Metal industry effluent is the most hazardous class of industrial wastewater due to heavy metals (Cr6+, Cr3+, Ni, Zn, Cu, Pb, Cd) and cyanide — toxic to humans and environment even at trace levels.
Typical parameters: pH 2-12 (acid/base baths), Cr6+ 50-500 mg/L, Ni 20-200 mg/L, Zn 50-300 mg/L, CN 10-100 mg/L, oil 50-300 mg/L, phosphate 100-1,000 mg/L. Volume is low (10-200 m³/day) but concentrations are extreme.
Metal treatment is therefore built on chemical reduction/oxidation + neutralization + precipitation. Biology is not used due to metal toxicity. Arsistek delivers sector-specific design for galvanizing, hard chrome, zinc plating, decorative finishing and PCB manufacturing.
Critical Features of Metal Wastewater
Metal industry effluent requires stream segregation — each bath type needs a different chemistry. Chromium needs reduction, cyanide needs oxidation, acid-base baths only neutralization. Separate collection lines are mandatory.
Cr6+ (chromate, dichromate) is highly toxic (carcinogenic). It must first be reduced to Cr3+ (SO2, NaHSO3, ferrous sulfate) then precipitated as hydroxide. Cr6+ cannot be discharged directly.
Cyanide does not degrade naturally. Alkaline chlorination (NaOCl + NaOH, pH > 10) converts CN to cyanate then to N2 and CO2. Wrong pH releases lethal HCN gas — pH alarms are critical.
Metal Treatment Process Flow
Streams segregated and processed in sequence:
- Chrome line: pH 2-3 → Cr6+ reduction (SO2/NaHSO3, ORP < +250 mV)
- Cyanide line: pH 10-11 → NaOCl + NaOH oxidation (ORP > +600 mV)
- Other heavy metals (Ni, Zn, Cu) collected separately
- All streams merge in neutralization (pH 8.5-9.5)
- Polyelectrolyte dosing + flocculation
- Lamella or clarifier
- Sand filter + selective ion exchange if needed
- Final pH adjustment + discharge
- Filter-press sludge dewatering
Chemicals & Equipment
- Cr6+ reduction: Sodium metabisulfite (Na2S2O5), ferrous sulfate
- CN oxidation: Sodium hypochlorite (NaOCl 12%), caustic
- Neutralization: Slaked lime Ca(OH)2 or NaOH
- Precipitation: Polyaluminum chloride (PAC) + polymer
- Tertiary: Ion-exchange resin for Ni/Cu recovery
- Equipment: ORP-controlled dosing pumps, dual pH probes (safety), filter press, lamella clarifier
- Automation: SCADA + PLC, real-time pH/ORP/flow
Hazardous Waste Management & Metal Recovery
Sludge from metal effluent is classified as hazardous waste (code 19-08). It must go to licensed disposal facilities tracked by national hazardous-waste forms. Disposal is expensive ($600-1,500/ton) so sludge minimization matters.
Valuable metals like Ni and Cu can be recovered via ion-exchange resin or electrochemical cells. Recovered metal sold to certified recyclers offsets investment in 2-4 years.
Regulations: Hazardous Waste Control Regulation, sector discharge tables (Table 14 for galvanizing), Chemical Substance Regulation. ISO 14001 is recommended.
Metal WWTP Advantages
Metal Sector References
Heavy-metal treatment references at galvanizing, chrome plating and electronics plants.
Frequently Asked Questions
Even small galvanizing plants from 10 m³/day are served by compact package systems. Low volume but high concentration management is the design driver.
Real-time ORP (Redox Potential) sensor. Full Cr6+ to Cr3+ reduction drops ORP below +250 mV. Auto dosing follows the signal. Backup sensor + alarm required.
Absolutely not. Contact with acid produces lethal HCN gas. Must undergo alkaline chlorination and verified CN < 0.1 mg/L. Non-compliance carries criminal liability.
Metal working brings rolling oil and cutting fluid residues. UF membrane or cartridge filters separate them mechanically — residual emulsions are broken with chemical demulsifiers.
Pure nickel trades at $15-22/kg. A plant recovering 200 kg/month earns $40,000-50,000/year. Resin investment pays back in 2-3 years.
Yes. PCB plants use ammoniated copper etchant, formaldehyde and EDTA chelators. Chelated metals resist hydroxide precipitation — acid cracking + ozone oxidation are required. Specialized design needed.