NL2012273C2 - Microalgae granules. - Google Patents

Description

Title: Microalgae granules

The invention is in the field of production of phototrophic microbial cultures, such as algae. In particular, the invention is directed to the cultivation and subsequent harvesting of algae from the dispersion they grow in.

Microalgae (also referred to as microphytes) are microscopic algae, typically found in freshwater and marine systems. The term ‘microalgae’ covers in this respect both microalgae (eukaryotic photosynthetic microbes) and bacteria (oxygenic photosynthetic bacteria, such as cyanobacteria), as well as all other phototrophic microbial cultures. Microalgae are species which exist individually, or in chains or groups. Depending on the species, their sizes can range from about one micrometer (gm) to a few hundreds of micrometers.

Over the last few decades an extensive amount of research has been carried out in an attempt to develop biofuels and other biobased products from sustainable resources. A variety of different biomasses from different, sources have been researched for the production of different biofuels including biodiesel, bio-ethanol, biogas, bio-hydrogen, bio-oil and bio-syngas. Biofuel sources such as sugar based ethanol and palm oil (or other agrocrops such as soybean, rapeseed and sunflower) were found to have the disadvantages that they compete with food crops and impact biodiversity and nature.

The use of aquatic phototrophic microbial cultures, in particular algae, particularly microalgae, is generally seen as more environmentally sound because primary production with algae can be more efficient than with higher plants. In particular the farming of algae enables higher areal yields and provides the possibility of using non-arable land or salt water environments.

Although microalgae are promising as a durable source for fuels and other valuable chemical components, they are hard to separate from the aqueous medium they grow in.

It is an object to obtain microalgae cultures in the form of particles that show improved separability properties. These particles are also referred to herein as microalgae granules.

It was found that this and other objects can be met by a process for obtaining granules of phototrophic microorganisms. This process comprises subjecting a starting culture of phototrophic microorganisms to selective pressure in a growth medium, and removing non-settling cultures from said medium, while allowing cultures that settle inside said medium to remain in said medium.

Although the invention is described with particular reference to microalgae, it is to be understood that it can be applied to other phototrophic microorganisms, such as other types of algae cultures as well.

Microalgae granules in accordance with the present invention offer superb properties is sohd/hquid separation. The selection is based on ability to settle (“settleability”). In accordance with the invention algae are grown in a reactor (or an environment further referred to as reactor) and are allowed to settle for a certain time. The reactor can be of any shape and size commonly encountered in the art, and may for instance be in the form of an open pond. The non-settled algae cultures are removed and the settled algae cultures remain in the system. The cultures that are removed are typically in the form of free cells or floes. Repeating this over various cycles leads to selection of microalgae, which form the granules in accordance with the present invention. The selection of the proper algae cultures may also be done for instance by using a fluidized bed, wherein the upward water flow is adjusted so that the non-settling or slowly-settling cultures are washed out over the top of the bed, whereas the settling cultures remain in the system.

The invention thus provides the application of selective pressure to such an extent that microalgae granules are produced, which are characterized by a very high settling velocity.

Growing microorganisms in granules has been described for instance in WO-A-2004/024638, incorporated herein by reference. This document however does not teach or suggest to apply this approach to phototrophic microorganisms, such as algae. WO-A-2004/024638 relates to a method for the treatment of waste water comprising an organic nutrient. According to the WO-A-2004/024638, the waste water is in a first step fed to sludge granules, after the supply of the waste water to be treated the sludge granules are fluidized in the presence of an oxygen-comprising gas, and in a third step, the sludge granules are allowed to settle in a setthng step. This makes it possible to effectively remove not only organic nutrients but optionally also nitrogen compounds and phosphate.

WO-A-2013/012329, incorporated herein by reference, describes a method for producing an open phototrophic culture with improved storage compound production capability. This known process comprises subjecting a starting culture to selective pressure, thus giving a competitive advantage to storage compound producing species, by subjecting said starting culture to a cycle of alternating dark phases and fight phases and providing limitation of availability of essential growth nutrients in one or more of said fight phases.

Typical settling velocities previously reported for microalgae range from 0.5 to 2 cm/minute (see for instance Van Den Hende et al. (Biotechnology and Bioengineering 108(2011)549-58) and Vafigore et al. (Water Research 46(2012)2957-64)).

In accordance with the present invention it is possible to have granules showing a settling velocities of more than 10 cm/min, preferably more than 15 cm/min, even more preferably more than 18 cm/min, for instance 20-100 cm/min. As a consequence of the process of the invention, the algae cultures of the present invention have to settle much faster and will become granules rather than floes. Floes are different from granules in that floes are more loosely clumped together, have a more open structure and therefore settle slower.

Figure 1 shows an example of an algae culture developing in accordance with the present invention after various points in time.

The first solid liquid separation of microalgae after cultivation is costly. Algae need light and in practice microalgae cultures are known to be fight limited at concentrations exceeding about 1-5 g/dm3 (the concentration of algae becomes too high to permit light throughout the medium). As a result, the product stream is obtained as a very diluted stream.

In the art, the first solid liquid separation is commonly carried out by means of centrifugation, filtration or similar techniques. In accordance with the present invention these can be replaced by fast, low cost and low energy consuming settling, which relies on gravity alone. The granules of the present invention also have improved separation properties for use in equipment operated by centrifugal forces.

Surprisingly it was found possible to produce algae granules having a dimension of about 0.02 to 2 mm (largest dimension, which correspond to the diameter for spherical particles). Preferably the granules have a diameter of 100-500 pm, e.g. about 200 pm. Typically the granules are spherical or almost spherical. Typically each granule comprises 1000 to 10000 algae cells.

The processes of the present invention have an advantage in that they do not require the sterilization of equipment and incoming and outgoing flows. The selective mechanisms ensures the functionality of the process is maintained. Other microorganisms, such as bacteria may be present in the granules as well. It is possible that these other microorganisms and algae live in symbiosis, for instance because bacteria may add to the mechanical stability of the granules, while the algae provide the energy required for growth. It is also surprising that the method of the invention results in granules having such a high setthng velocity, which by result may lead to a short settling time of several minutes or less, e.g. 45 seconds, depending on the design of the reactor.

The invention can be used for either high-value compounds or low-value compounds usually produced in bulk. The invention may find use in pure or mixed cultures, xenic or axenic cultures, both with fresh or marine water or variations thereof.

Commercial application could be an alternative for the solid-liquid separation steps now used in algae cultivation. It could also be used as a step before the solid-liquid separation step now used to decrease the liquid flow at least a factor ten at conceptually lower costs.

Example A photobioreactor with microalgae floes was subjected to the following conditions: a settling phase of 2 minutes, a settling distance to stay in the system of 60 centimeter, a solid retention time of 12 days. Within seven cycles microalgae granules were formed. Figure 1 shows free cells (day 0), floes (day 6, 26, 69) and granules (day 75).

Claims (14)

A method for obtaining granules of phototrophic microorganisms, the method comprising the steps of subjecting a starting culture of phototrophic microorganisms to selective pressure in a growth medium, and removing non-settling cultures from said medium, while cultures that settle within said medium may remain in said medium. Method according to claim 1, wherein said phototrophic microorganisms are algae, preferably microalgae. A method according to any one of the preceding claims, comprising a cycle of steps of removing non-settling biomass until said cultures form pellets. A method according to any one of the preceding claims, comprising a cycle of steps in which settling biomass is selectively retained over non-settling biomass until said cultures form granules. A method according to any one of the preceding claims, comprising a cycle of steps in which biomass with a higher settling rate is selectively retained over biomass with a lower settling rate until said cultures form granules. A method according to any one of the preceding claims, wherein in addition to phototrophic microorganisms other microorganisms are present in said granules. The method of any one of the preceding claims, wherein said non-settling cultures are in the form of flakes. A method according to any one of the preceding claims, wherein said granules are subjected to conditions that promote the production of storage compounds. A method according to any one of the preceding claims which forms part of a waste water processing method. A method according to any one of the preceding claims that forms part of the production of customized chemical products or other compounds with a high value. A grain that can be obtained by the method according to any one of the preceding claims. A granule according to the preceding claim, with a settling speed of approximately 20 - 100 cm / minute. A granule according to claim 11 or 12 with a density of about 1030 - 1100 g / dm 3. A granule according to any one of claims 11 to 13 with a largest dimension of 0.02 to 2 mm.