Produced water and refinery effluent pack dissolved/dispersed hydrocarbons, high salinity, and heavy metals. Tighter discharge rules are pushing operators toward three biological workhorses—conventional activated sludge, moving bed biofilm reactors, and membrane bioreactors—each with distinct trade‑offs in robustness, footprint, and cost.
Oil and gas “produced water” and refinery streams aren’t just dirty—they’re complex. They mix dissolved and dispersed hydrocarbons such as BTEX and PAHs, phenols, heavy metals, and treatment chemicals with high TDS (total dissolved solids). Typical COD (chemical oxygen demand) and BOD (biochemical oxygen demand) range from the hundreds to thousands of mg/L, with oil and grease often in the tens of mg/L.
Regulatory pressure is rising. Offshore discharge guidelines often require dispersed hydrocarbons at ≲10–40 mg/L and BOD/COD in the tens of mg/L (mdpi.com), and Indonesia’s anticipated standards are tightening as well. That is steering design toward advanced biological treatment, usually after primary oil removal using systems such as screens and oil‑removal primaries and dedicated oil removal units.
On most projects, the short list is three technologies: conventional activated sludge (CAS), moving bed biofilm reactors (MBBR), and membrane bioreactors (MBR). Here’s how they really compare, backed by data and field trials.
Conventional activated sludge: proven but space‑hungry
CAS suspends microbial flocs in an aerated basin, then uses a clarifier to settle solids. In oil and gas service, it is a familiar route to remove organics and nitrify, often deployed with clarifiers and standard activated‑sludge setups.
When well acclimated and operated at long SRT (solids retention time), CAS can deliver high removals. One continuous aerobic treatment of produced water at SRT ≈20 days and MLSS (mixed liquor suspended solids) ~730 mg/L achieved ≈98–99% TPH (total petroleum hydrocarbons) removal (researchgate.net). Another trial reached 92% COD and 99% BTEX removal after acclimation (researchgate.net).
Trade‑offs are material. CAS needs large reactors and clarifiers, especially at high strength, and free‑floc biomass is vulnerable to dispersed oil, biocides, toxic shocks, or extreme salinity. Long SRTs (weeks) are often essential; shorter SRTs can sharply reduce nitrification and hydrocarbon removal. Effluent quality is moderate—typically 80–90% BOD/COD removal—and often fails stricter reuse goals (e.g., ≥30–50 mg/L BOD). CAS generates large sludge volumes, and without carriers or membranes it is relatively vulnerable to sudden toxic pulses, with effluent COD often 50–100 mg/L.
Moving bed biofilm reactors: shock‑tolerant and compact
MBBR uses free‑moving plastic carriers where biofilms attach; carriers are mixed in an aerated basin, with a secondary clarifier or post‑treatment to remove sloughed solids. The attached biofilm protects microbes and prevents washout, boosting resilience compared with CAS. Modern systems are sold as MBBR packages with media such as high‑surface‑area bio media or foam carriers like Levapor foam.
The performance data are notable. In an offshore field trial treating 100 g/L‑saline produced water with acclimated carriers, MBBR removed >90% BOD in only 1 hour HRT (hydraulic retention time) at an organic loading of 12 kg COD/m³·d, and the acclimated biofilm strongly reduced biomass washout under upsets compared with a CAS configuration (mdpi.com).
Lab studies echo that robustness: a synthetic produced‑water MBBR maintained >90% COD removal as TDS increased from 1.5 to 20 g/L (mdpi.com), and a bench study showed >90% TPH removal using carbon‑enhanced MBBR media versus ~57% without carbon (pubmed.ncbi.nlm.nih.gov). In practice, MBBR has handled moderate‑to‑high salinity—up to ~20–35 g/L TDS—with COD removals typically >80%.
Operationally, MBBR requires no sludge return pumping; only excess biofilm is wasted (researchgate.net). That simplicity, plus more biomass per unit volume, lets reactors be smaller, and many plants retrofit carriers into existing activated‑sludge tanks to boost capacity by 30–50% without new civil works. The downsides: vigorous aeration and mixing to suspend carriers raise energy demand—a 5 MLD MBBR case needed ~4763 units of aeration versus ~2418 for an equivalent CAS, roughly 2× the oxygen demand (thembrsite.com). Sloughing can cause effluent TSS fluctuations, and effluent polish is typically lower than MBR, with potential residual phenolics or COD. Capital cost is higher than CAS but lower than MBR. Like CAS, foams or biomass can still be affected by very toxic substances if not acclimated.
Membrane bioreactors: highest polish, smallest tanks
MBR couples a suspended‑growth bioreactor with micro/ultrafiltration membranes (submerged or sidestream) that retain virtually all biomass. Decoupling SRT from hydraulic residence lets plants run at high MLSS and produce effluent with near‑zero suspended solids; most bacteria/pathogens are retained. Commercial systems are available as MBR packages leveraging ultrafiltration membranes.
In petrochemical and refinery applications, MBRs routinely remove ≥80–90% COD and essentially all ammonia via nitrification, owing to long SRT. One study treating real petrochemical wastewater reported >80% COD removal and ~99.9% NH4–N removal (scholars.cityu.edu.hk). Models indicate a full‑scale MBR can have ~24–42% smaller building footprint than an equivalent CAS plant, with pure tank area about ~42% smaller (thembrsite.com). The specific carbon footprint is ~25–30% lower than CAS, reflecting less concrete and steel (thembrsite.com). Long SRT also means fewer sludge solids to dispose of due to higher endogenous decay.
There are cautions. Oily substances foul membranes, so primary oil removal is critical upstream; MBRs are sensitive to suspended solids and require scouring/pumping energy plus periodic chemical cleaning. In a 5 MLD comparison, aeration demand was ~4151 units for MBR versus ~2418 for CAS (~1.7×) (thembrsite.com). Membrane life and cleaning add to costs, and very toxic wastes can still inhibit biomass despite retention.
Comparative footprint and energy
Footprint ranking is clear: MBR < MBBR < CAS. Modeling shows MBR plants ~25–40% smaller in building volume than CAS at equal flow (thembrsite.com), and ~24–42% smaller in building footprint depending on whether ancillary equipment volume is included; on pure tank area, MBRs can be ~42% smaller (thembrsite.com). MBBR commonly fits into existing basins and avoids return sludge loops, reducing the space of a CAS upgrade, though it is typically not as compact as MBR.
Energy and Opex follow the biology: higher activity costs more air. One analysis showed MBBR aeration at ~4763 units for 5 MLD vs ~2418 for CAS (about 2×), while MBR was ~4151 vs 2418 (~1.7×) for the same flow (thembrsite.com). MBBR saves on sludge recycle pumping—no return loop is needed (researchgate.net)—but still needs mixing to keep carriers in motion. CAS can be cheapest per unit flow to run, but its larger footprint and the need for tertiary polishing steps can offset that advantage. Where clarifier capacity is a bottleneck, compact settlers such as clarifiers can be paired with MBBR or CAS.
Removal performance and robustness
All three can achieve high removals; how they get there differs. A well‑operated CAS at long SRT can remove ~90–99% petroleum organics (researchgate.net; researchgate.net). Acclimated MBBR can reach ~90–95% TPH removal with enhanced media at much higher organic loadings (pubmed.ncbi.nlm.nih.gov) and has delivered >90% BOD removal in 1 hour HRT at 12 kg COD/m³·d under 100 g/L salinity (mdpi.com). MBRs consistently post ≥80–90% COD removal and >99% nitrification in petrochemical wastewater (scholars.cityu.edu.hk), and in practice give the most consistent effluent.
On resilience, MBBR and MBR substantially outperform CAS. Biofilm carriers in MBBR protect microbes from washout; pilots saw negligible biomass loss under upsets while CAS lost flocs (mdpi.com). MBR’s membrane similarly retains all biomass regardless of hydraulic conditions. Salinity tolerance tracks with that: acclimated MBBR/MBR systems have handled salinities of tens of g/L with moderate performance drops; for example, COD removal declined from 94% to 82% as salinity rose from 100→250 g/L in one study (mdpi.com). Unacclimated CAS typically fails at such extremes.
Handling complex, toxic wastewater
For complex or refractory organics—such as alkylphenols or high‑molecular hydrocarbons—steady‑state biofilms (MBBR) and high‑SRT biomass (MBR) are generally better suited. MBR in particular has been used to meet phenol standards in petrochemical effluents (scholars.cityu.edu.hk). Heavy metals and biocides are not fully removed by any of the three; immobilized biofilms and high sludge retention can sequester some metals and reduce toxicity.
All three typically rely on upstream oil removal (e.g., hydrocyclones or skimming) to protect biology; many facilities deploy compact primaries and dissolved‑oil management ahead of the biology, where components such as DAF units or dedicated oil removal systems can support the biological core.
What’s winning in projects right now
For new builds and upgrades in oil and gas, engineers increasingly favor MBBR or MBR to hit strict COD/BOD and toxicity limits. Both have delivered >90% COD and >95% BOD removals in pilots (the paper notes “ridge‑water” trials) (scholars.cityu.edu.hk; mdpi.com). Where space is constrained or reuse is desired, MBR often wins on footprint and polish: modeling shows ~25–40% less building volume than CAS (thembrsite.com) while achieving >99% nitrification and very low effluent BOD.
The middle ground is MBBR: compact, resilient, and able to retrofit quickly using media carriers, with fewer moving parts than MBR and no sludge return. CAS remains viable when land is ample and waste is less toxic, but it carries the largest footprint and the greatest sensitivity to salinity and toxic shocks. In all cases, the biological choice follows the post‑oil‑removal target and site constraints—and increasingly, the ability to deal with the toxic, salty reality of modern produced water.
