Clarifier sludge in palm oil mills carries 4–10% oil by weight; one mill reported 0.36 t/hr of oil “lost” in sludge. Three‑phase decanter centrifuges capture that value while cleaning up the wastewater.
During clarification of press liquor, sludge from the clarifier underflow (CST underflow; clarifier tank underflow) typically contains 4–10% residual oil by weight (patents.google.com). In one case, a mill reported ~0.36 t/hr of oil “lost” in sludge (jurnalkelapasawit.iopri.org). The underflow emerges from the clarifier, and without further treatment much of that oil ends up in wastewater or ponds.
Traditionally, mills lean on static sludge separators, skimmer screens, or manual recovery ponds to thicken or skim sludge. Older 2‑phase sludge separators (disc centrifuges or screw presses) or filter/belt presses are sometimes used, but they require multiple units and leave higher oil in the waste water. As Sivasothy et al. note, clarifier sludge often contains ~4–10% oil, “the bulk of which can be recovered using [a] centrifuge,” otherwise it produces only “substantially de‑oiled sludge” (patents.google.com).
Three‑phase decanter centrifuge operation
A three‑phase decanter centrifuge (continuous machine that splits a slurry into oil, solids, and water using centrifugal force) handles clarifier underflow in one pass, producing separate oil, solids, and water streams. For example, a Westfalia “Topd” decanter is described processing clarifier underflow to yield a high‑oil phase, a dry solids cake, and nearly oil‑free water (patents.google.com).
One patent example reports a 3‑phase decanter producing “an oil‑phase, solids, and virtually oil‑free wastewater” from sludge (patents.google.com). Because three‑phase designs also integrate clarification and decanting in one unit—“separates the sludge into three phases” (oil, solids, water) in one step—there is no need for separate settling (patents.google.com), which saves space and process steps. In contexts where separate settling would otherwise be added, compact options such as a lamella settler are designed to reduce footprint, but the decanter approach removes that step entirely.
In practice, decanter cake remains ~78% moisture (www.researchgate.net) but with only ~12–14% oil on a dry basis (www.researchgate.net), reflecting that most oil is driven to the oil phase. Because the water stream is essentially oil‑free, downstream effluent treatment is simpler (far less oil to aeration ponds). That reduced oil load aligns with the function of oil removal systems when they are used downstream.
Comparative performance and equipment count
Decanters consistently outperform conventional sludge treatment in oil recovery and effluent quality. A mill relying on manual/pond recovery lost 0.36 t/h (~7.8 m³/day) of oil (jurnalkelapasawit.iopri.org)—a resource that decanting can recapture.
Compared to a standard 2‑phase sludge separator, a 3‑phase decanter delivered better waste quality and oil yield (jurnalkelapasawit.iopri.org). Nasution et al. (2022) report that one decanter unit plus support equipment (28 elements) can replace three conventional separators (3×) plus 22 support units, while handling the same throughput (jurnalkelapasawit.iopri.org).
Total power usage favors the decanter: three sludge separators draw ~3×(45–60 kW) whereas a single decanter draws ~22–55 kW (jurnalkelapasawit.iopri.org). Field trials show the decanter produces a cleaner effluent (lower oil in the discharge) than the sludge separators (jurnalkelapasawit.iopri.org), which lowers oil loss and reduces BOD in POME (palm oil mill effluent).
Capital cost and payback dynamics
The up‑front cost of a 3‑phase decanter is higher than a simple separator: about IDR 3.8–5.8 billion (~USD $0.26–0.40 million) per machine (jurnalkelapasawit.iopri.org). A basic sludge separator costs IDR 0.86–1.84 billion (~$0.06–0.13 M) (jurnalkelapasawit.iopri.org).
The return in recovered oil typically justifies the capital. Recovering even 1–2 t/day of extra oil translates to ~$0.7–1.4 million/year (at ~USD $800–900/t CPO), giving a payback of well under 1–2 years on a ~$300k–400k decanter. Nasution et al. reported ~7.8 m³/day lost oil for one mill (jurnalkelapasawit.iopri.org); recapturing most of that would yield ~$0.6–0.7 M per month at current palm oil prices. Operating costs are modest: with only a single ~50 kW motor instead of three ~50 kW units, power bills drop. Maintenance is more involved per decanter unit but simpler overall since one system replaces several.
In contrast, traditional ponds or filters incur hidden costs from oil waste and compliance. Capturing oil before it becomes POME also reduces treatment load—less effluent COD/BOD and fewer sludge‑disposal fees—streamlining downstream primary treatment. In short, while the decanter’s capex is ~3–5× higher than simpler separators (jurnalkelapasawit.iopri.org), its per‑unit cost of oil recovered is far lower. Companies estimate internal rates of return on decanter upgrades on the order of 25–50% based on oil saved and efficiency gains.
Bottom line for mill operators
Industry experience and research consistently find three‑phase decanters to be the most effective sludge‑treatment method (jurnalkelapasawit.iopri.org). They achieve higher oil recovery and lower effluent oil content than older separators or manual methods, at a reasonable total cost of ownership. For mills facing high oil prices and strict effluent standards, the investment is usually justified by extra oil recovered (often many tons per year) and reduced waste costs (jurnalkelapasawit.iopri.org) (jurnalkelapasawit.iopri.org).
Sources include Nasution et al. (2022) for comparative performance and cost data (jurnalkelapasawit.iopri.org) (jurnalkelapasawit.iopri.org) (jurnalkelapasawit.iopri.org); patents WO2015037980 and AU2015101376 describe decanter principles and quantify sludge oil content (patents.google.com); and a peer‑reviewed decanter cake analysis (Sahad et al., 2014) documents cake moisture (~78%) and oil content (~13% dry) (www.researchgate.net) (www.researchgate.net).
