subcategory. There are three main alternatives for cultivating photoautotrophic
algae: (1) open systems such as the routinely used raceway pond systems, (2) closed
systems involving PBRs, and (3) hybrid production systems which are combinations of the other two systems. Among these systems, open ponds like raceway
pond systems are the most common design employed for large-scale applications
(Pulz 2001). A typical raceway pond comprises a closed oval channel, open to the
air, and mixed with a paddle wheel to circulate the water and prevent sedimentation.
These ponds are usually shallow; i.e., *0.25–0.4 m deep, to facilitate light penetration and prevent self-shading by algal cells. Limited light penetration through the
algal broth would decrease the photosynthesis and consequently the biomass production. Some genetic manipulations aimed at remodeling photosynthesis apparatus
to enhance this trait were discussed in Sect. 3.1.2. High rate algal ponds (HRAPs)
are shallow, open raceway ponds. HRAPs have been originally used for the
treatment of municipal, industrial, and agricultural wastewaters; however, the algal
biomass produced from these systems could be converted through various pathways
to biofuels.
A semi-closed ocean system enriched by iron was introduced by the US patent
2014/0113331 leading to atmospheric CO 2 sequestration, reduced ocean acidity, as
well as efficient cultivation and harvesting of a high deal of algal biomass. Although
open systems in general look promising for commercial applications, their several
shortcomings led to a widespread search for alternative algal cultivation systems. In
light of that, tubular PBRs were introduced through which problems like susceptibility to contamination (as seen in open pond systems), large water consumption,
low CO 2 absorption efficiency, the presence of dark zones (or in another word, low
light penetration efficiency), and the resultant low photosynthesis efficiency were
overcome. The issue of surface–volume ratio, light, CO 2, and nutrients supply as
well as the development of tools to control temperature and pH have been extensively studied and have been the subject of numerous patents such as
US20090211150A1, US5104803A, US20090291485A1, US2010000571A1,
US20090029445A1, and US9605238B2.
Different configurations of tubular PBRs (i.e., horizontal, helical, and flat panels)
have also been introduced in a number of patents (e.g., US20100248333A1 and
US20080311649A1). Moreover, airlift reactors in which bubbles are used as
bubble-columns to mix the media were later introduced (US20110113682A1) to
overcome the problems associated with the large surface–volume ratio observed in
tubular PBRs. However, bubble-columns and airlift reactors require high gas flows
to ensure an efficient circulation would be taking place between the light and dark
zones. This could further impose a shearing force on the growing algal cells.
A novel PBRs design elaborated in the patent US20150275161A1 eliminated the
need for sparging and compressors for suspending cells and mixing carbon dioxide
through the introduction of attendant mixing by subtending wave motion. The
novel system resulted in reduced initial investment required as well as the elimination of the above-mentioned sharing force. Another example of novel PBRs is
discussed in WO2015056267A1.
14 Recent Patents on Biofuels from Microalgae
299
algae: (1) open systems such as the routinely used raceway pond systems, (2) closed
systems involving PBRs, and (3) hybrid production systems which are combinations of the other two systems. Among these systems, open ponds like raceway
pond systems are the most common design employed for large-scale applications
(Pulz 2001). A typical raceway pond comprises a closed oval channel, open to the
air, and mixed with a paddle wheel to circulate the water and prevent sedimentation.
These ponds are usually shallow; i.e., *0.25–0.4 m deep, to facilitate light penetration and prevent self-shading by algal cells. Limited light penetration through the
algal broth would decrease the photosynthesis and consequently the biomass production. Some genetic manipulations aimed at remodeling photosynthesis apparatus
to enhance this trait were discussed in Sect. 3.1.2. High rate algal ponds (HRAPs)
are shallow, open raceway ponds. HRAPs have been originally used for the
treatment of municipal, industrial, and agricultural wastewaters; however, the algal
biomass produced from these systems could be converted through various pathways
to biofuels.
A semi-closed ocean system enriched by iron was introduced by the US patent
2014/0113331 leading to atmospheric CO 2 sequestration, reduced ocean acidity, as
well as efficient cultivation and harvesting of a high deal of algal biomass. Although
open systems in general look promising for commercial applications, their several
shortcomings led to a widespread search for alternative algal cultivation systems. In
light of that, tubular PBRs were introduced through which problems like susceptibility to contamination (as seen in open pond systems), large water consumption,
low CO 2 absorption efficiency, the presence of dark zones (or in another word, low
light penetration efficiency), and the resultant low photosynthesis efficiency were
overcome. The issue of surface–volume ratio, light, CO 2, and nutrients supply as
well as the development of tools to control temperature and pH have been extensively studied and have been the subject of numerous patents such as
US20090211150A1, US5104803A, US20090291485A1, US2010000571A1,
US20090029445A1, and US9605238B2.
Different configurations of tubular PBRs (i.e., horizontal, helical, and flat panels)
have also been introduced in a number of patents (e.g., US20100248333A1 and
US20080311649A1). Moreover, airlift reactors in which bubbles are used as
bubble-columns to mix the media were later introduced (US20110113682A1) to
overcome the problems associated with the large surface–volume ratio observed in
tubular PBRs. However, bubble-columns and airlift reactors require high gas flows
to ensure an efficient circulation would be taking place between the light and dark
zones. This could further impose a shearing force on the growing algal cells.
A novel PBRs design elaborated in the patent US20150275161A1 eliminated the
need for sparging and compressors for suspending cells and mixing carbon dioxide
through the introduction of attendant mixing by subtending wave motion. The
novel system resulted in reduced initial investment required as well as the elimination of the above-mentioned sharing force. Another example of novel PBRs is
discussed in WO2015056267A1.
14 Recent Patents on Biofuels from Microalgae
299