Population Equivalent

Population equivalent converts an industrial discharge into the only currency a municipal plant really understands — people. One person contributes about 60 g of BOD per day in European practice and 0.17 lb/d in North American practice, so a dairy discharging 500 kg of BOD a day is loading the works like 8333 extra residents, whether or not a single house was built. The European Union's Urban Waste Water Treatment Directive is written entirely in these units: the 2000 p.e. and 10,000 p.e. thresholds decide what treatment a town must install.

The concept is the backbone of industrial surcharge billing. A brewery, cannery or rendering plant pays a sewer surcharge based on the pounds of BOD, TSS, ammonia and grease it sends, because those pounds consume capacity that would otherwise serve homes. The trap is quoting a population equivalent without saying which pollutant it is based on: a laundry might be 200 p.e. on BOD and 2000 p.e. on suspended solids, and a metal finisher might be trivial on both and impossible on metals. Also keep hydraulic and organic equivalents separate — flow-based p.e. runs 100–150 gallons or 150–250 litres per person per day, and an industry can be enormous on one basis and negligible on the other.