Dose, demand, residual
Dose = demand + residual
Demand = chlorine used up reacting with material and organisms.
Residual = chlorine left over, still able to disinfect.
★High demand = dirtier water = needs more dose to leave a residual.
Free, combined, total
Free chlorine = available, unreacted. Fast and strong.
Combined chlorine = bonded to ammonia (chloramine). Weaker, but lasts longer.
Total chlorine = free + combined
Combined holds a residual farther out in the distribution system.
Contact time & residual
★A disinfectant needs enough CONTACT TIME to inactivate organisms.
The clearwell gives contact time after filtration.
A residual keeps protecting water after it leaves the plant.
New main disinfection
A new main is disinfected at a high dose, then flushed.
★Passing clearance samples BEFORE it serves customers.
lbs = volume (MG) x dose (mg/L) x 8.34