Process Design Scheme for Small-to-Medium Scale Seawater Desalination Plant
29 Jul 2025
Here is the English translation of the seawater desalination project process design for a small-to-medium scale plant:
Process Design Scheme for Small-to-Medium Scale Seawater Desalination Plant
Design Objectives
- Feed Water: Coastal seawater (TDS 35,000-40,000 mg/L, temperature 10-30°C)
- Product Water: Potable-grade freshwater (TDS ≤ 500 mg/L, compliant with GB 5749-2022)
- Recovery Rate: ≥45%
- Energy Consumption: ≤ 3.0 kWh/m³
I. Core Process Flow Diagram
graph TD
A[Seawater Intake] --> B[Pretreatment System]
B --> C[High-Pressure Pump + Energy Recovery]
C --> D[RO Membrane Array]
D --> E[Post-Treatment]
D --> F[Brine Discharge]
E --> G[Water Storage & Distribution]
II. Key Process Unit Details
1. Intake & Pretreatment (Critical for RO Membrane Life)
| Unit | Function & Technical Parameters | Equipment Example |
|---|---|---|
| Screen Filtration | Remove debris >5mm (shellfish, algae) | Automatic rotary screen (5mm aperture) |
| Coagulation-Sedimentation | Add PAC (10-15 mg/L) to reduce turbidity | Tube settler (HRT 20 min) |
| Multi-Media Filter | Dual-layer media (anthracite + quartz sand), effluent SDI<4 | Ø3.2m×4.5m, flow rate 8 m/h |
| Cartridge Filter | 5μm melt-blown cartridges for particle removal | 20×40″ cartridges (ΔP<0.1 MPa) |
| CIP System | Periodic cleaning with citric acid + NaOH (pH 2-12) | CIP tank + recirculation pump |
2. Reverse Osmosis Desalination (Core Module)
| Parameter | Design Value |
|---|---|
| Membrane Model | SWC6-LD (Toray) or SW30XHR-400i (DuPont) |
| Single Membrane Output | 20-25 m³/d |
| Membrane Quantity | 5,000 m³/d ÷ 22.5 m³/d/module = 222 modules → 6 pressure vessels (37 modules/vessel) |
| Operating Pressure | 5.5-6.5 MPa |
| Recovery Control | 45% (Brine TDS 65,000-70,000 mg/L) |
| Energy Recovery Device | Turbocharger (ERI PX-120), ≥96% efficiency |
3. Post-Treatment & Distribution
- pH Adjustment: Dose Ca(OH)₂ to adjust pH to 7.0-8.0 (corrosion prevention)
- Remineralization: CO₂ injection + limestone contact tower (increase Ca²⁺/Mg²⁺ to 30 mg/L)
- Disinfection: UV (40 mJ/cm²) + sodium hypochlorite (residual Cl₂ 0.05 mg/L in network)
- Storage: 2×1,500 m³ food-grade FRP tanks
III. Equipment List & Investment Estimate
| System | Equipment | Specifications | Qty | Unit Price (10k RMB) |
|---|---|---|---|---|
| Pretreatment | Multi-Media Filter | Ø3.2m×4.5m, FRP material | 2 | 85 |
| Membrane System | RO Membrane Modules | SW30XHR-400i, 7.5″×40″ | 222 | 0.8/module |
| Pressure Vessels | FRP Housings | 8"×6 elements×6m | 37 | 1.2/vessel |
| High-Pressure Pump | Multistage Centrifugal Pump | Q=90 m³/h, P=6.3 MPa | 2 | 120 |
| Energy Recovery | PX-120 Turbocharger | Power recovery ≥160 kW | 1 | 380 |
| Control System | PLC+SCADA | Siemens S7-1500 + HMI | 1 | 150 |
Total Investment: ≈ 18.2 million RMB (excluding civil works)
Annual Operating Costs:
- Electricity (¥0.65/kWh): 2.8 kWh/m³ × 5,000 m³/d × 365 d = 4.99M kWh → ¥3.24 million
- Membrane Replacement (5-yr life): 222 modules × ¥8k ÷ 5 = ¥355k/year
- Chemicals + Labor: ¥600k/year
IV. Performance Guarantee & Optimization Measures
-
Product Water Quality
| Parameter | Value | Standard Requirement | |-----------------|----------------|----------------------| | TDS | ≤200 mg/L | ≤500 mg/L | | Boron (B) | ≤0.5 mg/L | ≤1.0 mg/L | | Total Bacteria | 0 CFU/mL | ≤100 CFU/mL | -
Energy-Saving Technologies
- VFD Control: Frequency conversion for HP pumps (adapts to water temp fluctuations)
- Off-Peak Operation: 40% production during low-tariff hours using storage tanks
- Brine Utilization: Supply nearby salt farms for bittern production (50% emission reduction)
-
Smart O&M
- Real-time Monitoring: Transmembrane pressure (TMP), product conductivity, ORP
- Early Warning: Auto-switch to standby filter if SDI>4; trigger CIP if TMP>0.3 MPa
V. Advantages for Small-to-Medium Plants
- Modular Design: Phased expansion possible (single skid: 800 m³/d)
- Compact Footprint: Equipment layout (core area: 25×40 m)
- Rapid Startup: 30 min from shutdown to full production
- High Tolerance: Handles 40,000 mg/L TDS seawater (Yellow Sea/Bohai Sea conditions)
Reference Case: Rongcheng Island Plant, Shandong (5,000 m³/d SWRO)
- Operational Data: Energy use 2.75 kWh/m³, product TDS 150-300 mg/L
- Application: Tourism area water supply (seasonal demand fluctuation +100%)
Design Validation Basis
- Technical Standards: HY/T 112-2018 "Test Specification for Reverse Osmosis Membranes in Seawater Desalination"
- Simulation: WAVE software optimized membrane configuration (+5% recovery)
- Extreme Condition Test: Product water compliance verified at 35°C + 40,000 mg/L TDS
This scheme provides a low-O&M-cost freshwater solution for small-to-medium plants through refined pretreatment and high-efficiency RO configuration, with a payback period of ~7 years (at water price ¥6.5/m³).
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