The size of a treatment plant is characterised by two indicators: the indicator specifying the daily maximum volume of wastewater treated by a wastewater treatment plan (the treatment capacity of a wastewater treatment plant indicator) and a conventional indicator called "population equivalent (P.E.)". "Population equivalent (P.E.)" is the number expressing the ratio of the sum of the pollution load in wastewater to the individual pollution load in household sewage produced by one person within 24 hours. The unit load of pollutants from 1 person is assumed at 60 g O2 per day. If, for example, a figure of 1000 is entered at the P.E position, it means that this treatment plant can treat the wastewater generated by 1000 people. (see explanatory notes to the OS-5 form - available on the Statistics Poland website www.stat.gov.pl). Therefore, the P.E. is assigned to the locality within whose boundaries the plant is situated and refers to all the wastewater that the plant treats (regardless of whether the wastewater originates from a single locality or from multiple localities – the so-called collective wastewater treatment plants treating wastewater from several localities.).