Why Glycol-Contaminated Water Is a DAF Problem, Not a Settling Problem
Ethylene and propylene glycol carryover from engine coolant dumps, PET polyester process water, and deicing washdown leaves a stream that conventional API separators and lamella clarifiers cannot treat to compliance. Glycol depresses surface tension and stabilises as sub-100 µm emulsified droplets, with the dispersed phase resisting Stokes-law settling even after 60–90 minutes of residence time. Field data from refinery cooling-tower blowdown shows free-glycol fractions routinely push influent COD above 1,000 mg/L and BOD₅ above 400 mg/L — well past the typical 500 mg/L COD industrial sewer threshold and damaging to downstream RO membranes through fouling and flux decline.
Microbubble flotation is the documented industrial response to this colloidal window. The 2015–2021 TecnoLógicas systematic review of dissolved air flotation (DAF) confirms that microbubble flotation is the dominant industrial removal route for sub-100 µm colloidal and oily contaminants across agricultural, municipal, and industrial sectors. For glycol specifically, DAF should be treated as a free-oil and free-glycol scavenger upstream of biological or membrane polish, not as a final polishing step. A properly configured DAF cell drops oil and free glycol 80–95% and protects the MBBR, MBR, or RO that follows from organic overload.
Reference DAF Parameter Envelope for Glycol Streams
Glycol DAF pilot parameters should be based on the envelope in Table 1. These values come from the Durban University of Technology (DUT) oil-water optimisation work and the TecnoLógicas review, mapped onto glycol's physical behaviour (lower interfacial tension, higher viscosity than mineral oil). Microbubbles in the 40–60 µm band are small enough to attach to sub-100 µm glycol/oil droplets — bubbles larger than ~80 µm rise too fast and provide poor collision efficiency with colloidal contaminants (TecnoLógicas review, 2015–2021).
| Parameter | Recommended Range for Glycol Streams | Documented Optimum | Source |
|---|---|---|---|
| Microbubble size | 40–60 µm | ~50 µm | TecnoLógicas 2015–2021 review |
| Saturator pressure | 300–500 kPa | 350 kPa (continuous); 400 kPa typical industrial default | DUT oil-water jar/OFAT study |
| Air-to-water recycle ratio | 5–15% | 10% | DUT oil-water jar/OFAT study |
| Flotation (hydraulic) residence time | 5–15 min | 15 min | DUT oil-water jar/OFAT study |
| Coagulant dose (PAC or cationic PAM) | 30–50 mg/L | 30–45 mg/L (BBD); 50 mg/L (OFAT) | DUT BBD and OFAT work |
| Operating pH | 4–7 | 5.0 | DUT oil-water jar/OFAT study |
| Expected removal (oil & free glycol) | 70–95% | 85% treatability at BBD optimum | DUT BBD study |
Three of the six factors above — pH, recycle ratio, and saturator pressure — are the dominant variables in a DAF jar test; coagulant dose interacts with pH. The DUT Box-Behnken design (BBD) returned 85% treatability at lower coagulant dose (30–45 mg/L) and moderate pressure (300–425 kPa) than the OFAT single-factor optimum, which is the right band for glycol given that destabilised glycol droplets do not need aggressive chemistry to float once pH is at 5. Zhongsheng field pilots on PET polyester wastewater (2025-09) confirm that the 350–400 kPa / 10% recycle / pH 5 corner is the working point for streams with COD between 800 and 2,000 mg/L.
Acid-DAF-Coagulation vs Coag-DAF-Acid: Which Layout for Glycol?
The DUT study documented two train configurations that perform differently on glycol. Acid-DAF-Coagulation (acidification and microbubble flotation first, coagulant dosing downstream as a polishing stage) is the layout the data supports for emulsified glycol. The destabilised emulsion is floated before polymer bridging, so the bubble-droplet collision happens on a clean surface and free glycol reports to the float layer rather than being re-stabilised by overdosed polymer.
Coag-DAF-Acid (coagulant dosed upstream, DAF cell, then acidification) works for free-floating oils that have already coalesced, but underperforms on tightly emulsified glycol because the polymer coats the droplet before the bubble has a chance to attach. On oily refinery wastewater the DUT continuous pilot hit 85% treatability under the Acid-Coagulation-DAF layout — the same pattern Zhongsheng has observed on PET and glycol coolant streams. Practical recommendation: route ethylene/propylene glycol coolant dumps and PET process water through an acid-DAF-coagulation train, and reserve coag-DAF-acid for light free-oil skimming duty where droplet size is already above ~150 µm.
| Configuration | Sequence | Best Suited Stream | Documented Treatability | Notes for Glycol |
|---|---|---|---|---|
| Acid-DAF-Coagulation | pH adjust → DAF → coagulant polish | Emulsified glycol, refinery oily wastewater, PET process water | Up to 85% (DUT continuous pilot) | Destabilised emulsion floated before polymer bridge; preferred train |
| Coag-DAF-Acid | Coagulant → DAF → pH adjust | Free-floating oils, light skimming duty | Lower on emulsified feeds | Risk of polymer over-coating sub-100 µm glycol droplets; not recommended for glycol coolants |
Coagulant and Flocculant Pairing for Glycol-Bearing Influent
Glycol streams respond well to a two-chemistry program consisting of a hydrolysing metal coagulant for charge neutralisation and a low-dose cationic flocculant to bridge destabilised droplets to the rising bubble swarm. Polyaluminium chloride (PAC) at 30–50 mg/L is the workhorse in the pH 5–7 window; cationic polyacrylamide (PAM) at 1–3 mg/L is the bridging aid. A Zhongsheng automatic chemical dosing system holding both reagents on a flow-paced setpoint keeps the dose on target as glycol surges from batch dumps arrive.
For high-strength glycol (COD above 5,000 mg/L, common in polyester oligomer washwater and concentrated deicing runoff), add a pre-oxidation step to break the glycol into shorter-chain organics before DAF. Fenton at H₂O₂/Fe²+ ratios near 2:1 or ozone at 20–50 mg/L have been integrated with DAF in the WIT Press livestock-ozone work, where ozone was dosed ahead of the saturator to drop COD and reduce bubble-droplet inhibition. Whatever chemistry is chosen, run a jar test that mirrors the 5–15 min flotation window from the DUT study — the OFAT envelope is a starting point, not a guarantee on your specific influent.
Coupling DAF to Polishing for Reuse vs Compliant Discharge
DAF effluent requires a downstream step to meet either the reuse water quality envelope or the discharge threshold. The choice of polish depends on the final application, and the decision is best made based on COD and SDI rather than on flow alone.
- Reuse (closed-loop cooling-tower make-up): DAF → MBBR or MBR for residual glycol/COD reduction → carbon filtration → cooling-tower make-up. MBR is the right pick when the Silt Density Index (SDI) of the DAF effluent is still above ~3, because the membrane step also catches biomass and any un-floated oil sheen that would foul downstream carbon beds.
- Compliant discharge (industrial sewer): DAF → biological polishing (WSZ, SBR, or MBBR) → disinfection to bring COD under the typical 500 mg/L refinery/industrial sewer threshold and BOD₅ under 250 mg/L.
- High-strength glycol (COD above 10,000 mg/L): route toward ZLD-style trains with evaporation/crystallisation after biological reduction, since DAF alone cannot make the COD cut.
The float stream from a glycol DAF cell is voluminous — typically 2–5% of the influent flow as a wet sludge — and benefits from a downstream filter press for dewatering before disposal. The earlier background on How Does a DAF Clarifier Work? Industrial Process Flow, Micro-Bubble Physics & 95%+ TSS Removal Explained covers the sludge handling side in more detail. For a side-stream comparison on a different oily feed, see the DAF Configuration for Solvent Stripper Dilute: 2026 Reuse & Discharge Guide; for a high-COD textile case where MBR is the polish of choice, the MBR Configuration for Reactive Dyeing Bath Dump: 2026 Reuse & Discharge Guide documents the downstream train.
Frequently Asked Questions
What bubble size, recycle ratio, and saturator pressure are optimal for glycol DAF?
Operate at 40–60 µm microbubble size, 8–12% air-to-water recycle (10% optimum), and 350–400 kPa saturator pressure. The DUT OFAT study reported pH 5, 10% A/W, 350 kPa, 15 min flotation, and 50 mg/L coagulant as the single-factor optimum for oily refinery wastewater, with 85% treatability at the BBD corner of 30–45 mg/L coagulant and 300–425 kPa.
Should coagulant be dosed before or after the DAF cell on glycol streams?
Acid-DAF-Coagulation (pH adjust and float first, coagulant polish downstream) is the preferred train for emulsified glycol and PET process water. The DUT continuous pilot achieved 85% treatability under this layout; Coag-DAF-Acid is reserved for free-floating oils above ~150 µm where the emulsion is already loose.
Can a DAF alone meet cooling-tower make-up or refinery discharge limits on glycol water?
DAF rarely meets these limits alone. While it reliably removes 80–95% of free oil and free glycol, residual COD and BOD₅ still exceed the typical 500 mg/L industrial sewer COD threshold and will foul RO membranes at any meaningful recovery. Route DAF effluent to MBBR/MBR for reuse or to biological polishing plus disinfection for discharge, and consider ZLD trains for COD above ~10,000 mg/L.
