The extraction of sludge in any treatment plant of biological type, is essential to guarantee the non-exit of regime of the process in specific periods.

Extraction of sludge by self-priming tanks with transfers to larger sewage treatment plants is now uneconomical due to transport costs and the actual discharge rates. On the other hand, the low concentration of solids required for the transfer requires the water to move about 97%, a value that requires the evacuation of large volumes, compared to what is actually retained in the treatment plant itself.

Depending on the type of sludge and the production volume of the WWTP, different technologies are used; From the extraction systems through VIT filter bags for low productions, to systems of higher yield of dryness like presses or press filters.