BOD Removal Efficiency

Percent removal is the number on the discharge permit and on the front page of every monthly operating report. A plant taking 220 mg/L of BOD down to 18 mg/L is removing 202/220 = 91.8%, and US secondary treatment standards demand at least 85% removal of both BOD and TSS along with a 30 mg/L monthly average. Primary settling on its own is worth 25–35% of BOD and 50–65% of TSS; conventional activated sludge adds the rest.

The formula has a blind spot that regulators had to legislate around: percent removal is a ratio, so a community with dilute wastewater (heavy infiltration, a lot of groundwater in leaky sewers) can produce a beautifully low effluent and still fail the 85% test, while a strong-waste town can pass the percentage while discharging more pounds than its neighbour. That is why permits carry both a concentration limit and a percent-removal limit, and why the pounds-per-day loading is the honest measure of what reaches the river. Use concentrations from flow-proportional composite samples; a grab sample at 10 a.m. on a Tuesday flatters or damns a plant depending only on when the town takes its showers.