Design Scheme for RO Membrane Process Water Supply System in a Small-Scale Fertilizer Plant

1.0 Design Basis and Objectives

• Source Water: Typically groundwater or surface water, characterized by high hardness, high alkalinity, and potential presence of iron, manganese, and colloidal silica.

• Points of Use: Boiler feed water, process formulation water, cooling tower makeup water, etc.

• Product Water Quality Target: Meets industrial boiler or specific process requirements.

  • Key Parameters: Conductivity ≤ 50 µS/cm, Hardness ≈ 0 mmol/L, SiO₂ ≤ 1 mg/L.

• Core Challenge: Preventing rapid fouling and failure of RO membranes due to scaling from calcium carbonate, calcium sulfate, and silicates.

• Core Process: Enhanced Pretreatment & Softening + Fouling-Resistant Reverse Osmosis

• Design Philosophy: Economical, durable, scaling-prevention prioritized, with moderate automation for easy operation and maintenance by plant personnel.

2.0 Recommended Process Flow

This scheme adopts the core process route of "Chemical Softening & Clarification + Filtration + RO Desalination", specifically enhancing pretreatment for high-hardness, high-alkalinity water. The process flow is clear, as illustrated below:

flowchart TD
subgraph PreTreatment[Pretreatment & Softening System - Core Anti-scaling Stage]
    A[Raw Water<br>Ground/Surface] --> B[Raw Water Tank]
    B --> C[Raw Water Pump]
    C --> D[Coagulant/Flocculant Dosing]
    D --> E[Pipeline Mixer]
    E --> F[Mechanical Flocculation Clarifier<br>or High-Efficiency Settler]
    F -- Supernatant --> G[Clear Water Tank]
    G --> H[Multi-Media Filter]
    H --> I[Na Ion Exchange Softener<br>or Weak Acid Cation Unit]
    I --> J[Antiscalant/Reducing Agent Dosing]
    J --> K[Cartridge Filter 5μm]
end

subgraph RO[Reverse Osmosis Desalination System]
    K --> L[RO High-Pressure Pump]
    L --> M[Fouling-Resistant RO Unit]
    M -- Concentrate --> N[Concentrate Tank<br>Reuse for Dosing or Flushing]
    M -- Permeate --> O[RO Permeate Tank<br>Process Water]
end

subgraph PostTreatment[Post-Treatment System - Optional]
    O --> P[Decarbonator<br>CO2 Removal]
    P --> Q[Mixed Bed or Secondary RO]
    Q --> R[High-Purity Point of Use]
end

Step-by-Step Process Explanation:

1. Pretreatment & Softening System (Key to Scaling Prevention)

• Purpose: Thoroughly remove suspended solids and colloids, and reduce hardness (calcium and magnesium ions) and alkalinity to levels safe for RO membrane operation.

• Chemical Softening & Clarification: Dose lime (Ca(OH)₂) and soda ash (Na₂CO₃) to chemically convert most calcium and magnesium ions into calcium carbonate and magnesium hydroxide precipitates, which are removed in the clarifier. This also effectively reduces silica content.

• Multi-Media Filtration: Intercepts residual fine flocs from the clarifier effluent, ensuring effluent turbidity <1 NTU.

• Ion Exchange Softening (Safety Measure): After clarification, residual hardness may still exceed limits. Install a Sodium Ion Exchange Softener or a Weak Acid Cation Exchange Unit to completely remove residual hardness (≤0.03 mmol/L), providing absolutely safe feed water for the RO membranes. This is the most reliable anti-scaling choice for small fertilizer plants.

• Chemical Dosing & Final Filtration: Dose antiscalant to prevent minor scaling and reducing agent (if residual chlorine is present). The 5μm cartridge filter is the final protective barrier.

2. Reverse Osmosis (RO) Main Unit (Primary Desalination Unit)

• Purpose: Remove over 95% of dissolved salts, producing water that meets boiler or process requirements.

• Design Features: Use wide-channel, fouling-resistant brackish water membrane elements. Employ conservative design flux (15-18 LMH) and set recovery rate based on feed water quality (typically 60-70%), leaving ample margin against scaling.

• Concentrate Handling: The concentrate can be collected and reused within the fertilizer plant for preparing chemical solutions, flue gas desulfurization, or site flushing and dust suppression, achieving wastewater reduction.

3. Post-Treatment System (Optional based on requirements)

• Decarbonator: If the product water is for boilers, it is necessary to remove dissolved CO2 from the RO permeate to raise pH and reduce corrosion.

• Mixed Bed or Secondary RO: For higher purity requirements (e.g., high-pressure boilers), a Mixed Bed Ion Exchange unit or a secondary RO system can be added after the primary RO for deep desalination.

3.0 Key Equipment and Design Parameters

4.0 Operating Cost and Economic Analysis (Estimate based on 10 m³/h production)

Note: Compared to purchasing industrial steam or the costs associated with equipment cleaning and downtime losses due to scaling, this cost offers significant advantages.

5.0 Scheme Advantages Summary

1. Outstanding Anti-scaling Performance: The dual safeguard of "Chemical Softening + Ion Exchange" fundamentally solves the challenge of RO membrane scaling caused by hard water in fertilizer plants, greatly extending membrane life.

2. Stable and Reliable Operation: The process flow is mature, equipment selection is robust, offering strong adaptability to raw water fluctuations, suitable for small plant conditions.

3. Economical and Practical: Capital investment and operating costs are within the manageable range for small plants, and resourceful reuse of concentrate further reduces costs.

4. Moderate Automation: Fully automatic softeners and RO systems reduce technical demands on operators, simplifying routine maintenance.

Important Recommendations:

Before project implementation, a full water quality analysis over at least one complete hydrological year must be conducted to accurately determine softening chemical dosages, regeneration cycles, and RO system design parameters. Regularly monitoring the normalized performance (permeate flow, salt rejection, differential pressure) of the RO system is key to maintaining long-term stable operation.