Reverse Osmosis System Design for Coking Wastewater Treatment
14 Aug 2025
Reverse Osmosis System Design for Coking Wastewater Treatment
Scale: 20,000 m³/day (High-concentration coking wastewater)
Objective: Achieve COD≤50 mg/L, TDS≤100 mg/L, recovery rate ≥75%, compliance with GB 16171-2012 Coking Chemical Industry Emission Standards
I. Coking Wastewater Characteristics & Challenges
| Contaminant | Concentration | Membrane Fouling Risk |
|---|---|---|
| COD | 3,000–6,000 mg/L | Organic fouling/scaling |
| NH₃-N | 200–500 mg/L | Biofouling |
| CN⁻ | 10–50 mg/L | Toxic breakthrough |
| Phenolics | 500–1,500 mg/L | Membrane swelling |
| Oils & Grease | 50–200 mg/L | Clogging |
| TDS | 8,000–15,000 mg/L | Scaling/high osmotic pressure |
II. RO Process Flow (Four-Stage Barrier)
graph LR
A[Coking Wastewater] --> B[Enhanced Pretreatment] --> C[1st RO] --> D[2nd RO] --> E[Brine Minimization] --> F[Compliant Discharge]
B -->|Hardness/Silica Removal| G[Chemical Softening + Media Filtration]
B -->|COD Degradation| H[Catalytic Ozonation + Fenton]
C -->|Permeate to 2nd RO| D
C -->|Concentrate to Minimization| E
D -->|Concentrate Recycle| C
E -->|Evaporation Crystallization| I[Salt Resource Recovery]
III. Core Unit Design Specifications
(1) Enhanced Pretreatment
| Unit | Technology | Control Target |
|---|---|---|
| Chemical Softening | Ca(OH)₂ + Na₂CO₃ dosing (pH 10.5–11.0) | Hardness ≤50 mg/L (as CaCO₃) |
| Multimedia Filter | Anthracite + quartz + magnetite (8 m/h) | Oil ≤5 mg/L, SS ≤5 mg/L |
| Catalytic Ozonation | TiO₂/activated carbon catalyst, O₃ dose: 80 g/m³ | COD removal ≥40% |
| Fluidized-bed Fenton | H₂O₂/Fe²⁺ molar ratio 3:1 (pH 3.5±0.2) | Phenolics removal ≥90% |
(2) RO System Configuration
| Parameter | 1st RO (Fouling-Resistant) | 2nd RO (Brackish Water) |
|---|---|---|
| Membrane Model | DOW FILMTEC™ FR440 | Hydranautics LFC3-LD |
| Array | 2:1 staging (6 elements/vessel) | 1:1 |
| Flux (LMH) | 15–18 | 20–24 |
| Recovery Rate | 60% | 85% |
| Pressure (MPa) | 3.0–3.5 | 1.2–1.8 |
| Concentrate Path | To brine minimization | Recycle to 1st RO inlet (≤30%) |
(3) Brine Minimization System
- Technology: DTRO (Disc-Tube RO) + MVR Evaporation
-
DTRO Design:
- Model: PALL S-RoMem™ 8040 (pressure tolerance: 7.0 MPa)
- Flux: 12–15 LMH, recovery ≥50%
- Rejection: COD >98%, TDS >99%
-
MVR Crystallization:
- Evaporation efficiency: 20 kg H₂O/kWh
- Output: NaCl/Na₂SO₄ mixed salt (purity >95%)
IV. Fouling Mitigation Strategies
-
Chemical Cleaning Protocol
- CIP Trigger: When TMP increases >30% or flux decreases >15%
-
Cleaner Formulations:
Organic fouling: 0.1% NaOH + 0.03% sodium dodecyl sulfate (40°C, pH 11.5) Inorganic scaling: 2% citric acid (pH 4.0, ammonia-adjusted) Biofouling: 1% NaHSO₃ + 0.025% Na-EDTA
-
Antiscalant Optimization
- Blend: 2 ppm PTP-2000 (phosphinopolycarboxylic acid) + 4 ppm MDC220 (RO-specific)
-
Online Monitoring
- Langelier Saturation Index (LSI) <1.8
- Continuous SDI₁₅ monitoring (<5)
V. Control & Instrumentation
# PLC Control Logic
if TMP_1stRO ≥ 3.5 MPa:
Initiate 1st RO CIP
if Conductivity_2ndRO ≥ 100 µS/cm:
Open 2nd RO concentrate discharge valve
if Online_Cyanide ≥ 0.1 mg/L:
Intensify Fenton dosing Key Instruments:
- Free cyanide analyzer (HACH Amtax)
- High-sensitivity TOC monitor (GE Sievers M9)
- AI-based fouling prediction (ΔP + flux algorithm)
VI. Techno-Economic Indicators
| Parameter | Value |
|---|---|
| Total system recovery | ≥75% |
| OPEX per tonne | 12.8 RMB/m³ (incl. evaporation) |
| Membrane lifespan | 1st RO: 3 years, 2nd RO: 5 years |
| Salt production rate | 3.2 t/day (20,000 m³/day scale) |
VII. Risk Mitigation
| Risk | Countermeasure |
|---|---|
| Cyanide breakthrough | Activated carbon adsorption (iodine value ≥1,000) post-pretreatment |
| DTRO scaling | Seed crystal addition pre-evaporation (CaSO₄ density: 1.5 kg/m³) |
| Membrane swelling | Real-time phenolic monitoring; emergency resin adsorption if >50 mg/L |
| Salt impurity risk | NF pretreatment (TFC-SR2 membrane; SO₄²⁻/Cl⁻ separation >90%) |
VIII. Acceptance Criteria
-
Effluent Quality:
- RO permeate: COD≤30 mg/L, NH₃-N≤5 mg/L, TDS≤80 mg/L
- Crystallized salt: Compliant with GB/T 5462-2015 Industrial Salt Standards
-
Performance Validation:
- ASTM E2696: Annual salt passage increase <7%
- HJ 2022-2013: RO engineering specifications
Note: Optimized based on Veolia-Baosteel Coking Wastewater Project (20,000 m³/day). Operational data: RO continuous runtime >8,000 hrs; CIP interval ≥45 days.
Reference Documents:
➤ [Coking Wastewater ZLD Technical Whitepaper](Virtual Link)
➤ [High-Salinity Coking WW Membrane Fouling Control Guide](Virtual Link)
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