Zero Liquid Discharge Plant (ZLD) is an advanced wastewater treatment process designed to completely eliminate liquid waste from an industrial facility by recovering clean water and converting residual solids into manageable solid waste (salts, sludge, or crystals).

Zero liquid discharge plant is increasingly used in:

• Power plants

• Pharmaceuticals

• Textile & dyeing industries

• Chemical manufacturing

• Desalination plants

• Refineries

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Purpose of Zero Liquid Discharge Plant

• Achieve total water reuse in water-scarce regions

• Comply with strict environmental discharge regulations

• Recover valuable byproducts (e.g., sodium sulfate, lithium salts)

• Reduce pollution and water intake

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ZLD Process Flow (Typical)

Pre-Treatment → Primary Recovery (RO/NF)
→ Secondary Concentration (Evaporation)
→ Crystallization → Solids Handling
→ Permeate Recovery → Reuse

Each of these stages is modular and varies based on the influent characteristics, desired recovery, salts present, and economics.

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Key Components in ZLD (Zero Liquid Discharge System)

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Energy Use

• RO Stage: 2–5 kWh/m³

• Evaporator: 25–35 kWh/m³ (electric) or equivalent thermal energy

• Crystallizer: 35–60 kWh/m³

Use of MVR (Mechanical Vapor Recompression) or steam helps reduce energy consumption.

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Advantages of a Zero Liquid Discharge Plant

• Zero wastewater discharge — fully compliant with regulations

• Water recovery >95%

• Recovers valuable salts or chemicals

• Suitable for high TDS brine treatment

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 Challenges

• High capital and operational cost

• Energy intensive

• Requires skilled operators

• Disposal of non-recyclable solids may still be needed

Core Process Stages in a Zero Liquid Discharge Plant & Technologies

1. Pre-Treatment

Purpose:

• Remove suspended solids, organics, oil/grease, and scale-forming ions to prevent fouling of downstream RO/thermal units.

Technologies:

• Coagulation–Flocculation

• Dissolved Air Flotation (DAF)

• Media & Cartridge Filters

• Activated Carbon Filtration

• Ultrafiltration (UF) (removes particles >0.01 µm)

• Softening: Lime-soda or Na-based softeners to remove Ca²⁺, Mg²⁺

• Antiscalant Dosing: Prevents precipitation in RO membranes

Design Notes:

• Pre-treatment is tailored to the specific feedwater matrix.

• Common antiscalants: polyacrylates, phosphonates.

• pH adjustments are common before RO.

2. Primary Recovery – Reverse Osmosis (RO)

Role:

• High-pressure membrane separation to recover up to 80% of clean water.

• Concentrates dissolved solids into reject brine.

Technology:

• Two-pass RO systems are common

• NF may be used in tandem if divalent ion separation is prioritized

Design Parameters:

Notes:

• Scaling elements: CaSO₄, CaCO₃, SiO₂, BaSO₄

• Use SDI (Silt Density Index) to monitor pre-treatment performance (<3 preferred)

3. Brine Concentration – Evaporation Systems

When RO reject reaches osmotic limits (~60,000 ppm), thermal systems are used to further reduce liquid volume.

Options:

• Mechanical Vapor Recompression (MVR) Evaporators

• Multi-Effect Evaporators (MEE)

• Forced Circulation Evaporators (FCE)

Working Principle:

• Brine is boiled at reduced pressure, vapor is collected and condensed.

• Recovered condensate is reused.

• Vapor recompression (in MVR) reduces energy demand by reusing latent heat.

Design Parameters:

Design Notes:

• Tube material: SS316L or duplex SS for corrosion resistance

• Flow: Typically forced circulation (FC) to prevent scaling

4.  Crystallization

Role:

• Convert remaining concentrate into solid crystals.

• The final step in achieving zero liquid discharge.

Types:

• Forced Circulation Crystallizers (FCC)

• Vacuum Crystallizers (low-temp operations)

• Oslo-type Crystallizers (used for bulk salt crystallization)

Operating Conditions:

Notes:

• Anti-scaling and nucleation agents are used to promote uniform crystal formation

• Solids are separated via centrifuges or filter presses

5. Solids Handling & Disposal

Process:

• Crystals are dewatered, dried (optional), and stored

• Disposal via landfill, cement kilns, or reuse in industrial processes

Equipment:

• Centrifuges (scroll-type or basket)

• Vacuum belt filters

• Thermal dryers (rotary, fluidized bed)

Key Factors:

• Salt purity (recovered NaCl/Na₂SO₄ can be reused)

• Toxic constituents require secure landfill

• Sludge classification as hazardous or non-hazardous

6.  Recovered Water Reuse

• Condensate from evaporation is typically low TDS (<100 ppm)

• Post-polishing: Activated carbon, UV/ozone, or polishing RO

• Reused in boilers, cooling towers, process water, or irrigation (if safe)

Instrumentation & Automation for Zero Liquid Discharge Plant

A ZLD system uses advanced automation to ensure energy, flow, and quality optimization:

Controlled via PLC/SCADA systems for process interlocks, alarms, data logging.

Energy Considerations in a Zero Liquid Discharge Plant & Optimization

• Energy is the largest OPEX component in ZLD

• Recovery strategies include:

  • MVR in evaporators (saves >70% energy vs. single-effect)

  • Heat recovery exchangers

  • Solar drying or solar-assisted evaporators

  • Use of biogas from anaerobic sludge digesters

ZLD System Design – Engineering Considerations

1. Influent Characterization

   • Comprehensive water analysis (TDS, COD, scaling ions, organics)

   • Phase diagrams for salt solubility

2. Material Selection

   • Corrosion-resistant alloys: SS316L, duplex SS, Hastelloy

   • Non-metallic options for highly acidic/alkaline waste

3. Redundancy

   • N+1 design for critical pumps, blowers

   • Dual RO skids for cleaning/maintenance flexibility

4. Sludge Management

   • Integration with sludge digesters or incinerators

5. Space & Load Flexibility

   • Modular skids and containerized systems for remote/expansion-ready sites

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Real-World Applications

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Summary Table