The clarification station’s oily, 90°C overflow dominates palm oil mill effluent. Mills corral it with oil traps, long-retention ponds, and biogas capture to hit strict discharge limits.
The “milky” mix that feeds a palm oil clarifier is more water than most operators might think. Studies show it carries roughly 35–45% oil and 45–55% water, and in practice that means about half the mass fed into the clarifier is water (www.mdpi.com). At around 90°C, floater oil is skimmed off, leaving a dense, brownish effluent — a mixed oil/water/solids stream — that doesn’t get treated alone. It’s sent downhill with everything else.
All wastewater streams — heavily polluted clarifier overflow, sterilizer condensate, and hydrocyclone wash water — are combined as raw palm oil mill effluent (POME) for common treatment (www.mdpi.com) (www.mdpi.com). The clarifier wastewater becomes part of the overall POME load, typically moving first through oil-trapping (primary separation) and then a sequence of acidification/cooling, anaerobic, and aerobic stages (www.mdpi.com).
Wastewater streams and first-stop ponds
Sharifah et al. report the oil from the de‑oiling tank is fed to an oil‑trapping pond, after which the combined effluent passes through acidification, cooling, anaerobic, and aerobic (facultative) ponds before final discharge (www.mdpi.com). In mill practice, the “mix and move” step is deliberate: any remaining free oil is intercepted up front, and only then do operators lean on biological stages for the heavy lifting.
Primary oil interception aligns with typical plant “front end” design — screens, skimmers, and oil capture — the kind of kit grouped under physical separation systems. Once free oil is trapped, the remaining oily water heads downstream with sterilizer condensate and kernel hydrocyclone effluent as raw POME (www.mdpi.com).
How much POME — and from where
Scale is everything. Malaysian mills generated roughly 42–58 million tonnes of POME in 2021 — about 2.5–3.5 tonnes of POME per tonne of crude palm oil (CPO) — with Indonesia, the world’s largest producer, generating several times more (on the order of 100 Mt/yr) (www.mdpi.com). Literature estimates say clarification wastewater accounts for around 60% of raw POME, sterilizer condensate about 36%, and hydrocyclone wash roughly 4% (www.researchgate.net).
Clarifier effluent characteristics and loads
The combined POME — and by extension the clarifier stream — is a dark brown, viscous slurry that’s 95–96% water (www.mdpi.com). Typical raw POME carries about 0.6–0.7% free oil by weight (6–7 g/L on average) and 2–4% total suspended solids (mostly fruit fibers) (www.mdpi.com). Chemical oxygen demand (COD, a measure of total oxidizable organics) ranges roughly from 15,000 to 100,000 mg/L, while biochemical oxygen demand (BOD, a measure of biologically degradable organics) runs about 10,000 to 43,750 mg/L (www.mdpi.com).
Oil and grease span from a few hundred up to about 18,000 mg/L (www.mdpi.com). Temperatures out of process routinely hit ≈80–90°C and need cooling, and pH tends to be acidic at roughly 3.5–5.0 (www.mdpi.com). Minor constituents give POME its color and reactivity: carotene around 8 mg/L, phenolics around 5,800 mg/L, and lignin around 4,700 mg/L are cited (www.mdpi.com).
No separate pretreatment, then biological digestion
Normal practice is no separate chemical pretreatment of the clarifier stream; it’s treated together as POME. Key steps are to intercept free oil with trap ponds or skimmers, combine effluents in sealed holding tanks, cool to below about 45°C, and then feed the anaerobic–aerobic pond train for biodegradation and biogas capture (www.mdpi.com) (www.mdpi.com). Primary oil removal is often paired with purpose-built skimming gear — in mill parlance, oil removal systems — before the biological stages shoulder the main load.
Cooling, mixing, and transfer are the unglamorous linchpins that keep the train stable; mills lean on practical wastewater ancillaries for these tasks while keeping the process at steady temperature and flow.
Pond train configuration and retention time
Typical sequences look like this: an acidification/cooling pond holds raw POME for roughly 5–7 days, often with recycled anaerobic effluent mixed in, and a cooling tower drops temperature into the 35–40°C range before digestion (www.mdpi.com). Anaerobic ponding (without oxygen) follows for a total of about 40–60 days, tackling the bulk of COD/BOD at long residence times (~54–60 days in research examples) (www.mdpi.com). Aerobic or facultative ponds (with oxygen) add roughly 20 days of polishing, and total hydraulic retention time (HRT, the time water spends in the system) lands around 100–120 days (www.mdpi.com).
In early ponds, the clarifier stream’s oil often concentrates at the surface; mills skim it and return it to oil processing (www.mdpi.com). The overall route is classic biological treatment — the kind commonly summarized as anaerobic and aerobic digestion systems — scaled to extreme organic loads.
Performance, energy, and integration
Modern pond trains can reduce organic loads dramatically: operators report >95% COD removal with suitable retention (www.mdpi.com). Combined anaerobic–aerobic systems in pilot studies reach >98% COD/BOD removal with effluent BOD below about 50 mg/L (www.mdpi.com).
Biogas (methane) from the anaerobic stage is typically captured and flared or used for power — now a standard sustainability play in Indonesia and Malaysia. A typical POME anaerobic pond can yield roughly 20–100 Nm³ biogas per tonne POME (depending on COD), translating to about 400–800 kWh of energy per tonne of CPO produced. One cited scenario: a 60 t/h mill in Malaysia could generate about 2–5 MW of power from POME anaerobic digestion.
Regulatory targets and the compliance gap
Because incoming loads are extreme (POME is often 100× stronger than municipal sewage in COD terms), discharge targets are tight. In Malaysia (DOE Act 1974), treated POME must achieve BOD below 100 mg/L, TSS below 400 mg/L, and Oil & Grease below 50 mg/L (www.mdpi.com). Malaysia sets no direct COD limit, implying full removal to meet BOD and O&G constraints.
Indonesian standards are similarly strict for palm effluent: BOD ≤100 mg/L, COD ≤350 mg/L, TSS ≤250 mg/L, Oil & Fat ≤25 mg/L (www.karbonaktif.org). That means the clarifier-heavy POME mix must be stripped by roughly 95–99% to comply, a gap that the long-retention biological train is designed to close.
Design implications and mill-scale math
High oil can trigger foaming and sludge issues, so mills emphasize oil skimming and maximize anaerobic residence time. Consider a 30 t/h mill: at about 1.5 m³ POME per tonne of fresh fruit bunch (FFB), it might generate roughly 80–100 m³ of POME per hour with COD around 60,000 mg/L — a load that demands serious treatment volume or advanced bioreactor capacity (www.mdpi.com) (www.mdpi.com).
In summary, clarifier wastewater is managed as part of POME: collected with sterilizer and hydrocyclone effluents, de‑oiled in trap ponds, cooled, and treated through staged anaerobic–aerobic ponds, with oil skimming and biogas capture along the way (www.mdpi.com). Given COD up to about 10^5 mg/L and oil & grease up to about 1–18×10^4 mg/L, long retention and consistent operation are non‑negotiable to meet BOD ~50–100 mg/L and Oil/Grease ~25–50 mg/L limits (www.mdpi.com) (www.karbonaktif.org).
Sources: peer‑reviewed reviews and regulatory summaries on POME — Sharifah Mohd. et al., Processes 9(5):739 (2021) (www.mdpi.com) (www.mdpi.com); Dominic & Baidurah, Biology 11(4):525 (2022) (www.mdpi.com) (www.mdpi.com); Indonesian effluent standards (www.karbonaktif.org); and composition/pond performance summaries (www.mdpi.com) (www.mdpi.com) (www.researchgate.net).
