Refineries are under pressure to shrink, dewater, and valorize oily sludge — and the data show oil recovery and energy generation are both on the table. An integrated plan can cut disposal volumes by >90%, reclaim hydrocarbons, and feed steam networks, while meeting strict B3 (hazardous waste) rules.
Refinery operations generate large volumes of oily sludge — a stubborn emulsion of oil, water, and solids. The U.S. EPA formerly estimated ~30,000 t/year of sludge per refinery (pmc.ncbi.nlm.nih.gov). In Indonesia alone, with ~860,000 bpd (barrels per day) crude output (c. 340 Mt/yr), on the order of 51,000 m^3 of oil‑impacted sludge is produced annually (www.researchgate.net).
This sludge is classified as hazardous (B3) waste under Indonesian law, mandating recycling or safe disposal (www.hhp.co.id) (peraturan.bpk.go.id). It typically carries >30% oil by weight plus resins and toxic compounds such as heavy metals and cyanides (www.eeer.org) (pmc.ncbi.nlm.nih.gov). Unmanaged, it causes corrosion, loss of tank capacity, and serious environmental harm when landfilled or leaked (www.eeer.org) (pmc.ncbi.nlm.nih.gov).
Regulatory pressure is rising. Indonesia’s MOEF Reg. 9/2024 on B3 waste emphasizes waste minimization and recovery (www.hhp.co.id). A holistic plan therefore must reduce sludge volume at source, dewater and recover oil, and treat remaining solids or recover energy via thermal processes. The sections below summarize the technologies and results that underpin such a plan, with data and examples drawn from published work.
Policy drivers and plant programs
Indonesia’s rules (Permen LHK 6/2021, Permen LHK 9/2024) require oil and gas companies to minimize sludge generation and recover materials wherever feasible (www.hhp.co.id). One response is PT Pertamina’s Sludge Oil Recovery (SOR) program at Plaju (RU III), which stipulates sludge must contain >20% oil to be processed for recycling; sludges failing this criterion go to licensed incineration or disposal (www.researchgate.net).
Process modifications can curb sludge at the source. Pertamina RU VI Balongan added heated transfer lines and “box heater” tanks, cutting sludge generation by 790 t/yr in 2022 at a cost of Rp2.715 billion (library.universitaspertamina.ac.id). That demonstrates predictable reduction per investment and supports compliance with waste and pollution budgets.
Sludge composition and energy value
Refinery sludges vary by unit. Tank-bottom and API (American Petroleum Institute) separator sludges typically contain 20–95% water, 5–70% oil, and 5–10% solids (wax, metals, catalysts) (www.researchgate.net). FT‑IR (Fourier‑transform infrared) and ultimate analyses from one Indonesian refinery sludge (RU VI Balongan ARHDM unit) reported ~28.3% moisture, 71.1% volatile (mostly hydrocarbons), negligible ash (0.53% fixed carbon), and ~86% C / 13.4% H by weight (www.researchgate.net) (www.researchgate.net).
Globally, sludges often contain ~30–50% oil by dry weight (www.eeer.org) (pmc.ncbi.nlm.nih.gov). Their energy content is ~12–13 MJ/kg (megajoules per kilogram), similar to light fuel oils, making energy recovery attractive (www.mdpi.com). Heavy metals (Zn, Fe, Cr, Ni, Pb, etc.) and toxic organics are typically concentrated in the residue, requiring controlled handling (www.eeer.org) (pmc.ncbi.nlm.nih.gov).
Primary separation and conditioning
Before dewatering, many plants employ primary physical separation (screens, skimming, gravity steps) to remove debris and free oil (pmc.ncbi.nlm.nih.gov). For this front end, integrated packages are available for primary treatment; one example is primary physical separation systems.
Chemical conditioning is crucial to improve dewaterability, using coagulants and flocculants to enhance particle capture (www.eeer.org). Plants commonly dose polymer with precision; a dosing pump helps maintain stable feed chemistry.
Gravity settling is a frequent first step prior to mechanical dewatering; a clarifier can provide the residence time needed to thicken sludges before the high‑shear stages.
Dewatering trains and performance
Centrifuges (solid‑bowl decanters or scroll/disc separators) are widely used. In a refinery test, a scroll centrifuge thickened an 11.6% solids feed into a 33.6% solids cake, an ~80% gain in solids concentration (nepis.epa.gov). A related evaluation found a scroll centrifuge typically removed ~30% of suspended solids and a high‑speed disc separator removed ~91% of oil (nepis.epa.gov). Mechanical centrifugation yields oil recovery rates on the order of 40–70% (arabjchem.org). It is fast and continuous but energy‑intensive and sensitive to sludge viscosity. High‑speed disc centrifuges can produce relatively dry solids (30–50% solids), but require frequent maintenance when contents are abrasive or waxy (nepis.epa.gov). Operating costs have been cited around $100–300 (presumably per ton of sludge) (arabjchem.org).
Belt filter presses use gravity drainage and squeeze rolls. Typical belt‑press cake concentrations are moderate: ~15% solids from dilute activated sludge and ~28% from primary sludge (www.climate-policy-watcher.org). A design example showed a carrier capacity ~272 kg/m²·h yielding ~22% solids cake with 96% solids capture (www.climate-policy-watcher.org).
Plate/frame filter presses operate in batch mode and deliver the driest cakes: 40–60% solids, up to 65% if heated. They are slower and more capital‑intensive but can dramatically shrink disposal volume.
Acid addition can improve dewaterability by charge neutralization (www.eeer.org). Novel methods (electro‑dewatering, microwave drying) also show promise, though refinery‑scale data are limited. In practice, combinations are used: chemical conditioning plus centrifuge, or two‑stage centrifugation.
As a systems approach, a dewatering train runs gravity/settling to flocculation to centrifugation or belt filtration, then a press — typically achieving 20–35% solids in the residual cake (nepis.epa.gov) (www.climate-policy-watcher.org). For add‑on dryness and handling benefits, many facilities consider packaged sludge treatment steps within the train.
Where coagulant and flocculant make the difference, fit‑for‑purpose chemistries are used; examples include coagulants and flocculants tailored for oily matrices.
Hydrocarbon recovery methods
Mechanical separation doubles as oil recovery. In one EPA pilot, a disc centrifuge fractionated API separator sludge with ~91% oil recovery (nepis.epa.gov). Field practice uses heated decanters; research data show up to ~70% oil recovery by centrifuge alone (arabjchem.org).
Solvent extraction uses organics such as heptane, toluene, or MEK to dissolve oil, with lab reports of >85% oil recovery under optimized conditions (pmc.ncbi.nlm.nih.gov). VOC losses and solvent recycling are the trade‑offs.
Surfactant approaches — including bio‑surfactants — break emulsions. Combined ultrasonic‑surfactant treatment achieved 82–90% oil extraction in Shanghai University work (pmc.ncbi.nlm.nih.gov). Field “EOR” (enhanced oil recovery) style surfactant flooding of tank bottoms has recovered ~80% of hydrocarbons (pmc.ncbi.nlm.nih.gov). For chemical breaking of emulsions in such trains, a refinery may specify a demulsifier.
Flotation adds uplift to the separation step. Plants implementing flotation often consider dissolved air designs; a DAF unit is one embodiment of this approach.
Thermal methods such as heating to 200–300 °C release water and mobilize oil; continuous systems can vaporize light ends. Microwave‑assisted pyrolysis delivered an oil with calorific value ~44.4 MJ/kg and a char ~16.7 MJ/kg in recent studies (pmc.ncbi.nlm.nih.gov). Freeze–thaw cycles, especially with waxy sludges, have reported ~50–60% oil recovery in cold climates (pmc.ncbi.nlm.nih.gov).
Biological approaches (landfarming or bioreactors) are less used due to toxicity and modest efficiency. Some co‑digest refinery sludge with municipal sludge to produce biogas, but high salinity and metals are challenges (www.mdpi.com).
Combined processes can approach near‑total oil recovery — for instance, ultrasound plus freeze–thaw plus centrifuge (pmc.ncbi.nlm.nih.gov). A standout data point: 30,206 bbl of crude were recovered from 32,786 bbl of tank‑bottom sludge (~92% recovery) via optimized fluid extraction (pmc.ncbi.nlm.nih.gov). In Indonesia, Pertamina RU VI’s process changes reduced sludge by 790 t in 2022 (library.universitaspertamina.ac.id), and SOR‑style programs recover multiple thousands of barrels per year of oil from refinery sludges, offsetting disposal costs and contributing feedstock (www.researchgate.net). Where free oil is part of the matrix, a dedicated oil-removal stage can be integrated upstream of polishing.
Recovered oil (typically light hydrocarbons and paraffins) is recycled as refinery feed or fuel under SOR program rules (www.researchgate.net).
Thermal conversion and final disposal
Incineration (sludge‑to‑energy) in controlled systems — often fluidized beds — destroys organics and recovers heat. An Iranian refinery study found 4,000 t of sludge (over 5 years) contained ~12,400 Gcal of energy (~14.4 GWh) (www.mdpi.com). Emission controls (scrubbers, ESPs) are mandatory given PAH/dioxin risks (www.eeer.org) (www.eeer.org). If tied into the steam network, incineration reduces external fuel use, but byproducts (ash, flue gas) remain under B3 control under MOEF 9/2024 (www.hhp.co.id).
Gasification (sub‑stoichiometric combustion) produces a syngas for boilers or turbines; pyrolysis produces oil and char. Catalytic or microwave‑assisted pyrolysis can boost oil yields (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In all thermal routes, residue ash (5–10% of input) holds metals and remains hazardous.
Stabilization/solidification (e.g., cement mixing) is a last resort, immobilizing contaminants but increasing volume by ~50%. Overall, incineration or pyrolysis is often preferred for final disposal, destroying organics and recovering energy (www.mdpi.com) (www.eeer.org). Roughly, incinerating 1 t of oily sludge (~13 MJ/kg) yields ~4 GJ of heat; at 30% boiler conversion, that is ~334 kWh of electricity. A medium refinery generating 10,000 t/yr could produce ~3–4 GWh/yr, though moisture and system losses lower actuals. In Indonesia, such facilities need MOEF licensing and often community liaison due to emissions concerns.
Performance metrics and economics
Oil recovery efficiency: field and lab reports indicate 40–90% recovery is achievable. Mechanical centrifugation typically recovers ~40–70% (arabjchem.org). Adding chemical demulsification and flotation steps can push overall recovery toward 80–90%. Advanced lab methods (ultrasound + surfactant) report ~82–90% (pmc.ncbi.nlm.nih.gov) or transient >90% in seconds (pmc.ncbi.nlm.nih.gov). In scale‑up, a combined process (centrifuge + solvent wash) might yield ~75%. Quantitatively, recovering 75% of oil reduces sludge mass roughly by the oil mass fraction (e.g., ~50% if the sludge is half oil).
Dewatering performance: typical trains reduce volume by 10x or more. Belt presses concentrate 2–3% feed to 20–30% cake (www.climate-policy-watcher.org). Centrifuges deliver similar cake solids (~30%). Example: a 100 t/day wet sludge stream at 3% solids yields 10–15 t/day of dewatered cake. Solids capture reaches 90–99% if flocculation is optimized.
Energy recovery: at ~13 MJ/kg, 1 t of sludge yields ~4 GJ of usable energy (steam), enough for ~300–400 kWh of electricity. A 10,000 t/yr sludge stream could supply ~3–4 GWh/yr.
Waste reduction: co‑processing in cement kilns often achieves >95% volume reduction (into clinker). In SOR programs, recovered oil (>20% of sludge) is returned, and the remaining “dried” sludge is <5% of original volume. A data point from PT Plaju’s SOR: complete utilization of recovered oil and a reduction of disposed sludge from several hundred tons/yr to near zero, with a reported 100% recovery rate of usable oil from targeted sludges (www.researchgate.net) (www.researchgate.net).
Cost and compliance: reducing sludge saves B3 tipping fees. The Balongan upgrade (Rp2.7 billion) deferred 790 t in one year (library.universitaspertamina.ac.id), implying a saving ~Rp3.4 million per ton (including avoided treatment and landfill). Installing a centrifuge system (capex ~$1–3 M) can pay back within a few years via recovered oil and avoided waste fees. A life‑cycle analysis in the literature suggests sludge incineration is economically favorable when energy yields and strict landfill limits are included (www.mdpi.com) (www.eeer.org).
Integrated sludge management plan
Source reduction: modify operations to minimize sludge formation (e.g., heat tracing of tanks, improved separation efficiency, reuse of intermediate streams). Pertamina’s RU VI example: ~Rp2.7 billion (~$0.2 M) to save 790 t sludge (library.universitaspertamina.ac.id).
Dewatering train: implement flocculation and a combination of centrifuge + belt press (or filters) to remove >90% of water. Target ≥25% solids in cake; polymer optimization is key. Where chemical aids are used, appropriate coagulants can be paired with flocculants for capture.
Oil recovery suite: route 20–30%‑solids cake to oil recovery. Use a heated decanter centrifuge to skim free oil/water; follow with solvent wash or drying to reclaim additional hydrocarbons. Maintain >20% oil cut for reintegration or blending as refinery fuel under SOR criteria (www.researchgate.net).
Reuse and recycling: return recovered oil to the refinery (blend into vacuum gas oil after desalting if needed) or fire as supplemental fuel. Where emulsions persist, a demulsifier step can be aligned with the recovery protocol.
Thermal conversion (sludge‑to‑energy): incinerate the residual dry cake in a controlled unit (fluidized bed or co‑processing in a cement kiln). Capture waste heat to produce steam/electricity. Emission controls and B3 byproduct management are mandatory (www.mdpi.com) (www.hhp.co.id).
Ash handling: test incinerator ash for metals; if B3, dispose in a secured hazardous landfill. Expect 5–10% of input mass as ash.
Monitoring and optimization: use continuous monitoring (e.g., online calorimetry/COD, chemical oxygen demand) to adjust moisture and age for maximum oil yield; deploy periodic lab analysis (e.g., bomb calorimeter) to update the energy balance (www.mdpi.com).
Bottom line and sources
Combining physical dewatering with chemical and thermal processes can turn a hazardous waste problem into recoverable products. Expect drastic reductions in disposed waste (>90% less volume sent offsite), oil recovery yields measured from tens to hundreds of barrels monthly (plant‑dependent), and meaningful energy generation. One study found a 5‑year sludge stockpile of 4,000 t had ~12,400 Gcal potential, equivalent to ~5 MW continuous (www.mdpi.com). The business case hinges on local waste and energy prices, but international experience and Indonesian regulations favor oil/energy recovery over landfilling, aligning with a circular‑economy approach: less B3 liability, more value recovered (pmc.ncbi.nlm.nih.gov) (www.mdpi.com) (www.eeer.org).
Sources cited include comprehensive reviews, case studies, and regulatory documents: (www.researchgate.net) (www.eeer.org) (www.mdpi.com) (www.researchgate.net) (www.climate-policy-watcher.org) in addition to specific performance and regulatory references linked throughout.
