WO2013158795A1

WO2013158795A1 - Harvesting and dewatering algae using a two-stage process

Info

Publication number: WO2013158795A1
Application number: PCT/US2013/037023
Authority: WO
Inventors: Nicholas Eckelberry; Jose Sanchez
Original assignee: Originoil, Inc.
Priority date: 2012-04-17
Filing date: 2013-04-17
Publication date: 2013-10-24
Also published as: CN104768877A; MX2014012607A; KR20140144742A; AU2013249299A1; JP2015515375A; IN2014MN02073A

Abstract

The present invention is generally directed to an apparatus for harvesting algae using a two-stage approach. The two-stage approach includes a flocculation stage and a dewatering stage. The flocculation stage is implemented within a first-stage flocculation tank in which algae suspended within a growth medium is flocculated. The flocculated algae is then fed to a second-stage flotation tank in which electrodes are used to produce hydrogen and oxygen bubbles which attach to the flocculated algae causing the flocculated algae to float to the surface. The mat of floating algae can then be skimmed off the surface of the growth medium.

Description

HARVESTING AND DEWATERING ALGAE

USING A TWO-STAGE PROCESS

There is a history of separating materials from liquid suspension in several industries, including the wastewater treatment industry and algae farming industry.

Processes involved in achieving separation can vary, along with the desired end result.

For example, in the wastewater treatment industry, the desired result is typically treated water that can be released into the environment. In contrast, in the algae farming industry, the primary desired result may be the harvest of a usable biomass for energy production.

There is a long history of electro-flocculation in the wastewater industry. It has been found to be an effective method of separating solids from fluids in the secondary stage of remediation. This waste stream contains organic material of all types and algae are considered a nuisance generated by the high nitrate count common in the stream. Therefore, efforts in algae eradication usually does not include preservation of the integrity of the mass for further uses such as pharmaceutical or other high value feedstock.

In electro-flocculation, as commonly used in wastewater treatment, a metal ion or cation is added to improve flocculation by increasing conductivity of the matrix. The following cations have lower electrode potential than H+ and are therefore considered suitable for use as electrolyte cations in these processes: Li+, Rb+, K+, Cs+, Ba2+, Sr2+, Ca2+, Na+ and Mg2+ (sodium and lithium are frequently used as they form inexpensive salts). Other metals are used in conjunction with electro- flocculation to assist in precipitation of solids from the waste water, such as iron oxides and other oxidants. These metals are extremely effective at precipitating solids out of solution; however, they taint the product and the water itself with an inorganic chemical that then must be removed or otherwise processed in the tertiary waste treatment phase.

In practice, the current used by the wastewater systems for electro-flocculation is generally low, typically under 1 amp, as the processes are carried out in large ponds and/or in conjunction with massive fluid flows typical to a waste treatment plant which can be in the millions of gallons per day. Due to the sheer size of the plants and Ohms law (I=V/R) the current requirement and the scale of the process, it is not practical to utilize high-energy electro-flocculation systems for extended periods of time. Furthermore, the deterioration and scaling of electrolytic plates operating at high current for extended periods of time precludes the effective use of this technology at high amperage. The conductivity of the waste flow therefore must be enhanced by metal ions as discussed above to lower the energy requirement and make the process practical.

In algae product farming and harvesting, the considerations are reversed as the biomass in suspension is considered an asset whose qualities must be preserved, and the use of metals taints the product irreversibly. Most methods used to dewater the algae in suspension therefore consist of centrifuge, membrane filtration, air drying with possible chemical processing and decontamination.

One approach used to dewater algae is known as Dissolved Air Flotation (DAF). Typically, this flocculation method involves the use of coagulants, emulsifiers or other chemicals in tandem with a curtain of air generated from pumps or cyclones. While this method is generally more effective from an energy standpoint than centrifuge techniques, it has the inherent drawback of requiring both chemicals and an independent tank. Furthermore, the effectiveness of the DAF system as a continuous system is hampered by the creation of bubbles as a source of turbulence within the reactor. The solution to this problem has been to increase the size of the flotation which leads to larger and larger footprints.

Additionally, the use of chemicals in dewatering commonly prevents or limits reuse of the growth water. The related prior application no. 13/274,094, filed October 14, 2011 and titled Systems, Methods, and Apparatuses for Dewatering, Flocculating, and Harvesting Algae Cells, which is incorporated by reference, discloses some forms of electro-magnetic flocculation systems. That application focuses on cell lysing as the end product.

The harvesting of microorganisms and intracellular products of microorganisms such as algae shows promise as a partial or full substitute for fossil oil derivatives or other chemicals used in manufacturing products such as pharmaceuticals, cosmetics, industrial products, biofuels, synthetic oils, animal feed, and fertilizers. However, for these substitutes to become viable, methods for harvesting the cells, including steps of recovering and processing of intracellular products must be efficient and cost-effective in order to be competitive with the refining costs associated with fossil oil derivatives. Current extraction methods used for harvesting microorganisms such as algae to ultimately yield products for use as fossil oil substitutes are laborious and yield low net energy gains, rendering them unviable for today's alternative energy demands. Such previous methods can also produce a significant carbon footprint, exacerbating global warming and other environmental issues. These prior methods, when further scaled up, produce an even greater efficiency loss due to valuable intracellular component degradation and require greater energy or chemical inputs than what is currently financially feasible from a microorganism harvest. For example, the cost per gallon for microorganism bio-fuel is currently approximately nine times the cost of fossil fuel.

All living cells, prokaryotic and eukaryotic, have a plasma transmembrane that encloses their internal contents and serves as a semi-porous barrier to the outside environment. The transmembrane acts as a boundary, holding the cell constituents together, and keeps foreign substances from entering. According to the accepted current theory known as the fluid mosaic model (S.J. Singer and G. Nicolson, 1972, incorporated herein by reference), the plasma membrane is composed of a double layer (bi-layer) of lipids, an oily or waxy substance found in all cells. Most of the lipids in the bilayer can be more precisely described as phospholipids, that is, lipids that feature a phosphate group at one end of each molecule.

Within the phospholipid bilayer of the plasma membrane, many diverse, useful proteins are embedded while other types of mineral proteins simply adhere to the surfaces of the bilayer. Some of these proteins, primarily those that are at least partially exposed on the external side of the membrane, have carbohydrates attached and therefore are referred to as glycoproteins. The positioning of the proteins along the internal plasma membrane is related in part to the organization of the filaments that comprise the cytoskeleton, which helps anchor them in place. This arrangement of proteins also involves the hydrophobic and hydrophilic regions of the cell.

Intracellular extraction methods can vary greatly depending on the type of organism involved, their desired internal component(s), and their purity levels. However, once the cell has been fractured, these useful components are released and typically suspended within a liquid medium which is used to house a living microorganism biomass, making harvesting these useful substances difficult or energy-intensive. In most current methods of harvesting intracellular products from algae, a dewatering process has to be implemented in order to separate and harvest useful components from a liquid medium or from biomass waste (cellular mass and debris). Current processes are inefficient due to required time frames for liquid evaporation or energy inputs required for drying out a liquid medium or chemical inputs needed for a substance separation. Additionally, such processes are commonly limited to batch processing and are difficult to adapt for continuous processing systems.

Accordingly, there is a need for a simple and efficient procedure for dewatering microorganisms, such as algae, so that they can be harvested and their intracellular products can be recovered and used as competitively-priced substitutes for fossil oils and fossil oil derivatives required for manufacturing of industrial products.

BRIEF SUMMARY

In one embodiment, the present invention is implemented as an apparatus for harvesting algae using a two-stage process. The apparatus includes a flocculation tank in which the first stage of the two-stage process occurs. The flocculation tank comprises a reactor tube for creating an electric field within a growth medium containing suspended algae, the electric field causing the algae to flocculate. The apparatus also includes a flotation tank in which the second stage of the two stage process occurs. The flotation tank comprises a tank containing a plurality of electrodes which cause the formation of gas bubbles which attach to the flocculated algae and lift the flocculated algae to the surface of the growth medium. The flotation tank is connected to the flocculation tank to allow the flocculated algae to flow from the flocculation tank into the flotation tank.

In another embodiment, the present invention is implemented as a method for harvesting algae using a two-stage process. A growth medium containing suspended algae is supplied into a flocculation tank. The flocculation tank comprises a reactor tube for creating an electric field within the growth medium, the electric field causing the algae to flocculate. The growth medium containing flocculated algae is transferred into a flotation tank. The flotation tank comprises a tank containing a plurality of electrodes which cause the formation of gas bubbles which attach to the flocculated algae and lift the flocculated algae to the surface of the growth medium. The floating algae are then removed from the surface of the growth medium.

In another embodiment, the present invention is implemented as an apparatus for removing ammonia from a fluid. The apparatus comprises a reactor tube for creating an electric field within a fluid containing ammonia. The reactor tube includes a cathode and an anode, the anode comprising a titanium ruthenium alloy. When the electric field is created, the anode causes the creation of free chlorine within the fluid leading to the oxidation of the ammonia into nitrite and nitrate. The apparatus also includes a flotation tank connected to the reactor tube. The flotation tank comprises a tank containing a plurality of electrodes which cause the formation of gas bubbles.

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and other advantages and features of the invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

Figure 1A illustrates a two-stage algae harvesting apparatus having a first stage flocculation tank and a second stage flotation tank;

Figure IB illustrates side views of various possible configurations of electrodes within the second stage flotation tank;

Figure 1C illustrates a side view of the first stage flocculation tank;

Figure 2A illustrates the first stage flocculation tank when filled with a growth medium containing suspended algae;

Figure 2B illustrates the first stage flocculation tank when the algae is flocculated in a batch mode;

Figure 2C illustrates the first stage flocculation tank when the algae is flocculated in a continuous flow mode;

Figures 3A-3D illustrate the process, performed within the second stage flotation tank, of dewatering the flocculated algae using hydrogen bubbles to float the flocculated algae to the surface; and

Figure 4 illustrates an actual implementation of a two- stage algae harvesting apparatus in accordance with one or more embodiments of the present invention.

DETAILED DESCRIPTION

The algae harvested in this manner are free of harmful substances that are often required in other algae harvesting methods. Additionally, because harmful substances are not used in the two-stage process, the nutrient-rich growth medium can be reused in subsequent algae harvesting.

The apparatus of the present invention can be configured in various sizes. However, in many embodiments, the apparatus can be sized so that it is relatively portable to allow its use in virtually any location. In this way, many entities can employ the apparatus to produce an algae biomass without requiring a large area of land and/or large amounts of electricity as is often required in other harvesting approaches.

Figure 1A illustrates an example configuration of an apparatus 100 that harvests algae using the two-stage approach. Apparatus 100 includes two primary components: a first stage flocculation tank 101, and a second stage flotation tank 102.

A growth medium containing suspended algae is input into first stage flocculation tank 101. This growth medium can be obtained in virtually any manner. For example, a dedicated unit for growing algae within water can be connected to first stage flocculation tank 101, or a growth medium otherwise obtained can be directly supplied to first stage flocculation tank 101.

The suspended algae is flocculated (i.e. caused to form clumps) within first stage flocculation tank 101. This flocculation can be caused using an electric current produced by electrodes as will be further described below. Once the algae is flocculated to a desired degree, the growth medium containing the flocculated algae is fed into second stage flotation tank 102.

Second stage flotation tank 102 produces gas (e.g. hydrogen and oxygen) bubbles which rise through the growth medium. While rising, the bubbles attach to the flocculated algae and lift the flocculated algae to the surface. This process results in a mat of algae forming at the surface of the growth medium. Finally, the algae can be collected using conveyors 115 and 116 as will be further described below.

Figure 4 illustrates an actual implementation of an apparatus in accordance with one or more embodiments of the present invention.

First Stage Flocculation Tank

As shown in Figure 1A, flocculation tank 101 includes two primary components: a cathode 105 formed by an outer cylinder (e.g. an enclosed pipe or tube), and an anode 106 formed by an inner cylinder (e.g. a pipe or other enclosed cylindrical shape) that is contained within the outer cylinder. Accordingly, the growth medium flows between cathode 105 and anode 106 as shown by the arrows in Figure 1A. Other shapes other than cylinders can also be used as long as a fluid pathway is formed between the two components. Also, in some embodiments, multiple inner cylinders can be used for anode 106. In some embodiments, the surfaces of cathode 105 and anode 106 which are in contact with the growth medium can include grooves (e.g. rifling) which may decrease the occurrence of build-up on the surfaces.

Figure 1C illustrates a cross-sectional side view of flocculation tank 101. As shown a space exists between cathode 105 and anode 106 through which the growth medium flows. In some embodiments, this space can be between .5 mm and 200 mm wide. A voltage is applied to each of cathode 105 and anode 106 to cause an electric current to pass through the growth medium. This electric current causes the suspended algae in the growth medium to flocculate (i.e. to clump together). In some embodiments, as the algae pass through flocculation tank 101, the cells are exposed to both a magnetic field, causing a cellular alignment, and to an electrical field which induces cellular current absorption. These effects can cause the cells to flocculate.

Figures 2A-2C illustrate how this flocculation can occur. As shown, a source 210 of growth medium containing suspended algae is connected to flocculation tank 101. Alternatively, growth medium could be supplied manually to flocculation tank 101. The shading in Figure 2A indicates that the algae are initially suspended in the growth medium.

Figure 2B illustrates the case where the growth medium is treated in a batch mode. In the batch mode, flocculation tank 101 is initially filled with growth medium containing suspended algae. The growth medium is then subject to the electrical fields generated by cathode 105 and anode 106 until the desired level of flocculation has occurred. In some embodiments, the flocculated algae can be between 1 and 4 mm in size. Then, the growth medium with the flocculated algae is transferred to second stage flotation tank 102. Accordingly, Figure 2B illustrates that the growth medium within flocculation tank 101 contains clumps of algae which are ready to be transferred to flotation tank 102.

Figure 2C, in contrast, illustrates the case where the growth medium is treated in a continuous flow mode. In the continuous flow mode, the algae can be flocculated in the same manner as in the batch mode (e.g. by applying an electric current to the growth medium). However, the growth medium can be continuously flowed into flocculation tank at an appropriate rate so that, by the time the growth medium reaches the opposite end of the flocculation tank, the algae has been sufficiently flocculated. This is shown in Figure 2C with the growth medium at the left end having a similar degree of flocculation as the growth medium in source 210 and the degree of flocculation increasing towards the right end. Regardless of the mode used to flocculate the algae, flocculation tank 101 can be configured with controls for automatically determining the appropriate settings to ensure that the algae is sufficiently flocculated before exiting flocculation tanks 101. For example, in batch mode, flocculation tank 101 can automatically determine an appropriate duration of time to treat the growth medium or appropriate voltage levels to apply to cathode 105 and anode 106. Similarly, in continuous flow mode, flocculation tank 101 can automatically determine an appropriate flow rate and appropriate voltage levels to apply to cathode 105 and anode 106.

In at least one embodiment, the flow rate through flocculation tank 101 can be 0.1 ml/second per ml of volume. In other embodiments, however, the flow rate is at least 0.5 ml/second per ml of volume or at least 1.0 ml/second per ml of volume. In still other embodiments, the flow rate through the volume is at least 1.5 ml/second per ml of volume. In yet other embodiments, the flow rate through the volume exceeds 1.5 ml/second per ml of volume. In at least one additional embodiment, the flow rate can be controlled by controlling the pressure using a pump or other suitable fluid flow mechanical devices.

In some embodiments, the supplied voltage can be pulsed on and off repeatedly to cause extension and relaxation of the algae cells. According to such embodiments, voltages can be higher and peak amperage lower while average amperage remains relatively low. In such embodiments, this condition or controlled circumstance reduces the energy requirements for operating the apparatus and reduces wear on the anode and cathode pair or pairs. In at least one embodiment, the frequency of the pulses is at least about 500 Hz, 1 kHz, 2 kHz, or 30 kHz. In other embodiments, the frequency is less than 200 kHz, 80 kHz, 50 kHz, 30 kHz, 5 kHz, or 2 kHz. Ranges for the pulse frequency can be any combination of the foregoing maximum and minimum frequencies according to various embodiments.

In some embodiments, an electrical pulse is repeated in frequency to create an electromagnetic field and electrical energy transfer between the electrodes. As this pulsed electrical transfer occurs, an electromagnetic field is produced resulting in the elongation of the algae cells due to their polarity according to certain embodiments. According to further embodiments, the suspended algae absorb electrical input which causes internal cellular components and their liquid mass to swell in size. In such embodiments, and due to swelling, an internal pressure is applied against the transmembrane, however this internal swelling is to be considered as only momentary according to certain embodiments as it is relieved during an off frequency phase of the pulsed electrical input. As mentioned above, in some embodiments, rapid repeating of the on and off electrical frequency rearranges components and creates and/or increases the polar regions in the algae cells. In some embodiments, continuous frequency inputs further produce internal pressures caused by expanded internal component swelling which eventually creates the magnetic/electrostatic attraction causing coagulation/flocculation of the treated cells.

Although this specification primarily describes that the first stage leaves the algae cells intact during the flocculation process, it is also possible to lyse the algae cells during flocculation. For example, by varying the voltage levels/frequency applied to cathode 105 and anode 106 and/or varying the time that the algae cells are subject to the electric current formed between cathode 105 and anode 106, the algae cells can be lysed to thereby release the internal contents of the algae cells. Accordingly, in some embodiments, apparatus 100 can be used to lyse, flocculate, and dewater algae cells.

Second Stage Flotation Tank

Once the algae are flocculated in the growth medium, the growth medium is transferred to flotation tank 102. An electrical field can be applied to the growth medium within flotation tank 102 using electrodes. The electric field increases interface potential between solvent and solute and creates micron-sized bubbles of hydrogen and oxygen gas which lift the flocculated algae to the surface. The algae form a mat at the surface allowing for easy removal of the algae. Also, the mat of algae includes a substantial amount of hydrogen and oxygen gas. The algae can be used with this gas present, or further downstream processes can be performed to recover the gas. For example, the gas can be recovered and used to power apparatus 100 thereby minimizing the energy requirements for using apparatus 100.

Referring again to Figure 1A, flotation tank 102 includes a cathode plate 111 and a series of stacked anode 112 and cathode 113 rods. Figure IB illustrates side views of other configurations of electrodes that can be used within flotation tank 102. For example, at the top left corner of Figure IB, the configuration depicted in Figure 1A is shown. In some embodiments, a plate can be used in place of the rods.

Various other configurations of electrodes can be used. For example, a single cathode and a single anode, two cathodes and a single anode, a single cathode and two anodes, two cathodes and two anodes, or other combinations include one or more cathodes and one or more anodes.

As shown in Figure IB, some embodiments provide a two-by-three electrode arrangement, with two vertical columns of three electrodes. The top and bottom rows of electrodes can be cathodes and the middle row can include two anodes. Various other such anode-cathode configurations can be used in embodiments of flotation tank 102. Generally, combinations of between 1 and 20 anodes and between 1 and 20 cathodes can be used depending primarily on the size of flotation tank 102.

Flotation tank 102 also includes conveyor 115 (having rakes 115a and 115b) and conveyor 116 which are used to remove the algae cells from flotation tank 102 and into collector 114 as will be further described below. Other means for removing the algae from the surface of the growth medium can also be used as in known in the art.

Figures 3A-3D illustrate flotation tank 102 to provide an example of how the flocculated algae can be floated to the surface. Figure 3A illustrates the state of flotation tank 102 when a growth medium containing flocculated algae is passed into flotation tank 102. As stated above, prior approaches for separating algae from the growth medium are difficult, expensive, and oftentimes harmful to the algae making them unsuitable to recover algae that is intended for certain purposes. In contrast, the present invention provides a simple and safe process for recovering the algae cells. This process includes applying an electric field to the growth medium using electrodes 111, 112, and, in some cases, 113.

Figure 3C illustrates the state of flotation tank 102 after the flocculated algae cells have floated to the surface. Figure 3C also illustrates that the remaining growth medium underneath the floating clumps is substantially clear to indicate that this process is highly effective at separating the algae from the growth medium. The growth medium, which is nutrient dense, can then be reused.

Finally, Figure 3D illustrates an example of how the floating algae cells can be removed. As shown, this removal can be performed using rakes 115a, 115b which are rotated over the surface of the growth medium to rake the algae cells towards conveyor 116. Conveyor 116 is rotated to transfer the raked algae cells into collector 114 where it can be retrieved for further processing. Accordingly, this process yields a highly dewatered biomass that can be easily transported and used. Figures 3A-3D generally represent the process as being performed in batches (i.e. the entire growth medium is fully flocculated before any new algae cells are added). However, in some embodiments, this process can be performed on a continuous basis such as by periodically adding new growth media containing flocculated algae.

Gas bubble formation can be facilitated by strategically placing the electrodes in proximity to one another. For example, in some embodiments, the cathode(s) and anode(s) are spaced between about 0.1 inches and about 36 inches apart, between about 0.2 inches and about 24 inches apart, about 0.5 inches and about 12 inches apart, about 0.5 inches and about 6 inches apart, about 3 to about 8 inches apart, about 1 inch to about 3 inches apart, or variations and combinations of these ranges or values within these ranges. The ratio of separation may vary depending on the conductivity of the growth medium and/or the power levels applied to the electrodes. For example, the more saline or conductive the growth medium, the smaller the gap is required for hydrogen and/or oxygen production. In some configurations, the placement of two or more cathodes near a single anode can increase turbulence about the anode, creating a heightened mixing effect that can assist in aggregating and lifting the algae cells.

An operating voltage of between about 1 and about 30 volts, about 1 and about 24 volts, about 2 to about 18 volts, about 2 to about 12 volts, or combinations and intermediate ranges within these ranges, can be applied. For example, a voltage of about 4 volts, 6 volts, 8 volts, 10 volts, 12 volts, 14 volts, 16 volts, 18 volts, 20 volts, 22 volts, 24 volts, 26 volts, 28 volts, 30 volts, and/or combinations of these voltages or ranges encompassing these voltages can be applied. The amperage may vary and generally be between about 1 A to about 20 A, about 2 A to about 15 A, or combinations or intermediate ranges within these ranges. The actual current may reasonably vary depending on the density of the growth medium and its relative conductivity.

In some configurations, it can be desired to provide pulsed power to the electrods. To pulse power, the frequency of pulsing can be varied as can the duty cycle. In this context, the term duty cycle refers to the relative lengths of the on and off portions of each power cycle, and can be expressed, for example, as a ratio of the duration of the on portion of the cycle to the total time for the cycle, or as a ratio of the duration of the on portion of the cycle to the off portion of the cycle, or by stating the on and off durations, or by stating wither the on or off duration and the total cycle duration. Unless otherwise stated or is clear from the context, duty cycle will be stated herein as the ration of on duration to off duration for a cycle.

Accordingly, with embodiments that cycle an electromagnetic field on and off, the duty cycle can be about 1:1, about 1:1.1, about 1:1.2, about 1:1.3, about 1:1.4, about 1:1.5, about 1: 1.6, about 1:1.7, about 1:1.8, about 1:1.9, about 1:2, about 1:2.5, about 1:3, about 1:4, about 1:5, about 1:6, about 1:7, about 1:8, about 1:9, or about 1:10. Additionally, the duration of the duty cycle can be varied based upon the flow rate, volume, and/or characteristics of the growth medium.

Additional Features Or Variations

The electrodes can be made of a metal, composite, or other material known to impart conductivity, such as, but not limited to silver, copper, gold, aluminum, zinc, nickel, brass, bronze, iron, lead, platinum group metals, steel, stainless steel, carbon allotropes, and/or combinations thereof. Non-limiting examples of conductive carbon allotropes can include graphite, graphene, synthetic graphite, carbon fiber (iron reinforced), nano-carbon structures, and other form of deposited carbon on silicon substrates. In some configurations, the anode and/or the cathode can serve as a sacrificial electrode which is used in the flocculation and/or bubble generation processes. As such, electrodes can include consumable conductive metals, such as iron or aluminum.

In some embodiments, the electrodes (e.g. cathodes 105, 111, 113 and anodes 106, 112) can be comprised of a catalyst-coated metal such as iridium oxide coated titanium. Such metals can enhance the efficiency of the process. For example, by using iridium oxide coated titanium on the anode, the creation of gas bubbles can be facilitated.

Also, in some embodiments, one or more of the electrodes in flotation tank 102 can include numerous perforations or surface textures which allow the growth medium to pass through it. Such perforations and texturing provide an increase in the number of edges on the electrodes, which may facilitate bubble formation. For example, the one or more anodes may be formed as a mesh, grid, or other porous structure. The mesh may include relatively large openings that are larger than a typical clump of algae or sludge particulates in the growth medium. This configuration can advantageously allow for faster flow rates since it allows for greater interfacial contact between the growth medium and the hydrogen generated by the anode. This configuration may be advantageous when a faster flow through is desired or when conductivity of the growth medium is low. Moreover, in some embodiments, growth medium may be introduced into flotation tank 102 at the center of the anode. In this way, the growth medium will flow out one or more holes in the anode and be exposed to gas bubbles.

Although the above described apparatus 100 has shown flotation tank 102 as a separate elevated tank, it is also possible to form the flotation tank as a trench (e.g. in the ground). Using a trench can allow for the processing of greater amounts of growth medium.

In some embodiments, the efficiency of flocculating and/or floating the algae can be increased by adding a protic solvent to the growth medium. For example, the growth medium may be injected with a dilute solution of a protic solvent such as formic acid, n-butanol, isopropanol, n-propanol, ethanol, methanol, and acetic acid, such as of approximately 0.05% by volume. This solution may be mixed into the growth medium at various times. However, in some cases, it is beneficial to add the protic solvent as the electric field of the flocculation process is generated, or just before the batch process occurs.

The above-described apparatus can also be used to remove ammonia from wastewater or other fluids such as in aquaculture environments. To accomplish ammonia removal, the one or more anodes of flocculation tank 101 can be made of a titanium ruthenium alloy. By using a titanium ruthenium alloy, free chlorine is produced in the growth medium when the voltage is applied to the cathode and anode. The free chlorine allows the ammonia to be oxidized eventually resulting in conversion of the ammonia into nitrate, nitrite, and some nitrogen gas.

It has been found that a current density of between 30-50 mA/cm² of the anode is generally preferred to maximize the oxidation of the ammonia into nitrate and nitrite. However, other current densities can also be used, and the ideal density will depend on various characteristics such as the temperature of the wastewater.

Although the removal of ammonia from the wastewater is primarily performed within flocculation tank 101, in such implementations, flotation tank 102 can still be used to remove other undesirable matter from the wastewater such as organic compounds.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

What is claimed:

Claims

1. An apparatus for harvesting algae using a two-stage process, the apparatus comprising:

a flocculation tank in which the first stage of the two stage process occurs, the flocculation tank comprising a reactor tube for creating an electric field within a growth medium containing suspended algae, the electric field causing the algae to flocculate; and

a flotation tank in which the second stage of the two stage process occurs, the flotation tank comprising a tank containing a plurality of electrodes which cause the formation of gas bubbles which attach to the flocculated algae and lift the flocculated algae to the surface of the growth medium, the flotation tank being connected to the flocculation tank to allow the flocculated algae to flow from the flocculation tank into the flotation tank.

2. The apparatus of claim 1, wherein the flocculation tank includes an outer cylinder forming a cathode and an inner cylinder contained within the other cylinder, the inner cylinder comprising an anode.

3. The apparatus of claim 2, wherein a pulsed voltage is applied to the cathode and anode to cause algae cells within the growth medium to flocculate.

4. The apparatus of claim 2, wherein the growth medium is pumped through the flocculation tank at a specified flow rate.

5. The apparatus of claim 1, wherein the electrodes of the flotation tank comprise a first cathode layer a second cathode layer and an anode layer positioned between the first and second cathode layers.

6. The apparatus of claim 5, wherein the anode layer is spaced between 1 inch and 10 inches from each cathode layer.

7. The apparatus of claim 5, wherein the first cathode layer comprises a cathode plate.

8. The apparatus of claim 5, wherein the anode layer comprises a plurality of spaced rods.

9. The apparatus of claim 5, wherein the second cathode layer comprises a plurality of spaced rods.

10. The apparatus of claim 5, wherein the first cathode layer comprises a plurality of spaced rods.

11. The apparatus of claim 8, wherein each rod of the anode layer comprises iridium oxide coated titanium.

12. The apparatus of claim 8, wherein each rod of the anode layer includes one or more openings.

13. The apparatus of claim 8, wherein each rod comprises a mesh.

14. The apparatus of claim 1, wherein the gas bubbles comprise hydrogen and oxygen.

15. The apparatus of claim 1, wherein the flotation tank includes a conveyor having one or more rakes for raking the algae at the surface of the growth medium.

16. The apparatus of claim 15, wherein the flotation tank includes a second conveyor to which the algae is raked, the second conveyor lifting the algae from the flotation tank to a collector.

17. A method of harvesting algae using a two-stage process, the method comprising:

supplying a growth medium containing suspended algae into a flocculation tank, the flocculation tank comprising a reactor tube for creating an electric field within the growth medium, the electric field causing the algae to flocculate;

transferring the growth medium containing flocculated algae into a flotation tank, the flotation tank comprising a tank containing a plurality of electrodes which cause the formation of gas bubbles which attach to the flocculated algae and lift the flocculated algae to the surface of the growth medium; and

removing the floating algae from the surface of the growth media.

18. The method of claim 17, wherein the floating algae is removed using one or more rakes.

19. The method of claim 17, further comprising:

extracting gas from the removed algae.

20. An apparatus for removing ammonia from a fluid, the apparatus comprising:

a reactor tube for creating an electric field within a fluid containing ammonia, the reactor tube including a cathode and an anode, the anode comprising a titanium ruthenium alloy, wherein when the electric field is created, the anode causes the creation of free chlorine within the fluid leading to the oxidation of the ammonia into nitrite and nitrate; and

a flotation tank, connected to the reactor tube, the flotation tank comprising a tank containing a plurality of electrodes which cause the formation of gas bubbles.