Refineries’ New Water Math: Inside the API→DAF→Bio→O₃-AC Train Hitting COD ~50 mg/L

Refinery wastewater is a moving target — hydrocarbons, suspended solids, phenols, sulfides, ammonia, and the occasional slug of crude. The modern answer is a central wastewater treatment plant (WWTP) that strings together primary physical/chemical steps, secondary biological oxidation, and a tertiary polish, then automates the lot.

In Indonesia, PermenLH 19/2010 sets refinery effluent limits at BOD₅ ≤ 80 mg/L, COD ≤ 160 mg/L, oil & grease ≤ 20 mg/L, sulfide ≤ 0.5 mg/L, and NH₃–N ≤ 8 mg/L (id.scribd.com). In practice, advanced plants target tighter levels — final COD ~50 mg/L and TOC ~20 mg/L — to maintain compliance under real‑world variability (www.nature.com).

Treatment train fundamentals and targets

A typical refinery WWTP begins with influent screening and grit removal to protect downstream nozzles and pumps, where simple choices like an automatic screen or a headworks package can prevent early fouling. The core train follows: gravity oil–water separation (API or corrugated plates), dissolved air flotation (DAF), biological oxidation/nitrification, and a tertiary polish with filtration, membranes, adsorption, or advanced oxidation.

From recent case studies: API or CPI units remove roughly 60–80% of free oil; DAF with coagulants removes 80–90% of oil and TSS and can trim COD by ≈75% (www.mdpi.com; www.mdpi.com). Secondary biological systems remove dissolved organics (COD often <90%) and ammonia >90% with adequate sludge age and dissolved oxygen; integrated A/O+MBR systems have reported ≈94% removal of COD, BOD, and NH₄–N (www.nature.com; www.mdpi.com). Tertiary polishing (ozone/activated carbon/membranes) delivers final COD ~50 mg/L and TOC ~20 mg/L; free oil can fall below 1 mg/L with UF/MBR (www.nature.com).

Primary oil–water separation (API/CPI)

API separators (American Petroleum Institute standard) are gravity tanks sized so oil droplets ≥~150 μm rise under laminar flow governed by Stokes’ Law, typically with 30–60 minutes retention time. They have no submerged moving parts and very low energy draw, giving them resilience to large spills — but they can struggle with emulsified droplets, floor sludge, and maintaining quiescent hydraulics (sumiowater.com; sumiowater.com; pt.scribd.com). For 1,000 m³/h, a conservative design could allocate ~3,000 m² of footprint (e.g., 8 channels of 40 m × 4 m), and large plants often need multiple parallels (pt.scribd.com). Many sites augment skimming with compact modules akin to an oil-removal unit when space or spill risk dictates.

Corrugated plate interceptors (CPI) shrink footprint by adding inclined plates to shorten rise distance. In the same 1,000 m³/h scenario, CPI area is ~1,250 m² versus ~3,000 m² for API, with similar free‑oil removal; operators must manage moving scrapers and potential clogging with waxy solids, so CPI typically sits after a coarse tank or screen (pt.scribd.com).

DAF design, chemistry, and energy

Dissolved air flotation (DAF) injects micron‑scale bubbles to attach to fines and emulsified oil, lifting them for skimming. Optimized coagulant dosing (e.g., poly‑aluminum sulfate) at pH ~5–6 and air saturator pressures of 0.3–0.5 MPa routinely achieves >80% removal of residual oil, grease, COD, and TSS (www.researchgate.net). Full‑scale data show API+DAF trains removing ~90% of oil/solids and ≈75% COD, with oil often driven to <50 mg/L (www.mdpi.com; www.mdpi.com). In design, conventional DAFs handle 500–2000 m³/h, use upflow velocities of 1–2 m/h, and keep superficial rise ~1 m/h to maximize bubble attachment; recirculation power typically runs ~0.1 kWh/m³ (www.mdpi.com). De‑risking the step leans on a skid like a DAF unit, commercial coagulants and flocculants, and an accurate dosing pump for pH and polymer control.

DAF must be sized for peak oil loads and include automated skimmers and scum troughs to prevent re‑entrainment (www.mdpi.com; www.researchgate.net). One cited case combined API+DAF to achieve ~90% oil and solids removal and ~75% COD reduction (Dizayee, 2023; www.mdpi.com).

Secondary biological treatment choices

After primary separation, biology takes aim at dissolved organics and nitrogen. Activated sludge (AS) systems keep sludge retention time (SRT) long — 10–20+ days — for slow‑growing nitrifiers, with dissolved oxygen (DO) around 2–3 mg/L. Typical refinery influents carry COD in the 500–2,500 mg/L range; well‑run AS removes >85–95% BOD and 80–95% NH₄–N under adequate SRT/DO (www.nature.com). As a platform choice, an activated-sludge system remains the workhorse for many refineries.

Sequencing batch reactors (SBRs) package fill‑and‑draw cycles to manage variability and combine nitrification/denitrification in one tank, while fixed‑film options like MBBR/BAF carry more biomass to resist shock loads. Membrane bioreactors (MBRs) retain all biomass behind an ultrafiltration (UF) barrier, operating at MLSS up to 15–20 g/L to stabilize nitrification; reported results include 82% COD and 89% BOD₅ removal on real refinery wastewater in an HF‑MBR (www.mdpi.com). For flexibility, options span an SBR train, an MBBR retrofit, or a high‑efficiency MBR platform.

Kinetics, configuration, and ammonia control

Designers typically load at 0.2–0.6 kg BOD/kg MLSS/day with HRT of 6–12 hours and SRT >15 days. Multi‑stage trains — A²/O (anaerobic/anoxic/aerobic) or anoxic→aerobic (A/O) — sequence carbon oxidation and denitrification, often with initial aerobic removal of 50–80% BOD, an anoxic stage using internal carbon/recycle, and a final aerobic nitrification. Yang et al. reported 75% COD and 94% NH₄–N removal by combining microaerobic hydrolysis–acidification with anoxic/oxic AS (www.nature.com).

Ammonia spikes can derail nitrification: a proprietary Indonesian concern (IKORC) found that NH₄–N above ~70 mg/L caused failure unless pretreated, recommending upstream air‑stripping as needed (pubs.acs.org). Where tertiary polishing of nitrogen is required, plants may add a final denitrification step or deploy ion‑exchange resins; integrating an N/P removal module can safeguard against eutrophication drivers.

Solid–liquid separation after biology

Downstream of bioreactors, clarifiers or membranes separate biomass. Sludge recycles maintain MLSS and wasting controls SRT. Space‑constrained sites lean on media‑rich processes (e.g., IFAS/MBBR) to keep MLSS high without oversized clarifiers, while compact settlers like a lamella settler help shrink footprints by up to 80% compared to conventional basins.

Tertiary filtration and adsorption

Polishing shifts focus to trace organics, color, clarity, and pathogens. Multimedia filters handle 1–5 m³/m²·h to knock down remaining suspended solids; pairing with granular activated carbon (GAC) strips dissolved hydrocarbons and phenols. Plants commonly specify media such as sand/silica and use activated carbon beds post‑biological treatment.

Membrane filtration tightens the barrier: UF/MF can push turbidity below 0.1 NTU and, in some cases, total oil under 1 mg/L; pairing UF with MBRs yields reuse‑ready clarity (www.mdpi.com). A standalone ultrafiltration module often doubles as pretreatment and as the final barrier in water reuse trains.

Advanced oxidation and disinfection

Ozone (O₃), UV/H₂O₂, Fenton, or photocatalysis target refractory organics (e.g., phenols, PAHs). Ozonation stages typically dose ~5–20 mg/L with several minutes’ contact, followed by quenching or biofiltration to address by‑products such as bromate; an O₃ + activated carbon (O₃‑AC) finish has delivered COD ~50 mg/L and TOC ~20 mg/L in full‑scale practice (www.nature.com). Disinfection — typically UV or chlorination — is applied after substantial clarification; UV doses ≥30 mJ/cm² provide pathogen control, with equipment options like a UV disinfection unit.

Reuse ambitions and membranes

Refinery reuse is surging: UF/MF trains can meet potable reuse benchmarks for many parameters when paired with disinfection (www.mdpi.com). For salt and trace ion control — or to push COD < 20 mg/L and TOC < 5 mg/L — designers turn to NF/RO as part of an integrated membrane system. Where brackish TDS dominates, a brackish-water RO stage or a hardness‑targeted nano-filtration bridge can complete a ZLD‑oriented stack.

Design targets and achieved performance

State‑of‑the‑art WWTPs commonly guarantee BOD₅ < 20 mg/L, COD < 60 mg/L, oil & grease < 5 mg/L, and NH₃–N < 2 mg/L (local limits apply), and can go lower for reuse (e.g., COD < 20 mg/L, TOC < 5 mg/L after RO). A multi‑stage enhanced system in Shanghai (ICBAF + hydrolysis + A/O + MBR + O₃‑AC) processed ~2,500 mg/L influent COD down to ~50 mg/L and removed >94% of COD, BOD₅, NH₄–N, and P, with the first aerated biofilter removing ~83% of BOD₅ (www.nature.com; www.nature.com; www.nature.com).

Sludge and side‑stream management

API/CPI and DAF steps yield oily sludges and scums; plants de‑oil and either send offsite as hazardous waste or treat on site via thickening, dewatering, and, in some cases, thermal drying or incineration. Biological trains generate secondary (waste activated) sludge; co‑thickening and centrifuges/filter presses are common. Optimized pretreatment (API/DAF) cuts sludge volume and disposal cost, a major OPEX driver (www.mdpi.com).

Supporting waste streams — neutralization residuals, filter backwash, and spent GAC regeneration water — require capture and return to headworks or solids handling; spent GAC can be thermally reactivated or landfilled, and metal hydroxide sludges are settled as solid waste. Consumables and spare parts across these systems sit under standard water/wastewater chemicals and plant ancillaries.

Instrumentation, automation, and safeguards

Modern plants deploy online pH, DO, redox, turbidity, oil‑in‑water, and nutrient analyzers (ammonia/nitrate) to modulate aeration and dosing in real time. SCADA/PLC controls coordinate blowers, pumps, and interlocks; autosamplers generate composites for lab verification. Upsets — crude slugs or caustic leaks — route to emergency equalization or neutralization tanks; covered tanks and scrubbers address odors. While CEMS focus on air, effluent analyzers and soft sensors (e.g., UV‑Vis surrogates for COD) are increasingly standard at outfalls.

Footprint, energy, and cost

Space matters. CPI designs can be ~60–75% smaller in footprint than API for the same flow (pt.scribd.com), and MBRs trade higher energy for smaller clarifiers. Energy is dominated by aeration (biology) and pressurization (DAF). One analysis pegs API/DAF pretreatment at ~0.09 kWh/m³ versus ~0.86 kWh/m³ for MBRs, with API+DAF consuming ~10× less energy and roughly 1/6 the lifecycle cost than an advanced polishing plant; “pretreatment is fifteen times more” cost‑efficient in the cited comparison (www.mdpi.com). Compact settlers such as a tube settler can further reduce basin footprints 3–4× where space is constrained.

Industry trends and water reuse

Trends point to deeper membrane integration (MBR, UF), advanced oxidation to tame recalcitrants, and movement toward zero‑liquid‑discharge in water‑stressed regions. Refinery water reuse initiatives can cut freshwater intake by 10–40% (values vary by site) according to a Fluence Corp. report cited by sensorex.com. Embedded analytics and smart sensors are being applied to optimize aeration and chemical dosing — including AI‑based NOx control concepts within nitrification strategies — as data becomes another lever on OPEX.

Representative performance snapshots

Wang et al. (Sci. Rep.) reported an ICBAF + hydrolysis + A/O + MBR + O₃‑AC train treating ~2,500 mg/L COD influent and removing 94% COD, BOD₅, NH₄–N, and P, with final COD ~50 mg/L and TOC ~20 mg/L; the first aerated biofilter alone removed ~83% of BOD₅ (www.nature.com; www.nature.com; www.nature.com). Razavi & Miri (HF‑MBR) observed 82% COD and 89% BOD₅ removal on real refinery wastewater (www.mdpi.com). An API+DAF pretreatment case demonstrated ~90% oil/TSS and ~75% COD removal (Dizayee, 2023; www.mdpi.com; www.mdpi.com).

Reference base and standards

Design and performance figures here are grounded in industry manuals (API 421), peer‑reviewed research on DAF and biological systems (www.researchgate.net; www.nature.com), and cost/energy comparisons contrasting pretreatment and MBR operations (www.mdpi.com; www.mdpi.com), alongside Indonesian effluent standards (id.scribd.com). All numerical values and performance benchmarks above are drawn directly from these sources.