Zero Liquid Discharge Water Treatment Process Design in Environmental Engineering

I. Design Philosophy and System Composition

1.1 Core Concept and Objectives of Zero Liquid Discharge (ZLD)

This scheme aims to construct a ZLD water treatment system characterized by "Maximum Water Source Recovery, Ultimate Pollutant Solidification, Optimized Energy Integration, and Intelligent Operation Management." The core lies in shifting from the traditional linear "treatment-discharge" model to establishing a closed-loop system of "Collection-Purification-Reuse-Concentration-Solidification," ultimately achieving zero discharge of liquid pollutants from the entire plant, while simultaneously recovering water resources, thermal energy, and valuable substances.

• Ultimate Objectives:

  1. Liquid Zero Discharge: All plant wastewater is treated. The produced freshwater is reused in production, and dissolved pollutants are converted into solid form (crystalline salts or stabilized mixed salts) for compliant disposal, with no wastewater discharged to the external environment.

  2. Efficient Water Resource Reuse: The system's comprehensive freshwater recovery rate is ≥ 95%, with reuse water quality meeting various grades of process water standards.

  3. Pollutant Detoxification/Resource Recovery: The final solid products are stabilized and rendered harmless, with an effort to separate and recover valuable components (e.g., sodium chloride, sodium sulfate).

  4. Lifecycle Cost-Effectiveness: Through process optimization, energy recovery, and intelligent operation, control capital and operating costs to the maximum extent possible while achieving the ZLD objective.

1.2 ZLD System Boundaries and Composition

This scheme is applicable to scenarios such as industrial parks and large industrial enterprises (e.g., power, chemical, metallurgical, coal chemical) facing severe water-saving and environmental protection pressures. The system treats composite wastewater, high-salinity wastewater, and reverse osmosis concentrate that require final disposal after on-site pretreatment.

A complete ZLD system typically consists of the following four core modules:

1. Pretreatment and Softening/Hardness Removal Module: The "firewall" ensuring stable system operation, primarily removing scaling and fouling factors such as calcium, magnesium, silica, heavy metals, and organics.

2. Integrated Membrane Concentration/Reduction Module: The system's "energy saver," using multi-stage membrane technologies to efficiently separate salts and water, significantly reducing the water volume entering the high-energy-consumption evaporation unit, thereby lowering overall energy consumption.

3. Evaporation/Crystallization Solidification Module: The system's "terminator," converting dissolved solids in the high-concentration brine into crystals or stabilized solids.

4. Intelligent Control and Energy Optimization Module: The system's "brain," enabling intelligent, full-process operation and cascaded energy utilization.

II. Modular Zero Liquid Discharge Process Route Design

Addressing complex and variable influent water quality, this scheme adopts a modular, flexible process route of "Customized Pretreatment + Multi-Stage Membrane Synergistic Concentration + Fractional Evaporation/Crystallization." Its core advantage lies in the ability to flexibly combine process units based on specific water quality, salt composition, and reuse requirements, achieving optimal techno-economic performance.

2.1 Full-Process Modular ZLD Framework

III. Detailed Technology of Core Process Modules

3.1 Module A: Advanced Pretreatment and Softening

• Functional Positioning: This module is the "lifeline" for the long-term stable operation of the ZLD system. Its failure will directly lead to scaling, fouling, and corrosion of the subsequent expensive membrane and evaporation systems.

• Key Technology Units:

  • Chemical Softening: Employs a "Two-Stage Precipitation Softening Process." Stage 1 adds lime to remove magnesium, silica, and some calcium. Stage 2 adds soda ash and caustic soda to deeply remove calcium ions to <50 mg/L. pH is controlled at 10.5-11.5. The generated sludge is disposed of after thickening and dewatering.

  • Advanced Oxidation: For refractory COD, uses "Ozone Catalytic Oxidation" or "Electrochemical Oxidation" to break down macromolecular organics, improving biodegradability or achieving direct mineralization, thereby reducing membrane organic fouling.

  • Precision Filtration: Employs an "Ultrafiltration" system to ensure feed water to the RO system has SDI<3 and turbidity<0.2 NTU.

3.2 Module B: Integrated Membrane Concentration/Reduction

• Functional Positioning: Separates and recovers a large amount of water from the wastewater with relatively low energy consumption (compared to evaporation), achieving high concentration of dissolved salts. This is the core to reducing the scale and energy consumption of the evaporation unit.

• Key Technology Combinations:

  • Mainstream Route: "RO + DTRO/HERO." Primary RO uses brackish water membranes with ~70% recovery. DTRO (Disc Tube Reverse Osmosis) or HERO (High-Efficiency Reverse Osmosis) serves as the high-pressure concentration unit, tolerating high TDS and hardness, concentrating the brine to near saturation (TDS 150k-200k mg/L).

  • Specialized Route: "NF/ED Salt Fractionation Pretreatment." After RO concentration, Nanofiltration (NF) is used to separate divalent salts (e.g., Na₂SO₄) from monovalent salts (e.g., NaCl), or Electrodialysis (ED) is used for further concentration and preliminary salt fractionation. This creates favorable conditions for subsequent fractional crystallization, significantly enhancing resource recovery value.

3.3 Module C: Evaporation/Crystallization and Solidification

• Functional Positioning: Achieves the final transformation of dissolved solids from liquid to solid state, representing the "last mile" of ZLD.

• Key Technology Selection:

  • Fractional Crystallization (Resource Recovery Route):

    • Freeze Crystallization: Suitable for sodium sulfate systems. The concentrate is cooled to 0-5°C to precipitate sodium sulfate decahydrate (Glauber's salt), which can yield anhydrous sodium sulfate (salt cake) after melting and dehydration.

    • Thermal Fractional Crystallization: Utilizes the difference in solubility of NaCl and Na₂SO₄ at different temperatures and concentrations. By controlling the crystallization sequence in an MVR evaporation system, sodium chloride and sodium sulfate products are obtained separately.

  • Forced Evaporation Solidification (Disposal Route):

    • Mechanical Vapor Recompression (MVR) Evaporation: The mainstream choice, offering high energy efficiency. An MVR evaporator is used to evaporate the concentrate to dryness, producing mixed salts.

    • End-of-Pipe Solidification: The mixed salts are dried and formed, then undergo stabilization/solidification treatment. They are disposed of in a hazardous waste landfill upon meeting the Standard for Pollution Control on Hazardous Waste Landfill, or disposed of as general solid waste after characterization.

3.4 Intelligent Control and Energy Optimization Module

• Digital Twin and Intelligent Control: Establish a full-process digital twin model. Based on data from online water quality instruments (ion chromatography, hardness, silica, TOC, etc.), optimize chemical dosing, membrane system recovery rates, and evaporator operating parameters in real-time for preventive control.

• Energy System Integration: Recover waste heat from evaporator condensate to preheat influent; use high-efficiency vapor compressors for MVR systems; in plants with waste heat resources (e.g., thermal power plants), prioritize "Bypass Flue Evaporation" or use low-grade heat sources to drive MED, significantly reducing operating costs.

IV. Techno-Economic Analysis

• Capital Cost: The unit water treatment investment is extremely high, typically in the range of 8,000 - 20,000 RMB/(ton/day). The investment is primarily concentrated in the evaporation crystallization system (40-50%), the integrated membrane system (30-40%), and the high-standard pretreatment and control systems.

• Operating Cost: 15 - 40 RMB per ton of raw wastewater. Main components are:

  • Electricity (40-60%): Mainly for high-pressure membrane pumps, MVR compressors, various blowers, and pumps.

  • Steam/Heat Source (20-30%): If supplemental heat is required for MVR.

  • Chemical Costs (10-20%): Softening agents, antiscalants, acids/alkalis, oxidants.

  • Maintenance & Depreciation (10-15%): Membrane replacement, equipment overhaul.

• Economic Benefits:

  • Direct Revenue: Water and discharge fee savings from high freshwater recovery; revenue from sales of fractionated crystalline industrial salts.

  • Indirect Benefits: Meeting environmental requirements, ensuring continuous plant operation; enhancing corporate social image and sustainability rating.

• Conclusion: ZLD is an environmental protection project with "high capital investment and high operating costs." Its primary drivers are regulatory policies and water scarcity pressures. Optimizing processes, integrating energy, and recovering resources can significantly improve its economics, but the payback period is typically long (>8 years).

V. Implementation Strategy and Recommendations

1. Water Quality First, Pilot Testing is Mandatory: ZLD processes are highly dependent on influent water quality. Comprehensive water quality analysis and pilot testing over several months are essential to verify pretreatment efficacy, membrane flux and recovery rates, salt fractionation feasibility, and final salt quality.

2. Modular Design, Phased Construction: Adopt a modular design philosophy, allowing phased implementation based on water quality changes and funding availability. For example, first construct the "Pretreatment + RO" for high-ratio reuse and reduction, then add the evaporation crystallization unit as needed.

3. Prioritize Pretreatment, Select Reliable Suppliers: The reliability of the pretreatment system is key to project success. Core equipment (membranes, evaporators, compressors, instruments) should be selected from technologically mature suppliers with a proven track record.

4. Build an Intelligent Water Management Brain: The planning and construction of an advanced central control system and intelligent water management platform must be synchronized. This is a necessary tool for managing the complex ZLD system and achieving long-term stable and economical operation.

5. Clarify Crystalline Salt Disposal Route: The disposal or resource recovery pathway (agreement) for the final crystalline or mixed salts must be clarified in the early design stage. This is a prerequisite for passing environmental impact assessments and ensuring stable operation.

This scheme, through systematic modular design, provides a technically advanced, adaptable, and cost-considered comprehensive solution framework for achieving Zero Liquid Discharge in environmental engineering. It represents the necessary technological choice for addressing the most stringent environmental challenges and moving towards sustainable water resource management.