WO2014022689A1

WO2014022689A1 - Photobioreactor system for removal of nitrogen and phosphorus

Info

Publication number: WO2014022689A1
Application number: PCT/US2013/053266
Authority: WO
Inventors: John C. Mcardle; Sherry K. BLIGHT; Mick F. ARTHUR; Micah P. MCCREERY; Stephanie A. Smith; Randy L. Jones
Original assignee: Battelle Memorial Institute
Priority date: 2012-08-03
Filing date: 2013-08-01
Publication date: 2014-02-06

Abstract

Methods and systems for removing phosphorus and nitrogen from a wastewater stream are disclosed. The wastewater stream is fed to a photobioreactor containing phosphate-accumulating microorganisms that are present as activated sludge or as fixed biofilms. Under light and anaerobic conditions, phosphorus will be captured by the microorganisms. Denitrification bacteria in the photobioreactor also convert nitrate to nitrous oxide and nitrogen gas. Denitrification produces alkalinity that can be used in a downstream aerobic nitrifying reactor. The resulting mixed liquor stream from the photobioreactor is then sent to an aerobic reactor, where different bacteria convert ammonia to nitrite and nitrate, increasing acidity. The mixed liquor stream from the aerobic reactor is separated into sludge that can be used as fertilizer and supernatant that has been reduced in nitrogen and phorphorus. Some of the sludge can be recycled to the photobioreactor and to the aerobic reactor.

Description

PHOTOBIOREACTOR SYSTEM FOR REMOVAL OF NITROGEN AND

PHOSPHORUS

BACKGROUND

[0001] This application claims priority to U.S. Provisional Patent Application Serial No. 61/679,269, filed August 3, 2012. The contents of that application are hereby fully incorporated by reference herein.

[0002] The present disclosure relates to a photobioreactor system for removing nitrogen and phosphorus from a liquid or fluid. The system uses bacteria to capture phosphorus and convert the nitrogen into a gas. Depending on the preferred operation, either one separate photobioreactor, or one photobioreactor and one aerobic reactor connected to each other, are applicable for this disclosure.

[0003] Inadequate nutrient discharge management from both point and non-point sources has significantly contributed to the degradation of surface water bodies. The U.S. Environmental Protection Agency (EPA) estimates that more than 100,000 miles of rivers and streams, 2.5 million acres of lakes, reservoirs and ponds, and 800 square miles of bays and estuaries are impaired due to high levels of nitrogen and phosphorus.

[0004] There is a growing recognition of the benefits of treating wastewater as a sustainable resource for water, energy, and nutrients rather than as a waste stream for treatment and discharge. For example, it is now common practice to treat municipal wastewater to a level that it can be used as an indirect source of drinking water or to augment local irrigation. As another example, anaerobic digester (AD) technologies are used to produce methane, a renewable energy source, from municipal biosolids and livestock waste. AD is a biological process that converts organic matter in manure into valuable biogas (e.g. methane), a renewable energy that can be used for heating or converted to electricity or compressed natural gas (CNG), which can be used as an alternative motor vehicle fuel. AD releases a large portion of the phosphorus from the manure into an aqueous effluent, leaving behind a solid material that can be used for animal bedding or as a soil amendment. In some locations, sufficient agricultural land area is available to apply this phosphorus rich effluent at beneficial agronomical rates, thereby substituting for and reducing the need for petroleum based chemical fertilizer. However, in locations where land availability is limited or the available land is already saturated with nutrients, phosphorus must be extracted from the effluent as a concentrated product for transport to another location for beneficial land application.

[0005] Controlling nutrient discharges to protect water quality and recover bodies of surface water for their designated use is an increasingly important driver of environmental regulations. Phosphorus, in particular, has come under increasingly stringent discharge restrictions because of its association with harmful algal blooms and eutrophication that cause severe degradation of water bodies.

[0006] In addition, nutrients, and specifically phosphorus, have significantly increased in value over the past few years, providing economic incentive to recover these values. For example, the price of phosphorus fertilizer has increased by more than 3 fold over the past ten years. Global production of fertilizer has increased from less than 10 million tons in 1950 to 80 million tons in 1990 and is projected to increase to 135 million ton by 2030. To meet this demand, large quantities of phosphorus- containing minerals are mined and converted to fertilizer. This is an ultimately unsustainable practice as there is a limited supply of such phosphate-containing minerals and continued mining will eventually result in a global shortage.

[0007] Increasing regulations to reduce nutrient discharge from municipal and livestock wastewater operations, as well as increased economic incentives to recover valuable nutrients, have resulted in a market need for more economical nutrient control technologies that are capable of reducing effluent phosphate and nitrogen concentrations to very low levels.

BRIEF DESCRIPTION

[0008] Described in various embodiments are devices for and methods of removing phosphorus and nitrogen from a wastewater stream. A photobioreactor uses phosphate-accumulating microorganisms in the form of natural or artificial biofilms or in the form of a suspended solid. Upon exposure to light and anaerobic conditions, such microorganisms can capture phosphorus and build other potentially valuable products. Denitrifying bacteria also convert nitrates to nitrous oxide or nitrogen gas, separating nitrogen from the wastewater stream. The mixed liquor stream from the photobioreactor can then be sent to an aerobic reactor which converts ammonia into nitrite or nitrate that can be recycled to the photobioreactor. Phosphates are captured in the sludge recovered from the anaerobic reactor and/or the aerobic reactor. The resulting supernatant has a reduced concentration of phosphorus and nitrogen.

[0009] Disclosed in various embodiments is a method of removing phosphorus and nitrogen from a wastewater stream, comprising: feeding the wastewater stream to an internal volume of a photobioreactor; exposing phosphate-accumulating microorganisms and denitrification bacteria in the internal volume to light under anaerobic conditions to capture phosphorus, convert nitrate in the wastewater stream to nitrous oxide or nitrogen gas, and form a mixed liquor stream; and separating the mixed liquor stream into sludge and supernatant, the supernatant containing a reduced amount of phosphorus and nitrogen. Optionally, a portion of the mixed liquor stream or the sludge can be recycled back to the photobioreactor.

[0010] The light in the photobioreactor can be provided using light sources that emit only at wavelengths between 700 nm and 950 nm. The light sources can be light emitting diodes. In some embodiments, the light is provided by light sources that are located throughout the internal volume so that no location in the internal volume is greater than a threshold distance from a light source.

[0011] The phosphate-accumulating microorganisms can be purple non-sulfur bacteria, such as Rhodopseudomonas palustris, Rhodobacter sphaeroides, Rhodobacter capsulatus, or Rhodospirillum rubrum. The phosphorus can be captured in the form of phosphate granules. The purple non-sulfur bacteria may also produce a polyhydroxyalkanoate.

[0012] In other embodiments, the light is provided by a light source located within a light pipe that extends from a first end of the internal volume towards a second end of the internal volume. Radial fins can extend radially from a sidewall of the light pipe. Circumferential fins can extend from the radial fins. Thus, different surface arrangements can be provided for natural biofilms to grow.

[0013] The phosphate-accumulating microorganisms may be immobilized in a matrix to form a synthetic biofilm. In embodiments, the matrix is alginate, and is in the shape of beads. The phosphate-accumulating microorganisms can alternatively be in the form of natural biofilms on a packing material in the internal volume. The packing material may be in the form of particles, sheets, or hollow fibers. In yet other variations, the phosphate-accumulating microorganisms and denitrification bacteria are in the wastewater stream.

[0014] The phosphate-accumulating microorganisms can be in the photobioreactor in the form of a suspended biomass.

[0015] The photobioreactor may be at an ambient temperature range of 27°C to 34°C. Nitrogen sparging of the internal volume of the photobioreactor may be used to maintain anaerobic conditions. The internal volume of the photobioreactor can be from about 1 liter to about 10,000 liters, or can be 5,000 liters or greater, or can be 10,000 liters or greater.

[0016] The residence time of liquids in the photobioreactor may be from about 1 minute to about 1 hour, or even longer. The residence time of sludge in the photobioreactor may be from about one day to about 10 days, or even longer.

[0017] The pH of the mixed liquor stream exiting the photobioreactor can be from about 7.0 to about 8.5. Oxygen gas may be present in the photobioreactor in an amount of 0.5 mg/L or less.

[0018] The mixed liquor stream from the photobioreactor may in some embodiments be sent to an aerobic reactor, where nitrifying bacteria convert ammonia in the mixed liquor stream into nitrite or nitrate. The mixed liquor stream is separated into sludge and supernatant downstream of the aerobic reactor.

[0019] The nitrifying bacteria in the aerobic reactor can be Nitrosomonas or Nitrobacter bacteria. The oxygen gas may be present in the aerobic reactor in an amount of about 0.5 mg/L or greater. The alkalinity produced in the photobioreactor can be used to help control the pH of material in the aerobic reactor. In particular forms, the aerobic reactor is a sequencing batch reactor. The pH of the mixed liquor stream exiting the aerobic reactor can be from about 7.5 to about 8.5.

[0020] The mixed liquor stream is then separated into sludge and supernatant in a clarifier downstream of the aerobic reactor. A portion of the sludge can be recycled back to the photobioreactor or to the aerobic reactor. The clarifier can be a settling tank, a filter press, or a centrifuge. [0021] Alternatively, the mixed liquor stream is separated into sludge and supernatant in a clarifier directly downstream of the photobioreactor. Again, the clarifier can be a settling tank, a filter press, or a centrifuge.

[0022] The phosphate-accumulating microorganisms and the denitrifying bacteria can be independently added as an inoculum to the internal volume of the photobioreactor, or be present in the wastewater stream fed to the internal volume of the photobioreactor.

[0023] Also disclosed herein in different embodiments is a system for removing phosphorus and nitrogen from a wastewater stream, comprising a photobioreactor, an aerobic reactor, a clarifier, and a return loop. The photobioreactor comprises: an outer casing, a first end wall, and a second end wall that define an internal volume; a fluid inlet, a fluid outlet, a gas inlet, and a gas outlet connecting to the internal volume; and a plurality of light sources distributed throughout the internal volume. The aerobic reactor comprises: a reactor inlet, an air inlet, and a recycle inlet connecting to a mixing tank; wherein the reactor inlet connects to the fluid outlet of the photobioreactor. The recycle inlet is used for recycling material back to the photobioreactor. The clarifier separates a mixed liquor stream into sludge and supernatant. The return loop recycles mixed liquor from the aerobic reactor back to the photobioreactor.

[0024] The aerobic reactor can be a sequencing batch reactor, where the clarifier is part of the aerobic reactor. Alternatively, the clarifier can be a separate settling tank, filter press, or centrifuge downstream of the aerobic reactor.

[0025] The plurality of light sources in the photobioreactor can be distributed so that no location in the internal volume is greater than a threshold distance away from a light source. The light sources sometimes emit only at wavelengths between 700 nm and 950 nm. The light sources can be light emitting diodes, or can be fiber optic pads that transmit light from an external source.

[0026] In various embodiments, the light sources are located within light pipes that extend from one end wall into the internal volume towards the other end wall. Radial fins can extend radially from a sidewall of the light pipe. Circumferential fins can extend circumferentially from the radial fins. [0027] Synthetic biofilms can be placed within the internal volume of the photobioreactor. The synthetic biofilms can be made of phosphate-accumulating microorganisms immobilized in a matrix. The phosphate-accumulating microorganisms can be purple non-sulfur bacteria, such as Rhodopseudomonas palustris, Rhodobacter sphaeroides, Rhodobacter capsulatus, or Rhodospirillum rubrum. The matrix can contain alginate. The synthetic biofilms can be in the shape of beads.

[0028] The gas inlet and the fluid inlet may be located in the second end wall, and the fluid outlet can be located in the first end wall. Alternatively, the gas inlet can be located at a lower end of the photobioreactor.

[0029] The outer casing can be opaque. The system can further comprise a mechanical agitator and/or a floor above the second end wall. The internal volume may have a volume of from about 1 liter to about 10,000 liters, or a volume of 5,000 liters or greater. The system can further comprise a mesh container for placement within the internal volume.

[0030] The internal volume of the photobioreactor can contain a packing material. The packing material can be in the form of particles, sheets, or hollow fibers.

[0031] The air inlet of the aerobic reactor can be connected to an air pump.

[0032] These and other non-limiting aspects and/or objects of the disclosure are more particularly described below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The following is a brief description of the drawings, which are presented for the purposes of illustrating the exemplary embodiments disclosed herein and not for the purposes of limiting the same.

[0034] FIG. 1 is a diagram illustrating a synthetic biofilm in the shape of a bead or sphere.

[0035] FIG. 2 is a picture showing the synthetic biofilm in the form of spheres. The spheres are made from microorganisms and alginate.

[0036] FIG. 3 is a cross-sectional view of an exemplary embodiment of a photobioreactor of the present disclosure. This embodiment includes multiple light pipes. [0037] FIG. 4 is a cross-sectional view of another exemplary embodiment of a photobioreactor of the present disclosure. This embodiment has a single light pipe with multiple fiber optic pads.

[0038] FIG. 5 is a cross-sectional view of another exemplary embodiment of a different photobioreactor, where a fiber optic pad is used as the light source.

[0039] FIG. 6 is a cross-sectional view of the embodiment of FIG. 5, but with the encapsulated bacteria beads placed in the internal volume.

[0040] FIG. 7 is another embodiment of a light pipe having radial fins.

[0041] FIG. 8 is a top view of the light pipe of FIG. 7.

[0042] FIG. 9 is another embodiment of a light pipe having radial fins and circumferential fins.

[0043] FIG. 10 is a top view of the light pipe of FIG. 9.

[0044] FIG. 11 is a diagram illustrating one system for practicing the methods of the present disclosure.

[0045] FIG. 12 is a diagram illustrating another system for practicing the methods of the present disclosure.

DETAILED DESCRIPTION

[0046] A more complete understanding of the processes and apparatuses disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the existing art and/or the present development, and are, therefore, not intended to indicate relative size and dimensions of the assemblies or components thereof.

[0047] Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.

[0048] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. [0049] Numerical values in the specification and claims of this application should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.

[0050] All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of "from 2 grams to 10 grams" is inclusive of the endpoints, 2 grams and 10 grams, and all the intermediate values). The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value; they are sufficiently imprecise to include values approximating these ranges and/or values.

[0051] As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about" and "substantially," may not be limited to the precise value specified, in some cases. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. The modifier "about" should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression "from about 2 to about 4" also discloses the range "from 2 to 4."

[0052] It should be noted that many of the terms used herein are relative terms. For example, the terms "inlet" and "outlet", or "upstream" and "downstream", are relative to a direction of flow, and should not be construed as requiring a particular orientation or location of the structure. Similarly, the terms "upper" and "lower" are relative to each other in location, i.e. an upper component is located at a higher elevation than a lower component.

[0053] The present disclosure relates to methods and systems that can be used to remove phosphorus and nitrogen from a wastewater stream. Generally, phosphate- accumulating microorganisms (PAOs) and denitrifying bacteria in the internal volume of a photobioreactor are used to capture phosphorus and to convert nitrates into nitrous oxide or nitrogen gas. The PAOs can capture phosphorus in the presence of light and under anaerobic conditions. The mixed liquor from the photobioreactor is then sent to an aerobic reactor, where ammonia in the mixed liquor is converted to nitrite/nitrate (ΝΟ₂7Νθ₃ ^') that can be sent back to the photobioreactor if desired. The phrase "mixed liquor" as used herein refers to the aqueous stream containing biosolids and other solid materials that exists from a bioreactor and which can be subsequently separated into a supernatant aqueous phase and a sludge solids phase. The mixed liquor is separated into sludge and supernatant. The sludge can be recycled to the photobioreactor, disposed of as waste, or used as a fertilizer.

[0054] The phosphate-accumulating microorganisms (PAOs) can arrive in the photobioreactor in at least three different ways. First, the PAOs may be placed in the photobioreactor as artificial or synthetic biofilms of a given shape. Second, the PAOs may be placed in the photobioreactor as a suspended biomass. This assisted introduction of the PAOs into the photobioreactor can be referred to as inoculation, or as adding an inoculum. Finally, the photobioreactor may contain a packing material. PAOs naturally present in the wastewater stream can adhere to the packing material and form natural biofilms. Depending on the condition of the biomass, the photobioreactor (PBR) can alternatively be referred to as an activated sludge reactor (ASR) if the biomass is in suspension or as a fixed film reactor (FFR) if the biomass is in a biofilm.

[0055] The phosphate-accumulating microorganisms (PAOs) may be placed in the photobioreactor as artificial or synthetic biofilms of a given shape. Generally speaking, the microorganisms are immobilized in a matrix, which can then be shaped as appropriate (e.g. beads). This both improves handling of the components and increases phosphorus capture.

[0056] The term "biofilm" refers to an aggregate of microorganisms that are embedded within a self-produced matrix of an extracellular polymeric substance. This extracellular polymeric substance generally contains extracellular DNA, proteins, and polysaccharides. The photobioreactor uses artificial biofilms composed of microorganisms immobilized in a matrix. The terms "artificial" and "synthetic" are used interchangeably to indicate that the matrix in which the microorganisms are embedded or immobilized is made of a material that is not naturally produced by the microorganisms. Conversely, the term "natural" is used to indicate that the matrix in which the microorganisms are embedded or immobilized is self-produced.

[0057] The phosphate-accumulating microorganisms used in the photobioreactor to capture phosphorus are purple non-sulfur (PNS) bacteria. These bacteria are metabolically versatile organisms that grow on a variety of carbon substrates (e.g. CO, CO₂, hydrocarbons) under aerobic or anaerobic conditions. They are typically pigmented with bacteriochlorophyll a or b, together with other carotenoids, and may have colors ranging between purple, red, brown, and orange. During photosynthesis, hydrogen is typically used as the reducing agent.

[0058] The microorganisms in the synthetic biofilms used with the photobioreactor are phosphate-accumulating microorganisms (PAOs). They incorporate phosphorus into their biomass during growth, just like all bacteria, but also sequester large amounts of phosphorus into polyphosphate storage granules. Polyphosphate accumulates in the cells of PAOs when the organism is not limited for energy. In particular, PNS photoheterotrophic microorganisms can accumulate polyphosphate granules under anaerobic conditions in the presence of light (i.e. photoheterotrophic growth). Some examples of suitable bacteria include Rhodopseudomonas palustris, Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodospirillum rubrum. It should be noted that some PNS bacteria can also produce and sequester polyhydroxyalkanoate (PHA) plastics when exposed to excess amounts of sugar substrate. This can also be a valuable byproduct.

[0059] The microorganisms can be immobilized in a matrix (i.e. encapsulated) to form the synthetic biofilm. The matrix is made of a material that is both gas-permeable and water-permeable. In particular embodiments, the matrix can be described as a gel. An exemplary material suitable for the matrix is alginate, which is also known as algin or alginic acid (CAS# 9005-32-7), is commercially available, and can absorb a large quantity of water. Other exemplary materials suitable for the matrix include sol-gel silica, carrageenan, latex, polyvinyl alcohol, polystyrene sulfonate, and mixtures of these materials (including with alginate). Sol-gel silica refers to using silica in a sol-gel procedure to obtain a three-dimensional network containing both a liquid phase and a solid phase. Exemplary silicates include tetraethyl orthosilicate (TEOS). The microorganisms can be contained within the network. Carrageenan refers to polysaccharides which can gel, and which can be obtained from seaweed. Other materials, which are both gas-permeable and water-permeable, may also be used to immobilize the microorganisms.

[0060] The synthetic biofilm aids in retaining structural integrity and bacterial viability. The cell density, cell placement, and consistency of the synthetic biofilm can also be controlled. The synthetic biofilm can be formed quickly, without the need for a long incubation period while waiting for a natural biofilm to be created. In addition, the microorganisms are immobilized in the biofilm instead of living in the growth medium that typically circulates within the bioreactor vessel. The synthetic biofilm can also be shaped into any desired configuration or shape. In particular embodiments, the synthetic biofilms used in the photobioreactor are in the shape of spheres. One advantage to using such synthetic biofilms is that they provide an easy way to collect the desired products. Such products may accumulate within the spheres, and can be collected by removal of the spheres from the photobioreactor.

[0061] FIG. 1 is a diagram illustrating an artificial biofilm in the form of a sphere. This diagram is not drawn to scale. The sphere 100 is made up of a matrix 110 in which microorganisms 120 are immobilized. It is contemplated that the microorganisms can be either homogeneously dispersed throughout the volume of the sphere, or can be concentrated in the center of the sphere. The sphere may have a diameter 130 of from about 0.1 millimeters (mm) to about 20 mm, including in specific embodiments a diameter of from about 1.5 mm to about 2.5 mm.

[0062] FIG. 2 is a picture of a test tube containing purple non-sulfur (PNS) bacteria encapsulated into spheres containing 2% alginate. This value refers to the final percentage of alginate (w/v) in the sphere that is mixed with the bacteria. The spheres are made dropwise and fall into a 100 mM CaCI₂ solution that solidifies the drops. In specific embodiments, the artificial biofilms used in the photobioreactor are PNS bacteria encapsulated in an alginate matrix and shaped as spheres.

[0063] Materials for making the various components of the artificial biofilm as disclosed herein are known in the art, as are methods for making the artificial biofilm itself. [0064] The phosphate-accumulating microorganisms (PAOs) can alternatively be placed in the photobioreactor as a suspended biomass. In this regard, the suspended biomass can be activated sludge which has been previously treated to remove excess water. Sludge refers to semi-solid material which generally contains a large number of microorganisms.

[0065] Lastly, the photobioreactor may contain a packing material. PAOs naturally present in the wastewater stream can adhere to the packing material and form natural biofilms. The packing material can be, for example, in the form of particles, sheets, or hollow fibers. The packing material can be made of natural or synthetic materials, and can be fixed in place or permitted to float within the internal volume of the photobioreactor. The packing material provides high surface area for PAOs to adhere to.

[0066] The photobioreactor exposes the phosphate-accumulating microorganisms (PAOs) to appropriate conditions which encourage them to capture phosphorus. The photobioreactor can used to remove phosphorus from waste water and retain it for use in other applications, for example as plant fertilizer. The photobioreactor also contains denitrifying bacteria that also operate under anaerobic conditions to convert nitrate to nitrous oxide (N₂O) and nitrogen gas (N₂). It should be noted that the nitrous oxide and nitrogen gas also act to maintain the anaerobic conditions. The amount of oxygen gas (O₂) in the photobioreactor should be 0.5 mg/L or less.

[0067] FIG. 3 is a cross-sectional view of an exemplary embodiment of a photobioreactor 300 of the present disclosure. The photobioreactor includes an outer casing 310, a first end wall 320, and a second end wall 330. An internal volume 340 is present within the photobioreactor, in which the phosphate-accumulating microorganisms (PAOs) and denitrifying bacteria are located. The internal volume will vary in size depending on its application. For example, a bench scale photobioreactor may be as small as from about 1 liter to about 1.5 liters. However, commercial photobioreactors may have an internal volume of 5,000 liters or greater, or 10,000 liters or greater. A plurality of light sources (not shown) are located within the internal volume, and are distributed so that no location in the internal volume is greater than a threshold distance away from a light source. The photobioreactor may be considered as having a lower end 302 and an upper end 304. Here, the first end wall and the second end wall can be described as being disc-shaped.

[0068] The outer casing is formed from an outer wall 312 and has a radial lip 314 at each end. The outer wall 312 may be of any shape, and here is cylindrical. The outer casing 310 is joined to the first end wall 320 and the second end wall 330 at each radial lip 314. It is contemplated that the outer casing is opaque, or in other words that light does not need to penetrate the outer casing to illuminate the internal volume. This allows a wider range of materials to be used for constructing the outer casing. However, if desired, the outer casing may be design to allow for the penetration of certain wavelengths of light to enter the internal volume. In particular embodiments, the inner surface 316 of the outer casing is reflective.

[0069] A gas inlet 342, a fluid inlet 344, and a fluid outlet 346 are also present leading to the internal volume 340. Inert gas (usually nitrogen) can be sparged to maintain anaerobic conditions within the internal volume. The gas can also be used to mix the solid materials (artificial biofilms/beads, biomass, packing material) within the internal volume with the wastewater. The wastewater is rich in phosphorus, and the dwell time in the photobioreactor is controlled to permit the PAOs to capture the phosphorus. The exiting sludge stream has a greatly reduced concentration of phosphorus compared to the incoming wastewater.

[0070] The gas inlet 342 is usually present at the lower end 302 of the photobioreactor so that gas can passively rise through the beads and fluid in the internal volume. In particular embodiments, the gas inlet 342 is in the second end wall 330. The gas inlet may be on the central axis 305 of the photobioreactor. The fluid inlet 344 and fluid outlet 346 are generally located at opposite ends of the photobioreactor, and on opposite sides of the photobioreactor as well. In particular embodiments, the fluid inlet 344 is in the second end wall 330 and the fluid outlet 346 is in the first end wall 320. This configuration generally ensures that the sludge stream exiting the internal volume 340 has achieved the desired residence time needed for phosphorus capture to occur. Filters may be placed upstream of the gas inlet and fluid inlet to remove solid particles and prevent blockage within the photobioreactor. [0071] A plurality of light sources is present in the internal volume of the photobioreactor to provide internal illumination. Many conventional systems are illuminated with broad-spectrum light, such as incandescent or fluorescent bulbs or sunlight. Broad-spectrum light includes many wavelengths which are usually not used by the microorganisms, and also generates excess heat. This wasted energy increases operating costs for the system. It is contemplated that the photobioreactors of the present disclosure use light sources that produce only the wavelengths of light that are needed for the microorganism of choice, significantly decreasing input energy costs. In specific embodiments, the light sources emit only at wavelengths between 700 nm and 950 nm. In this regard, bacteriochlorophyll pigments of purple non-sulfur bacteria are excited around 850 nm. This provides the required light for culturing and maintaining the bacteria as well as maximizing phosphorus removal. In more particular embodiments, the light sources emits only light at wavelengths between 700 nm and 950 nm and in a width of 100 nm (for example from 800 nm to 900 nm) or a width of 50 nm.

[0072] The light sources can generally be any type of illumination. In particular embodiments, it is contemplated that the light sources are light emitting diodes (LEDs). In other embodiments, the light sources are fiber optic pads. A fiber optic pad contains individual plastic fibers that transmit light from an external source. The fibers are woven into a mat that produces a pad which is lighted over the entirety of either one side of the pad or both sides of the pad. The fiber optic pad channels light from the external source into the internal volume of the photobioreactor.

[0073] In FIG. 3, the light sources are placed within light pipes 350. Here, two of four light pipes are visible. The light pipes extend from the first end wall 320 into the internal volume towards the second end wall 330. It should be noted that there is usually a gap 352 between the end or cap 354 of the light pipe and the second end wall. This facilitates separating only one of the end walls during operation of the photobioreactor. The light pipe itself is formed from a sidewall 356 that surrounds a pipe volume 358 and is capped at its distal end. This construction encases the light sources so that they do not contact the bacteria-containing beads, any fluids, or any gases in the internal volume. The artificial biofilms/beads are present in the interstitial spacing between the outer casing and the light pipes. The sidewall and cap may be made of a transparent material, or may be used to filter the light so that only wavelengths between 700 nm and 950 nm reach the internal volume.

[0074] The light sources can be distributed throughout the internal volume such that no location in the internal volume is greater than a threshold distance away from a light source. In this regard, testing has shown that even in pure water, light at the wavelengths of interest (700-950 nm) only penetrates a distance of 1.5 cm. Penetration depth decreases rapidly as a function of turbidity, and it is likely that the phosphorus- containing waste water will be turbid. The light sources are thus distributed so that the provided light penetrates substantially the entire internal volume. For example, in embodiments, no location in the internal volume is greater than 1 meter, or 10 cm, or 5 cm, or 2 cm, or 1.5 cm, or 1 cm away from a light source. This distance will also affect the number of light sources, and can be varied for cost purposes as well.

[0075] It should also be noted that a floor 332 is present above the second end wall. The floor is constructed to permit the fluid and gas to flow through the floor, but prevent the inlets from being blocked by the material in the internal volume. For example, the floor may be a mesh.

[0076] A mesh container (not shown) may be placed within the internal volume. The synthetic biofilms/beads, biomass, or packing material can be loaded into the mesh container and then placed into the photobioreactor. These components are generally denser than water and so should remain at the bottom of the reactor, i.e. in the mesh container. This would enable more convenient removal of the components from the internal volume, without the need to also empty the photobioreactor of any fluid. In this regard, an end wall can be removed to permit access to the internal volume and the mesh container. The mesh container would be shaped as appropriate to avoid the light pipes. As previously mentioned, the internal volume may range from about 1 liter to about 10,000 liters, or higher as desired. The artificial biofilms/beads, biomass, or packing material may take up a volume of from about 40% to about 80% of the internal volume of the photobioreactor.

[0077] It may be desirable for the phosphate-accumulating microorganisms (PAOs) to be moved while they are present in the internal volume. For example, it may be beneficial to move them past the light sources so that they can be more evenly irradiated, as well as to encourage better mixing with the phosphorus-containing wastewater stream. This agitation can be accomplished, for example, by bubbling the gas through the internal volume and the fluid. In other embodiments, a mechanical agitator (not shown) can be used to move the solids. For example, an Archimedean screw (a helical surface surrounding a shaft) could be located within the internal volume. If present, the mechanical agitator should be located so as not to contact the light sources. Also not shown is a pressure valve which may be present to release any gas from the internal volume. For example, the valve may be used to release the nitrous oxide / nitrogen gas produced by the denitrifying bacteria.

[0078] It should be noted that while FIG. 3 illustrates the use of multiple light pipes, it is possible that only one light pipe can be present, with the light pipe simply having a larger diameter such that the distance between the sidewall of the light pipe and the sidewall of the outer casing is optimum. For example, the sidewall of the outer casing could be made of a material that permits light to penetrate into the internal volume. FIG. 4 illustrates one such embodiment having a single light pipe 350 containing the plurality of light sources. Here, the outer wall of the outer casing is removed. This embodiment depicts both LEDs 360 and fiber optic pads 370 being used to provide light. In addition, the light pipe in this embodiment extends completely between the first end wall and the second end wall, so that the artificial biofilms/beads would be located in an annular space.

[0079] FIG. 5 is a cross-sectional view of a different embodiment of the photobioreactor. Compared to FIG. 3, this figure illustrates a version where a fiber optic pad 370 is used as the light source. A port 372 is present in the first end wall 320 to permit a cord 374 to travel from the external source (not shown) to the pad. Also, the nitrogen sparger is depicted as traveling throughout the internal volume, rather than simply below the floor as in FIG. 3. FIG. 6 is a cross-sectional view of the embodiment of FIG. 5, but with the encapsulated bacteria beads 380 placed in the internal volume 340.

[0080] Although described above with reference to synthetic biofilms/beads, it is contemplated that the photobioreactors described herein can also be used to distribute light to naturally occurring biofilms. In particular, some embodiments are contemplated in which the internal volume includes additional surfaces on which bacteria can adhere and develop as a natural biofilm.

[0081] FIG. 7 and FIG. 8 show one such embodiment. FIG. 7 is a perspective view, and FIG. 8 is a plan view. In this embodiment, radial fins 392 extend radially from the sidewall 356 of the light pipe 350. The outer casing 310 is also shown.

[0082] FIG. 9 and FIG. 10 show another such embodiment. FIG. 9 is a perspective view, and FIG. 10 is a plan view. Here, radial fins are present as in FIG. 7. Circumferential fins 394 also extend from the radial fins 392. These radial fins and circumferential fins provide additional surface area upon which bacteria can adhere and form biofilms. This construction permits the fins to be removed from the internal volume so that the bacteria, phosphate granules, and possibly the bioplastics can be harvested.

[0083] The wastewater stream that is fed into the photobioreactor can be municipal wastewater, industrial wastewater, high or low strength wastewater, or effluent from an anaerobic digester (centrate or filtrate). These generally differ in the types of materials or the concentrations of the materials in the wastewater.

[0084] In operation, the phosphate-accumulating microorganisms (PAOs) are mixed with the wastewater stream in the internal volume of the photobioreactor. The light sources are activated to provide energy so that the PAOs perform their desired function under photosynthetic conditions. The denitrifying bacteria also convert the nitrates to nitrous oxide and nitrogen gas to remove nitrogen from the wastewater. The waste water provides the necessary nutrients to the PAOs. Carbon dioxide (CO₂) may also be produced. Nitrogen gas is sparged into the internal volume as needed to obtain and maintain anaerobic conditions within the photobioreactor. The ambient temperature within the photobioreactor (i.e. in the internal volume) is from 27°C to 34°C. The fluids may have a hydraulic residence time in the photobioreactor of from about 1 minute to about 1 hour, but can also be longer than 1 hour. The sludge (not the PAOs) in the photobioreactor may have a residence time in the photobioreactor of from about 1 day to about 10 days, but can also be longer than 10 days. Generally, the residence time for each phase (liquid, solid) is determined by the requirements of the overall system. [0085] The process steps in the photobioreactor are envisioned as follows. First, the organisms will be grown in the photobioreactor to the stationary phase of growth, using sunlight or artificial illumination for energy and deriving nutrients from the wastewater. With continuous illumination for an estimated two days, the organisms will accumulate both polyphosphates and possibly bioplastics like polyhydroxyalkanoates (PHA). After this time it is expected that the organisms, which will have sequestered a great deal of phosphorus while also producing PHA granules, may begin to slough from the photobioreactor because they have reached maximum biofilm densities possible. Illumination will be discontinued, and the majority of the organisms will be removed from the bioreactor. The wastewater is sent downstream for further processing. Meanwhile, within the bioreactor, enough organisms will have been retained to "reseed" the reactor, meaning that the organisms will enter into a second cycle of illumination and photo heterotrophic growth, followed by collection of PHA-containing biomass that results. The biomass also contains phosphorus, and could be used for fertilizer.

[0086] With continuous operation, it is predicted that as much as 90% of the phosphorus may be removed from the wastewater. Efficiency of removal will depend upon incoming phosphorus concentrations and other conditions of operations, which may include coupling of the photobioreactor with existing biological phosphorus removal processes. A significant advantage of this approach can be the addition of a high-value product stream to the treatment process, through the extraction of the PHA. The photobioreactor can also be used in a polishing step at the end of the wastewater treatment process, when the wastewater contains for example only 1 to 2 ppm of phosphorus. The photobioreactor can reduce the concentration of phosphorus even further.

[0087] Next, the mixed liquor from the photobioreactor can be sent directly to a clarifier that separates the mixed liquor into a supernatant and a sludge stream.

[0088] Alternatively, the mixed liquor from the photobioreactor, which contains supernatant and sludge, is sent to an aerobic reactor. The mixed liquor will also contain phosphate-accumulating microorganisms (PAOs) that have accumulated phosphate storage granules in their cells. It should be noted that the denitrification process that occurs in the photobioreactor produces alkalinity that increases the pH of the mixed liquor in the anaerobic reactor. For efficient denitrification, pH of the photobioreactor is maintained at about 7.0 to about 8.5.

[0089] The aerobic reactor differs from the photobioreactor by the presence of oxygen. The amount of oxygen gas (0₂) in the aerobic reactor should be at least 0.5 mg/L, and is usually 1 .0 mg/L or higher. The aerobic conditions select for bacteria that sequentially convert ammonia to nitrite and nitrate. Such bacteria include Nitrosomonas and Nitrobacter. This nitrification process produces acidity that decreases the pH of the mixed liquor in the aerobic reactor. For efficient nitrification, pH of the aerobic reactor is maintained at about 7.5 to about 8.5. Alkalinity produced in the photobioreactor is available to help control the pH of the material in the downstream nitrification (i.e. aerobic) reactor. The nitrate is recirculated back to the photobioreactor, where the denitrifying bacteria can remove the nitrogen as a gas.

[0090] Physically, the aerobic reactor includes a mixing tank. An aerobic reactor inlet connects to the fluid outlet of the photobioreactor to bring the mixed liquor from the photobioreactor into the mixing tank. An air inlet connected to an air pump allows for aeration of the mixed liquor stream. A recycle inlet is used to recycle material (e.g. nitrate) back to the photobioreactor.

[0091] Next, the mixed liquor stream of the aerobic reactor is separated into sludge and supernatant using a clarifier. It is contemplated that this can be accomplished by at least two different structures.

[0092] First, the aerobic reactor may be a sequencing batch reactor. In other words, the aerobic reactor acts as the clarifier. In this physical system, the inlet is closed so that no additional mixed liquor enters the mixing tank, and the suspended solids in the mixing tank are allowed to settle. This results in sludge and supernatant. The supernatant can exit through an outlet valve. In this type of setup, there are usually two mixing tanks, with a bioselector connecting to the inlet of both tanks. The bioselector directs the flow of mixed liquor from the photobioreactor to either mixing tank. One mixing tank will be in a mixing mode to form nitrite/nitrate, while the other mixing tank will be in a settle/separating mode to form sludge and supernatant.

[0093] In the other structure, the clarifier is a separate device that is downstream of the aerobic reactor. One exemplary device is a settling tank. The mixed liquor stream enters the settling tank, which separates the mixed liquor into sludge and supernatant. Another device is a filter press, which generally works in a batch manner. The mixed liquor stream enters a space between plates. The plates are clamped together, applying pressure that separates the sludge from the supernatant. The supernatant runs out of the space, leaving the sludge behind in a cake form. The sludge is removed, and the device can be used again. A third device is a centrifuge, which separates the sludge and supernatant based on density. The phosphorus is concentrated in the sludge, which can then be disposed of or used for various applications (e.g. fertilizer). The concentration of phosphorus and nitrogen is significantly reduced in the supernatant compared to the original wastewater stream. As desired, the sludge is disposed of or returned to the photobioreactor and/or aerobic reactor as needed.

[0094] The anaerobic photobioreactor, aerobic reactor, and clarifier can be physically separated into stand-alone apparatuses, or may be combined into one large structure. It is contemplated that these devices can be made as one structure of the desired shape (e.g. rectangular, circular, orbital) with different zones for each function (e.g. lighted anaerobic or aerobic zones).

[0095] FIG. 11 is a diagram illustrating one system 1100 for removing phosphorus and nitrogen. The system includes a photobioreactor 1110, an aerobic reactor 1130, and a clarifier 1150. Wastewater 1102, light 1104, inert gas 1106, return sludge 1152, and a mixed liquor recycle loop (for nitrate) 1132 can enter the photobioreactor. A mixed liquor stream 1140 exits the photobioreactor, as does gas (e.g. N₂0, N₂, C0₂) 1108. If natural biofilms are used, then it is also possible that biofilm mass containing phosphate granules could be removed from the packing material, either by mechanical removal or by simple sloughing. This is denoted as sludge 1112 that exits the photobioreactor as well.

[0096] The mixed liquor stream from the photobioreactor 1140 enters the aerobic reactor 1130, as does air 1136. Return sludge 1152 from the clarifier 1150 can enter the aerobic reactor as well. A mixed liquor recycle loop 1132 returns nitrate back to the photobioreactor, and gas 1134 may exit as well. The mixed liquor exits the aerobic reactor (reference numeral 1142) and enters the clarifier 1150. [0097] In the clarifier, the mixed liquor stream 1142 from the aerobic reactor is separated into product sludge 1154 and supernatant 1156 reduced in nitrogen and phosphorus content. These two products exit the clarifier. The product sludge can be used as fertilizer (reference numeral 1158) or recycled back to provide desirable microorganisms as needed to the photobioreactor or the aerobic reactor (reference numeral 1152).

[0098] FIG. 12 is a diagram illustrating the other system for removing phosphorus and nitrogen. The system 1200 includes a photobioreactor 1210 and two sequencing batch reactors (SBRs) 1230, 1250. Again, wastewater 1202, light 1204, inert gas 1206, return sludge 1252, and a mixed liquor recycle loop (for nitrate) 1232 can enter the photobioreactor. A mixed liquor stream 1240, gas 1208, and sludge stream 1212 can exit the photobioreactor. The sludge stream enters a bioselector 1220 which determines which SBR 1230, 1250 the flow will enter. Air 1236 can enter the SBR for mixing and creating aerobic conditions. Recycled mixed liquor 1232 containing nitrate can exit the SBR in mixing mode 1230/1250 for return to the photobioreactor. Gas 1234 can also exit the SBR. Sludge 1254 and supernatant 1256 exit the SBR in settle/separating mode 1250/1230. The sludge can be separated into product sludge 1258 or return sludge 1252.

[0099] The present disclosure has been described with reference to exemplary embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

CLAIMS:

1. A system for removing phosphorus and nitrogen from a wastewater stream, comprising:

(a) a photobioreactor (300, 1110) comprising:

an outer casing (310), a first end wall (320), and a second end wall (330) that define an internal volume (340);

a fluid inlet (344), a fluid outlet (346), and a gas inlet (342) connecting to the internal volume; and

a plurality of light sources distributed throughout the internal volume;

(b) an aerobic reactor (1130) comprising:

a reactor inlet, an air inlet, and a recycle inlet connecting to a mixing tank;

wherein the reactor inlet connects to the fluid outlet of the photobioreactor; and

wherein the recycle inlet is used for recycling material back to the photobioreactor;

(c) a clarifier (1150) for separating a mixed liquor stream (1142) into sludge (1154) and supernatant (1156); and

(d) a return loop (1132) for recycling mixed liquor from the aerobic reactor back to the photobioreactor.

2. The system of claim 1 , wherein the plurality of light sources in the photobioreactor is distributed so that no location in the internal volume is greater than a threshold distance away from a light source.

3. The system of any of claims 1-2, wherein the light sources emit only at wavelengths between 700 nm and 950 nm.

4. The system of any of claims 1-3, wherein the light sources are located within light pipes (350) that extend from one end wall into the internal volume towards the other end wall.

5. The system of claim 4, wherein radial fins (392) extend radially from a sidewall (356) of the light pipe.

6. The system of any of claims 1-5, wherein the gas inlet (342) and the fluid inlet (344) are located in the second end wall (330), and the fluid outlet (346) is located in the first end wall (320).

7. The system of any of claims 1-6, further comprising a floor (332) above the second end wall.

8. The system of any of claims 1-7, further comprising a mesh container for placement within the internal volume.

9. A method of removing phosphorus and nitrogen from a wastewater stream, comprising:

feeding the wastewater stream (1102) to an internal volume (340) of a photobioreactor (300);

exposing phosphate-accumulating microorganisms and denitrification bacteria in the internal volume to light (1104) under anaerobic conditions (1106) to capture phosphorus, convert nitrate in the wastewater stream to nitrous oxide or nitrogen gas, and form a mixed liquor stream (1140); and

separating the mixed liquor stream into sludge (1154) and supernatant (1156), the supernatant containing a reduced amount of phosphorus and nitrogen; and optionally recycling a portion of the mixed liquor stream (1132) or the sludge (1152) back to the photobioreactor.

10. The method of claim 9, wherein the light is provided by a light source located within a light pipe (350) that extends from a first end of the internal volume towards a second end of the internal volume, and wherein radial fins (392) extend radially from a sidewall (356) of the light pipe.

11. The method of any of claims 9-10, wherein the phosphate-accumulating microorganisms are immobilized in an alginate matrix (110) in the shape of beads (100).

12. The method of any of claims 9-10, wherein the phosphate-accumulating microorganisms are in the form of natural biofilms on a packing material in the internal volume, wherein the packing material is in the form of particles, sheets, or hollow fibers.

13. The method of any of claims 9-12, further comprising nitrogen sparging of the internal volume of the photobioreactor to maintain anaerobic conditions.

14. The method of any of claims 9-13, wherein the mixed liquor stream from the photobioreactor is sent to an aerobic reactor, where nitrifying bacteria convert ammonia in the mixed liquor stream into nitrite or nitrate; and

wherein the mixed liquor stream is separated into sludge and supernatant downstream of the aerobic reactor.

15. The method of claim any of claims 9-14, wherein the mixed liquor stream is separated into sludge (1154) and supernatant (1156) in a clarifier (1150) downstream of the photobioreactor (1110).