POME oil, acid oil and soapstock are widely traded residue feedstocks for biodiesel, HVO and oleochemical use, and they are often confused. They come from different stages of the oil supply chain and are treated differently by fuel regulations. This article explains what each one is, how they compare, and how they are bought and sold. For the wider context, see our buyer's guide to waste and residue feedstocks.
Palm oil mill effluent (POME) and POME oil
Palm oil mills process fresh fruit bunches by sterilising them with steam, stripping and pressing the fruit, and clarifying the crude oil. These steps generate a large volume of wastewater, known as palm oil mill effluent, which is treated in ponds or anaerobic digesters. The effluent carries a small fraction of residual oil, which floats and can be skimmed from ponds and recovery tanks.
That recovered oil is traded as POME oil, sometimes also called sludge oil. It is dark, often high in free fatty acids (FFA) because it has been in contact with water and heat for long periods, and it carries more moisture and dirt than crude palm oil. Its fatty acid profile is close to palm oil, so its iodine value is typically around 50–55 g I2/100 g.
POME is listed in Annex IX Part A of the EU Renewable Energy Directive, which means fuels made from it can count towards the advanced-biofuel sub-target. That value creates a strong incentive to present crude palm oil as POME oil. Every mill can only generate a limited quantity of POME oil relative to its throughput, so auditors and buyers compare declared volumes with the mill's plausible capacity, and look for tell-tale signs such as FFA that is too low for a genuine effluent oil.
Soapstock: the by-product of chemical refining
Crude vegetable oils such as soybean, rapeseed, sunflower or cottonseed oil are often refined chemically. In the neutralisation step, caustic soda reacts with the free fatty acids to form sodium soaps, which are separated from the oil by centrifuge. That heavy phase is soapstock: a mixture of soaps, entrained neutral oil, phospholipids, water and other impurities.
Soapstock is wet, alkaline and unstable, so it is rarely shipped far in that form; it is usually converted into acid oil at or near the refinery, or used locally where permitted.
Acid oil: soapstock that has been split
Acid oil is made by acidulating soapstock, usually with sulphuric acid. The acid converts the soaps back into free fatty acids, and the oily layer is separated from the acidic water. The result is a dark, high-FFA oil that is traded by its source, for example soybean acid oil, rapeseed acid oil or sunflower acid oil, or as mixed soapstock acid oil.
Two features matter to buyers. First, FFA varies widely, because it depends on how much neutral oil was carried into the soapstock. Second, residual mineral acid and sulphate can raise sulphur and make the oil corrosive, which matters for biodiesel plants that must meet a 10 mg/kg sulphur limit in the finished fuel.
Acid oil should not be confused with fatty acid distillates such as PFAD, which come from the physical refining of palm oil. Both are high in FFA, but distillates are generally cleaner, lighter in colour and lower in MIU.
Typical ranges compared
The figures below are indicative only; they show common orders of magnitude, not a standard.
- POME oil: FFA often 20–50% or more; moisture and impurities often higher than in used cooking oil; iodine value around 50–55.
- Soapstock (wet): water content high and variable; fatty matter present largely as sodium soaps; usually traded locally on a fatty-matter basis.
- Acid oil: FFA typically 50–80%; total fatty matter usually 90–95% or more; MIU commonly up to 3–5%; sulphur ranging from tens to hundreds of mg/kg.
- PFAD, for comparison: FFA typically 70–90%; MIU usually 1–2% or less; iodine value around 45–60.
How these materials are traded
Residue feedstocks are usually sold against a basis specification, with adjustments or rejection above agreed limits. For acid oil, the basis is often total fatty matter (TFM) together with an MIU maximum; for POME oil, FFA and MIU are the central figures. Iodine value, sulphur, phosphorus and, for HVO buyers, chlorine and metals are added as the conversion route requires.
The regulatory status is part of the product. POME oil sold as an Annex IX Part A feedstock needs a proof of sustainability and a traceable chain back to the mill. Acid oils are not listed in Annex IX as such; their treatment as residues, processing residues or co-products varies between countries and schemes, so the same material can carry very different values in different markets. Ask for the certificate scope with your quotation, and confirm with your authority how the material will be classified. Our explainer on Annex IX Part A vs Part B covers the framework.
These materials move in flexitanks, ISO tanks and bulk parcels. Acid oils are corrosive, so confirm tank and flexitank compatibility, and plan heating for palm-based lots.
Where they are used
- Biodiesel: after acid esterification to reduce FFA, followed by transesterification.
- HVO and SAF: after pretreatment to remove phosphorus, metals, chlorine and other contaminants; see our page on feedstock pretreatment and refining.
- Oleochemicals: fatty acid splitting and distillation, soap making and metallic soaps.
- Animal feed: some acid oils and soapstocks are used as feed fats where regulations permit, subject to feed-safety controls.
What to check before buying
- The source oil and production route, including whether the material is acid oil, soapstock or a distillate.
- FFA, MIU, TFM, iodine value, sulphur and any contaminants your process is sensitive to, with named test methods.
- Tank, flexitank and gasket compatibility for acidic material.
- Sustainability documents, the classification your market will accept, and evidence of the originating mill or refinery.
- Sampling at loading and discharge, retained samples and a referee laboratory.
Frequently asked questions
What is the difference between soapstock and acid oil?
Soapstock is the wet, soapy by-product of neutralising crude vegetable oil with caustic soda. Acid oil is soapstock that has been split with sulphuric acid, turning the soaps back into free fatty acids. Acid oil is drier, more stable and easier to transport.
Is POME oil the same as crude palm oil?
No. POME oil is recovered from the wastewater of palm oil mills. It is higher in FFA, moisture and impurities than crude palm oil, and it has a different regulatory status. Genuine POME oil cannot be produced in unlimited volumes by any single mill.
Why can acid oil have high sulphur?
Acid oil is made with sulphuric acid. Residual acid and sulphate compounds can remain in the oil if washing and separation are incomplete, which raises the sulphur content.
Are acid oils eligible for renewable fuel incentives?
It depends on the market. Acid oils are not listed in Annex IX as such, and their classification differs between countries and certification schemes. Confirm the accepted category with your authority before contracting.