Galvanizing’s zinc problem meets a conventional fix: filtration, pH 9–10, clarification

Hot‑dip galvanizing is water‑intensive, using ≈0.5–3.0 m³ per tonne of steel to quench or clean zinc‑coated steel (iwaponline.com) (iwaponline.com). Worldwide, ≈60% of ~13 million tonnes of Zn produced per year ends up in galvanizing—about ∼72–75 kg Zn per tonne of steel (iwaponline.com).

Cooling/rinse tanks alone use ~2,300 L freshwater per tonne (≈2.3 m³/t, iwaponline.com), and most of that—95%—becomes wastewater (~400–3000 L/t steel, iwaponline.com). The quench effluent typically carries zinc as fine particulates (scrap, spatter) and dissolved Zn (often 10–100’s+ mg/L), plus other metals (Fe, Cu, etc.) and elevated pH from alkaline rinse.

One study reported Zn in galvanizing rinses from ~70 up to 33,800 mg/L (iwaponline.com) (iwaponline.com). Indonesian regulations (e.g., Permen LH 5/2014) typically limit Zn in effluent to low mg/L (e.g., ~5 mg/L or less), necessitating >90% removal in practice.

Quench effluent loads and target

Designing for the numbers above means removing both suspended zinc particles and dissolved zinc species. The objective is straightforward: meet low mg/L limits on Zn with a robust, end‑of‑pipe sequence that aligns with typical galvanizing flows and concentrations.

Coarse‑to‑fine filtration train

The first step is physical filtration to remove suspended Zn particles. Plants typically start with coarse screening around 0.5–1 mm before fine filtration; this protects downstream media because metal hydroxide flocs and zinc spatter clog filters easily (nepis.epa.gov). For coarse solids control, operators often deploy an in‑channel screen such as a manual screen.

Bag or cartridge filters in the 10–100 µm range capture micron‑scale particulates. As polishing stages that maintain low headloss, cartridge filters are a common choice. In practice, well‑designed bag/cartridge systems remove 85–99% of total suspended solids (TSS; suspended particles measured gravimetrically) and turbidity (www.researchgate.net).

For larger flows, media filters add throughput and resilience. Dual‑media beds such as sand/silica filters handle higher solids loading while stabilizing effluent clarity. Key design numbers: expect filtrate to achieve >90% solids removal; change‑out frequency depends on load, with metals‑finishing filters often cleaned or replaced when differential pressure (ΔP) reaches ~50–80 kPa.

A real‑world test underscores the gains: with alum coagulant, turbidity dropped from 557 to 1.4 NTU (99.7%) (www.researchgate.net). Even without coagulant, particulate Zn can still be >90% removed by proper pore‑size selection. Where plant layouts require an integrated headworks, packaged wastewater physical separation systems provide screens and primary solids removal upstream.

Hydroxide precipitation chemistry

Dissolved Zn²⁺ is removed by pH adjustment—raising pH to 8.5–10—to form zinc hydroxide (Zn(OH)₂, an insoluble precipitate). Zinc hydroxide solubility is minimized around pH ≈9–10; for most zinc systems, precipitation is essentially complete by pH 9.0–9.5 (www.researchgate.net). Plants dose caustic alkali—NaOH or Ca(OH)₂—via a metered system such as a dosing pump to reach ~pH 9–10, depending on buffering.

Bench data show that raising to pH 9.5 with appropriate coagulant can drop Zn from ~80 mg/L to ~3.7 mg/L (95.3% removal) (www.researchgate.net), and even pH≈9.0 gives ~93.8% removal (effluent Zn <5 mg/L) (www.researchgate.net). For perspective, an EPA example notes that adjusting a metal‑rich stream to pH 9 would reduce dissolved lead to ~0.2 mg/L versus 0.8 mg/L at pH 7.5 (nepis.epa.gov).

Dosing follows stoichiometry: roughly 2 moles of OH⁻ per mole Zn to form Zn(OH)₂. In practice, the alkali requirement is often on the order of 100–300 mg NaOH per liter for wastewater with tens of mg/L Zn, depending on initial pH and buffer capacity. To agglomerate fine Zn(OH)₂ flocs, plants add iron or aluminum salts from a coagulants program, sometimes supported by a flocculant polymer.

One study found a low alum dose (50–150 mg/L) plus 10 minutes of mixing dramatically improved clarity and removal (www.researchgate.net). Typical hydraulics use rapid mixing for 30–60 seconds followed by slow mixing for 5–10 minutes to ensure precipitation and floc growth.

Clarifier sizing and performance

After precipitation/flocculation, the slurry flows to a clarifier (sedimentation tank) that separates zinc hydroxide sludge from clarified water. A hydraulic retention time of 0.5–2 hours is common for metal‑hydroxide settling, and the surface overflow rate is typically kept ≤ 0.05–0.2 m³/m²·h (360–1,440 m³/m²·d) to allow floc settling.

As an example, at 0.1 m³/m²·h, a 10 m³/h flow needs ~100 m² of area (10 m² per 1 m³/h). In practice, much lower surface loading (e.g., 0.01 m³/m²·s, or 0.36 m³/m²·h) is used for fine precipitates. The weir overflow rate should be <0.5 m³/m²·h to prevent carryover. A conventional clarifier provides the required detention and solids control for this duty.

Where footprint is constrained, plate clarifiers can save space. A lamella settler enables the same separation with a compact plan area, while a tube settler increases clarifier capacity by increasing effective settling surfaces.

With well‑formed flocs, clarifiers can remove >95% of precipitated Zn(OH)₂. In the literature, combined flocculation/clarification routinely achieves >90–95% Zn removal (www.researchgate.net) (www.researchgate.net). One study reports final effluent Zn ~3–4 mg/L after full clarification (www.researchgate.net) (www.researchgate.net). Residual TSS in clarified effluent is typically <10–20 mg/L, often <5 mg/L.

Sludge handling and yield

The clarifier bottom yields zinc hydroxide sludge that is gelatinous; thickening and dewatering are recommended. A belt or plate press can dewater the sludge to >20% solids for disposal or zinc recovery.

A simple yield estimate helps size handling systems: removing 1 kg of Zn yields ~1.6 kg Zn(OH)₂ (molar mass 99 vs Zn 65, so sludge weight ~2.5× Zn mass). For example, treating 80 mg/L Zn in 100 m³ generates ~8 kg Zn (as ≈20 kg Zn(OH)₂) in daily sludge.

Outcomes, compliance, and figures

Combined treatment—[filtration plus precipitation plus settling](https://beta.co.id/en/blog/inside-a-galvanizing-plants-quench-tank-fix-filters-highph-chemistry-and-a-clarifier-that)—can reduce Zn from tens of mg/L down to below discharge limits. A lab‑scale treatment cut Zn from 79 to ~3.7 mg/L (95.3% removal) (www.researchgate.net); even at pH 9.0, 93.8% removal was achieved (≈4.9 mg/L) (www.researchgate.net). Post‑treatment pH is adjusted (neutralized) to meet discharge rules in the 6.0–9.0 range, an operation supported by water‑treatment ancillaries in most plants.

Other dissolved metals and contaminants (Cu, Fe, etc.) also precipitate at the same pH range, delivering multi‑metal removal (www.researchgate.net). In full‑scale HDG facilities using conventional end‑of‑pipe treatment (coag‑floc, sedimentation, filtration), cooling rinse streams at ~98–272 mg/L Zn enter treatment and discharge at ~<10 mg/L (iwaponline.com). EPA plating guidance likewise presumes hydroxide precipitation can meet limits, reflecting its status as “best practicable technology” (nepis.epa.gov).

Key design figures consolidate the approach: chemical dose around ~200–300 mg/L Ca(OH)₂ (or equivalent NaOH) for 50–100 mg/L Zn; pH setpoint ∼9.0–9.5 as indicated by solubility chemistry (www.researchgate.net); total mixing and reaction time ~10–20 minutes; clarifier detention of ~0.5–2.0 hours; surface loading ~0.1 m³/m²·h (360 m³/m²·d) or lower; overall removal >95% Zn and >99% TSS/turbidity (www.researchgate.net). Trials also showed >69% COD and >99% turbidity removal (www.researchgate.net).

Sources and context

The flow, concentration, and removal figures above are drawn from Indonesian industry studies, EPA guidance, and galvanizing process surveys. The 2020 pilot study on ZnO waste cited here (www.researchgate.net) (www.researchgate.net) underpins treatment performance, while global zinc and water‑use statistics (iwaponline.com) (iwaponline.com) set the operating context. These peer‑reviewed and government sources align the design parameters with practical outcomes and regulatory bounds.