Urban wastewater treatment - Extensive technologies and tertiary treatments

This article looks at a group of domestic wastewater treatment processes often considered less conventional: extensive techniques.

These techniques are characterized by their ability to purify water without the need for energy-intensive devices. They exploit two main biological systems:

• the first is based on fixed cultures of micro-organisms on solid supports with a small grain size (fine support);

• the second uses so-called free microbial cultures, i.e. those that do not adhere to a support, for which photosynthesis, thanks to solar energy, plays an essential role in the production of the oxygen needed to decompose pollutants.

The significant advantage of these systems lies in their ability to operate with little or no electricity input, making them both ecologically sustainable and economical. However, this rule does not apply to the aerated lagoon system. Unlike other extensive methods, this technology requires an external energy supply to operate the aerators or air-injection devices, thus oxygenating the water artificially.

Another distinctive feature of these extensive techniques is their application of particularly low pollutant loads per surface area compared with so-called intensive treatment methods. It is this low surface load that enables treatment to be carried out in a way that resembles natural self-purification phenomena.

We also highlight the advantages and constraints associated with their use, in an approach that seeks more sustainable wastewater treatment solutions adapted to the local context.

The final section looks at the techniques used to eliminate pollutants not degraded by the biological stage, i.e. tertiary/complementary treatments. This section is common to the series of articles on urban wastewater treatment.