How is chlorine demand calculated when raw water is 0.75 MGD, chlorinator feed rate is 4.0 mg/L, and chlorine residual is 1.8 mg/L?

To calculate chlorine demand, you need to determine how much chlorine is consumed by the water before reaching a specific residual level after chlorination. The chlorine demand is essentially the difference between the amount of chlorine added and the amount remaining as a residual.

In this case, the chlorinator feed rate is provided as 4.0 mg/L, which means that 4.0 mg/L of chlorine is being added to the water. Additionally, the chlorine residual, which is the amount of chlorine that remains after reacting with the water and any contaminants, is measured at 1.8 mg/L.

To find the chlorine demand, you subtract the chlorine residual from the chlorinator feed rate:

Chlorine Demand = Chlorinator Feed Rate - Chlorine Residual

Chlorine Demand = 4.0 mg/L - 1.8 mg/L

Chlorine Demand = 2.2 mg/L

This calculation indicates that 2.2 mg/L of chlorine has been consumed by the water due to reactions with organic materials, bacteria, and other compounds, hence this value represents the chlorine demand. Understanding this calculation is critical for operators to ensure proper chlorination, maintain water quality, and optimize chlorine use in very small water systems.