Per-Capita Wastewater Flow

Divide the plant's daily flow by the population it serves and you get the design number every collection-system engineer starts from. A 2 MGD plant serving 20,000 people runs 100 gallons per capita per day, which is the classic North American domestic figure; Ten States Standards recommends designing on at least 100 gpcd, and European practice uses 150–250 litres. The solver reports this one in gallons per person per day, the unit it is universally quoted in.

The interesting thing about this number is what makes it wrong. Real per-capita flows have fallen steadily since the 1990s — low-flush toilets, efficient washers and metered billing have taken many communities from 100 gpcd down to 60–70 — so a plant sized on 1975 assumptions is often hydraulically oversized and organically undersized, because the pollutant load did not fall with the water. In the other direction, a per-capita flow of 200 gpcd on a residential sewershed is not evidence of long showers, it is evidence of infiltration and inflow: groundwater leaking into cracked pipes and stormwater entering through illicit roof drains. The diagnostic is to compare dry-weather midnight flow with the daytime peak; a sewer that never goes quiet at 3 a.m. is a sewer full of groundwater.