Microalgae — A Natural Answer To Purifying Wastewater
By Dr. Rachel Whitton
Algae is a major problem for many wastewater treatment plants and water managers. It may also be a solution.
Cranfield Water Science Institute at England’s Cranfield University conducts research and teaching on the science, engineering, and management of water in municipal, industrial, and natural environments, including wastewater treatment. Traditional methods of wastewater treatment can be energy-intensive, often requiring the use of chemicals, and many researchers working in the sector are currently looking closely at sustainable, chemical-free alternatives, including microalgae. One such project is AquaNES, a European collaboration across four different sites that aims to accelerate innovation in wastewater treatment by combining natural treatments with engineered processes. While other groups in the consortium are exploring the use of alternative solutions — such as natural wetlands — for wastewater purification, the team at Cranfield is looking closely at microalgae, evaluating its potential as a sustainable treatment option and analyzing its overall contribution to a circular economy.
Harnessing Natural Advantage
Using microalgae in this context takes advantage of the natural ability of algae to efficiently consume nutrients such as nitrogen and phosphate from the surrounding environment during their growth. These nutrients are also found within wastewater effluent and their removal is necessary to prevent the deterioration of the quality of natural waters when the treated effluent is discharged. In theory, once water treatment is complete, the algal biomass can be recovered and used for other purposes, such as energy generation following anaerobic digestion, or soil enhancers. However, in practice, removing the algae from the system is a challenge that many researchers are trying to overcome; algal cells naturally repel one another so remain in suspension. One option is to add chemicals to the treated wastewater that overcome the repulsion and allow the algae cells to clump together and settle. Another uses the application of air, which causes the algae to float so that they can then be skimmed from the surface and removed. The team at Cranfield University is now working on something slightly different — an approach that encapsulates the microalgae biomass into beads made from a natural resin, sodium alginate. These beads can be fed into the wastewater treatment process and, once treatment is complete, the system can be switched off, allowing them to naturally settle and be recovered without the addition of further chemicals.
Optimizing Conditions With Photobioreactors
The microalgae strain being studied for the bead technology is already present in wastewater, and this eliminates the risk of the aquatic ecosystem being disturbed by the introduction of foreign species. Optimizing the growth of this algae is vital to the overall efficiency of the system — the more microalgae that is produced through growth, the more nutrients can be removed from the wastewater. When the project started, it was difficult to replicate the external environmental conditions inside a lab. The team, based in England, could not rely on natural light alone for algal growth and instead used artificial lights and a flask on the benchtop, attempting to change the light profile and intensity simply by moving the flask nearer or farther away from the lamps. This, however, did not help with controlling temperature. Discovering Algenuity, a local company manufacturing specialist laboratory photobioreactors that allowed flexible operational parameters, including lighting and temperature, was a real breakthrough. Using the company’s Algem system, they are now able to replicate various environmental conditions — literally set by entering the geographical coordinates of wastewater treatment works all around the world — and can examine how they affect the biomass yield and improve reproducibility. Even the difference between conditions in June and December can be set at the click of a button, giving a true reflection of real-life conditions.
The current process under review feeds wastewater effluent into the bioreactor, where it is treated by the microalgae beads, and the purified water is collected. The wastewater is then tested to see how the algae are performing at removing nitrogen and phosphorus, and to study and select the best lighting and temperature conditions for optimal treatment. In addition to the bead technology, further experiments examining the growth of the suspended algae can be tracked by the optical density function, shining a light through the sample and measuring light absorption, which is then correlated to an increase in biomass. The reactors have changed the institute’s approach to growth optimization, opening up many new options for experiments.
Walkaway Convenience And Energy Conservation
The walkaway automation capabilities of the reactors allow complete experiments to be programmed in advance, improving time management and enabling researchers to perform other tasks. Experiments can run for months at a time virtually unattended, which has completely changed the way the research group works. The reactors also house two flasks, which allows two experiments to be completed at once, making it much quicker and easier to get data and guiding the course of the research. As a bonus, the reactors support research on how the amount of energy and the costs associated for the artificial lighting can be reduced. Specific light wavelengths can be selected and tested to see how they affect algal growth, potentially reducing the amount of energy that is required. There is also a flashing light function that creates photoperiods, which can significantly reduce lighting energy usage.
Scaling Up For A More Sustainable Future
The next stage of the project is already underway; the team at Cranfield University is working with a U.K. water authority to evaluate the performance of this approach at a wastewater treatment works to see if microalgae can really make a difference on a large scale. It’s a collaborative effort working toward a more sustainable future and the institute is excited to be part of it.
To find out more about Rachel’s work at Cranfield, visit www.cranfield.ac.uk/people/dr-rachel-whitton-531715. To find out more about Algenuity, visit https://www.algenuity.com.
About The Author
Dr. Rachel Whitton, research fellow in applied biology at Cranfield Water Science Institute, has a background in microalgae for wastewater treatment, specializing in the innovative solution of immobilization. Her work involves cultivating and characterizing microalgae for nutrient remediation. Outputs from her research have resulted in a large-scale demonstration plant — operated by Severn Trent Water — and inclusion of the technology in the AquaNES EU project, which demonstrated synergies in combined natural and engineered systems.