Oil and gas facilities now design for gravity and flotation up front, biology in the middle, and membrane or carbon polishing at the end — because permits from BOD≈10 mg/L to oil & grease <0.2 mg/L are no longer outliers. The prize: ≥90–95% removal with an eye toward reuse and even zero‑liquid‑discharge.
For every single barrel of oil, operations generate roughly 3–6 barrels of “produced” and process water — a volume that pushed U.S. output to ≈3,400 billion liters in 2019 alone (www.spglobal.com | www.spglobal.com). What’s in it isn’t gentle: free and emulsified hydrocarbons, dissolved organics (high BOD/COD), ammonia (often tied to natural gas processing), salts and sometimes heavy metals.
Regulators in Indonesia illustrate the direction of travel. PP 82/2001 and related PermenLK drive stringent discharge limits — often BOD≤20–30 mg/L, oil/grease ≤0.2–5 mg/L, NH₃–N ≪1–2 mg/L for sensitive receivers — and one marine permit cited BOD₅≈9–11 mg/L and oil & grease <0.2 mg/L in the treated effluent (www.researchgate.net).
Meeting those numbers reliably has coalesced around a predictable train: primary physical separation (API separators, flotation) to strip free/dispersed oil; secondary biological oxidation to knock out dissolved organics and ammonia; and tertiary polishing (filtration, adsorption or membranes) to cross the finish line. The modern twist is sustainability — targeting ≥90–95% removal while minimizing sludge, energy, and footprint — and a rising bias toward water reuse and even ZLD (zero‑liquid‑discharge).
Influent profile and permit targets
Design starts with the influent mix and the permit. That means free and emulsified oil, BOD/COD (biochemical/chemical oxygen demand, measures of organic load), ammonia as NH₃–N, salts, and occasional metals; and targets such as BOD≤20–30 mg/L, oil/grease ≤0.2–5 mg/L, and NH₃–N ≪1–2 mg/L for sensitive discharges in Indonesia, with a real‑world marine case reporting BOD₅≈9–11 mg/L and oil & grease <0.2 mg/L (www.researchgate.net). Up front, plants lean on rugged physical kit — including screens and grit removal — often packaged as primary separation systems for oil and gas service.
API separators and DAF sizing
The textbook starter is an API oil–water separator (American Petroleum Institute design). A long, shallow tank uses gravity governed by Stokes’ Law to split oil, water and solids; proper API design (per API Std. 421) keeps enough volume — often several hours retention — to coalesce >150 µm droplets (www.filtsep.com; nepis.epa.gov). Typical performance is 40–65% removal of emulsified oil without chemicals, and with feeds <1,250 mg/L oil, effluent often lands <100 mg/L (nepis.epa.gov; skimoil.com). API plus coagulant can boost oil removal to >80% (nepis.epa.gov), and large “gun‑barrel” or multi‑compartment tanks reach ~90% for gross oil and settleable TSS, with settled sludge scraped and disposed (www.filtsep.com; nepis.epa.gov).
Polishing emulsions falls to DAF — Dissolved Air Flotation — where microsized air bubbles (<50 µm) attach to oil droplets and colloids for skimming (www.filtsep.com). A well‑operated DAF often drives oil/grease to single‑digit mg/L, though a refinery case reported DAF effluent oil at ≈22–24 mg/L while removing nearly all coarse oil (pmc.ncbi.nlm.nih.gov). Chemical programs — polymers/coagulants metered via a dosing pump — are tuned to maximize >90% oil removal, commonly with coagulants in the DAF feed.
Key metrics: API tanks typically provide 30–60 minutes retention for droplet rise; DAF units run ~10–20 minutes hydraulic residence with 10–20% recycle to generate bubbles. Combined, oil/grease usually falls from hundreds or thousands of mg/L to O(10–50) mg/L and TSS to <50 mg/L (nepis.epa.gov; pmc.ncbi.nlm.nih.gov). Upstream screening protects equipment — many plants fit an automatic screen for debris — and flow equalization buffers dampen surges from gas plant washwater or rollbacks.
Keeping oil out of the bioreactors isn’t optional: hydrocarbons as high as 50–100 mg/L can disrupt biological processes, so industry practice is ≥80–90% total oil removal before biology (www.filtsep.com; www.filtsep.com; nepis.epa.gov). Where compact primary polishing is needed, packaged DAF options such as a DAF unit or an integrated oil removal package are standard.
Biological oxidation and nitrogen control
With oil largely out, biology takes the stage to remove soluble organics and nitrogen. Conventional activated sludge and MBBR/MBBR hybrids typically deliver ~95% BOD and COD removal (www.filtsep.com). If the final discharge target is BOD₅≈25 mg/L (common for industrial effluents), the influent BOD should be ≲500 mg/L for a single aerobic stage (www.filtsep.com), with a two‑stage (anaerobic then aerobic) setup used for higher‑strength loads — the initial anaerobic step typically removes ~75% BOD/COD and the train can handle BOD/COD up to ~2,500 mg/L (www.filtsep.com).
Full‑scale activated sludge plants in upstream fields routinely report effluent BOD₅ <20 mg/L and NH₄–N often <1–2 mg/L with nitrification. Typical aerobic design runs a long sludge age, SRT (sludge retention time) of 10–20+ days, to degrade complex organics and nitrify; MLSS (mixed liquor suspended solids) sits around 3–8 g/L, F/M (food‑to‑microorganism) ~0.05–0.2, and detention time 6–12 hours, with oxygen demand often 1–1.5 kg O₂ per kg BOD oxidized — making blowers a major OPEX line. Standard options include activated sludge basins and MBBR reactors, while anaerobic digestion stages front‑end high‑strength streams.
Where permits tighten or footprints shrink, MBR (membrane bioreactor) systems rise. Bench studies on petrochemical wastewater report >80% COD and 99+% ammonia removal in MBRs (www.sciencedirect.com); by retaining essentially all biomass on the membrane, full‑scale MBRs typically yield effluent BOD/COD <5–10 mg/L and nitrate near background (www.sciencedirect.com). Plants specify MBR packages when reuse or ultra‑low solids are required, typically followed by a secondary clarifier if not integrated.
Ammonia management and stripping contingencies
Ammonia (NH₃/NH₄⁺) often originates in TEG (triethylene glycol) regeneration, H₂S scavenger trains, or tailgas scrubbers. Nitrifying bacteria are sensitive: above ~50–70 mg/L NH₃–N, nitrifiers suffer severe inhibition, and one refinery study judged nitrification “economical and controllable” up to ~50 ppm NH₃–N (pmc.ncbi.nlm.nih.gov). To ride out spikes, designers add an ammonia stripping bypass: Bahrami et al. (2020) described a simple air‑stripping column with NaOH pH adjustment downstream of DAF that knocked down ~200+ mg/L ammonia to levels the biological system could handle (pmc.ncbi.nlm.nih.gov; pmc.ncbi.nlm.nih.gov), with gas‑phase NH₃ recovered or vented to a stack. Caustic addition is typically metered by a dosing pump.
Absent extreme loads, high‑DO aeration with long SRT and an anoxic zone oxidizes ≥95% ammonia to nitrate and, if needed, denitrifies to N₂. Dedicated nutrient removal stages appear where nitrogen limits tighten or reuse drives ultra‑low nutrients. Advanced monitoring — online BOD/COD proxies and ammonia analyzers — and automated nitrate/NH₃ feedback control are increasingly standard to tune aeration and bioadditives.
Tertiary filtration, carbon, and disinfection
Post‑biology, tertiary polishing closes the gap. Filtration targets fines — many plants use dual‑media beds such as sand filters and anthracite media — followed by GAC (granular activated carbon) to adsorb phenols, solvents, and nitrated organics. Well‑run GAC filters often achieve >90% removal of trace organics; bench tests show 80–95% removal of low‑level organics from refinery wastewater. Specifying activated carbon downstream of the secondary clarifier is a common path to low COD.
Disinfection rounds out the train for discharge or reuse. UV systems deliver chemical‑free inactivation at low operating cost; plants typically deploy ultraviolet disinfection if residuals are discouraged, or chlorination where residuals are required. In most plants, this level of polishing hits BOD <10–20 mg/L, COD <50–100 mg/L, oil & grease <1–5 mg/L, NH₃–N <1 mg/L, and TSS <10 mg/L; in Indonesia, compliance often maps to class II/III receiving water standards or specific permits such as the cited seawater case with BOD≈10 mg/L and oil & grease <0.2 mg/L (www.researchgate.net).
Membrane polishing, reuse, and ZLD
When the goal shifts to reuse, membrane polishing steps in. A secondary RO (reverse osmosis) stage can produce very low‑salinity water; studies report RO‑polished effluent with TDS <100 mg/L suitable for boiler feed or injection (pubs.acs.org). Plants commonly frame this as integrated membrane systems anchored by a brackish‑water RO, adding hardness control via a water softener when required. A full ZLD stack — RO plus evaporation/crystallization — can recover >90–95% water and produce salt solids, though at higher energy cost; industrial ZLD plants in power and chemicals routinely achieve <100 mg/L TDS and reuse water for boiler feed (pubs.acs.org).
The compounding effect matters: each added stage drives another order‑of‑magnitude drop. As an example, aerobic biological removal of ~95% BOD followed by RO (≈95% salt rejection) yields final BOD on the order of 1–2 mg/L with very low salinity.
Sizing, power, and controls
As a planning rule, each 10 MGD (mega‑gallon/day) of oilfield wastewater at 500 mg/L BOD would need roughly a 10,000–20,000 m³ aeration basin with associated clarifiers, producing effluent BOD <20 mg/L and oil <5 mg/L. Across mature plants, biology removes >95% BOD, ~90–98% ammonia with nitrification, and 75–90% total nitrogen with denitrification.
Operations are increasingly digital: turbidity, UV254, residual chlorine, and online ammonia/COD proxies feed automated aeration and dosing control. Support systems — from wastewater ancillaries to flocculants at the clarifier and bio‑consumables — underpin stability while minimizing sludge and energy.
Industry trendlines and outcomes
Asia‑Pacific facilities are pushing toward “zero discharge” or high‑recovery reuse. Implementing tertiary RO and brine recovery can achieve >90–95% water recovery, albeit at higher OPEX, and oilfields are testing analogous high‑recovery schemes (membrane distillation, evaporation). The outcome of the tiered approach is predictable — effluent BOD reduced by ≈90–98%, COD by 80–95%, and ammonia oxidized or stripped to meet permits — and aligns with Indonesian requirements and global best practice (www.filtsep.com; nepis.epa.gov; www.sciencedirect.com; pmc.ncbi.nlm.nih.gov; www.researchgate.net; pubs.acs.org).
Sources: Authoritative engineering reviews and case studies (www.filtsep.com) (nepis.epa.gov) (www.sciencedirect.com) (pmc.ncbi.nlm.nih.gov) (www.researchgate.net), industry reports, and regulatory comparisons underpin the design figures cited.
