This is the boss. No calculator. Every flow, volume, area and concentration tonight has been chosen so the divisions fall out in your head, and the sanitary identity 1 mg/L = 1 g/m³ does all the unit work there is.

One plant, four lines, and every answer feeds the next: the flow gives the primary clarifier its detention time, the same flow over the settling surface gives the overflow rate, the flow and the influent strength give the BOD load, and that load against the biomass in the aeration basin gives the F/M ratio. Write each line down as you go — one slip at the headworks rolls all the way to the blowers.

Note what the last line actually asks. It does not want another number; it wants a decision: is this aeration basin inside the conventional band of 0.2 to 0.5, or is it overloaded and short of biomass. The arithmetic exists only to earn you the right to say so. And watch the two traps this chapter has been laying — a load is not a loading, and the primary clarifier's comfortable detention time says nothing at all about the basin downstream.

A bonus mark follows, one line long. Some nights it turns the BOD load into a population. Other nights it goes back to the clarifier for the third of its three numbers: the flow divided by the length of weir crest it leaves over.

Everything in the chapter rides in this one walk. 5★ under par here is the chapter crown. Clock in.