AU8150787A - A method for improving recovery by centrifugation of microorganisms from suspension in aqueous solutions - Google Patents

Description

A METHOD FOR IMPROVING RECOVERY BY CENTRIFUGATION OF FRAGILE MATERIALS FROM SUSPENSION IN AQUEOUS SOLUTIONS

The present invention relates to a method of obtaining naturally occurring fragile material to be used, for example, in the harvesting of marine or salt tolerant species. In particular, the present invention relates to harvesting unicellular algae, such as, for example, of the genus Dunaliella, from fluids, such as, for example, brine, in order to recover materials of som economic value, such as, for example, carotenoids, particularly beta-carotene from the harvested material. Although the present invention will be described with particular reference to the harvesting of algae of the genus Dunaliella, it is to be noted that the present invention is not so limited and extends to include other methods of obtaining the naturally occurring raw material, and to raw materials other than the specific genus described herein.

Previous attempts to recover algae from brine have not entirely been successful for one or other reasons. One previous attempt is the subject of Australian Patent No. 486,999, which discloses a filtration process for the recovery of caroteniferous algae from brine suspensions in which a filter aid is added to a suspension of the algae prior to the recovery of the combined filter aid and algae by filtration. However, one disadvantage of this process includes the tendency of the microscopic algae to block the filter, even in the presence of the filter aid, which reduces the flow rate through the filter material. A further disadvantage is that large volumes of filter aid must be mixed with the algae in order to obtain reasonable filtration from which the desired algae is not readily separable after filtration which adds a further step to the overall process of obtaining the algae in a usable form.

Centrifugation is another cell harvesting technique which has been attempted, but has been found unsatisfactory because the action of most centrifuges will rupture the algal cell membrane, and because separation efficiency of the algal cells from the brine is poor.

In one broad form the invention comprises an apparatus for the centrifugation of fragile material from a liquid medium, so constructed as to cause minimum shear during the centrifugation.

In another form the invention comprises a method of separating fragile material from a liquid medium by means of diluting the material to be collected in filtering the material through a centrifuge with minimum shear.

In one embodiment of the present invention there is provided an improved method for harvesting algae of the genus Dunaliella, which eleviates the problems of previously attempted methods related to this material. According to the preferred embodiment of the present invention, there is provided a number of "alterations to conventional centrifugation practices. The combined effect of these alterations makes centrifugation cost effective for separating species of the genus Dunaliella and other materials.

The majority of industrial centrifuges cause unacceptably high levels of cell rupture to Dunaliella species, or any other fragile materials. Members of the genus Dunaliella do not have a cell wall, but are surrounded by a thin elastic membrane or periplast, this makes the cells particularly susceptible to rupture.

Shear and cavitation within the centrifuge are two major reasons for damage to fragile materials. The amount of damage can be minimised by alterations to the machine, and by choosing a machine with specific features to minimise shear. One or several or all of the following features help reduce damage to fragile materials: 1. Hermetic centrifuge design which restricts the entry of air thereby limiting cavitation. 2. Feed and discharge pumps, whether they be an integral part of the centrifuge or separate to the centrifuge, should be of a design which minimises shear. 3. The liquid entering the machine should be gradually accelerated in angular velocity, to minimise shear as the liquid enters the bowl. Gradual acceleration in angular velocity can be achieved by introducing liquid into the bowl through a rotating hollow spindle as, for instance, found in certain hermetic milk separators.

Certain centrifuges can be altered to reduce cell rupture by removing feed and/or discharge paring discs.

The paring discs are centripetal pumps in the centrifuge, they create shear which damages fragile materials. When the feed paring disc is removed, it would be necessary to feed the machine using an external pump of a type which would cause minimal shear.

The separation efficiency of Dunaliella and other materials from brine or other media using a centrifuge can, in some circumstances, be improved dramatically by dilution of the media. Dilution of the media causes a number of changes to the physical characteristics of the algae and the continuous phase (media) which affect sedimentation velocity:

1. Dilution of the culture media causes lowering of the density of the media. The lowering of the density of the media should lead to an increase in the difference in density between the algal cells and the media, thereby increasing sedimentation velocity. 2. Viscosity of the media will be reduced by dilution, this will increase sedimentation velocity of algal cells. 3. Dilution of algal culture media is reported to cause swelling of the algal cells. The swelling of the algal cells effectively increases particle diameter, therefore increasing sedimentation velocity.

The increased sedimentation velocity caused by the dilution will lead to an improvement in separation efficiency.

An application of the invention for commercial harvesting of Dunaliella species is represented in Figure

1. This manner of harvesting Dunaliella has a number of advantages over other harvesting methods, including filtration and flocculation methods. These advantages include:

1. Minimisation of the number of unit steps in harvesting. 2. The Dunaliella cells not recovered during the harvesting operation are not damaged, hence, they can be immediately returned to the growing ponds as seed inoculum.

3. Dilution of the culture, as recommended in this invention, is an advantage in most Dunaliella production situations. Dunaliella species are naturally occurring, and usually cultivated, in an environment where evaporation rate is extremely high. Hence, dilution of the growing media is necessary to maintain constant salinity. Dilution and mixing of the Dunaliella salina culture during the harvesting operation reduces the number of unit operations in algal production, and improves mixing of dilution water and brine before return into growing ponds.

EXAMPLE 1

Operating "Alfa Laval" Model 2181 centrifuge with and without feed paring disc.

4 A Dunaliella salina culture, containing 6.9 x 10 cells. Ml was pumped into an "Alfa Laval" Model 2181 centrifuge. The pump was a "Mono" helical rotor pump which was coupled to a "Toshiba" Inverter to allow accurate variable flow rate.

The centrifuge was first tested with the paring discs on the feed discharge removed. In the culture discharged from the centrifuge it was found that there was negligible cell damage.

The centrifuge was then tested with the feed paring disc inserted and the discharge paring disc removed. A feed nozzle designed for maximum throughput was inserted into the feed paring disc. Culture from the discharge was examined microscopically. Ruptured cells and active cells were counted, and it was noted that significant cell rupture was caused when the centrifuge was operated with the feed paring disc inserted. The results are shown in Figure 2.

EXAMPLE 2

Separation efficiencies at different dilution. A Dunaliella salina culture, containing 7.2 x 10 cells, ml- , was pumped into an "Alfa Laval" Model

2181 centrifuge with both feed and discharge paring discs removed. Before entering the centrifuge the algal culture was mixed with freshwater in a "Lightnin" static mixer. Flowrates of fresh water and culture were controlled by identical variable speed "Mono" helical rotor pumps. A diagram indicating the equipment used in this test is given in Figure 3.

The amount of beta-carotene in the discharge from the centrifuge was determined (the beta-carotene concentration being proportional to the concentration of Dunaliella salina cells) . From these results the amount of beta-carotene recovered in the centrifuge was calculated. The results of this test are given in Figure 4. It can be seen that dilution of the culture increases separation efficiency markedly at flowrates over 500 litres. hr^~ (for example, at a culture flowrate of 3,000 litres. hr^~ with no dilution 3.6 grams of beta-carotene. hr^~ was recovered. At a culture flowrate of 3,000 litres. hr^~ with dilution of 1 part culture to 1 part freshwater 11.7 grams of beta-carotene, hr- was recovered) .

It should therefore be obvious that the present invention provides an improved method of recovery by centrifugation of fragile materials from suspension in aqueous solutions of about 30 parts per thousand or greater, which comprises the steps of:

(a) Dilution of the culture prior to feeding into the 'centrifuge to improve separation efficiency; and (b) Utilising a centrifuge in which the shear is minimised.

This can be caused by the removing from conventional centrifuges of the feed paring discs, the feed nozzle and the discharge paring disc. Further, the feed to the centrifuge could be by way of a low shear pump as a distinct entity to that of the centrifuge, whereby the aqueous solution is gradually accelerated in angular velocity to approximately match up to that of the centrifuge itself.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall into its spirit and scope.

Claims (13)

THE CLAIMS :

1. An apparatus for the centrifugation of fragile material from a liquid medium so constructed as to cause minimum shear during the centrifugation.

2. An apparatus according to claim 1 comprising hermetic seals to restrict the entry of air therein to limit cavitation.

3. An apparatus according to claim 1 or 2, wherein the feed and discharge pumps are designed to minimise shear.

4. An apparatus according to any one of the preceeding claims wherein means are provided to gradually accelerate, in angular velocity, the liquid entering the apparatus.

5. An apparatus according to claim 4, wherein said means comprises a rotating hollow spindle.

6. An apparatus according to any one of the preceeding claims wherein the apparatus comprises external feed and discharge pumps of a type which causes minimal shear, and the apparatus has its paring discs removed.

7. A method for the centrifugation of fragile material from a liquid medium whereby the fragile material is separated from the medium with minimum shear during the centrifugation.

8. A method according to claim 7 including the step of gradually accelerating in angular velocity the liquid entering the apparatus.

9. An apparatus according to claim 7 or 8 using feed and discharge pumps which are designed to minimise shear.

10. A method according to any one of claims 7 to 9 including the step of restricting the entry of air to the apparatus to limit cavitation.

11. A method according to any one of claims 7 to 10 including the step of diluting of the media.

12. A method according to any one of claims 7 to 11 wherein the fragile material is unicellular algae.

13. A method according to claim 12 wherein the algae is of the genus Dunaliella.