Edible oil refinery wastewater treatment with DAF
Edible oil refining produces the highest FOG loads of any common industrial effluent — up to 4,000 mg/L. It is also the one duty where the float has genuine recoverable value, which changes the economics of the whole plant.
A dissolved air flotation unit on edible oil refinery effluent removes 92–99 % of fats, oils and grease and 85–95 % of suspended solids, typically dosed with 150–600 mg/L polyaluminium chloride plus 2–5 mg/L anionic polymer, at a surface loading rate of 5–8 m/h. Effluent from the acidulation and washing stages must first be pH-corrected from as low as 2.5–4.0 into the coagulation window of 6.0–8.0. Recovered float typically contains 15–40 % oil and is normally routed to soapstock or biodiesel feedstock rather than to disposal.
Where the load comes from
Physical refining and chemical refining produce very different effluent. Chemical (alkali) refining, which still dominates in much of Africa for palm, sunflower, cottonseed and groundnut oil, generates far more aqueous waste because neutralisation and washing are wet operations.
| Process stage | Flow share | FOG mg/L | COD mg/L | pH |
|---|---|---|---|---|
| Degumming | 10 – 20 % | 500 – 2,000 | 3,000 – 10,000 | 3.0 – 5.0 |
| Neutralisation / soapstock split | 15 – 30 % | 2,000 – 15,000 | 10,000 – 50,000 | 2.5 – 4.5 |
| Washing water | 40 – 60 % | 300 – 1,500 | 1,500 – 6,000 | 5.0 – 9.0 |
| Deodoriser condensate / barometric | 10 – 25 % | 100 – 800 | 500 – 3,000 | 4.0 – 6.5 |
| Floor and tank washing | 5 – 15 % | 200 – 2,000 | 1,000 – 5,000 | 6.0 – 11.0 |
| Combined, balanced | 100 % | 500 – 4,000 | 2,000 – 10,000 | 4.0 – 7.0 |
Pretreatment: oil interception before flotation
Do not send free oil to a DAF. A flotation unit is designed to remove emulsified and finely dispersed oil at tens to hundreds of mg/L; feeding it free-phase oil at thousands of mg/L floods the skimmer, blinds the float and drags oil into the effluent. The correct sequence is:
- Gravity oil interceptor or API separator to remove free oil above roughly 150 µm. Recovers the bulk of the value at almost no operating cost.
- Balance tank, 8–12 h HRT, mixed, to even out the pH swing between acidulation batches and washing.
- pH correction to 6.0–8.0 on the balance tank outlet. Caustic demand is significant; budget for it properly.
- Coagulation and flocculation, then flotation.
Where the stream is a stable emulsion — common after high-shear pumping or where surfactant is present from cleaning — acid splitting at pH 2.5–3.5 with sulphuric acid before re-neutralisation breaks the emulsion far more cheaply than coagulant alone. This is standard practice on soapstock liquor and is often worth applying to the combined stream too.
Chemical regime
| Chemical | Typical dose | Purpose |
|---|---|---|
| Sulphuric acid (splitting) | To pH 2.5 – 3.5 | Breaks soap emulsion, liberates free fatty acid |
| Caustic soda / lime | To pH 6.0 – 8.0 | Re-neutralisation into coagulation window |
| Polyaluminium chloride | 150 – 600 mg/L | Charge neutralisation of residual emulsion |
| Ferric chloride (alternative) | 200 – 700 mg/L | Where phosphorus from degumming is also targeted |
| Anionic polyacrylamide | 2 – 5 mg/L | Floc bridging; oil-rich floc needs more polymer than most streams |
Worked sizing example: 100 t/day refinery
- Effluent volume. 100 t/day crude oil at 1.5 m³ effluent per tonne = 150 m³/day. Balanced over 20 h: design flow 7.5 m³/h, peak 12 m³/h.
- Load after interception. FOG reduced from 2,500 to 900 mg/L by the interceptor; TSS 600 mg/L. Solids plus oil to the DAF: 7.5 × (0.900 + 0.600) = 11.3 kg/h.
- Flotation area. At 6 m/h (conservative, because oil-laden floc is fragile): 7.5 ÷ 6 = 1.25 m². Select 1.5 m².
- Air demand. A/S of 0.05 kg air/kg solids — high, because oily floc needs generous air: 11.3 × 0.05 = 0.57 kg air/h.
- Recycle flow. At 5.5 bar and 35 °C, usable dissolved air ≈ 92 mg/L. Recycle = 0.57 ÷ 0.092 = 6.2 m³/h — a recycle ratio of 83 %. High recycle ratios are normal on oily streams; do not let a supplier quote you 15 % here.
- Float recovery. Roughly 11 kg/h at 8–15 % dry solids, of which 15–40 % is recoverable oil. At 250 operating days this is 25–30 tonnes/year of acid oil — a real revenue line at biodiesel feedstock prices.
- Expected effluent. FOG 20–70 mg/L, TSS 40–90 mg/L, COD 900–3,000 mg/L. A biological stage is normally required for COD consent.
Materials of construction
Acid splitting, sulphate-rich liquor and low pH excursions make this an aggressive service. SS304 is the practical minimum; SS316L is the correct answer where acid splitting is practised in the same stream. Carbon steel with epoxy will fail at the liquid line and around the acid dosing point within two to four years, whatever the lining datasheet says, because the lining is only as good as the site’s ability to maintain it.
Specify a heavy-duty skimmer with variable speed and a heated or steam-traced sludge line: recovered oil-rich float at ambient African night temperatures still sets hard enough in an unheated line to block it.
Frequently asked questions
How much oil can be recovered from edible oil refinery effluent?
Between the gravity interceptor and the DAF float, a well-run plant recovers the equivalent of 0.3–1.0 % of throughput as acid oil or soapstock. For a 100 t/day refinery that is 75–250 tonnes per year. Value depends on free fatty acid content and the local biodiesel or soap market, but it is frequently enough to pay for the effluent plant’s chemicals and power.
Why is the recycle ratio so high on oily streams?
Because the air-to-solids ratio required to float oily floc is 0.04–0.06 kg air per kg solids, roughly three to ten times what a municipal stream needs. Air demand scales directly with A/S, and recycle flow scales directly with air demand. Recycle ratios of 50–100 % are normal on edible oil effluent. A supplier quoting 15–20 % has sized the unit on a municipal assumption and it will underperform.
Can DAF handle palm oil mill effluent (POME)?
Not on its own. Raw POME carries 40,000–90,000 mg/L COD and 4,000–10,000 mg/L oil and grease at 80–90 °C — two orders of magnitude beyond a flotation duty. POME requires oil recovery, cooling and anaerobic digestion first. A DAF is a sensible polishing stage after anaerobic treatment, where it removes carried-over biomass and residual oil before discharge or land application.
What pH should edible oil effluent be at the DAF inlet?
6.0 to 8.0, held steady. Polyaluminium chloride works across most of that window; below pH 5.5 aluminium hydrolysis is incomplete and coagulation is poor, above 8.5 the floc redisperses. Since the raw stream swings between pH 2.5 and pH 11, this is a control problem rather than a chemistry problem, and it is solved in the balance tank with automatic dosing on the outlet.