Review of Literature
2.7 Processing of microalgal biomass for biodiesel generation: Current harvesting and conversion technologies
2.7.1 Various dewatering methods used for algal harvesting
The high cost of dewatering is attributed to dilute nature of the broth with less biomass fraction (103 to 108 cells mL-1) and the negative charge of cells with excessive extracellular algal materials that keep them stable in a dispersed state (Danquah et al., 2009).
The major techniques available for harvesting and to concentrate the algal cells include gravity settling, centrifugation, flocculation, filtration, flotation, electrocoagulation, electrolysis and electrophoresis (Pragya et al., 2013). The harvesting method selected should be applicable for wide range of algal species and should utilize less energy and chemicals.
Table 2.6 Comparison of various dewatering techniques on the basis of their efficiencies in concentrating microalgae (table obtained and modified from Uduman et al., 2010)
Method Microalgae yield Water removal Energy utilized (kWh m-3) Gravity sedimentation 0.5-1.5% tss 16 Low (0.1)
Centrifugation 12% tss 120 High (8)
Flocculation 95% 200-800 Low
Filtration 1-6% tss 15-60 Low (0.4)
Pressure filtration 5-27% tss 50-245 Moderate (0.9) Tangential filtration 70-89% 5-40 High (2.06)
Flocculation-Flotation 90% N/A High (10-20)
Electroflotation 3-5% 300-600 Very high
Electrocoagulation 95% N/A Moderate (0.8-1.5)
Electrolysis >90% N/A Low (0.33)
tss – total suspended solids
Table 2.6 compares various dewatering techniques available based on their energy utilization and recovery efficiencies. Gravity sedimentation is the most energy efficient harvesting method that settles out the suspended cells using gravitational force, forming concentrated slurry at the bottom and clear liquid at top (Uduman et al., 2010). In general, all the algal cells may have tendency to settle down at the bottom through gravitational force
however, the rate of settling depends upon the species characteristics. Therefore, the efficiency is enhanced by using lamellar separators and sedimentation tanks for oleaginous algal systems. Addition of flocculants on the other hand increases the rate of sedimentation to a greater extent which is being commercially used (Pragya et al., 2013). Centrifugation is an alternate method that utilizes centripetal acceleration to separate the algal cells with removal efficiencies up to 90 % in very less time frame of 2 to 5 minutes (Pragya et al., 2013). However, the process is highly energy intensive and not suitable for large commercial scale biodiesel production (Uduman et al., 2010). Filtration is another process which retains the algal cells and allow the water to pass through the filters. Depending on the type of filters used and the flow pattern there are several filtration forms which include microfiltration, ultrafiltration, dead end filtration, vacuum filtration, tangential flow filtration and pressure filtration. It is economical to filter large and filamentous algal cells through simple vacuum filtration, while the filtration of small sized cells are too expensive as it requires complex filters with very small pore size and frequent backwashing and cleaning (Wyatt et al., 2012). Tangential and pressure filtration methods are considered to be the energy efficient methods that have ability to concentrate up to 90 % of microalgal cells with very minimal membrane fouling (Uduman et al., 2010).
Flocculation is a process that aggregates the algal cells to form flocs which is applicable to many species and large broth volumes (Uduman et al., 2010). Microalgal cells carry an electronegative charge on their surface which may vary between 2.5 to 11.5 and therefore, addition of flocculants neutralizes their surface charge and enables particle bridging resulting in cell aggregation (Pragya et al., 2013). Different flocculating agents are available that have differential influence on flocculation process which include inorganic (alum, ferric sulfate and lime) and polymeric forms (Purifloc, Zetag 51, Dow 21M, Dow C- 31, Chitosan) (Uduman et al., 2010). Polymeric form of flocculants include both ionic and
non-ionic molecules which works by forming electrostatic or chemical bonding forces and the efficiency depends up on their charge density and chain length of the polymer. Addition of iron or aluminium based coagulants causes the charge neutralization in the algal cells based upon their charge density. Another interesting method is the autoflocculation in which the algal cells spontaneously sediments to form flocs and is often associated with elevated pH or excretion of polymeric macromolecules (Park et al., 2011). Changing the environmental pH or low temperature conditions alters the cell wall composition of the algae thereby inducing aggregation of the cells. Addition of NaOH increases the pH to alkali side which induces many algal cells to aggregate themselves (Chen et al., 2011; Vandamme et al., 2012). Recent study on bioflocculation induced by the co-culturing of Nannochloropsis cells with bacterium is an energy efficient strategy for algal harvesting (Wang et al., 2012).
Flotation is an alternate phenomena in which the air bubbles generated carries the solid particles such as algal cells to the upper surface of a suspension which can be skimmed off (Uduman et al., 2010). This method has been found to more efficient than sedimentation for many microalgal systems (Pragya et al., 2013). Depending upon the bubble size, the flotation can be divided in to dissolved air flotation, dispersed flotation and electrolytic flotation (Chen et al., 2011). Dissolved air flotation is the commonly used along with chemical flocculation as the effectiveness of this method depends on the particulate size ie with larger particle size, higher efficiency is achieved. Dispersed air flotation method forms the bubbles of size 700-1500 µm which interacts with the negatively charged algal cells. By increasing the cationic charge on the bubbles increase the interaction and effectiveness of algal cell removal (Rawat et al., 2011). Ozone is used in the dispersed air flotation and harvesting Chlorella vulgaris using ozone flotation method resulted in an increase in lipid content also. However, use of ozone for flotation is not economically feasible for biodiesel production (Rawat et al., 2011). Electrolytic flotation method involves the use of a cathode
from which H2 ions are released that attracts the negatively charged algal cells and moves them to the surface.
Electrocoagulation and flocculation involves the dissolution of anode to form metal cations which interacts with the negatively charged algal and enables aggregation. The method is suitable for high cell densities and marine species as saltwater lowers power input required (Vandamme et al., 2011). Fouling of the cathode or anode is the major problem in using such electrode based methods for harvesting.