Coagulation in water treatment: effect of temperature on chemical coagulant dose In conventional clarification (e.g., alum or ferric salts), how does the required coagulant dosage typically change as the temperature of the polluted/raw water increases (viscosity drops and flocculation kinetics improve)?

Introduction / Context:
Coagulation–flocculation is a fundamental step in water and wastewater treatment. Operators often ask how seasonal temperature changes affect the chemical dose of coagulants (such as alum, ferric chloride, or polyaluminum chloride). Temperature influences viscosity, mixing, collision frequency, and the formation/settling of flocs. Understanding this relationship helps optimize chemical cost and finished water quality.

Given Data / Assumptions:

• Same raw water source and comparable quality except for temperature.
• Conventional treatment with rapid mix, flocculation, and sedimentation.
• No dramatic shock loads of organics or turbidity beyond typical seasonal variation.

Concept / Approach:
As temperature rises, water viscosity decreases, which increases Brownian motion and reduces hydraulic resistance during aggregation. This raises the probability of effective collisions between destabilized particles, improving floc growth at a given coagulant dose. Warmer water also hastens hydrolysis reactions for metal-salt coagulants, reducing the dose needed to reach target zeta potential and floc strength.

Step-by-Step Solution:

Verification / Alternative check:
Jar tests performed at different temperatures typically show equal or better turbidity removal at a lower coagulant dose as temperature increases. Plant records frequently reflect lower winter performance and higher doses, whereas summer operation often permits dose cuts while maintaining residual criteria.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming temperature alone dictates dose. Alkalinity, NOM character, and turbidity also drive demand; always confirm with jar tests. Over-reduction of dose can increase filtered-water turbidity or aluminum/iron residuals.

Final Answer:
decreases.