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Cultivation considerations

Generally, three types of microalgal cultivation systems can be distinguished:

  • • open systems such as ponds, raceways and high rate algal ponds (HIRAPs) traditionally used in aquaculture and for the commercial production of microalgae
  • • closed systems: tube or plate photobioreactors, where the algal biomass is generally cycled through a solar compartment and a mixing compartment, which allows for degassing and nutrient addition
  • • hybrid systems: which are essentially open systems but operate under positive air pressure compared to the outside, making it less likely for contaminants to invade the system (da Rosa et al., 2011; Henrard et al., 2011; see also Chapter 4).

All cultivation systems have their advantages and disadvantages. Disadvantages of open systems are: prone to invasions, shallow, making mixing and gas solubilisation difficult, high water loss due to evaporation, large land requirements, low biomass productivities and often poor temperature control. Open systems also have significant advantages. The shallow depth allows for effective degassing of the photosynthetically produced oxygen, which can inhibit photosynthesis if it accumulates in the system, evaporative water loss provides a means of non-energy derived cooling, most microalgal species investigated can be grown in these systems and they are inexpensive in terms of CAPEX (Christenson and Sims, 2011; Weissman and Goebel, 1987). However, evaporative water loss and the large area requirement, particularly for biomass use for biofuel production, are of environmental concern considering future freshwater resources

(Murphy and Allen, 2011). To avoid these negative impacts, it would be mandatory that evaporative water loss is compensated for using non-potable wastewater and that system operation must occur on non-arable land. Currently, open systems are used for the commercial production of B-carotene mainly using the chlorophyte Dunaliella salina, production of the chloroxybacterium Arthrospira platensis and the chlorophyte Chlorella sp. as a health food supplements (Table 5.2). Reported long-term operation averages for the eustigmatophyte Nannochloropsis oculata are 20 g dry weight m-2 day-1, which still significantly exceeds productivities of even the most productive terrestrial oil crops (CSIRO, 2011), make such systems potentially useful to also secure high-quality aviation fuel, an area the aviation industry is actively pursuing. With reference to the sustainability of aviation fuel, it is noteworthy that the CSIRO considers bio-derived jet fuels the only sustainable replacement for fossil oil-derived aviation fuels, which will not interfere with arable land use for human food production and can be generated in sufficient quantities to make this a possibility (CSIRO, 2011).

Closed systems are believed to have significant advantages over open cultivation systems in that they are considered to be less prone to contamination, do not suffer from evaporative water loss, show higher productivities on a volume and area basis due to improved light penetration and biomass resuspension (Carvalho et al., 2006). Disadvantages of these systems are that current systems are relatively small scale, only very few organisms can be successfully cultivated, mixing and degassing (build up of photoinhibitory concentrations of photosynthesis-derived oxygen) is still problematic and energy-intensive, require extensive ground preparations for their set up and cooling due to the small volumes in tubular and thin plate solar compartments, are highly technical and very expensive requiring highly trained personnel, which almost prohibits operating them in less developed countries.

In general, improved productivities are typically not large enough to offset the higher costs of CAPEX and OPEX (energy requirements), making it energetically and economically unattractive to use them for the production of low-value end products, such as fuels (Xu et al., 2009). Volumetric daily productivities of closed photobioreactor systsms are being advertised as 4-6 g dry weight L-1 day-1; however, long-term multi-year production records are lacking, which makes it unclear whether these productivities could be maintained year round. Regardless, as volumes in closed production systems are typically 10-20 times smaller than open systems, but costs are 10 times higher, it is questionable if this increased productivity would actually stand out compared to the reported long-term year-round productivities of open systems’ 0.5-1 g dry weight L-1 day!, which for lower value products is most likely not the case. In terms of cost and volumes, closed photobioreactors are attractive for the cultivation of microalgal biomass for the high to very high value product market where much smaller biomass or compound quantities are required to strike economical success. As such, to date, commercial-scale cultivation is restricted to the freshwater chlorophyte Haematococcus pluvialis for the production of the antioxidant astaxanthin (Li et al., 2011).

Given the economical and energetic drawbacks of closed systems, current research also focuses on developing hybrid systems, which are essentially a semi-closed cultivation system where a positive air displacement between the system and the outside should restrict air-borne contamination. Another definition of hybrid system exists where the term describes a closed photobioreactor tasked with maintaining biomass for the inoculation of open systems for short-term cultivation in order to curb contamination (Singh and Dhar, 2011). Regardless of the definition used for hybrid systems, they are likely to be similarly expensive with regards to energy used for culture resuspension and will also suffer from similar rates of evaporative water loss, displaying approximately twice the price tag of commercial-scale open systems. In essence, however, these systems have inherited the positive sides of the open cultivation systems and more of the advantages of the closed system. This makes these systems economically attractive for the mid-price range product market, as contamination is one of the major economic losses associated with open cultivation systems. Whether these systems display appropriate productivities remains to be shown, but initial results show that horizontal systems, which are comparable in depth and volumes to commercial raceways, show similar productivities and that these can be increased fivefold and more if cultivation occurs in vertically oriented systems (data not shown). The latter systems, however, are of much lower volume, thus it remains to be demonstrated whether vertical hybrid systems of similar volumes to horizontal ones and raceways would maintain this aerial productivity advantage.

It is also possible to grow many microalgal species (e.g. the chlorophytes Chlamydomonas rheinhardtii or Chlorella protothecoides) heterotrophically in fermenter-style cultivation systems on glucose or acetate in the absence of light, which increased lipid productivity around 24-fold (Xiong et al., 2008) compared to photosynthetically grown microalgae with high lipid productivities, such as the green alga Tetraselmis sp., a marine species belonging to the class Prasinophyceae (Huerlimann et al., 2010). While this approach shows immense promise for the production of low-value end products such as biodiesel, there are no ecological advantages to promote this to a commercial scale considering rising atmospheric CO2 concentrations and the competition for arable land and irrigation-derived sugar, as heterotrophic growth generates CO2 and the approach would enter the food versus fuel debate if conducted on a large enough scale to substantially contribute to renewable biofuels to meet growing future demands in industry and for general transport. In addition, the approach requires axenic (bacteria-free) cultures, which will be challenging to maintain on an industrial scale. Furthermore, the beneficial allelopathic interactions between the microalgae and their bacterial flora are lost in axenic cultivation, which leads to the cultivation of strains that are tolerant to this loss, thereby restricting strain choice. In addition, the demand for organic carbon would, at the required scales, negatively impact on sugar prices and arable land committed to carbohydrate production for fuel rather than human food, which has already been criticised with regards to the use of corn for bioethanol production (Liao et al., 2011). Even if life cycle and economic analysis were favourable, at this stage, the negative aspects outweigh the positive aspect of fuel security.

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