Oil and gas plants facing g/L‑level toxics are stabilizing biology and clearing tough effluent limits by pairing sour‑water stripping with powdered activated carbon in the bioreactor. The data show fewer upsets, faster treatment, and compliance with Indonesia’s COD ≤200 mg/L, sulfide ≤0.5 mg/L, ammonia ≤5 mg/L, phenol ≤2 mg/L standards.
Produced water and sour water in oil and gas operations often carry hydrogen sulfide (H₂S), ammonia (NH₃), phenols, volatile organics like benzene and toluene (BTEX), metals, and caustics at concentrations that can derail biology. Stripping these “shock” toxins upstream, then dosing powdered activated carbon (PAC—finely divided activated carbon used in suspension) into the bioreactor, has emerged as a high‑stability, low‑cost path to meet tight permits (yokogawa.com; nepis.epa.gov).
Indonesia’s Permen LH 19/2010 sets representative refinery/produced‑water limits at COD ≤200 mg/L, oil & grease ≤25 mg/L, sulfide ≤0.5 mg/L, NH₃‑N ≤5 mg/L, phenol ≤2 mg/L (researchgate.net). A field study reported phenol ~11.6 mg/L after simple filtration—well above the 2 mg/L limit (researchgate.net; 123dok.com). Dissolved H₂S and hydrocarbons are major freshwater toxicants that can cause acute and chronic effects (sciencedirect.com), underscoring the need to strip concentrated toxics and then polish trace residues to ppb–ppm.
Regulatory limits and toxic load
Typical constituents span H₂S and NH₃ (often up to g/L in sour water), soluble phenols, BTEX, cyanide/thiocyanate (from sour‑water stripper and caustic units), heavy metals (e.g., Ni, Cr), and residual caustics like NaOH (yokogawa.com; 123dok.com). Jakarta’s regulations (Permen LH 19/2010) align with international practice—COD ≤200 mg/L, oil & grease ≤25 mg/L, sulfide ≤0.5 mg/L, NH₃‑N ≤5 mg/L, phenol ≤2 mg/L—requiring ≥90–99% removal upstream of biology and final polishing for compliance (researchgate.net). Continuous H₂S/NH₃ monitoring is recommended to catch toxic surges.
Sour‑water stripping configuration
Sour water from crude units, FCC and cokers is typically degassed (removing H₂, CO₂, light hydrocarbons) and sent through steam strippers. An acid‑gas stripper at pH ~7–8 removes H₂S, followed by an ammonia stripper at pH ~10–11 for NH₃ (>95–99% removal is achievable in practice). Vendors note a well‑designed system removes “essentially all ammonia and hydrogen sulfide” (yokogawa.com). H₂S gas is recovered and routed to sulfur recovery or sulfuric acid units (yokogawa.com). Design conditions are critical—the “ideal pH” tradeoff often favors separate towers or staged pH control for adequate removal of both gases (yokogawa.com).
Air stripping of volatile organics
VOCs such as benzene, toluene and C₁–C₃ aromatics are commonly air‑stripped early. With air‑to‑water ratios of ~20–100:1 at 30–50ºC, single‑stage towers can remove >90% BTEX; one dataset reported >99% benzene, >93% toluene and ~93% xylene under moderate conditions (researchgate.net). Designing for benzene to <1 mg/L is common at hazardous‑waste sites, and similar performance is expected in refinery polishing trains.
Oil and solids separation steps
Free oil and settleable solids are removed via API separators, skimming and hydrocyclones; emulsified oil is toxic and complicates downstream steps. Maintaining oil/water separators reduces foulants in aeration, where residual oil can degrade floc and add COD (nepis.epa.gov). Facilities commonly target biological influent at ≤10–25 mg/L oil & grease, supported by coalescing systems such as an oil‑removal unit. For suspended solids control, a properly sized clarifier or a compact DAF can assist; dissolved air flotation typically achieves 95%+ removal of suspended solids and oils with 1–3 hour detention (dissolved air flotation).
Chemical precipitation and oxidation
Where needed, metals can be precipitated by pH adjustment and easily‑oxidized toxics treated with mild oxidation. Advanced oxidation of stripped sour water (Fenton or UV/H₂O₂) has been shown to near‑completely remove phenols and reduce phytotoxicity (patents.google.com). These steps are typically secondary to the core sour‑water and VOC stripping.
Bioreactors with powdered activated carbon
Post‑pretreatment, activated sludge or biofilm systems degrade remaining organics. Adding PAC directly to aeration—creating a carbon‑augmented mixed liquor—adsorbs residual toxics and buffers shocks, producing more uniform effluent (nepis.epa.gov). Options include conventional activated sludge, MBBR, and SBR configurations; membrane systems (MBR) combine biology with ultrafiltration and can produce reuse‑quality water (membrane bioreactor).
Adsorption is rapid: bench tests show PAC removed ~80% phenol in ~10 minutes (researchgate.net). In pilot and full‑scale studies, PAC captured refractory organics and stabilized performance; a Sun Oil refinery trial reported effluent BOD down ~76% and TSS down ~56% versus baseline, with COD down ~36% (nepis.epa.gov). One review concluded PAC “protected biological systems from shock or toxic loadings,” with downstream toxicity bioassays improving to no fish mortality at 100% effluent (nepis.epa.gov; nepis.epa.gov).
Beyond refinery matrices, carbon‑augmented reactors have removed ~99% of persistent pharmaceuticals that bypassed conventional steps (carbamazepine, diazepam) (sciencedirect.com). A recent AC‑AS (activated carbon–activated sludge) study reported 90% removal of COD, DOC and aniline achieved 30–58 hours faster than the control reactor without PAC (sciencedirect.com). PAC also forms dense bioflocs that settle better; long‑term sludge volumes fell ~60–70%, with operators noting less foaming and steadier aeration (nepis.epa.gov; nepis.epa.gov).
Cost has been modest: historical PAC dosing ran about $0.02–0.05 per m³ treated (1.7–4.3 cents per 1,000 gal), with savings from lower sludge handling and avoided bypass treatment delivering rapid ROI; some sites regenerate via granular activated carbon columns (nepis.epa.gov). Activated carbon media selection remains central; specialty grades for refining organics are available (activated carbon).
PAC implementation parameters
PAC is typically fed continuously to aeration at ~10–100 mg/L depending on loading; lab tests determine capacity for site‑specific toxics. A controlled feed via a metered dosing pump and adequate mixing keep carbon suspended, with recycle in return sludge for mature systems. In membrane bioreactors, PAC reduces membrane fouling while adsorbing toxics (MBR). Routine MLSS, pH, and effluent COD/BOD checks guide optimization; studies report no deterioration of nitrification due to PAC.
Measured outcomes and tightening standards
Reported gains include 56% lower effluent suspended solids, 76% lower BOD, and 36% lower COD in the Sun Oil pilot with PAC (nepis.epa.gov); phenol is often cut by >90% when adsorption is paired with biodegradation, and PAC alone removed ~80% phenol within minutes in bench tests (researchgate.net). Carbon‑augmented reactors achieved 90% removal of complex organics and ammonia roughly 30–58 hours faster than controls (sciencedirect.com), supporting steadier effluents despite influent spikes.
ASEAN and Indonesian regimes increasingly align petroleum effluents with other industrial streams; failure to drive phenol from ~10 mg/L down to <2 mg/L triggers the need for advanced steps (researchgate.net; 123dok.com). Plants pairing robust pretreatment (sour strippers, oil removal) with PAC‑polished biology have consistently met Indonesian Permen thresholds, with one refinery reporting that undiluted effluent passed bioassays that previously showed 100% mortality after PAC installation (nepis.epa.gov; nepis.epa.gov).
Economically, quicker stabilization reduces tertiary footprints (e.g., UV, ozone) and improves uptime; reclaiming H₂S/S offers additional value. Case studies cite payback in months via chemical and sludge savings, with large plants reporting up to ~$50K/yr reduced sludge‑disposal costs (nepis.epa.gov; nepis.epa.gov).
Design and operations checklist
1. Complete upstream stripping: sour‑water units optimized for steam rate and reflux can remove >95–99% H₂S/NH₃; stripped water targets H₂S <1 ppm and NH₃ <1 ppm, with separate or staged pH towers (acid gas ~pH 7–8; ammonia ~pH 10–11) for optimal removal (yokogawa.com; yokogawa.com). Air or vacuum stripping for VOCs should aim at >90% benzene removal, commonly to <0.1–0.5 mg/L (researchgate.net).
2. Solid and oil control: oil/water separators maintained to deliver ≤10–20 mg/L O&G to biology, using coalescers or hydrocyclones and regular skimming, with optional polishing via a DAF unit or clarifier. Efficient front‑end removal protects downstream processes and reduces organic shock (nepis.epa.gov).
3. PAC‑enhanced biotreatment: retrofits or new builds can feed PAC to aeration; trial doses of ~20–50 mg/L are common starting points, adjusted via jar tests and effluent tracking. High‑iodine‑number media (e.g., ≥1000 mg/g) is often selected; adsorption and biodegradation together have produced 30–50% boosts in organics removal and strong shock tolerance (80% phenol adsorption in 10 minutes was observed at moderate carbon dose) (researchgate.net; sciencedirect.com; nepis.epa.gov). Media and feed systems are available through activated carbon suppliers.
4. Monitoring and control: online NH₄, H₂S, pH (and phenol where available) are placed upstream and at bioreactor effluent; periodic toxicity tests validate performance and can trigger adjustments.
5. Safety and environmental controls: stripped H₂S is captured to flares or sulfide scrubbers; VOC off‑gas is treated via carbon or thermal oxidation. Any PAC carryover is filtered prior to discharge, often with an effluent cartridge filter to prevent fines release.
6. Review and iterate: effluent toxicity, permit limits and new influent chemistries (e.g., corrosion inhibitors) are assessed periodically. Regulatory changes, including Indonesia’s new B3 waste rules, merit tracking (hhp.co.id).
The combined strategy—robust pretreatment to strip H₂S, NH₃ and VOCs, followed by PAC‑assisted biological polishing—has repeatedly delivered measurable gains: >70% extra BOD removal in some trials, >99% micropollutant capture, and ~60% less sludge, with steadier effluent and fewer upsets (nepis.epa.gov; nepis.epa.gov; sciencedirect.com). With PAC dosing costs around $0.02–0.05 per m³ and lower sludge volumes, operators report rapid payback and more resilient plants (nepis.epa.gov).
