Patrick V. Brady, Mark P. McHenry, M. Carolina Cuello and Navid R. Moheimani

Abstract Industrial-scale microalgae production will likely require large energy­intensive technologies for both culture and biomass recovery; energy-efficient and cost-effective microalgae dewatering and water management are major challenges. Primary dewatering is typically achieved through flocculation followed by sepa­ration via settling or flotation. Flocculants are relatively expensive, and their presence can limit the reuse of de-oiled flocculated microalgae. Natural flocculation of microalgae—autoflocculation—occurs in response to changes in pH and water hardness and, if controlled, might lead to less-expensive “flocculant-free” dewa­tering. A better understanding of autoflocculation should also prompt higher yields by preventing unwanted autoflocculation. Autoflocculation is driven by double­layer coordination between microalgae, Ca+2 and Mg+2, and/or mineral surface precipitates of calcite, Mg(OH)2, and hydroxyapatite that form primarily at pH > 8. Combining surface complexation models that describe the interface of microalgae: water, calcite:water, Mg(OH)2:water, and hydroxyapatite:water allows optimal autoflocculation conditions—for example pH, Mg, Ca, and P levels—to be iden­tified for a given culture medium.

P. V. Brady (H)

Geoscience Research and Applications Group, Sandia National Laboratories,

Albuquerque, USA

e-mail: pvbrady@sandia. gov

M. P. McHenry

School of Engineering and Information Technology, Murdoch University,

Perth, Australia

M. Carolina Cuello • N. R. Moheimani

Algae R&D Centre, School of Veterinary and Life Sciences, Murdoch University, Perth, Australia

© Springer International Publishing Switzerland 2015

N. R. Moheimani et al. (eds.), Biomass and Biofuels from Microalgae,

Biofuel and Biorefinery Technologies 2, DOI 10.1007/978-3-319-16640-7_13

13.1 Introduction

Microalgae achieve a much higher biomass productivity when compared to con­ventional terrestrial biofuel crops (Fon Sing et al. 2011), although several issues that must be resolved include developing cost-effective dewatering and processing technologies, and the associated commercial and environmental challenges (Grif­fiths and Harrison 2009; Moheimani et al. 2011; Vasudevan and Briggs 2008). Cost-effective and energy-efficient dewatering of microalgae, nutrient recycling, and control of effluent wastewater are becoming major challenges to microalgae producers (Borowitzka and Moheimani 2010; Charcosset 2009; Clarens et al. 2010; Wyman and Goodman 1993; Xiong et al. 2008). Open pond microalgae production can become expensive due to variable capital, operational, and downstream pro­cessing costs derived from the low microalgae cell densities (Lee 2001; McHenry

2010) , and if not optimised, industrial microalgae production will consume large volumes of water through evaporative loss (Chisti 2007; Clarens et al. 2010), generate effluent and become an energy-intensive process (Borowitzka and Mo- heimani 2010; Charcosset 2009; Clarens et al. 2010; McHenry 2013; Wyman and Goodman 1993; Xiong et al. 2008). It is generally not well understood that mic­roalgae production will require large energy-intensive technologies for both culture and biomass recovery (Chisti 2007), including heat exchangers, scrubbing, pres — surisation, pipeline construction, and pumping of flue gases (as a potential source of CO2) to be intensively managed (McHenry 2010, 2013), and microalgae production sites must be carefully chosen to optimise industrial resources, natural resources, and environmental conditions to facilitate post-harvest processing (Borowitzka 1992).