Cultivation of filamentous cyanobacteria (blue-green algae) in agro-industrial wastes and wastewaters: A review
Introduction
Microalgae, a broad category encompassing eukaryotic microalgae and cyanobacteria, can be cultivated to produce biomass for a wide range of applications, including animal and human nutrition, the health sector, cosmetics and agriculture (biofertilizers) [1], [2], [3], [4]. In parallel, an important application for the cultivation of microalgae is the production of biomass for energy purposes. Microalgae produce biomass, which can be converted into energy or an energy carrier through a number of energy conversion processes. They include thermochemical conversion (gasification, direct combustion and pyrolysis), biochemical conversion (anaerobic fermentation, anaerobic digestion and photobiological hydrogen production) and esterification of fatty acids to produce biodiesel [5], [6], [7], [8], [9], [10], [11], [12].
Microalgae biomass contains considerable amounts of proteins [13] and on the basis of biomass composition the quantity of nitrogen (N) required as fertilizer is estimated to be 8–16 tons N/ha, which means that microalgae production involves enormous amounts of N fertilizers. The use of such large quantities of fertilizer for microalgae cultivation raises questions about their environmental impact [14], [15]. Furthermore, the use of fertilizer contributes to the cost of algal biomass production. For example the use of fertilizer constitutes nearly half of the overall cost of Spirulina cultivation [16]. In order to reduce the use of fertilizer, wastewaters rich in N and phosphorus (P) can be used as a cultivation medium, while at the same time microalgae can be used to reduce the inorganic and organic load of these wastewaters, thereby providing a method of biological wastewater treatment [17], [18], [19], [20], [21], [22], [23], [24].
A serious drawback to unicellular micro-algal cultivation is the harvesting of the biomass due to the microscopic dimensions of microalgae (0.5–30 μm) [25], [26]. In essence, harvesting means that the algal biomass is separated from the liquid cultivation medium. As a result algal biomass is concentrated or dewatered, forming a slurry that consists of 5–15% dry solids [7]. Harvesting of biomass from the broth is thought to contribute 20–30% of the total cost of biomass production [25]. Filamentous cyanobacteria, with dimensions of around 200 μm can help reduce the harvesting problem because they may be harvested relatively easily by filtration. In addition, some filamentous cyanobacteria form aggregates and can be harvested by sedimentation or by flotation [27], [28], [29].
Today research is focused on the cultivation of microalgae rich in lipids in order to produce biodiesel [6], [9], [30]. Although cyanobacteria are not rich in lipids (up to 20%), they have relatively high biomass productivity. Microalgae with high biomass productivity may generate energy more efficiently by means of other types of energy conversion besides biodiesel production technology [10], [11], [14], [31]. In addition, the majority of the existing techniques for lipid extraction, in order to produce biodiesel, require the drying of slurry harvested from the algal biomass [7], [14], [25]. The drying of the biomass is an energy consuming process and must be taken into consideration [32]. Therefore, using wet concentrated biomass may be a more cost-effective way to produce biofuels. Biomass energy conversion technologies in which wet algal biomass could be applied include anaerobic fermentation [11], anaerobic digestion [33] and thermochemical liquefaction [34].
Moreover, the composition of algal biomass in lipids, proteins and carbohydrates can be affected and consequently manipulated by various cultivation factors. Each biomass energy conversion technique is suitable to a specific type of biomass composition. For instance, anaerobic fermentation technology requires carbohydrates, which are fermented into alcohol [11]. Thus for this technology biomass with a high carbohydrate content is desirable. On the other hand, in anaerobic digestion the substrate must have a carbon to nitrogen ratio (C:N) suitable for the digestion process [33]. Therefore, in respect to biomass energy conversion, the biomass composition would be manipulated in order to produce the most suitable composition for each of the energy conversion technologies.
This paper aims to present basic knowledge for the culture of cyanobacteria, to review the factors that affect biomass composition and to give useful references for further research into this topic. This review concerning the cultivation of filamentous cyanobacteria focuses on two areas: the production of biomass using agro-industrial wastes and wastewaters as substrate and on the reduction of the organic and inorganic load of these agro-industrial wastes and wastewaters.
Section snippets
Photosynthesis and carbon metabolism
Photosynthesis is a process conducted by photoautotrophs in which inorganic compounds and light energy are converted to organic matter. Microalgae are oxygen producing photosynthetic organisms, which means that they use light energy to extract protons and electrons from water (H2O) to reduce CO2 in order to form organic molecules (glucose). The organic matter is formed according to the stoichiometric formula:
The photosynthesis process can be divided into two
Cyanobacteria genera
Cyanobacteria (or cyanophyceae) are non-motile, planktonic, occasionally forming blooms and belong to the kingdom of eubacteria and to the division of cyanophyta. They are also gram-negative and are common in some extreme environments. Cyanobacteria are a large and morphologically diverse group [3], [54], [55], which can thrive in all kinds of waters with some species thriving in freshwater while others thrive in brackish water or the marine environment. The habitats and the ecological
Cultivation factors
In order for a culture to be successful, various environmental and operational factors, which affect the biology and habitats of the organisms, must be taken into account. These factors also affect cyanobacterial biomass productivity as well as biomass composition. The most important factors are: nutrients, pH and alkalinity, light and cultural cell density, temperature, and contamination by other microorganisms.
Agro-industrial wastes and wastewaters
The agro-industrial sector generates considerable amounts of W&WW, most of which are rich in organic or/and inorganic pollutants. The intensification of agro-industrial production and the disposal to land of the W&WW generated have raised a number of environmental issues, including eutrophication, surface and ground water pollution, odour pollution, gas emissions etc. High concentrations of livestock in small areas generate enormous quantities of waste, which are insufficient for land disposal
Conclusions
Cyanobacteria can make a significant contribution to the treatment of agro-industrial W&WW, reducing considerably their inorganic and organic pollutants. The produced cyanobacterial biomass, which may be rich in carbohydrates and/or proteins can possibly contribute to the production of biofuels besides biodiesel. The manipulation of the composition of cyanobacterial in order to suit a specific biomass energy conversion is feasible by controlling the cultivation conditions. However, a serious
Acknowledgments
G. Markou worked on this study as part of his Ph.D. in process, which is partially funded founded by Greek State Scholarships Foundation (I.K.Y.). We thank Dr. Despo Kritsotaki for her help with English. The text was finally edited by Jonathan Smith, CORDIS Science Editor. This work is dedicated to newborn Sophia.
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