Tannery DAF pilot: measured performance
This case records pilot tests on real tannery wastewater after screening, homogenisation and sulphide oxidation. Location and operator identity are deliberately withheld; the test conditions and measured water-quality data are retained because they are unusually detailed for tannery flotation.
At an influent flow of 2.0–2.9 m³/h with 50–100% treated-water recycle, dissolved air flotation reduced suspended solids from 2,680 to 640 mg/L and COD from 2,110 to 1,150 mg/L. The measured reductions were 76.1% TSS and 56.5% COD.
DAF configuration and operating window
The pilot DAF had a 1.8 m² net flotation zone and approximately 1.2 m³ flotation volume. It used a pressurisation tank, pressure-release zone, treated-effluent recycle tank and a variable-speed mechanical scraper for both floated and bottom solids.
| Operating item | Recycle test | Direct-pressurisation comparison |
|---|---|---|
| Raw influent flow | 2.0–2.9 m³/h | 3.6 m³/h |
| Recycle ratio | 50–100% | None |
| Total flow through cell | 3.8–4.1 m³/h | 3.6 m³/h |
| Pressure | 5 bar | 2.5–3.0 bar |
| Air flow | 0.30–0.45 Nm³/h | 0.05–0.10 Nm³/h |
| Surface hydraulic loading | 1.1–1.6 m³/m²·h | 2.0 m³/m²·h |
| Flotation detention time | 25–30 min | 20 min |
Measured influent and DAF effluent
| Parameter | Influent after equalisation | DAF effluent | Measured reduction |
|---|---|---|---|
| Settleable solids | 102 mL/L | 0.4 mL/L | 99.6% |
| Suspended solids | 2,680 mg/L | 640 mg/L | 76.1% |
| COD | 2,110 mg/L | 1,150 mg/L | 56.5% |
Chemistry and operation lessons
The best efficiency/cost balance in the recycle test was reported near 400 mg/L alum, without polyelectrolyte. In the direct-pressurisation test, the reported chemical programme was 300 mg/L alum plus 0.5 mg/L polyelectrolyte.
The important operational lesson is that more air was not automatically better. Excessive air flow created turbulence and destabilised the float layer, causing some sludge to settle. The tests found that raw tannery wastewater could be pressurised directly, but chemical addition after pressurisation and careful scraper speed were important to avoid shearing the floc.
How to use this case in real design
This was a pilot, not a plant capacity guarantee. It is valuable because it reports the hydraulic conditions, pressure, recycle, air-to-solids range and laboratory results on actual tannery wastewater. For design, treat the reported 1.1–1.6 m³/m²·h as a conservative pilot reference and validate the chemical dose, air-to-solids ratio and float stability with a jar test and a pilot where raw wastewater varies significantly.
This page is based on an independent public tannery-treatment pilot report. The operating setting and participant details have been redacted on this website. Numbers shown are reported measured results from the stated test conditions. They demonstrate process capability; they are not a performance warranty for another tannery or an unsegregated wastewater stream.