Each Sermonde and Gestal Unit has a leachate pre-treatment system.

The organic fraction of the waste admitted to landfills and a waste-to-energy center undergoes decomposition processes through fermentation or anaerobic digestion (because it is carried out without oxygen), resulting in two effluents: a liquid, the leachate, and a gaseous, the biogas.

Pre-treatment may involve:

- Laguning with oxygen injection.

- Treatment by nitrification – denitrification using an aeration tank and an anoxic tank.

- Physico-chemical treatment with sludge removal by flotation.

After this pre-treatment, the effluent is sent to the municipal sewage treatment network.