DK180237B1

DK180237B1 - A method and an apparatus for recovering at solid organics from a flow of an effluent of partly digested biomass.

Info

Publication number: DK180237B1
Application number: DKPA201770365A
Authority: DK
Inventors: Jessen Jürgensen Erik; Christian Svendsen Tore
Original assignee: Bio Aqua As
Priority date: 2017-05-23
Filing date: 2017-05-23
Publication date: 2020-09-04
Also published as: WO2018215040A1; EP3630938A1; DK201970806A1; DK201770365A1

Abstract

An apparatus is operated in a method that comprises the steps of providing a first flotation tank (5) with a first riser pipe (11) erected in a first flotation chamber, providing a continuous flow (E) of effluent (2) to the first flotation tank (5) via the first inlet (10), providing a first mixture (M1) of a first liquid medium (LM1) with microbubbles (15) of a first flotation vehicle (FV1) in form of a first inert, anoxic or low-oxygen gas and pumping the first mixture (M1) to the first flotation tank (5), and allowing the microbubbles to attach to organics inside the first riser pipe to form aggregates that can float and settle at the top zone of the first flotation tank and can be further digested in the digester of the biogas plant. A further processing and refining of a lower phase can take place in a second flotation tank in a similar manner. Process gas is the gas already present in the digester and dispersion water for microbubbles is water re-circulated from a second lower phase formed in the second flotation tank in response to a second formation of floating aggregates.

Description

. DK 180237 B1 The present invention relates to a method of continuously recovering at least solid organics from a flow of an effluent from a biogas plant and re-circulating the recovered substances to the biogas plant.

The invention further relates to an apparatus for performing the method.

A biogas plant converts biomasses, such as manure, waste and other organic feedstock into biogas by anaerobic digestion in a digester with anaerobic organisms, which digest material inside a closed system, or by fermentation of biodegradable materials. As the organic materials decompose in the absence of oxygen methane gas is created. Thus the biogas is primarily methane and carbon dioxide that can subsequently be combusted or oxidized with oxygen to be used as a fuel and to make electricity, or be used as a chemical feedstock for producing petrochemicals.

The effluent however still contains a substantial amounts of valuable organics wherefrom biogas could have been produced if economical feasible.

In an exemplary continuous method an average retention time of 20 days in a fully stirred digester is assumed typical. In such a digester one twentieth of the biomass is replaced every day by the daily in- and outflow. About 20% of the biomass pumped into the digester on day 1 will have been pumped out again already after five days. This biomass will therefore not be fully digested. However, about one third of the biomass will still be in the reactor after 20 days. This relationship is decisive of how much of the theoretically potential gas production of the biomass can be achieved before the biomass is pumped out again.

, DK 180237 B1 So the biomass in a fully stirred digester of the biogas plant will never be completely digested. The digester effluent from the biogas plant may for some readily digested biomass have a content of organic solids as little as about 2 — 5 wt/vol %, the rest being water. However, the digester effluent is often much higher in suspended organics. A centrifuge can be used to recover organics. The filtrate from the centrifugation can be subjected to lime clarification, additional centrifugation, ammonia stripping, and a spiral reverse osmosis (RO) unit. There are however severe problems with these conventional methods, such as that the centrifuge plugs up, the use of which then is discontinued, with result that the overall production comes to an expensive stop. Another problem is that the clarifier often does not remove enough suspended solids. In the further processing of the filtrate or effluent the ammonia- stripping column and the spiral RO membrane may be fouled.

A known method for recovering product from an effluent from a biogas plant is disclosed in International patent application no. WO2011/19373. The separation method utilizes an aerated inlet mixture of fluid and solids together with one or both of a coagulating agent and a flocculating agent. This aerated inlet mixture is fed into a cylindrical, upright, separation vessel at four spaced-apart locations at a center part of said vessel, so as to be injected tangentially along the cylinder wall and force aggregates towards the top of the vessel. Tangentially injection of the inlet mixture promotes the formation of solid aggregates so that the inlet mixture becomes separated into an upper float layer and a lower clarified layer. The aerating gas that contributes in the gas flotation separation of the solid contaminant by attaching to the solid particles aids in forming an aggregate mass of solids and gas microbubbles. The density difference between the aggregate mass

2 DK 180237 B1 and the fluid medium provides a buoyant force to drive the separation process. The aerating gas, e.g. methane, is generally added to the inlet fluid as a dissolved gas at an elevated pressure relative to the operating pressure of the separation vessel. Upon a reduction in pressure by e.g. traversing an orifice plate upon entering the separation vessel at each four inlets, the aerated gas leaves solution and forms the aerated gas micro-bubbles. The upper float layer is withdrawn from the vessel when the height of the upper float layer exceeds the height of an overflow conduit and forms a concentrated solids effluent. The lower clarified layer is withdrawn from the separation vessel as a clarified fluid effluent. The separation process is performed continuously using a control process that maintains a relatively stable distribution between the lower clarified layer and the upper float layer. The upper phase can be re-circulated as a feed substrate to a biogas plant however such re-circulated matter is polluted with the coagulating agent and a flocculating agent.

Although this known continuous method and system suggest a method to recover and re-circulate solids of the upper float layer to a digester, the use of coagulating agent and flocculating agent is not desired, on the one hand due to costs and on the other hand due to pollution and contamination of the upper float layer by being part of the coagulated and flocculated agglomerates of the digestible substrate. Furthermore, the inlet mixture needs to be injected tangentially at several points at a center section of the separation vessel or at an even higher location to achieve the swirl that is needed to continuously stir of the large reaction chamber to create the solid aggregates in the large reactor chamber, and move those towards the upper float layer. For this known method a single injection position close to the bottom of separation vessel would only create a small or inferior swirland limited level of aggregation. Thus this known method and apparatus are complex, and there is high need to carefully keep control of the operating parameters.

W09220628 disloses a method of recovering solids organic from a flow of an effluent from a biogas plant and re-circulating the recovered solid substances to the biogas plant, wherein the recovery is performed by flotation using anoxic gases.

There is a continuing and increasing trend to rely on renewable energy and the number of biogas plants is increasing and there is a demand for environmentally acceptable methods and apparatuses for treating waste from a biogas plant.

A main aspect of the present invention is to provide an efficient method and an efficient apparatus to increase and optimize the gas production from the biogas plant.

It is yet an aspect of the present invention to provide an improved method and apparatus for collecting solids suspended in the effluent from a production plant or manufacturing site, including from a biogas plant, without using a centrifuge.

It is yet an aspect of the present invention to provide an improved and economical attractive method and apparatus for producing biogas from the organics of the effluent from the biogas plant.

It is yet an aspect of the present invention to provide a method and apparatus for use with a biogas plant to optimize cleaning of effluents from said biogas plant.

It is yet an aspect of the present invention to provide a method and apparatus that reduce the need for secondary storage capacity of effluent and waste from a biogas plant.

. DK 180237 B1 It is yet an aspect of the present invention to provide a method and an apparatus for performing flotation of at least organics in a continuous feed of effluent to a flotation tank without chemical additions and stirring.

The novel and unique whereby these and other aspects are achieved according to the present invention consists in the provision of a method that comprises the steps of a) providing a first flotation tank with a first flotation chamber, which first flotation tank comprises at least - a first inlet located at a bottom zone of said first flotation tank and a first outlet located at a center zone of the first flotation tank, and - a first riser pipe erected in the first flotation chamber, which first riser pipe has a first riser pipe inlet and an opposite first riser pipe outlet, b) providing a continuous flow of effluent to the first flotation tank via the first inlet, c) providing a first mixture in form of a first liquid medium mixed with microbubbles of a first flotation vehicle in form of a first inert, anoxic or low-oxygen gas and pumping the first mixture to the first flotation tank, d) passing the first mixture through a first nozzle means located at the first riser pipe inlet or at the first inlet for reducing the pressure of the first mixture that rises through the first riser pipe and releasing the microbubbles into the effluent in the first riser pipe, e) allowing the microbubbles to ascend through the first riser pipe, and the first mixture to separate into a first upper phase and a first lower phase in the first flotation chamber, and f) re-circulating the first upper phase to the biogas plant. Within the context of the present invention "microbubbles” are bubbles smaller than one millimeter in diameter, but larger than one micrometer. Within the context of the present

DK 180237 B1 invention the term "organics” refers to organic matter that is capable of decay, the product of decay, or substances composed of organic compounds. The terms “organics”, "organic matter” and "organic substances” are used interchangeable in the 5 present application. The term "methane potential” refers to the volume of methane biogas produced during anaerobic degradation in the presence of bacteria of a sample initially introduced, expressed under Normal conditions of Temperature and Pressure (0°C, 1013 hPa).

The composition of the biomass being digested in the biogas plant may vary considerably and is not exactly known. So in all biogas plants some solid organic matter escapes the digester without having been digested and converted fully into biogas. How much organics that escape depend amongst others on the reaction conditions and the reaction parameters of the digestion process, including the composition of the biomass, the digestion time, the reaction temperature, pH, stirring, potential inhibitor concentrations, the feed speed and density of the biomass, etc. The more complicated the molecules of the biomass are the longer it takes a microorganism to break the molecule down. So normally digestion of organics in an effluent from a biogas plant are incomplete and the effluent can have a substantial residual caloric content and methane potential represented by undigested organic matter. Even 1f the concentration of solid organic matter in the effluent is low after primary digestion of the biomass, such as e.g. just 2% wt/vol or even lower, the caloric content and methane potential of said organics are still valuable to derive from the effluent. By simply being able to recover and re-circulate the residual organics of the biomass that are present in the effluent from the biogas plant to the digester after initial digestion, the caloric content and methane potential of a larger part of the biomass can be utilized, and the need for expensive secondary storage capacity for effluent is reduced.

; DK 180237 B1 In the method of the present invention the effluent from the biogas plant is mixed with a first mixture containing microbubbles of a first gaseous flotation vehicle dispersed in a first liquid medium. The first mixture is fed to the first inlet of the first flotation tank, which first inlet is in fluid communication with the first riser pipe inlet with the first nozzle means. The micro-bubbles of gas in the first mixture is released in the first riser pipe upon passing the first nozzle means, which passage also reduces the feed pressure of the first mixture. The first mixture with microbubbles dispersed therein then ascends the first riser pipe erected in the first flotation tank at a pressure equalized by the first nozzle means to typically about the water column pressure existing in the first flotation tank.

During the ascending of the microbubbles of gas in the first mixture in the first riser pipe, said first riser pipe serves as a confined first reaction chamber where microbubbles and organics of the first mixture are kept in a close contact to facilitate and promote formation during rising of the microbubbles through the first riser pipe of aggregates formed by the attachment of the microbubbles to the organic matter. The microbubbles make the aggregated substances buoyant and carries said aggregates gently upwards through the first riser pipe directly into the first upper phase as a first flotation layer that settles, e.g. as a foam, as a top surface layer at a top zone of the first flotation tank. The first liquid phase, which is the first lower phase, has a lower density than the first flotation layer, which is the first upper layer, so the first liquid phase flows towards a bottom zone of the first flotation tank as a first lower phase when the reacted first mixture leaves the first riser pipe outlet. The first lower phase is substantially water containing dissolved, not yet digested organic matter, inorganics and optionally some indigestible matter that cannot be attached to the microbubbles. The selected gas keeps same anaerobic conditions

2 DK 180237 B1 as in the digester. No additional stirring that could disintegrate the formed aggregates or swirls for enhancing contact between gas and organics are needed, nor is any flocculation agents and coagulation agents used.

The first riser pipe serves as a separate temporary confined reaction and flotation chamber wherein the microbubbles induce aggregation of residual solid organic matter in the effluent from the biogas plant by attaching to said residual organics during the rising. The retention time of the organics during passage of the first riser pipe suffice for creating coherent floatable aggregates containing said organics, which floatable aggregates are driven upwards due to, on the one hand the driving force of the microbubbles flowing upwards, and on the other hand due to the aggregates having a lower density than the liquid first lower phase. The first riser pipe preferably has the first riser pipe inlet aligned with the first inlet and is shorter than the overall height of the first flotation tank to leave a head space for the first upper phase, and optionally also for establishing a distinctive distance between the first upper phase and the first riser pipe outlet so that aggregates can continuously float freely upwards and out through said first riser pipe outlet without plugging or clogging of the first riser pipe.

A thorough dispersing of microbubbles of selected gases of the first flotation vehicle in the first liquid medium is e.g. obtained by means of a pump, preferably a turbine pump. The first liquid medium with its content of gas microbubbles is typically pumped to the first inlet of the first flotation tank by the pump at a pressure substantially higher than the water column pressure in the first flotation tank, e.g. a pressure about 3 — 5 bar. A suitable turbine pump is e.g. the MICRO/NANO BUBBLE GENERATOR obtainable from Rotary Trading Company, Industrieterrein 't Woud 28, 3232 LN Brielle, The Netherlands.

9 DK 180237 B1 Rotary Trading Company is the European Distributor for Nikuni pumps.

The first nozzle means reduces the pressure of the first mixture and releases the microbubbles of gas of the first mixture into the effluent at the first riser pipe inlet.

The incoming pressure of the first mixture to the first flotation tank is preferably reduced at the first riser pipe inlet to a pressure close to the water column pressure in the first flotation tank so that the first mixture is not driven so fast towards the first riser pipe outlet that formation of aggregates of organics of the effluent and gas microbubbles of the first mixture are hindered and/or are limited, and so that disintegration of such floatable aggregates are prevented.

The first nozzle means for obtaining the reduction of the pressure of the first mixture into the first riser pipe include e.g. a first nozzle positioned inside the first riser pipe at the first riser pipe inlet.

The orifices of the first nozzle provide restrictions for the incoming flow of the first mixture to control the pressure of the incoming gaseous first mixture when it enters the first riser pipe.

The outlet pressure from the first nozzle of the first mixture is balanced substantially to the pressure of the water column head as there is nothing to restrict flow on the exit side of the orifices.

The flow of first mixture that passes through the orifices to the exit side of the orifices at reduced pressure, depends on the pressure of the first mixture when it hits the orifice(s) of the first nozzle, and of e.g. the size, shape and design of the first nozzle.

Because the flow of first mixture is restricted by the orifices of the first nozzle, it follows that the pressure on the exit side of the orifices is less than that on the inlet side, thus the pressure is reduced and the microbubbles are released into the effluent.

An effluent from a biogas plant is however not a homogenous mass and the orifices of the first nozzle means may clog.

To

10 DK 180237 B1 prevent clogging of the orifices of the first nozzle, the first nozzle may conveniently be self-cleaning, in that the at least one slot can be opened pneumatically on a timer-controlled basis. The first nozzle means can then operate uninterrupted and under same conditions at all times. The opening and size of the orifices of the first nozzle can be controlled by being subjected to pressurized gas, e.g. pressurized methane directed through the orifices. The pressure of the pressurized gas directed towards the orifices of the first nozzle may be adjusted in accordance with the static pressure in the first flotation tank to open, close and/or re-size the orifices. The first upper phase settles at the top zone of the first flotation tank and is harvested, and optionally pressurized, and re-circulated to the biogas plant for further digestion.

Thus floating matter is re-used to make more biogas. The above method may further comprises the steps of g) providing a second flotation tank that defines a second flotation chamber, which second flotation tank comprises at least - a second inlet located at a bottom zone of said second flotation tank and a second outlet located at a center zone of the second flotation tank, and - a second riser pipe erected in the second flotation chamber, which second riser pipe has a second riser pipe inlet and an opposite second riser pipe outlet, h) transferring the first lower phase from the first flotation tank to the second flotation tank, i) providing a second mixture in form of a second liquid medium with microbubbles of a second flotation vehicle in form of a first inert, anoxic or low-oxygen gas and pumping the second mixture to the second flotation tank, j) passing the second mixture through a second nozzle means located at the second riser pipe inlet for reducing the pressure of the second mixture that rises through the second

11 DK 180237 B1 riser pipe and releasing the microbubbles into the first lower phase in the second riser pipe, k) allowing the microbubbles to ascend through the second riser pipe, and the second mixture to separate into a second upper phase and a second lower phase in the second flotation chamber, and 1) re-circulating at least a fraction of the second upper phase to the biogas plant.

Even though the first mixture has been separated in the first flotation tank the first lower phase may still contain some organic matter and thus still represent a source of bio-energy and methane potential. Accordingly the first lower phase may conveniently be subjected to yet a flotation process in a second flotation tank.

Preferably the first flotation tank has a conical bottom zone and optionally also a conical top zone. The first inlet can then be expediently located at the tapered tip of the bottom zone. The second flotation tank may have the same design, but can in the alternative be of a different design, such as an oblong tank that exposes a large surface area at the top zone for accumulation of floating aggregates.

In the second flotation tank the first lower phase can conveniently be treated in a similar manner as the effluent was in the first flotation tank. The second flotation tank can thus work and be operated in the same manner as the first flotation tank, as described in details above.

The first flotation vehicle and the second flotation vehicle may conveniently be the same gas and come from the same source, but can in the alternative be different gases.

The first inert, anoxic or low-oxygen gas and the second inert, anoxic or low-oxygen gas are selected from the group of gases

DK 180237 B1 including methane and nitrogen and CO,, wherein methane is most preferred.

Combinations of the above-mentioned cases are also within the scope of the present invention.

The first inert or low-oxygen gas and/or the second inert or low-oxygen gas can bea product gas from the biogas plant, such as a clean gas, e.g. methane, or a gas mixture preferably containing methane.

The second nozzle means for obtaining the reduction of the pressure of the second mixture into the second riser pipe includes e.g. a second nozzle positioned inside the second riser pipe at the second riser pipe inlet.

The first nozzle means and the second nozzle may be of the same kind as the first nozzle means and the first nozzle, including the second nozzle be self-cleaning, as described above, and be subjected to a compressed gas stream to release the microbubbles of the second mixture into the first lower phase added to the second riser pipe from the first flotation tank.

The first and second mixtures with their content of microbubbles are preferably pumped by the same pump that created these mixtures of flotation vehicle and liquid media to the first nozzle and the second nozzle, respectively.

The pump puts the first and second mixture under pressure at which they arrive at the first nozzle and the second nozzle, respectively.

The first nozzle means and the second nozzle means, respectively, are preferably of the kind being able to provide a counter pressure, whereby a pressure drop across a respective nozzle of a nozzle means 1s obtained, which pressure drop advantageously may serve to further reduce the size of the microbubbles.

The microbubbles that attach to organics to obtain floating aggregates of organics are at least to some extent trapped in the first upper phase and in the second upper phase, respectively, which upper phases then contain microbubbles of gases which can come from the biogas plant itself and can be

12 DK 180237 B1 re-circulated to the biogas plant again in a closed loop. No biogas is lost to the environment in that the methane potential of the effluent is derived to the largest possible extent and biogas re-used in the flotation steps of the present invention.

The first liquid medium and the second liquid medium may be the same, be of the same kind or be different. Preferably the first liquid medium and the second liquid medium is taken from the second lower phase that is withdrawn from the second flotation tank and pumped by a pump that adds microbubbles of a first inert, anoxic or low-oxygen gas to the liquid media and pumps the obtained mixtures into the first flotation tank and the second flotation tank, respectively. A preferred pump for this purpose is a turbine pump, preferably the turbine pump from Nikuni mentioned above. The vacuum at the suction side is responsible for the intake of inert, anoxic or low-oxygen gases into the pump chamber where it is mixed into the liquid media as microbubbles. The pump pressurize the so obtained mixture. So a flow of a mixture of a flow of gas and a flow of lower second phase is created by one single turbine pump. This flow of mixture is divided into a flow of a first mixture and a flow of a second mixture of same composition, which flow of first mixture and flow of second mixture are pumped by the pump via the respective first and second nozzles into the respective riser pipes of the first and second flotation tanks to further reduce the size of the gas microbubbles created by the pump. The method of the present invention can include one or more of the further steps of m) discharging at least a fraction of the second lower phase from the second flotation tank as waste, optionally discharging the entire second lower phase, and/or n) using at least a fraction of the second lower phase as the first liquid medium and/or second liquid medium.

1 DK 180237 B1 The entire first upper phase can be cleaned further of organics by passing the first upper phase to yet a flotation tank and re-doing the cleaning process.

After having been subjected to the further flotation treatment in the second flotation tank the incoming first upper phase from the first flotation tank has been separated into a second upper phase and a second lower phase. At least some of this second lower phase can be re-used as the first liquid media.

The method of the present inventions can re-use methane or other gases developed from digestion of biomass in the biogas plant or derived from one or both of the flotation tanks as the flotation vehicles. The method can advantageously also derive and re-use/re-circulate any water that are present in any step of the method of the present to the highest possible extent as process water e.g. to adjust viscosity and density of the raw effluent entering the first flotation tank and/or as part of a liquid media. The need for supplement of large amounts of process water is thus reduced. Thus costs for process water can be kept at a minimum.

Both the process stream of gas for creating microbubbles and the process streams of the liquid media are not flow streams that need to come from an additional source. Once the method is up and running water and gas can, to any chosen preferred degree, be circulated in a loop between biogas plant, first flotation tank and second flotation tank.

The invention further relates to an apparatus for continuously recovering at least organics from an effluent from a biogas plant, wherein the recovering apparatus comprises - a first flotation tank defining a first flotation chamber, which first flotation tank has a first bottom zone opposite a first top zone, a first center zone between the first bottom zone and the first top zone, a first upper discharge

Ls DK 180237 B1 conduit at the first top zone, a first inlet provided at the first bottom zone, and a first riser pipe erected in the first flotation tank, - the first riser pipe has a first riser pipe inlet and an opposite first riser pipe outlet, preferably the first riser pipe inlet is aligned with the first inlet, - means for providing a flow of effluent to the first flotation tank, - means for mixing a first flotation vehicle in form of a first inert, anoxic or low-oxygen gas with a first liquid medium to form a first mixture containing microbubbles of said gas, - means for pumping the first mixture into the first flotation tank, - a first nozzle means at the first riser pipe inlet or at the first inlet for reducing the pressure of said first mixture that rises through the first riser pipe and for releasing the microbubbles in the effluent, - means for harvesting and/or discharging the first upper phase at the first top zone, - means for re-circulating at least a fraction of the harvested and/or discharged first upper phase to the biogas plant, and - means for discharging the first lower phase from the first flotation tank.

The preferred designs of flotation tanks, nozzles and pumps are described in relation to the method discussed above.

The apparatus may according to the present invention further comprise - a second flotation tank that defines a second flotation chamber, and which second flotation tank has a second bottom zone opposite a second top zone, a second center zone between the second bottom zone and the second top zone, a second upper discharge conduit at the second top zone, a

16 DK 180237 B1 second inlet provided at the second bottom zone, and a second riser pipe erected in the second flotation tank, - the second riser pipe has a second riser pipe inlet and an opposite second riser pipe outlet, - means for providing a flow of first lower phase to the second riser pipe, - means for mixing a second flotation vehicle in form of a second inert, anoxic or low-oxygen gas with a second liquid medium, which preferably is the same liquid medium as the first liquid medium, to form a second mixture containing microbubbles of said gas, - means for pumping the second mixture into the second flotation tank, - a second nozzle means at the second riser pipe inlet for reducing the pressure of said second mixture that rises through the second riser pipe and for releasing the microbubbles in the first lower phase inside the second riser pipe, - means for harvesting and/or discharging the second upper phase at the second upper zone.

Further optional means can be selected from the group of means including - means for re-circulating at least a fraction of gases of the flotation vehicles from any of the first flotation tank and the second flotation tank to the product stream of the biogas plant or to the liquid media, - means for re-circulating the second upper phase to the biogas plant, - means for discharging at least a fraction of the second lower phase from the second flotation tank, and/or - means for re-circulating at least a fraction of the second lower phase from the second flotation tank as a first liquid medium and/or a second liquid medium.

1 DK 180237 B1 A considerable amount of process water and costs for treating and adding process water can be saved in the method of the present invention by re-using at least some of the second lower phase as the liquid media of the first liquid medium and/or the second liquid medium, or for suspending and/or diluting the effluent further.

Re-circulating of an upper phase may include re-circulating floating aggregates for further digestion in the biogas plant and re-circulating gases of flotation vehicles to the product stream of the biogas plant or to the liquid media.

The second flotation tank can preferably be coupled in series with the first flotation tank so that the first lower phase directly enters the second riser pipe of the second floatation tank.

The first flotation tank and the second flotation tank can communicate so that liquid content in a respective flotation tank continuously is balanced in response to adding matter and removing matter in any of the flotation tanks.

If e.g. a fraction of the second lower phase is taken out of the second flotation tank replenishment of first lower phase from the first flotation tank will inherently take place so that a substantial constant amount of first lower phase continues to exist in the second flotation tank.

In a similar manner the feed of effluent from the biogas plant can be balanced in response to the withdrawal of first lower phase from the first flotation tank into the second flotation tank.

So the apparatus and continuous method of the present invention can be self- balancing.

The speed of the effluent to the first flotation tank, the speed of first lower phase to the second flotation tank, the speed of gaseous first and second liquid media to the respective first and second flotation tanks, the amount of re- circulated upper and lower phases and fractions thereof, and the size of the fractions of the second lower phase that is notls DK 180237 B1 re-circulated, but goes to waste in order not to accumulate indigestible matter in the process mass, are mutually adjusted in condition of each other. An air trap can be provided in the communication line between the first flotation tank and the second riser pipe in the second flotation tank to keep gases in the first flotation tank. An adjustable overflow may serve as a means to control the liquid level in one or both flotation tanks.

In a preferred embodiment the means for mixing a liquid medium with a flotation vehicle to create microbubbles in said liquid medium includes a pump means, preferably the pump means include a turbine pump, preferably the turbine pump has an inlet gauge of between -0.02MPa - 0.03MPa and an outlet gauge of between

0.4MPa — 0.6MPa.

The invention will now be described in further details with reference to the drawing in which the sole figure schematically shows an example of an apparatus 1 for continuously recovering at least solid substances from an effluent 2 from a biogas plant 3 in the method of the present invention.

The effluent 2 is partly digested biomass still having a content of dry matter, e.g. about 2,5 —- 3 wt/vol%, containing organics that still have a methane potential.

The flow E of effluent 2, which is indicated in the figure in fat full line, go to the first flotation tank 5 that delimits a first flotation chamber 5’ within a cylindrical center zone 6, a conical bottom zone 7 and an opposite conical top zone 8, above which a collection chamber 9 collect gases rising from the content in the first flotation tank 5. The collection chamber 9 prevents valuable gases from escaping to the surrounding atmosphere and serves as a temporary storage chamber for the later re-circulating of the gases to other

19 DK 180237 B1 steps of the method, to the biogas product line and/or to components of the apparatus of the present invention.

The flow E of effluent 2 enters the first flotation tank via a first inlet 10 in a tapering of the conical bottom zone 7. A first riser pipe 11, that has a first riser pipe inlet 12 and an opposite first riser pipe outlet 13, is erected in the first flotation tank 5, so that the first inlet 10 is aligned with the first riser pipe inlet 12 to provide fluid communication between said inlets 10,12 whereby the effluent 2 can rise through the first riser pipe 11. A first nozzle 14 is disposed at the first riser pipe inlet 12, close to said first inlet 10, preferably the first nozzle 14 is arranged inside or just inside the first riser pipe 11.

A flow of a first liquid media LM1 is provided with dispersed microbubbles 15 by means of turbine pump 16. The first liquid medium LM1 and a flow of first flotation vehicle FV1, which is taken from the methane-containing product stream P from the biogas plant 3, is subjected to a turbulent mixing by means of the turbine pump 16, to create a high concentration of methane- containing microbubbles in the first liquid media LM1, thereby obtaining the first mixture M1.

The first mixture is pumped as a first mixture M1 at a pressure of e.g. about 4 - 5 bar by the turbine pump 16 to the first nozzle 14 and into the effluent 2 in the first riser pipe 11, whereby the pressure of the first mixture Ml is reduced and the microbubbles 15 is released, preferably at reduced size, into the effluent 2 confined in the first riser pipe 11. Once released into contact with organic matter of the effluent 2 the microbubbles 15 attach to said organic matter to form aggregates that float upwards through the first riser pipe 11 and out through the first riser pipe outlet 13 to settle as a first upper phase 17 in the conical top zone 8 of the first flotation tank 5. A first lower phase 18 is then created below

20 DK 180237 B1 the first upper phase 17, which first lower phase 18 has a reduced content of organics. The gases of the microbubbles 15 that rises through the first riser pipe 11 åis collected in the collection chamber 9 wherefrom the gases can be re-circulated to the product stream P of the biogas plant 1, as indicated by dotted line Gl. The first upper phase 17 is harvested, optionally pressurized to remove water and capture any methane or other gases of the first upper phase. Captured gases can be directed to the biogas product stream P or be used in other gas demanding steps of the method. The water derived from the first upper phase can be used as process water for adjusting the density of the biomass or in any water-demanding step of the method. The first upper phase, whether it has been further treated or not, is pumped, by means of hose pump 19 back to the biogas plant 3 for digestion, as indicated by dash-dotted line UP1. The first lower phase 18 is transferred to a second riser pipe 20 erected in the second flotation tank 21, as indicated by arrow A, e.g. via a first outlet 22 of the first flotation tank 5, and into the second flotation tank 21 via a third inlet 23a. In the present example the first outlet 22 and the third inlet 23a are coaxial, but in other embodiments the axis of said first outlet 22 and the third inlet 23a are not aligned. Pump means (not shown) may be inserted for this purpose. In the present case the second flotation tank 21 is an oblong tank that delimits a second flotation chamber 21’ within a second center zone 24, a second top zone 25, and a second bottom zone

26. The oblong design of the second flotation tank 21 provides a large surface area for accumulation of aggregates of organics of the first lower phase 18 and microbubbles 15 at the second top zone 25. The second riser pipe 20 has a second riser pipe inlet 27 and an opposite second riser pipe outlet 28, and is erected in the

21 DK 180237 B1 second flotation tank 21, so that first lower phase 18 can enter the second riser pipe inlet 27 when said first lower phase 18 enters the second flotation tank 21 via its third inlet 23a close to the second riser pipe inlet 27. In the present example of an apparatus 1, the third inlet 23a is located in a sidewall of the second bottom zone 26. The third inlet 23a can in the alternative be located next to the second inlet 23b in the bottom wall of the second flotation tank 21 and pumped from the first outlet 22 of the first flotation tank

5. In yet an alternative the third inlet 23a is the same as the second inlet 23b. The first lower phase 18 can rise through the second riser pipe 20. A second nozzle 29 is disposed at the second riser pipe inlet 27, close to said riser pipe inlet 27, preferably inside the second riser pipe 20.

A flow of a second liquid medium LM2 is provided with dispersed microbubbles 15 of a second flotation vehicle FV2 by means of turbine pump 16. In the present embodiment the flow of second liquid medium LM2 is the same as the flow of first liquid medium LM1 and the flow of first flotation vehicle FV1 is the same as the flow of second flotation vehicle FV2. The second liquid medium LM2 and a flow of second flotation vehicle FV2, which is taken from the methane-containing product stream P from the biogas plant 1, is subjected to a turbulent mixing by means of the turbine pump 16, to create a high concentration of methane-containing microbubbles in the second liquid media LM2, thereby obtaining a second mixture M2. The same turbine pump thus produces first and second identical mixtures M1,M2 of liquid medium LM1,LM2 and microbubbles 15 of gas, preferably of gas derived from the method itself or directly from the biogas plant 3. Emphasis is made that each flotation tank 5;21 may in the alternative be fed by individual turbine pumps and use separate flows of flotation vehicles and liquid medium.

2 DK 180237 B1 The second mixture M2 is pumped at a pressure of e.g. about 4 - bar by the turbine pump 16 via the second inlet 23b at the bottom of the second flotation tank 21 to the second nozzle 29 and into the first lower phase 18 in the second riser pipe 20, 5 whereby the pressure of the first mixture M2 is reduced and the microbubbles 15 is released, preferably at reduced size, into the first lower phase 18 confined in the second riser pipe 20.

Once released into contact with residual organic matter of the first lower phase 18 the microbubbles 15 attach to said organic matter to form aggregates that float upwards through the second riser pipe 20 and out through the second riser pipe outlet 28 to settle as a second upper phase 30 in the second top zone 25 of the second flotation tank 21. A second lower phase 31, which has now been created below the second upper phase 30, which second lower phase 31 has a yet further reduced content of organics compared to the first lower phase 18, exits the second flotation tank 21 via second outlet 32 at second center zone 24, and a fraction is re-circulated to a turbine pump 16 as first liquid media LM1 and/or second liquid media LM2 for addition of microbubbles of gas, as indicated by dashed line in the figure. A fraction of about 0,5% of second lower phase 31 may continuously be discharged as waste Wl and a fraction of about 15% of same lower phase be used as liquid media LM1;LM2.

The gases of the microbubbles 15 that rises through the second riser pipe 20 may be collected in the head space 33 above the second upper phase 30 wherefrom the gases can be re-circulated to the product stream P of the biogas plant 3, as indicated by dotted line G2. The second upper phase 30 is harvested, optionally pressurized to remove water and capture any methane or other gases of the second upper phase 30. Captured gases can be directed to the biogas product stream P or be used in other gas demanding steps of the method. The water derived from the second upper phase 30 can e.g. be used as process water for adjusting the density of the biomass in the biogas plant 3, or of the effluent 2, or be used in any other water-demanding step

D3 DK 180237 B1 of the method. The second upper phase 30, whether it has been further treated or not, is pumped, by means of the same hose pump 19 used for pumping the first upper phase 17, or by means of another pump, back to the biogas plant 3 for digestion, as indicated by dash-dotted line UP2. The first nozzle 14 and the second nozzle 29 may be of the same kind, preferably being self-cleaning, as described above. The first nozzle 14 is subjected to compressed gas, as indicated by arrow C1, through a first valve 34 to counteract the static pressure in the first flotation tank and aid in releasing the microbubbles 15 of the first mixture M1 into the effluent 2 in the first riser pipe 11 by controlling the opening and closing of the orifices (not shown) of the first nozzle 14, whereby also the size of the microbubbles is reduced. Similarly, the second nozzle 29 is subjected to compressed gas, as indicated by arrow C2, through a second valve 38 to counteract the static pressure in the second flotation tank 21 and aid in releasing the microbubbles 15 of the second mixture M2 into the first lower phase 18 in the second riser pipe 20 by controlling the opening and closing of the orifices (not shown) of the second nozzle 29, whereby also the size of the microbubbles is reduced.

The digestion of biomass involves a lot of water and use of make-up water. The majority of process water and make-up water are the same water as the effluent from the biogas plant brought along, and which is used again in the method of the present invention as dispersion water, thus as liquid media, which is water captured and re-circulated again and again until concentrations of particulates and of non -digestible matter in the second lower phase 31 reaches an upper limit beyond which the method is no longer favorable. At such a stage the first flotation tank 5 can be emptied completely or partly, or a first sediment phase (not shown) at the first bottom zone 7 be

D4 DK 180237 B1 discharged via discharge line W2 at said first bottom zone via discharge valve 37. A similar process can take place via discharge line W3 from the second flotation tank 21 through second discharge valve 36. Preferably the discharge valves 37,36 are ball valves. An exemplary first flotation tank may e.g. have a total height of 3.2 m, a diameter at the center diameter of 1.2 m, and a total volume of 2,5 m'. The first riser pipe may e.g. have a total height of 1300 mm and a diameter of 315 mm. The second flotation tank may e.g. have a total height of 1.8 m and a total volume of 4,9 m'. The second riser pipe may be similar to the first riser pipe.

Such an apparatus can process an effluent flow of 5.5 m /h. The flotation vehicles are expediently gases from the biogas plant or from the apparatus developed during its operation, so separate sources of gas for formation of microbubbles in the liquid media are not needed. Accordingly, the present invention optimizes the yield of biogas from the biogas plant at minimum costs using an apparatus and method based on recovery and re- circulation of products in form of water and gases for use in process streams of the apparatus and method itself. This novel method and apparatus to recover, re-circulate and otherwise treat the organics in the effluent from a biogas plant is a fast and efficient way to optimize the yield of a biogas plant. The method and apparatus are economical attractive, pollute the environment at a minimum, do not negatively influence the digestion process and utilize the caloric content and methane potential of a biomass at its optimum.

DK 180237 B1 A further advantage is that the method and apparatus are easily and inexpensively implemented at existing biogas plants. The apparatus can be a mobile unit that can be used at different biogas plants upon demand. Such a unit can be rented or leased, 5 or be shared by several owners so, that e.g. a farmer not need to invest a substantial amount in such processing equipment. Gases of the microbubbles taking part in the flotation reaction in any of the flotation tanks can be collected and be re- circulated to the biogas plant, e.g. added to the biogas product stream of the biogas plant, wherefrom the gas(es) used as the flotation vehicles to create microbubbles in the liquid media also can be taken initially. The collected gases can also be re-used in the method of the present invention.

A further huge advantage is that the slow growing methanogens that are attached to the organic fraction of the effluent can be re-circulated. Due to the anaerobic conditions in the flotation tank the methanogens are immediate ready for further methane production when re-circulated as included in the upper phase to the reactor of the biogas plant. The biogas plant in itself is not part of the apparatus of the present invention. The apparatus can be provided as a rather compact unit that can be connected to the waste and products streams of the biogas plant in a very easy and simple manner. In a first aggregation process in the first flotation tank aggregates of organics in the effluent and microbubbles of gas from the product stream from the biogas plant float into the upper zone and are transported back for further digestion in the biogas plant thereby boosting biogas formation. The effluent is thus cleaned of some further organics but may still have a content of organics worth recovering. This takes place in a similar manner in a second flotation tank in fluid connection with the first flotation tank. Process gas is the

D6 DK 180237 B1 gas already present in the digester of the biogas plant and dispersion water for microbubbles is water re-circulated from a second lower phase of the second flotation tank formed in the second flotation tank in response to a second formation offloating aggregates in a second aggregation process.

It should be noted that although the invention is described in relation to a biogas plant and in relation to treating the effluents from such a biogas plant in view of recoveringmatter, whether it be liquid matter, gaseous matter or solid matter, from the effluent, other anaerobic digesters that produce effluents having a content of matter that can be recovered for re-use in the productions itself or be used for other purposes, are contemplated within the scope of thepresent invention.

Recovered organic matter may be re- circulated to produce more biogas for combustion, energy production or value added products.

Claims

> DK 180237 B1 Requirements

A method for continuously recovering at least organic dry matter from a flow (E) of an effluent (2) from a biogas plant (3) and recirculating the recovered solids to the biogas plant (3), which method comprises the steps of a ) providing a first flotation tank (5) with a first flotation chamber (57), said first flotation tank (5) comprising at least a first inlet (10) located at said bottom flotation tank (5) bottom zone (7), and a first outlet (22) located at the central zone of the first flotation tank (5), and a first riser (11) upright in the first flotation chamber, said first riser (11) having a first riser inlet (12) and an opposite first riser outlet (13), b) providing a continuously flow (E) of effluent (2) to the first flotation tank (5) via the first inlet (10), Cc) provide a first mixture (M1) in the form of a first liquid medium (LM1) mixed with microbubbles (15) from a first flotation aid (FV1) in the form of a first inert, anoxic or low-oxygen gas and pumping the first mixture (M1) to the first flotation tank (5), d) passing the first mixture (M1) through a first nozzle means (14; C1; 34) located at the first riser inlet (12) or at the first inlet (10) for reducing the pressure of the first mixture (M1) rising through the first riser (11) and releasing the microbubbles (15) into the effluent (2) in the first riser (11), e) allowing the microbubbles (15) to rise through the first riser (11) and the first mixture (M1) to

- DK 180237 B1 divide into a first upper phase (17) and a first lower phase (18) in the first flotation chamber (57), and f) recirculate at least a fraction of the first upper phase (17) to the biogas plant ( 3).

The method of claim 1 further comprising the steps of g) providing a second flotation tank (21) defining a second flotation chamber (217), said second flotation tank (21) comprising at least - a second inlet (23b) to said second flotation tank (21). ), and a second outlet (32) located at the center zone (24) of the second flotation tank (21), and - a second riser (20) upright in the second flotation chamber (217), said second riser (20) having a second riser inlet ( 27) and an opposite second riser outlet (28), h) transferring the first lower phase (18) from the first flotation tank (5) to the second flotation tank (21), i) providing a second mixture (M2) in the form of a second liquid medium (LM2) mixed with microbubbles (15) of a second flotation aid (FV2) in the form of a first inert, anoxic or low-oxygen gas, and pump the second mixture (M2) to the second flotation tank (21), j) passing the second mixture (M1) through a second nozzle means (29; C2; 38) locally at the second riser inlet (27) to reduce the pressure of the second mixture (M2) rising through the second riser (20) and release the microbubbles (15) into the first lower phase (18) of the second riser. (20), k) allowing the microbubbles (15) to rise through the second riser (20) and the second mixture (M2) to separate in a second upper phase (30) and a second lower phase (31) in the second flotation chamber (217), and l) recirculate at least a fraction of the second upper phase (30) to the biogas plant (3).

J DK 180237 B1

A method according to any one of claims 1 or 2, wherein the first nozzle means (14; C1; 34) includes a first nozzle (14) and / or the second nozzle means (29; C2; 38) includes a second nozzle (29) , wherein one or both of the first nozzle (14) and the second nozzle (29) have at least one opening in the form of a slot, through which microbubbles (15) in the first mixture (M1) and the second mixture (M2), respectively , is released into the effluent (2) in the first riser (11) and in the first lower phase (18) in the second riser (20), respectively.

A method according to any one of claims 2 or 3, wherein - the first inert, anoxic or low-oxygen gas and the second inert, anoxic or low-oxygen gas are the same or different, which inert, anoxic or low-oxygen gases are selected from the group of gases including methane, nitrogen and CO, and combinations thereof, and / or - the first liquid medium (LM1) and the second liquid medium (LM2) are the same or different, preferably both liquid media are taken out (LM1; LM2) from them second lower phase (31).

A method according to any one of the preceding claims 1-4, wherein the first inert, anoxic or low-oxygen gas and / or the second inert, anoxic or low-oxygen gas is a product gas (P) from the biogas plant (3).

A method according to any one of the preceding claims 2 - 5, wherein each of the first upper phase (17) and the second upper phase (30) contains gases which are recycled to the biogas plant (3).

A method according to any one of the preceding claims 2-6 further comprising one or more of the steps of

30 DK 180237 B1 m) derive at least a fraction of the second lower phase (31) from the second flotation tank (21) as waste (W1), optionally discharge the entire second lower phase (31), and / or n) use at least a fraction of the second lower phase (31) as the first liquid medium (LM1) and / or second liquid medium (LM2).

Apparatus (1) for continuous recovery of at least solids from an effluent from a biogas plant (3), wherein the recovery apparatus (1) comprises - a first flotation tank (5) defining a first flotation chamber (57), which first flotation tank (5) has a first bottom zone (7) opposite a first top zone (8), a first middle zone (6) between the first bottom zone (7) and the first top zone (8), a first upper discharge line (UP111) at the first top zone (8), a first inlet (10) provided in the first bottom zone (7), and a first riser (11) standing upright in the first flotation tank (5), - the first riser (11) has a first riser inlet (12) and an opposite first riser outlet (13), preferably the first riser inlet (12) is in line with the first inlet (10), - means (4) for providing a flow (E) of effluent (2) to the first flotation tank ( 5), - means (16) for mixing a first flotation aid (FV1) 1 in the form of a first inert, anoxy sk or low-oxygen gas with a first liquid medium (LM1) to form a first mixture (M1) containing microbubbles (15) of said gas, - means (16) for pumping the first mixture (M1) into the first flotation tank (5), - a first nozzle means (14; C1; 34) in the first riser inlet (12) or in the first inlet (10) for reducing the pressure of said first mixture (M1), which

B1 ascends through the first riser (11) and to release the microbubbles (15) in the effluent (2) in the first riser (11), - means (19; UP1) for harvesting and / or discharging the first upper phase (18) at the first top zone (8), - means (19; UP1) for recirculating at least some of the harvested and / or discharged first upper phase to the biogas plant (3), and - means for discharge the first lower phase (18) from the first flotation tank (5).

The apparatus (1) of claim 8, further comprising - a second flotation tank (21) defining a second flotation chamber (217), and said second flotation tank (21) having a second bottom zone (26) opposite a second top zone (25), a second middle zone (24) between the second bottom zone (26) and the second top zone (25), a second upper discharge line (UP2) at the second top zone (25), a second inlet (23) to the second flotation tank (21), and a second riser (20) upright in the second flotation tank (21), - the second riser (20) has a second riser inlet (27) and an opposite second riser outlet (28), - means for providing a feed stream of the first lower phase (18) to the second riser (20), - means (16) for mixing a second flotation aid (FV2) in the form of a second inert, anoxic or low-oxygen gas with a second liquid medium ( LM2), which is preferably the same liquid medium as the first liquid medium, to form a second mixture g (M1) containing microbubbles (15) of said gas, - means (16) for pumping the second mixture (M2) into the second flotation tank (21), - a second nozzle means (29; C2; 38) at the second riser inlet (27) to reduce the pressure of said

180237 B1 second mixture (M1) rising up through the second riser (20) and for releasing the microbubbles (15) in the first lower phase (18) inside the second riser (20), - means for harvesting and / or deriving a second upper phase (30) at the second upper zone (25).

An apparatus (1) according to claim 8 or 9, which apparatus (1) further comprises one or more means selected from the group of means including - means for recirculating at least a fraction of flotation auxiliary gases from any of the first flotation tank, and the second flotation tank for the product stream (P) from the biogas plant (3) or for liquid media (LM1; LM2), - means (19) for recirculating the second upper phase (30) to the biogas plant (3), - means (W3; 36) for discharging at least a fraction of the second lower phase (31) from the second flotation tank (21), and - means (16) for recirculating at least a fraction of the second lower phase (31) from the second flotation tank (21) as a first liquid medium (LM1) and / or a second liquid medium (LM2).