Saturator pressure and recirculation time on municipal effluent
Most published DAF work varies the chemistry and holds the hydraulics constant. This one does the opposite, and the result is a useful corrective: on a stream with free oil in it, the air side of the process carries more of the load than most suppliers admit.
A pilot dissolved air flotation unit treating municipal effluent removed 84.4% of oil and grease, 88.9% of BODâ‚…, 88.7% of COD and 85% of suspended solids by varying saturator gauge pressure between 1 and 5 bar and recirculation time between 1 and 20 minutes. Dissolved air in the recycle reached 102.3 mg/L with a saturation factor of 0.8, attributed to packing the absorption column with Raschig rings.
The influent was 800 mg/L total solids, 590 mg/L TSS, 450 mg/L oil and grease, 360 mg/L BODâ‚… and 710 mg/L COD. pH and dissolved solids were reported as stable through treatment.
Influent and result
| Parameter | Influent | Removal achieved | Implied effluent |
|---|---|---|---|
| Total solids | 800 mg/L | Reported stable | — |
| Total suspended solids | 590 mg/L | 85% | ≈ 89 mg/L |
| Oil and grease | 450 mg/L | 84.4% | ≈ 70 mg/L |
| BOD₅ | 360 mg/L | 88.9% | ≈ 40 mg/L |
| COD | 710 mg/L | 88.7% | ≈ 80 mg/L |
| pH | — | Stable | — |
| Dissolved solids | — | Stable | — |
The number worth noting
An 88.7% COD removal on a municipal stream is well above the 40–70% we publish as the normal expectation for particulate-dominated COD, and far above the 20–40% typical where COD is largely soluble. That is not a contradiction — it tells you the COD in this effluent was overwhelmingly particulate and lipid-bound, which is exactly what a 450 mg/L oil and grease figure implies.
The transferable point is the diagnostic, not the percentage: filtered COD as a fraction of total COD predicts what flotation can achieve on your stream more reliably than any other single measurement. Ask for both numbers before believing any supplier’s COD promise. See brewery effluent for the opposite case, where soluble COD dominates and flotation cannot reach the consent alone.
On the air side
The saturation factor of 0.8 is the useful engineering detail. It is the fraction of theoretical air solubility actually achieved in the saturator, and it is the term most often assumed rather than measured when a plant is sized.
- An unpacked saturator typically achieves 0.5–0.7. A packed one achieves 0.8–0.9. This study reports 0.8 using Raschig rings.
- The difference is not academic. Sizing a recycle system on an assumed factor of 0.9 and building an unpacked vessel that delivers 0.6 leaves the plant a third short of its design air, which shows up as thin, grey float that no coagulant adjustment will fix.
- On African sites the temperature correction compounds it: air solubility falls roughly 20% between 20 °C and 32 °C. A plant sized on 0.9 at 20 °C and running at 0.6 and 32 °C has less than half its intended air.
Our sizing calculator applies the temperature correction automatically. The saturation factor is the term you should be asking your supplier to state explicitly rather than bury.
Caveats
- Pilot scale, not full scale. The study is an optimisation exercise on a pilot rig.
- No coagulant regime is reported in the abstract. The removals appear to be achieved on air and hydraulics alone. On a stream with less free oil, chemical conditioning would be necessary to reach comparable numbers.
- The influent is unusually oily for a municipal stream. Do not read 84.4% oil and grease removal across to a domestic sewage works.
- Removal percentages are reported as best achieved across the tested pressure and recirculation range, not as sustained averages.
Source
Efficiency of a DAF System in Removing Organic Matter and Lipid Compounds from Municipal Effluent, Water (MDPI), volume 17, issue 24, article 3474, 2025. Open access. Read the full paper.
All influent and removal figures are as reported. Implied effluent concentrations, the filtered-COD diagnostic and the saturation-factor commentary are ours.