Refineries use roughly 246–340 liters of water per barrel of crude, and about 30–40% of that ends up as effluent. Meeting tight oil, BOD, COD and ammonia limits now means three stages: primary oil/solids removal, secondary biological treatment, and tertiary polishing—with membranes increasingly closing the loop for reuse.
Water is the silent workhorse of oil refining, and it is getting a more exacting job description. Global studies put refinery water use at about 246–340 L per barrel of crude, with roughly 30–40% discharged as effluent (www.sciencepublishinggroup.com). In Indonesia, the Ministry of Environment Regulation No. 19/2010 sets refinery effluent standards at BOD₅ (five‑day biochemical oxygen demand) ≤80 mg/L, COD (chemical oxygen demand) ≤160 mg/L, oil & grease ≤20 mg/L, NH₃‑N (ammonia nitrogen) ≤8 mg/L, and phenol ≤0.8 mg/L (id.scribd.com). Developed‑country targets are often similarly stringent; oil is typically <10 mg/L.
That backdrop has pushed a clear, three‑step playbook: remove free and emulsified oil up front (primary); oxidize dissolved organics and nitrify ammonia (secondary); and polish for discharge or reuse (tertiary). A 2024 review highlights how advanced membrane systems—especially ultrafiltration–reverse osmosis (UF‑RO)—are increasingly used to produce high‑grade reclaimed water (www.sciencepublishinggroup.com).
Primary oil and solids separation
The mission here is to strip free oil, grease, and coarse solids to protect what follows. Screening and gravity separation lead the way. Facilities often start with front‑end screens; a continuous unit such as an automatic screen removes debris >1 mm without interrupting flow.
The API separator (American Petroleum Institute style gravity oil–water separator) is the workhorse. Designed per API RP 421—length‑to‑width ≥5:1, horizontal flow <3 ft/s, and depth‑to‑width ≈0.3–0.5 (docsbay.net)—it removes 60–99% of free oil (www.watertechnologies.com). Effluent oil typically lands ≤50–200 mg/L, with designs aiming for <200 mg/L and ≈50 mg/L often seen in practice (www.thewastewaterblog.com). Expect 10–50% TSS (total suspended solids) removal and a modest 5–30% COD drop (www.watertechnologies.com).
Design details matter: multiple skimmer/scraper flights continuously remove floating oil and convey settled solids to a sludge hopper (docsbay.net). Skimmed oil can be 90–95% pure, suitable for recovery or incineration. Where footprint allows, sedimentation hardware such as a clarifier or coalescing plates (corrugated plate interceptors) helps enhance separation.
Flotation for emulsified oil
Emulsified and fine oil not captured by gravity units typically goes to dissolved‑air flotation, DAF (microbubble flotation), or IGF (induced gas flotation). With coagulation—e.g., alum or polyaluminum chloride at ~10 mg/L—DAF operated near pH≈5 and 300–500 kPa saturator pressure removes >80% of remaining oil, organics, and TSS (www.researchgate.net). On API effluent, typical DAF performance is ~70–85% oil removal, ~80–85% TSS removal, and 10–60% COD/BOD removal (www.watertechnologies.com). In practice, DAF plus coagulant dosing usually drives oil & grease to <10 mg/L.
DAF float—a frothy concentrate of oil/grease and sludgy solids—runs ~4–6% solids and heads to sludge processing. Refiners dosing polyaluminum chloride often use a DAF unit in tandem with a PAC coagulant program and controlled addition via a dosing pump for stable performance.
Flow equalization and pH control
A surge/equalization tank buffers flow and load swings—crucial insurance against upsets—and is a logical place to adjust pH (acidify to ~6–7) to optimize coagulants and protect biology. Oil‑water storage/settling tanks, or corrugated plate interceptors, can serve as intermediate skimmers during large spills. Equalization capacity of at least 24–48 hours is recommended later when sizing a central plant.
Overall, primary treatment should remove roughly 90–99% of free‑floating oil and 30–70% of TSS before the bioreactor (www.watertechnologies.com). Veolia data note an API+DAF train removing ~85–95% of oil and cutting BOD/COD by ~20–70% (www.watertechnologies.com).
Secondary biological oxidation and nitrification
Post‑primary, expect FOG (fats, oils, grease) ≲20 mg/L but elevated BOD and COD. Biological treatment—activated sludge or biofilm processes—does the heavy lifting on dissolved organics and ammonia. Conventional activated sludge or MBBR (moving bed biofilm reactor) basins are typically sized for >8 h hydraulic retention, MLSS (mixed liquor suspended solids) ~2–4 g/L, and DO (dissolved oxygen) ~2 mg/L. Activated sludge achieves ~85–95% BOD removal and 50–95% COD removal from primary effluent (www.watertechnologies.com), often pushing effluent BOD to single‑digit mg/L (frequently <20 mg/L).
Veolia reports secondary ASP (activated sludge process) often removes ~80–99% of residual oil and ~95–99% of phenol (www.watertechnologies.com). An implementation based on an activated sludge aeration basin remains the default, while biofilm options such as MBBR increase process stability under variable loads.
Many modern refineries adopt MBRs (membrane bioreactors) for compact, high‑performance treatment. Anoxic–aerobic MBR pilots have demonstrated ≈97% COD and ≈96% oil removal (www.sciencepublishinggroup.com), while full‑scale MBRs have shown NH₃‑N <0.5 mg/L in the effluent (www.sciencepublishinggroup.com). Typical MBR settings use SRT (sludge retention time) >20 days and 0.04–0.1 mm pore membranes, with permeate turbidity <0.2 NTU (nephelometric turbidity unit). Compact, integrated packages such as membrane bioreactors are thus a route to reuse‑grade effluent.
Nutrient removal pathway
Ammonia in crude‑derived wastewater is often 5–20 mg/L NH₃‑N. To meet an 8 mg/L NH₃‑N limit (id.scribd.com), complete nitrification typically requires ~24–48 hours of aerated contact. If total nitrogen is a concern, many designs add an anoxic denitrification zone. A sequence of anoxic MBBR + aerobic MBBR has been shown to achieve near‑complete nitrate removal as well (www.sciencepublishinggroup.com). Modular options such as nutrient removal packages can be integrated into the biological train.
Sludge production and handling
Biological treatment generates secondary sludge at ~0.5–0.7 kg dry solids per kg BOD removed (Veolia table; www.watertechnologies.com). Sludge is thickened to ~3–5% dry solids before disposal or incineration. Removing FOG in primary reduces oily sludge in the bioclarifier; any oil that reaches the bioreactor can accumulate in sludge flocs and may require controlled wasting.
Tertiary filtration and adsorption
For discharge polish, rapid granular media filters remove remaining suspended solids and oil droplets. From secondary clarifier effluent, granular media filtration delivers ~75–95% TSS removal and ~65–95% oil removal (www.watertechnologies.com). In practice, turbidity can drop to <5 NTU and oil to <5 mg/L, with BOD/COD typically falling a further 20–30%.
Media trains can be configured with sand/silica filtration to remove 5–10 micron particles. Multilayer beds often incorporate anthracite media for depth filtration and low headloss.
Activated carbon—a classic adsorbent—mops up residual organics. Post‑sand filtration, GAC (granular activated carbon) can remove >90% of trace phenol and PAHs, with Veolia citing 91–98% BOD and 86–94% COD removal after secondary+GAC polishing (www.watertechnologies.com). Carbon beds such as activated carbon can bring total phenols to the low µg/L range if bed life and regeneration are maintained.
Membrane polishing for reuse
Where reuse is the goal—or discharge is ultra‑strict—membranes are the state of the art. UF (ultrafiltration) at 0.01–0.1 µm locks out suspended matter and oil; NF (nanofiltration) and RO (reverse osmosis) then strip salts and small organics. A hybrid UF–RO train is highlighted as among the most promising refinery reuse routes (www.sciencepublishinggroup.com), with permeate showing near‑zero oil, <10 mg/L COD, and minimal conductivity. One refinery reported UF‑RO reuse water meeting boiler/make‑up specs.
As building blocks, pretreatment UF skids such as ultrafiltration safeguard downstream membranes. For salt and organics removal, integrated RO/NF systems are typical; RO options for industrial effluent include brackish water RO where TDS is within range, and specialty elements such as FilmTec membranes or Toray membranes depending on specification.
Advanced oxidation and disinfection
When recalcitrant organics persist, AOPs (advanced oxidation processes) such as ozone or H₂O₂/UV can break down phenols or sulfides while providing disinfection. For pathogen control alone, UV or chlorination are options. Note: high chlorine residual can form organochlorines, so UV is often preferred for disinfection of sensitive environments. Compact UV reactors such as ultraviolet systems offer chemical‑free disinfection with a 99.99% pathogen kill rate at low operating cost.
Design outcomes and performance metrics
A well‑designed refinery WWTP targets effluent quality of ≤50 mg/L BOD₅, ≤100 mg/L COD, oil <10 mg/L (preferably <5 mg/L), and NH₃‑N <2–5 mg/L before discharge (id.scribd.com). In pilots, anoxic–oxic MBR runs recorded COD removal ~97% (from ~1000 mg/L to ~25 mg/L) and oil removal 96.6% (www.sciencepublishinggroup.com), far below Indonesian limits.
Removal efficiencies stack up: overall FOG removal is typically ~98–99%; overall BOD removal ~95%; ammonia removal ~90%. One case study showed >95% BOD removal with final effluent <3 mg/L (www.sciencepublishinggroup.com). Veolia table values suggest combined primary+secondary remove ≥90% of BOD and essentially all oil (www.watertechnologies.com) (www.watertechnologies.com).
Capacity, energy, and footprint planning
“Central” plants should be sized for peak wet‑weather flow because refinery drainages can spike. A practical rule of thumb: ~0.5–1.0 m³/hour per barrel‑per‑day capacity. For a 100,000 bbl/d refinery, that equates to ~5,000–10,000 m³/day wastewater and a plant flow of ~200–400 m³/hr. Include at least 24–48 h equalization to handle surges.
Aeration (secondary) and membranes (if used) dominate energy use. Advanced systems (MBR, RO) add OPEX but shrink footprint. Tradeoff example: MBR aeration is ~30–50% more energy than ASP, but avoids clarifier space. In humid tropics, cooling deodorization or VOC capture from tanks may also be needed.
Industry adoption trends
Data show rising uptake of high‑rate and membrane processes to conserve water. A 2024 review counted ~82 published studies on refinery effluent reuse (2002–2023), with Asia leading (Iran, Iraq, China, India) (www.sciencepublishinggroup.com). About half the reviewed applications used membrane‑based steps (UF, RO) (www.sciencepublishinggroup.com), underscoring regulatory and resource‑efficiency drivers. UF‑RO reclaimed water can offset freshwater demand by supplying boilers or cooling systems.
Practical design checklist
Primary clarifier/API unit: install a 5:1 L:W oil separator with coalescing plates if space allows. Include automatic skimmers and sludge hoppers. Target residual oil <100 mg/L after API. Gravity separation hardware and oil skimming equipment such as oil removal systems can bolster free‑oil capture.
DAF polishing: use DAF with chemical feed after API and design for ~15–30 minutes retention; expect ~80% removal of emulsified oil and organics (www.researchgate.net). Coagulants applied upstream can be sourced from programs like PAC/ACH.
Equalization basin: provide 1+ days of flow buffering with pH adjustment; instrumentation and tanks in wastewater ancillaries help standardize chemical handling and mixing.
Secondary activated sludge: design for BOD loading ~1.0–1.5 kg/m³·d, MLSS ~2–3 g/L, DO 2–3 mg/L. Include nitrification (approximately 24 h contact). Aim for >90% BOD removal. Equipment packages such as activated sludge aeration systems provide a familiar baseline.
Advanced aerobic process: consider MBBR or MBR to achieve >95% removal and a compact layout. Pilot tests have shown MBR effluents far exceeding standards (www.sciencepublishinggroup.com) (www.sciencepublishinggroup.com).
Tertiary polishing: add multimedia filtration (or cloth filters) followed by activated carbon. For reuse water, add UF + RO. Ensure final oil ≤5 mg/L and BOD/COD meet client targets (typically 10–20 mg/L). Media and carbon building blocks include sand/silica filters and activated carbon; membrane pretreatment and desalting stages are built with UF and downstream nanofiltration or RO.
Monitoring: use continuous oil‑in‑water monitors, plus monthly lab tests for BOD, COD, NH₃, phenol, and more to prove compliance.
Sludge handling: thicken sludge from API and DAF; dewater by centrifuge or filter press and incinerate or co‑process. Activated sludge excess can be stabilized (digested) if needed.
Compliance and calculation notes
Following these guidelines and current technologies, a refinery WWTP can reliably meet Indonesia’s standards (id.scribd.com). Specific removal data—API separator oil removal 60–99% (www.watertechnologies.com), DAF ≥80% oil/organic removal (www.researchgate.net), secondary BOD removal ~90% (www.watertechnologies.com), tertiary solids removal ~80–95% (www.watertechnologies.com)—should be used in design calculations to size units and predict effluent quality. Rigorous monitoring, smart primary separation, robust biology, and targeted polishing enable environmental compliance, cut freshwater intake, and improve long‑term operational efficiency.
Sources: a 2024 systematic review on refinery wastewater reuse (www.sciencepublishinggroup.com) (www.sciencepublishinggroup.com); industry design guidelines (www.watertechnologies.com) (docsbay.net); and Indonesian regulations (id.scribd.com). All numeric values and performance figures are drawn from these sources.
