Algenol (Florida, USA) has been continually refining a process for continuous growth and harvest of ethanol excreted by modified cyanobacteria. In this process, ethanol is released by the organism in the vapour phase and then captured for extraction using a novel distillation process, eventually the spent microalgae bio­mass is converted to fuel using a variant of the HTL process. Algenol claims the following figures for their pilot plant (http://www. algenol. com/):

• Yield: 8000 gallons per acre of total liquid fuel production (80,000 L/ha) of which 85 % is ethanol and the remaining 15 % is hydrocarbons

• Cost: $1.27 per gallon

Two other companies pursuing milking-based projects are Joule unlimited and Proterro who are focused on chemicals/fuels and sugars as feedstock to traditional biofuel processes, respectively. In all of these approaches, the process is funda­mentally different as the milking process extracts the oils, ethanols or other chemicals of interest from the growth medium without killing the microalgae. As a comparison, the traditional microalgae production systems ‘kill’ the ‘cow’ (microalgae) to extract the ‘milk’ (oil) rather than keeping the cow (microalgae) productive and continually harvesting the milk (oil) (Moheimani et al. 2013a). Milking addresses the shortfalls of the existing production systems in two major ways:

• Nutrients—Only the products of interest are removed (which typically contain very low N and P), and as a result, there is a limited requirement for fertilisers. Only water, CO2 and sunlight are required to continually produce the compounds

• Dewatering—Microalgae are typically not removed from the culture to be milked to limit the need for dewatering (Moheimani et al. 2013b).

These systems are currently at various stages of early development. Despite this, the potential of these novel approaches to address the major issues with traditional methods warrants their continued investigation.