The main advantages of microalgae growth compared to land plants are the ability to grow on arid land using saline water (Fig. 1.1). This means that microalgae cultures will not compete with food crops over agricultural land and freshwater. However, microalgae, the same as any other photosynthetic organism, would still require fertilisers (especially nitrogen and phosphorous) to grow. If grown in sea water, macronutrients are necessary to be added to the culture to achieve high growth rate. Borowitzka and Moheimani (2013a) indicated that for producing

100,0 bbl of algal oil year 1, there is a need for 14,447 and 219 tons of nitrogen (as NaNO3) and phosphorous (as NaH2PO4), respectively. Such a high volume of fertilisers will significantly affect the overall cost of production. Furthermore, phosphorous is a non-renewable resource, and at current rates of extraction, global commercial phosphate rock reserves may be depleted in less than 100 years (Cordell et al. 2009). That means that algae cultures, irrespective of their product, will be in direct competition with food crops over fertilisers. Obviously, one very important consideration in developing any potential large-scale algae production facility is the recycling of the medium (Fig. 1.1). Recycling medium especially post-extraction/conversion would allow the recycling of a large amount of fertilisers especially if the wet biomass is being converted to biodiesel and biomethane (Fig. 1.1). Furthermore, there is a possibility of combining microalgae cultivation with wastewater treatment. Combining microalgae cultures with wastewater treat­ment plants (domestic or animal waste) can provide microalgae with required nutrients and result in lower cost wastewater treatment than traditional approaches.

The potential of combining microalgae cultures and domestic wastewater treatment was first proposed in 1960s with the main interest to produce biofuel (Oswald and Golueke 1960). There are currently some facilities around the world (i. e. New Zealand, USA) using high rate algal ponds (HRAPS) for treating tertiary domestic wastewater. In general, microalgae growth in tertiary-level wastewater treatment can significantly reduce the electromechanical cost of treatment (Craggs et al. 2013). Another advantage of using microalgae in the domestic wastewater treatment process is more efficient nutrient removal and sunlight-driven disinfection (Davies-Colley et al. 2005). Animal waste (i. e. piggery waste) can also be treated using microalgae cultures. The environmental impacts of intensive pig production can be significant. A poorly managed piggery may risk wastewater pollution to local waterways, produce odour emissions and release greenhouse gases into the atmosphere (Maraseni and Maroulis 2008). Wastewater generated through high- intensity pig production is high in ammonia and phosphorous while also having high chemical and biological oxygen demands (Olguin et al. 2003). High phos­phorous levels have been shown to correlate to high turbidity levels giving the effluent a dark colour (Ong et al. 2006). One wastewater treatment system that is gaining acceptance in Australian piggeries is anaerobic digestion ponds. These systems typically consist of a covered pond containing wastewater which is bio­logically treated by heterotrophic microorganisms in the absence of oxygen. The covered digesters allow the production and capture of biogas including methane and carbon dioxide. The benefits obtained from these ponds are the removal of solids through settling, capture of biogas for use as a biofuel and the reduction of odour emissions. The utilisation of methane as a fuel source can effectively reduce dependence on energy sources from outside the piggery. One challenge is that the anaerobic digestion effluent from piggeries is very high in ammonium (toxic to most organisms). If a process incorporating CO2 uptake such as algae culture was to be adopted, ideally CO2 (generated via burning CH4 or separated from the raw biogas stream) will be captured and reused within the piggery. A recent review of wastewater management in Australian piggeries recommended that along with anaerobic digestion, microalgae culture systems should be investigated further as a potential component of the Australian piggery wastewater management strategy (Buchanan et al. 2013).

To date, all trials on culturing microalgae on undiluted and untreated anaerobic digestion piggery effluent (ADPE) have failed to gain widespread acceptance in the industry. On the other hand, there are reports of the successful microalgal culti­vation on piggery anaerobic digestate after dilution with freshwater (Park et al.

2010) . Interestingly, in some cases, the digestate was diluted more than 15 times with freshwater. In the context of an Australian piggery system, such a method would never be practical due to the shortage of freshwater. Ayre (2013) isolated three microalgae capable of growing on undiluted, sand-filtered, piggery anaerobic digestate. This proof-of-concept study clearly illustrated the potential for culturing microalgae in such effluent with a high ammonium content. The produced algae biomass on piggery anaerobic digestate will sequester carbon and remove nutrients (i. e. nitrogen and phosphorous). The produced biomass could alternatively be used as pig feed, although the biomass pathogen load would need to be closely moni­tored (Buchanan et al. 2013). Another potential application for the biomass is the co-anaerobic digestion with the piggery waste.