Zero Liquid Discharge (ZLD) Process Design Scheme for Electroplating Wastewater

1.0 Design Basis and Challenge Analysis

• Wastewater Sources and Characteristics:

  • Cyanide-containing Wastewater: Discharged from processes such as cyanide copper plating, zinc plating, and gold plating. Highly toxic and must be pre-treated for cyanide destruction.

  • Chromium-containing Wastewater: Discharged from processes like chromium plating and passivation. Hexavalent chromium (Cr6+) is a strong carcinogen and must be reduced to trivalent chromium (Cr3+) before precipitation.

  • Comprehensive Wastewater: Mixed wastewater containing nickel, copper, zinc, tin, acids/alkalis, oils, surfactants, and various complexing agents (e.g., EDTA, tartaric acid). This is the most challenging stream to treat.

• Core Challenges:

  1. Deep Removal of Heavy Metals: Ensuring effluent heavy metal ion concentrations meet surface water or reuse standards.

  2. Destruction of Complexing Agents: Stable heavy metal complexes interfere with precipitation and membrane separation processes.

  3. High Salinity and Scaling Potential: High concentrations of salts (Na₂SO₄, NaCl) lead to severe scaling during membrane concentration and evaporation stages.

  4. Complex Pretreatment: Waste streams must be segregated and treated separately to prevent toxic substances from reacting with each other or causing interference.

• Design Objective: Zero Liquid Discharge (ZLD), product water reuse in the production line, heavy metal recovery or safe solidification, crystalline mixed salts disposed as hazardous waste.

• Core Process Route: Segregated Pretreatment → Advanced Oxidation for Complex Breaking → Deep Purification → Multi-Stage Membrane Concentration → Evaporation/Crystallization

2.0 Complete ZLD Process Flow Design

The key to ZLD for electroplating wastewater lies in "Segregated treatment to reduce toxicity, Staged concentration to lower costs". The core process includes wastewater segregation, categorized toxin destruction, combined advanced treatment, membrane-based volume reduction, and ultimate solidification.

The diagram below clearly illustrates the complete flow from wastewater collection to final ZLD achievement.

flowchart TD
 

Step-by-Step Process Explanation:

Phase 1: Wastewater Segregation and Pretreatment (Eliminate Toxicity, Remove Interference)

1. Cyanide-containing Wastewater Treatment:

   • Process: Two-Stage Alkaline Chlorination.

   • Procedure: Under pH > 10 conditions, add sodium hypochlorite (NaClO) to oxidize cyanide (CN⁻) to cyanate (CNO⁻), and further to carbon dioxide and nitrogen. This completely eliminates cyanide toxicity and its inhibitory effect on subsequent biological processes.

2. Chromium-containing Wastewater Treatment:

   • Process: Chemical Reduction and Precipitation.

   • Procedure: Under acidic conditions (pH 2-3), add a reducing agent (e.g., sodium bisulfite NaHSO₃, ferrous sulfate FeSO₄) to reduce highly toxic hexavalent chromium (Cr6+) to less toxic trivalent chromium (Cr3+). Then, add alkali to adjust pH to 8-9, forming chromium hydroxide precipitate.

3. Comprehensive Wastewater Treatment:

   • Key Step: Advanced Oxidation for Complex Breaking.

   • Procedure: For wastewater containing complexing agents like EDTA or tartaric acid, conventional precipitation is ineffective. Fenton Oxidation or Ozone Catalytic Oxidation is required. Strong free radicals (·OH) break the complex structure, "releasing" heavy metal ions for subsequent precipitation.

Phase 2: Combined Deep Treatment and Solid-Liquid Separation

• Mix the pretreated wastewater streams in an equalization tank for homogenization.

• Neutralization/Precipitation: Add NaOH/Ca(OH)₂ to control pH between 8.5-9.5, causing all heavy metal ions (Cu²⁺, Ni²⁺, Zn²⁺, Cr³⁺, etc.) to co-precipitate as hydroxides.

• Coagulation/Flocculation: Add PAC and PAM to form large, dense flocs.

• Solid-Liquid Separation: Achieve sludge-water separation via a high-efficiency clarifier or membrane filtration (e.g., Tubular Microfiltration, TMF). The chemical sludge is dewatered and disposed of as hazardous waste. The supernatant at this stage meets conventional discharge standards but becomes the "feed water" for the ZLD system.

Phase 3: Staged Membrane Concentration (Achieving Significant Volume Reduction)

1. Deep Purification: The supernatant passes through multi-media filtration, activated carbon filtration (to remove residual organics), and cartridge filtration (5µm) to ensure SDI₁₅ < 3, providing qualified feed water for the RO system.

2. Primary RO: Uses fouling-resistant brackish water RO membranes, with recovery set at 60-70%. The permeate conductivity is significantly reduced and can be directly reused for electroplating pretreatment or general cleaning.

3. Secondary/Tertiary RO (High-Salinity Concentration):

   • Primary RO concentrate enters a more fouling-resistant secondary RO, with recovery around 60%.

   • Secondary RO concentrate enters Disc Tube Reverse Osmosis (DTRO) or High-Pressure Spiral RO, which tolerate higher salinity and fouling. The overall system recovery can reach 85-90%, concentrating the final brine to 10-15% of the original wastewater volume. This step is crucial for reducing evaporator load and energy consumption.

Phase 4: Evaporation/Crystallization (Ultimate ZLD)

• Process Selection: Mechanical Vapor Recompression (MVR) Evaporator is the mainstream choice due to its high energy efficiency (~40-70 kWh per ton of water evaporated).

• Procedure: The high-concentration RO brine is evaporated. Water vapor condenses into high-quality distillate, which is returned to the product water tank. Dissolved solids (mainly Na₂SO₄, NaCl, and trace heavy metal salts) continuously concentrate until they reach supersaturation and crystallize.

• Products:

  • Condensate: High-quality pure water for reuse.

  • Crystalline Mixed Salts: Mixed salts containing trace heavy metals, must be disposed of as hazardous waste by licensed contractors or undergo salt separation and purification (technically complex and costly).

3.0 Key Equipment and Design Parameters

4.0 Scheme Advantages and Economic Analysis

Technical Advantages:

1. Complete Elimination of Environmental Risk: Achieves zero wastewater discharge, heavy metals and toxins are solidified, no secondary pollution.

2. Efficient Water Resource Reuse: >90% of wastewater is reclaimed, significantly reducing freshwater intake.

3. Strong System Shock Load Resistance: Multi-barrier design ensures stable final product water quality even with front-end fluctuations.

Brief Economic Analysis (Example based on 100 tons/day wastewater treatment):

• Total Investment: Approximately 8 - 15 million RMB (varies greatly with scale and configuration).

• Operating Cost per Ton of Water: 25 - 45 RMB/ton (Conventional treatment costs ~8-15 RMB/ton, excluding reuse and ZLD).

  • Main components: Electricity (dominated by evaporation), chemical costs, membrane replacement, hazardous waste disposal (mixed salts and sludge).

• Return on Investment: Mainly from saved discharge fees, avoided environmental penalties, reduced freshwater purchase costs, and the social benefits and compliance assurance brought by sustainable development.

Summary and Implementation Recommendations

ZLD for electroplating wastewater is a technically feasible but economically demanding systematic engineering project. The key to successful implementation lies in:

1. "The Front End Determines the Back End": Thorough waste stream segregation and pretreatment are the foundation for the long-term stable operation of the entire system.

2. "Membrane Concentration is the Core of Efficiency": Efficient membrane systems recover most of the water at a relatively low cost, directly determining the scale and energy consumption of the evaporation system.

3. "Final Solidification Ensures Compliance": Evaporation/crystallization is the necessary means to finally achieve "zero liquid," and a safe disposal path for mixed salts must be secured.

Recommendation: Before project initiation, comprehensive water quality analysis and pilot testing are essential to verify the process flow, determine key parameters (e.g., chemical dosage, membrane flux, evaporator scaling tendency), thereby optimizing the design and controlling investment and operational risks.