WO2012072432A1

WO2012072432A1 - Continuous flow system for production of microalgae

Info

Publication number: WO2012072432A1
Application number: PCT/EP2011/070505
Authority: WO
Inventors: Peter Williams; David Bowers
Original assignee: Bangor University
Priority date: 2010-12-04
Filing date: 2011-11-18
Publication date: 2012-06-07
Also published as: GB2486187A; GB2486187B; GB201020547D0

Abstract

The present invention discloses a continuous flow system for growing algae in an algae farm. It provides a simple and energy efficient system for the mass production of photosynthetic microalgae.

Description

CONTINUOUS FLOW SYSTEM FOR PRODUCTION OF MICROALGAE.

FIELD OF THE INVENTION.

This invention relates to the field of the mass cultivation of algae for the purpose of producing biodiesel and other algal biomass and biomass-derived products. It discloses a continuous flow system for the mass production of photosynthetic microalgae.

DESCRIPTION OF THE RELATED ART.

Micro-organisms divide by binary fission, thus exhibiting an exponential development of general formula

wherein

B_t and B₀ are the biomasses respectively at time t and time 0, μ is the growth rate and t is the time. The biomass is thus increasing exponentially with time as represented by the solid line in Figure 1 .

The aim in mass culture of algae is to maintain the culture growth rate and physiological state constant and also, very importantly, to maintain constant biomass concentration. This can be achieved by continuously diluting the culture in order to satisfy equation ln(V_t/V₀) = ln(B_t/B₀) = μΐ wherein V_t and V₀ are respectively the volume of the growth container at time t and at time 0. The biomass concentration B/V is thus constant with time as represented by the dashed line in Figure 1 .

Because growth is driven by sunlight in algal culture, the depth needs to be kept as constant as possible. As a consequence, the culture surface area A needs to increase in the same proportion as the culture volume as represented by equation ln(A_t/Ao) = In(WVo) = ln(B_t/B₀) = μΐ wherein A_t and A₀ are respectively the area of the culture surface at time t and at time 0.

This could be achieved by increasing the culture vessel diameter progressively with time but would produce major practical problems.

In prior art systems, it was achieved by transferring the culture stepwise to progressively larger culture systems as represented in Figure 2. This however was applied to small sized systems as the culture systems rapidly reached an unmanageable size.

Another solution was to transfer the culture in an increasing number of similar-sized culture systems, set up in a cascade as represented schematically in Figure 3A, the biomass concentration and total biomass are represented respectively in Figures 3B and 3C.

In either solution, the culture transfer was carried out by linking the various units by piping and associated valves and pumps. This set up consumed a significant part of the overall operating budget and a large amount of energy.

In yet another system, described in US-A-5,981 ,271 , an outdoor thin-layer cultivation of algae was distributed on consecutive inclined cultivation areas under turbulent flow. The suspension flowing from the lowest cultivation level was conveyed to a collecting tank from which it was pumped back to the upper edge of the highest cultivation tank, thereby forming a closed loop.

BRIEF DESCRIPTION OF FIGURES.

Figure 1 represents the algal growth curve (solid line) expressed in kg/m³ as a function of time and the biomass concentration (dashed curve) in a continuously diluted culture.

Figure 2 represents an algal biomass plant in Eilat, Israel, showing raceways of increasing size.

Figure 3A represents schematically a cascading arrangement enabling stepwise dilution.

Figure 3B represents the biomass concentration as a function of time.

Figure 3C represents the total biomass as a function of time.

Figure 4 represents a comparison between the existing systems and a continuous flow system. A represents a three stage system of separate culture raceways, B is a system wherein the raceways at each stage are combined into a single reservoir, C represents a linear continuous flow system and D represents a continuous system folded into a serpentine raceway at each stage.

Figure 5 represents the serpentine canal according to the preferred embodiment of the present invention. In embodiment A all seawater is carried to the head of the serpentine canal and is injected where needed at the beginning of each stage. In embodiment B seawater runs into a pipe and is pumped directly from that pipe where needed at the beginning of each stage.

Figure 6 represents a possible design organising the flow around the bends.

Figure 7 represents the set up of the comparative example wherein the cultures are grown in combined front end bioreactors feeding recirculation raceways. The system shown in the figure comprised 80 of 38 m x 390 m raceways and the equivalent number of bioreactors. The interconnections between the bioreactors and the raceways and the supplies of growth culture medium to both are not shown for simplicity,

SUMMARY OF THE INVENTION.

It is an objective of the present invention to grow organisms in a continuous flowing canal rather than in a re-circulating system.

It is another objective of the present invention to reduce the piping length needed to operate the algae farm.

It is a further objective of the present invention to decrease the number of valves and pumps needed to operate the algae farm.

It is also an objective of the present invention to reduce the space needed for the farm by folding the flowing canal on itself in a serpentine manner.

It is yet another objective of the present invention to reduce the cost and energy needed to operate an algae farm.

In accordance with the present invention, the foregoing objectives are realised as described in the independent claims. Preferred embodiments are described in dependent claims.

DETAILED DESCRIPTION OF THE INVENTION.

Accordingly, the present invention discloses a system for growing algae in an algae farm, based on continuous flow, said system comprising a plurality of canals each canal being the same or different and having an increasing width, said width increasing either continuously or stepwise or a combination, wherein the system algae/seawater is kept moving from input to output at a speed of at least 0.05m/s, wherein the inoculum is continuously injected at the inlet of the system and wherein the biomass concentration is periodically returned to its starting value by dilution.

In this invention, the organisms are grown in a flowing system rather than in a recirculating system as used in present algae farms. In the present system, there is thus an increase in cell biomass with position downstream. To keep the culture in active growth it needs to be diluted, either continuously or at intervals, along the length of the canals. The depth of the canal has to be maintained constant in order to keep desired flow conditions. The width of the canal therefore increases with increasing dilution, either continuously or in a stepwise manner.

The growth systems presently used require mechanical mixing to maintain the level of turbulence necessary to keep the cells in suspension. Flow ranging between 0.05 m/s and 0.2 m/s provides an adequate level of mixing. Preferably it ranges between 0.08 m/s and 1 .8 m/s and more preferably it is of about 0.15 m/s equivalent to 13 km per day. At mean flow rates below 0.05 m/s, parts of the flow becomes laminar, at mean flow rates greater than 0.2 m/s, the slope or the power required to induce and drive the flow becomes prohibitively high as discussed by Oswald (1988). In past procedures the organisms were grown in a re-circulating canal and the motion was provided by mechanical mixing, typically paddle wheels that were the simplest and most energy efficient systems. The power required to induce the preferred flow of 0.15 m/s flow was of the order of 0.03 watts/m² as calculated by Oswald.

In the present invention characterised by a continuous flow, said flow can be driven by the same system as that used in the prior art such as for example paddle wheels or pumps. Alternatively and preferably it can be induced by gravity if the slope between the head of the flow and its outlet is designed to maintain the required flow throughout the length of the canal. Present day technology allows the slope to be adjusted within the millimetre range. The existing systems and the systems according to the present invention are compared in Figure 4-A, B, C and D that disclose respectively a prior art system consisting of separate cascading tanks (A), a prior art system consisting of several separate tanks of increasing size (B), a continuous linear system of increasing width according to the present invention (C) and a continuous system according to the present invention wherein the canal is folded back to form a serpentine (D).

A linear canal, as represented in Figure 4C, although feasible, presents practical problems, both spatial and operational, because its length is necessarily over 50 km. A more compact structure is therefore preferable. This is achieved in the present invention by folding the canal on itself in a serpentine manner.

In a preferred embodiment according to the present invention, represented in Figure 5, each canal is folded to create a serpentine-type canal (10) enclosed in a structure comprising at least

- a first side (1 ) and a second side (2) positioned opposite to the first side and said first and second sides being aligned to the direction of the overall flow;

- a third side (3) and a fourth side (4) positioned opposite to the third side and said third and fourth sides being transverse to the direction of the overall flow ; wherein said structures are divided in their direction parallel to the overall flow into n sub-sections (1 1 , 12, 13), wherein n is at least equal to 2, and wherein the length L_x+ of section (x+1 ) is longer than that L_x of section x and wherein ratio q = L_x+ /L_x is adjusted to the growth rate of the biomass, and q may be the same or different in different sub-sections;

wherein in each sub-section, the serpentine canal comprises p folds (20) of about equal width wherein p is the same or different in each subsection and is preferably a number of at least 6, preferably at least 10, and, at each fold, optionally, a device designed to push water forward into the next fold,

wherein the ratio of fold's width F_x+ in sub-section x+1 to that F_x of section x is of the same order of magnitude as q, and is preferably equal to q;

said structure further comprising a first pump (40) for injecting sea water and inoculum at the beginning of the first sub-section and additional pumps (50, 60) for injecting additional sea water at the beginning of each sub-section; said structure further comprising a recycling system comprising a pump (41 ) for retrieving a fraction of the material comprising sea water and inoculum at the end of the first sub-section and injecting it back at the beginning of said first sub-section (42);

said structure comprising an outlet (70) for collecting sea water and grown algae and said structure being characterised in that it is designed to allow the stream of sea water and algae to flow continuously from inlet to outlet.

Preferably the structure enclosing the serpentine canal is an elongated rectangle with its long side parallel to the direction of the overall flow, optionally with rounded corners.

Preferably, the structure also includes a feedback device to retrieve a fraction of the flow exiting each sub-section and send it back to the beginning of said sub-section in order to stabilise the flow.

When the flow is driven by gravity, the fluid motion is provided by the slope of the structure. Energy is thus required to pump the water up to the head of the flow. Calculations show that the power required to lift water to the starting position as represented in Figure 5A is very small. It is of the order a few hundredths of watts/m².

The power requirement can be about halved, if rather than lifting all the seawater to the full height, i.e. to the inlet of the first stage, it is only lifted to the top of the individual sub-sections where needed, as represented in Figure 5B.

The length of the canal is determined and constrained by several factors, the most important being: a. the flow velocity required to maintain the cells in suspension;

b. the time required for the diluted culture to grow back to its original biomass. The flow velocity is largely determined by the hydrodynamics and a flow ranging between 0.05m/s and 2 m/s is adequate to maintain the growing organism in suspension, preferably of 0.08 m/s to 1 .8 m/s with an optimal flow of about 0.15m/s or 13 km/day.

The time necessary for the diluted culture to grow back to its original biomass is governed by both the scale of the stepwise dilution and the cell growth rate. A suitable dilution ranges between 2-fold and 10-fold, preferably about 3-fold as this keeps the cells in active growth. Typical growth rates range between 0.3 d^"1 and typically 1 d^"1. For example a growth rate of 0.55 d^"1, necessitates a time period of 2 days to grow back and a 3-fold dilution thus requires a canal length of about 26 km per sub-section for a flow velocity of 0.15 m/s.

As the growth rate of the culture is influenced by the amount of sunshine and temperature, it is advantageous to develop the algae farms in areas where the average temperatures range between 20 and 25 ^°C. These temperatures are found in latitudes ranging between 30 and 35 degrees. As these farms require a large space, they are preferably developed in costal desert or unused areas.

Because of the dilution taking place at the beginning of each sub-section, the width of the canal in each sub-section must increase to accommodate the increasing amount of fluid. Flow considerations impose limits on the width and length of each arm of the serpentine canal. Typical arm widths of consecutive sub-sections, in a three sub-section system, can for example be selected as 3 m, 9.5 m and 30 m respectively, in order to keep a constant growth increase, with an arm length of about 750 m. Any other dimensions can be envisaged depending upon the terrain available and the growth rate of the organisms used.

The energy necessary to initiate and maintain the flow at the selected speed can be provided by pumps or paddle wheels as in the prior art plants. As the present system provides a continuous flow, it offers the possibility to generate the flow by gravity by creating a slope between inlet and outlet. The reduced cost is however accompanied by a reduced flexibility of operation. Because the slope is fixed, the only means of adjusting the flow velocity is by increasing the depth of the culture.

A number of equations can be used to predict the relationship between the gradient in a shallow channel and the resultant flow. They all give similar predictions. The Manning equation is used in the present description.

The slope s can be written as

s = u².n²/R^{4 3}

wherein u is the flow velocity, n is the Manning coefficient selected as 0.01 for a smooth plastic surface as determined by Oswald and R is the hydraulic radius defined as

R = z.w/(w+2.z)

wherein w is the width of the channel and z is the water depth.

Present day lining of the canals is preferably selected from plastic as it provides a thin, durable and inert layer.

Typical slopes necessary to generate the needed flow velocity range between 0.5 m/km and 5 m/km in the case of a serpentine-type structure.

In a preferred embodiment according to the present invention, the flow is organised in order to minimise the dead areas. The flow round the bends is subject to centrifugal forces. It is therefore beneficial to provide smooth curves. The outer walls of the channel are thus preferably rounded as well as the sharp turn at the inner radius as represented in Figure 6. Additionally, guides may be provided to reduce centrifugal forces, even though the angular velocities are low.

Advantageously, mechanical propulsion by paddle wheels may also be used. The incoming stream of culture medium, at the first stage, is inoculated with the desired culture at the selected cell concentration. Once set up, the inoculum can be sustained by any known mechanism. A feedback loop draws the culture from the end of the first sub-section to the beginning of said first sub-section as represented in Figure 5 (top right-hand side of each figure). In such embodiment, the biological component of the system is self-sustaining. However it would also cycle contaminants so provision would need to be made in such circumstances to introduce fresh contaminant-free culture. If, for example, the growth at each subsection, is approximately 3-fold, then 1 /3^rd of the culture needs to be returned by the feedback loop. The removal of 1 /3^rd of the culture requires that the volume flow at the first sub-section be increased by 1 /3, in order to pass the appropriate volume onto the second sub-section. That can be achieved by modifying the length or width of the canal in the first sub-section.

The production of biomass depends upon the amount of photons received by the system. If solar exposure increases, the biomass production increases, leading to a thickening of the culture and therefore a reduction of subsequent production and vice-versa. Such system is thus self-regulating. It may however be desirable to gain control of the system and impose a growth rhythm. This can be achieved by either of two methods.

The system can be designed with a minimum slope to accommodate the slowest possible growth rate and comprise paddle wheels or pumps to accelerate the flow when production rate increases. Alternatively, the production rate can be measured at the outlet of each stage and a fraction of the culture sent back to the beginning of said stage if the production rate has not reached the target rate. The amount of culture sent back is proportional to the difference between target rate and observed rate. The volume flow is then adjusted accordingly as explained for the first subsection.

The present set up offers the additional advantage of reducing the risk of contamination likely to occur through the pumps and connecting pipes in the prior art systems. It also reduces the maintenance work necessary to keep the pumps and pipes in good operational condition.

The algae farms, in addition to producing the required lipids also produce large amounts of proteins and carbohydrates. The proteins may advantageously used to prepare animal feed and the carbohydrates can be used to generate biopolymers.

Examples- Example according to the present invention.

Flow calculations have been carried out on a 3-stage system according to the present invention and for an algal growth rate of 0.55 d^"1. The arms of the serpentine canal were 750m long in each stage, with 35 turns per stage. The widths of the canal in the 3 stages were respectively of 3 m, 9.5 m and 30 m in order to be proportionally equal to the step-wise dilution and the canal depth was of about 0.15 m. The overall length of the canal was thus of 78 km whereas the overall length of the serpentine system was of about 1 .5 km with a elevation of 2.3 m between inlet and outlet and with a total area of 1 .125 km². The serpentine structure also reduced the number of pumps by a factor of at least 100 when the flow was induced by mechanical means or removed all the pumps when the flow was induced by gravity. The length of piping necessary to carry the seawater was also reduced by a factor of at least 5.

Six units as described hereabove covered an overall area of 6.75 km². The residence time in each unit was of 6 days, assuming an algal growth rate of 0.55 days^"1. The productivity was estimated at 50g biomass/m².d, therefore producing 350 tonnes dry weight of algal cell material per day. Assuming a lipid content of about 35%, typical of algal material, the calculated lipid yield was of about 120 tonnes of lipid per day.

Comparative example. Figure 7 is a schematic drawing of part (80 units) of a 456 unit system; the whole system occupying 6.55 km² and producing 1 ,000 barrels/day (135 tonnes of oil/day). The two stage system comprised 456 units at each stage, wherein the raceways' dimensions were about 40 m x 350 m. Transfers between the two stages required 3710 pumps and 30km of 50cm diameter tubing. The circulation of the second stage system was achieved with 456, 15m wide, paddle wheel impellors. Collectively these amount to some 30 to 40 % of the capital costs and 35% of the operating costs.

The yield of this unit is comparable to the proposed serpentine canal growth system shown in Figure 5.

The present invention thus provides a similar amount of lipid per unit area as the prior art facilities but at a much reduced construction and operating cost and much simpler design and most importantly reduced power consumption. The non-lipid residue of the algal biomass contains protein and other valuable biochemicals (e.g. carotene) that can also be exploited

Claims

CLAIMS.

1 . A system for growing algae in an algae farm that comprises a plurality of

canals each canal being the same or different and having an increasing width, said width increasing either continuously or stepwise or a combination, wherein the system algae/seawater is kept moving from input to output at a speed of at least 0.05m/s, wherein the inoculum is continuously injected at the inlet of the system, wherein the biomass concentration is periodically returned to its starting value along its path by dilution and characterised in that the flow in each canal is continuous.

2. The system of claim 1 wherein each canal has a constantly increasing width.

3. The system of claim 1 wherein each canal has a width that increases by

steps.

4. The system of any one of the preceding claims wherein the system

algae/seawater is kept in motion by gravity.

5. The system of any one of claims 1 to 3 wherein the system is kept in motion mechanically.

6. The system of any one of the preceding claims wherein the inoculum is reinjected at the beginning of the system by a feedback loop.

7. The system of any one of claim 1 or claims 3 to 6 wherein each canal is

folded to create a serpentine-type canal (10) enclosed in a structure having an elongated shape comprising at least:

a first side (1 ) and a second side (2) positioned opposite to the first side and said first and second sides being aligned to the direction of the overall flow; a third side (3) and a fourth side (4) positioned opposite to the third side and said third and fourth sides being transverse to the direction of the overall flow ; wherein said structures are divided in the direction parallel to the overall flow into n sub-sections (1 1 , 12, 13), wherein n is at least equal to 2, and wherein the length L_x+ of section (x+1 ) is longer than that L_x of section x and wherein ratio q = L_x+ /L_x is adjusted to the growth rate of the biomass, and q may be the same or different in different sub-sections;

wherein in each sub-section, the serpentine canal comprises p folds (20) of about equal width wherein p is the same or different and is an even number of at least 6, preferably at least 10, and, at each fold, a device designed to push water forward into the next fold,

said structure further comprising a first pump (40) for injecting algae diluted in sea water at the beginning of the first sub-section and additional pumps (50, 60) for injecting additional sea water at the beginning of each sub-section;

said structure further comprising a recycling system comprising a pump (41 )for retrieving a fraction of the material comprising sea water and inoculum at the end of the first sub-section and injecting it back at the beginning of said first sub-section (42);

8. The system of claim 7 wherein the structure enclosing the serpentine canal is an rectangle with its long dimension parallel to the overall flow.

9. The system of claim 7 or claim 8 wherein the dead zones are minimised by rounding the bends at each fold.

10. The system of any one of the preceding claims further comprising at the end of each stage a device for re-injecting a fraction of the culture to the beginning of said stage in order to keep the growth rate constant.

1 1 . A method for growing algae in an algae farm the uses the continuous system of any one of claims 1 to 10.

12. Use of the system of any one of claims 1 to 10 to reduce the cost and energy required to operate the algae farm.