Commercial microalgae biofuel production is dependent on the unique microalgal species chosen, the management of biotic and abiotic conditions, production costs (energy, nutrients, water, land, chemicals, etc.), co-located industrial facilities, and downstream applications and markets (Borowitzka 1999; Kunjapur and Eldridge 2010; McHenry 2010, 2013). Because the concentration of microalgae in open pond systems is so low [usually between 0.5 and 2 g/L (Fon Sing et al. 2011)], dewatering and harvesting/processing will heavily influence the evolution of upstream microalgae production, strain selection, water/nutrient recycling tech­nology, and other processes (Moheimani et al. 2013). Primary dewatering is typi­cally achieved through flocculation, followed by separation from the water via settling or floatation (Moheimani et al. 2013). Some microalgae naturally flocculate, while others respond to chemical flocculants or non-chemical methods (Brady et al. 2014; Moheimani et al. 2013; Sukenik and Shelef 1984). For example, Origin Oil and Diversified Technology have used pulsed electric fields to enhance flocculation and lyse microalgae (Moheimani et al. 2013). During post-primary dewatering, the remaining water in microalgae pastes is usually removed by heating, which if not using low grade or waste heat can also include a high-energy burden (McHenry 2013; Moheimani et al. 2013). Secondary dewatering or mechanical drying gen­erally incudes physical separation of the water and the microalgae using centrifuges or decanters that are often energy intensive and relatively inefficient (Moheimani and McHenry 2013; Moheimani et al. 2013; Solix Biofuels 2011b). However, this area of research is a fast-moving field with some companies including Evodos achieving mechanical centrifuge electricity consumption reductions when dewa­tering water and microalgae to a paste (Moheimani et al. 2013). Evodos centrifuges exhibit practically zero cell shearing, thermal damage, without chemicals and collecting minimal bacteria requiring only <1 kWh/m, and 5.5 kWh/kg DW (Evodos 2011). Nonetheless, for industrial-scale volumes of water, this energy consumption per unit of output remains prohibitively high.

There are numerous competing approaches focussed on reducing dewatering energy demands in microalgae production (McHenry 2013). For example, General Atomics, a long-time defence contracting company based in California, is evaluating Algaeventure Systems microalgae “harvesting, dewatering, and drying” (HDD) technology with the ability to reduce dewatering energy consumption considerably using unique conveyer centrifugal technology (Li 2012; Moheimani et al. 2013). Algaeventure Systems Inc. is a spin-off company from Univenture, Inc., which is a plastic packaging manufacturer, a large-potential high-value microalgae-based co­product potentially cross-subsidising biofuel production (Moheimani et al. 2013). Taking another unique method to dewater and process biofuel precursors is the company Algenol, which has developed a unique DIRECT TO ETHANOL® process using a 15-m-long and 1.5-m-wide semi-transparent polyethylene film outdoor photobioreactor containing treated sea water, microalgae, nutrients, with a volume of air above the water. Algenol uses a “hybrid” microalgae that reportedly produce ethanol intracellularly which diffuses through the cell wall into the culture medium and evaporates along with water into the internal air volume. The water/ethanol solution condenses on the inner surface of the photobioreactor and is collected, concentrated, and distilled (Moheimani and McHenry 2013). Algenol states that their DIRECT TO ETHANOL® process produces around equal amounts of freshwater and ethanol, with collaborators membrane technology research (MTR) using their Bio — Sep™ technology—a membrane distillation technology. Similarly, Solix’s from Colorado has developed a “LumianTM AGSTM” photobioreactor which is a series of water-filled metal tanks supporting semi-submerged transparent microalgae cultures circulated by weighted rollers, with independently controlled air and CO2 from a co­located coal-bed methane production facility (Moheimani and McHenry 2013; Solix Biofuels 2011a). The twenty 36-m-long 200-L bags sum to a total culture capacity of around 4 kL (Solix Biofuels 2011c). Solix’s Colorado Coyote demonstration facility can produce a maximum of 28 kL/ha/year of microalgal oil, with culture peak yield rates equivalent to around 19 kL of oil/ha/year, or around 5 g of oil per m2 per day (Moheimani and McHenry 2013). However, Solix is primarily a culturing technology provider and has not focussed on downstream energy and dewatering technology.

Another unique production approach is underway in New Zealand, where a private company, Aquaflow, is specialising in harvesting wild microalgae from municipal sewage ponds and high-nutrient waters. Aquaflow solely relies on water remediation and equipment sales rather than microalgae as their business model. In 2008, Aquaflow partnered with Honeywell UOP to use UOP/Eni Ecofining™ and the Canadian company Ensyn’s rapid thermal processing (RTP™) fast biomass pyrolysis (*75 % bio-oil output by volume) to provide liquids for Honeywell’s Green Die­sel™ and Honeywell Green Jet™ fuel production (Ensyn 2011), using hydropro­cessing to produce catalysts and thermal energy for output separation (Moheimani et al. 2013; Regalbuto 2011). Aquaflow currently operate with around 60 ha of open mixed sewage and municipal and agro-industrial waste oxidation ponds (serving a population of 27,000 and water flows of 5 GL/year.) The ponds are continuously harvested using systems built inside a 40 ft sea container using dissolved air flotation and polyelectrolyte flocculation (70-90 % recovery), followed by a belt press for extraction, processing wastewaters at a rate of 35 m3/h to produce a wild microalgae liquid concentrate at 8-10 % microalgae by volume (Moheimani et al. 2013).

To avoid the associated high energy and material costs of dewatering, integrated microalgal biorefineries have received much attention in the literature (Chisti 2007; Wyman and Goodman 1993), and recent approaches have focussed on hydrothermal liquefaction of relatively low-lipid biomass from mixed microalgae species to pro­duce a crude oil replacement in the presence of water at medium temperatures and pressures (Biller and Ross 2011). de Boer et al. (2012) showed that in situ hydrolysis and esterification of wet biomass and hydrothermal liquefaction was the most ener­getically feasible process in a comparison of four of the most promising methods to convert microalgae into biodiesel, including pulsed electric field-assisted extraction, transesterification, and in situ acid catalysed esterification of dry biomass. However, the present fundamental diversity of approaches for microalgae dewatering and the associated energy efficiency options demonstrate that microalgae dewatering is far from a mature field on the verge of commercialisation. This research discusses an alternative approach that is a potential tool for achieving low-cost primary dewatering of microalgae for economic biofuel production using inexpensive “chemical-free” approach to autoflocculation that is appropriate to large-scale industrial production systems using existing bulk agricultural commodities.