Virtuous coupling of water sanitation and microalgae culture

Around 80% of the world's wastewater is discharged into the natural environment without adequate treatment. Inadequate sanitation can lead to the spread of serious diseases such as cholera, dysentery, typhoid, intestinal worm infections and polio. Contamination of domestic water due to inadequate purification systems thus caused more than 1,000 child deaths every day worldwide in 2012, mainly in Africa and East Asia. The discharge of wastewater nutrients such as nitrogen and phosphorus into the natural environment also encourages the excessive growth of algae, which eventually suffocates the bodies of water they invade and causes the death of aquatic fauna.

While effective systems for removing pollutants and pathogens do exist, they are often too costly and energy-intensive to be used sustainably in the poorest countries, where the proportion of untreated wastewater is also highest. In the case of developed countries, sanitation still poses problems of cost, environmental impact and the recovery of pollutants from wastewater in the form of nutrients.

Low-cost, low-maintenance, low-energy solutions are therefore needed. To meet these needs, wastewater treatment systems using microalgae were first developed in California in the 1950s. Outdoor ponds, known as High Rate Algal Ponds (HRAPs), are used for secondary biological treatment of domestic or industrial wastewater. They do away with the artificial aeration used in activated sludge systems by using photosynthetic organisms such as microalgae to produce the oxygen needed to break down pollutants. Microalgae also consume some of the pollutants targeted by water treatment, such as carbon, nitrogen, phosphorus and sulfur, which are necessary for the proper functioning and structural elements of microalgae cells.

The small environmental footprint and low cost of this type of system would make it possible to replace some existing, sub-optimal technologies, or to equip regions that do not have them. Nevertheless, the light required for microalgae photosynthesis comes solely from the sun, making HRAP technology dependent on light and temperature variations throughout the day/night cycle and the seasons, resulting in variable but significant performance.

The aim of this article is to present the main challenges and promises of wastewater treatment systems using microalgae. After an initial overview of the French wastewater treatment market, the HRAP technology, its ability to respond to certain wastewater treatment issues and the functioning of the microalgae-bacteria consortium are detailed. The purification performances currently achieved by various HRAPs operating on a large scale are compiled, before discussing possible ways of reusing the treated wastewater leaving...