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Wastewater purification plant. © INRA, BERTIN Jean

The purification of domestic wastewater: the LBE is innovating

Scientists from the LBE and their American colleagues have just made a major discovery regarding the treatment of domestic wastewater, which has been published in Scientific Reports and Environmental Science & Technology.  They have demonstrated the role of granules enriched in cyanobacteria that can treat the pollution found in wastewater.

Updated on 05/03/2018
Published on 12/20/2017

Between 2 and 3% of all the electricity consumed in developed countries is used to operate plants to treat domestic wastewater. In particular, this energy is used to supply bacteria with the oxygen they need to mineralise organic matter and eliminate nitrogen. The bacteria develop normally in the form of aggregates referred to as activated sludge, which are separated from the treated water in settling tanks.

Scientists in the LBE (Environmental Biotechnology Laboratory) and their American colleagues have discovered that some photosynthetic bacteria may aggregate in a particular manner in space. This discovery means they can propose an ingenious and promising alternative based on the ability of these bacteria to both form granules that can easily be separated from the treated water and supply oxygen in situ by photosynthesis** to the depollution ecosystem.

Microbial photogranules rich in cyanobacteria…

The story started serendipitously with a vial containing activated sludge that had been forgotten on a windowsill. Within a few weeks, the scientists observed the transformation of this sludge into an almost spherical granule. They they reproduced this phenomenon using sludge from different sources.

Microscopic analysis of these granules revealed the presence of an external layer that was mainly made up of motile, filamentous cyanobacteria* forming a mat of interlaced filaments that guaranteed its structural cohesion.

Under turbulent hydrodynamic conditions close to those which prevail during the treatment of wastewater, the scientists also observed the propagation and growth of these photogranules which were similar to those obtained under static conditions.

… for innovative use in the treatment of domestic wastewater

These photogranules produce oxygen by photosynthesis and at the same time convert the carbon dioxide arising from the degradation of organic matter in the polluted environment into biomass***. These two properties could be of considerable value to the treatment of domestic wastewater insofar as in situ oxygen production could be advantageous in replacing an input of air. After anaerobic digestion, the biomass thus generated could serve as fertiliser (nutrient-containing digestate) and as a source of renewable energy (methane from biogas). It is now necessary to control the mechanism underlying the formation of the photogranules and to intensify the bioprocess so that this opportunity can be placed on an industrial footing.

The use of photogranules to treat wastewater overturns the traditional role that has devolved to purification plants; within the same innovative process it couples depollution with a low energy footprint, nutrient recovery and the recycling of organic matter that can be further exploited. In addition, it offers a potential response to the challenges of the sustainability of a bioeconomy based on the development of controlled, eco-designed bioprocesses that integrate innovative, more flexible and more robust technologies using fewer resources.

Cyanobacteria*, in a few words

Since appearing about 2.5 to 3 billion years ago, cyanobacteria have contributed to the expansion of current life forms on Earth thanks to their ability to produce oxygen by photosynthesis. Cyanobacteria display a considerable diversity of morphologies and physiologies. They also play a crucial role in the functioning of ecosystems because of their capacity to fix atmospheric carbon and nitrogen.

Photosynthesis**: the process by which organic matter is produced from carbon dioxide under the action of light. Plants achieve photosynthesis thanks to the chlorophyll they contain.

Biomass***: represents all organic matter whether it is of plant or animal origin. It may be found in forest, marine and aquatic environments, hedges, parks and gardens, industries generating by-products, organic waste or livestock effluents.

Contact(s)
Scientific contact(s):

Press Relations:
INRA News Office (33 (0)1 42 75 91 86)
Associated Division(s):
Microbiology and the Food Chain
Associated Centre(s):
Montpellier

REFERENCE

Kim Milferstedt, W. Camilla Kuo-Dahab, Caitlyn S. Butler, Jérôme Hamelin, Ahmed S. Abouhend, Kristie Stauch-White, Adam McNair, Christopher Watt, Blanca I. Carbajal González, Sona Dolan, Chul Park. The importance of filamentous cyanobacteria in the development of unusual oxygenic photogranules. Scientific Reportsdoi:10.1038/s41598-017-16614-9.