While a large part of the world is concerned about the effects of climate change due to global warming, and summits are held between governments and organizations to discuss the issue, it goes almost unnoticed that more than 80% of the wastewater generated is discharged into lakes and seas without receiving any treatment, and that in addition, millions of people in the world have difficulty accessing water and lack basic sanitation services. This is an alarming situation that should also be high on the agenda.
Direct disposal of wastewater. Source: Wikimedia Commons.
Perhaps we trust in the high capacity of water bodies to assimilate pollutants, but this is not sustainable. In this regard, different designs of wastewater treatment plants have been explored to help massify their use, since conventional systems are very expensive, require chemicals and are not very efficient. In this sense, the use of microalgae is bursting into the world of water sanitation as a hopeful biotechnological alternative, since in their research and development phase they are proving to be very efficient, only sunlight is needed to purify the water and at the same time they transform pollutants into valuable compounds and capture CO2.
How a wastewater treatment plant works
Conventional wastewater treatment systems consist of at least three main stages:
Primary treatment: this is a treatment stage where coarse solids and solid particles carried by the water as it travels through the sewer are removed from the water, physical and mechanical separation processes such as roughing, decanting, desanding and oil and grease separation are used.
Secondary treatment: is based on the use of biological methods for the elimination of organic matter. Aerobic processes (in the presence of oxygen) and anaerobic processes (in the absence of oxygen) are used to promote the activity of certain bacteria that feed on the biodegradable organic substances present in the wastewater. Some of the most common processes in this stage are activated sludge reactors, oxidation lagoons and biodiscs, among others.
Tertiary treatment: these are based on chemical methods whose objective is the elimination of pathogenic agents for human health or the environment, such as the removal of heavy metals and infectious agents. For this purpose, methods such as microfiltration, ozonation and disinfection by ultraviolet light, to name a few, are used.
Image of a conventional treatment plant. Source: Wikimedia commons.
However, although these processes allow us to have treated water of sufficient quality to be reused in some processes or to discharge it safely into the environment, their implementation is scarce because they consume a lot of energy, require periodic maintenance and generate a lot of waste. Therefore, efforts have been made to advance more sustainable methods that can be integrated into a circular economy strategy.
In this sense, microalgae can enter the process and directly replace secondary and tertiary treatments, and are capable of absorbing carbon, reducing energy expenditure and producing valuable compounds.
Microalgae – Nannochloropsis sp. Source: Wikimedia commons.
Microalgae are microscopic plants that feed on carbon (from CO2) nitrogen (from nitrates), phosphorus (from phosphates); for this reason we may have read or seen that they multiply uncontrollably in polluted water bodies, and that is why the water in these reservoirs looks green. That is to say, they not only eliminate these pollutants from wastewater but also produce a large amount of biomass rich in these elements, which can then be used as ecological fertilizers.
On the other hand, they do not require energy since they only need sunlight or injecting oxygen as in other biological processes, since they extract it from the water by themselves.
For this reason, their use has been proposed in treatment plants to save energy, capture carbon and generate valuable bioproducts instead of just waste. They are therefore a better alternative than the use of landfills.
Many water-stressed regions are considering water reuse, so we need to develop and improve tertiary purification methods and ensure better quality water for their use. And microalgae have already been shown to remove 95% of pollutants such as carbon, nitrogen and phosphorus, as well as being effective in eliminating certain pathogens and emerging pollutants, saving up to 40% of energy costs.
Microalgae research. Source: Wikimedia commons.
Although development stages and further research are still lacking, microalgae look promising for integrated water management and contribute to the achievement of the UN Sustainable Development Goal: Ensure availability and sustainable management of water and sanitation for all.
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Wearewater.org. Microalgas: biotecnología para el saneamiento universal.