Co-Authored By; Bonface G. Mukabane, Benson B. Gathitu, Urbanus Mutwiwa, Paul Njogu and Stephen Ondimu
Microalgae represent a sustainable biofuel source because of their high biomass productivity and ability to sequester carbon dioxide from the air and remove water born pollutants. This paper reviews the current status of microalgae cultivation systems, including the advantages and disadvantages of both open and closed systems. The key barriers to commercial cultivation of microalgae and the way forward is also discussed.
Increased interest in biofuels is mainly driven by; the fluctuating oil prices and recognition of the fact that the global fossil fuel reserves are getting exhausted, concerns about environmental pollution and resultant environmental change due to fossil fuel emissions and the provision of alternative outlets for agricultural producers.
Global biofuel production has been increasing rapidly over the last decade, but the expanding biofuel industry has recently raised pertinent concerns. In particular, the sustainability of many first-generation biofuels; fuels made from food and feed crops and mainly vegetable oil, has been increasingly questioned over concerns such as reported displacement of food crops, effects on the environment and climate change . In general, there is growing consensus that if significant emissions reductions in the transport sector are to be achieved, biofuel technologies must become more efficient in terms of net life cycle greenhouse gas emission reduction while at the same time be environmentally and socially sustainable. It is increasingly understood that most first-generation biofuels, except sugarcane ethanol, will likely have a limited role in the future transport fuel mix .
Biodiesel is a mixture of fatty acid alkyl monoesters (FAMEs) derived from vegetable fatsand oils. It can be used as a replacement of petro-diesel because of their structural similarity. Biodiesel is produced using vegetable oil, plant oil, and animal fat. Biodiesel is an alternative fuel for diesel and most diesel engines can use 100% biodiesel . The main feedstock currently used for biodiesel production includes palm oil, sunflower, rapeseed, soybean, and canola seed. A great challenge of using vegetable oils for biodiesel production is the availability of crop land for oil production to produce enough biodiesel that significantly replaces the current fossil fuel consumption . Chisti  estimated that it would take 24% of the existing crop land in the US to grow oil palm that is considered as a high yield oil crop or over three times of the current cropland in the US to grow soybean to produce enough biodiesel that would replace 50% of the transportation fuel in the US. Several studies have been conducted on using alternative oils such as waste oils from restaurants and kitchens and microalgal oils for biodiesel production . Shah et al.  investigated the utilization of restaurant waste oil as a precursor for sophorolipid and biodiesel production. Zhang et al.  evaluated the Biodiesel production from waste cooking oil including economic analysis. Miao and Wu  studied biodiesel production from heterotrophic microalgal oil. A great advantage of using microalgal oil over vegetable oils for biodiesel production is that the production of algal oil does not need cropland and has much higher oil yield per acre of land because the microalgae can be grown in 3 dimensions in photobioreactors . However, a big challenge of biodiesel production using algal oil is that the cost of algal oil production is extremely high . The goal of the present paper is to review recent development in microalgae production systems and identify strategies for further development.
- Microalgae cultivation systems
Annual oil production from high-oil microalgae can be in the range of 58 700 to 136 900 litres per hectare . If this microalgal oil is used for biodiesel production, it would take approximately 1.0 – 2.5% of the current cropland in the US to meet 50% of the US transportation fuel needs, which is much more feasible than the current oil crops . Commercially growing microalgae for value-added products is usually conducted in open ponds (raceways) or closed photobioreactors (PBRs) under autotrophic (making complex organic nutritive compounds from simple inorganic sources by photosynthesis) or heterotrophic (cannot synthesize its own food) conditions at relatively warm temperature (20 – 30 0C) . In autotrophic microalgal cultivation, the microalgae need sunlight (energy source), CO2 (carbon source) and nutrients (P, N and minerals) for their photosynthesis and generate oxygen. The main difference of growing heterotrophic microalgae from autotrophic ones is the carbon source. The former requires organic carbon source such as glucose to support its growth. Normally autotrophic microalgae are grown for biodiesel production, mainly because they use CO2 as their carbon source for growth . Therefore, the whole cycle of growing microalgae for biodiesel production and combustion of biodiesel as fuel would generate zero net carbon dioxide emission to the atmosphere. However, sometimes heterotrophically grown microalgae can make much more oil than autotrophic ones. Miao and Wu  reported the heterotrophic growth of Chlorellaprotothecoides resulted in a significant increase of oil content of microalgae from 14.5% under the original autotrophic growth to 55.2% (dry weight).
In a photobioreactor microalgal growth system, pure high-oil microalgae are grown in closed glass or plastic tubular bioreactors. Nutrient water is circulated in the bioreactors for keeping the microalgae from settling and for the growth of the microalgae. Natural sunlight is usually the energy source for microalgal growth . Although artificial illumination to the photobioreactors is viable, it is much more expensive than natural illumination. Pure microalgal culture can be maintained in the photobioreactors. Heat exchanger is usually necessary to maintain an adequate temperature in the photobioreactors. A high concentration of microalgal biomass can be achieved in photobioreactors. In that case high dissolved oxygen may inhibit the microalgal growth, so degassing system is usually necessary to release oxygen from the water .
Open (raceway) ponds are similar to oxidation ditches used in wastewater treatment systems. They are large, open basins of shallow depth and a length of at least several times greater than that of the width . Raceway ponds are typically constructed using concrete shell lined with polyvinyl chloride. Dimensions range from 10 to 100 m in length and 1 to 10 m in width with a depth microalgal growth of 10 to 50 cm . Ponds are kept shallow as optical absorption and self-shading by the algal cells limits light penetration through the algal broth . Waste waters from animal operations and municipalities can be used for growing microalgae. Water recirculation or agitation is necessary to keep the microalgae from settling. Microalgal biomass concentration in the ponds is usually low compared to the photobioreactors. Wild algae and/or bacterial contamination is normally challenging in the open ponds (Table 2.1) [1, 7].Oswald considered the open pond to be the most viable method of combining algal cultivation and wastewater treatment in the 1950s 
Table 2.1: A comparison of growing microalgae in open ponds and photobioreactors [1, 7]
|Estimated productivity (g.m-2.day-1)||11||27|
|Advantages||Low energy||Pure algal culture|
|Simple technology||High volumetric productivity|
|Well researched||Small area required|
|Disadvantages||Low productivity||High energy|
|Large area required||Less researched|
|High water use|
Photobioreactors (PBRs) are more commonly used for growing algae for high value commodities or for experimental work at a small scale. Recently, however, they have been considered for producing algal biomass on a large scale as they are capable of providing optimal conditions for the growth of the algae [10, 11]. A closed reactor allows species to be protected from bacterial contamination. Shallow tubing allows efficient light utilization. Bubbling CO2 provides high efficiency carbon uptake and water loss is minimized in closed reactors. PBRs provide very high productivity rates compared with raceway ponds. In their life-cycle assessment (LCA) study, Jorquera et al.  estimated volumetric productivity to be at least eight times higher in flat-plate and tubular PBRs. The reason why PBRs have not become popular is due to the energy and cost intensity of production and operation. PBRs require afar higher surface area for the volume of algal broth compared with alternative infrastructure. Much higher volumes of material are therefore required which in turn requires a higher capital energy input and increases environmental impacts . During operation, algal biomass must be kept in motion to provide adequate mixing and light utilization. These increase productivity but also require additional energy for pumping. So far in comparison to raceway ponds the benefits of PBRs do not outweigh the necessary energy requirements identified in the LCA study published by Jorquera et al. .A net energy ratio (i.e., energy produced/energy consumed) of 8.34 has been reported for raceway ponds as compared to a net energy ratio of 4.51 and 0.20 for flat-plate and tubular photobioreactors, respectively . It is likely that ponds will continue to provide the most effective infrastructure for algal cultivation due to their low impact design and low energy input requirement. PBRs will continue to be important however, for laboratory work, developing cultures and producing biomass with higheconomic value. As research continues it may also be possible to develop infrastructure that willprovide the benefits of both PBRs and open ponds together.PBRs are of different configurations including flat plate, column and tubular . In both open and closed microalgae culture systems, light source and light intensity are vital for the performance of phototrophic growth of microalgae. The development of optical trapping system, light delivery and lighting technologies, which improve the distribution and absorption and the advent of some new photobioreactors, will improve the efficiency of photosynthesis . In addition, gas-liquid mass transfer efficiency is another critical factor affecting CO2 utilization and thus the phototrophic growth . Cheng et al.  constructed a 10 L photobioreactor integrated with a hollow fiber membrane module which increased the gas bubbles retention time from 2 s to more than 20 s, increasing the CO2 fixation rate of Chlorella vulgaris from 80 to 260 mg.L-1.h-1.
- Future perspectives of microalgal biodiesel production
To improve the economics of microalgal biodiesel production, more research and development are compulsory to reduce the costs of growing microalgae and to competently control culture contamination when grown in open ponds. The research and development efforts probably need to focus on the following areas:
- Selection and development of high-yield, oil-rich microalgae: Oil-rich microalgal species can be enhanced through cultivation and genetic engineering to increase the oil content in their biomass without compromising the biomass production rate .
- Enhancement of the tolerance oil-rich microalgae to high and/or low temperatures: Most microalgae prefer to grow at the temperatures of 20-30 0 When the temperature is higher than 30 0C, which happens very frequently during the sunny days in photobioreactors, heat exchangers have to be operated to cool down the microalgal culture to sustain a high microalgae growth. Installation and operation of the heat exchangers significantly add cost to the whole microalgal biomass production. Selection and modification of microalgae to aid them grow fast at high temperatures would probably eradicate the heat exchangers and contribute to the cost reduction of microalgal biomass production .
- Enhancement of the tolerance of oil-rich microalgae to the high concentration of oxygen: When microalgae grow under autotrophic conditions, they produce oxygen that dissolves in water to yield a super saturated dissolved oxygen concentration in the media, sometimes 4-5 times of the air saturation value. A combination of high dissolved oxygen with intense sunlight impedes the growth of the microalgae and destroys the microalgal cells. To prevent the inhibition and damage to the microalgae, a degassing system is necessary to keep the dissolved oxygen at a suitable level in the growth media. Increasing the tolerance of the microalgae to the high dissolved oxygen concentration in the media could also decrease the cost of microalgal biomass production .
- Improvement of the competitiveness of oil-rich microalgae against wild algae and bacteria: In open pond microalgae production, the contamination of wild algae and bacteria is very challenging. If the growth media is contaminated by wild algae and/or bacteria, the wild algae and bacteria will devour the nutrients in the media and significantly diminish the yield of the desired microalgae. Improving the competitiveness of the oil-rich microalgae against the wild algae and bacteria and deterring the wild algal and bacterial activities in the media for growing the microalgae also has a potential to reduce the cost of microalgal biomass production .
- Improvement of the engineering of the microalgae growth systems: Both microalgae growing systems presently used for microalgal biomass production, photobioreactors and open ponds have rooms for improvement. When microalgae grow in tubular photobioreactors, some of them stick on the wall of the tubes, significantly decreasing the penetration of light to the growth media and resulting in a lower yield of the microalgal biomass. Cost-effective materials which inhibit the microalgae from attaching the surface should be explored to maintain a high growth rate of the microalgae. The main drawback of growing microalgae in open ponds is contamination. Greenhouse ponds can be an effective system to avert contamination and to increase the microalgal density in the growth media .
- Development of cost-effective microalgae harvesting systems: Harvesting microalgal biomass contributes markedly to the total costs of the biomass production. Current technologies ordinarily involve coagulation, filtration and centrifugation, which are costly. Innovative cost-effective harvesting systems need to be explored to significantly reduce the cost of microalgal biomass harvesting .
- Application of the biorefinery model to microalgal biodiesel production system: Microalgal biomass contains lipids (oil), carbohydrates, proteins and other minor components such as minerals and vitamins. Oil is used for biodiesel production. Other constituents can be processed into value-added products. After oil extraction, the residues which are rich in carbohydrates, proteins and minor nutrients can be used to produce animal feed. They can also be utilized for biogas production through anaerobic digestion. Special high-value organic chemicals could be extracted from the residues and should be explored to increase the revenue of the microalgae-to-biodiesel process. All these byproducts have capabilities to improve the economics of the microalgae-to-biodiesel process .
- Combine microalgae cultivation with wastewater treatment.The microalgae could therefore provide a means of improving the waterquality of raw or partially treated effluent as well as providing livestock feed and/or biomass for energy generation.
Microalgae are a sustainable energy resource with great potential for CO2 fixation and wastewater purification. This review discussesmicroalgae cultivation systems, challenges and anticipated future developments in the microalgae to biodiesel approach. For biodiesel production to have a significant impact on renewable fuels, technologies must be developed to enable large scale algae biomass production. Further efforts on microalgae biodiesel production should focus on reducing costs in large-scale algal biomass production systems –this is being investigated at The Jomo Kenyatta University of Agriculture and Technology, Kenya. Combining microalgae mixotrophic cultivation with sequestration of CO2 from flue gas and wastewater treatment approach to algal biomass conversion will improve the environmental and economic viability.
The authors are grateful for financial support of the National Council of Science, Technology and Innovation (NACOSTI) of Kenya.
 J.J. Cheng,“Advanced Biofuel Technologies: Status and Barriers.” Development Research Group. The World Bank, September, 2010.
 International Energy Agency (IEA) Bioenergy, “From 1st-to 2nd-Generation Biofuel Technologies: An Overview of current Industry and R and D Activities.”2008.
 Y. Chisti,“Biodiesel from microalgae.”Biotechnology Advances,vol. 25, pp. 294-306, 2007.
 V. Shar, M. Jurjevic and D. Badia, “Utilization of restaurant waste oil as a precursor for sophorolipid production.”Biotechnology Progress, vol.23, no.2, pp. 512 – 515, 2007.
 Y.Zhang, M.A. Dube, D.D. McLean and M. Kates, “Biodiesel production from wastecooking oil: 2. Economic assessment and sensitivity analysis.”BioresourceTechnology,vol. 90, pp. 229–240, 2003.
 X. Miao andQ. Wu, “Biodiesel production from heterotrophic microalgal oil.”Bioresource Technology, vol. 97, pp. 841-846, 2006.
 O. Jorquera, A. Kiperstok, E.A. Sales, M. Embirucu and M.L. Ghirardi, “Comparative energy lifecycle analyses of micoalgal biomass production in open ponds and photobioreactors.”Bioresour. Technol., vol. 101, pp. 1406-1413, 2010.
 R. Slade and A. Bauen,“Micro-algae cultivation for biofuels: Cost, energy balance, environmental impacts and future prospects.”Biomass Bioenergy,vol. 53, pp. 29-38, 2013.
W.J.Oswald, H.B. Gotaas, H.F. Ludwig and V. Lynch,“Algae symbiosis in oxidation ponds. 3.Photosynthetic oxygenation.”Sewage Ind. Wastes, vol.25, pp. 692–705, 1953
 O. Jorquera, A. Kiperstok, E.A. Sales, M. Embirucu and M.L. Ghirardi,“Comparative energylifecycle analyses of microalgal biomass production in open ponds and photobioreactors.”Bioresour. Technol., vol.101, pp. 1406–1413, 2010.
 K. Soratana, and A. E. Landis,“Evaluating industrial symbiosis and algae cultivation from a life cycleperspective.”Bioresour. Technol., vol.102, pp.6892–6901, 2011.
 X. Wu, R. Ruan, Z. Du and Y. Liu,“Current Status and Prospects of Biodiesel Production from Microalgae.”Energies, vol. 5, pp. 2667-2682, 2012.
 J. Masojidek, S. Papacek, M. Sergejevova, V. Jirka, J. Cerveny, J. Kunc, J. Korecko,O. Verbovikova, J. Kopecky andD. Stys, “A closed solar photobioreactor for cultivation of microalgae under supra-high irradiance: Basic design and performance.”J. Appl. Phycol.,vol. 15, pp. 239-248, 2003.
 L.H.Cheng,L. Zhang, H.L. Chen andC.J. Gao, “Carbon dioxide removal from air by microalgae cultured in a membrane-photobioreactor.”Sep. Purif. Technol., vol. 50, pp. 324-329, 2006.