The surface chemistry that makes DAF work: collision/entrapment/growth capture, contact angle and Young's equation, DLVO and zeta potential, and how coagulant/polymer conditioning engineers floc hydrophobicity.
Producing a perfect microbubble cloud is wasted unless the bubbles actually attach to the particles you want to remove. Attachment is a surface-chemistry problem: hydrophobic surfaces — oils, fats, and polymer-conditioned flocs — bind bubbles 5–20× more efficiently than bare hydrophilic mineral or biological surfaces. The whole purpose of coagulation and flocculation ahead of a DAF is to engineer particle surfaces and floc structure so that bubble capture becomes thermodynamically favourable.
Particles are captured by three parallel mechanisms, all of which a good design promotes:
Whether a colliding bubble stays attached is set by the contact angle θ, governed by Young’s equation:
cos θ = (γSG − γSL) ⁄ γLG
A larger contact angle (more hydrophobic surface) means a stronger, more stable bubble–particle bond and a higher probability the aggregate survives to the surface.
Before bubbles can attach, the suspension must be destabilised. Raw colloids carry a negative surface charge (zeta potential) that keeps them apart by electrostatic repulsion — the DLVO energy barrier. Coagulation collapses that barrier; flocculation then builds capturable aggregates:
Coagulant (alum, ferric, PACl) neutralises charge so particles can approach. Polymer/flocculant bridges them into open, low-density flocs and — critically — presents hydrophobic patches that bubbles love. Dose is tuned to the iso-electric point (zeta near zero) for charge, then trimmed for floc structure and hydrophobicity. Over-dosing re-stabilises the colloid; under-dosing leaves a hydrophilic, poorly-capturable particle.
| Particle / surface | Character | Contact angle | Relative attachment efficiency |
|---|---|---|---|
| Free & emulsified oils, FOG | Strongly hydrophobic | High | Very high (5–20×) |
| Polymer-conditioned flocs | Hydrophobic patches | Moderate–high | High |
| Coagulated mineral flocs | Partly hydrophobic | Moderate | Good |
| Bare biological / mineral colloids | Hydrophilic, charged | Low | Poor — needs conditioning |
Attachment efficiency is the fraction of bubble–particle collisions that result in a stable aggregate; conditioning chemistry can move a stream from “poor” to “high”.
Coagulant and polymer types and doses are selected from jar tests and zeta-potential measurement on the actual stream — never assumed.
Rapid mix for coagulation (high G), gentle flocculation (low G, longer time) to grow strong flocs without shearing them apart before the cell.
Flow-paced dosing with feedback (streaming-current or zeta) holds the iso-electric point as load varies, avoiding the re-stabilisation cliff.
Coagulant upstream for charge neutralisation; polymer just before the contact zone so flocs reach the bubbles intact and hydrophobic.
Because attachment is decided by surface chemistry, we treat the conditioning step as part of the DAF design, not an afterthought. Reynolds & Bauhm characterises the stream, runs jar and zeta testing, and proposes a coagulant/polymer programme and injection scheme tuned for charge neutralisation and floc hydrophobicity — so the microbubbles the saturator works so hard to make actually capture the solids. Explore our DAF coagulation & flocculation and chemical dosing calculator for the practical detail.
Reynolds & Bauhm can review your saturator, release and contact-zone design, run the CFD and propose a sized, validated solution. Send us your flow, stream analysis and target effluent.
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