Engineers are carving reactors into aerobic and anoxic zones and hitting >90% nitrogen removal—if they control oxygen, pH/alkalinity, and carbon with precision. Case studies show 82–90% total N removal and effluent ammonia in the low single digits.
Oil and gas wastewaters often arrive with high ammonia (NH4+) and little easily biodegradable organics. The playbook now is biological: stage an aerobic nitrification zone up front and an anoxic denitrification zone downstream, then manage the handoff so reliably that the biology barely notices the swings in plant load.
In practice, it’s an activated‑sludge (or biofilm) reactor split into two (or more) compartments. The influent first sees an aerobic basin, dissolved oxygen (DO) about 2–4 mg/L, where autotrophic nitrifiers convert NH4+ → NO2− → NO3−. The stoichiometry is textbook—NH4+ + 2.0 O2 → NO3− + 2H+ + H2O—with about 4.57 mg of O2 consumed per mg NH4–N (nepis.epa.gov). That oxidation generates acidity, roughly 7.14 mg as CaCO3 per mg‑N (nepis.epa.gov), so pH control is not optional.
The nitrified effluent, now rich in NO3−, moves into an anoxic zone (DO ≪0.5 mg/L). Denitrifying heterotrophs use the nitrate as an electron acceptor and push it to N2 gas. Because many oil/gas streams lack easy carbon, designers commonly dose methanol or acetate to supply about 2.5–3.0 mg COD per mg NO3–N and target an M/N ratio around 3.0 (COD:N) for ≈95% nitrate removal (nepis.epa.gov) (lower ratios of ~2.6 suffice for partial removal, nepis.epa.gov). To make nitrate meet carbon, plants recirculate nitrified water into the anoxic tank at 1–3× the influent flow. This staged (often step‑feed) design routinely removes >90% of nitrogen.
Two‑zone biology and stoichiometry
Nitrifier biomass must be retained: these cultures grow slowly, so operators keep a high sludge age or use attached‑growth carriers. Mixed‑liquor MLVSS is often kept in the thousands of mg/L, or fixed biofilm media are used, to safeguard the nitrification step.
On the denitrification side, the oxygen target is essentially zero. Any residual aeration in the anoxic tank nullifies denitrification, so facilities rely on strict DO separation between zones. Designers lean on DO controllers or differential aeration to hold that line and protect the aerobic/anoxic split.
Evidence from pilots and SBRs
One membrane‑aerated reactor treating a high‑nitrogen feed (370–390 mg/L COD, 500–600 mg/L total nitrogen) hit 82% total N removal—about 100 mg N/L·d—without external carbon addition (www.researchgate.net). In laboratory mobile‑bed SBRs, nitrification efficiencies of 44–63% during acclimation with high COD/N have been reported (iwaponline.com), while well‑optimized systems routinely drive effluent NH4–N into the low single digits.
It matters because standards are strict: the Indonesian oil/gas wastewater standard KEP‑42/1996 requires effluent NH3‑N well below 10–15 mg/L, reinforcing the need for full nitrification/denitrification. Advanced systems—MBBR, MABR, and SBR—have logged N‑removals above 80% on high‑strength waste (www.researchgate.net) (pubs.acs.org).
Pre‑treatment for oil and solids
Oil, solvents, H2S, heavy metals, and suspended solids can inhibit nitrification; solids and oil droplets should be screened or floated off before biological treatment. Many facilities install automated headworks; for debris removal >1 mm, an automatic screen helps protect the biology upstream.
To address dispersed oil, dissolved‑air flotation is commonly paired with primary systems; a compact DAF unit offers the kind of surface loading and separation that reduce inhibitory carryover into the aerobic zone. Integrated primary trains (screens, oil removal, and primary basins) are standard in waste‑water physical separation prior to nitrification/denitrification.
Control points: oxygen, pH/alkalinity, carbon
DO setpoints anchor performance. Design targets are usually ≥2 mg/L DO in the nitrification zone (nepis.epa.gov), with theoretical oxygen demand at 4.57 mg O2 per mg NH4–N plus a margin for biomass growth (nepis.epa.gov). Too low DO (<1 mg/L) will stall nitrification. In the anoxic tank, DO is ≈0 (often <0.5 mg/L).
Nitrification consumes alkalinity—about 7.14 mg CaCO3 per mg‑N nitrified (nepis.epa.gov)—and can drag pH below 6.5, inhibiting nitrifiers. Plants typically dose sodium bicarbonate or lime to keep pH ≈7–8, raising wastewater alkalinity to about 150–200 mg/L as CaCO3. Denitrification pushes back on acidity (it consumes proton as NO3− is reduced). Continuous pH monitoring is essential; an unexplained pH crash often signals nitrification upsets.
Carbon is the denitrifier’s fuel. With low BOD feeds, operators target an M/N ratio near 3:1; methanol dosing around 3.0 mg COD per mg NO3–N removed is common for ≈90–95% nitrate conversion (nepis.epa.gov). Where hydrocarbon‑contaminated water raises influent COD:N, internal carbon can suffice; otherwise, methanol or acetate is added. Precise metering helps: a dedicated dosing pump simplifies COD feed control to prevent NO3− breakthrough.
Temperature, salinity, and biomass retention
Biological rates climb with temperature. Nitrifiers are active about 15–35 °C; below ~10 °C nitrification slows sharply, and above >40 °C they can be impaired. Warm wastewaters from gas treating often nitrify easily. Denitrification also slows below ~10 °C. In tropical Indonesia (25–30 °C ambient), rates are generally favorable.
Salt matters too. Nitrifiers can tolerate high salinity—complete NH4 removal even up to ~100 g NaCl/L—but fail above ~125 g/L (www.liebertpub.com). Retention matters as much as resilience: typical design SRTs are 10–15 days or more to avoid washout. A high mixed‑liquor MLSS (3–5 g/L or more) stores biomass; over‑aeration or too‑short SRT can bleach out nitrifiers, causing NH4 spikes.
Biofilm systems hold the slow growers in place. Moving‑bed designs retain nitrifiers on carrier media; in practice, a moving bed bioreactor (MBBR) bypasses the washout risk inherent to low SRT suspended‑growth plants.
Hydraulics, mixing, and time in tank
Hydraulic discipline is non‑negotiable. Both zones require good mixing (mechanical or via diffused air). Laminar paths or short‑circuiting cut contact time and depress rates. Typical aerobic hydraulic retention time (HRT) is 8–24 hours; anoxic HRT is ~1–4 hours, adjusted to match nitrate conversion. Sludge wasting is tuned to maintain the target SRT.
Internal nitrate recycles make the zones work together: treated water from the aerobic nitrifier is typically recycled into the anoxic tank at 1–3× flow to align nitrate with available carbon. Batch configurations, such as a sequence batch reactor (SBR), handle variable loads while preserving the aerobic/anoxic sequencing discussed above.
Shock loads and toxicants
Ammonia shocks or toxins can crash nitrification. “Ammonia shocks are the only crisis that disrupts nitrification,” one study notes (pubs.acs.org). Influent NH4+ must be equalized or partially removed—e.g., by an upstream stripper—to avoid >70–100 mg/L spikes (pubs.acs.org). Denitrifiers are somewhat more robust but will fail if oxygen intrudes or if carbon is exhausted.
Primary systems are the first line of defense against inhibitory carryover. Where free oil is persistent, targeted oil removal can protect downstream nitrifiers by preventing emulsions from entering the aerobic zone.
A refinery’s blueprint for reuse
In one refinery train, a pre‑stripper removed about 50% of incoming NH4+ by air‑stripping. The remaining ammonia was biologically nitrified in aeration basins and denitrified in anoxic zones, enabling water reuse for cooling and saving “millions of cubic meters” of fresh water (pubs.acs.org). It’s a snapshot of how rigorous zone control turns compliance into conservation.
Measured outcomes and rates
Under optimized conditions, one pilot reactor achieved nearly complete (≈82–90%) total N removal (www.researchgate.net). In practice, effluent NH4–N can often be driven below 1–3 mg/L with modern systems. Removal rates in the tens of mg N/L·d are typical; highly loaded industrial reactors can approach ~100 mg N/L·d transfer, as in the membrane‑aerated example above (www.researchgate.net).
Operating takeaway
Success hinges on balancing nitrifier and denitrifier activity via DO control, pH/alkalinity dosing, and carbon dosing. Meeting an effluent TN or NH4 limit requires sufficient reactor volume plus real‑time control of DO/pH and provision of enough carbon—principles that hold whether the plant is conventional activated sludge or biofilm‑based. When those parameters are met, biological nitrification/denitrification remains a cost‑effective, sustainable way to meet nitrogen limits in oil/gas wastewater.
