Whether coagulation worked is only confirmed 3–4 hours later, by turbidity at the end of the sedimentation basin. Until then, operators judge dosing by eye and experience — a subjective, non-quantitative call.
If dosing equipment malfunctions in that window, turbidity in the settled water can rise before anyone notices. To stay safe, plants often dose more chemical than needed — or, when quality slips, burn more energy at the filtration stage. Both cost money.