The size of a plant depends on the number of apartments connected to the plant or the number of people living in the apartments.

Related to the design of a wastewater treatment plant an apartment is a self-contained suite of rooms with kitchen and bathroom. We calculate with 2 PE (population equivalent) for apartments smaller than 60 m² and with 4 PE for bigger ones. If more people live in an apartment we calculate with the number of people constantly living there. 

1 PE corresponds to a water consumption of 0.15 m³/d and a pollution of 60 g BOD₅/d and 11 g N/d.