Thanks to the threats posed to human health and the environment, and the economic costs of its treatment, wastewater from domestic, industrial and agricultural processes has been considered a serious problem for a long time. However, a paradigm shift in the past few years has led to a new concept of wastewater, not as a problem but as a source of energy and other precious resources, including water itself.

Wastewater is increasingly seen as one of the solutions to many of the big sustainability and climate change issues we are facing. Researchers around the world are developing technologies that make products recovered from wastewater both socially acceptable and economically viable. The mundane world of wastewater treatment has suddenly become the glamorous world of the cyclical, green economy.

One of the stars of this new paradigm is microalgal, attracting considerable attention because of its positive characteristics for biotechnology systems, such as wastewater treatment and biofuel production.

Why are microalgae-based technologies so attractive?

What really makes the use of algae a thriving technology is that these microorganisms have the potential to efficiently remove nutrients from wastewater, and provide a biomass energy source. Additionally, when combined with the conventional activated sludge systems, the algae-bacteria symbiosis can reduce the electrical energy demands from aeration, which can represent more than 50 percent of the total energy of wastewater treatment plants.

The positive interactions are clear when microalgae provide, through photosynthesis, the oxygen necessary for aerobic bacteria to biodegrade organic pollutants, consuming in turn the carbon dioxide released from the bacterial activity.

Biomass produced in photobioreactors can be used for several purposes, including biogas substrate, biofuels, fertilizers and biopolymers, which can be converted into packaging materials, and have the advantage of being renewable. While the application of by-products derived from algae are technically feasible, their economic feasibility is still under discussion.

Positive aspects and drawbacks

 The great advantage of this biotechnology is that most of the algae species proven to be effective are easily available, especially in countries that lack infrastructure for wastewater treatment. As photosynthesis is a key process for microalgae cultivation, these systems are ideal for regions with high temperatures and sunlight exposure.

The use of a microalgae-bacteria symbiosis has been proved to provide good quality treated water by removing organic matter, nutrients (including nitrogen and phosphorus, the main causes of eutrophication in water bodies), and some hazardous contaminants and pathogens. The composition of harvested biomass is directly related to the quality of valuable products obtainable from it.

The main challenges for the application of microalgae in wastewater treatment are the harvesting of the algae, due to the settling characteristics and operational conditions; the control of biomass composition is complicated by the selection of the desired species; while the definition of an optimal ratio of algae and bacteria biomass, micro-pollutants removal, and the possible need for external CO₂ present additional obstacles.

Although there are several studies on this topic, further clarifications are needed to prove the feasibility of microalgae-based systems in full scale. What are the perspectives for the next years? Is microalgae application in wastewater treatment a promising alternative for our current scenario?

Given global population growth, increasing consumption and scarce natural resources, we need to develop effective resource recovery technologies in order to promote sustainable progress. Algae-based technologies are great options for treating wastewater and generating useful products with low cost and high efficiency. Further research should be encouraged in order to expand the application of these alternatives worldwide and enhance current systems.

In this way, these novel technologies will contribute to a better understanding of the water-energy nexus in a sustainable way, and while inspire the development of innovative solutions in the future.

The Workshop on Microalgal Based Wastewater Treatment Facilities at the 13^th IWA Leading Edge Technology (LET) Conference on Water and Wastewater Technologies was a step in the right direction, as it provided experts from different countries the opportunity to share their findings and set research priorities to resolve remaining challenges around microalgae’s applicability. This year, from the 27th to the 31st of May 2018, LET will document recent developments and applications on Microalgal Based Wastewater Treatment Facilities. Check it out: www.iwa-let.org