Chlorine Dose, Demand and Residual

Chlorine added to water does not stay chlorine. It is consumed by iron, manganese, sulphide, ammonia, and every scrap of organic matter it meets, that consumption is the demand, and only what survives shows up on a DPD test as residual. The bookkeeping is a straight sum: dose equals demand plus residual. Feed 3.3 mg/L into a water with 2.5 mg/L of demand and you keep 0.8 mg/L residual, which is a typical distribution target. There is no way around the demand; it has to be satisfied before a single tenth of a milligram of residual appears, which is why a well with high iron can swallow 4 mg/L and still test zero at the tap.

The practical procedure is a chlorine demand study: dose a set of bottles of the actual water at increasing rates, wait the real contact time (usually 30 minutes, but overnight for a distribution main) and test each. The plot of residual against dose is flat until demand is satisfied, then rises with unit slope. Two field cautions. Demand is not a fixed property: it climbs in warm weather, after a main break, and with every degree of surface-water turbidity, so the study needs repeating seasonally. And test at the far end of the system, not at the plant: the residual that matters legally and microbiologically is the one still standing at the last service connection.