Inside a Galvanizing Plant’s Quench-Tank Fix: Filters, High‑pH Chemistry, and a Clarifier That Hits 95% Zinc Removal

Quench tanks don’t look complicated. Yet the effluent they release is a regulatory minefield: both suspended and dissolved zinc, and a pH that drifts around neutral. The data are blunt — rinse or quench streams can carry on the order of 50–150 mg/L Zn (pubmed.ncbi.nlm.nih.gov). One study of galvanizing rinse water found ~100 mg/L Zn at pH ~5–6 (pubmed.ncbi.nlm.nih.gov).

The benchmark in Indonesia is strict: Permen LH 5/2014 prescribes Zn ≤5 mg/L for surface discharge, and 10 mg/L for less sensitive discharges (studylibid.com). In practice, quench effluent is nearly neutral (pH ~6–9) but carries fine Zn oxide/hydroxide particles (spatter, dross) plus dissolved Zn from carryover. All Zn must be removed or precipitated before discharge.

Quench effluent composition and limits

Those 50–150 mg/L Zn figures are not edge cases; they are typical of rinse or quench streams (pubmed.ncbi.nlm.nih.gov). The ~100 mg/L Zn at pH ~5–6 case underscores why neutralization alone is not enough. The discharge target is clear: ≤5 mg/L Zn (or 10 mg/L in less sensitive discharges) and pH between 6.0 and 9.0 (studylibid.com).

Mechanical pre‑filtration and cartridge stage

Designs start by skimming out solids before chemistry. A coarse screen or mesh traps dross; many plants favor an automatic screen for continuous removal of debris >1 mm. Downstream, fine filtration at 5–50 µm via bags, multimedia, or a cartridge filter captures sub‑millimeter zinc particles.

The plating industry’s playbook is consistent: insoluble byproduct precipitants are removed by physical separation via clarification and/or filtration (platinghome.com). Properly sized filters intercept free‑streaming zinc solids and can remove the bulk of particulate Zn (>80–90% of total suspended solids) (platinghome.com).

For example, a 10 m³/h flow passing through a 10 µm filter can lower TSS to <10 mg/L and remove coarse Zn precipitates by >90%, although performance depends on influent TSS. Where media filtration is preferred, dual‑media beds such as sand/silica are a common pretreatment to catch fines before chemistry. For materials compatibility and sanitary handling, many specify stainless cartridge housings.

pH adjustment and hydroxide precipitation

Dissolved Zn²⁺ is stripped out by raising pH to precipitate Zn(OH)₂. Metals‑treatment guides recommend a precipitation target around pH 9–9.5 (sterc.org). For galvanizing effluent, pH ~9.2–9.5 is optimal — above ~8.5 for Zn(OH)₂ formation but below ~11 where Zn(OH)₂ re‑dissolves (sterc.org).

In practice, strong base (lime or NaOH) is metered while mixing; dedicated chemical dosing pumps make pH control repeatable. Stoichiometrically, 1 mg Zn requires ~1.13 mg Ca(OH)₂ (one mole Zn per mole Ca(OH)₂). Thus, 100 mg/L Zn needs ≈114 mg/L Ca(OH)₂. For flows on the order of 5–20 m³/h, a well‑mixed neutralization tank of a few cubic meters (providing 30 min or more contact time) is typical (sterc.org). STERC explicitly recommends ≥30 min mixing with lime (sterc.org), and a two‑stage pH control is often used: rough neutralization to pH ~6–7, then final adjustment to ~9.5.

Coagulants and polymers accelerate settling. In practice, 20–100 mg/L of Fe³⁺ or Al³⁺ salts (ferric chloride, alum) plus a polymer aid is common; one Indonesian study added 50 mg/L coagulant (likely FeCl₃ or PAC) with stirring for 10 min (researchgate.net). Plants typically jar‑test to optimize dose; that study achieved Zn (79→3.7 mg/L) and turbidity (557→1.4 NTU) at pH 9.5, 50 mg/L coagulant, 10 min reaction (researchgate.net). The chemistry is: Zn²⁺ + 2 OH⁻ → Zn(OH)₂ (s).

Where aluminum coagulants are preferred, many operations deploy PAC (polyaluminum chloride) — see PAC for industrial wastewater — alongside core coagulant and flocculant programs. For scale of dosing, raising pH from 7 to 9.5 in a 10 m³/h stream containing 50 mg/L Zn would require ~0.5 kg/h of Ca(OH)₂ and produce on the order of 1 kg/h of Zn(OH)₂ sludge.

Clarifier sizing and performance targets

Precipitated Zn(OH)₂ and flocculated solids are separated by settling. A gravity clarifier or lamella settler is standard. Design guidelines point to a surface overflow rate around 30–50 m³/m²·day; at 10 m³/h (240 m³/day), a clarifier loading of 40 m³/m²·day requires ~6 m². In practice, many designs provide 30–60 min hydraulic retention time, with a sludge blanket depth of ~1 m and peripheral weirs (sterc.org). Typical metal‑hydroxide floc settling velocities of ~0.5–1.0 mm/s inform the overflow loading.

Performance can be very high. In the cited study, the clarifier produced a final turbidity of 1.42 NTU (down from 557) — a 99.7% suspended solids removal — and effluent Zn of 3.7 mg/L (from 79 mg/L influent) (researchgate.net). Many industrial clarifiers achieve >90–95% removal of metal hydroxide sludge, and lamella plates can boost capacity. Plants commonly specify a clarifier for gravity separation, or a lamella settler where a smaller footprint is required.

After settling, clear supernatant is decanted. Effluent pH is then neutralized to ~6.5–8 before discharge — a small dose of acid (e.g., H₂SO₄) or CO₂ often suffices — with metering handled by a dosing pump. In Indonesia, effluent pH is typically required between 6.0 and 9.0 (studylibid.com).

Sludge production and handling

The settled solids — Zn(OH)₂ plus coagulant floc — form a slurry or sludge. Sludge mass typically tracks at ~1.5–2× the mass of Zn removed (reflecting Zn in Zn(OH)₂ plus added Fe/Al hydroxides). As a rule of thumb, removing 1 kg Zn yields roughly 1.5 kg Zn(OH)₂. Many plants send this to a filter press and routinely dry sludge to >15% solids for disposal or recycling. If zinc recovery is of interest, technologies exist to further process the sludge.

Outcomes, sizing example, and compliance

With proper design, a galvanizing quench‑water system can cut Zn concentrations by >95%. In the Indonesian study, pH 9.5 precipitation with 50 mg/L coagulant gave 95.3% Zn removal, yielding 3.7 mg/L Zn — safely below the 5 mg/L regulatory cap (researchgate.net; studylibid.com). Clarifier effluent turbidity was ~1–2 NTU (researchgate.net), and well‑operated systems typically achieve treated effluent Zn in the low mg/L range (often <2–5 mg/L) and TSS <50 mg/L.

Quantitatively, if a plant processes 20 m³/hour of quench water at 100 mg/L Zn, the Zn load is 2000 g/h. Stoichiometric Ca(OH)₂ needed is ~2.26 kg/h (for 100% removal), while actual coagulant (e.g., FeCl₃) might be ~1–2 kg/h (based on 50–100 mg/L dosing). A reactor (30–60 min) and clarifier (1–2 m³ volume) could reduce Zn by ~95%. Seasonal or maintenance trends (e.g., higher drag‑out during certain processes) should be anticipated by flexible dosing.

Key metrics — flow rate, Zn mg/L, and the desired effluent — drive sizing: 15–30 min of strong mixing and ~0.5–1.0 h clarifier retention typically suffices. STERC’s guidance on mixing (≥30 min with lime) and residence volume reinforces that baseline (sterc.org). Ongoing monitoring (daily effluent tests for Zn, pH, TSS) ensures compliance with Indonesian standards (Zn ≤5 mg/L or 10 mg/L, pH 6–9) (studylibid.com).

Bottom line design

In summary, a conventional treatment train — solid filtration → high‑pH precipitation with coagulant → clarifier — effectively cleans galvanizing quench water. Data‑backed designs (e.g., Ratnawati et al. 2020) show >95% zinc removal at pH ~9.5 with modest coagulant dose (researchgate.net). With 15–30 min of strong mixing, ~0.5–1.0 h clarifier retention, and careful jar‑testing, plants routinely deliver clear effluent (<2 NTU, Zn a few mg/L) aligned with Indonesian limits (studylibid.com).