Wastewater from the oil clarification station dominates Palm Oil Mill Effluent (POME) by volume and pollution load — and dictates how mills design treatment, recovery, and compliance. Data show sky‑high oil and COD, and why most mills blend this stream with others before biological digestion.
Industry: Palm_Oil | Process: Crude_Oil_Clarification_&_Purification
In palm-oil mills, the oil purification line ends with a deceptively simple step: clarification at about 90 °C that strips out water and fine solids from crude palm oil (CPO). The catch is what’s left behind. The station’s discharge — a hot, brown, viscous slurry — is the single biggest component of Palm Oil Mill Effluent (POME), contributing roughly 60% of total effluent volume in many mills (www.researchgate.net) (www.mdpi.com).
The numbers underscore why it matters. Typical raw POME — dominated by clarification effluent — averages ~51,000 mg/L COD (chemical oxygen demand, an oxygen-equivalent measure of oxidizable pollutants), ~25,000 mg/L BOD₅ (biochemical oxygen demand over 5 days), and ~6,000 mg/L oil and grease (www.mdpi.com). Mills produce on the order of 2.5–3.5 tonnes of POME per tonne of CPO (www.mdpi.com), which puts the clarification station squarely in the crosshairs of environmental controls and recovery plans.
Clarification flows and mill integration
After high-pressure pressing, the liquid oil–water–fiber slurry passes through sand traps, clarifiers, and purifiers at ∼90 °C to remove residual mesocarp fiber and water. As CPO is skimmed from the top of the clarifier tank, the remaining discharge (water, oil droplets, fine fiber) is routed to POME storage; many mills first pass it through an oil trap or de‑oiling sump to recover additional floating oil before main treatment (www.mdpi.com).
From there the combined waste — clarification effluent + trap overflow + decanter laundry water — is pumped with sterilizer condensate and hydrocyclone (kernel separator) washings to anaerobic treatment ponds. Sterilizer condensates contribute about ~36% of total flow and hydrocyclone washings ~4%; clarification/purification contributes ~60% by volume (www.researchgate.net) (www.mdpi.com). The raw stream (often called “trap pond” or “acidification pond”) is very hot (60–80 °C) and acidic and proceeds through cooling, anaerobic, and aerobic ponds — or modern closed‑digester systems — before discharge or reuse (www.mdpi.com).
(www.researchgate.net) (www.mdpi.com) Figure 1. Palm‑oil mill wastewater flows. Clarification effluent (the largest source, ≈60% of POME volume (www.researchgate.net)) is mixed with sterilizer‑condensate (≈36%) and hydrocyclone wash (≈4%) into the POME treatment system. (Source: adapted from literature (www.researchgate.net) (www.mdpi.com)).
High‑strength wastewater characteristics
The water leaving the clarification station is a high‑strength wastewater: a brown, viscous slurry with entrained oil droplets, colloidal fibers, dissolved carbohydrates, proteins, and lipids. Typical analysis shows ~95–96% water (www.mdpi.com) (www.researchgate.net), total suspended solids (TSS, insoluble particles) ~20–50 g/L (2–5%) from fine mesocarp debris and kernel bits (www.mdpi.com) (www.mdpi.com), and oil & grease (FOG, fats/oils/grease) around 0.6–0.7% by volume, averaging about 6,000 mg/L; ranges span <0.1% to ~1% (~1,300–18,000 mg/L) even after primary oil recovery (www.mdpi.com).
Organic loads are extreme: typical raw POME (largely clarification effluent) shows COD ~15,000–100,000 mg/L (average ~50,000 mg/L) and BOD₅ ~10,000–44,000 mg/L (average ~25,000 mg/L) (www.mdpi.com). Nutrients include organic nitrogen (~0.5–0.8% typically) and inorganic N (NH₃) in the tens of mg/L, plus plant nutrients (K, Mg, etc.) and trace metals (www.researchgate.net). The stream is acidic (pH ~4–5) due to organic acids (www.researchgate.net).
In practice, raw clarification wastewater may clock COD ~40,000–60,000 mg/L and oil/grease ~4,000–8,000 mg/L; its dark color and odor reflect carotene and phenolics (www.mdpi.com). Reviews confirm POME’s acidity and high load of organics and FOG (www.researchgate.net) (www.mdpi.com). Clarifier outflow is one of the strongest streams in the mill — often the highest FOG load and the second‑highest pollutant concentration after decanter solid waste (www.climate-policy-watcher.org) (www.mdpi.com).
Combining with other mill streams
Clarifier wastewater is not treated in isolation. A sump or pump pit collects clarification effluent, decanter filtrate, hydrocyclone wash, and sterilizer condensate; the raw POME is conveyed to the first pond or anaerobic digester. The first pond (acidification/oil‑trap) cools the hot effluent and captures free oil; many mills include a dedicated de‑oiling tank or oil separator, then route the bulk liquid through sequential anaerobic and aerobic treatment (www.mdpi.com). In enclosed zero‑discharge systems, the mixed POME instead feeds anaerobic digesters for biogas recovery followed by polishing.
By volume, about 60% of total POME enters as clarification effluent (www.researchgate.net). For a 30 t/h FFB mill (2.5 t CPO/h), total water use is ~15–20 m³/h, of which ~8–12 m³/h becomes wastewater; ~5–7 m³/h would be clarifier effluent and hydrocyclone water, with the rest from condensate. Typical mills report total POME on the order of 3–5 m³ per tonne of FFB (www.mdpi.com) (www.mdpi.com). All streams are mixed before treatment; no separate “clarifier‑only” pond is used in practice.
Pre‑separation and flotation performance
Because this stream drives most of the treatment requirements, operators minimize oil carry‑over and suspended solids before ponding. Typical management steps include sand traps, hydrocyclones, belt‑press filters, or dissolved‑air flotation (DAF) to remove coarse solids and coalesce oil droplets. Suppliers position compact units like DAF systems for precisely this duty, where 2–3 atm air injection can remove 80–90% of TSS and 50–70% of oil/fat in trials (www.researchgate.net) (www.researchgate.net).
Optimized flotation with “polyalum” flocculants has cut COD by ~30–35% and oil/grease by >50% from raw effluent in lab studies (www.researchgate.net). Coagulant aids such as coagulants and flocculants are routine in DAF optimization in this context. In one bench trial, lengthening contact time from 15 to 60 minutes raised COD removal from ~23% to ~34% (www.researchgate.net). Oil traps and separators upstream reduce scum formation in ponds; equipment families for separating free oil include industrial oil‑removal systems.
Ponding, digestion, and biogas recovery
In Malaysia and Indonesia, ponding (open lagoons) still predominates — more than 80% of mills — because of low capital costs (www.mdpi.com). In the anaerobic series (acidogenic/acetogenic to methanogenic), bacteria break down fats and sugars; roughly 95+% of the oxygen demand in mill effluent can be removed anaerobically into methane when the process is optimized (www.climate-policy-watcher.org). Biogas from POME is typically 55–70% CH₄ (methane) (www.researchgate.net) and is often captured for boiler or power use in larger mills.
Residual oil/grease tends to float and is skimmed; facultative/aeration ponds oxidize remaining organics. Final effluent discharges are usually BOD ~20–50 mg/L after ~90–120 days total retention (www.mdpi.com) (www.mdpi.com). Modernized lines may swap open ponds for enclosed digesters with downstream polishing; turnkey biological digestion systems mirror the same anaerobic‑then‑aerobic sequence.
Operating constraints: temperature, pH, nutrients
Clarification water enters POME at ~60–80 °C. The first pond must accept the heat or mills use heat exchange; stable methanogenesis needs ~30–35 °C, so cooling in the acidification pond is typical (www.mdpi.com). The low pH (~4) in raw POME means acidification ponds are needed; buffering (including from recycled digested material) or neutralization is addressed before discharge. Clarifier waste adds nutrients (N, P, K), which can aid microbes; testing also checks trace metals to ensure compliance (e.g., Cu, Fe, oil/grease limits) (www.researchgate.net).
Regulatory limits and performance metrics
Malaysian law (Environmental Quality Act 1974) limits final POME discharge to COD ≤100 mg/L, BOD ≤50 mg/L, TSS ≤400 mg/L, and oil/grease ≤50 mg/L (www.mdpi.com). Indonesia has tightened standards (recent regulations target COD ≤350 mg/L for treated POME, with stricter targets phased in) (www.researchgate.net). Achieving these low limits requires essentially complete biodegradation of the organic load — which is why pre‑separation and robust biological trains are central. If needed, secondary polishing (e.g., constructed wetlands, aerobic polishing) is added.
The business outcomes are tangible. Removing 80–90% of TSS and 50–70% of oil by flotation has been shown to cut COD by ~30–35% and can lower required hydraulic retention time by 20–30%, saving land and capital; bench work also shows COD removal improving from ~23% to ~34% as DAF contact time is raised from 15 to 60 minutes (www.researchgate.net) (www.researchgate.net) (www.researchgate.net). Capturing residual oil early not only eases pollution but also adds revenue; mills report that every 1 t/d of oil recovered early translates into saving about $300–$500 per month in avoided treatment costs (via lower COD load).
Summary: the clarifier’s outsized role
The clarification station’s wastewater is a high‑strength, oil‑rich stream — roughly two‑thirds of a mill’s POME by volume — and carries much of the pollutant load. Its management is integrated: preliminary oil traps and solids removal, blending with other mill streams, then anaerobic–aerobic treatment. The analytical profile — COD in the tens of thousands mg/L and oil/grease in grams per liter (www.mdpi.com) (www.researchgate.net) — mandates aggressive pre‑separation and tuned biology. In practice, that means maximizing oil recovery at source, leveraging flotation with aids like coagulants, and optimizing bioreactor conditions to meet tightening discharge limits — and to recover energy along the way. The core flows, compositions, and treatment outcomes summarized here are reported across peer‑reviewed reviews and technical reports (www.mdpi.com) (www.mdpi.com) (www.researchgate.net) (www.mdpi.com).
