Modern refineries now run mini treatment works to turn oily, toxic effluent into discharge‑compliant — and often reusable — water. The winning recipe: rugged primary oil removal, high‑rate biology, and membrane‑ready polishing.
Oil refining consumes water at industrial scale — one review put world oil/petrochemical refining at ≈3.99×10^7 m^3/day in 2020 (~16% of global water use) (www.sciencedirect.com). An estimated 80–90% of that ends up as contaminated wastewater (www.sciencedirect.com), forcing refineries to treat effluent to tight discharge limits — and increasingly, to reuse it.
The influent is no simple mix. Refinery wastewater carries a “complex mixture of hydrocarbons, sulphides, ammonia, oils, suspended and dissolved solids, and heavy metals” (pubmed.ncbi.nlm.nih.gov) with free and emulsified oil often in the tens to hundreds of mg/L and soluble BOD/COD (biochemical/chemical oxygen demand) in the hundreds to low thousands of mg/L. Regulations bite: Indonesia’s Permen LH 19/2010 sets oil‑and‑grease around 15–25 mg/L (facility‑dependent), alongside tight BOD/COD and ammonia caps (123dok.com). Generic industrial guidance often targets BOD <30–50 mg/L, COD <100–200 mg/L, oil & grease <5–20 mg/L, and NH₃–N <10–15 mg/L for discharge. In practice, modern plants aim for single‑digit mg/L for oil and BOD and just a few mg/L of ammonia to enable reuse.
With water scarcity rising, recycling has become a design driver: a Brazilian analysis showed effective water consumption near ≈0.9 m^3 per m^3 of oil processed (≈254,000 m^3/day for Brazil’s 2009 refinery output) (www.intechopen.com). The U.S. EPA notes that membrane bioreactors can produce “clarified and potentially reusable” water (pmc.ncbi.nlm.nih.gov).
Refinery effluent composition and standards
The contaminant suite drives unit selection. Typical designs are built to drop oil and grease from tens–hundreds of mg/L down to single digits, strip 75–95% of soluble organics, and oxidize ammonia to low single‑digit mg/L. Local “baku mutu” limits (e.g., Permen LH 19/2010 in Indonesia) anchor the spec (123dok.com), while global practice pegs effluent at BOD <30–50 mg/L, COD <100–200 mg/L, oil & grease <5–20 mg/L, and NH₃–N <10–15 mg/L for discharge.
Large refineries deploy centralized trains that can also facilitate high‑quality reuse across the site. One strategy in water‑scarce regions is zero liquid discharge (ZLD), which pushes beyond polishing into brine concentration and solids handling.
Primary oil–water separation (API/CPI and DAF)
The work starts with gravity separation. An API (American Petroleum Institute) separator removes free oil (Stokes’ law) and settles coarse solids; designs target oil droplets ≥150 μm and can remove 60–99% of free oil (www.watertechnologies.com). Corrugated‑plate interceptors (CPI) intensify coalescence in a tighter footprint — one analysis sized CPI at ~1/4 the volume of a conventional API for the same 1,000 m³/h flow (pt.scribd.com).
Refinery trains typically then go to Dissolved Air Flotation (DAF) to lift out emulsified and fine oil. With coagulant/flocculant dosing, DAF removes an additional ~70–85% of residual oil while capturing much of the suspended solids (50–80%) (www.watertechnologies.com). In full‑scale studies, API+DAF removed about 90% of oil & grease and 90% of TSS, with ~75% COD reduction across pretreatment alone (pmc.ncbi.nlm.nih.gov). Skipping pretreatment forces bioreactors to face high oil, risking toxicity and poor settling.
Design details matter: laminar hydraulics, several minutes of residence time, and routine skimming for both oil and sludge. Primary outlets often hit <50 mg/L oil; DAF can push this to <5–10 mg/L, and total suspended solids (TSS) to ~<100 mg/L after pretreatment (pmc.ncbi.nlm.nih.gov, www.watertechnologies.com). Energy use is modest: combined API+DAF clocked ~0.091 kWh/m³, roughly 10× lower than membrane bioreactors in one analysis (~0.86 kWh/m³) and ~6× cheaper on an NPV basis (pmc.ncbi.nlm.nih.gov).
Equipment choices follow that logic: coarse separation and skimming systems align with primary physical separation, free‑oil capture maps to oil removal hardware, and emulsified oil treatment typically relies on a DAF unit with chemical aid from coagulants dosed via an accurate dosing pump.
Secondary biological oxidation (BOD and ammonia)
Post‑oil removal, biology does the heavy lifting on dissolved organics and nitrogen. Conventional activated sludge (AS) and moving‑bed biofilm reactors (MBBR) run aerobic biodegradation to cut BOD and nitrify ammonia (oxidizing NH₄–N to nitrate). Typical secondary systems remove ~75–95% of BOD; after primary pretreatment, influent BOD is often <200 mg/L, so ≾20 mg/L BOD in secondary effluent is attainable (www.watertechnologies.com).
MBBR performance reported in pilots shows 70–89% removal of residual BOD (final 40–75 mg/L) when followed by oxidation polishing (www.researchgate.net). Ammonia drops similarly: 45–86% removal to 2–6 mg/L across varying loads (www.researchgate.net). Targets often set NH₃–N <5 mg/L, and many plants design for <1–5 mg/L with sufficient dissolved oxygen (DO ~2–4 mg/L) and sludge age; SRT >15 days is typical to retain nitrifiers.
Refinery biology also oxidizes sulfides (to sulfate) and many volatile organics (BTEX, phenols). Recalcitrant compounds may need longer solids retention or downstream polishing. Practical design anchors include ~0.5–1.0 kg BOD/(m³·day) loading, MLSS ~3,000–5,000 mg/L, and aeration energy ~0.3–0.5 kWh/m³. Proper seeding and nutrient balance stabilize performance.
Process selections range from a conventional activated sludge system to a moving‑bed bioreactor, often within broader anaerobic/aerobic digestion trains. Operators frequently support startups with biological aids such as nutrient supplements when load variability or toxicity is a concern.
Tertiary polishing and reuse options
To reach stringent discharge or reuse specs, refineries add a polishing step. Filtration (e.g., sand/gravel) and membranes strip fine solids and colloidal oil; ultrafiltration (UF) is common as a barrier before high‑pressure desalination. In reuse‑driven plants, a UF skid often follows secondary clarifiers or membrane bioreactors to capture residual TSS and protect downstream units.
Adsorption with activated carbon targets dissolved organics such as phenols and trace hydrocarbons, providing an additional 10–50% COD/BOD reduction and pushing oil & grease toward ~1 mg/L in practice. A GAC tower or BAC (biological activated carbon) stage complements oxidation steps; many plants specify activated carbon polishing before membranes.
Advanced oxidation processes (AOP) — ozone, UV/H₂O₂, Fenton — generate hydroxyl radicals to break down refractory organics. Studies report 50–90% cuts of persistent compounds in refinery flows post‑biology, and AOP is often placed after biological treatment to avoid biomass inactivation. Membrane bioreactors (MBR) have also been shown to produce “clarified and potentially reusable” effluent (pmc.ncbi.nlm.nih.gov), making a membrane bioreactor an established tertiary route when footprints are constrained.
For high‑quality reuse, membranes take the final step. Nanofiltration (NF) and reverse osmosis (RO) remove salts and organics; RO typically removes >95% of remaining salts with permeate BOD <5 mg/L and oil & grease <1 mg/L, at the cost of higher energy (~3–6 kWh/m³) and concentrate management. Plants integrate these via modular membrane systems, selecting nanofiltration where appropriate and using brackish‑water RO for desalting of secondary/tertiary effluent.
Disinfection remains standard. Chlorination or UV ensures coliform control and suppresses biofouling in reuse loops; many facilities deploy UV disinfection units as a chemical‑free barrier. Where granular filters are favored before membranes or discharge, media such as sand filters provide a low‑energy polish.
Performance benchmarks and plant economics
The multi‑stage train is well characterized. Oil & grease leaves primary units at <50 mg/L and finishes at <5–10 mg/L after DAF; reuse trains can reach <1–5 mg/L with filters and carbon (pmc.ncbi.nlm.nih.gov, www.watertechnologies.com). BOD commonly lands <30 mg/L and COD <100 mg/L (or lower for reuse), with ~90% BOD removal typical from a well‑run AS/MBBR (www.watertechnologies.com). Ammonia targets trend <10 mg/L; many plants design for <1–5 mg/L through sufficient oxygenation and sludge age, verified by online NH₃ monitoring.
Solids control follows a similar arc: TSS drops to ~<100 mg/L after pretreatment, then ≾10 mg/L post‑secondary and ~<5 mg/L with filters. Sludge yield is predictable at ~0.4–0.6 kg VSS per kg BOD removed; for example, removing 100 kg BOD/h at 90% generates ~36 kg VSS/h.
Energy and cost shape the flowsheet. Pretreatment (API/CPI + DAF) consumes about ~0.091 kWh/m³ in one study, versus ~0.86 kWh/m³ for MBR; flotation’s specific energy was ~10× lower and ~6× cheaper (NPV) than MBR polishing for the same wastewater volume (pmc.ncbi.nlm.nih.gov). Aeration typically runs ~0.3–0.5 kWh/m³, while advanced membranes add ~1–5 kWh/m³ depending on reuse goals. This underwrites a common strategy: use gravity/flotation to remove the bulk load cheaply, and reserve costly membranes for final polishing or reuse.
Ancillary selections (screens, skimmers, chemical dosing, monitoring) underpin reliability; facilities often standardize on robust wastewater ancillaries to minimize downtime and maintain compliance.
Design notes, trends, and control
Equalization and bypass capacity safeguard biology during shock loads (e.g., firewater flushes). Digital control and ML‑based optimization — including online oil alarms and BOD/NH₃ analyzers — are increasingly standard. Hybrid trains are rising: MBBR or integrated UASB (upflow anaerobic sludge blanket) + aerobic for energy recovery, and ZLD pilots in water‑stressed regions. Indonesia’s new refinery projects (e.g., bright field facilities) are trending toward high water‑recycle designs, with membrane and AOP pilots to meet both reuse and local discharge limits (pmc.ncbi.nlm.nih.gov).
In summary, the state‑of‑the‑art refinery WWTP pairs rugged primary oil‑water separation (API/CPI + DAF) with high‑rate biological treatment (activated sludge or MBBR) and tertiary polishing (filtration, GAC/AOP, membranes), as documented across industry literature (www.aquasust.com, pmc.ncbi.nlm.nih.gov). The combined system removes >90% of oil, BOD, and TSS, and >80% of ammonia, delivering effluent that meets strict discharge standards — or even reuse quality (pmc.ncbi.nlm.nih.gov, www.watertechnologies.com). The final flowsheet must be built on actual influent data and local regulation.
For a modern refinery, that is now the baseline: a central, staged treatment plant ready to protect the river — and, increasingly, to supply clean water back to the process.
Sources: industry reviews and studies underpin these performance and design ranges (pubmed.ncbi.nlm.nih.gov; pmc.ncbi.nlm.nih.gov; www.watertechnologies.com; www.sciencedirect.com; www.sciencedirect.com).
