For the treatment of domestic wastewater, the main dimensioning criterion for a biological treatment plant is the organic load resulting from the organic load per day and per inhabitant, multiplied by the number of inhabitants connected to the installation.
This organic load is quantified via the Biochemical Oxygen Demand (BOD5), a protocol aiming to measure the quantity of biodegradable organic matter contained in water.
However, the theoretical daily load of BOD5 produced by an inhabitant is very variable according to the geographical area, in particular according to the size of the agglomeration and the standard of living of the inhabitants.
For example, the European directive of May 21, 1991 defines the inhabitant equivalent as an organic load equivalent to 60 grams of BOD5 per day and per person.
In Morocco, it is common to size wastewater treatment plants with a theoretical organic load of 30 grams of BOD5 per day and per inhabitant, but this value may actually be higher in developed urban areas.
A study of the physicochemical quality of raw wastewater from five cities in the Chaouia – Ouardigha region (Morocco), for example, made it possible to define the pollution ratio per inhabitant, varying between 27 and 37 mg / inhabitant depending on the city.