Classifications

• • C—CHEMISTRY; METALLURGY
  • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
  • C12M—APPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
  • C12M29/00—Means for introduction, extraction or recirculation of materials, e.g. pumps
  • C12M29/06—Nozzles; Sprayers; Spargers; Diffusers
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03D—FLOTATION; DIFFERENTIAL SEDIMENTATION
  • B03D1/00—Flotation
  • B03D1/14—Flotation machines
  • B03D1/1431—Dissolved air flotation machines
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03D—FLOTATION; DIFFERENTIAL SEDIMENTATION
  • B03D1/00—Flotation
  • B03D1/14—Flotation machines
  • B03D1/24—Pneumatic
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03D—FLOTATION; DIFFERENTIAL SEDIMENTATION
  • B03D1/00—Flotation
  • B03D1/14—Flotation machines
  • B03D1/24—Pneumatic
  • B03D1/242—Nozzles for injecting gas into the flotation tank
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F3/00—Biological treatment of water, waste water, or sewage
  • C02F3/02—Aerobic processes
  • C02F3/12—Activated sludge processes
  • C02F3/20—Activated sludge processes using diffusers
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F3/00—Biological treatment of water, waste water, or sewage
  • C02F3/02—Aerobic processes
  • C02F3/12—Activated sludge processes
  • C02F3/22—Activated sludge processes using circulation pipes
  • C02F3/223—Activated sludge processes using circulation pipes using "air-lift"
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F3/00—Biological treatment of water, waste water, or sewage
  • C02F3/28—Anaerobic digestion processes
  • C02F3/2866—Particular arrangements for anaerobic reactors
  • C02F3/2873—Particular arrangements for anaerobic reactors with internal draft tube circulation
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F3/00—Biological treatment of water, waste water, or sewage
  • C02F3/32—Biological treatment of water, waste water, or sewage characterised by the animals or plants used, e.g. algae
  • C02F3/322—Biological treatment of water, waste water, or sewage characterised by the animals or plants used, e.g. algae use of algae
• • C—CHEMISTRY; METALLURGY
  • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
  • C12M—APPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
  • C12M21/00—Bioreactors or fermenters specially adapted for specific uses
  • C12M21/04—Bioreactors or fermenters specially adapted for specific uses for producing gas, e.g. biogas
• • C—CHEMISTRY; METALLURGY
  • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
  • C12M—APPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
  • C12M27/00—Means for mixing, agitating or circulating fluids in the vessel
  • C12M27/18—Flow directing inserts
  • C12M27/24—Draft tube
• • C—CHEMISTRY; METALLURGY
  • C12—BIOCHEMISTRY; BEER; SPIRITS; WINE; VINEGAR; MICROBIOLOGY; ENZYMOLOGY; MUTATION OR GENETIC ENGINEERING
  • C12M—APPARATUS FOR ENZYMOLOGY OR MICROBIOLOGY; APPARATUS FOR CULTURING MICROORGANISMS FOR PRODUCING BIOMASS, FOR GROWING CELLS OR FOR OBTAINING FERMENTATION OR METABOLIC PRODUCTS, i.e. BIOREACTORS OR FERMENTERS
  • C12M47/00—Means for after-treatment of the produced biomass or of the fermentation or metabolic products, e.g. storage of biomass
  • C12M47/02—Separating microorganisms from the culture medium; Concentration of biomass
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03D—FLOTATION; DIFFERENTIAL SEDIMENTATION
  • B03D1/00—Flotation
  • B03D1/14—Flotation machines
  • B03D1/1493—Flotation machines with means for establishing a specified flow pattern
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03D—FLOTATION; DIFFERENTIAL SEDIMENTATION
  • B03D2201/00—Specified effects produced by the flotation agents
  • B03D2201/007—Modifying reagents for adjusting pH or conductivity
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/52—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities
  • C02F1/5236—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities using inorganic agents
  • C02F1/5245—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities using inorganic agents using basic salts, e.g. of aluminium and iron
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/66—Treatment of water, waste water, or sewage by neutralisation; pH adjustment
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E50/00—Technologies for the production of fuel of non-fossil origin
  • Y02E50/30—Fuel from waste, e.g. synthetic alcohol or diesel
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
  • Y02W10/00—Technologies for wastewater treatment
  • Y02W10/10—Biological treatment of water, waste water, or sewage
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
  • Y02W10/00—Technologies for wastewater treatment
  • Y02W10/30—Wastewater or sewage treatment systems using renewable energies
  • Y02W10/37—Wastewater or sewage treatment systems using renewable energies using solar energy

Definitions

• This invention relates to the generation of fine bubbles and their application in processes, one being an anaerobic digester and another being a particle, in particular, an algal floe, separator, especially in the context of airlift loop bioreactors.
• Bubbles of gas in liquid are frequently required in many different applications and usually, but not exclusively, for the purpose of dissolving the gas in the liquid.
• WO2008/053174 describes method and apparatus for generating microbubbles in liquid, and suitable applications in which such microbubbles may be employed. These range from aeration and cleaning of water, oil lifting in wells, and bioreactors and fermenters.
• Small bubble generation has application in the sewage treatment industry, in which it is desired to dissolve oxygen in the water being treated. This is to supply respiring bacteria that are digesting the sewage. The more oxygen they have, the more efficient the digestion process.
• bioreactors and fermenters generally where they are sparged for aeration or other purposes.
• yeast manufacturing industry has this requirement, where growing and reproducing yeast bacteria need constant oxygen replenishment for respiration purposes.
• Another application is in the carbonisation of beverages, where it is desired to dissolve carbon dioxide into the beverage.
• a process not looking to dissolve the gas but nevertheless benefiting from small bubbles is in the extraction of hard-to-lift oil reserves in some fields which either have little oil left, or have the oil locked in sand.
• bubbles are in particle separation from a liquid suspension of the particles, of which an example application is algal separation. This might be desired for one or both of two reasons. A first reason is to clarify water contaminated with algae. A second reason is to harvest algae grown in water.
• the process is not limited to algal separation; any mixer comprising solid particles can comprise the use of bubbles which attach to suspended particles and carry them to the surface from which they may be scraped, either to recover the particles or clarify the water.
• attachment of the bubbles to the particles is problematic, particularly if the particles are charged when they may simply bounce off bubbles and not attach to them.
• DAF Dissolved air flotation
• An anaerobic digester is a processing unit in a wastewater treatment plant where organic matter is broken down via anaerobic bacteria in the absence of oxygen.
• the biodegradation of organic matter in an anaerobic digester takes place through four steps.
• the first step is an hydrolysis stage which converts the complex organic matter in to a simple state.
• the second step is the acidogenesis stage. In this stage, the product of the first stage converts into volatile fatty acids. Volatile fatty acids are converted into acetate in the third step by acetatogenic stage. Finally the acetate and carbon dioxide with hydrogen produced in second step convents into methane and carbon dioxide in the methanogenesis stage.
• Each stage is mediated by specific type of bacteria. Each bacteria requires a specific environment. Methanogenesic bacteria are more sensitive to change of operating conditions. However, there are general operating conditions, such as temperature, pH, Carbon-nitrogen ratio, and ammonia etc, appropriate for all the bacterial consortia.
• the hydraulic retention time of a mesophilic anaerobic digester is approximately 20 days. Then the sludge discharges as effluent.
• the digested sludge (effluent) contains organic matters (biodegradable), anaerobic bacteria and some dissolved gases, for instance carbon dioxide and hydrogen sulphide. The presence of these dissolved gases has negative impact on piping and the downstream processing units. Corrosion is one potential problem in piping metals.
• the generation of biogas continuously in digested sludge during transfer creates a gas-liquid mixture. Even if a small phase fraction of gas, it degrades the performance of pumps due to cavitation phenomena.
• Anaerobic digestion is an important source of methane in the search for green energy. It is UK Government policy to recycle 50% and recover energy from 25% of human waste by 2020. Anaerobic digestion breaks down food and plant waste to produce biogas, a mixture of methane and carbon dioxide which is burnt to produce electricity and a residual material which can be used as a soil improver. BRIEF SUMMARY OF THE DISCLOSURE
• an anaerobic digester comprising a liquid fermenter tank for anaerobic microorganisms and a diffuser of a microbubble generator to introduce bubbles of non-oxygen containing gas into the digester whereby methane and acid gases produced by the digestion are exchanged across the bubble surface to strip such gases from the liquid when said bubbles connect with a header space of the tank.
• said gas is nitrogen or another inert gas that merely strips the fermenter of oxygen to promote the onset of anaerobic digestion.
• said gas is or comprises the biogas generated and released into a head space of the fermenter tank.
• This typically comprises approximately 60% methane and 40% carbon dioxide. While reintroduction of bubbles of such gas into the fermenter does little to change the equilibrium of such gases in the liquid, it provides an escape route for the methane which in physical terms should be gaseous at room temperatures and the temperature of anaerobic digestion (typically 35°C) it typically clings to particles such as the microorganisms themselves. Locally positioned bubbles permit methane to escape the liquid phase. Inside the bubbles, which thus grow in this environment, the balance of carbon dioxide remains so that it is also stripped from the liquid, along with acid gases.
• said gas is also is nitrogen or another inert gas that serves in this event to strip methane and hydrogen sulphide. Not only does this reduce the acidity of the remaining liquid but also it promotes further growth of the methanogenesic bacteria enhancing the methane output.
• microbubbles introduced into the reactor increases methane yields, while at the same time 'sinking' waste C0 2 .
• Such a process has the possibility to increase or even double the amount of methane extracted through anaerobic digestion.
• Methane tends to adhere to the microorganisms and biomass in the reactor, rather than be released to the gas headspace.
• carbon dioxide microbubbles it is found that, in addition to removing the methane already produced more effectively, the production rate of methane increases.
• the dissolved carbon dioxide is taken up as food by methanogenesic bacteria, thus increasing their production of methane.
• Microbubbles speed up gas exchange by providing more food to the bioculture, but also by removing the methane, which has an inhibitory effect on metabolism.
• fertilizer recovered from anaerobic digestion has hitherto been dried and trucked to farms, despite the fact that the digestate is about 90% water and would benefit from other transport mechanisms such as piping. This is necessary because the digestate is highly acidic and corrosive. By stripping out the acid gases, leaving the digestate substantially neutral, this barrier to transport via pumps and pipes is removed, improving the attraction of the anaerobic digestion process and the production of more environmentally sustainable electricity. It also supports sustainable agriculture by recovering the nutrients, particularly phosphates and potassium, which are not renewable without recycling.
• sludge includes micro-organisms, organic matters, elements and suspended solids
• the flow generated by the bubbles may be non-turbulent laminar, having a Reynolds number less than 2000, the Reynolds number being based on the liquid flow velocity, its constitutive properties and the pore diameter of the diffuser.
• Sufficient bubbles must be introduced, of course, and sufficient proportion of them must be small enough (in the range 10 to 30 ⁇ ) to be retained in suspension to provide a site for microorganisms to attach and enable the transfer of methane from their surface into the bubble.
• step d) may be repeated daily and the period of time in step d) may be between 30 and 120 mins.
• the bubbles introduced in step d) may comprise carbon dioxide, indeed, more than 90% carbon dioxide, or essentially pure carbon dioxide.
• step d) may be repeated a final occasion with said bubbles comprising substantially only nitrogen gas, whereby remaining methane, carbon dioxide and hydrogen sulphide is stripped from the digestate to neutralise the pH of the digestate.
• the digestate can be pumped from the tank and transported by pipeline to an irrigation array for land fertilisation. This is much less energy consuming than drying the digestate and transporting it by road.
• anaerobic reactor While the anaerobic reactor has application in the digestion of waste, it also has application in yeast fermentation to (bio) ethanol and in anaerobic ammonium oxidation (anammox).
• the carrier gas introduced into the digestion is nitrogen diluted by C0 2
• the bacteria uses the dissolved C0 2 as a carbon source, resulting in greater anammox efficiency, i.e. production of N 2 from nitrates and nitrates in an aqueous medium.
• the carbon dioxide may be sourced from one of: power production from combustion of the methane produced by said digester; and from sequestered carbon dioxide from another source, for example power station waste gas.
• the bubbles preferably have a size in the range 10 to 100 ⁇ . At least a proportion of the bubbles may have a size in the range 10 to 30 ⁇ accounting for at least a gas holdup of 0.05%. Preferably, at least a proportion of the bubbles have a size in the range 10 to 30 ⁇ accounting for at least a gas holdup of 0.5%.
• the tank may have sides and a base and the liquid in the tank may have a top surface above which is a header space.
• the diffuser may be disposed in the liquid at the base of the tank and be arranged to inject bubbles of gas into the liquid in the tank whereby the apparent density of the liquid above the diffuser is reduced by the bubbles thereby creating a flow of the liquid, which flow is: up the tank in a riser section thereof,
• a physical divider is possible, comprising, for example, a draft tube in a cylindrical tank
• a physical divider is not, in fact, absolutely necessary because the requisite circulation occurs naturally, when the flow is entirely laminar.
• the divider is simply the boundary separating the rising flow from the returning downcomer flow, which separate effectively from one another without turbulence or significant mixing in these conditions.
• bubble size needs to be less than about 100 ⁇ in diameter.
• more than 90% of the bubbles having a size in the range 10 to 100 ⁇ diameter (preferably 20 to 40) and a bubble density of at least 50 million per cubic metre, optionally at least 100 million.
• the gas holdup is less than 2%, optionally less than 1 %.
• Holdup is the overall volume of gas in bubbles per unit volume of liquid.
• a suspended particle separation tank may comprise: a floor having a floor area; liquid in the tank having suspended particles; and a diffuser of a microbubble generator, which diffuser is disposed at the base of the tank and is arranged to inject bubbles of gas into the tank to reduce the apparent density of the liquid above the diffuser, thereby creating a flow of the liquid and gas bubbles that is a non-turbulent laminar flow having a Reynolds number less than 2000, wherein more than 90% of the bubbles have a size in the range 10 to 100 ⁇ diameter and a bubble density of at least 50million per cubic metre, the Reynolds number being based on the liquid flow velocity, its constitutive properties and the pore diameter of the diffuser.
• the Reynolds number may be less than 200.
• more than 90% of the bubbles have a size in the range 20 to 50 ⁇ diameter.
• the bubble density may be at least lOOmillion per cubic metre.
• the gas holdup is less than 2%, optionally less than 1 %.
• the diffuser may be disposed across substantially the entire floor area of the tank.
• a specific example of a suspended particle separation tank is an algal separation tank.
• a method of particle separation comprising the steps of providing a separation tank as defined above comprising an aqueous mixture of suspended particles to be separated from the water, adding a flocculant to cause the particles to coagulate in floes and adjusting the acidity of the mixture to a predetermined pH appropriate for the selected flocculant, and injecting bubbles into the tank, optionally across substantially the entire area of the floor of the tank, wherein more than 90% of the bubbles have a size in the range 10 to 100 ⁇ diameter (preferably 20 to 40) and a bubble density of at least 50million per cubic metre and wherein the flow created by the bubbles injected is a non-turbulent laminar flow having a Reynolds number less than 2000.
• the Reynolds number is based on the liquid flow velocity, its constitutive properties and the pore diameter of the diffuser.
• the bubbles introduced create non-turbulent laminar flow having a Reynolds number less than 200.
• the Reynolds number is less than 20.
• the benefit of a non-turbulent introduction of the bubbles is two-fold. Firstly, little energy is employed, so that the cost of the process is minimised, and secondly the flux rate of the bubbles is such that they gently approach floes of coagulated material without disturbing them. Indeed, the problem of attachment is avoided because the particles (floes) are much larger and so the bubbles can gently lift from underneath the floe rather than having to chemically attach and connect. This is not to suggest that chemical attachment does not occur, but rather that time is permitted for the attachment to occur through the gentle pressure caused by the constant, albeit small, force of gravity. The tendency of bubbles to bounce off or slip by particles is reduced.
• the method is continued until a desired level of clarity of the water remains and at least until the sludge layer on the surface of the water comprising the particles separated from the water has thickened and thinned.
• thickened and thinned is meant that the solids density of the sludge layer has increased through compression from below by accumulated gas bubbles lifting the sludge against the surface and consequential squeezing out of water from the sludge layer, and whereby the thickness of the sludge layer reduces.
• the flocculant may be a metallic inorganic coagulant, such as iron or aluminium salts, added in solution. Coagulation is achieved with the coagulants dissociating into Fe 3+ and Al 3+ respectively as well as other soluble complexes having varying high positive charges. Essentially, the rate and extent to which these trivalent ions and other complexing species adsorb onto colloidal surfaces is pH dependent. At room temperature, under acidic pH, trivalent species-Fe 3+ and Al 3+ .are the dominant species in the continuous phase. These predominant trivalent species are the most effective in colloidal charge neutralization and attach to the negatively charged algal cell.
• a metallic inorganic coagulant such as iron or aluminium salts
• said separation tank is also a fermenter tank comprising algae, which are grown through photosynthesis.
• the algae are harvested when required by introducing the flocculant into the acidity-regulated liquid so as to causes the algae to coagulate together in floes which are lifted to the surface by the microbubbles. With a laminar flow of the fermenter liquid, the bubbles do not disrupt the floes and effective clarification of the liquid can be achieved and the algae subsequently harvested by scraping from the surface.
• the present invention provides a method of separation of particles suspended in a liquid, the method comprising the steps of:
• microbubbles of pH-adjusting gas which gas comprises molecules or radicals which, when dissolved in the liquid across the bubble/liquid boundary, adjust the pH in a desirable direction;
• the microbubbles are introduced into the liquid with sufficiently low energy, and the bubbles are sufficiently small so that they rise in the liquid sufficiently slowly under the influence of gravity, that a stationary boundary layer of liquid pertains and remains around the bubble as the bubbles rise in the liquid, and whereby a pH gradient develops across the boundary layer through dissolution of said molecules or radicals from said bubbles in the liquid;
• the boundary layer is sufficiently thick that particles approaching the bubble contact liquid in the boundary layer at a desirable pH of the liquid so that electrostatic repulsion between the microbubble and particle is minimised and microbubbles and particles collide and attach so that said particles are raised with the microbubbles and separate from the liquid more effectively than would be the case if the average pH of the liquid was pre-adjusted to the average value achieved by said introduction of said microbubbles.
• the microbubbles not only lift the particles but they also adjust the pH of the liquid in the vicinity of the microbubble to prevent repulsion between bubble and particle and encourage attachment between them.
• the adjustment of the overall pH of the liquid is much less than traditionally understood to be required, because it is only in the vicinity of bubble that such adjustment is needed. This is only possible, however, where the bubbles are introduced in non-turbulent conditions, otherwise the boundary layer in which a gradient can be established to overcome electrostatic repulsion is too thin to prevent such repulsion. Bubbles can only be in this condition when they are small and when they are created with a low energy process.
• the invention provides a corresponding particle separation system to separate particles from an aqueous suspension or colloid of the particles, the system comprising:
• microbubbles the injection into the liquid of microbubbles of a pH-adjusting gas, said microbubbles, wherein the microbubbles are small enough that flow of liquid around the bubbles as the bubbles rise in the liquid does not prevent a boundary layer of liquid remaining stationary around the bubbles;
• the desirable pH is determined by the kind of a coagulant or flocculant added to liquid to facilitate coagulation or flocculation of the particles.
• the gas preferably is or contains carbon dioxide.
• the desired pH may be between 5 and 7.
• the coagulant may be ferric chloride.
• the system may comprise a suspended particle separation tank, the tank comprising: a floor having a floor area;
• a diffuser of a microbubble generator which diffuser is disposed at the base of the tank and is arranged to inject said microbubbles of gas into the tank to reduce the apparent density of the liquid above the diffuser, thereby creating a flow of the liquid and gas bubbles that is a non-turbulent laminar flow having a Reynolds number less than 2000, wherein more than 90% of the bubbles have a size in the range 10 to 100 ⁇ diameter and a bubble density of at least 50million per cubic metre, the Reynolds number being based on the liquid flow velocity, its constitutive properties and the pore diameter of the diffuser.
• the tank may also be a liquid fermenter tank for microorganisms, wherein the tank has sides and said floor, the liquid in the tank has a top surface above which is a header space, and said flow is:
• Particles are in suspension in a liquid, and fail to coagulate naturally and separate out without external influence, because they are or become charged, positively or negatively. They develop a Stern layer on their surface of identical but opposite charges to the charge of the particle itself. A shear or slipping plane of mixed charges develops around the Stern layer, and further from the particle a diffuse layer of mixed charges exists of potential dependent on the pH of the bulk liquid.
• the electrical potential difference in the shear plane between a colloidal particle and liquid bulk is known as the zeta potential, Z, and it decreases away from the particle. It is given by:
• v E electrophoretic velocity of migrating particle, ⁇ /s
• K z a constant that is 4 ⁇ or 6 ⁇
• ⁇ dynamic viscosity of water, N.s/m 2
• ⁇ permittivity relative to a vacuum ( ⁇ for water is 78.54)
• ⁇ 0 permittivity in a vacuum, 8.854188 x 10 "12 C 2 /J.m, or N/V 2
• the zeta potential is essentially exponential, but if it is sufficiently high, sufficiently far from the surface of the particle then another particle or a bubble cannot approach without being repulsed by electrostatic forces. However, as the pH is appropriately adjusted, an isoelectric point of zero zeta potential may be developed close to the particle so that the particle can be approached. By arranging for bubbles to carry with them their own local environment of liquid with appropriate acidity, the isoelectric point can be "squeezed" between the bubble and a particle so that they can contact and stick by virtue of the hydrophobic nature of the particle.
• the fluidic oscillator comprises a diverter supplied with the gas under constant pressure through a supply port that divides into respect output ports, and including means to oscillate flow from one output port to the other, and
• Sonic and ultrasonic vibrations are high frequency and may not be as effective in generating bubbles. Although high energies can be imparted, the most effective detachment of bubbles is with longer stroke (higher amplitude) oscillations, rather than higher frequencies.
• said oscillations effected by the fluidic oscillator are effected at said frequency between 1 and 3000 Hz, preferably between 5 and 500 Hz, more preferably between 10 and 30 Hz.
• the bubbles formed are between 0.1 ⁇ and 100 ⁇ in diameter, more preferably between 30 and 80 ⁇ .
• said oscillation is of the type that has between 10% and than 30% backflow of gas from an emerging bubble.
• said oscillation preferably is of the type that has between 10% and 20% backflow of gas from an emerging bubble.
• This is preferably provided by an arrangement in which a fluidic oscillator divides flow between two paths, at least one of said paths forming said source.
• Backflow here means that, of a net gas flow rate from said conduit of x mV 1 , (x+y) mV 1 is in the positive direction while (-y) mV 1 is in the negative direction, 100(y/(y+x)) being defined as the percentage backflow.
• backflow is largely inevitable, particularly with the arrangement where flow splits between paths, since there will always be some rebound. Indeed, such is also a tendency with bubble generation since, with the removal of pressure, back pressure inside the bubble will tend to cause some backflow. Indeed, backflow here means at the conduit opening, because backflow may vary by virtue of the compressibility of the gas.
• the bubble production of the present invention injects at ⁇ 2 bar into the -1.3 bar tank, and it is gas, not liquid, that is injected so there is no liquid turbulence.
• the present invention achieves the onset of the bubble creation with little more than the energy to create the bubble and the momentum needed to overcome the head of liquid above it. Consequently, each bubble can be injected at not much more than its terminal rise velocity.
• the present invention employs bubbles that rise in laminar flow. Moreover, at low gas flow rates, the gas phase holdup can be less than about 1 %. Given that the traditional levels of gas phase holdup in dissolved air flotation is in the order of 10-12%, there is no comparison with the bubble flux; that is, there is much less kinetic energy/momentum injected with the present invention, so that, on the face of it, there is much less lift force as well. It is therefore unexpected that the present invention achieves comparable separation performance and rates to traditional DAF.
• one theory is that the floes that form with a laminar flow of few small bubbles do not break up (because of the lack of the turbulent destruction mechanism) and therefore achieves comparable separation performance and rates. Additionally, because energy dissipation rates are typically proportional to the Reynolds number of the liquid flow, based on the diameter of the exit pore/nozzle, the present invention provides a Reynolds number in the range of 10-100, whereas conventional DAF has exit- nozzle Reynolds numbers of 10,000-100,000. It is therefore expected that the present invention dissipates -1000-fold less energy. The capital cost of equipment, due to working at pressures less than 2 bar, is substantially less than working at pressures of 6 bar for DAF.
• Figure 1 is a plan view of a suitable diverter to oscillate gas in a method in accordance with the present invention
• Figure 2 is a graph of oscillation frequency plotted against feedback loop length for one arrangement of the diverter shown in Figure 1 ;
• Figure 3 is a graph of bubble pressure against bubble volume for conduit openings of two different diameters
• Figure 4 is a bubble generator plate of an alternative arrangement of the present invention.
• Figure 5 is an end view showing the relative dimensions of the liquid and gas conduits of the bubble plate shown in Figure 4;
• FIG. 6 is a schematic illustration of the overall arrangement employing the bubble plate of Figures 4 and 5;
• Figure 7 is a schematic illustration of the overall arrangement of a preferred embodiment of the present invention.
• Figure 8 is a cross section through a bubble generator of the system of Figure 7;
• Figure 9 is a section through a bubble generator according to the fourth aspect of the present invention.
• Figures 10 a and b are respectively a schematic perspective view of a diffuser employed in a method according to the present invention and a side section showing bubble pinch-off;
• Figures 11 a and b are respectively side sections, (a) to (e), through an elastic membrane showing the development of a bubble, and a graph of differential gas/liquid pressure ⁇ across the membrane at each of the stages of bubble formation shown in Figure 11 a;
• Figure 12 is a schematic representation of the experimental set-up of an algal floatation separation unit in accordance with an embodiment of the present invention
• Figures 13 a and b are histograms of (a) bubble size distributions and (b) bubble density from a stainless steel mesh diffuser used in the set up of Figure 12;
• FIGs 14i a to c are photographs of the flotation unit of the set up of Figure 12, showing the separation at three different key stages: (a) a few moments after flocculated algal cells were introduced into the unit; (b) after 12 minutes, clearly showing the algae sludge blanket minutes and where small floes are predominant; and (c) a third stage after 30 minutes, marked by much slower separation as relatively smaller floes but intense sludge thickening and thinning is observed;
• Figures 14ii is a graph over the three stages referred to in Figure 14i, of residual biomass concentration in the tank of the set up of Figure 12, for three different coagulants (aluminium sulphate, ferric sulphate and ferric chloride)
• Figures 15 a to c are graphs of recovery efficiency at 150mg/L coagulant dose against time at varying pH levels for all three metallic coagulants mentioned with respect to Figure 14ii;
• Figures 16 a to c are plots of algae recovery efficiency as a function of pH at different coagulant concentrations
• Figures 17 a to c are graphs of algae recovery efficiency at pH 5 as a function of time at varying coagulant concentrations for the three metallic coagulant types;
• Figure 18 is a schematic diagram of an airlift tank with internal concentric flow loop
• Figures 19 a to c are snapshots of simulated gas concentration in the tank (across a half section thereof) at (a) diameter 20 ⁇ and (b) after 100 ⁇ over a development period from 0, 40, 80 and 120 sec after commencement, and (c) after 120 sec for different bubble diameters (20, 60, 100, 120 and 140 ⁇ );
• Figures 20a and b are graphs showing (a) the mixture density at different bubble diameters (20, 100, and 200 ⁇ ) and (b) the volume gas fraction across the downcomer section at different bubble diameters (20, 100,200 ⁇ );
• Figures 21 a to d are graphs showing (a) the velocity gas profile in cross-section riser zone after 120 sec at different gas bubble diameter (20, 100, 200, 400, 600, 800, 1000 ⁇ ), (b) the velocity profile at a certain point in the riser zone after 120 sec, (c) the velocity liquid profile across the riser zone after 120 sec at different gas bubble diameters (20, 100, 200, 400, 600, 800, 1000 ⁇ ), and (d) the velocity profile in certain point in riser zone after 120 sec;
• Figures 22a and b are graphs showing (a) the gas volume fraction at a certain point in downcomer zone at different gas bubble diameters (20, 60, 100, 120, 140 ⁇ ) after 120 sec, and (b) the depth of penetration (hp) of microbubbles into downcomer zone at different bubbles sizes (20, 60, 100, 120 and 140 ⁇ ⁇ );
• Figure 23 is a schematic illustration of the experimental apparatus relating to an anaerobic digester in accordance with the present invention
• Figures 24 a and b are graphs showing (a) methane production from the digesters of Figure 23 with and without bubble injection, and (b) methane production per day before and after one hour nitrogen sparging in the digester of Figure 23;
• Figures 25 a and b are graphs showing (a) carbon dioxide produced from the digester, with and without nitrogen injection, and (b) hydrogen sulphide produced from anaerobic digestion, again with and without nitrogen sparging;
• Figure 26 illustrates the distribution of charges around a particle showing different layers of ions, and an approaching microbubble
• Figure 27 is an illustrative graph of zeta potential against distance from a particle surface.
• Figure 28 is a graph illustrating a typical variation of zeta potential against pH of a liquid containing the particle.
• a fluidic diverter 10 is shown in section, comprising a block 12 in which passages indicated generally at 14 are formed.
• An inlet passage 14a has a supply 16 of fluid under pressure connected thereto by an inlet port 18.
• Two outlet passages 14b, c branch from the inlet passage 14a.
• Two control passages 14d,e oppose one another on either side of the inlet passage just in front of the branch 14f between the two outlet passages 14b, c.
• the control passages are supplied by control ports 20d,f which are interconnected by a closed loop conduit 22.
• the arrangement shown in Figure 1 conveniently comprises a stack of several PerspexTM plates each about 1.2 mm thick and laser cut with the outline shape of passage 14. Top and bottom cover plates close and complete passage 14 and hold the stack together, the bottom (or top) one being provided with the ports 18, 20d, 20f, A, and B. However, it has been shown experimentally that the arrangement scales up effectively and is within the ambit of the person skilled in the art.
• Figure 2 illustrates the variation of frequency of oscillation of one system employing air as the fluid in the diverter of Figure 1 , with a control loop of plastics material of 10 mm internal diameter and an airflow of 10 litres per minute. Frequencies between 5 and 25 Hz are easily achieved, but also in the range of a few Hz to 5000 Hz. Again, the arrangement is capable of being scaled-up to provide significant airflows in this range of oscillation frequency.
• a suitable diffuser 30 is shown in Figure 8, which comprises a housing 32 of shallow, hollow cylindrical form and having a central inlet opening 34 for connection to the tubing 17.
• the chamber 36 formed by the housing 32 is closed by a porous disc 38, which may be ceramic, or a sintered metal.
• Such bubble diffusers are known and in use in the water treatment industry, and such products are available, for example, from Diffuser Express, a division of Environmental Dynamics Inc of Columbia, MO, USA.
• the present invention may have application in numerous other fields in which a gas needs diffusing into a liquid.
• the equality of the bubble size or their absolute minimisation in size may not be imperative. Rather, the capacity to retro-fit the arrangement may be more important.
• the arrangement illustrated in Figures 4 and 5 may be employed.
• FIG. 3 two plots are shown of internal pressure against bubble size being formed from two apertures of different size (0.6 and 1.0 mm).
• the pressure increases substantially linearly with increasing volume until the bubble reaches a hemispherical shape. Thereafter, however, pressure decreases as the bubble grows further.
• a bubble can have two sizes. More importantly, however, if two bubbles are growing from two ports that are supplied by a common source in parallel with one another then as the pressure increases with growing bubble size, the growth of the two bubbles in parallel is stable. However, once the bubble reaches hemispherical the stable growth ends and as one bubble continues to grow its pressure diminishes.
• a diffuser 50 comprises a plate 52 having a top surface 54 in which a right-angled groove 56 is formed, with each of its sides 58,60 being angled at 45° to the top surface 54.
• two supply passages 62,64 also lying parallel, and disposed one on either side of, the groove 56.
• Rising up from each passage are a plurality of ports 62a, 64a.
• Ports 64a are relatively narrow and open in the middle of the face 60 of the groove 56.
• Ports 62a are relatively broad and open at the base of the groove 56.
• the passage 62 and the ports 62a are arranged so that the direction of discharge of fluid from port 62a is parallel the face 60 of the groove 56.
• Passage 62 may be larger than passage 64, but the ports 62a are certainly larger than the ports 62b.
• the reason for this is that the passage 62 is arranged to carry liquid, the liquid in which the diffuser 50 is sited.
• the passage 64 on the other hand, carries gas.
• the arrangement is such that the diameter of the gas port 62b is small, according to the desired size of bubble to be formed, and possibly as small as 0.5 mm or less depending on the technique employed to form the port 64a.
• the holes can be drilled mechanically to about 0.5 mm, but other methods exist to make smaller holes if desired.
• a tank 80 of liquid 82 has a diffuser 50 in its base.
• a gas supply 16 supplies gas under pressure to a diverter 10 of the kind shown in Figure 1 , and whose two outputs A,B are connected to passages 64,62 respectively by lines 86,88 respectively.
• connection B has a bleed 84 to the environment above tank 80, so that its pressure is substantially ambient. Consequently, line 88 fills with liquid to the height of the liquid in the tank 80. Indeed, when the air supply 16 is turned off, so does the outlet A and consequently the diverter 10 is located above the level of the liquid in the tank.
• the ports 62a are larger simply because of the increased resistance of the liquid to flow, but also because a large flow pulse, rather than a narrow flow jet, is better at knocking off bubbles.
• each pulse into output A is arranged such that a hemispherical bubble forms at the mouth of each port 64a.
• a jet of water issues from the mouth of each port 62a and is directed against the side of the bubble on the ports 64a and knocks them off.
• the bubbles 90 so formed are therefore very small, or at least much smaller than they would otherwise be, and of very even size distribution.
• Figure 9 illustrates a bubble generator 1000, in which a plate 12' has a conduit 64' having a plurality of ports 64a' connecting the conduit 64 with the liquid 82 in which bubbles are to be formed.
• the conduit 64' is connected via tube 86' to a source of gas under pressure greater than the pressure of the liquid in the ports 64a', so that there is a net flow of gas along the conduit 64'.
• the gas is also oscillating by virtue of a fluidic mechanism (not shown in Figure 9) such as the diverter 10 of Figure 1.
• a glass diffuser 150 is constructed from two sheets of glass 152, 154 adhered face to face, in which, on one sheet 154, channels 156, 158 have been etched, so that, when connected as shown, a large conduit 156 is formed from which several smaller conduits 158 depend and emerge at surface 160 of the diffuser 150.
• a diverter such as that shown in, and described above with reference to, Figure 1
• bubbles are formed at the openings 162 of each conduit 158. If the channels 158 are approximately 60 microns in depth and width, bubbles of a corresponding diameter are pressed from the conduits 158. If the gas flow is oscillated as described above, bubbles of that size break off.
• FIG. 1 1 some existing diffusers employed in waste water cleaning, such as those illustrated in Figures 7 and 8, have a membrane (38, in Figure 8 and in Figure 11 a) which has a number of slits cut through it.
• the mode of operation is already oscillatory to some extent, even with a steady gas flow, as the pressure distends the membrane, opens the slits and, as bubbles pinch off, there is a certain rebound of the lips of the slit before a new bubble begins.
• the differential pressure ⁇ across a slit 170 increases from zero as shown at (a).
• the gas begins to deform the membrane 38 and it is forced through the slit commencing the formation of a bubble 90.
• the membrane deforms further, as shown in (c) accelerating the growth of the bubble.
• the pressure differential begins to decrease so that the natural rebound of the elastic membrane is facilitated, closing off the bubble 90 as shown at (d).
• the membrane returns to the position shown at (a), and (e) but in the latter with the bubble 90 released.
• Figure 1 1 b shows a preferred form of square wave pressure development that is potentially the result of both the fluidic arrangement and slitted membrane, and shows the potential pressure positions at each stage of bubble development illustrated in Figure 11 a.
• Flotation has become the mainstay for colloidal particle separation from an aqueous solution.
• the key subprocess is the generation of microbubbles that attach to hydrophobic particles, resulting in buoyant aggregates which then rise to the surface of the flotation cell, where following bubble rupture, the particles are recovered (Dai et al., 2000). Recovery of valuable end-products has been the centre of attraction in flotation separation.
• Algae in particular, are a reasonable target for flotation separations for biomass processing, but as yet untried with the dense solutions produced from algal cultivation.
• Pienkos and Darzins (2009) highlight harvesting and dewatering operations as a key challenge for economic algal biofuels processing.
• the density can reach 10g/L of dry biomass, which is substantially higher than DAF removal of fine particles in water purification.
• Gudin and Thepenier (1986) estimated that harvesting can account for 20-30% of the total production cost.
• Molina et al. (2003) present possibly the closest technique to microflotation for algal harvesting - flocculation and bioflocculation followed by sedimentation. Flotation is often viewed as "inverted" sedimentation.
• the Jameson Cell (Yan and Jameson, 2004) is an induced air flotation process which also achieves high separation performance for microalgae (98%) and phosphorus.
• algae When present in effluent water, algae could be a pernicious contaminant in potable water treatment otherwise, but could be regarded as a raw material given the numerous products obtainable from the unicellular organism such as ⁇ - carotene (Borowitzka, 1992) glycerol, biomass and in particular, biofuel from lipid (Chisti, 2007). While most previous works have focused on the production of biomass from algae (Zimmerman et al., 2011 b), only few researchers have been concerned with harvesting biomass and lipid from algae. Whether it is for potable water treatment or recovery of algae for biofuel, flotation separation is a viable means for harvesting algae.
• Dissolved air flotation in particular is the most efficient and widely employed flotation option.
• the process essentially requires dissolving air in water at very high pressure. By so doing, the solution becomes supersaturated; leading to nucleation of microbubbles as soon as pressure is reduces at the nozzle.
• this process is energy intensive, due to the high pressure required for air dissolution in water as well as the work done by the pump in feeding the saturator with clarified water.
• FIG. 12 A schematic representation of the bench scale dispersed air flotation unit is shown in Figure 12.
• the main rig components comprise: a flotation cell 170, microbubble generator comprising a fluidic oscillator 172 and 40mm stainless steel baffle distributor diffuser 174.
• the fluidic oscillator 172 (Tesar et al. 2006,(Tesaf and Bandalusena, 201 1) measures: 10cm x 5cm x 5cm in length, height and width respectively while the flotation unit measures: 50cm by 9cm in height and diameter respectively.
• the tests were conducted with the diffuser placed at the bottom of the flotation cell 170.
• the oscillator mid-port 176 was linked by a 0.5m feedback loop 178.
• the supply in the form of a compressor 179 delivered air at a pressure of 0.8bars and a supply flowrate of 85L/min through the oscillator. Microbubbles were generated under oscillatory flow by connecting the diffuser to the outlet of the fluidic oscillator.
• microalgae culture was taken for further processing.
• Two litres of microalgae sample at room temperature (20°C) was mixed to break lumps and disperse the cells homogenously in solution following sedimentation and clustering of cells as a result of prolonged storage. Coagulation and flocculation followed for 4mins and 10mins respectively following pH adjustment.
• the broth was gradually introduced into the flotation column to a height of 30cm above diffuser before the microbubble generator was turned on.
• the diffuser used in this study was made of Perspex material and measures 40mm in diameter and overlaid with a stainless steel mesh (Plastok, UK) with pore size of 38 ⁇ and an open area of 36%. Broth samples were collected every three (3) minutes from sample ports SP1 ,2,3 and 4, and measured with the calibrated spectrophotometer DR 2800 (HACH Lange) to assay absorbance at 663 and 640 nm wavelength. Recovery efficiency (R) was determined using the formula:
• C, and C f are the initial and final algae concentrations respectively.
• the acoustic bubble sizer (Dynaflow, Inc.) was developed to meet challenges in the optical method caused by cloudy liquid. By exploiting the ability of bubbles to affect acoustic propagated waves, bubble size and population can be extracted at varying frequencies (Wu and Chahine, 2010).
• the device consists of a pair of transducer hydrophones 177, made of piezoelectric materials inserted in a polyurethane material to prevent contact with water. Both hydrophones are connected to a computer 175 via a control box. The transmitting hydrophone generates short bursts of sound signals within a set frequency which are then received by the second hydrophone after travelling through the liquid.
• the signals are then analysed by special in-built software for processing the phase velocity and attenuation within the desired frequency range to estimate the size distribution of bubbles.
• the acoustic bubble sizer (ABS) was used in this study for bubble characterisation.
• the two sets of flat hydrophones 177 used (measuring: 7.5x 7.5 x 2.5cm, optimal operating frequency range from 70 ⁇ 200 kHz and corresponding bubble size of 34-100 ⁇ ) were mounted vertically (9cm apart) on either side of the flotation column 170. Three (3) runs were undertaken to determine bubble size distribution under oscillatory conditions.
• Microbubble generation is an essential part of flotation separation.
• Figure 13a presents the distribution of bubble size generated under oscillated air supply conditions.
• the single peak graph shows a positive skew of bubble size distribution which reveals the dominance of 24 ⁇ sized bubbles.
• the smallest bubble produced was 24 ⁇ , while the largest size measured was 260 ⁇ .
• average bubble radius was 86 ⁇ with 60% of the bubbles approximately 74 ⁇ .
• the average bubble size generated with the fluidic oscillator is approximately twice larger than the diffuser 174 pore size (which is 38 ⁇ ).
• the average bubble size achieved was approximately 28 times larger than the diffuser pore size (ie over 1 mm).
• the bubble density graph presented in Figure 13b was determined by measuring the population of bubbles in the column and the results showed that 20-40 ⁇ bubbles made up 95% of the total bubble density, while 5% consisted of bubbles greater than 40 ⁇ in a bubble size distribution of 20-260 microns.
• the narrow distribution range of bubble size not only strongly suggests the production of largely non-coalescent but more particularly, relatively uniformly sized microbubbles.
• the difference in bubble size is simply attributable to the fluidic oscillator.
• the bistable device facilitates microbubble production by oscillating a stream of the continuous air supply.
• the pulse generated due to the oscillation helps to knock-off bubbles at the developmental stage. Without oscillation, bubbles tend to move irregularly, leading to increased bubble- bubble interaction and coalescence leading to larger bubbles. Regular detachment results in less coalescence because the bubbles are more uniformly spaced and sized.
• the level of inertial force in the pulse can be tuned so that bubbles emerge with little excess kinetic energy over the terminal rise velocity (Parkinson et al., 2008).
• Stage 1 is simply attributable to the large surfaces of floes which readily render them susceptible to bubble collision and adhesion, bubble formation at particle surface, microbubble entrapment in aggregates and bubble entrainment by aggregates.
• (Edzwald, 2010) reported these bubble-particle interaction mechanisms in the review of flotation as a wastewater treatment. These large floes also engage in sweep flocculation as they travel upwards under the lift of microbubbles; hence the exponential biomass recovery efficiency recorded at the early stage.
• Stage 2 After half the separation time (being the sum of Stages 1 and 2 together), the amount of large floes decreases markedly. During the next, straight-line, phase (Stage 2), smaller floes become prevalent in the flotation unit.
• surface sludge build-up continues, thickening the sludge blanket. As more bubbles rise to the top, these bubbles compress the sludge layer from underneath, reducing the water content of the sludge.
• the third key stage is primarily characterised by intensive sludge thickening and thinning.
• intensive sludge thickening and thinning By that is meant increasing density of the sludge layer, and hence reducing depth, which makes separation of the sludge easier to achieve.
• the majority of the particles have been separated, ending the separation phase, whereby microbubble rise velocity is increased, since very few particles are present to cause rise retardation.
• the rate of water removal from the sludge is thus high as it is compressed.
• the sludge layer is reduced to almost a quarter of the initial size.
• Chemical pre-treatment is essential in decreasing the effect of repulsive charge between bubbles and floes (except see below).
• the success of chemical pre-treatment depends on pH, because pH determines the solubility of chemical constituents of nutrient and metals in solution and influences the form and quantity of ions produced.
• Optimum pH and coagulant dosing reduces the charge on particles to about zero causing particles to be more hydrophobic (Edzwald, 2010).
• trials were conducted across different pH levels and results reported in Figure 15.
• Figures 15 and 16 present the flotation results for three metallic coagulants.
• the effect of pH on algal removal efficiency from Figure 15(a) showed that with aluminium sulphate coagulant, efficiency increases with decrease in pH to the lowest at pH 7 before rising again as pH increases to 9.
• Optimum recovery result of 95.2% was obtained at pH 5 with efficiency gradually decreasing to 71.9% at pH6 and 50.6% at pH 7.
• pH 8 however, a sudden increase to 74.6% was obtained and 81.5% at pH 9 indicating the other peak of result with aluminium sulphate.
• Data from Figure 15(b) (ferric sulphate) can be compared with Figure 15(a) which showed a similar trend in the effect of pH on algal recovery efficiency.
• Airlift bioreactors have many advantages over stirred tanks. For instance, there are no moving parts inside the reactor, low cost of installation and maintenance, and low energy consumption. However, bio-reactors would benefit from increased efficiency of mass and heat transfer rate in gas-liquid processes. Enhancement of mass transfer rate in gas-liquid interface has been traditionally dependent on increasing interfacial area between gas and liquid phases. The use of microbubbles not only increases surface area to volume ratio, but, also, increases mixing efficiency through increase in the liquid velocity circulation around a reactor. The mixing process in bioreactors is an important and critical factor in determining the efficiency of fermentation process and the nature of design which plays an active role in providing a suitable environment for micro-organisms. The traditional mixing method (i.e.
• stirred tanks may yield better performance in the degradation process, yet when the process energy requirement is weighed against the energy obtained from biogas produced, these processes become economically unviable. Therefore, the reduction of the energy required for mixing is one the most challenging targets that is faced by advanced developments of bioprocess applications.
• airlift reactor has been used in several industrial applications, and it has been the most appealing option for any gas-liquid contacting process. It has been noticed that using airlift reactor intensifies the efficiency the process compared to stirred tanks.
• airlift reactors can be classified into two main types: airlift external loop reactor, in which the circulation takes place in separate conduits; and, airlift internal loop reactor, which is provided with a tube or a plate to create the conduit (channel) inside a single reactor for circulating the liquid inside the reactor.
• the latter is shown in Figure18 and comprises a tank 180 containing a biological liquid medium 182 providing a head space 184.
• a gas diffuser 186 is provided at the floor 188 of the tank supplied with an oscillating supply 189 of gas from a source (not shown) whereby bubbles 190 of gas may be introduced.
• a baffle or draft tube 192 divides the tank 180 into a riser section or region 194, immediately above the diffuser 186, and a surrounding annular downcomer region 196.
• the tank may be circular cylindrical, with a diameter D
• the draft tube may be likewise circular cylindrical with a diameter d, each centred on the axis A of the tank at which the diffuser is also positioned.
• the draft tube has a top edge 198, spaced from the surface 200 of the biological liquid medium 182, and a bottom edge 202 spaced from the diffuser 186 and bottom 188 of the tank.
• a toroidal path is thus established comprising the riser section 194, over the top edge 198 of the draft tube, down the downcomer section 196, and under the bottom edge 202 of the draft tube back into the riser section.
• Bioreactor design requires accuracy in choosing the dimensions and materials required for manufacturing due to the complexity of the medium.
• the biological medium is a multiphase mixture, which consists of solid, liquid and gas, as well as having different microorganisms that need suitable environmental conditions. It is conceivable under such situations to provide reliable control systems for pH and temperature monitoring, in addition to maintain the process under anaerobic conditions (if required).
• a cylindrical bioreactor shape as airlift gas injection was used in the current study.
• the ratio (D/d) of the diameter (D) of the bioreactor to the draught tube diameter (d) was 0.7.
• the volume of reactor was 15 litres, while 8-9 litres were working volume leaving 6-7 litres in the head space.
• a laminar bubbly flow model interface was used for modelling of the two-fluid flow regimes (e.g. mixture from gas bubbles and liquid), driven by gravitation through the density difference between gas-bubble-containing liquid in the riser section 194 and depleted-gas- bubble-containing liquid in the downcomer section 196.
• # is liquid volume fraction (m 3 / m 3 ), p is density of liquid, ⁇ the velocity of liquid phase (m/s), t is time (sec), P is pressure (Pa), % is dynamic viscosity of liquid phase (Pa.s) and g the gravity(m/s 2 ).
• the liquid holdup coefficient (0 ; ) is about unity. Therefore, the change of & can be neglected in the following equation.
• M m is the molecular weight of the gas bubble
• J? is the ideal gas constant (8.314 J/(mol.K) and T the temperature of gas (K).
• the gas volume fraction is estimated by the following equation
• the retention time of the gas bubbles increases dramatically (e.g. doubled, if it is assumed that the rotation of these bubbles has only been for one cycle).
• the residence time of gas micro-bubbles in the "downcomer" zone would be longer than that for the riser zone, if the gravity force is considered.
• the buoyancy force of the gas bubbles is balanced with their drag force caused by flowing liquid; thus, leading to stationary states of the bubbles velocity, which cause the residence time of these bubbles to increase.
• the controllable size of micro-bubbles generated by fluidic oscillation would add another advantage to gaslift bioreactor system by increasing the mass and heat transfer not only in the riser region but also in the downcomer region.
• the goals of the mixing system in biological processes include prevention of the formation of thermal stratification, maintaining uniformity of the pH, increase of contact between feed and microbial culture, and preventing fouling and foaming.
• some of biological media are viscous liquids, of high density, and contain solids, grits etc, thus, mixing of these materials thoroughly in order to achieve the desired objectives requires a great effort and energy.
• using a bubbling system for mixing of such media is inefficient at certain flow rates. Owing to generation of foams at the top of the culture surface, an increase of induced gas flow rate becomes necessary, perhaps rendering the entire bioprocess uneconomical.
• FIG. 21 a shows a gas velocity profile in the riser zone, at 0.12 m height level and between 0 and 0.06 m radius from the centre of the tank at different bubble sizes.
• Figure 21 b shows the gas velocity in the Y (vertical) direction at different bubble sizes in certain points in the riser zone.
• Figures 21 c and d show the liquid velocity profile at different bubbles sizes within similar areas, rise times, and distances mentioned above.
• Penetration depth (hp) of the micro-bubble into the downcomer zone was also investigated in the present study. Depth of penetration represents, and can be an indicator, of enhanced efficiency of the mixing system in an airlift bioreactor as a result of the increased residence time in this region. Greater transfer rates of heat and mass would be achieved by higher residence times.
• that gas volume fraction increases in the downcomer zone with decreasing the bubbles size.
• Figure 22a presents the gas volume fraction in downcomer region at various bubble diameters (20, 60, 100, 120 and 140 ⁇ ).
• the penetration of the microbubbles into downcomer depends on their diameter. For example, the depth of penetration of microbubbles with diameter 20 ⁇ was more than was observed for the microbubbles diameter of 100 ⁇ as shown in Figure 22b, and the snapshots of gas concentration in Figure 19c.
• the simulation data showed a maximum velocity of liquid in Y-axis (along axis A) that could be achieved with a ratio of 0.6 (m/m) is higher than that observed with ratio of 0.7, 0.8, and 0.9 (m/m).
• a narrow entrance between the diffuser and draft tube also contributed in increasing the velocity of liquid phase in the riser region.
• Anaerobic digestion of already digested sludge by processing in an airlift bioreactor is used for nutrient and energy recovery from biomass. It is used to breakdown organic matter into methane (CH 4 ), carbon dioxide (C0 2 ), hydrogen sulphide (H 2 S). Digested sludge is dried and used for fertilizer. There are four biodegradation stages. The rate of gas generation through mesophilic anaerobic digestion is generally high, yet the remaining dissolved gases in a digested sludge have a pejorative effect on the environment when they are eventually released, as well as causing operational difficulties. The generation of biogas in an already digested sludge causes cavitation phenomena in pumps.
• An airlift bioreactor is used as anaerobic digester in the present invention to remove the produced gases from digested sludge, with a resultant reduction in pathogens and odour, as well as improvement digested sludge for fertilizer.
• ALRs have many valuable benefits in comparison with stirred tanks for instance: there are no moving parts inside the reactor, low cost of installation and maintenance, and low energy required.
• using an airlift reactor enhances the mixing efficiency. The process is preferable to agitation by stirring in conventional tanks on power consumption grounds.
• the experimental data discussed below shows that the cumulative methane production of an airlift anaerobic digester is about 30% more than the observed in the conventional anaerobic digester, and even greater efficiency is achieved, as discussed further below, by nutrient supply.
• each reactor comprises a digester tank 280, 282, with the tank 280 of the reactor D1 being arranged as the tank described above with reference to Figure 18.
• the sparger 286 was supplied with gas from a nitrogen generator 220.
• Each tank 280,282 was monitored for pH and temperature through probes 222, 224 and respective controls 226,228. Gas evolved from each tank was collected in collector tanks 240, which each was connected through a selectively engageable gas analyser 242.
• wastewater generally comprises lipids, polysaccharides, protein and nucleic acids which are bio-degraded by anaerobic bacteria to produce biogas and effluent, which used as fertilizer.
• facultative anaerobes including methanogenic bacteria and organic particulates should be present in the sludge.
• the primary clarifier in waste treatment provides particulates and many anaerobes including methane- produce bacteria, whilst the secondary clarifier provides many facultative anaerobes.
• the digested sludge was collected from the outlet stream of a full-scale mesosphilic digester from a wastewater treatment plant in Sheffield city, UK. Digested sludge has methanogenesic bacteria but with low concentration of substrates.
• Figure 24a shows that during 170 hours, the cumulative methane production from the airlift anaerobic digester 280 was about 29 % higher than observed in the control anaerobic digester 282.
• a large amount of methane obtained from the airlift anaerobic digester occurred during the sparging with nitrogen for one hour daily as shown as in the Figure 24b.
• the essential ingredient in the biological medium is water with a composition of 90- 95% depending on the type of bioprocess. For instance, water content in sludge is around 95%, while 5% consists of micro-organisms, organic matters, elements and suspended solids. Micro-organisms feed on the organic matter and elements to produce gases by metabolic processes. Carbon dioxide, methane, and hydrogen are highest composition gases produced from fermentation process. The ability of these gases to stay in the liquid phase is related to their relatively solubility, which is 1.45 g gas/kg water in the case of carbon dioxide, 0.0215 g gas/kg water in the case of methane and 0.00155 g gas/kg water in the case of hydrogen.
• C0 2 is relatively highly soluble compared with CH 4 and H 2 . Thus, it will be stay in the liquid phase longer as dissolved aqueous gas (C0 2(aq) ) .
• C0 2(aq) dissolved aqueous gas
• Released carbon dioxide reacts with water to produce carbonic acid. Kinetically, the conversion to carbonic acid is very slow, just 0.2% of carbon dioxide converts to carbonic acid and its ions, while 99.8% remains as dissolved gas.
• Carbonic acid is a diprotic acid, dissociating into bicarbonate and carbonate ions, and producing two hydrogen atoms ionisable in water.
• Figure 25a shows the production of carbon dioxide from the anaerobic digester with and without nitrogen sparging.
• the figure shows that the bubbling system in anaerobic digestion contributes to increasing the carbon dioxide in biogas production.
• the efficiency with the bubbling system was 350% more than with the control digester.
• the complex characteristics of the sludge have played an important role in stripping of all gases.
• H 2 S The high solubility of hydrogen sulphide contributes to remaining in the sludge as H 2 S(aq).
• H 2 S dissolves in sludge, the pH, also, would drop due to releasing a hydrogen ion and forming a weak acid.
• the behaviour of the solubility of hydrogen sulphide is very similar to carbon dioxide because both gases form a diprotic acid in water.
• Sulphate dissolved with a high concentration can inhibit generation of biogas produced from the anaerobic digestion of wastewater.
• the most important reason leading to this inhibition is that the sulphate dissolved in wastewater encourages growth of sulphate-reducing bacteria, which consume acetic acid and hydrogen that would otherwise be consumed by methanogenesic bacteria.
• This competition between the sulphate-reducing bacteria and the methane-producing bacteria for the consumption of the hydrogen and acetic acid can be illustrated thermodynamically through the equations:
• Figure 25b shows the hydrogen sulphide removal from digested sludge during nitrogen bubbling. The figure indicates that with one hour of nitrogen sparging with fine bubbles, there is a stark increase in the removal of hydrogen sulphide compared to a conventional digester.
• methane is "strongly" gaseous at room and warmer temperatures (anaerobic digestion is exothermic and can elevate temperatures of digesters to circa 35°C).
• methane is also "sticky", with respect to particles and bacteria surfaces, and therefore does not easily escape the liquid phase after its release by the producing bacteria. Collisions with biogas bubbles however provide an opportunity for methane to escape the liquid environment and return to the gas phase and thus enlarge the bubbles cycling through the digester.
• the concentration of carbon dioxide, and other gases such as hydrogen sulphide therefore inevitably also decreases within the bubbles as methane is absorbed. By that means, a concentration gradient is restored across the bubble surface driving more dissolved gases into the bubble.
• pure carbon dioxide may well contain impurities. Nevertheless, not only do the bubbles of such gas extract the methane, just as biogas or nitrogen does, but also they provide additional fuel to encourage growth of the methanogenesic bacteria. Consequently, the yield of methane is yet further enhanced.
• the bubbling of gas into the anaerobic digester may be undertaken once or twice daily over a period of perhaps one hour on each occasion. If the bubbles are small, in the order of 10-30 ⁇ then they have such a slow rise rate in the tank that they will remain in place for up to 24 hours. During that period they are resident in the digester and, if sufficient quantity of bubbles are injected, and the bubbles are small enough, microorganisms throughout the digester can access bubbles to shed themselves of the inhibiting methane.
• the same anaerobic processes can be employed in the anammox process, for the digestion of nitrates by producing nitrogen.
• the application of the anammox process lies in the removal of ammonium in wastewater treatment and consists of two separate processes.
• the first step is partial nitrification (nitritation) of half of the ammonium to nitrite by ammonia oxidizing bacteria:
• a negatively charged colloid particle 300 has closely and firmly packed opposite ions 310 (positively charged) surrounding the particle surface, referred to as the Stern layer. This is followed by a layer 320 of relatively less strongly held ions, found just away from the particle surface. These two arrangements of charges are referred to as the double layer 315. Further away from the double layer 315 there exists loose ions 330 that result in the formation of a diffuse layer.
• a shear plane extends from the Stern layer to the diffuse layer.
• the shear plane is loosely attached to the particle relative to the Stem layer, but is unsusceptible to an external velocity gradient in the liquid, and is therefore bound to the particle as the particle moves within the liquid continuous phase.
• the electrical potential difference between the colloidal particle in the shear plane and the liquid bulk is known as the zeta potential and decreases away from the particle as shown in Figure 27.
• the zeta potential is a measure of the electrical charge of a colloidal particle.
• a denotation of the potential stability of the colloidal system can be given by the magnitude of the zeta potential and it can be mathematically expressed as in Eq.1 above.
• the differentiating factor between a stable and an unstable suspension can be taken as +30 mV or -30 mV.
• Mean zeta potential for colloidal particles in wastewater ranges from -12 to +40 mV.
• a crucial factor influencing the particle zeta potential however is the medium pH.
• IEP isoelectric point
• Hydrophobic aggregation is similar to froth flotation where particles are held in close proximity to be selectively hydrophobised. The particles undergo strong agitation. Non-polar oil could be an additive to improve aggregate strength.
• Other types of hydrophobic aggregation include: emulsion flotation, shear flotation, oil-extended flotation, spherical agglomeration, carrier flotation and two liquid extraction.
• Coagulation differs from selective flocculation in that the addition of an electrolyte causes a decrease in electrostatic repulsion between particles.
• the energy barrier between particles that prevents agglomeration is overcome by coagulant addition.
• the disadvantage associated with this method of particle agglomeration is that it produces heterocoagulation, and so it is mainly employed in fields other than the mineral industry. Nonetheless, aggregation by coagulation is still the most widely applied technique of the three sorts but choice of technique ultimately depends on the recovery process as well as the desired end product.
• Particle destabilization by the addition of a coagulating or flocculating agent occurs by four (4) known mechanisms viz: the compression of the electrical double layer, adsorption and charge neutralization, adsorption and inter-particle bridging and the enmeshment in a precipitate.
• coagulant and flocculants there are two main categories of coagulant and flocculants viz: organic and inorganic coagulants and organic flocculants
• Metal salts are the most common coagulants available and are still widely employed in water purification with aluminium salts being the most commonly used. These cations hydrolyse rapidly in the liquid medium and interact with particles, neutralising their net surface charge.
• aluminium salts When aluminium salts are added to an aqueous solution a rapid hydrolysis reaction occurs to form other dissolved Al ions.
• the main Al-hydroxide precipitates that result following dissolution of the metal salts are: Al 3+ ; AI(OH) 2+ ; AI(OH) 1 2+ ; AI(OH) 1 4" and the amorphous AI(OH) 3(am) .
• Al species distribution in an aqueous solution is however pH dependent. In acidic pH, Al 3+ is the predominant species present. But with increase in pH, Al ions with lower positive charge become dominant. As pH exceeds 6.5, the most active species are the AI(OH) 1 4 ⁇
• Speciation of coagulants can also be temperature dependent. In cold water, positively charged Al species dominate, but this is less important than pH. However, what has not been appreciated in the past is that it is not the pH in the bulk liquid that matters but only the pH in the region between particles.
• a bubble is just another particle, although with different zeta potential compared with particles in suspension in a liquid.
• a bubble carrying a gas that permits active species to cross the boundary between the bubble and liquid in which it is immersed will affect, first, the pH in the immediate environment of the bubble. If that permits a bubble to approach a particle before electrostatic repulsion takes place, the attraction of particle to the gas phase by virtue of its hydrophobic nature permits attachment of the particle to the bubble before it is electrostatically repelled.
• the zeta potential (or isoelectric point) is squeezed between the bubble carrying the pH-adjusting gas and the particle.
• a microbubble 400 is seen approaching the particle 300. Because the bubble is so small (circa 40 ⁇ in diameter) and has been injected into the liquid with little energy, it has surrounding it a boundary layer 410 that is, to all intents and purposes, stationary with respect to the bubble. Boundary layers are usually defined as the radial position relative to the bubble centre where 99% of the free stream velocity is achieved. Within the boundary layer, it is well known that diffusion dominates over convection of mass. The bubble contains carbon dioxide that establishes, by diffusion of ions from the bubble, a pH gradient across the layer 410, between a minimum pH at the bubble surface 420, towards the bulk liquid pH in the diffuse layer 330 around the particle 300.
• the zeta potential from the particle surface 312 may be as shown in solid line 500 in Figure 27.
• the zeta potential between the particle and bubble is squeezed towards that shown by dotted line 510 in Figure 27, as H + ions surrounding the bubble in the boundary layer 410 invade the diffuse layer 330.
• the electrostatic repulsion of the bubble by the particle that would pertain if the bubble contained a non-pH-adjusting gas is inhibited, possibly sufficiently for the bubble and particle to attach.
• the particle is generally hydrophobic, once the double layer 315 is breached, the particle 300 attaches to the bubble 400 and thereby can be lifted with it, along with other particles, and ultimately be taken to the surface of the liquid in the tank.
• the particles to which the above application applies may be residual waste particles in an anaerobic digester as described above or algal species grown in bioreactors as described above, but the invention is not limited to such particles.
• GUDIN C. & THEPENIER, C. 1986. Bioconversion of solar energy into organic chemicals by microalgae. Advances in biotechnological processes, 6, 73-110.
• MIETTINEN T., RALSTON, J. & FORNASIERO, D. 2010. The limits of fine particle flotation.
• MOLINA GRIMA E., BELARBI, E. H., ACIEN FERNANDEZ, F. G., ROBLES MEDINA, A. &
• PERNITSKY D. J. & EDZWALD, J. K. 2006. Selection of alum and polyaluminum coagulants: principles and applications.
• PIENKOS P. T. & DARZINS, A. 2009. The promise and challenges of microalgal-derived biofuels. Biofuels, Bioproducts and Biorefining, 3, 431-440.
• G. & POHL, P. I. 201 Critical conditions for ferric chloride-induced flocculation of freshwater algae. Biotechnology and Bioengineering, n/a-n/a.
• ZIMMERMAN W. B., HEWAKANDAMBY, B. N., TESAR, V., BANDULASENA, H. C. H. & OMOTOWA, O. A. 2009.
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