Microalgae Harvesting Methods for Industrial Production of Biodiesel: Critical Review and Comparative Analysis

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Microalgae biomass can be used to produce numerous value added products such as biodiesel, bioethanol, biogas and bio hydrogen, fish feed, animal feed, human food supplements and skin care products. Production of value added products from microalgae biomass requires growing and recovery of the algae biomass and extraction and downstream processing of the desired product. However, the major obstacle for using microalgae biomass on an industrial-scale for the production of biodiesel and other value added products is the dewatering step which accounts for 20-30% of the total costs associated with microalgae production and processing. The aim of this study was to review the current methods used for harvesting and concentrating microalgae and to perform a comparative analysis in order to determine the most efficient and economically viable dewatering methods for large scale processing of microalgae biomass. The harvesting techniques investigated included sedimentation, vacuum filtration, pressure filtration, cross flow filtration, disc stack centrifugation, decanter centrifuge, dispersed air floatation, dissolved air flotation, fluidic oscillation, inorganic flocculation, organic flocculation, auto-flocculation, bio-flocculation electrolytic coagulation, electrolytic flocculation and electrolytic floatation. Eight criteria were used for evaluation of these microalgae harvesting techniques: (a) dewatering efficiency (b) cost (c) toxicity (d) suitability for industrial scale (e) time (f) species specificity (g) reusability of media and (h) maintenance. Each criterion was assigned a score between 7 and 15 based on its degree of importance. Higher values were given to the criteria that were deemed most important for development of an efficient and economic large scale dewatering method for microalgae whereas lower values were given to criteria that were deemed necessary for determining a suitable method but were considered less important. The results indicated that of the 16 methods evaluated, 4 scored values of 80/100 and above and were deemed suitable for harvesting microalgae on an industrial scale. Three were physical techniques (disc stack centrifuge (87/100), cross flow filtration (84/100), decanter centrifugation (82/100)) and the forth was the organic flocculation (80) method. These techniques were deemed suitable for large scale use because of their effectiveness, low operational costs, suitability for numerous species, rapidness, minimal maintenance requirement and being environmentally friendly. The other methods were deemed unsuitable because they are not effective in dewatering a wide array of microalgae species, not suited for large volumes, costly and require high maintenance. Although each of the optimum techniques was deemed suitable for harvesting of microalgae on its merit, a combination of methods can also be used to enhance the recovery efficiency and improve the economics. The use of organic flocculation as an initial harvesting step to concentrate the algae suspension and the centrifugation (or filtration) as a secondary dewatering step will reduce the time and costs associated with dewatering. Flocculation allows for effective removal of algae from large amounts of liquid media and as such the costs associated with energy intensive centrifugation and filtration techniques (used individually) can be reduced by using them as secondary techniques since less volumes of microalgae suspension will undergo the secondary treatment.