Navid Moheimani

Algal cultivation for treating agricultural waste offers distinct advantages over conventional treatment options. These include the capacity for carbon dioxide capture and the potential reuse of nutrients through the production of valuable algal biomass. Piggery wastewater (WW) is of particular interest due to its characteristically extreme pollutant potential, typically exceeding concentration levels of other agricultural effluents, and a rich history of experimental work going back to the 1970s. The associated challenges and potential for success in this field also have important ramifications for the processing of many other agricultural, industrial, and commercial WWs. This review takes into account research involving raw effluent, the development of anaerobic digestate (AD) as the preferred first‐stage treatment approach, current challenges such as water conservation via minimizing dilution while coping with excessively high ammonia concentrations, and the potential for nutrient limitations such as phosphorous. Other aspects of algal cultivation such as the choice of photobioreactor and some of the relevant economic factors are also discussed.

The global seaweed market is currently valued at USD $11 billion annually and utilizes about 29 million tonnes of seaweed (dry weight) for a variety of applications (Ferdouse et al., Globefish Res Programme 124:I, 2018). By 2025 the global market is estimated to reach USD $30.2 billion dollars (Bloomberg, 2020). The current Australian seaweed market is valued at US$3 million, which is relatively small, contributing only 0.03% to the global market. However, the Blue Economy CRC and Marine Bioproducts CRC together have resulted in a public/private funding of nearly AUD $600 million over the past 3 years with the aim of expanding towards a billion-dollar seaweed market in Australia within the next decade. Furthermore, a recent CSIRO Data 61 report identified ’Microalgal and Macroalgal Resources’ as an emerging growth sector, specifically in the state of Queensland (Naughtin et al., 2021). Currently, most of the seaweeds on the Australian market are imported, and this sector has seen a rapid expansion from 2005 to 2017. This growth highlights the increasing demand for seaweed products in Australia and potential for the expansion of domestic producers. International seaweed markets are also expanding rapidly at a rate of 12% and so offer significant export opportunities (IMARC, 2021). Despite the huge domestic biodiversity of seaweeds and their use by First Nations peoples, the commercialisation of native seaweed products has been limited. To date, seaweed cultivation and research has been broadly managed by the Department of Aquaculture and Marine Sciences, and more recently this has expanded to specific organisations (e.g. CSIRO, AgriFutures, ASI) and an increasing number of universities (e.g. the combined marine cooperative research centers (CRCs) have been approved for a total of AUD $580 million development over the next 10 years for marine bioproduct development and the blue economy with a consortium of ~100 industry, government and research partners). These collaborative networks with seaweed production companies provide evidence of a growing interest and demand for seaweed-based products in Australia and internationally. The two major seaweeds cultivated in Australia today comprise Durvillaea potatorum and Undaria pinnatifida. D. potatorum (commonly known as Bull Kelp) is a native brown seaweed species, which is collected primarily on King Island (Tasmania). Sustainable harvesting both generates an income of about AUD $2.63 million annually and helps preserve kelp forest ecosystems, motivating further research and sustainable cultivation. Both species are widely used to produce thickening agents for the food and cosmetics industry. In Queensland, New South Wales and Victoria there are currently a number of promising seaweed-based research projects developing seaweed-based products ranging from 3D bio-printing technology, livestock feed (methane emissions mitigation) and optimisation of seaweed mariculture systems. This chapter reviews current trends and future perspectives of seaweed-based products in the Australian market, highlighting the limitation of import and export restrictions and the importance of expanding investments into research on native strains. A growing domestic Australian seaweed industry not only offers to create regional jobs and reduce the cost of imports but has significant opportunities to become an export industry, extending value chains in the near future.

Whilst aquaponics may be considered in the mid-stage of development, there are a number of allied, novel methods of food production that are aligning alongside aquaponics and also which can be merged with aquaponics to deliver food efficiently and productively. These technologies include algaeponics, aeroponics, aeroaquaponics, maraponics, haloponics, biofloc technology and vertical aquaponics. Although some of these systems have undergone many years of trials and research, in most cases, much more scientific research is required to understand intrinsic processes within the systems, efficiency, design aspects, etc., apart from the capacity, capabilities and benefits of conjoining these systems with aquaponics.

Models are useful and cost effective tools for understanding the behaviour of a system. As microalgae are potential raw material sources for biofuel production, the need for a rigorous model to predict their growth performance in various conditions is critical. Despite good agreements between models and indoor microalgal experiments, when it comes to the real life outdoor cultivation of microalgae most of these models have not been validated and hence cannot be trusted. Among all the conditions affecting microalgal growth, light is the most significant one and its role in outdoor culture becomes more important as the light intensity varies significantly during the day. In this study a new microalgal growth model is derived based on the energy balance of an outdoor raceway pond, to predict the growth rate of Tetraselmis in terms of cell density as a function of daily light exposure. Two sets of data are used in this study, one for training the model and estimating its parameters and the other to test the model and validate its performance. The results indicate that despite a few cases of inaccuracy, in most cases the model is able to accurately predict culture cell density.

Chemical energy can be produced from solar energy via photosynthesis. Solar energy can also be converted into electricity via photovoltaic devices. These two mechanisms would seem to compete for the same resources. However, due to differences in the spectral requirements, there is an opportunity to coproduce both electricity and chemical energy from a single facility. We propose to introduce an active filter or solar panel above a microalgae pond to generate both electricity and chemical energy. There are several advantages to such technology including reduced heating (saving freshwater) and an independent electricity supply. Additionally, by channeling targeted illumination back into the microalgae ponds, we can double the amount of light absorbed by the microalgae. This can result in increased biomass productivity.

Various microalgae species have shown a differential ability to bioremediate atmospheric CO2 . This chapter reports biomass concentration , biomass productivity , and CO2 fixation rates of several microalgae and cyanobacteria species under different CO2 concentrations and culture conditions. Research indicates that microalgal species of Scenedesmuss obliquss , Duniella tertiolecta , Chlorella vulgaris , Phormidium sp. , Amicroscopica negeli , and Chlorococcum littorale are able to bioremediate CO2 more effectively than other species. Furthermore, coccolithophorid microalgae such as Chrysotila carterae were also found to effectively bioremediate CO2 into organic biomass and generate inorganic CaCO3 as additional means of removing atmospheric CO2 . Important factors to increase the rate of CO2 bioremediation such as initial cell concentration , input CO2 concentration , and aeration rate are reviewed and discussed.

Industrial-scale microalgae production will likely require large energy-intensive 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 double-layer 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.

The current prices of microalgae oils are much higher than oils from higher plants (vegetable oils) mainly due to the high cost of photoautotrophic cultivation of microalgae. However, many strains of microalgae can also grow and produce oil using organic carbons, as the carbon source under dark (heterotrophy) or light conditions (mixotrophy). Lipid productivities of most strains of microalgae are higher in culture systems that incorporate heterotrophic metabolisms (presence of organic carbon source) than under photoautotrophic conditions. This is because for many strains, cell growth rates and final cell concentrations are higher in heterotrophic cultures than in photoautotrophic cultures. Furthermore, in some cases, the oil contents of the cells are also higher in cultures incorporating heterotrophic metabolisms. It has also been reported for some strains that the quality of oil produced in the presence of organic carbon sources are more suitable for biodiesel oil production than those produced under photoautotrophic conditions. Thus, heterotrophy can be used to reduce the cost of biodiesel oil production, but the effectiveness of the various organic carbons in supporting cell growth and oil accumulation depends on the strain and other culture conditions. Use of wastewaters for cultivation of microalgae can further substantially reduce the cost of production (since they contain carbon, nitrogen, and other nutrients) and also reduce the requirement for freshwater. Generally, many factors such as nitrogen limitation, phosphate limitation, silicon limitation, control of pH, and low temperature can be used to increase oil accumulation, although their effectiveness depends on the strain and other culture conditions.

Microalgae cultivation is a promising methodology for solving some of the future problems of biomass production (i.e. renewable food, feed and bioenergy production). There is no doubt that in conjunction with conventional growth systems, novel technologies must be developed in order to produce the large-scale sustainable microalgae products. Here, we review some of the most promising existing microalgae biomass growth technologies and summarise some of the novel methodologies for sustainable microalgae production.