiments, the perforated support  220  can include or be formed from a material selected from plastics, metals, and combinations thereof. Plastics such as polycarbonate, polylactic acid, polyvinyl chloride, polyolefins, and other thermoplastic or thermoset materials can be desirable for cost and ease of manufacturing considerations (e.g., via  3 D printing or other additive manufacturing). Metals such as stainless steel can be desirable for strength, longevity, and reusability considerations. 
     As noted above, the number and size of the open areas  224  can be selected to control resistance to fluid flow, prevent filter clogging, control fluid retention time, etc. In an embodiment, the perforated support  220  has a ratio of open area  224  to closed (solid surface  222 ) area in a range of 20:80 to 80:20. The areas are generally defined in a plane perpendicular to the direction of fluid flow through the perforated support  220 , for example in a direction normal to the plate or other major surface of the support  220 . Thus, about 20-80% (e.g., at least 20, 30, 40, or 50% and/or up to 50, 60, 70, or 80%) of the surface area of the perforated support  220  is open and available for fluid flow therethrough. Correspondingly, about 20-80% (e.g., at least 20, 30, 40, or 50% and/or up to 50, 60, 70, or 80%) of the surface area of the perforated support  220  is the solid surface  222  upon which the capture fungus  230  can grow and adhere. Thus, the fractional solid surface  222  area of the support  220  is desirably high to provide a higher capture capacity for microalgae, and it is balanced with open area  224  to allow sufficient fluid throughput through the fungal filter  210 . 
     The fungal filter  210  further includes a capture fungus  230  on the solid surface  222  (e.g., adhered thereto) of the perforated support  220 , for example having been grown as a living fungus directly on the solid surface  222  in a suitable culture medium  300 . The fungus  230  is able to bind or otherwise capture microalgae that it contacts during operation of the filtration system  10 , for example via its fungal mycelium. Suitable fungi and corresponding microalgae for capture are described in more detail below. In a particular embodiment, however, the capture fungus  230  includes a  Mortierella  fungus.  Mortierella  species are widespread soil fungi that are safe to plants or animals and humans.  Mortierella  fungi form dense biofilms along culture surfaces, and being indeterminate in growth form, can grow into the size and shape of their incubation container to make standard size fungal filters  210 . Many  Mortierella  strains can be used for human nutraceuticals because of their outstanding productivity in polyunsaturated fatty acids. Further, several species of  Mortierella  fungi are able to capture microalgae such as  Nannochloropsis microalgae  (e.g.,  Nannochloropsis oceanica ) with high-efficiency. This marine alga has a high oil yield and makes valuable omega-3 fatty acid eicosapentaenoic acid (EPA). Thus, when the capture fungus  230  itself forms a useful bioproduct (e.g.,  Mortierella  fungus or otherwise) along with the microalgae (e.g., a  Nannochloropsis microalgae  or otherwise), the collective capture fungus  230 -bound microalgae  232  biomass can be harvested or otherwise recovered together from a coated fungal filter  212  after use, and the combined fungus/microalgae biomass can be extracted or otherwise processed to obtain desired bioproducts formed by the fungus  230  and well as the microalgae  232 . 
     The fungal filter  210  is adapted to be removed and replaced from or with the modular filtration unit  200 , or from the filter housing  100 , for example during operation of the filtration system  10  or during an intermittent shutdown of the filtration system  10 . 
     In embodiments as illustrated in  FIGS. 3 and 7  and described in more detail below, the fungal filter  210  is removably engaged (e.g., slidably engaged) with the modular filtration unit  200  from an external region E of the filter housing  100 . For example, a filter housing  100  can include a plurality of externally accessible and internal slots  140  or other orifices for insertion/installation of a new/fresh fungal filter  210  and removal of a used/coated fungal filter  212  after accumulation of microalgae  232  bound to the capture fungus  230 . Thus, the modular filtration unit  200  can be in the form of a slidable drawer including the fungal filter  210  as a component thereof such that removal a modular filtration unit  200  having a coated fungal filter  212  can be followed by replacement/insertion of a modular filtration unit  200  having a new or fresh (e.g., microalgae-free) fungal filter  210  for continued operation of the filtration system  10 . 
     In embodiments as illustrated in  FIGS. 1, 2, and 4-6  and described in more detail below, the modular filtration unit  200  can include a modular housing unit  240  defining a portion of the filter housing  100  and containing the fungal filter  210 . The plurality of modular filtration units  200  (e.g., individual modular filtration units  200 A,  200 B, etc.) can be removably connectable and disconnectable to each other by any suitable means, for example threaded connectors, latch connectors, pin or screw connectors, etc. Such connectors  242  are generally illustrated in the figures as complementary structures  242 A,  242 B, for example including complementary threaded/screw connections, complementary pin/slot connections, etc. Such removable connections allow for temporary disassembly of a portion of the filter housing  100  to permit removal and replacement of the fungal filter  210 . The removable connections also allow the filtration system  10  to be assembled with any desired number of modular filtration units  200 , for example where a higher or lower number of filtration stages are desired based on the nature of the filter feed stream (e.g., concentration and/or type of microalgae in feed). 
     The disclosure further relates to a method for harvesting microalgae using a microalgae filtration system  10  according to any of the various disclosed embodiments. An influent or feed stream  20  including microalgae therein is fed via the inlet  110  to the filter housing  100  (e.g., into the internal volume V thereof). The influent  20  is generally an aqueous feed stream containing (primarily) water and microalgae suspended or otherwise distributed therein. The influent  20  can include further components depending on the source of the feed stream. For example, when the influent  20  is fed from an upstream bioreactor producing the microalgae as a product, the influent  20  can include other components from the bioreactor reaction medium, for example nutrients or other growth medium components for microalgae growth, metabolic byproducts from microalgae growth, etc. Similarly, when the influent  20  is fed from a natural body of water, the influent can include other components from the natural water source, for example bacteria or other microorganisms present in the aquatic environment, salt from a saltwater source, etc. 
     The influent or feed stream  20  travels through the internal volume V of the filter housing  100  and along the continuous fluid flow path  130  across the plurality of fungal filters  210  in the filtration system  10 . As the microalgae-containing fluid passes through the filter housing  100 , it contacts the capture fungus  230  of the fungal filters  210 , whereupon the dispersed microalgae is captured by and bound to the capture fungus  230  as bound microalgae  232 . The resulting structure in the filtration system  10  is a coated fungal filter  212 , which corresponds to the original or fresh fungal filter  210  (i.e., free from microalgae), but further including bound microalgae  232  on the capture fungus  230 . 
     After capture and removal of microalgae from the fluid passing through the filter housing  100  and contacting multiple fungal filter  210  stages, the resulting fluid is withdrawn from the filtration system  10  as an effluent or product stream  30 . The effluent  30  is likewise generally an aqueous product stream containing (primarily) water, but it contains relatively less microalgae than the influent  20 . In various embodiments, at least 70% of the microalgae in the influent  20  is captured by the fungal filters  210  when forming the corresponding coated fungal filters  212 . For example, at least 70, 80, 90, or 95% and/or up to 90, 95, 98, 99, or 100% of the microalgae in the influent  20  is captured. Alternatively or additionally, the concentration of microalgae in the effluent  30  can be 30% or less than that in the influent  20 . For example, the concentration of microalgae in the effluent  30  can be at least 1, 2, 5, or 10% and/or up to 5, 10, 20, or 30% of the concentration of microalgae in the influent  30 . 
     The coated fungal filters  212  are removed from the filtration system  10  or from the filter housing  100  to harvest and recover the bound microalgae  232  thereon. The coated fungal filters  212  can be removed at any suitable time during operation of the filtration system  10 , for example after sufficient bound microalgae  232  has accumulated on the capture fungus  230 , in which case the overall capture and removal efficiency of the filtration system  10  might begin to decrease without removal and replacement of the coated fungal filters  212 . A particular coated fungal filter  212  can be removed separately from or along with its corresponding modular filtration unit  200  as described above. Harvesting the bound microalgae  232  suitably includes scraping or otherwise mechanically removing the bound microalgae  232  from the perforated support  220 . In an embodiment, the collective capture fungus  230  and bound microalgae  232  biomass on the fungal filter  212  can be removed together from the perforated support  220  during harvesting. The harvested biomass can be used as mixture of fungus and algae. For example, when capture fungi producing useful bioproducts are combined with oil-producing microalgae, the collective algae-fungi feedstocks harvested from the coated fungal filters  212  can be processed simultaneously for food, nutraceutical, and animal feed markets. 
     In an embodiment, the method further includes replacing the removed coated fungal filters  212  with fresh fungal filters  210 . The fresh fungal filters  210  generally include a new or different perforated support  220  having a newly grown capture fungus  230  thereon which is suitably free from microalgae. Such fresh fungal filters  210  can be prepared from other coated fungal filters  212  that have been previously removed from the filtration system  10 , harvested to remove and recover the capture fungus  230  and bound microalgae  232 , sterilized, and then incubated in a culture medium  300  to re-grow a new capture fungus  230  on the support  220 . In some embodiments, removing the coated fungal filters  212  and replacing them with fresh fungal filters  210  can be performed while the filtration system  10  is still in operation. For example, in the embodiments illustrated in  FIGS. 3 and 7 , removal of a single modular filtration unit  200  with a coated fungal filter  212  for replacement can be performed while influent  20  continues to pass through the filtration system  10  and microalgae is captured on the other modular filtration unit  200  still inserted in the filter housing  100 . In some embodiments, removal of a coated fungal filter  212  can be performed while the filtration system  10  is temporarily halted, for example halting influent flow when changing filters, such as when disassembly and reassembly is required to remove and replace a filter. 
     Flow of the process fluid through the filtration system  10  can be driven by any suitable means, for example by gravity, an external pressure source, or a combination thereof. In some embodiments, gravity alone can be a sufficient driving force for flow through the filter housing  100  (i.e., with the inlet  110  at a higher relative elevation than the outlet  120 ), in particular because large velocities are not required for effective operation of the filtration system  10 . Lower velocities can promote extended contact time and capture efficiency between the influent  20  and the capture fungus  230 , and they also limit fluid shearing forces that can detach or otherwise damage a capture fungus  230  attached to its support  220 . Nonetheless, in some embodiments, positive pressure or a suction pressure (e.g., via a pump or otherwise) can be used to assist flow through the filtration system  10 , in particular if care is taken to limit or prevent damage to the capture fungus  230 . 
     Further features and embodiments of the disclosed apparatus and methods are illustrated in specific figures and described in more detail below. 
       FIG. 1  illustrates an embodiment of a modular filtration unit  200  according to the disclosure. The modular filtration unit  200  has a screw design to increase the ease of setup of the filter system. It utilizes a stacking system where fungal filters  210  (not illustrated in  FIG. 1 ) can be inserted into filtration units  200  that can be connected with each other through the use of threads  242 A,  242 B just like a screw. As illustrated, the top of the filtration unit  200  includes a threaded portion  242 A that is complementary to an interior threaded receiving portion  242 B (internal location denoted by dashed lines) at the base of the filtration unit. The top of the filtration unit  200  further includes a surface or recess  244  where the fungal filter  210  is positioned and seated (e.g., via locator slots  246  and complementary pins (not shown)) and then fixed in position once another modular filtration unit  200  is connected/screwed on top. The illustrated modular filtration unit  200  was fabricated from polylactic acid via  3 D printing. 
       FIG. 2  illustrates another embodiment of a modular filtration unit  200  according to the disclosure. The modular filtration unit  200  has a funnel design to eliminate leaking of microalgae solution on the sides of the fungal filters  210  (not illustrated in  FIG. 2 ), by incorporating a funnel structure or surface  244  at the bottom of the unit to ensure the solution that was filtered was applied towards the center of the fungal filter  210  in the unit below it. This modular filtration unit  200  design also includes slots  242 A (illustrated at the top of the unit) that can be used to screw in and secure taller stacks of pods or filtration units  200  via complementary extruding or outwardly extending pins  242 B (illustrated at the bottom of the unit). Each filtration unit  200  is designed to accommodate a fungal filter  210 , which sits on a supporting mesh plate at the top of the funnel, for example at or on the surface  244 . The illustrated modular filtration unit  200  was fabricated from polylactic acid via  3 D printing. 
       FIG. 3  illustrates an embodiment of a filtration system  10  according to the disclosure. Panel (A) illustrates a cabinet design for the filter housing  100 , which was designed for prolonged use and ease of fungal filter  210  removal and replacement, for example via removal and replacement of one or more corresponding modular filtration units  200 A- 200 E from a plurality of filtration units  200  each including a fungal filter  210 . A main factor for consideration during prolonged use is oversaturation of the fungal filter  210  over time, which leads to the need for the filter  210  to be replaced in order to continue operations. The cabinet design also facilitates proper alignment of the fungal filter  210  stack within the filter housing, which can otherwise be difficult in a cylindrical column. A series of slots  140  on the external surface  142  of the housing  100  cabinet provides access for insertion of new fungal filters  210  and removal of algae-coated fungal filters  212  after extended use. Openings at the top and bottom of the housing  100  cabinet (not shown) provide an inlet  110  and an outlet  120  for movement of fluid through the apparatus via a continuous flow path  130 . Panel (B) illustrates a modular filtration unit  200  and corresponding fungal filter  210  as a drawer structure that can be slidably inserted and removed from the various stacking locations within the cabinet. The illustrated drawer filtration unit  200  embodiment has a perforated support  220  with 6.35 mm-diameter holes or open areas  224  spaced every 12.7 mm on the solid surface  222  of the support  220  to allow for filtration and fluid through-flow. Two 6.35 mm-diameter cylinders or alignment/sliding means  226  run the length of the drawer, and these cylinders  226  have a complementary matching or receiving cylinder set (not shown) inside the cabinet which acts as a track for the drawer to facilitate insertion, removal, and alignment of a given filtration unit  200  when its corresponding coated fungal filter  212  is in need replacement. 
       FIGS. 4-6  illustrate further embodiments of a modular filtration unit  200  according to the disclosure.  FIG. 4  illustrates a modular filtration unit similar  200  to that of  FIG. 1  with threaded screw connectors  242 A,  242 B. The mesh plate or perforated support  220  of the fungal filter  210  includes pins  246 B for positioning, seating, and alignment in receiving holes  246 A at the top of the filtration unit  200 . The right side of the figure illustrates multiple modular filtration units  200 A- 200 C stacked together to form a multistage filtration system  10 , further including top and bottom reservoirs for holding influent  20  and collecting effluent  30 , respectively.  FIG. 5  illustrates stackable modular filtration units  200  similar to those of  FIG. 4 . Instead of threaded screw connectors, however, pins  242 A and receiving holes  242 B are used to position, align, and connect consecutive filtration units  200 , and corresponding linkers  246 C (shown as a bracket with two holes for screws, etc.) can be used to fasten adjacent filtration units  200 , for example via additional receiving holes  242 B on an outer surface of the units  200  as shown. A recess  244  in the filtration unit receives the mesh plate fungal filter  210 , which is seated and fixed in place when the linkers  246 C are attached.  FIG. 6  illustrates a modular filtration unit  200  that houses multiple fungal filters  210 . Two shell housing unit pieces  240  contain multiple complementary recess or groove sets  244  for receiving mesh plate fungal filters  210  (only one of which is shown in  FIG. 6 ), and threaded connectors  242 A at the top and bottom can be used to seal the filtration unit  200  (e.g., via a threaded ring as illustrated) and/or connect it to adjacent filtration units (not shown). 
       FIG. 7  illustrates a side cross-sectional view of an embodiment of a filtration system  10  similar to that shown in  FIG. 3 . The filter housing  100  has a cabinet design and the corresponding modular filtration unit  200 /fungal filter  210  has a drawer design. The plurality of modular filtration units  200  is illustrated as including individual modular filtration units  200 A- 200 D. A series of slots  140  on both the external surface  142  of and within the housing  100  cabinet provides access for insertion of new fungal filters  210  and removal of coated fungal filters  212  after extended use. For example,  FIG. 7  illustrates the sliding removal of the modular filtration unit  200 D which has a coated fungal filter  212  with accumulated bound microalgae  232  on the capture fungus  230 . After removal of the filtration unit  200 D, the combined fungus  230  and microalgae  232  biomass can be harvested and removed from the perforated support  220  of the filtration unit  200 D. While the filtration unit  200 D is removed, the filtration system  10  can continue to operate as the influent  20  being fed via the inlet  110  still passes through the filtration units  200 A- 200 C, where microalgae can still be captured as bound microalgae  232  to provide the corresponding effluent  30  with reduced microalgae content withdrawn via the outlet  120 . While the removed filtration unit  200 D is being processed as part of the harvesting and optional fungal re-growth steps, a replacement filtration unit  200 D with a fresh fungal filter  210  can be inserted into the housing  100  so that operation of the filtration system  10  can continue with its normal complement of filtration units  200  (e.g., four units  200 A- 200 D in the illustrated embodiment). In cases where the effluent  30  still contains a substantial amount of uncaptured microalgae, all or at least a portion of the effluent  30  can be further processed for microalgae removal, for example by recycling all or at least a portion of the effluent  30  to the inlet  110  of the filtration system  10 , or by feeding all or at least a portion of the effluent  30  to the inlet of a second filtration system (not shown) in a downstream serial relationship to the illustrated filtration system  10 . 
     As shown in  FIG. 7 , the filtration system  10  can include further components to assist with process flow control and/or monitoring. For example, a flow rate meter  252  can be used to measure and control total flow rate through and retention time within the filtration system, for example with a corresponding valve in addition to or incorporated in the meter  252 . Sensors  254  and  256  can be positioned at various locations on the filter housing  100  so that they can monitor (e.g., in real time) conditions within the internal volume V of the filter housing  100 . For example, an optical meter  254  can be used to measure cell density or concentration of microalgae suspended in the fluid within the internal volume V. When positioned at various locations of the filter housing  100 , such optical meters  254  can provide information related to microalgae concentration gradients and/or separation/capture efficiency within the filtration system  10 . Alternative or additionally, a camera  256  can be used to observe the surface of a fungal filter  210  and detect the accumulation over time of bound microalgae  232  on the capture fungus  230 , which in turn provides information regarding a relative degree of filter saturation and timing for removal, harvesting, and replacement of a given filtration unit  200 /fungal filter  210 . More than one camera  256  can be used, for example to provide multiple views of a single filter and/or views of different filters to determine a filter-specific timing for removal, harvesting, and replacement. 
       FIG. 8  illustrates side cross sectional views of perforated supports  220  according to the disclosure. Panel (A) illustrates a perforated support  220  prior to binding a capture fungus  230  thereto and/or after harvesting fungus  230 /microalgae  232  therefrom. Panel (B) illustrates a perforated support  220  with capture fungus  230  thereon in the form of a fungal filter  210 , for example a fresh fungal filter  210  with newly re-grown fungus  230  that is ready for re-insertion and use in a filtration system  10 . Although the figure illustrates the capture fungus  230  grown only on a single (top) surface of the support  220 , the fungus  230  more generally can grow on and attach to any available surface of the support  220  (e.g., top, sides, bottom). Panel (C) illustrates a perforated support  220  with capture fungus  230  and bound microalgae  232  thereon in the form of a coated fungal filter  212 . Once sufficient bound microalgae  232  has accumulated on the capture fungus  230 , the coated fungal filter  212  can be remove from the filtration system  10 , and the collective fungus  230 /microalgae  232  biomass can be harvested therefrom, which in turn results in the structure illustrated in panel (A) (i.e., a bare perforated support  220  that can be sterilized and then have new fungus  230  re-grown thereon). 
     U.S. Publication No. 2018/0346954 provides disclosure related to various fungi and algae that can be used as the capture fungus and corresponding microalgae in the disclosed filtration system, and it is incorporated herein by reference in its entirety. 
     A variety of fungi can be employed as the capture fungus for the fungal filter. In some embodiments, a single type (e.g., genus or species) of fungus is used as the capture fungus for the fungal filter. In other embodiments, multiple types (e.g., genus or species) of fungi are used as the capture fungus for the fungal filter, for example including blends or mixtures of fungi on a single perforated support or including different types of fungi on different perforated supports in a filtration system. In some cases, the fungus can be a basidiomyccte, ascomycete, or zygomycete. For example, one or more fungi can be a member of a genus such as:  Aspergillus, Blakeslea, Botrytis, Candida, Cercospora, Cryptococcus, Cunninghamella, Fusarium  ( Gibberella ),  Kluyveromyces, Lipomyces, Morchella, Mortierella, Mucor, Neurospora, Penicillium, Phycomyces, Pichia  ( Hansenula ),  Puccinia, Pythium, Rhodosporidium, Rhodotorula, Saccharomyces, Sclerotium, Trichoderma, Trichosporon, Xanthophyllomyces  ( Phqffia ), or  Yarrowia . For example, the fungus can be a species such as:  Aspergillus terreus, Aspergillus nidulans, Aspergillus niger, Atractiella  PMI152 , Blakeslea trispora, Botrytis cinerea, Candida japonica, Candida pulcherrima, Candida revkaufi, Candida tropicalis, Candida utilis, Cercospora nicotianae, Clavulina  PM1390 , Cryptococcus curvatus, Cunninghamella echinulata, Cunninghamella elegans, Flagelloscypha  PM1526 , Fusarium fujikuroi  ( Gibberella zeae ),  Grifola frondosa  GMNB41 , Kluyveronmyces lactis, Lecythophora  PM1546 , Leptodontidium  PMI413 , Lachnum  PM1789 , Lipomyces starkeyi, Lipomyces lipoferus, Mortierella alpina, Mortierella elongata  AG77 , Mortierella gamsii  GBAus22 , Mortierella ramanniana, Mortierella isabellina, Mortierella vinacea, Mucor circinelloides, Neurospora crassa, Phycomyces blakesleanus, Pichia pastoris, Puccinia distincta, Pythium irregulare, Rhodosporidium toruloides, Rhodotorula glutinis, Rhodotorula graminis, Rhodolorula mucilaginosa, Rhodolorula pinicola, Rhodotorula gracilis, Saccharomyces cerevisiae, Sclerotium rolfsii, Trichodenna reesei, Trichosporon cutaneum, Trichosporon pullans, Umbelopsis  PMI120 , Xanthophyllomyces dendrorhous  ( Phqffia rhodozyma ),  Yarrowia lipolytica , or a combination thereof. In some cases, the fungus is not  Geosiphon pyriformis.    
     In some cases, the fungus employed is a multi-celled fungi. For example, the fungus employed can have tissues and/or structures such as hyphae. Many fungi is made up of fine, branching, usually colorless threads called hyphae. Each fungus can have vast numbers of these hyphae, all intertwining to make up a tangled web called the mycelium. The mycelium is generally too fine to be seen by the naked eye, except where the hyphae are very closely packed together. However, in some cases the fungus need not be a multi-celled fungus. For example, the fungus can be a one-celled organism such as a yeast. 
     In some cases, the fungus can be one or more of  Mortierella elongata, Mortierella elongata  AG77 , Mortierella gamsii, Mortierella gamsii  GBAus22 , Umbelopsis  sp.,  Umbelopsis  PM1120 , Lecythophora  sp.,  Lecythophora  PM1546 , Leptodontidium  sp.,  Leptodontidium  PM1413 , Lachnum  sp.,  Lachnum  PM1789 , Morchella  sp.,  Saccharomyces cerevisiae, Atractiella  sp.,  Atractiella  PMI152 , Clavulina, Clavulina  PM1390 , Grifola frondosa, Grifola frondosa  GMNB41 , Flagelloscypha  sp.,  Flagelloscypha  PM1526, and combinations thereof. 
     The capture fungus can be bound or otherwise adhered to the perforated support by any suitable means to form the corresponding fungal filter. Suitably, the perforated support can be immersed in a fungal growth medium, as the capture fungus is generally capable of attaching/adhering to most solid surfaces during fungal filter formation/growth. The capture fungi can form dense biofilms along culture surfaces, growing into the size and shape of their incubation container to adhere to the perforated support and form the fungal filter. Suitable growth or culture medium for the capture fungus is not particularly limited and can be selected by the skilled artisan. For example, the capture fungus can be cultured in a culture medium that contains some carbohydrate, such as some sugar. The sugar can be any convenient sugar or a combination of sugars. Examples include dextrose, sucrose, glucose, fructose or a combination thereof. The amount of sugar can be included in amounts of about 1 g/liter to about 20 g/liter, or of about 3 g/liter to about 18 g/liter, or of about 5 g/liter to about 15 g/liter. In an embodiment, the capture fungus can be grown in potato dextrose broth (PDB) media (12 g/L potato dextrose broth, 5 g/L yeast extract, pH 5.3).  FIG. 9  illustrates this process in which an initially uncoated or otherwise fungus-free perforated support  220  (left side of  FIG. 9 ; see also panel (A) of  FIG. 8 ) is placed or immersed in a fungal growth or culture medium  300  that contains floating or suspended fungal mycelia  310  of the desired capture fungus. As the culture medium  300  with immersed supports  220  therein is incubated, capture fungus  230  grows and adheres to the solid surfaces  222  of the perforated support  220 . The right side of  FIG. 9  shows the corresponding fungal filter  210  including the perforated support  220  with the capture fungus  230  thereon (see also panel (B) of  FIG. 8 ). After sufficient growth and accumulation of capture fungus  230 , the fungal filter  210  is removed from the culture medium  300  and can be inserted or otherwise installed into a filtration system  10 , for example as a fungal filter  210  replacing a coated fungal filter  212  (see panel (C) of  FIG. 8 ) to be removed and harvested. 
     A variety of microalgae in an aqueous feed can be captured by and bound to the capture fungus during use of the filtration system. Examples of microalgae genera that are particularly suitable for capture and harvest using the filtration system include  Nannochloropsis, Chlorella , and  Chlamydomonas . Depending on the source of the influent, the aqueous feed to the filtration system can include a single type of microalgae or multiple types of microalgae. For example, an influent can be obtained from a bioreactor, which could be operated to produce a single type of desired microalgae that is then captured and harvested using the filtration system. In other embodiments, a bioreactor could also be operated to produce a desired blend of different types of microalgae (e.g., based on production of different desired metabolic or biomass products) that are then captured and harvested using the filtration system. In other embodiments, the influent can be obtained from a natural water source where microalgae are present, for example freshwater or saltwater marine environments, such a pond, lake, river, estuary, gulf, sea, ocean, etc. In such cases, the natural water source can include any of a variety of microalgae present in the environment, which microalgae are then captured and harvested using the filtration system. 
     Examples suitable algae include diatoms (bacillariophytes), green algae (chlorophytes), blue-green algae (cyanophytes), and golden-brown algae (chrysophytes). In addition, a fifth group known as haptophytes may be used. Specific non-limiting examples of bacillariophytes capable of lipid production include the genera  Amphipleura, Amphora, Chaetoceros, Cyclotella, Cymbella, Fragilaria, Hantzschia, Navicula, Nitzschia, Phaeodactylum , and  Thalassiosira . Specific non-limiting examples of chlorophytes capable of lipid production include  Ankistrodesmus, Botryococcus, Chlorella, Chlorococcum, Dunaliella, Monoraphidium, Oocystis, Scenedesmnus , and  Tetraselmis . In one aspect, the chlorophytes can be  Chlorella  or  Dunaliella . Specific non-limiting examples of cyanophytes capable of lipid production include  Oscillatoria  and  Synechococcus . A specific example of chrysophytes capable of lipid production includes  Boekelovia . Specific non-limiting examples of haptophytes include lsochrysis and  Pleurochrysis . In some cases, an alkenone-producing alga, for example, a species of the lsochrysis family which includes, but not limited to, lsochrysis galbana, lsochrysis sp. T-Iso, and lsochrysis sp. C-Iso can be employed. Other examples of alkenone-producing algae include Emiliania huxleyi and Gephyrocapsa oceanica. In some cases, the algae is not a cyanobacterium. For example, the algae may not, in some cases, be  Nostoc punctiforme.    
     Further examples of algae can be species of  Amphipleura, Amphora, Aquamortierella, Chaetoceros, Charophyceae, Chlorodendrophyceae, Chlorokybophyceae, Chlorophyceae, Coleochaetophyceae, Cyclotella, Cymbella, Dissophora, Embryophytes, Endogaceae, Fragilaria, Gamsiella, Hantzschia, Klebsormidiophyceae, Lobosporangium, Mamiellophyceae, Mesostigmatophyceae, Modicella, Mortierella, Mucor, Navicula, Nephroselmidophyceae, Nitzschia, Palmophyllales, Prasinococcales, Prasinophytes, Pedinophyceae, Phaeodactylum, Pyramimonadales, Pycnoccaceae, Pythium, Phytophthora, Phytopythium, Rhizopus, Thalassiosira, Trebouxiophyceae, Ulvophyceae, Zygnematophyceae , or a combination thereof. 
     In some cases, the algae is a photosynthetic algae. For example, the alga type employed can be a strain of  Nannochloropsis oceanica , for example  Nannochloropsis oceanica  CCMP1779. 
     Because other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the disclosure is not considered limited to the example chosen for purposes of illustration, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this disclosure. 
     Accordingly, the foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications within the scope of the disclosure may be apparent to those having ordinary skill in the art. 
     All patents, patent applications, government publications, government regulations, and literature references cited in this specification are hereby incorporated herein by reference in their entirety. In case of conflict, the present description, including definitions, will control. 
     Throughout the specification, where the compositions, processes, kits, or apparatus are described as including components, steps, or materials, it is contemplated that the compositions, processes, or apparatus can also comprise, consist essentially of, or consist of, any combination of the recited components or materials, unless described otherwise. Component concentrations can be expressed in terms of weight concentrations, unless specifically indicated otherwise. Combinations of components are contemplated to include homogeneous and/or heterogeneous mixtures, as would be understood by a person of ordinary skill in the art in view of the foregoing disclosure. 
     PARTS LIST 
     
         
           10  microalgae filtration system 
           20  influent/feed source 
           30  effluent/product stream 
           100  filter housing (internal volume V) 
           110  filter housing inlet 
           120  filter housing outlet 
           130  continuous fluid flow path 
           140  opening or slot 
           142  external surface (external region E) 
           200  modular filtration units (e.g.,  200 A,  200 B,  200 C, . . . ) 
           210  fungal filter 
           212  coated fungal filter 
           220  perforated support 
           222  perforated support solid surface 
           224  perforated support open area(s) 
           226  alignment/sliding means such as complementary track components 
           230  (capture) fungus 
           232  microalgae bound to (capture) fungus 
           240  modular housing unit 
           242  connection means for modular housing unit(s) such as threads, slots, pins, etc. (e.g.,  242 A,  242 B, . . . ) 
           244  surface or recess 
           246  locating/positioning/seating means for fungal filter(s) such as slots, pins, etc. (e.g.,  246 A,  246 B, . . . ) 
           252  flow rate meter and optional valve or flow controller 
           254  sensor/optical cell density meter 
           256  sensor/filter saturation camera 
           300  culture/fungal growth medium 
           310  floating/suspended fungal mycelia