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Shallow Air Flotation units, like other air flotation systems, often require the addition of chemical reagents to enhance particle-bubble attachment and improve separation efficiency. The choice of chemicals depends on the characteristics of the wastewater and the target treatment goals. Here’s a detailed overview of commonly used chemicals and their roles:
1. Coagulants
Purpose: Neutralize electrostatic repulsion between colloidal particles (e.g., fine suspended solids, oils) to form larger aggregates (microflocs).
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Inorganic Coagulants:
- Aluminum-based: Aluminum sulfate (alum), polyaluminum chloride (PAC). Effective for removing suspended solids and some organic matter. Work best at pH 5–8.
- Iron-based: Ferric chloride, ferrous sulfate. Suitable for high-TSS wastewater and can enhance oil removal. Operate in a wider pH range (4–11) but may cause coloration if not fully reacted.
- Mechanism: Release metal ions (Al³⁺, Fe³⁺) that destabilize charged particles, promoting initial aggregation.
- Considerations: Dosage must be optimized to avoid excessive sludge production or residual metal ions in effluent.
2. Flocculants
Purpose: Bridge microflocs into larger, denser flocs that attach more effectively to air bubbles.
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Polymeric Flocculants:
- Cationic: Used for negatively charged particles (common in municipal wastewater, food processing effluents).
- Anionic: Suitable for positively charged particles (e.g., some industrial sludges).
- Non-ionic: Versatile for neutral particles but less commonly used in flotation.
- Key Parameters: Molecular weight (high MW enhances bridging) and charge density must match particle characteristics. Dosage is typically lower than coagulants (e.g., 0.1–10 mg/L).
- Synergy with Coagulants: Often used sequentially (coagulant first, flocculant second) to improve floc strength and bubble adhesion.
3. pH Adjustment Agents
Purpose: Optimize the pH range for coagulant/flocculant effectiveness.
- Acids: Sulfuric acid, hydrochloric acid (lower pH to match coagulant’s optimal range, e.g., alum works best at pH 5–6).
- Bases: Sodium hydroxide (NaOH), lime (CaO) (raise pH for iron-based coagulants or to precipitate metals).
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Impact on Flotation:
- pH affects particle charge, coagulant solubility, and bubble stability. For example, oil droplets may carry stronger negative charges at higher pH, requiring more coagulant.
- Incorrect pH can lead to poor floc formation or re-dispersion of particles.
4. Demulsifiers (for Oil-Water Separation)
Purpose: Break down stable oil-in-water emulsions (common in petroleum, metalworking, or food industries) to release free oil droplets for flotation.
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Types:
- Organic solvents (e.g., alcohols) or specialized surfactants that disrupt emulsion stability.
- Often used in pre-treatment stages before flotation to enhance oil droplet aggregation.
- Consideration: Compatibility with subsequent flocculants is critical to avoid chemical interference.
5. Bubble Modifiers
Purpose: Improve bubble-particle attachment by adjusting solution chemistry or bubble properties.
- Surfactants: Non-ionic surfactants (e.g., polyglycol ethers) can reduce surface tension, promoting bubble-floc adhesion. However, excessive use may stabilize foam excessively, complicating sludge removal.
- Salts: Adding electrolytes (e.g., NaCl) can reduce double-layer repulsion between bubbles and particles, enhancing attachment efficiency in low-ionic-strength wastewaters.
6. Specialized Additives
- Phosphate Removers: In some cases, chemicals like calcium chloride or magnesium salts may be added to precipitate phosphate as insoluble compounds (e.g., calcium phosphate), which can then be removed via flotation.
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Oxidants: Hydrogen peroxide or ozone may pre-treat refractory organics to make them more amenable to flocculation and flotation.
