Microalgae cultivation can be done in open-culture system, such as lakes or ponds, and in highly control closed-culture system called photobioreactors (PBRs). A photobioreactor is a reactor in which a phototrophs (microbial, algal or plants cells) are grown or used to carry out phootobiological reaction.
Open-culture systems are normally less expensive to build and operate, more durable than large production capacity when compared with closed systems. Ponds use more energy to homogenize nutrients and the water level cannot be kept much lower than 15 cm (or 150 L/m2) for the microalgae to receive enough solar energy to grow. Raceway pond, the most widely used photobioreactor for commercial production of microalgae, requires 1.5 km2 to fix the CO2 emitted from a 150 MW thermal power plant. Thus, it is important to maximize both volumetric productivity and photosynthetic efficiency to reduce the capacity of the system. However it is not an easy task because the two objectives are contradictory.
Generally ponds are more susceptive to weather conditions, not allowing control of water temperature, evaporation, and lighting. They may produce large quantities of microalgae, but occupy more extensive land area and are more susceptible to contaminations from other microalgae or bacteria. Moreover, it is expected that mass transfer limitation could slow down the cell growth of microalgae since atmosphere only contains 0.03-0.06% CO2.
PBRs are flexible systems that can be optimized according to the biological and physiological characteristics of the algal species being cultivated. On a PBR, direct exchange of gases and contaminants, such as microorganism and dust, between the cultivated cells and atmosphere are limited or not allowed by the reactor walls.
PBRs are considered to have several advantages over open ponds.
- Offer better control over culture conditions and growth parameter (pH, temperature, mixing, CO2, O2)
- Prevent evaporation
- Reduce CO2 losses
- Allow to attain higher microalgae densities or cell concentrations
- Higher volumetric productivities
- Offer a more safe and protected environment
- Preventing contamination or minimizing invasion by competing microorganisms
However, PBRs suffer from several drawbacks that need to be considered and solved. Their main limitations include:
- Oxygen accumulation
- Difficulty in scaling up
- High cost of building, operating, and algal biomass cultivation
- Cell damage by shear stress and deterioration of material used for the photo-stage
Table below makes a comparison between PBR and ponds for several culture conditions and growth parameters.
A combination of open ponds and PBRs is probably the most logical choice for cost-effective cultivation of high yielding strains for biofuels. Inoculation has always been a part of algal aquaculture. Open ponds are inoculated with a desired strain that was invariably cultivated in a bioreactor, whether it be as simple as a plastic bag or a high tech fiber optic bioreactor. Importantly, the size of the inoculum needs to be large enough for the desired species to establish in the open system before an unwanted species. Sooner or later though contaminating species will end up dominating an open system and it will have to be cleaned and re-inoculated. Therefore to minimize contamination issues, cleaning or flushing the ponds should be part of the aquaculture routine, and as such, open ponds can be considered batch culture.
This process has been demonstrated by Aquasearch (Hawaii, USA) cultivating Haemotococcus pluvialis for the production of astaxanthin. Half of the Aquasearch facility was devoted to photobioreactors and half to open ponds. H. pluvialis is grown continuously in photobioreactors under nutrient sufficient condition and then a portion is transferred to nutrient-limited open ponds to induce astaxanthin production. Enough nutrients are transferred with the inoculum for the culture to continue to grow for 1 day, and after 3 days when astaxanthin level peak, the open ponds are harvested, cleaned, and then re-inoculated. This approach is also very suitable for biofuel production as under low-nutrient conditions algae rapidly start to convert energy from the sun into chemical energy stored as lipids as a means of survival.
- Second generation biofuels: high-efficiency microalgae for biodiesel production
- Microalgae for biodiesel production and other applications: a review
- Review of advances in biological CO2 mitigation technology
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