Industrial wastewater treatment: When oil and grease are present in an emulsified state (not free-floating) in waste water, which process is most appropriate for effective removal at the primary/secondary stage?

Introduction / Context:
Oil and grease in industrial effluent appear as free oil (readily separable), dispersed droplets, or stable emulsions. The removal method depends on the physical state. Free oil is skimmed easily, whereas emulsified oil requires breaking the emulsion before separation. Selecting the correct unit process prevents downstream fouling and enhances biological treatment performance.

Given Data / Assumptions:

• Oil/grease is in an emulsified state (stable droplet size with surfactants/emulsifiers).
• Objective is to remove it prior to secondary biological treatment.
• Plant has conventional clarification equipment.

Concept / Approach:
Emulsion breaking typically employs chemical coagulation and pH adjustment to destabilize droplets, followed by flocculation and gravity separation (settling or dissolved air flotation). Coagulants (e.g., alum, ferric salts, PAC blends) neutralize surface charges, allowing coalescence and removal. Skimming targets free oil layers only; biological oxidation does not remove oil efficiently without pretreatment and can be inhibited by oils; chlorination is a disinfectant and does not break emulsions.

Step-by-Step Solution:
Diagnose the oil state: emulsified, not separable by gravity alone.Choose chemical destabilization: add coagulants/adjust pH to break the emulsion.Follow with separation: settle out the formed flocs or use flotation.

Verification / Alternative check:
Jar tests demonstrate rapid turbidity and oil removal after coagulant addition and mixing regimes. Many refinery and metalworking plants incorporate API separators for free oil and a DAF or clarifier with chemicals for emulsified oil.

Why Other Options Are Wrong:
Biological oxidation: Sensitive to oils and surfactants; not a primary emulsion breaker.Skimming: Only effective for free oil layers, not stable emulsions.Chlorination: Disinfection; does not address emulsion stability.

Common Pitfalls:

• Attempting to skim emulsions, leading to poor removal and downstream foaming.
• Neglecting pH control, which is critical for optimal coagulant performance.

Final Answer:
settling out using chemical reagents.