Keep oily and biological sludges apart, squeeze them differently, and send the solids to an incinerator with heat recovery. That’s the pragmatic blueprint emerging from case studies and regulatory realities.
Oil and gas facilities quietly generate a waste problem with industrial scale. Each large refinery can produce on the order of 30,000–40,000 tonnes of oily sludge per year (US EPA estimate; pmc.ncbi.nlm.nih.gov). Indonesian regulations treat oily sludge as hazardous B3 waste under the Environmental Ministry’s rules (e.g., Permen LHK 6/2021 revoking Kep.Paramen 128/2003 on petroleum‑contaminated waste; peraturan.bpk.go.id), mandating bonded storage and licensed transport.
The fix starts with discipline: keep two streams separate. Primary (oily) sludge comes off oil/water separators; secondary (biological) sludge emerges from secondary treatment (e.g., microbial clarifiers). Segregation matters because oily sludge is typically hazardous while biological sludge can be non‑B3 if it meets waste criteria.
The payoff from separation is operational, too. Oily sludge can be chemically de‑oiled and handled by high‑shear dewatering, while biological sludge can run through conventional thickening and presses without introducing oil back into clean circuits.
Segregated collection and tankage design
Primary oily sludge typically runs 5–15% solids and is rich in TPH (total petroleum hydrocarbons). In practice, it is collected by dedicated sumps and storage pits downstream of an API separator (a gravity oil‑water device) or DAF unit. Facilities often frontload this with screens, oil skimming, and primary settlers; for packaged options, systems under waste‑water physical separation cover screens, oil removal, and primary treatment.
Coarse debris control at intakes reduces downstream fouling; continuous removal is the job of an automatic screen when flows and loading fluctuate.
Dissolved‑air flotation (DAF) concentrates solids and oils before sludge storage; a compact system such as a DAF unit provides 95%+ suspended solids and oil removal with 1–3 hour detention.
Free oil recovered ahead of sludge handling is routed out of the waste line; refinery operators rely on oil removal systems to separate free oil to below typical discharge limits. Secondary sludge is pumped from clarifiers into separate thickeners or holding tanks, keeping it isolated from oil contact.
Dual dewatering trains and conditioning chemicals
Each stream gets its own dewatering path. For primary oily sludge loaded with fine clay and emulsified oil, a common scheme is (1) decant free water and oil (API separator or centrifuge), (2) add demulsifier/polymer and lime as needed, then (3) dewater. Accurate chemical feed — from demulsifiers to polymers — is enabled by a metering dosing pump.
For biological sludge, gravity thickeners or dissolved‑air flotation concentrate biomass before dewatering by decanters or presses. Conditioning chemistry is standard: flocculants to enhance particle aggregation and improve solids capture, with co‑conditioning as needed.
Collection design is explicit: two separate piping/trenching systems should be provided. Oily sludge drains (balancing tanks) are piped to an oily storage tank and then to its own sludge thickener; biological sludge from clarifiers goes to a separate sludge digester/thickener. This preserves demulsifier dosing control and prevents undesirable recycling of organics into the oil stream. Skids, tanks, and pumps are commonly grouped under waste‑water ancillaries for integration and maintenance access.
Centrifuge throughput, cake, and power profile
Centrifuges (decanter/scroll) run continuously and handle large flows, typically 19–44 L/s or 300–700 gpm per machine (nepis.epa.gov). One refinery case used a scroll centrifuge on oily sludge, raising solids from 11.6% in the feed to 33.6% in the cake (nepis.epa.gov).
In municipal tests, a solid‑bowl unit on a 50/50 mix of primary/secondary sludge typically achieved 15–21% cake solids (nepis.epa.gov), although industrial oily sludges with heavy particulates can yield higher. Polymer demand is modest in some studies (~4 g polymer/kg sludge), producing a pumpable slurry cake (nepis.epa.gov).
Power is the trade‑off: decanters often consume 10–15 kWh/ton‑dry (energy per tonne of dry solids), but they offer continuous operation and flexibility (thickening or dewatering mode). Drawbacks include limited cake dryness (often ≤30–40%), higher unit capital cost, and bearing maintenance.
Filter press cake dryness and conditioning
Filter presses (recessed‑plate, diaphragm, membrane) are batch workhorses that produce the driest cakes. In one industrial test, a high‑pressure recessed plate press reached 36% cake solids with lime and ferric chloride conditioning (nepis.epa.gov); a diaphragm press reached 40% on the same basis. Conditioning chemicals and feed control are standard practice for operators, including metal salts and alkaline aids under coagulants.
On the same sludge, a belt press delivered only 20–30% cake solids (nepis.epa.gov). Separately, a tertiary filter‑press run (2.4% feed solids) obtained about 41% cake solids with lime dosing (nepis.epa.gov).
Presses can achieve very low moisture cakes (50–60% dry) with near‑complete solids capture (>99%). Throughput is constrained by batch cycles (multiple presses can be staged). Energy is relatively low (pumps and hydraulics), but operating costs are driven by reagents and cloth washing.
Belt filter presses: polymer demand and energy
Across trials, belt presses often produce 18–30% cake solids (nepis.epa.gov; nepis.epa.gov). A review found properly operated belts yield 35–45% for easily dewatered sludges (e.g., calcium carbonate type) but only 20–30% for more difficult sludges (nepis.epa.gov).
Belts often require higher polymer doses than centrifuges (e.g., 6 vs. 4 g/kg in one study; nepis.epa.gov) and deliver lower throughput. Their advantages are lower capital cost and lower energy use (~1–3 kWh/ton), but they can clog on sticky oily sludges and yield lower solids recovery.
Performance summary and selection trade‑offs
As a rule, filter presses give the driest cakes — often 35–50% solids (nepis.epa.gov; nepis.epa.gov). Belt presses are moderate (20–30% solids; nepis.epa.gov). Centrifuges are variable (15–45% solids), with one oily sludge case around 33% (nepis.epa.gov).
In one side‑by‑side, a belt press versus a centrifuge on the same sludge yielded 18% vs. ~X% cake (both 95% solids recovery), with the belt needing about 50% more polymer (6 vs. 4 g/kg; nepis.epa.gov). Large continuous flows may favor decanters, while high dryness needs and smaller batches favor filter presses. For oily sludge, attention to polymer type and dose — and, where needed, high‑pressure wall scrubbers (membrane presses) — maximizes oil expulsion.
Incineration, volume reduction, and heat recovery
Once dewatered, oily sludge solids may be incinerated to destroy organics, recover energy, and cut final disposals. Incineration yields >95% reduction of wet volume, while dry solids mass typically falls by ~40–75% (nepis.epa.gov).
The calorific value is material: recent refinery sludge tests show ~3,100 kcal/kg (~13 MJ/kg; mdpi.com). One example calculated that 4,000 t of refinery sludge (wet) contained ~12,400 Gcal (~14 GWh) of energy (mdpi.com). Moisture lowers net heat; incinerators typically use auxiliary fuel to evaporate water but can generate steam or electricity from recovered heat.
Heat recovery designs have “significantly increased the cost‑effectiveness” of sludge incineration (nepis.epa.gov). In a circular‑economy context, incineration defrays energy costs and shrinks landfill loads, provided flue trains include robust emission controls for NOx, SOx, dioxins, and metals in line with regulations.
Permitting and integrated energy use
For Indonesian operations, any sludge incinerator must comply with ambient air standards (Lim. Emisi Permen LHK) and obtain B3 waste incinerator permits. Proper design with afterburners and scrubbers is critical to meet these limits.
In summary, incineration offers >95% wet volume reduction (nepis.epa.gov) and leverages the sludge’s tens of MJ/kg energy (mdpi.com; nepis.epa.gov), but at the cost of capital and strict pollution controls. A holistic plan therefore routes recovered heat/steam back to the plant to improve economics, as indicated by the reviewed literature (mdpi.com; nepis.epa.gov).
Sources and regulatory notes
Figures and case data are drawn from EPA technical references and industry reviews (nepis.epa.gov; nepis.epa.gov; nepis.epa.gov; nepis.epa.gov; nepis.epa.gov; mdpi.com; nepis.epa.gov). Regulatory classification of oily sludge as hazardous B3 waste follows the Environmental Ministry’s framework (peraturan.bpk.go.id), while production scale is contextualized by the US EPA estimate (pmc.ncbi.nlm.nih.gov).
