Perhaps the largest challenge regarding process modelling and cost estimation of microalgal products, is the uncertainty regarding downstream processing (Williams and Laurens, 2010).
Currently the largest cost in processing algal biomass is ‘dewatering’ (Wyatt et al., 2012) which can account for 30% of the production cost (Parmar et al., 2011). This is a consequence of the dilute and neutrally buoyant nature of microalgal cultures, which typically reach a cell density of between 103-108cells ml-1 (Pulz, 2001), or 0.1-4wt% (Wyatt et al., 2012). For each kg of raw algal biomass, approximately 1000kg of water must be processed (Parmar et al.,
31 2011). Producing concentrated biomass with current technology requires a large energy input, significantly adding to costs and upsetting the energy balance (Uduman et al., 2011).
Currently dewatering uses a few steps to concentrate cultures into a slurry or paste. Compared to other suspended particles, the variable characteristics of algal cells may require different treatments for different applications (Uduman et al., 2010). Characteristics of algae which impact treatment are: morphology, motility, surface charge, cell density, and extracellular organic matter (EOM) composition and concentration (Henderson et al., 2008). Major techniques such as centrifugation, flocculation, filtration, screening, gravity sedimentation, flotation and electrophoresis, may differ in suitability for a particular species or application (Table 1.7).
Table 1.7: A comparison of water removal and energy use for various microalgal dewatering techniques.
Modified from Uduman et al (2010).
Flocculation 200-800 Low (varies) High Expensive flocculants, may
need purification
Centrifugation 120 Very High (8.00) High High energy input
Gravity sedimentation
16 Low (0.10) Low Process is slow
Filtration 15-60 Low (0.40) Medium Filters need replacing
Pressure filtration 50-245 Low (0.88) High Filters need replacing Tangential flow
filtration
5-40 High (2.06) Med-High High energy, filters need replacing
Flocculation-floatation (DAF*)
n/a High (10-20) Med-High Electrodes need replacing
Electrocoagulation n/a Medium-High
(0.80-1.5)
High Electrodes need replacing Electrofloatation 300-600 Very High (n/a) High Electrodes need replacing Electrolytic
Centrifugation whilst efficient (80-90% recovered in 2-5min), is highly energy intensive and therefore not suitable for the production of a biofuel (Uduman et al., 2010). Filtration of larger or filamentous species of algae would be economical, requiring filters with larger pore sizes giving faster flow rates (Uduman et al., 2010). However filtration is often too expensive for small celled species (2-50µm diameter), due to the fine filters and regular backwashing required to prevent blockages (Wyatt et al., 2012). Tangential flow filtration involves recircling of retentate across a membrane, to keep cells in suspension and minimize fouling, useful for shear senstive algae (Uduman et al., 2010).
32 Flocculation (the aggregation of algal cells) is applicable to many species and large culture volumes (Uduman et al., 2010). The majority of algal cells are 5-50µm diameter with a stable electronegative surface charge between pH 2.5-11.5 (Uduman et al., 2010). Flocculants may be inorganic (ferric sulphate, alum, lime) or polymeric (i.e. Purifloc, Zetag 51, chitosan) and work by charge neutralization and particle ‘bridging’. Varying the charge, charge density and length of biopolymers make polymeric flocculants much more versatile. Nature of the flocculant, concentration, algal species and in particular culture pH, can have an effect on efficiency (Uduman et al., 2010). The addition of flocculants has great potential, however depending on the end use of the product may require subsequent separation of the flocculant from harvested biomass (Wyatt et al., 2012), in particular for biodiesel or neutraceuticals whereby toxic or expensive elements may interfere with further downstream processing or final product quality (Uduman et al., 2010). As a result there is great interest in the use of chitosan as a flocculant as it has low toxicity and is easily biodegradable (Cheng et al., 2007).
Autoflocculation of microalgal cultures is highly desirable, and often associated with an increased pH or excreted macromolecules (Park et al., 2011). Microalgae are capable of altering their cell wall composition in response to their environment (Cheng et al., 2007). A high pH can cause autoflocculation in C. vulgaris, with the presence of magnesium hydroxide appearing essential for initiation (Vandamme et al., 2012). A novel study by Lee et al. (2009) found that under stress, symbiotically cultured microbes produced an extracellular polymeric material which encouraged flocculation of Pleurochysis carterae. Another recent study has found that the co-culturing of Nannochloropsis oceanica with a bacterium causes efficient flocculation without affecting lipid content of the microalga (Wang et al., 2012). Stress can also induce flocculation in cultures of Scenedesmus vacuolatus (data not shown). Algae are capable of regulating their buoyancy according to various stimuli and have been shown to settle differently under light and dark conditions (Harith et al., 2009), such factors could have an effect on floc sedimentation. Alterations to the cell wall of microalgae can greatly affect the efficacy of flocculants and alterations in culture conditions can be employed to improve the efficiency of flocculation (Cheng et al., 2007).
Flotation is a form of physio-chemical separation, whereby small particles <500µm (e.g. algal cells) attach to the surface of microscopic gas bubbles generated in the suspension, (achieved by forcing though small nozzles) which ‘float’ to the surface of a suspension and can be skimmed off (Uduman et al., 2010). Dissolved air floatation (DAF) is commonly used in
33 industry and effluent treatment, when used with algae it is often combined with chemical flocculation (Uduman et al., 2010). Electro coagulation-flocculation (ECF) involves the dissolution of reactive anode electrode to form metal cations that neutralise negatively charged microalgal cells (Uduman et al., 2010). The process is strongly improved by high turbulence, high cell densities, decreased pH and choice of anode metal (i.e. aluminium (Al 3+) > iron (Fe 3+)) (Uduman et al., 2010). ECF is best suited to marine species as saltwater lowers power input required (Vandamme et al., 2011). Electrolytic floatation generates bubbles of H2 at the cathode, creating and carrying microalgal flocs to the surface (Uduman et al., 2010).
Cell size and cell wall structure present a significant energy barrier to product extraction (Razon and Tan, 2011), often requiring mechanical disruption to extract valuable biomolecules within (Parmar et al., 2011). Physical methods for rupturing cells include freeze thawing, grinding, pressing, beadbeating and homogenisation; and chemical methods include solvent extractions, supercritical fluid extraction and direct transesterification (Parmar et al., 2011). All techniques typically require dry biomass to increase efficiency (Dragone et al., 2010), which in the case of microalgal biomass comes with a high energy penalty. One way to reduce the energy input is to use methods suitable for use with wet biomass (Scott et al., 2010), yet osmotic shock is largely inefficient (Parmar et al., 2011) and ultrasound is expensive and energy intensive (Dragone et al., 2010). Enzymatic extractions although attractive are unlikely to ever become industrially viable due to cost, yet can give valuable clues as to the structure and composition of the cell wall (Parmar et al., 2011) (further discussed in Chapter 3). Extraction methods are further discussed in Chapter 7.