Volumetric Organic Loading Rate

Volumetric loading answers the question a designer actually asks: how big does the tank have to be? Take the pounds, or kilograms, of BOD arriving each day and divide by the reactor volume. A flow of 4320 m³/d at 200 mg/L delivers 864 kg BOD/d, and in a 1200 m³ aeration basin that is 0.72 kg BOD/(m³·d). Conventional activated sludge sits at 0.3–1.0, extended aeration well below 0.3, high-rate systems above 1.5, a rock trickling filter around 0.2–0.5, and a mesophilic anaerobic digester on volatile solids at 1.6–3.2.

The value of the number is that it collapses two variables, flow and strength, into one, so it exposes what a hydraulic detention time hides. Two plants with identical two-hour detention times are not comparable if one treats 150 mg/L domestic sewage and the other 900 mg/L cannery waste. Push the loading past design and the symptoms are textbook: dissolved oxygen collapses, filamentous organisms take over because they out-compete floc-formers at low DO, sludge stops settling, and solids leave over the clarifier weir. The classic mistake is loading a digester by volume of sludge pumped rather than by pounds of volatile solids: a thin, watery feed can double the pumping and still starve the bugs, while a good thickener can overload the same digester at half the flow.