In conventional surface-water treatment plants, which chlorination practice is regarded as the standard step applied after filtration to ensure a disinfectant residual in the distribution system?

Introduction / Context:
Chlorination strategies can be placed at different points in the treatment train to achieve specific goals (algae control, taste/odor management, primary disinfection, residual maintenance). The standard practice that ensures pathogen inactivation of filtered water and maintains a residual in the network is post-chlorination.

Given Data / Assumptions:

• Conventional treatment: coagulation, flocculation, sedimentation, filtration.
• Need for a stable disinfectant residual entering the distribution system.
• Compliance with microbial safety objectives.

Concept / Approach:
Post-chlorination after filtration treats clarified, low-turbidity water, maximizing disinfectant efficacy and permitting a controlled residual to persist in the distribution system for protection against recontamination. Other placements (pre-chlorination) serve different objectives and may form undesirable by-products if not managed carefully.

Step-by-Step Solution:

Verification / Alternative check:
Standard operating procedures and texts identify post-chlorination as the default/standard disinfecting step; alternative strategies are supplemental or situational.

Why Other Options Are Wrong:

• Plain or pre-chlorination alone may not assure residual after filtration and can increase by-product formation.
• Double chlorination is not universally required and is not termed the standard practice.

Common Pitfalls:
Confusing pre-chlorination (process aid) with the final disinfection and residual maintenance step; overlooking contact time and residual monitoring needs.

Final Answer:
post-chlorination (after filtration, before distribution)