Water and Wastewater Treatment – What is flocculation? Choose the statement that best describes the process of flocculation in environmental engineering practice.

Introduction / Context:
Flocculation is a key unit process in water and wastewater treatment. It bridges coagulation and clarification by gently mixing water so that micro-flocs grow into settleable flocs, improving sedimentation, dissolved air flotation, and filtration performance.

Given Data / Assumptions:

• We are considering the generic definition of flocculation as used in treatment trains.
• Gravity separation (settling or flotation) follows flocculation.
• Coagulant addition may precede flocculation to destabilize particles.

Concept / Approach:

Fine colloids are stable due to surface charges. Coagulation neutralizes these charges, and flocculation provides controlled hydraulic energy (characterized by velocity gradient G and Gt) to promote inter-particle collisions, forming larger, denser flocs that are more readily removed by gravity processes.

Step-by-Step Solution:

Verification / Alternative check:

Jar tests optimize coagulant dose and flocculation gradients; improved turbidity removal and filter run length confirm proper floc formation.

Why Other Options Are Wrong:

(a) describes one application (algae removal) rather than the definition; it can be true in context but is not the generic meaning. (c) is incorrect because (a) alone does not define the process. (d) is false since (b) correctly states the definition.

Common Pitfalls:

Overmixing (breaking flocs), insufficient Gt, poor coagulant dosing, or skipping alkalinity control, all of which reduce floc size and settleability.

Final Answer:

A process that promotes aggregation of fine particles into larger flocs to enhance gravity removal