This is the first plant in Spain to use the tunnel composting technique to treat the sludge from a wastewater treatment plant. Annexed to the plant in Teia, Barcelona, it receives the sludge from various treatment plants located nearby to produce compost for farming purposes.

The composting process is carried out inside the tunnels once they have been filled and hermetically closed. For better aeration, the sludge is mixed with a support material such as wood chips that are sifted off at the end of the process so they can be used again. Air is injected into the tunnels through nozzles located in the floor. The system allows fresh air to be mixed with part of the recycled air coming from the tunnels.

The process is controlled automatically taking into consideration the sludge and air temperatures, the amount of oxygen in the recycled air and the incoming air flow. A high quality compost is obtained after the sludge has been in the tunnels for two weeks.