This question asks about the sequence of water treatment processes, specifically relating to the process that follows de-chlorination. Let's break down the different chlorination methods mentioned in the options to understand their relationship with de-chlorination.

Understanding Chlorination Processes

Chlorination is a common method used in water treatment to kill harmful microorganisms. Different techniques are employed depending on the treatment goals:

• Pre-chlorination: Chlorine is added before other water treatment processes like sedimentation or filtration. It helps in reducing the formation of disinfection byproducts (DBPs) later and aids in controlling algae and improving coagulation.
• Post-chlorination: Chlorine is added after the main treatment stages, typically just before the water enters the distribution system. This provides a residual level of chlorine to protect the water from re-contamination.
• Super-chlorination: This involves applying a much higher dose of chlorine than normally required for disinfection. The purpose of super-chlorination is to destroy ammonia, taste and odor-causing compounds, and other resistant organic matter.
• Double-chlorination: This usually means applying chlorine at two different points in the treatment process, for instance, both pre- and post-chlorination.

De-chlorination Explained

De-chlorination is the process of removing residual chlorine from treated water. This is necessary when the chlorine level is too high, as excessive chlorine can affect the taste and odor of the water and potentially be harmful.

Connecting Super-chlorination and De-chlorination

The process of super-chlorination intentionally uses a high concentration of chlorine. Because this level is significantly higher than what is desired in the final drinking water, a subsequent step is required to remove the excess chlorine. This removal step is precisely de-chlorination.

Therefore, de-chlorination is typically performed after super-chlorination to bring the chlorine residual back to a safe and acceptable level.

Considering the sequence:

• Pre-chlorination happens before treatment.
• Post-chlorination happens after treatment, providing a residual.
• Super-chlorination involves a high dose, requiring a follow-up step.
• Double-chlorination involves two points of application, not necessarily linked directly to de-chlorination in the same way super-chlorination is.

The process that necessitates and is followed by de-chlorination is super-chlorination.