Comprehensive RO Membrane Process Design for Zero Liquid Discharge (ZLD) of Dyeing and Printing Wastewater
19 Jun 2025
Complete RO Membrane Process Design for Zero Liquid Discharge (ZLD) of Dyeing and Printing Wastewater
Process Framework
A[Dyeing and Printing Wastewater] --> B[Pre-treatment System] --> C[RO Membrane Desalination Chain] --> D[Concentrate Evaporation Crystallization] --> E[Crystallized Salt Resource Utilization] --> F[RO Permeate Reuse]
Pre-treatment System
B1[Screening] --> B2[Equalization Tank] --> B3[Catalytic Ozone Oxidation] --> B4[Coagulation and Sedimentation] --> B5[Hydrolysis Acidification - A/O] --> B6[UF Ultrafiltration]
RO Membrane Desalination Chain
C1[Primary HSRO High Salinity RO] --> C2[Concentrate Salt Separator] -->|High Sulfate| C3[NF Nanofiltration Salt Separation] --> C4[Secondary HSRO Concentration] -->|High Chloride| C4
Evaporation Crystallization
D1[MVR Evaporator] --> D2[Sodium Sulfate Crystallization] --> D4[Freezing Crystallization] --> D3[Sodium Chloride Crystallization]
Key Process Unit Design
1. Enhanced Pre-treatment System (Ensuring Stable RO Operation)
| Unit | Technical Parameters and Functions |
|---|---|
| Catalytic Ozone Oxidation | Ozone dosage 80-120 mg/L + Catalyst MnO₂/Activated Carbon, breaking chelation and chain scission (COD removal rate >40%, color removal >90%) |
| Coagulation and Sedimentation | PAC (300 ppm) + PFS (100 ppm) + Cationic PAM (5 ppm), removing colloids/suspended solids (SS <10 mg/L) |
| Biological Treatment | Hydrolysis acidification HRT = 12h + A/O process HRT = 24h, degrading small molecular organic compounds (B/C ratio increased to 0.3+) |
| Ultrafiltration (UF) | PVDF outside-pressure membrane (pore size 0.03 μm), permeate SDI15 <3, turbidity <0.2 NTU |
2. Salt Separation RO Membrane Chain Design (Achieving Salt Resource Utilization)
| Membrane System | Functions and Parameters |
|---|---|
| Primary HSRO | Anti-fouling seawater membrane (e.g., DOW SW30XHR-440i), operating pressure 4-6 MPa, recovery rate 60% |
| Concentrate Salt Separator | Diverting high sulfate liquid (>70%) and high chloride liquid (>80%) based on ion concentration ratio (Cl⁻/SO₄²⁻) |
| NF Nanofiltration Salt Separation | Retaining SO₄²⁻ (NF90 membrane, retention rate >95%), passing Cl⁻ to sodium chloride concentrate |
| Secondary HSRO | Segmental design (4:2 arrangement), concentrating sulfate to 15% and chloride to 18% |
3. Evaporation Crystallization System (Core of ZLD)
| Unit | Technical Solution |
|---|---|
| Sodium Sulfate Crystallization | MVR evaporator (steam compression ratio 1:6), producing anhydrous sodium sulfate (purity >98%, meeting GB/T 6009-2014) |
| Sodium Chloride Crystallization | Freezing crystallization (-5℃), producing industrial salt (purity >99.2%, meeting GB/T 5462-2015) |
| Mixed Salt Solidification | Pressing mixed salts (containing organics) into blocks → hazardous waste landfill (content <5%) |
Fouling Prevention and Energy-saving Technologies
| Risk | Control Measures |
|---|---|
| RO Membrane Organic Fouling | Pre-treatment catalytic ozone oxidation + addition of nonionic surfactant (OP-10 1-2 ppm) before RO |
| Inorganic Scaling | Scale inhibitor for HSRO section (HEDP + PAA composite formula), silica inhibitor (polyether-modified siloxane) before NF |
| Biological Contamination | UV disinfection throughout the system (40 mJ/cm²) + intermittent chloramine shock (maintaining residual chlorine 0.05-0.1 mg/L) |
| Heat Recovery | MVR condensate water (80℃) → preheating RO feed water (30→60℃), energy saving 35% |
Operation Results and Resource Utilization Indicators
| Material Flow | Component | Treatment Effect | Resource Utilization Path |
|---|---|---|---|
| RO Permeate | TDS ≤200 mg/L | Water quality meets GB 4287-2012 indirect discharge standard | Reused for dyeing vat rinsing/cooling water |
| Sodium Sulfate Crystallization | Na₂SO₄ ≥98% | Brightness >82, Cl⁻<0.8% | Used as dyeing auxiliaries/glass production raw material |
| Sodium Chloride Crystallization | NaCl ≥99.2% | Moisture <0.3%, Ca²⁺<0.02% | Used as raw material for ion-exchange membrane caustic soda production |
| Mixed Salt Solid Blocks | Organic content <5% | Leachate toxicity meets (GB 5085.3-2007) | Hazardous waste code HW12 (compliant landfill) |
Economic Analysis (Based on a Scale of 1,000 m³/d)
| Item | Cost (Ten Thousand Yuan) | Notes |
|---|---|---|
| Equipment Investment | 2,800 | Including evaporation crystallization system (55%), RO membrane chain (30%) |
| Operating Cost per Ton of Water | 32.8 yuan/ton | Energy (58%), chemicals (22%), maintenance (20%) |
| Annual Revenue | 10.5 million yuan/year | Reused water (0.8 yuan/ton) + crystallized salts (sodium sulfate 600 yuan/ton, sodium chloride 400 yuan/ton) |
| Investment Payback Period | 4.2 years | Government ZLD subsidies can shorten to 3 years |
Engineering Implementation Key Points
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Salt Separation Pre-treatment: Use EDR electrodialysis pre-concentration instead of the primary HSRO unit (suitable for water quality with TDS >50,000 mg/L)
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Membrane Cleaning Strategy:
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Acid Cleaning: 2% Citric acid + 0.5% HCl (for calcium/iron scale removal), activated carbon adsorption required
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Alkaline Cleaning: pH 12 NaOH + 0.1% EDTA + 1% SDS (for organic removal), preferred
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Biocide Cleaning: 1% Peroxyacetic acid (for biological contamination)
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Intelligent Control:
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PLC control logic example:
if conductivity(HSRO concentrate) > 100,000 μS/cm: Automatically start the secondary HSRO system if pressure_drop(NF) > 1.5 * initial value: Trigger pulse flushing + switch to standby membrane set
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Material Selection: RO pressure pipes use 2507 super duplex steel, evaporators use TA2 titanium material (resistant to Cl⁻ corrosion)
Notes:
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PPC (Persistent Pollutants) content in wastewater should be tested; if PCBs >1 ppm, activated carbon adsorption should be added.
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Market research on crystallized salt sales should be conducted to avoid accumulation of mixed salts.
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Modular containerized ZLD integrated equipment is preferred (to reduce land occupation, suitable for retrofitting in old plants).
This solution combines the advantages of membrane salt separation and thermal crystallization, achieving pure salt recovery and eliminating the production of mixed salts, providing a truly sustainable zero liquid discharge solution for dyeing and printing enterprises.
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