WO2015172256A1

WO2015172256A1 - Methods and apparatus for biomass growth

Info

Publication number: WO2015172256A1
Application number: PCT/CA2015/050470
Authority: WO
Inventors: Wing-Yam WONG; Chi Wah Wong
Original assignee: Sro Tech Corporation
Priority date: 2014-05-12
Filing date: 2015-05-12
Publication date: 2015-11-19

Abstract

The invention relates to a biofilm growth system comprising a rack with at least two horizontal panel layers mounted on said rack, inlet and outlet manifolds and a holding tank connected to said inlet manifold. The biofilm growth system further comprises a harvest system comprising a spring loaded blade for scraping algae from said horizontal panel layers and a vacuum means in association with the spring loaded blade. The biofilm growth system is for use in growing biofilm generating organisms such as algae. Said biofilms are able to remove impurities from wastewater that has been obtained from water and/or sewage treatment facilities. The biofilm growth system is additionally designed to use flue gas as a C02 source to increase biofilm growth on said horizontal panel layers.

Description

METHODS AND APPARATUS FOR BIOMASS GROWTH

TECHNICAL FIELD

[0001] The present application pertains to the production of biofilm-forming microorganisms. More specifically, the present application relates to the design and methods of an attached growth biofilm apparatus for the cultivation of microalgal species. The apparatus can be modified by adding further growth panels, for the treatment of post-primary settled wastewater or tertiary effluent prior to final discharge to the natural waterway.

BACKGROUND OF THE INVENTION

[0002] Microalgae are microscopic unicellular organisms capable of converting solar energy to chemical energy via photosynthesis (Richmond 2004). Microalgae can contain numerous bioactive compounds that may be harnessed for commercial use including: proteins, lipids, carbohydrates, carotenoids, antioxidants and vitamins. The potential of microalgal photosynthesis for the production of valuable compounds or for energetic use is widely recognized due to their more efficient utilization of sunlight energy as compared with higher plants. Our dependence on fossil fuels and the resultant anthropogenic changes have lead to the search for sustainable replacements (Pulz and Gross 2004). First generation biofuels made from sugar, starch, or vegetable oils, are proving largely unsustainable, whereas cultivated or farmed phototropic organisms, such as microalgae, could not only be a sustainable fuel source, but also provide a feedstock for a variety of other valuable products. Other products include nutraceuticals, pharmaceuticals, plastics, animal feeds, etc (Chisti 2007; Gouveia and Oliveira 2009).

[0003] According to the National Renewable Energy Laboratory's (NREL) Aquatic Species Program (Sheehan et al. 1998), the potential for microalgae as a sustainable fuel source is evident: 1. Microalgae grow at a rapid rate. Agriculturally speaking, microalgal crops grow 20 to 30 times faster than any other food crop.

2. Accumulated biomass is very high, capable of producing 6,000 gallons of oil and 98 tons of meal per acre every year. This ranges from 30 to 100 times greater production than other alternative biofuel sources, such as soybeans, corn, etc. 3. Microalgae biomass provides a rapidly harvestable biofuel feedstock. Microalgal cultures can reach harvest size in as little as 48 hours and appropriately designed cultivators can continuously harvest the biomass.

4. Microalgae have the ability to absorb large amounts of carbon dioxide (C0₂ sequestration) while growing. Approximately 180 tons of CO2 are absorbed annually from the atmosphere per acre of microalgae, in addition to other greenhouse gases.

5. Appropriately designed cultivators make very efficient use of water; 85-97% of all water can be recovered and reused.

6. Microalgae biomass derived oil is suitable for use in existing petrochemical refineries and distribution systems. In comparison, ethanol is an aggressive solvent that requires modifications to existing infrastructure, resulting in additional cost.

7. Microalgae biomass derived meal is high in protein (39%) and is suitable for use as animal feed and as nutritional supplements.

8. Microalgae-based fuels are considered to be carbon neutral. When burned, they offer a 50-80% reduction in particulate emissions versus fossil fuels, with no loss of power. Carbon emissions from microalgae derived fuel is offset by the CO2 absorbed from the atmosphere during its cultivation.

9. Microalgae-based fuel is naturally sulfur-free (sulfur needs to be removed from some types of petroleum crude oil, increasing the cost of refining).

10. Just 15,000 square miles of microalgae farms could replace all the petroleum used in the United States per year, according to the US Department of Energy. That is approximately one-sixth the size of Minnesota and less than one-seventh the area now used to harvest all the corn across the United States.

[0004] Current methods of large-scale microalgal cultivation are primarily of the open photobioreactor (open-pond) and closed photobioreactor types; however both have their own significant shortcomings. Although comparatively inexpensive to build, large plots of land are required for open-pond systems, which have high evaporative loss, lack temperature and light control, have inadequate mixing and are susceptible to contamination by other microorganisms

(bacteria, fungi, other undesirable microalgal species) and dust. Thus, instability in culture conditions and low photosynthetic and carbon sequestration efficiencies, often result in low overall productivity. Closed photobioreactors can often achieve higher cell concentrations compared to open-ponds, and are most-often constructed as transparent tubes or flat containers

(Pulz 2001). Tubular photobioreactors generally use glass or plastic bent into different shapes of small diameter all connected together, with a gas exchange zone for adding C0₂ and nutrients and removing the O2, and an air-lift device for media circulation. Tubular photobioreactors suffer from high material cost and frequent cell death due to inefficient gas exchange, shear stress and poor large-scale mixing, while biofouling of the reactor walls also reduces light penetration and makes cleaning problematic. Flat-plate photobioreactors have improved upon tubular designs by increasing the light specific surface area thereby increasing the productivity of photosynthesis. Low power ventilation also promotes mixing, turbulence and reduces shear stress. A flat-plate photobioreactor has enhanced heat and mass transfer efficiency, greater O2 control, and more simplified cleaning and maintenance over the tubular bioreactors. Disadvantages of the flat-plate design are the necessity of direct light supply and utilization efficiency, leading to reduced reactor sizes. Finally, both open and closed photobioreactors still suffer the major issue of dilute microalgal suspensions, which require high cost methods of separation and drying of the accumulated biomass (Banerjee et al. 2002; Irving 2010). [0005] Microalgae cultivated in suspension must be harvested from very dilute solutions, concentrated and dried prior to any further processing for downstream products. Current separation methods include filtration, sedimentation, centrifugation, dissolved air flotation, addition of electrolytes or polymers to induce coagulation and flocculation, and multiple combinations thereof (Olguin 2012). Separation by filtration is difficult because of the small size of planktonic microalgae and the sedimentation rates of microalgae are too slow for separation on a cost-effective time scale. Dissolved air flotation requires high energy and large volumes of electrolytes or polymers are required to adequately flocculate the microalgae. Finally, centrifugation is currently the most popular method used to separate microalgae from the aqueous growth media, however, high upfront capital costs, power requirements and frequent maintenance make it uneconomical for large-scale sustainable use (Christenson and Sims 2011).

[0006] The cultivation of microalgae as a biofilm is a potential means to overcome the shortcomings of conventional open-pond and closed photobioreactor culture systems (Ozkan et al. 2012). Compared to suspended culture systems, microalgal biofilm systems at similar culture densities require significantly reduced water volumes and dewatering energy requirements, leading to a more streamlined process with significantly reduced downstream costs. Current microalgal biofilm production is typically inoculated and immobilized onto phycocolloid beads or gels, or in polyurethane polystyrene or polyvinyl foams, but there have been many other trial substrates.

[0007] To further enhance the economics and societal benefits of these systems and microalgae as a biofuel stock, wastewater or brackish water can be used as a growth medium (Sheehan et al. 1998). Wastewater is polluted water released from residential, commercial and municipal/ industrial/ agricultural sources that must be adequately treated before being discharged back into the environment or re-utilized. Its treatment includes, but is not limited to, the reduction or elimination of organic matter, pathogens, metals and other pollutants (pharmaceuticals, hormones, pesticides, ammonia, salts, metal ions etc.) (Christenson and Sims 2011 ; Pittman et al. 2011).

[0008] There are a variety of methods for the treatment of wastewater, but the most effective and economic methods involve the biological processing thereof using biomass of some description, utilizing aerobic, anoxic, anaerobic processing or a combination of. Generally, the biomass is enclosed in a reactor, or as a biofilm on a suitable substrate. The biomass can be comprised of bacteria, microalgae and other microorganisms which naturally remove pollutants through biomass assimilation (Noue et al. 1992; Hoffmann 1998). In addition to the environmental benefits of biologically treating wastewater with microalgae and bacteria, harvesting of said biomass can be a valuable bioproduct or feedstock for the production of biofuels. [0009] When wastewater flows over a surface, microalgae, bacteria and other microorganisms can attach and grow on the surface whilst consuming contaminants/pollutants. Extracellular polymeric substances (EPS), like the polysaccharide alginate, are secreted by microorganisms and compose a matrix. These EPS permit the formation of biologically active layers (biofilms) ranging in thickness from microns to millimeters. Subsequently, the accumulated microalgal biomass can be harvested (Munoz et al. 2009).

[0010] Existing wastewater biofilm treatment systems include single or sequencing batch biofilm reactor (SBBR) systems with stationary or rotating bioreactors with bed internal supports (membranous, rigid plastic growth plates, hollow polymer filaments) or fluidized bed support (plurality of small biofilm retaining objects i.e. sand, glass/ceramic/polymer/alginate beads, activated carbon), algal flow-way (AGF) systems, and biomembrane reactors, etc.

However, many systems are still limited in that harvesting of the biomass or product is not possible without high cost (Craggs et al. 1997; Munoz and Guieysse 2006). [001 1] Microalgae are phototrophic microorganisms, meaning they use CO2 as inorganic carbon source and sunlight as an energy source to synthesize organic compounds. This process called photosynthesis res ults i n 0₂ being released as a by-product. The balanced overall equation of photosynthesis is described as:

6C0₂ + 6H₂0 -> C₆H₁₂0₆ + 60₂ (1) where [C₆Hi₂0₆] represents g l uc o s e , the smallest building block of a carbohydrate and is the first organic product of photosynthesis. These sugar molecules are starting points for anabolic reactions to synthesize biomass compounds. Further auxiliary energy is required for these metabolic pathways. The organic carbon content in microalgae under non-limiting nutrient conditions is about 50% by weight. In this case, the stoichiometric C0₂ demand is 1.83 kg (0.5 · 44 g/mol C0₂ 12 g/mol C) per each kilogram of dry algal biomass produced.

[0012] The conversion efficiency of light into biomass is expressed by the photoconversion efficiency (PCE) and is defined as the energy gained by a conversion process compared to the available sunlight supplied to the conversion process. In the case of microalgae, it is the ratio of the lower heating value of dry mi cro algae biomass divided by the sunlight supplied per ground area to the microalgae cultivation:

Biomass (W/m²)

PCE =

Sunlight (W/m²)

Establishing maximal PCE under sunlight is of great importance because it implies attaining maximal biomass yield achievable from a given area of microalgal culture.

[0013] The entire energy from incident sunlight cannot be converted into biomass because of several physiological properties of both microalgae and higher plants that reduces the efficiency of photosynthesis. Due to the pigment absorption properties, only the photosynthetic active radiation (PAR) part of the solar radiation spectrum can be used for photosynthesis. The PAR consists of photons in the wavelength range between 400 and 700 nm resulting in a 55% loss of the total incident radiation. The aim of light absorption by the pigments is to excite the chlorophyll molecules to eject electrons so the radiation energy can be converted into chemically stored energy in the form of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH₂). ATP and NADPH₂ are needed to fix CO2 and to synthesize further biomass. Only photons with wavelengths of 680 and 700 nm (red) exhibit the exact energy content required for the excitation of chlorophyll. Higher frequency photons (blue) exhibit higher energy content as required and are used with less efficiency than red photons. Excess energy is dissipated as heat or fluorescence, a mechanism known as non- photochemical quenching. Consequently, there is an energy loss of 21% of the absorbed PAR energy due to the degradation of PAR to the chlorophyll excitation energy at 700 nm. Furthermore, about 35% energy losses are generated by the several enzymatic steps needed for the fixation of CO2 into carbohydrate. In this case a photon demand of 8 photons to fix one mole of CO2 to carbohydrates is required (Weyer et al. 2010).

[0014] The addition of all the energy losses described above, gives a theoretical maximum PCE of 12.4%, a value often referred to as photosynthetic efficiency (PE). The resulting achievable PE value is independent of the photobioreactor technology or process strategy and cannot be improved by genetically engineering of the microalgal cells.

[0015] The real PCE in outdoor algal cultures decreases to a value between 1.5% and 5% depending on the photobioreactor system applied (open-pond or closed systems), mixing conditions and climate conditions. Reasons for the further losses of radiation are the reflection of light on water bodies at the reactor surface (10%), photosaturation and photoinhibition (40%) and respiration (20%). If the PCE loss due to photosaturation can be reduced, for example by means of light dilution, short time PCE of 8% can be measured.

[0016] The overall PCE can be summarized as follows:

Photosynthetically Degradation absorbed Carbohydrate

Active radiation PAR photons to 700nm synthesis

Max

100% ► ► 35.5% ► PCE

λ = 400-700 nm (-21%) (-65%) 12.4%

(-55%) Cell activity

Reflection (respiration) (photosaturation and photoinhibition) on surface

11.2% 8.96% 5.4%

Max. (-10%) _k (-20%) (-40%) PCE

(dependent on environmental conditions, process and reactor design, these are

12.4% variable PCEs )

The variable PCE factors, i.e. reflection on surfaces and cell activities as described are some of the key aspects of the present invention.

SUMMARY

[0017] The present invention discloses the design of a modular rack mount apparatus for the cultivation of biofilms. The apparatus is comprised of flat panels made of polyethylene or high density polystyrene or other polymeric material of a contact angle of less than 90° in respect to hydrophobicity. The panels are stack mounted in a rack thus reducing areal space requirements. The addition of a transparent polymeric filter above the top panel reduces infrared radiation and evaporation, thus enabling the achievement of higher PCE by reducing reflection, minimizing photosaturation, photoinhibition and water evaporation. Lighting sensors and supplementary lighting can be added to the lower panels allowing the optimization of PAR and enhancing growth.

[0018] In another aspect, there is disclosed the harvesting of the microalgal biofilm by mechanically scraping the surface at a certain interval once the biofilm has achieved substantial growth. Retention of some of the biomass (<20%) on the substratum surface will enhance the re-growth of the microalgae. The harvested biomass has a significantly higher density compared to open-pond and other enclosed bioreactor harvested microalgae. This high-density biomass reduces the dewatering cost thus making the bio-process significantly more cost effective.

[0019] In another aspect, a Programmable Logic Controller and sensors are used to monitor and control the lighting intensity, pH, carbon dioxide injection, flow rates and other parameters e.g nitrogen (N) and phosphorus (P) if required prior to delivery to the biofilm panels. BRIEF DESCRIPTION OF THE DRAWINGS:

[0020] Figure 1 line diagram of a general commercial application of microalgae and where the disclosed attached growth biofilm apparatus can be a substitute.

[0021] Figure 2 is a line diagram of a general wastewater treatment application of microalgae and where the disclosed attached growth biofilm apparatus can be a substitute.

[0022] Figure 3 is a schematic drawing of an example embodiment of the disclosed attached growth biofilm apparatus.

[0023] Figure 4 is a schematic drawing of panel configuration arrangements for different applications. [0024] Figure 5 is a line diagram of a system according to one example embodiment of the present disclosure.

DETAILED DESCRIPTION

[0024] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0025] As used herein, the singular forms "a", "an" and "the" include plural references unless the context clearly dictates otherwise.

[0026] Influent means nutrients (bulk fluid) to the inlet manifold, and effluent means bulk fluid leaving the manifold. [0027] The term "biofilm", "biomass" can represent any number of species comprising autotrophic, heterotrophic and mixotrophic species.

[0028] "Microalgae" primarily refers to, but is not limited to species of the Chlorophyta division.

[0029] The term microorganism can be microalgae or bacteria. [0030] The present disclosure provides methods and apparatus for the growth of biomass. The apparatus has a flexible and modular design compared to present microalgal growth systems. Similar to commercial solar panel power floor mount systems, a structural support rack is used to house the panel layers. However, multiple panel layers are stacked below each other with a preferred distance of about 1/5 to the length (width) of length of the rack. The apparatus is designed for phototrophic biomass growth in a commercial biomass application where selected microalgal species is used. In this type of application (Figure 1), a transparent enclosure is required in order to prevent the introduction of air borne contaminants. The apparatus can also be used for the treatment of wastewater (Figure 2). In this application, phototrophic- heterotrophic microalgae and bacteria grow symbiotically on the panel substratum.

[0031] Figure 1 is an overview of the process and requirements for Commercial Application of Microalgae. Firstly, a microalgae species is selected 111. Selection of microalgae species depends on the intermediate and conversion products. Next the land requirement 112 is determined. Compared to traditional open-pond culture which requires large land usage, cultivation as an attached biofilm in a tiered multilayer system permits much greater biomass production at a significantly reduced land area with lower water requirements. Through photosynthetic metabolism 113, microalgae will absorb C0₂ and release 0₂. Microalgae can fix CO2 efficiently from a variety of different sources, including (1) the atmosphere, (2) CO2 from industrial exhaust gases (e.g. flue gas and flaring gas), and (3) fixed CO2 in the form of soluble carbonates (e.g., NaHC03 and Na2C03). The cultivation of microalgae requires additional nutrients and a medium for growth 114. Depending on the species, the growth media is either freshwater or marine-based. Nutrients supplied to microalgal cultures must include the inorganic elements that make up the algal cell in addition to other macronutrients, vitamins, and trace elements. While there is limited reports on optimal levels of nutrients required for mass algal cultures, normally the important macronutrients required are nitrogen (N) and phosphorus (P) (ratio 16: 1 N: P), although other essentials may include sulphur, iron, magnesium and silicon. Cultivation 120 of microalgae can be accomplished in a variety of systems. The primary types of cultivation are open and closed systems 121, being exposed to the environment or enclosed separate from external abiotic and biotic factors. In the disclosed system, an attached growth biofilm apparatus can be used to replace current culture methods. Wet microalgae 130 has to be harvested, depending on the process used, for example, settling, flotation, centrifugation will have different degrees of moisture content. Harvesting and drying 140 of the biomass are major obstacles to the sustainable production of microalgae. The attached biofilm production system proposed here significantly reduces the water volume, simplifies drying by reducing the high energy requirements compared to centrifugation, filtration, etc. Next the biomass is extracted and separated 150. Microalgae have the potential to supply the large demand for biofuels in the future. Some species can produce high quantities of high quality products for a variety of uses. The microalgae-derived intermediate products 160 include lipid extraction products, such as hydrocarbons, fatty acids, glycerol, proteins, and polysaccharides. Microalgae are also an attractive feedstock for conversion 170 to a variety of products; bioconversion products, such as alcohols, organic acids and methane; and catalytic conversion products, such as paraffins, olefins and aromatics.

[0032] Figure 2 is an overview of the process and requirements for Wastewater Treatment Application of Microalgae. Wastewater influent 210 (e.g. sewage) refers to a fluid comprising one or more contaminants. Contaminants may include organic compounds, bacteria and metal ions. Wastewater may also include solid inert materials such as undissolved polymeric material, dirt, and sand. Treatment 220 of wastewater first involves pre-treatment 230 for the removal of solids. Pre-treatment of influent most commonly begins with passage through automated mechanically raked screens to remove all large debris. Pre-treatment may also include grit channels or chambers to allow settlement of sand, grit, stones, etc. Pre-treatment practices also may require the removal of scum (fats and grease), where skimmers collect the scum settled on the influent surface interface. For primary treatment, wastewater flows through settling/sedimentation tanks 240, whereby floating material and/or material that has settled out from the wastewater is separated from the wastewater. Conventionally, primary settled wastewater passes to aeration tanks 250 or basins which may contain anoxic, aerobic and/or mixed zones. Within these tanks wastewater is mixed with microorganisms in suspension or as an attached biofilm (bacteria/microalgae) in percolating media which consume the remaining organic matter in the wastewater as their nutrient supply. The aeration tank uses air bubbles to provide mixing and oxygen, both of which are needed for the microorganisms to multiply. The attached growth biofilm apparatus disclosed herein can be used to replace this conventional process. Sewage after aerobic digestion by microorganisms is subjected to secondary settling 260. Secondary treated effluent can be subjected to tertiary treatment 270. The attached growth biofilm apparatus disclosed herein can be ideal as tertiary treatment particularly for nitrogen and phosphorus removal. Sludge is sent to a digester 280 and further treated anaerobically by bacteria. Digested gas from anaerobic digestion can be used for combustion 290 to generate power. More commonly, treated sludge is mechanically dried/dewatered 292 to reduce volume by filter presses, centrifuges etc. The dewatered and treated sludge, or "cake", is then disposed 294 of, for example, sent to landfills, or beneficially used as a fertilizer or soil amendment, while the removed fluid, called "centrate," is typically reintroduced back to the aerobic treatment of the wastewater process. [0033] Figure 3 is a Schematic of the Attached Growth Biomass Apparatus 300.

[0034] Source carbon, nitrogen, phosphorus and other essential elements (nutrients) are mixed prior to discharge to the growth panel where the attached biomass resides. The biomass consumes the nutrients and multiplies. The final effluent can either be discharged, or partially recycled back to the inlet tank or subsequent panel layer for further treatment. A programmable logic controller is used to monitor and control the amount of nutrients, flow rates and environmental conditions as specified. Within a specific time, depending on the maturity and thickness of the biomass, harvesting of said biomass is performed by mechanical scraping. The harvested biomass is settled, ready for anaerobic digestion or further drying if required for other applications.

[0035] As described in Figures 1 and 2, the biofilm apparatus can be a standalone microalgal culture system or a subsystem within wastewater treatment system. The apparatus is composed of a number of components: (1) biofilm growth panels, (2) frame (rack), (3) harvester, (4) monitor and controller comprising programmable logic controller, pH sensors, carbon dioxide injectors, lighting and light sensors, pumps and (5) other major support components including piping, top protective infrared filter and an optional external cover. These components are described in more detail below.

[0036] Biofilm growth yield is usually expressed as weight/area/time. In Figure 3, horizontal flat panels 313 are used as the support substrate for biofilm growth. Substrata materials are non-toxic and have a contact angle below 90° from a hydrophobicity perspective. These materials may include polyethylene, polystyrene and other suitable polymeric materials. In an embodiment, each panel is approximately 0.91 m (3 ft) x 1.52 m (5 ft). From a practicality aspect, 8 (length) x 5 (width) panel arrays are used, giving (0.91 x 8 meter) x (1.52 x 5) m² size substratum panels 313 with an overall area of 55.33 m² (-600 ft²) per layer. The perimeter of the layer has a 0.076 m (3") liquid tight polymeric raised edge, thus providing 55.33 x 0.076 m³ bulk fluid capacity. In this embodiment, up to 5 substratum layers can be stacked in a rack giving a total area of 276.6 m² (-3000 ft²). However, due to the flexibility and modular design, scalability of the panel dimensions can vary depending on applications and locations.

[0037] The influent 301 is collected in a holding tank 302. The influent is adjusted for pH via a signal 325 before being delivered by a pump 308 at a designed flow rate through a pipe 311. The designed flow rate accommodates for differences in temperature. Depending on summer and winter temperatures, the return flow rate from the recycle pump can change thus more recycle flow will return to the influent holding tank for treatment. Biomass grows quicker under higher optimal temperatures and significantly reduced growth rates in cold temperatures, thus to compensate, more recycle flow would have to return for further nutrient reduction treatment. The influent flow over the panel layer is laminar where photosynthesis and biological metabolism take place. The influent is collected at the lower panel outlet manifolds 316, where it is either transported to the subsequent panel layers or to the effluent outlet settling tank via effluent outlet to settling tank 317. The biomass community creates a symbiotic environment where oxygen, a by-product of photosynthesis, and other metabolites are used by the phototrophs, heterotrophs or mixotrophs. The symbiotic and design properties of the system enable the biofilm to concentrate ions and dissolved organic carbons and C0₂ from the bulk fluid. As the nutrients are consumed by the microorganisms on the panel layers, nutrient concentrations diminish, with the performance similar to a multi-stage system. In the natural environment, biofilms often grow in low or oligotrophic conditions, thus very low level consent limits can be achieved in wastewater treatment applications, which suspended planktonic open- pond or enclosed photobioreactors and conventional activated sludge processes cannot meet. This apparatus enables secondary and tertiary treatment wastewater operations to be combined. Once the influent is delivered to the top panel, flow by gravity transports the wastewater and its diminishing constituent nutrients to the terminal end of the system. [0038] Frame (Rack): The frame can be used for a wide variety of ground mounting applications where panel layers are supported. In a preferred embodiment, 2" or 3" schedule 40 pipe is used to provide a sturdy, highly scalable frame. The frame can be 25 ft high providing up to five panel layers. The inlet manifold 312 and outlet manifold 316 sides of the panel layers are vertically adjustable, thus allowing modification of retention time. The panel layer spacing is approximately 5 ft. Depending on the application and requirements, starting from one panel layer and up to five or more panel layers can be installed. This modular approach allows capacity increases or meeting wastewater treatment consent limits easier to achieve. The frame holding the panel layer inlet side and the panel outlet side are vertically adjustable by screws thus allowing the flow by gravity in one direction.

[0039] Harvesting: Raceway open-pond, flat plate, tubular and attached biofilm photobioreactors have typical biomass concentrations of 0.35, 2.7, 1.1 and over 90 gram/liter respectively. Thus harvesting a biofilm with a reduction in water content is a significant advantage. The thickness of biofilm growth is not infinite due to diffusion limitations (nutrients, metabolites and irradiation) across the biofilm, but can grow up to 300 μηι. If the biofilm is not harvested at the appropriate interval, the fully mature biofilm can detach or sloughed off. When 80-90 % of the biomass is removed, the biofilm will regrow with proper nutrients and light. A spring loaded harvest blade 318 attached to a vacuum is used to harvest the biomass and deliver to the biomass holding

tank 319 for further settling prior to other applications as described in Figures 1 and 2. The supernatant is usually recycled back to the influent holding tank 302.

[0040] Monitors and Controllers: Many parameters affect the biomass growth; however, some environmental parameters can be measured and controlled on a continuous basis. A Programmable Logic Controller (PLC) 321 is used to optimize the operation of the process and apparatus. The pH sensor 304 is used to measure the pH of the influent, depending on the set point, a pH signal 323 is sent to the PLC which interlocks with the C0₂ signal 322. For example, pH is lowered by the inj ection of CO2 303 into the holding tank 302. The CO2 source could be flue gas after biogas combustion or other convenient source. The ammonium and nitrate sensor 305 with corresponding measurement and signal 324 are used to control the amount of recycled influents 306 which could be centrates from dewatering process or supernatant from the holding tank sources 320 by signal 328 via recycle pump 307. The ammonium and nitrate measurements are critical as nitrogen and phosphorus are essential components for the growth of algal biomass. The following stoichiometric reaction provides a general makeup of the algal biomass.

CO2 + 0.7H₂O + 0. 12NH₄ ⁺ + O.OIH2PO4- CH1.78O0.36N0.12P0.01 + I . I 8O2 + 0.1 1H⁺ Thus, the limiting component can be optimized from a mass balance perspective.

[0041] Optional light sources 314 and light sensors 315 are used to optimise the biomass growth via interlock signals 326, 327. As mentioned earlier, photosynthesis requires a minimum light intensity. High light intensity creates photosaturation and photoinhibition and lowers the PCE. The light sensors and light sources located between the panel layers will not likely receive direct light irradiation. 200 urnol nrV¹ is used as a default setting, however, this can be adjusted as optimal light intensity can depend on the microalgal species and the seasonal insolation variation. [0042] Other Support Components

[0043] A greenhouse glazed transparent polymeric sheet 309 is located above the first panel layer. As stated in above, the cover filters off some harmful wavelengths, thus improving the overall PCE. Another function of the cover is to act as a physical barrier to reduce evaporation and airborne matter landing onto the top panel layer.

[0044] An optional greenhouse glazed transparent polymeric cover 310 is used when a physical barrier is desirable depending on applications and locations. The polymeric cover can be used as a physical barrier for the avoidance of unwanted biotic and abiotic factors affecting the apparatus operation.

[0045] Figure 4 shows the schematic of panel configuration arrangements for different applications.

[0046] A typical panel configuration 410 arrangement for commercial microalgal growth application is shown on the left. The influent feeds in parallel through a common manifold to the panels 411, 412, 413, 414, 415, unused nutrients is recycled back to the holding tank.

[0047] A modified arrangement 420 for a combined secondary and tertiary wastewater treatment application is shown on the right. The influent discharge occurs in parallel through a common manifold to the top three panels 421, 422, 423. The effluents from the three panels are collected through a common manifold are directed to fourth 424 and/or fifth 425 panels in a sequential mode to the settling tank.

[0048] Example 1 Estimate the panel surface area for the treatment of a wastewater

[0049] As the basic energy source for phototrophic algae, light intensity and availability are crucial factors affecting the success or failure of microalgae cultures. The net growth of the microalgae cultures at very low light intensities is zero (compensation point) (Lee 1997). With increase in light intensities, photosynthesis increases until a point is reached where the growth rate is the maximum attainable (saturation point) (Goldman 1979; Lee 1999; Richmond 2000). Increasing the light intensity beyond the saturation point does not result in increased growth rates but can lead to photo-oxidation, which damages the light receptors, thereby decreasing the rate of photosynthesis and productivity. As an example, the following provides the annual insolation of a location (45.3°N; 75.6°W) (Table 1). In the summer months there is adequate photons/area for microalgae growth at a location in Ottawa. Regions within 40° latitude north and south provide a higher productivity.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec SUM

Solar Irradiation

2.08 3.1 4.8 6.56 7.79 8.02 7.54 6.6 5.17 3.61 2.36 1.76 59.39 (kWh/m²/day)

Solar Irradiation PAR (43%)

3.74 4.8 7.4 10.15 1 1.74 12.38 11.65 10.19 8 5.55 3.66 2.71 91.97 (MJ/m²/day)

Solar Irradiation PAR (PFD)

532.31 616.74 1052.38 1397.38 1670.34 1705.28 1658.11 1450.08 1101.32 789.28 503.29 385.46 12861.97 (mol photons/m²/month)

Table 1 Clear sky incident isolation on a horizontal surface

[0050] For a symbiotic system, O2 is required for respiration purposes. An important parameter of the microalgal biomass system is the quantum requirements of the photosynthetic process. According to Weyer et al (2010) for theoretical maximum algal oil production, 8 PAR photons are required to liberate one molecule of O2. However, considering photosaturation, low light intensity and other inefficiency factors, assuming 20 PAR photons are required per O2 molecule is hypothesized. This equates to a maximum oxygen quantum yield (QY02) of 0.05 02/photon. Thus,

ROA aigae = QYo2 * PFD (mol/m²/day);

Where RO ,A, algae IS the areal oxygen production by microalgae (mol/m²/day); PFD is the photo flux density (mol photons/m²/day).

[0051] The following stoichiometric reaction is used for freshwater microalgae with ammonium as nitrogen source:

CO2 + 0.7H₂O + 0.12NH₄ ⁺ + O.OIH2PO4- CH1.78O0.36N0.12P0.01 + I. I 8O2 + 0.11H⁺

The real amount of microalgae produced can be calculated with the above equation. Assuming an average algal composition of 7.8% N and 1.4% P (w/w based on microalgal biomass composition of CH1.78O0.36N0.12P0.01 ).

[0052] With the areal algal production known and the fraction of nitrogen (N) and phosphorus (P) present in the biomass, the uptake of the N and P from the wastewater is calculated as:

RN,A, algae = Rx,A, algae * FN,algae (wt/m²/day) Where R_N ,Α, algae IS the areal N uptake rate by the microalgae (wt/m²/day); R_x ,A, algae IS the areal microalgae production rate (wt/m²/day) and TN,algae IS the fraction of N in the microalgae biomass (wt/wt).

[0053] Assuming a population of 6,000 and wastewater generation is 260 L per capita.

Influent Chemical Oxygen Demand is 240 mg/L

Ammonium NH^4" is 40 mg/L

Phosphorus P is 10 mg/L

Using (CHi.₇80o.36No.i₂Po.oi ) as the microalgal biomass, the C: N: P ratio is 100: 12: 1 Thus nitrogen is the limiting nutrient.

Using the Fed Fisheries Act 2010 as a guide line where NH^4" is 1.25 mg/L From: RO,A, ai_gae = QY02 * PFD (mol/m²/day)

The PFD in (45.3°N; 75.6°W) location is 12861.97 (mol photons/m²/year) Which is 35.2 (mol photos/m²/day) Thus RO,A, aigae = 0.05 * 35.2 = 1.76 (mol photos/m²/day) CH1.78O0.36N0.12P0.01 is 21.53 gm which is one mole

Using RN,A, algae = Rx,A, algae * FN,algae (wt/m²/day)

= 21.53 * 1.76/1.18

= 32.1 (gm/ m²/day)

RN ,A, algae Rx ,A, algae * F]SI,algae (wt/m²/day)

= 32.1 * 7.8/100

= 2.5 (gm/ m²/day)

A = Q * (Nin - Nout ) / RN,A, algae (m²) = (6000* 260)/1000 * (40-1.25)/2.5

= 24180 m²

Per capita area required is 24180/6000 = 4.04 m² [0054] Referring now to Figure 5, an example setup of a rack mounted attached biomass growth system 500 will now be described. A feed pump 540 is connected to a feed tank 550 for pumping influent to a first panel 560 that is above a second parallel panel 570. A CO2 source 530 is connected to the feed tank 550 for adjusting the pH levels of the influent. A recycle tank 510 (or setting tank) collects effluent from the panels. Supernatant is pumped from the recycle tank 510 back to the feed tank 550 with a recycle pump 520.

[0055] Although certain methods, apparatus and articles of manufacture have been described herein, the scope of coverage of this disclosure is not limited thereto. To the contrary, this disclosure covers all methods, apparatus, and articles of manufacture fairly falling within the scope of the claims either literally or under the doctrine of equivalents.

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Claims

Claims:

1. A rack mounted attached biomass growth system comprising:

at least two open horizontal panel layers, each panel layer comprising an inlet manifold and an outlet manifold;

a rack for holding the panel layers;

and an influent holding tank connected to the inlet manifold of at least one of the horizontal panel layers.

2. The system of claim 1, wherein photoautotrophic, heterotrophic and mixotrophic species could grow symbiotically on the open atmospheric panel layers.

3. The system according to claim 1 or 2, wherein influent nutrients (bulk fluid) flows to the inlet manifolds of the panel layers either in parallel, in series or both depending on applications, and the effluent exits from the outlet manifolds.

4. The system according to any one of claims 1 to 3,wherein the panel layer inlet side and the panel outlet side are vertically adjustable.

5. The system according to any one of claims 1 to 4, further comprising a harvest system comprising a spring loaded blade (scraper) connected to a vacuum source, where the blade scrapes the film biomass.

6. The system according to any one of claims 1 to 5, further comprising a monitoring and control system comprising a Programmable Logic Controller which manages and process information collected for pH, ammonium, nitrate, and light sensors and uses the collected information to control C0₂ source injection, flow rate of centrates and/or supernatants and light source.

7. The system of claim 6, wherein the CO2 source injection injects flue gas.

8. The system claim 7, wherein the flue gas is from biogas combustion and/or other sources

9. The system according to any one of claims 1 to 8, wherein each panel layer comprises high density polyethylene, high density polystyrene or other transparent polymeric materials at a contact angle less than 90° in respect to hydrophobicity.

10. The system according to any one of claims 1 to 8, further comprising a greenhouse glazing transparent polymer sheet located above the top panel layer.

11. The system according to any one of claims 1 to 10, further comprising an extemal transparent polymer cover.

12. The system according to any one of claims 1 to 11, comprising a grouping of panel layers in parallel followed by a grouping of panel layers in series.

13. The system according to claim 6, further comprising a light sensor and supplementary florescent lights managed by the Programmable Logic Controller and installed within the panel layer space to provide light intensity of not less than 800 μηιοΐ rrrV¹.

14. Use of the system of any one of claims 1 to 13 for attached biomass growth.

15. Use of the system of any one of claims 1 to 13 for wastewater treatment.

16. Use of the system of any one of claims 1 to 13 for the treatment of secondary and tertiary influents.

17. Use of the system of any one of claims 1 to 13 for attached biomass growth, an influent holding tank is used for mixing influent, recycled effluent, pH control and C0₂ injection.