Coagulation with iron salts (ferric/ferrous): Identify the incorrect statement about performance, side effects, and applicable pH range in conventional water treatment.

Introduction / Context:
Iron-based coagulants (e.g., ferric chloride, ferric sulphate) are widely used to destabilise colloids, remove turbidity, and aid in colour/odour control. Knowing their operating pH window and side effects ensures reliable plant performance.

Given Data / Assumptions:

• Conventional surface-water treatment train (coagulation–flocculation–sedimentation–filtration).
• Typical pH control using alkali or acid as required.
• Use of iron salts in place of alum where appropriate.

Concept / Approach:

Iron coagulants are effective across a comparatively broad pH band (commonly about 4–9), broader than alum’s typical optimum near neutral. They can also oxidise or co-precipitate sulphides, while their acidifying effect may increase corrosivity if not corrected by alkalinity/pH adjustment.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals show jar-test pH optima spanning wider ranges for iron than alum. Plant operators routinely adjust alkalinity to control finished-water corrosivity.

Why Other Options Are Wrong:

• (a), (b), and (d) reflect established operational behavior.
• (e) is wrong because (c) is indeed incorrect.

Common Pitfalls:

• Assuming alum and iron coagulants share identical pH windows.
• Neglecting post-coagulation pH correction.

Final Answer:

Iron salts can be used only over a very limited pH range.