**Dosage of Chlorine in Water Treatment**

In water treatment processes, chlorine is commonly used as a disinfectant to kill harmful microorganisms and make the water safe for consumption. The dosage of chlorine refers to the amount of chlorine added to the water in order to achieve the desired level of disinfection. The dosage is usually measured in milligrams per liter (mg/L).

**Determining the Dosage**

To determine the dosage of chlorine needed, several factors need to be considered, including the desired residual chlorine level, contact time, and the chlorine demand of the water. The residual chlorine level is the concentration of chlorine that remains in the water after a certain contact time.

In this case, the residual chlorine after 10 minutes is given as 0.2 mg/L. This means that after 10 minutes of contact time, the concentration of chlorine in the water is 0.2 mg/L. To calculate the dosage of chlorine, we need to consider the chlorine demand of the water.

**Chlorine Demand**

Chlorine demand refers to the amount of chlorine required to react with the impurities present in the water. It is influenced by factors such as the organic and inorganic content of the water, as well as the pH level. The chlorine demand can vary from one water source to another.

To determine the chlorine demand, a chlorine titration test can be conducted. This test involves adding a known amount of chlorine to a water sample and measuring the residual chlorine after a certain contact time. By comparing the initial and final chlorine concentrations, the chlorine demand can be calculated.

**Calculating the Dosage**

Once the chlorine demand is determined, the dosage of chlorine can be calculated by adding the residual chlorine level to the chlorine demand.

For example, if the chlorine demand is 0.3 mg/L and the residual chlorine after 10 minutes is 0.2 mg/L, the dosage of chlorine would be:

Dosage = Chlorine demand + Residual chlorine
Dosage = 0.3 mg/L + 0.2 mg/L
Dosage = 0.5 mg/L

Therefore, the dosage of chlorine required to achieve a residual chlorine level of 0.2 mg/L after 10 minutes would be 0.5 mg/L.

**Conclusion**

In water treatment processes, the dosage of chlorine is crucial for achieving effective disinfection. By considering the residual chlorine level, chlorine demand, and contact time, the appropriate dosage of chlorine can be calculated. It is important to regularly monitor the residual chlorine levels and adjust the dosage accordingly to ensure the water remains safe for consumption.