Population equivalent
Population equivalent or unit per capita loading,, in waste-water treatment is the number expressing the ratio of the sum of the pollution load produced during 24 hours by industrial facilities and services to the individual pollution load in household sewage produced by one person in the same time. It measures the amount of oxygen-demanding substances in waste water which will consume oxygen as it bio-degrades, usually as a result of bacterial activity.
For practical calculations, it was assumed that one unit equalled 54 grams of Biochemical oxygen demand per 24 hours, when the United Nations defined the term in 1997. In Poland it is now assumed to be 60 grams per day, and this is also the case within the European Union, while in United States, a figure of 80 grams per day is normally used.
The assumption is that one person living in a normal house will produce 200 litres of effluent per day, and that will contain 60 grams of biochemically active material. Thus a family of four living in a normal house would be expected to produce 800 litres of effluent, containing 240 grams of biochemically active material. The values vary for other situations, and so in Ireland, one guest staying in a hotel would typically produce 250 litres of effluent with 75 grams of active material, and so would have a population equivalent of 1.25.