A gravity oil–water separator designed to API 421, followed by dissolved air flotation, routinely drives oil from hundreds of mg/L to single digits—opening the door to internal reuse and cutting freshwater demand.
In the calculus of refinery water, the combination is simple and potent: let gravity do the heavy lift, then let microbubbles finish the job. An API‑type separator for free oil, followed by dissolved air flotation (DAF) for emulsified oil and fine solids, is described as “the technology of choice” for primary oil and grease removal (thewastewaterblog.com).
On numbers, the train is striking. A well‑designed API can drop effluent oil to the tens or low hundreds of mg/L—often around 50 mg/L, with design targets up to 200 mg/L—before DAF typically cuts what remains by ~85–90%, landing final oil in the single‑digit mg/L range (thewastewaterblog.com; mdpi.com). In one representative mass balance, 500 mg/L O&G entering pretreatment falls to ~50 mg/L after gravity, then to ~7 mg/L after DAF (86% of the remainder removed) (mdpi.com).
That puts many facilities well below typical discharge expectations—U.S. refinery water rules often set O&G limits near 10–30 mg/L—and aligns with Indonesia’s PermenLH 19/2010, which requires oil & grease monitoring and likely values in the tens‑of‑ppm range (nepis.epa.gov; saka.co.id).
API gravity separation (API 421 parameters)
Stage one is a gravity API‑type separator (API 421 is the American Petroleum Institute design guideline) sized with length:width ≥5:1 and horizontal velocity ≤3 ft/min to let buoyant oil drops rise. Inlet velocity should stay ≤0.15 m/s to keep flow calm for separation (thewastewaterblog.com). The unit targets free oil droplets ≥150 µm and sheds settleable solids to sludge, using retention time and quiescent flow to do the work (thewastewaterblog.com).
Performance is well‑documented: APIs often bring effluent oil content to the tens or low hundreds of mg/L, with ~50 mg/L achievable in practice and up to 200 mg/L as a design target. Even under heavier loads, they remove ~80–90% of free oil by mass (thewastewaterblog.com). In refinery pretreatment trains, a conventional setup shows ~90% removal of oil and TSS (total suspended solids) across flotation stages, with the API responsible for most free‑oil reduction (mdpi.com). For primary equipment selection, refineries often pair gravity separation with purpose‑built oil–water primary treatment systems.
DAF for emulsified oil and fine solids
Stage two is dissolved air flotation (DAF), which attaches microbubbles to remaining emulsified oil and fine particulates so they float. Coagulant dosing and controlled turbulence are critical: bench trials on refinery wastewater achieved >80% removal of oil and TSS with pH ~5, coagulant at ~10 mg/L of aluminum, iron, or poly‑salts, saturator pressure 300–500 kPa, and air:water ratios ~5–15% (researchgate.net). In practice, polyaluminum sulfate (PAS) around ~10 mg/L raised oil removal to >80% in the same tests (researchgate.net).
Full‑scale data show that API+DAF removes ~90% of oil & grease (O&G) and ~90% of TSS, with ~75% COD (chemical oxygen demand) removal; the DAF step alone often achieves 85–86% for TSS and O&G and around 70% for COD, producing effluent oil commonly in single‑digit mg/L. Design bases frequently assume DAF effluent <10–20 mg/L O&G, and literature notes <10 mg/L is needed upstream of biological units (mdpi.com; suezwaterhandbook.com). Many plants standardize on a packaged DAF unit with automated chemical addition, using coagulants fed by an inline dosing pump for stability and control.
Alternate pretreatment options and limits
Facilities sometimes add compact plate devices such as corrugated‑plate interceptors (CPI/DCI) that act like high‑surface‑area gravity separators, hydrocyclones for higher‑density solids, or oil‑attracting media and coalescers upstream. For difficult emulsions, chemical breakers or acid adjustment (pH <7) can improve coalescence; some refineries deploy stacked‑disk centrifuges for rag‑layer naphthenic emulsions. Even so, API + DAF remains the industry standard and “the technology of choice” (thewastewaterblog.com), after which biological polishing via activated sludge is commonly considered if needed for downstream objectives.
Performance metrics and operating scale
Typical desalting brine can carry hundreds to thousands of mg/L of oil and fine solids, depending on crude quality. After gravity separation, 50–200 mg/L oil is common out of the API (thewastewaterblog.com). DAF then removes another ~85–90%. The illustrative 500→50→~7 mg/L O&G reduction underscores how API+DAF reliably reaches single‑digit mg/L O&G and TSS below ~10 mg/L (mdpi.com; suezwaterhandbook.com).
On flow, a refinery DAF may handle tens of m³/hr (cubic meters per hour). For context, a 100,000 bbl/d (barrels per day) crude unit can generate desalter water on the order of dozens of m³/hr. Reported field sampling typically shows >80% O&G and TSS removal across the DAF step, tracking the numbers above (researchgate.net; mdpi.com).
Reuse pathways and water savings
With oil and solids down to low ppm, the brine becomes a reuse candidate. Options span returning cleaned effluent to the desalter as wash water—Veolia notes recovered desalter water “may be returned to the desalter, improving its performance”—to using it as cooling tower or process makeup, subject to appropriate polishing (veolianorthamerica.com). A Tehran refinery pilot reported 95% salt (TDS, total dissolved solids) rejection and essentially 100% chloride removal with RO (reverse osmosis) after pretreatment, producing water comparable to city makeup for cooling (doaj.org). Where membranes are appropriate, refineries deploy RO systems for brackish streams and, for biopolishing, membrane bioreactors (MBR) that pair biological treatment with ultrafiltration—MBR “produces reuse quality water” in integrated setups.
Broader industry data support aggressive recycling. An EPA‑linked survey estimated cooling water recycling alone can cut raw‑water needs to ~20% of a once‑through system (>80% reuse) (nepis.epa.gov). Reviews emphasize that “final treated effluent can be reused for non‑potable purposes as an additional water resource, according to the degree of decontamination” and that global refinery water use is on the order of 4×10^7 m³/day (~16% of total industrial water) (sciencedirect.com; sciencedirect.com). The American Petroleum Institute has noted that complete internal water recycle is technically feasible and cost‑effective (nepis.epa.gov). Many refineries therefore target >80–90% recycle, edging toward zero liquid discharge in advanced installations.
Other non‑potable uses include tank or pipe flushing and dust suppression; some facilities reinject treated brine for reservoir support or for disposal where chemistries align. For plants routing API+DAF effluent into biology, standardized membrane systems and integrated activated‑sludge processes provide pathways to meet reuse goals alongside compliance.
Bottom line and compliance context
Across studies and field data, the API separator → DAF train typically removes ~90%+ of influent oil and solids, taking O&G from hundreds of mg/L down to single‑digit mg/L and TSS to ~<10 mg/L (mdpi.com; researchgate.net). Those residuals sit well below many industrial discharge norms (U.S. O&G often 10–30 mg/L) and track regulatory expectations in markets such as Indonesia (PermenLH 19/2010 requires O&G monitoring) (nepis.epa.gov; saka.co.id). Case studies indicate that recycling such treated water can offset most of a refinery’s freshwater draw (nepis.epa.gov; sciencedirect.com).
Definitions: API separator (gravity oil–water separator per API 421); DAF (dissolved air flotation using microbubbles for separation); O&G (oil and grease); TSS (total suspended solids); COD (chemical oxygen demand, a measure of organic load); TDS (total dissolved solids); m³/hr (cubic meters per hour); CPI/DCI (corrugated/parallel plate interceptors for compact gravity separation); MBR (membrane bioreactor combining biology with membranes).
