Palm Oil Mill Effluent (POME) is 95–96% water yet produced at massive scale; turning its sludge into compostable, nutrient-rich cake is cutting volumes and fertilizer bills. Case studies show the value of recovered sludge can offset recovery costs.
Industry: Palm_Oil | Process: Palm_Oil_Mill_Effluent_(POME)_Treatment
Processing 1 tonne of fresh fruit bunches (FFB) yields roughly 0.65 tonne of POME (barformula.com). POME is ≈95–96% water with only 4–5% total solids (organic matter) (researchgate.net) (researchgate.net). With global palm production in the tens of millions of tonnes per year, that translates into huge annual POME volumes and correspondingly large masses of sludge—on the order of ~50–65 kg dry solids per tonne FFB—accumulating in treatment ponds.
Regulation is the push. Indonesian standards (Permen LH 51/1995) require final POME effluent to meet BOD≤250 mg/L, COD≤500 mg/L, and TSS≤300 mg/L (where BOD/COD are measures of oxygen demand from organics and TSS is total suspended solids) (saka.co.id). Achieving these limits inevitably involves removing solids. Sludge builds up; dredging ponds is laborious and costly. Modern mills are switching to mechanical dewatering to shrink volumes and reclaim nutrients.
Mechanical dewatering objectives and options
The aim is simple: separate a clarified liquid and a solid “cake.” The cake has far less water, so it’s easier to handle or reuse; the liquid can be recycled or polished. Mechanical dewatering reduces transport and disposal costs and is central to modern sludge treatment.
Two workhorses dominate: screw presses and decanting centrifuges. There are also belt presses, multi-disc presses and geotextile tubes. Choice depends on scale, dryness targets and budget.
Screw press performance and costs
A screw press forces sludge against a perforated screen; water drains as solids compact. Typical slurry feeds are 5–10% solids, and presses can raise the cake to ~20–30% solids (dry matter, or “DM,” refers to the solids content; “wet basis” states percentage of solids by weight in the wet cake) (mdpi.com). In practice, ~20% DM is common; ~30% is near the upper limit (mdpi.com), with performance shaped by feed consistency, polymer flocculant dosing, and screen aperture (mdpi.com).
Energy draw is relatively low (~0.4–1.2 kWh per tonne of feed) (mdpi.com). Capital cost for a farm‑scale press is roughly €15k–65k (mdpi.com) with operating costs around £0.44 per tonne of slurry treated (mdpi.com). Maintenance is modest—few moving parts (mdpi.com). They’re less efficient at capturing fine colloids or dissolved nutrients but produce a stackable, transportable cake once solids exceed ~15% (mdpi.com).
Decanting centrifuge capture and dryness
A decanting centrifuge spins sludge at high speed so heavier solids compact on the bowl wall and discharge continuously. These units handle feeds up to ~15% solids without clogging and often deliver dryer, more nutrient‑concentrated cake than screw presses.
The trade‑off is power and price: ~2.2–5.1 kWh/tonne of feed (mdpi.com), capital roughly £50k–250k (mdpi.com), and operating cost about £2.21/tonne of feed—≈×5 the cost of screw‑pressing (mdpi.com). Capture of suspended solids is higher (often 60–90% vs 30–34% for screw presses) (mdpi.com).
Geotextile tubes and other systems
Belt or multi‑disc presses show up in some mills. A newer twist is geotextile tube (Geotube) dewatering: pump POME sludge into long, porous tubes laid on pads; water drains, leaving a consolidated “mulch.” A Malaysian case showed geotubes effectively dewater POME sludge without polymers, eliminating costly pond dredging, and yielding a dry, friable cake reusable as fertilizer (solmax.com). The operator reported “major cost savings” and that the “value of the recovered sludge offsets the recovery cost” (solmax.com).
Benchmark outputs and selection criteria
Typical outputs vary. Screw presses deliver ~18–28% solids (wet basis) from raw POME sludge (mdpi.com). Centrifuges often hit 25–35% (wet basis), and up to ~40% in some designs. Given raw POME sludge is ~4–5% solids (researchgate.net), dewatering can reduce sludge volume roughly 3–8‑fold.
Removal efficiency typically sits at 30–34% of feed solids to cake for screw presses and 60–90% for centrifuges (mdpi.com). Energy use is ~0.5–1.5 kWh/t for screw presses vs 2–5 kWh/t for centrifuges (mdpi.com). Capital costs run ~€15k–65k (screw) vs €60k–250k (centrifuge), and OPEX ~£0.4–0.5/t (screw) vs £2.2/t (centrifuge) (mdpi.com) (mdpi.com). Throughput spans hundreds to thousands of cubic meters per day per machine; sizing follows sludge production rate.
Scale and goals guide selection. For small/medium mills or limited capital, screw presses often offer best cost‑effectiveness (mdpi.com). Larger mills seeking maximum dryness or nutrient capture may justify centrifuges. Polymer flocculants (e.g., cationic polymers) are commonly added before either machine to aggregate fine solids and increase cake dryness; with flocculated POME sludge, centrifuges often approach the higher end of solids yield (close to ~30% or more) (mdpi.com). Geotube or filter presses suit sites with ample land and a desire for polymer‑free operation.
Nutrient value and safety profile
Dewatered POME sludge is a valuable biofertilizer. Analyses show treated POME sludge can contain ~2–3% nitrogen and ~1–2% potassium, plus appreciable P, Ca, Mg, Fe, etc. Heavy metals are generally very low, well under phytotoxic or food‑accumulation limits. Khairuddin et al. (2016) found POME sludge met WHO/FAO heavy‑metal standards and had “high amounts of nutrients such as Fe, K, Ca, Mg and P.” (researchgate.net). Another study noted that after composting or treatment, POME sludge poses minimal risk of toxic metals and can safely be used as fertilizer (researchgate.net).
Soils and crops respond. Co‑composting POME sludge with empty fruit bunch and decanter cake produced compost that increased oil palm soil fertility and yields; plot yields rose from ~23.2 to 25.5 t FFB/ha·yr with compost application (versus 25.5 t with inorganic NPK) (jopr.mpob.gov.my). Composted palm wastes significantly raised soil pH, organic C, total N, available P and exchangeable K/Ca/Mg (jopr.mpob.gov.my). In greenhouse trials, plants given POME sludge or precipitate often grew as well or better than on chemical NPK (researchgate.net). Studies also note that nutrient availability (especially N as ammonium/nitrate) increases during anaerobic treatment, making the liquid a good quick‑release N fertilizer.
Composting parameters and field application
Best practice is to compost or mature the dewatered sludge before land application. Raw POME solids remain biologically active (high BOD/COD). Mixing dewatered cake with carbon‑rich residues (empty fruit bunches, wood chips, biomass) balances C/N and improves aeration. Effective compost piles reach C/N ≈ 25–30; Indonesian organic fertilizer standards (Permen Pertanian No.70/2011) require mature compost (C/N < 25, pathogens absent, heavy metals under limits) (researchgate.net).
On plantations, POME‑cake compost is often applied in furrow or broadcast near palms. Typical loading rates are on the order of 5–10 tonnes dry cake/ha per application (adjusted by nutrient content), sometimes supplemented by mineral NPK as needed. Because tropical soils can be nutrient‑poor, adding organic sludge can significantly restore fertility. The exact rate should be based on nutrient analysis and maturity; agronomic trials often blend POME compost with minimal commercial fertilizer.
Economics and payback evidence
Using sludge as fertilizer saves on disposal costs and chemical fertilizer purchases. Agustina and Zuhroh (2016) estimated the nutrient value in POME solids could cover 20–50% of a plantation’s fertilizer demand (depending on POME volume and efficiency). Cost avoidance can offset capital/operational costs of the dewatering system; a Malaysian case reported that producing compost from POME sludge generated a revenue stream: “the recovered sludge’s value offsets the cost of recovery” (solmax.com). Some mills even registered composting POME under carbon‑offset schemes, since composting avoids methane emissions from ponds.
Numbers help prove payback. According to Lyons et al., mechanical separation has a processing cost ~5× lower per tonne than centrifugation (mdpi.com). Recovered nutrients can be valued against chemical prices to show net gain. Many mills find sludge dewatering and reuse becomes profitable through savings on dredging, new ponds and fertiliser purchases (solmax.com) (jopr.mpob.gov.my).
Implementation and monitoring parameters
Operationally, mills regularly dredge pond sludge into a feed tank, then pump through the dewatering unit. Monitor feed solids—ideally >4–5%—since extremely dilute slurry yields poor performance. Optimize press pressure, polymer dose, and screen/scroll settings for maximum dryness. Ensure dewatering equipment discharge (filter cake) meets biosolid handling specifications.
For a typical Indonesian medium‑to‑large mill, installing a mechanical dewatering unit (screw press or centrifuge) after the anaerobic/aerobic ponds is strongly advised (ponds are a form of biological digestion). This immediately shrinks sludge volume—often by 70–80% if achieving ~20–30% solids in the cake (mdpi.com)—and produces a relatively dry cake that’s easier to handle.
Plan immediate reuse of dewatered solids: allocate space for composting (windrows or static piles), ideally co‑compost with palm fiber/chips to adjust C/N. Target mature compost (C/N≈20–25, moisture ≈40%); test nutrient content and moisture; then apply to palm fields as a soil amendment rather than dispose. This closes the nutrient loop and provides measurable yield benefits (jopr.mpob.gov.my) (researchgate.net).
Monitor and optimize: test effluent before and after dewatering to confirm clarification (polished effluent will have much reduced TSS and BOD). For the cake/compost, analyze N, P, K and heavy metals periodically; ensure compliance with Permen Pertanian No.70/2011. Adjust dewatering and composting parameters (screen size, retention time, bulking agents) to maximize dry solids recovery and nutrient retention.
The bottom line: by pairing mechanical dewatering with composting and on‑site reuse, mills align with Indonesian standards, cut environmental impact, and often improve the plantation’s economics—backed by field data and case studies (solmax.com) (jopr.mpob.gov.my).
