In comparison with class II separators, class I separators provide a better cleaning performance (99.98 %) achieved via so-called oleophilic coalescence insets or similar devices. The process of coalescence (flowing together/drop merging) allows very finely dispersed droplets of light liquid to settle, forming an oil film.

Once this film reaches a certain size, the drop of oil cuts loose and rises to the surface in the form of a separable drop of oil. In comparison with class II separators, class I separators, if handled properly, provide better wastewater values (>5mg/l). The material should be cleaned and/or replaced regularly, in order to reduce the risk of the coalescence material clogging up.

Available nominal sizes (NS) from 3→ 150 l/s