Palm oil mill effluent treatment with DAF
The most concentrated common industrial effluent in Africa, and the one where flotation is most often specified in the wrong place. Where a DAF belongs on a palm oil mill is precise, and it is not at the front.
Raw palm oil mill effluent is far too concentrated for dissolved air flotation. At 40,000–100,000 mg/L COD and 15,000–25,000 mg/L suspended solids, it is an anaerobic digestion feedstock, not a flotation duty. Flotation belongs in two places on a palm oil mill: recovering residual oil from the de-oiling stage ahead of the ponds, and polishing anaerobically digested POME before discharge or land application.
On digested POME — typically 1,000–3,500 mg/L COD and 100–500 mg/L oil and grease — a correctly dosed DAF removes 80–90% of suspended solids and residual oil, which is where the published results in this range come from.
Influent characteristics
| Parameter | Raw POME | After anaerobic digestion | Flotation duty? |
|---|---|---|---|
| COD | 40,000–100,000 mg/L | 1,000–3,500 mg/L | Only after digestion |
| BOD₅ | 20,000–60,000 mg/L | 100–1,000 mg/L | Only after digestion |
| Total suspended solids | 15,000–25,000 mg/L | 500–2,000 mg/L | Only after digestion |
| Total solids | 30,000–50,000 mg/L | — | — |
| Oil and grease | 2,000–8,000 mg/L | 100–500 mg/L | De-oiling first, then flotation |
| pH | 3.4–5.2 | 6.5–8.0 | Raw POME needs correction |
| Temperature | 80–90 °C | 35–45 °C | Critical — see below |
| Total nitrogen | 500–800 mg/L | — | Biological |
For scale: a typical mill produces roughly 0.65–0.75 m³ of POME per tonne of fresh fruit bunches processed. A 30 t/h mill running 20 hours therefore generates around 400–450 m³/day, carrying a COD load of 20–40 tonnes. That is the organic load of a small town, from one factory.
The temperature problem
This is the design point that gets missed, and it is specific to palm oil. POME leaves the mill at 80–90 °C. Dissolved air flotation depends entirely on how much air will dissolve in the recycle stream, and air solubility falls sharply with temperature.
| Water temperature | Approximate air solubility at 1 atm | Relative to 20 °C |
|---|---|---|
| 20 °C | ≈ 24.3 mg/L | 100% |
| 32 °C | ≈ 19.5 mg/L | 80% |
| 40 °C | ≈ 17.5 mg/L | 72% |
| 60 °C | ≈ 14 mg/L | 58% |
| 80 °C | ≈ 12 mg/L | 49% |
The consequences follow directly. A flotation plant on hot POME either needs cooling ahead of it — normally a cooling pond, which most mills already have — or the recycle ratio and saturator pressure must be increased to compensate, at a real energy cost. Both are manageable. What is not manageable is discovering it at commissioning, which is what happens when a supplier sizes the recycle on a default temperature.
Ask any supplier quoting a DAF for POME duty what design temperature they used, and check it against your actual discharge temperature at that point in the train. Our sizing calculator applies the correction automatically.
Where flotation belongs in the train
| Stage | Purpose | Is DAF appropriate? |
|---|---|---|
| 1. Sterilisation and clarification condensate | Mill process | No — this is the source, not a treatment stage |
| 2. De-oiling / oil recovery tank | Recover residual crude palm oil | Yes. Flotation recovers oil that gravity separation leaves behind, and the recovered oil has direct value |
| 3. Cooling pond | Drop temperature from 80–90 °C to 40–45 °C | No — but essential if a DAF follows |
| 4. Anaerobic digestion or ponds | Remove 80–95% of COD; capture biogas where covered | No — this is the main treatment |
| 5. Polishing before discharge or land application | Remove residual solids and oil to meet consent | Yes. This is the classic DAF duty on POME |
| 6. Aerobic polishing or wetland | Residual BOD and nutrients | No |
Chemistry
- On the oil recovery duty, often no coagulant at all, or a very low polymer dose. The objective is recovering saleable oil, and coagulant contaminates it. Air and residence time do most of the work.
- On the polishing duty after digestion, the stream is near-neutral and a conventional regime applies: PAC at 80–250 mg/L with an anionic polymer at 2–5 mg/L is a reasonable jar-test starting point.
- Raw POME acidity matters if anyone insists on treating it directly. At pH 3.4–5.2, aluminium coagulants are outside their window and would need substantial alkali. A nonionic polymer is the more sensible chemistry on an acidic stream, because it does not depend on charge.
- Watch the phosphorus if the effluent goes to land. POME is applied to plantations as fertigation in many mills, which is a legitimate and valuable route — but where a discharge consent applies instead, iron dosing at the flotation stage handles phosphorus.
The economics that actually decide it
On most African mills, a DAF is not justified by compliance alone. It is justified by recovered oil.
Residual oil in POME typically runs 1–3% of the effluent stream after the mill’s own clarification. On 400 m³/day that is 4–12 m³ of oil per day at the theoretical maximum, of which a flotation stage might realistically recover a meaningful fraction. Even at modest recovery rates and a conservative crude palm oil price, the arithmetic frequently pays for the unit inside a year — which is a very different conversation from a compliance-only business case.
Two honest counterweights. Recovered oil from a flotation stage is lower grade than mill oil and may need reprocessing or may only be saleable for non-food use. And if you dose coagulant to improve removal, you contaminate the oil and lose the value — so the oil recovery duty and the compliance duty want different chemistry and are usually better as two separate stages.
Where the African mills are
Nigeria is Africa’s largest palm oil producer and among the largest globally, followed by Côte d’Ivoire, Ghana, Cameroon and the Democratic Republic of the Congo, with growing production in Sierra Leone, Liberia and Uganda. Most of these mills discharge to ponds under environmental consents that have tightened over the last decade, and many have ponds that were sized for a smaller mill than the one now feeding them.
Where land is available, extending the pond system is usually cheaper than any equipment. Where it is not — and increasingly it is not, because plantations expand into the land the ponds would have used — a polishing flotation stage on the digested effluent is the compact alternative. Check your own national limits: see discharge limits by country.
Frequently asked questions
Can a DAF treat raw palm oil mill effluent?
No, and any proposal that says otherwise should be questioned closely. Raw POME runs 40,000–100,000 mg/L COD and 15,000–25,000 mg/L suspended solids. Flotation is a separation process for streams one to two orders of magnitude weaker than that. The coagulant cost would be uneconomic and the float sludge volume unmanageable. POME’s primary treatment is anaerobic — ponds or a digester — and flotation belongs before it for oil recovery or after it for polishing.
Why does POME temperature matter so much for a DAF?
Because air solubility roughly halves between 20 °C and 80 °C, and dissolved air is the entire basis of the process. POME leaves the mill at 80–90 °C. A flotation unit sized on standard temperature data and fed hot POME delivers about half the air it was designed for, which shows up as a thin grey float and poor removal. Either cool the stream first — most mills already have a cooling pond — or size the recycle ratio and saturator pressure for the actual temperature.
What removal can I expect on digested POME?
On anaerobically digested POME at 1,000–3,500 mg/L COD, a correctly dosed flotation stage typically removes 80–90% of suspended solids and residual oil and grease, and a smaller share of COD depending on how much of the remaining COD is soluble. After digestion much of the easily degradable organic load has already gone, so the residual COD is more soluble than in the raw effluent and flotation reaches less of it.
Is recovered oil from a DAF saleable?
Often, but at a lower grade than mill oil and sometimes only for non-food use, depending on how it has been handled and whether coagulant was dosed. The important design consequence is that the oil recovery duty and the compliance polishing duty want different chemistry — coagulant improves removal but contaminates the oil. Where recovery value matters, run them as separate stages.
How much POME does a mill produce?
Roughly 0.65–0.75 m³ per tonne of fresh fruit bunches processed. A 30 t/h mill running 20 hours a day produces around 400–450 m³/day. Confirm against your own mill’s water balance, because dilution water practice varies considerably and it is the single biggest driver of both volume and concentration.