Galvanizing’s fluxing rinse water is loaded with zinc and ammonia. A two‑stage fix hits strict limits

Galvanizing’s fluxing stage leans on concentrated zinc chloride (ZnCl₂) and ammonium chloride (NH₄Cl) solutions, so the rinse water that follows predictably carries high dissolved zinc (Zn²⁺) and ammonium (NH₄⁺). EPA documentation notes kettle‑flux solutions can be ≈98% ZnCl₂ by weight (nepis.epa.gov), and that galvanizing rinse waters contain “zinc, lead, and iron, which must be treated as hazardous” (nepis.epa.gov).

That chemistry meets hard policy. Indonesian discharge regulations (PermenLH/PermenLHK) mandate low mg/L limits for heavy metals—e.g., zinc often ≲5 mg/L—and similarly restrict ammonia‑nitrogen to the low mg/L range (drawing on values cited in Ratnawati; we infer similar targets might apply for galvanizing effluent) (researchgate.net).

There is a playbook that works. Start with pH‑driven chemical precipitation of zinc as zinc hydroxide, then add biological treatment if very low ammonia is required. Below is what the data say about setpoints, removals, and unit process sizing—and what it means for compliance.

Fluxing rinse composition and standards

Flux rinses are heavy‑metal wastewater high in dissolved Zn²⁺ and NH₄⁺ because fluxing uses ZnCl₂ and NH₄Cl (nepis.epa.gov). Even dilute rinses can carry tens of mg/L Zn when upstream kettle‑flux is ≈98% ZnCl₂ by weight (nepis.epa.gov). One EPA study calls galvanizing rinse water “zinc, lead, and iron, which must be treated as hazardous” (nepis.epa.gov). Indonesian Ministerial standards referenced in Ratnawati place zinc in the low mg/L range; in an Indonesian ZnO‑factory case, influent at 79 mg/L Zn was precipitated to 3.7 mg/L (95% removal) meeting the Minister’s effluent standard (researchgate.net).

Chemical precipitation via pH adjustment

The core step is pH‑driven precipitation of Zn²⁺ as Zn(OH)₂. Raising pH to ~9–10 using Ca(OH)₂ or NaOH (pH is a measure of acidity/alkalinity) converts Zn²⁺ to solid Zn(OH)₂: Zn²⁺ + 2 OH⁻ → Zn(OH)₂ (researchgate.net). The optimal pH window is ~9.0–9.5; one study reported 93–95% Zn removal in that range (researchgate.net).

In Ratnawati’s tests, adding alkali and 50 mg/L polymer (PAC, polyaluminum chloride coagulant) achieved Zn removal from 79 to 3.71 mg/L (95.3% removal) within 10 minutes at pH 9.5 (researchgate.net). Even pH 9.0 alone yielded 93.8% removal (Zn ≈4–5 mg/L) with less reagent (researchgate.net). Plants cited by EPA “treated wastewater by lime and polymer addition and pH adjustment” with good results (nepis.epa.gov).

After precipitation, heavy‑metal hydroxides must be settled or filtered; Zn(OH)₂ particles have low settleability, so a coagulant or flocculant is usually added, and the settled sludge contains the metal hydroxides (researchgate.net). To control pH accurately in this step, facilities often rely on an automated dosing pump. For coagulation, industrial PAC formulations such as PAC for wastewater are commonly applied to grow settleable flocs. [Settling is typically performed in a clarifier](https://beta.co.id/en/blog/inside-a-galvanizing-plants-quench-tank-fix-filters-highph-chemistry-and-a-clarifier-that); equipment like a clarifier is designed to remove suspended solids by detention and gravity separation.

Sludge characteristics and handling

Galvanizing sludge typically contains ~8–18% Zn by dry weight, a concentration high enough for possible zinc recovery (mdpi.com). For stoichiometry, ~1.5 kg Zn(OH)₂ forms per kg Zn removed. The clarified effluent after solids removal contains mainly calcium and chloride ions. Plants often dewater and minimize the resulting sludge volume using solutions in the vein of sludge treatment.

Ammonia removal and biological polishing

Because the pH step targets metals, ammonia needs a separate lens. Raising pH shifts ammonium (NH₄⁺) to volatile ammonia gas (NH₃) (pKa≈9.25, where pKa is an acid–base equilibrium constant), so some stripping does occur; however, significant ammonia often remains. If discharge limits are very strict—on the order of a few mg/L NH₃–N—biological nitrification (biological oxidation of NH₄⁺→NO₂⁻→NO₃⁻) is advisable. Full‑scale systems routinely achieve >90% ammonia removal; even experimental counter‑current aeration configurations have reported 98% removal to <1 mg/L NH₃–N (nepis.epa.gov). Realistic designs—SBR (sequencing batch reactor), RBC, MBBR (moving‑bed biofilm reactor)—can reduce high ammonia (tens of mg/L) to single‑digit mg/L.

Heavy metals can inhibit nitrifiers, so adequate Zn removal first is crucial (mdpi.com) (mdpi.com). Once Zn is precipitated to ≲5 mg/L, toxicity to nitrifiers is minimal; Zn levels up to ~5 mg/L had little effect on AOB (ammonia‑oxidizing bacteria), whereas ~20 mg/L caused irreversible inhibition (mdpi.com). Where biological polishing is selected, utilities often deploy platforms like SBR or MBBR to provide robust nitrification at compact footprints.

If permits dictate near‑zero ammonia (e.g., <1–2 mg/L NH₃–N), integrate nitrification and, if needed, heterotrophic denitrification (biological reduction of NO₃⁻), recognizing that this rinse water has little BOD and may require external carbon (e.g., methanol). If financial or land constraints exist, one could consider gas‑stripping towers, which in tests reached similar ammonia removal (nepis.epa.gov); however, a biological reactor is generally more compact and cost‑effective for continuous flows in the context described.

Measured removals and operating ranges

Chemical precipitation can routinely remove >90% of dissolved Zn. In practice, initial Zn around ~80 mg/L dropped to ~3–5 mg/L (≈95%) in controlled studies (researchgate.net) (researchgate.net). Dosage—for instance, on the order of 1.1 kg Ca(OH)₂ per kg Zn—and polymer addition in the tens of mg/L should be scaled to the flow and concentration.

For ammonia, aerobic nitrification in appropriately sized reactors achieves >90% removal, and the cited counter‑current aeration test reached 98% removal to <1 mg/L NH₃–N (nepis.epa.gov).

Sizing example and compliance check

At 100 m³/day of flux effluent with 80 mg/L Zn and 50 mg/L NH₄–N, dosing roughly 10–12 kg/day Ca(OH)₂ (or NaOH) to reach pH 9–9.5 would precipitate ≈95% Zn, leaving ≤4 mg/L. The resulting Zn(OH)₂ sludge forms at ≈1.5 kg per kg Zn removed and must be dewatered. After solids removal, NH₄–N ~50 mg/L enters a biological reactor.

A 500 m³ biological unit with 6–8 h HRT (hydraulic retention time) and DO ~2 mg/L (dissolved oxygen) could nitrify >90% to <5 mg/L NH₄–N, yielding ~7 mg/L NO₃–N. Such performance meets strict limits in most jurisdictions in the context described. These data—95% Zn removal (researchgate.net) and ~98% NH₃ removal (nepis.epa.gov)—guide realistic sizing and compliance evaluation.

Sources and validation

Findings on flux composition and hazardous characterization are drawn from EPA reports (nepis.epa.gov; nepis.epa.gov). Ratnawati et al. (2020) demonstrate Zn precipitation at pH 9–9.5 with ≥95% removal and show 50 mg/L polymer (PAC) accelerating turbidity removal (researchgate.net; researchgate.net). EPA guidance also notes lime–polymer treatment of galvanizing effluent (nepis.epa.gov). Nitrification and stripping benchmarks come from U.S. EPA data (nepis.epa.gov). Metal toxicity thresholds for nitrifiers and sludge zinc content are summarized from MDPI sources (mdpi.com; mdpi.com; mdpi.com). Regulatory context is drawn from Indonesian MOE regulations cited in Ratnawati (researchgate.net) and general practice (e.g., Zn effluent ~5 mg/L). Each design choice (pH setpoint, coagulant dose, reactor size) should be validated via lab tests or pilot trials using actual flux wastewater.