by Dr Morley Muse

This piece appears in the May 2023 edition of Science Victoria magazine. All issues can be read online for free at rsv.org.au/Science-Victoria.

Microalgae have been gaining attention as a sustainable, less energy-intensive method for wastewater treatment.

They can remove contaminants without the need for oxygen, making them a reduced energy treatment option compared to traditional systems that require aeration.¹ Additionally, following treatment the recovered microalgae can be used for the production of biogas and biodiesel.² This provides an effective means of mitigating current negative environmental impacts of wastewater discharge.

Treating wastewater with microalgae involves growing them in the effluent, where they consume compounds containing nitrogen and phosphorus, as well as heavy metals, pesticides, and particular toxins (depending on the species). Usually considered as waste, the microalgae view them as valuable nutrients, and effectively remove these pollutants from the water.

Once the microalgae have reached their maximum growth potential, they can be harvested and processed to extract valuable resources such as lipids, which can be used to produce biodiesel. In addition to biodiesel, microalgae waste can also be used to generate biogas via anaerobic digestion. Anaerobic digestion involves the breakdown of organic matter in the absence of oxygen, resulting in the production of ‘biogas’, which is made up of methane (CH4) and carbon dioxide (CO2). The terms ‘biogas’ and ‘biodiesel’ refer to products that have the same chemical components of natural gas and diesel respectively, but are produced from a ‘renewable’ source.

The biogas can then be refined to produce ‘biomethane’ (i.e., methane from a renewable source), and CO2. The biomethane can then be used as a renewable energy source for electricity and heat production, while the CO2 can be re-bubbled as a carbon source, making the entire process carbon neutral.³

In addition to biogas, anaerobic digestion results in the production of other intermediate products, such as biohydrogen and volatile fatty acids (VFA).^{4, 5} VFAs can then be utilised across industries including energy, food, plastics, and pharmaceuticals, while biohydrogen can be used as an alternative fuel source. As biohydrogen can also be obtained from biogas, it means that it can be extracted from multiple points in the process of treating wastewater with microalgae.⁶

Conventional wastewater treatment methods, such as activated sludge treatment, require significant energy inputs and can be expensive to operate and maintain. In contrast, the use of microalgae-based wastewater treatment systems can reduce energy consumption and operating costs while also producing valuable resources.

Furthermore, the use of microalgae for wastewater treatment can help address the issue of nutrient pollution in waterways. Nutrient pollution from sources such as agricultural runoff and wastewater discharge can lead to harmful algal blooms (excessive growth of toxin-producing algae), which can have negative impacts on aquatic ecosystems and human health. By making use of microalgae’s ability to remove nutrients from wastewater, the risk of nutrient pollution in waterways can be reduced.

Despite its promise, one of the major challenges of using microalgae for energy production is the fact that they are quite sturdy: when extracting lipids for biodiesel production, the cell wall of most microalgae resists digestion by microbes, and is therefore more energy-intensive to break down (Muse, 2021).

To resolve this, a pre-treatment process is required to disrupt the cell wall structure and improve degradation. Biological pre-treatment (using enzymes and bacteria) is the most energy-efficient option and can offer several benefits for microalgae breakdown during anaerobic digestion or lipid extraction for biodiesel. When compared to chemical pre-treatment, the benefits include lower cost, and greater yield of methane and lipids.

In summary, extracting microalgae waste as a form of wastewater treatment offers a sustainable and cost-effective solution for mitigating the environmental impacts of wastewater discharge. The production of biogas, biohydrogen, VFAs, and biodiesel from microalgae waste provides opportunities for renewable energy generation and can help address issues such as nutrient pollution in waterways. The use of microalgae waste for wastewater treatment and energy production offers a range of benefits and opportunities for a more sustainable and resilient future.

Dr Morley Muse is a Chemical, Environmental and Renewable Energy Engineer with expertise in wastewater treatment. She is Director and Co-founder of iSTEM Consulting Pty Ltd, and a Board Director of Women in STEMM Australia.

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References:

1. Mohsenpour, S. F., et al. (2021). Integrating micro-algae into wastewater treatment: A review. Science of the Total Environment, 752, 142168. doi.org/10.1016/j.scitotenv.2020.142168
2. González-González, L. M., Correa, D. F., Ryan, S., Jensen, P. D., Pratt, S., & Schenk, P. M. (2018). Integrated biodiesel and biogas production from microalgae: Towards a sustainable closed loop through nutrient recycling. Renewable and Sustainable Energy Reviews, 82, 1137–1148. doi.org/10.1016/j.rser.2017.09.091
3. Biomass and Biofuels from Microalgae. (2015). In N. R. Moheimani, M. P. McHenry, K. de Boer, & P. A. Bahri, Biofuel and Biorefinery Technologies. Springer International Publishing. doi.org/10.1007/978-3-319-16640-7
4. Muse, M. (2021) Characterisation of Chlorella vulgaris cell wall breakdown to improve Anaerobic Hydrolysis. PhD thesis, Victoria University. vuir.vu.edu.au/42502/1/MUSE_Morley-thesis.pdf
5. Harirchi, S., et al. (2022). Microbiological insights into anaerobic digestion for biogas, hydrogen or volatile fatty acids (VFAs): a review. Bioengineered, 13(3), 6521–6557. doi.org/10.1080/21655979.2022.2035986
6. Wang, K., Khoo, K. S., Chew, K. W., Selvarajoo, A., Chen, W.-H., Chang, J.-S., & Show, P. L. (2021). Microalgae: The Future Supply House of Biohydrogen and Biogas. Frontiers in Energy Research, 9. doi.org/10.3389/fenrg.2021.660399