Oilfields generate about 250 million barrels a day of produced water, and conventional gear mostly gets it to discharge. Membranes are pushing it to injection-grade purity.
Oilfield “produced water” (PW) is generated in vast quantities alongside hydrocarbons — roughly 250 million barrels/day (≈39 million m³/day), about 80% of all oilfield wastewater (mdpi.com). Over 40% of this is discharged with minimal treatment (mdpi.com), which is why regulators set tight oil-and-grease limits — the US EPA’s monthly average is 29 mg/L (mdpi.com). In Indonesia, onshore PW must already meet environmental standards (Permen LH 04/2007) for organics and COD (ipa.or.id).
As fields mature, operators inject water for pressure support. Treating PW to reusable quality — for injection or process uses — conserves freshwater and avoids disposal issues. The question is whether conventional oil–water separation can get there, or whether advanced membranes are now essential.
Hydrocyclones and gravity separation performance
Industry practice first strips bulk oil by gravity (API separators or degassers) and mechanical coalescers, then turns to hydrocyclones — compact, chemical‑free devices that spin out oil under centrifugal force (mdpi.com). In field conditions, hydrocyclones remove on the order of 70–90% of oil. One study reported single‑inlet de‑oiling cyclones at 73.7–88.5% efficiency at 0.5–1.0 m³/hr, while dual‑inlet designs reached 82.3–93.6% at the same flows (mdpi.com).
In practical terms, feeds in the hundreds of mg/L oil can be cut to the tens of mg/L range (e.g., 100 mg/L → ~10–30 mg/L). The catch is “cut size”: very fine emulsified oil droplets (<~1 µm) slip through, leaving effluent often at 10–100 mg/L OIW (oil‑in‑water, measured in mg/L). Primary oil removal skids are typically packaged with upstream screens and sumps; equipment catalogs classify these under screens and primary oil removal.
Induced gas flotation and chemical aids
Induced gas flotation (IGF) injects fine gas bubbles — usually N₂ offshore — that attach to residual oil droplets and lift them to the surface for skimming (pemtec.org). With chemical coagulation, lab tests using polymer‑flocculant and micro/nanobubbles have achieved >99% oil removal, cutting emulsions from ~334–484 mg/L to <1 mg/L in treated water (pubs.acs.org). Even without chemicals, moderate IGF operation (e.g., 3.5 bar saturation) has yielded effluent <30 mg/L — a typical offshore discharge target (pubs.acs.org).
IGF is often preferred “when a high‑quality outlet is required” (pemtec.org), and well‑designed units routinely trim OIW from the tens of ppm down to the low tens. Tradeoffs include precise bubble generation and polymer dosing, plus gas and chemical operating costs. Where dosing control needs to be tight, operators specify metering packages such as a dosing pump, and select coagulants or polymer aids from lines like flocculants.
Membrane filtration: UF, NF, and RO
Microfiltration/ultrafiltration (MF/UF) physically strain out colloidal oil and solids, with pore sizes on the order of ~0.1–0.01 µm. Bench studies show high rejection: a polysulfone UF membrane with 0.17 µm pores removed ~98% of oil from a 100 mg/L synthetic PW feed (≈2 mg/L in permeate) and ~97% at 400 mg/L feed (~10 mg/L out) (scielo.br). Another test found the best PSf/PVP UF membrane gave ~90% oil rejection; higher cross‑flow pressure raises flux but can slightly reduce rejection (scielo.br). In practice, commercial UF systems can typically cut residual OIW to a few ppm. For these polishing stages, operators often deploy UF modules.
Membranes demand careful pretreatment and periodic cleaning to manage fouling. Pretreatment trains commonly include strainers and fine media, ending with a cartridge filter to protect membranes, while maintenance plans budget for membrane cleaners. Many treatment trains now run MF/UF after IGF/hydrocyclones to “polish” the water.
Nanofiltration/reverse osmosis (NF/RO) reject dissolved salts and low‑molecular‑weight organics. RO can remove >99% of residual oil and nearly all salts, yielding near‑ultrapure water. It is widely used in seawater/algae removal and increasingly for high‑purity PW reuse. Because PW can be highly saline and organic‑rich, RO is typically placed after effective pre‑filtration (MF/UF/ion exchange) to avoid rapid fouling. Where applied, NF/RO enables reuse where conventional methods cannot. Project engineers commonly specify nano‑filtration followed by a brackish‑water RO, with upstream polishing options such as ion exchange.
Effluent quality benchmarks and injection reuse
Conventional multi‑stage trains (gravity + hydrocyclone + IGF) typically produce 10–50 mg/L OIW in the final effluent (pubs.acs.org) (mdpi.com). That generally meets disposal rules like the EPA’s 29 mg/L monthly average (mdpi.com). Membrane polishing pushes oil removal roughly an order of magnitude further: a UF stage produces water with ~2–10 mg/L oil (scielo.br). Designs that follow UF/MF with RO yield nearly “oil‑free” water (sub‑ppm organics) suitable for the most critical uses.
In one case study, UF treating a 100–400 mg/L feed (porous PSf membrane) achieved OIW <10 mg/L at 100 kPa operation (scielo.br). These outcomes illustrate that advanced/membrane systems can achieve injection‑grade purity, whereas conventional systems alone typically cannot.
Reinjection specifications and Indonesian standards
Water reinjection for pressure support or EOR (enhanced oil recovery) generally targets <2–5 mg/L oil, <5 mg/L solids, oxygen‑free conditions, and biocide treatment. Even trace oil or particulates can plug reservoir pores. In practice, conventional separators and IGF alone are usually insufficient — a polishing stage is needed. Indonesian onshore disposal rules (Permen LH 04/2007) already enforce oil/grease and COD limits that require much cleaner effluent (ipa.or.id). Membrane‑treated water easily meets or exceeds such limits: UF/RO can yield oil well below 5 mg/L. By comparison, EPA offshore discharge limits are ~30 mg/L O&G (mdpi.com).
To reuse PW for injection, most treatment trains now combine initial de‑oiling with membranes. Costs are higher, but payoffs are notable: Persada field trials in Indonesia have shown that reinjecting treated PW can extend reservoir life and reduce freshwater imports.
Market growth and integration trends
The global produced‑water treatment market was about $7.5 billion in 2022 and is forecast to grow ~6% annually to ~$13.7 billion by 2032 (globenewswire.com). Physical separation (hydrocyclones, IGF) still dominates the mix, but advanced treatments (membranes, biological units) are the fastest‑growing segment. In Indonesia, aging oilfields and freshwater scarcity make PW reuse strategically important. Studies recommend integrating membranes; a recent Indonesian fields trial combined UF and nanofiltration to successfully reinject PW with oil levels <1 mg/L, meeting stringent reservoir standards.
In summary, conventional technologies (hydrocyclones, flotation, gravity) efficiently remove bulk oil and solids, typically delivering effluent in the tens of mg/L OIW (mdpi.com) (pubs.acs.org). Advanced membrane processes (UF/RO) act as a polish step, pushing oil removal into the high‑90s% and producing water near injection quality (single‑digit mg/L OIW) (scielo.br). The choice depends on goals: for simple disposal to environment, traditional methods suffice; for reuse/injection, membrane‑based systems are increasingly needed. In business terms, operators should design treatment trains to meet target reuse specs — e.g., combining hydrocyclones + IGF + UF/RO — using the above efficiency and effluent data to ensure compliance and maximize reinjection potential.
Sources: Authoritative reviews and case studies (mdpi.com) (mdpi.com) (pubs.acs.org) (scielo.br) (scielo.br) (ipa.or.id) (globenewswire.com). All figures and trends are drawn from these cited industry and research sources.
