Mass Loading: BOD, Solids or Metals

Concentration alone tells you nothing about the size of a problem; multiply it by flow and you get a mass, and mass is what a treatment process actually has to eat. In North America this is written lb/d = mg/L × MGD × 8.34, where 8.34 is the pounds in a US gallon of water. In SI it is even simpler, because 1 mg/L is exactly 1 g/m³: one megalitre a day at 200 mg/L BOD carries 200 kg of BOD per day. A 1 MGD plant at 200 mg/L is 3785 m³/d × 0.2 kg/m³ = 757 kg/d, which the imperial shorthand gives as 200 × 1 × 8.34 = 1668 lb/d. The same multiplication works for anything carried in the water: 0.44 mg/L of zinc in 5000 m³/d is 2.2 kg of zinc a day.

The solver works from a water density of exactly 1000 kg/m³, which is the sanitary-engineering convention, so it returns 8.3454 rather than the traditional rounded 8.34: a 0.065% difference nobody has ever cared about in a wastewater plant. What people do care about is the difference between a hydraulic overload and an organic overload. A plant can be well under its rated flow and badly over its rated pounds of BOD because a brewery, dairy or septage hauler discharged into the collection system; the flow meter looks fine and the aeration basin goes septic. That is precisely why industrial pretreatment permits are written in pounds per day, not in concentration.