Inside the steel mill that cleans oil, metals, and cyanide — by design

Modern integrated steel mills run on water — roughly ~28.6 m³ of process water per tonne of steel (www.mdpi.com) — and send back highly variable, pollutant‑rich waste streams that would overwhelm a municipal plant.

Coking and blast‑furnace operations load up organics (COD, BOD), ammonia, phenols, cyanide and oils far beyond typical municipal values. One coke‑oven wastewater sample clocked COD ~1,931 mg/L, BOD₅ ~1,242 mg/L, and free cyanide ~5.3 mg/L (iwaponline.com). By contrast, blast‑furnace blowdown in one study had moderate organics (COD ~54 mg/L) but very high salts (TDS ~6,445 mg/L) (iwaponline.com).

The major pollutants on the watchlist: COD, NH₃‑N, phenols, cyanide, TSS, oils and heavy metals (pmc.ncbi.nlm.nih.gov). Meeting strict discharge standards typically means BOD/COD in the tens of mg/L, oils <5–10 mg/L, cyanide <0.2 mg/L and heavy metals (Zn, Cu, Ni, Cr, etc.) <0.1 mg/L.

Influent equalization and screening

A large equalization basin (hours to days retention) is the shock absorber, buffering flow and strength peaks, blending streams, and stabilizing pH before downstream units. In practice, designs allow, e.g., 6–12 h of retention at peak flow to flatten surges.

Primary hardware starts with physical separation — screens and grit — to keep debris out of hydraulics, the domain of systems like waste‑water physical separation. Where debris loads fluctuate, plants standardize on continuous removal via an automatic screen.

API–DAF oil removal train

An API separator (gravity oil–water separator) goes first to lift free and non‑emulsified oil (droplets ≳150 µm) by buoyancy. Efficient API design (for example, length/width ≥5:1) can remove the bulk of floating oil and settle solids, often 60–80% or more of “free” oil depending on droplet size and retention time.

To tighten oil targets, a chemically aided Dissolved Air Flotation (DAF) follows. With coagulant addition (polyaluminium/ferric salt + polymer), DAF can treat influents with oil and grease up to ~500 mg/L and remove ≥90–95% of residual oil and TSS (parsianfarab.com). On the equipment side, compact skids such as a DAF unit are standard in these trains.

Multi‑stage oil removal — gravity then flotation — routinely drives effluent oil to ≪10 mg/L. Where gravity separation needs a boost, plants integrate dedicated oil removal modules to strip free oil before flotation.

Chemical precipitation of heavy metals

Metals come out next with pH adjustment and coagulant dosing: lime or caustic nudges pH to 8.5–10, and sulfide or ferrous iron pushes metals to form insoluble hydroxides/sulfides; ferric chloride or alum flocculants then agglomerate colloids. Coagulation–flocculation is a conventional, cost‑effective heavy‑metal treatment (pmc.ncbi.nlm.nih.gov).

For the coagulant, operators often choose polyaluminium salts, aligning with solutions like PAC for industrial wastewater. Floc build and settling benefit from tailored polymers, supported by programs built around flocculants.

Precision matters at this stage, so metal removal trains typically meter reagents through a dosing pump for stable pH control and coagulant feed.

The numbers guide design: one coke‑oven effluent in a cited study carried ~1.53 mg/L Zn (iwaponline.com). A well‑operated lime or sulfide precipitation step typically reduces dissolved Zn (and similar metals) by over 90%, aiming for <0.1 mg/L in the effluent. The metal‑rich sludge is thickened/filtered and disposed or recycled.

Anaerobic front‑end for high COD

For high‑strength organics, the plant leans on anaerobic oxidation. A high‑rate anaerobic reactor such as UASB (upflow anaerobic sludge blanket) or EGSB (expanded granular sludge bed), or an anaerobic baffled reactor, typically runs 20–40 °C, handles very high COD loads, and produces biogas. UASB systems commonly remove on the order of 60–90% of COD from extremely polluted streams.

In one full‑scale integrated steel WWTP, an anaerobic/anoxic front‑end achieved ~90% COD reduction and 88% NH₄⁺ removal (iwaponline.com). Anaerobic options in this slot include modular packages such as waste‑water biological digestion.

The anaerobic effluent still contains biodegradable organics and ammonia (from cyanide/chloride oxidation), which pass to aerobic treatment.

Aerobic nitrification and denitrification

A subsequent aerobic bioreactor — activated sludge (suspended growth), SBR (sequencing batch reactor), or oxidation ditch — completes COD removal and converts ammonia to nitrogen. Typical schemes include A²/O (anaerobic–anoxic–oxic) or similar hybrids.

In cited pilot/full‑scale plants, the combined system drove NH₄⁺‑N from ~278 mg/L down to ~1.7 mg/L (∼99% removal) and residual COD to <100 mg/L (iwaponline.com). Conventional basins align with solutions like activated sludge for BOD/COD polishing and nitrification.

Denitrification (an anoxic zone with supplemental carbon) removes produced nitrate, yielding total nitrogen in the low single digits mg/L. Overall, hybrid biological trains routinely achieve 85–95% reduction of COD/BOD and >90% of ammonia, often leaving effluent COD on the order of a few tens of mg/L (iwaponline.com).

Attached‑growth reactors — biofilm towers or MBBR (moving bed biofilm reactor) — are resilient under cyanide and phenol loads; options such as moving bed bioreactors (MBBR) support stable nitrification alongside carbon removal.

Cyanide and phenols management

Cyanide and volatile phenols, toxic at low ppm, also degrade across the bio‑process. One hybrid aerobic system removed ~99% of free cyanide (iwaponline.com). In practice, an anaerobic step can convert metal‑complex cyanides to simpler forms, and subsequent nitrification/oxidation degrades cyanate to N₂/CO₂.

Polishing filtration and adsorption

After biological treatment, solid/liquid filtration (e.g., sand filtration or membrane filtration) removes remaining flocs, yielding turbidity/SS <5 mg/L. Plants commonly add beds built on sand/silica filtration media to capture fines.

Where a tighter particulate cut is needed, operators deploy low‑pressure membranes, for example ultrafiltration, as a polishing step before final discharge or reuse.

Activated carbon adsorption targets trace organics and dissolved cyanide. GAC or powdered AC columns remove residual phenols, thiocyanates and complex organics, a role served by systems based on activated carbon. Adsorption trials on bio‑treated coke plant effluent achieved up to ~70% phenol and ~80% total cyanide removal at moderate contact times (www.mdpi.com). In full‑scale practice, multi‑stage carbon polishing (often combined with aeration/oxidation) typically reduces phenols and CN to below regulatory detection limits (<0.1–0.2 mg/L) on top of prior treatment.

Performance snapshot and compliance

The overall performance of this centralized scheme is high. Numerical examples from pilot/full plants indicate effluent routinely meeting strict targets: COD <50–100 mg/L, BOD <20–30 mg/L, NH₄⁺ <5 mg/L, cyanide <0.2 mg/L and metals <0.1 mg/L.

In one documented case, the treated effluent reached ~98 mg/L COD and ~1.7 mg/L ammonia (from influent 1,230 and 279 mg/L, respectively) (iwaponline.com). DAF pretreatment to 500 mg/L organics removed ~95% of oils/solids (parsianfarab.com), and AC polishing removed ~80% of remaining phenolics/CN (www.mdpi.com).

Process data thus imply consistently high removal efficiencies (>90% for organics and N, >99% for cyanide (iwaponline.com)), enabling final compliance well within typical Indonesian or international discharge standards (often requiring <0.1–0.5 mg/L for CN and heavy metals, tens of mg/L for COD/BOD).

Sources and citations

Authoritative industry and research reports were used. For example, Colla et al. report ∼28.6 m³/tonne water use in integrated steel (www.mdpi.com). Mondal et al. (2021) provide detailed effluent characterization (Table 1) for coke/blast‑furnace streams (iwaponline.com) (iwaponline.com). Reviews of treatment methods (Oladimeji et al., 2024; Pawar et al., 2022) detail coagulation for metals (pmc.ncbi.nlm.nih.gov) and hybrid bioreactors for COD/N and CN removal (iwaponline.com) (www.mdpi.com), confirming the high removal rates cited above. All cited values (e.g., percent removals, influent/effluent concentrations) are drawn from these peer‑reviewed sources to support the plant design strategy.