Three biological workhorses dominate refinery effluent treatment: conventional activated sludge, moving bed biofilm reactors, and membrane bioreactors. The trade-offs are footprint, sludge, and shock resistance—with MBRs delivering reuse-grade water at higher complexity and cost.
Refinery wastewater is a tough mix—oils, phenolics, salts, and toxicity spikes that can floor a plant. Yet three biological processes keep showing up in design rooms and tender books: conventional activated sludge (CAS), moving bed biofilm reactors (MBBR), and membrane bioreactors (MBR). The differences are stark. MBRs run with higher biomass and produce “pathogen-free effluents” that enable reuse and “reduce plant water footprint” (mdpi.com), typically in one‑third to one‑quarter the tank volume of CAS (mdpi.com). MBBR sits in the middle on footprint and cost—and is notably resilient to shock loads. CAS, the incumbent, is the simplest and cheapest to build but the bulkiest and most sludge‑intensive.
All three demand upstream oil/water handling; pretreatment is not optional. Designs typically incorporate primary oil/solids removal—think screens, separators, and skimmers—which aligns with physical separation for primary oil/water removal. Free oils also need attention before biology to avoid inhibition and fouling, a role served by dedicated oil removal steps.
Conventional activated sludge (CAS) baseline
CAS—large aeration tanks with suspended biomass plus clarifiers—is the most established process in refinery plants. Under steady influent loads it can achieve high organic removal: laboratory reactors have reported 94–95% COD (chemical oxygen demand) removal on petroleum refinery wastewater (researchgate.net). Typical mixed liquor suspended solids (MLSS, the biomass concentration) run ~2–4 g/L, which means relatively low biomass density and large reactor volumes.
In practice CAS systems struggle with highly toxic or variable refinery streams. Perturbations—like toxic shock or heavy oils—can suppress nitrification and slow recovery, since suspended sludge can be washed out or inhibited; fixed‑film or membrane processes tend to buffer such shocks better (researchgate.net) (researchgate.net).
Sludge is where CAS hurts. Biomass yield is typically ~0.5–0.7 g VSS/g COD removed (VSS = volatile suspended solids), driving high solids handling costs. One study found CAS produced ~135% more sludge than an equivalent MBBR system and ~230% more than an MBR‑based hybrid for comparable loads (mdpi.com). That excess sludge demands large clarifiers, thickening, and disposal—often 1–2% of influent flow wasted as sludge.
Advantages: CAS is well‑proven, relatively low‑capex (per m³/day), and simple to operate for stable municipal‑like loads. It can achieve strong COD/BOD (biochemical oxygen demand) removal (often >85–90%) and nutrient removal with proper design. Disadvantages: footprint and sludge production are the largest of the three. Typical refinery CAS plants might need 2–3× more aeration volume than MBR and much larger clarifiers. Effluent is limited by settling and tends to have higher residual TSS (total suspended solids) and organics than membrane effluents; in comparative tests, MBR effluent COD, NH₄‑N (ammonium nitrogen) and TSS were “always superior” and largely independent of influent spikes, whereas CAS showed higher and more variable outputs (researchgate.net). CAS can fail to meet very stringent limits (e.g., BOD <10–20 mg/L, Oil & Grease <5–10 mg/L) without additional polishing.
Attached-growth MBBR and shock resistance
MBBR (moving bed biofilm reactor) is an attached‑growth process in which biomass grows on mobile carriers (typically 40–70% of reactor volume) within an aeration tank, combining suspended and biofilm modes to retain high active biomass without washout. Effective biomass concentration (free + attached) is typically 6–10 g/L—higher than ~3 g/L in ASP (activated sludge process)—which shrinks reactor volumes to ~40–50% of a CAS plant for the same load (mdpi.com). The flow scheme is simple: one aeration basin with carriers and a physical screen, instead of a large clarifier, as delivered in packaged MBBR systems.
Sludge production is significantly lower. Under similar load conditions, one study found an MBBR produced ~23.7 g total sludge vs 37.2 g in CAS (≈35% less) (mdpi.com), and another report cited ~42% less biosolids than CAS (mdpi.com). Clarifiers, when used, can be much smaller (mdpi.com).
In refinery service, MBBR has proven robust. Cao et al. (2012) reported an MBBR treating petrochemical effluent at 1.0 kg COD/m³·d achieved >80% COD removal (down to <50 mg/L effluent), and even at 2.0 kg/m³·d it still removed ≈70%. They concluded “MBBR removal of COD was higher than ASP in all cases” and that MBBR showed “stronger resistance to organic load shocks” than CAS (researchgate.net). Schneider et al. (2011) found a pilot MBBR (carrier fill 60%, HRT [hydraulic retention time] 6 h) delivered effluent COD 40–75 mg/L (69–89% removal) on real refinery wastewater (researchgate.net).
Disadvantages: biofilm carriers need agitation and periodic cleaning; poor oxygen distribution can create dead zones on media. MBBRs still produce some suspended solids (sloughed flocs), so tertiary filtration or clarification is typically needed. Performance can drop if oils clog the biofilm. Capital and O&M costs are moderate—higher than CAS but typically lower than MBR.
MBR compactness and reuse-grade effluent
MBR couples suspended biological treatment with membrane filtration (usually micro/ultra‑filtration) to retain essentially all biomass. MLSS is routinely 7–12 g/L (mdpi.com), yielding extremely compact, high‑load reactors—roughly 25–50% of CAS volume for the same flow. Permeate quality is unmatched: TSS typically <3 mg/L and often BOD/COD <20 mg/L (mdpi.com). The membranes actively separate nearly all suspended and larger soluble substances (mdpi.com) (mdpi.com), producing “pathogen‑free effluents” and cutting the plant water footprint (mdpi.com).
MBRs excel with complex, toxic waste. High biomass and long sludge ages allow slow‑growing organisms (e.g., for phenol or high‑salinity degradation) to persist. In head‑to‑head tests, MBRs far outperformed CAS on trace organics: González et al. (2007) showed an MBR pilot removed ~94% of toxic alkylphenol surfactants (nonylphenols) versus ~54% in ASP—yielding effluent in the low µg/L range (researchgate.net). Reviews emphasize that “MBR effluent quality in terms of COD, NH₄⁺, and TSS was always superior” and largely insensitive to influent shocks (researchgate.net), and that MBRs “produce high quality effluent to meet strict discharge standards” with significantly smaller size (mdpi.com) (mdpi.com). The ultrafiltration step means virtually no solids or oil carryover, making MBRs excellent for stringent standards (e.g., BOD <5 mg/L, Oil & Grease <1–3 mg/L).
Advantages: effluent quality and compactness. MBRs eliminate secondary clarifiers—only membranes (often submerged) are used—delivered as integrated MBR systems. They enable reuse, which is why they are being promoted in Indonesia; for example, a new industrial park MBR achieves 85% reuse and complies with national standards (supremewatertech.com). One review also noted that “MBR effluent remains stable even if influent deteriorates” and referenced “low ML Flask, because almost all biomass is retained” (researchgate.net). Disadvantages: highest capital and O&M; membranes foul (especially with high oils/grease) requiring frequent chemical cleaning; energy use is higher (aeration to scour membranes and pumping), and spare parts/process expertise demands are greater. Economic analyses show MBR has lower footprint and long‑term OPEX but higher initial cost than CAS; it becomes cost‑optimal only for long‑lived plants or very stringent permits (researchgate.net). In short, MBR offers unparalleled treatment and space savings, but at the cost of complexity (fouling management) and tender cost. Where membranes are specified, the filtration step is commonly ultrafiltration, as supplied in UF modules and broader membrane systems.
Pretreatment and hybrid polishing trains
All processes need pretreatment (oil/water separation) before biological steps; if influent inhibitors (e.g., phenols, acids) are very high, staged operation or further pretreatment may be needed. MBRs are less susceptible to microbes washing out, but heavy oils can foul membranes. MBBRs cope well with variable organic loads and can degrade some compounds that CAS cannot; CAS is simplest when influent is relatively stable.
Regarding polishing, many refineries deploy hybrids: CAS or MBBR followed by post‑ozonation/activated carbon to reach reuse targets (researchgate.net) (researchgate.net). In those trains, the carbon block typically relies on high‑quality activated carbon media to strip residual organics.
Footprint, robustness, and cost calculus
Footprint: MBR is smallest—about one‑third to one‑quarter the tank volume of CAS (mdpi.com)—with MBBR intermediate (roughly half the volume of CAS at equivalent loading) and CAS largest (MLSS ~3 g/L). In fixed‑workload scenarios, MBR’s higher MLSS directly scales down reactor size (mdpi.com).
Treatment robustness to toxicity and load swings follows MBR ≥ MBBR > CAS. Studies report MBR effluents stable under shock, whereas CAS can see effluent degradation when feed quality changes (researchgate.net); MBBR shows “stronger resistance to organic load shocks” than ASP (researchgate.net).
Cost and scalability: CAS has lowest capital but highest O&M (sludge hauling). MBBR is middle‑ground: moderate capex, modest energy, lower sludge costs (mdpi.com). MBR has highest capex and energy use but can cut land cost and avoid further polishing costs by already meeting strict standards; long‑term total costs may be lower if standards tighten (researchgate.net) (mdpi.com).
Permits, land, and the Indonesian push
Tightening regulations and water scarcity in Indonesia are accelerating uptake of advanced biotreatment. National initiatives explicitly promote membrane systems to improve effluent quality (supremewatertech.com) (id.ionexchangeglobal.com), aligning with packaged membrane systems for industrial parks and refineries.
The choice ultimately depends on refinery size, discharge limits, land availability, and budget: CAS remains viable for less‑stringent permits; MBBR offers robustness with a moderate footprint; MBR delivers maximum removal and reuse potential at higher cost.
