Zero Liquid Discharge Systems for Textile & Dyeing Unit Effluents: Engineering & Design Guide

Executive Summary & Industrial Context

The textile processing and wet-dyeing industry represents one of the most water-intensive and chemically complex manufacturing sectors globally. Wet processing stages—including desizing, scouring, bleaching, mercerizing, dyeing, and printing—consume between 80 to 150 liters of fresh water per kilogram of processed fabric. The resulting wastewater is characterized by extreme chemical heterogeneity: high Total Dissolved Solids (TDS: $8,000 \text{ to } 30,000 \text{ mg/L}$), recalcitrant Chemical Oxygen Demand (COD: $1,200 \text{ to } 4,000 \text{ mg/L}$), high color index (1,000 to 3,500 Pt-Co units), strong alkaline pH ($8.5 \text{ to } 11.5$), and massive concentrations of monovalent and divalent inorganic salts, primarily Sodium Chloride (NaCl) and Sodium Sulfate (Na_2SO_4).

Driven by stringent environmental regulations issued by regulatory bodies such as the Central Pollution Control Board (CPCB) and State Pollution Control Boards (SPCB) in industrial clusters like Tirupur, Surat, and Ahmedabad, textile manufacturers must achieve Zero Liquid Discharge (ZLD). A ZLD system ensures that no liquid stream leaves the boundary limit of the plant; 95% to 98% of the effluent is recovered as high-purity condensate/permeate for process reuse, while the remaining dissolved inorganic solids are crystallized and recovered as dry salt cakes.

This engineering guide presents the process thermodynamic logic, mechanical design codes (ASME, TEMA, API), membrane integration, thermal evaporator sizing, and metallurgical selection rules required to execute reliable, non-scaling, energy-efficient Textile ZLD plants.

---

1. Effluent Characterization & Process Architecture

1.1 Textile Dyeing Effluent Profile

Textile effluents exhibit distinct chemical challenges compared to municipal or conventional chemical industry streams:

• Refractory Organics & Dyes: Complex synthetic dye structures (azo, reactive, phthalocyanine, anthraquinone) paired with leveling agents, surfactants, and PVA sizing chemicals.
• Inorganic Salt Loading: Heavy usage of NaCl or Na_2SO_4 as exhausting agents during reactive dyeing to force dye uptake onto cotton fibers.
• Scaling Species: Presence of Calcium (Ca^{2+}), Magnesium (Mg^{2+}), Hardness, Silica (SiO_2), and trace heavy metal ions.

                                  TEXTILE EFFLUENT ZLD PROCESS FLOWSHEET
                                  
  +-----------------+     +-------------------+     +--------------------+     +-------------------+
  | Raw Textile     | --> | Equalization &    | --> | Extended Aeration/ | --> | Advanced Oxidation|
  | Dyeing Effluent |     | Neutralization    |     | MBR Biological     |     | Fenton / O3       |
  +-----------------+     +-------------------+     +--------------------+     +-------------------+
                                                                                     |
                                                                                     v
  +-----------------+     +-------------------+     +--------------------+     +-------------------+
  | Purified Water  | <-- | Primary / High    | <-- | Softening & Ultra- | <-- | Color Separation  |
  | Reuse in Dyebath|     | Pressure RO (HPRO)|     | filtration (UF)    |     | Nanofiltration(NF)|
  +-----------------+     +-------------------+     +--------------------+     +-------------------+
                                    |                                                |
                              RO Reject (6-9% TDS)                               Dye Concentrates
                                    v                                                v
                          +-------------------+                            +-------------------+
                          | Thermal [MEE](/process/equipment/multi-effect-evaporator)       |                            | Sludge Dewatering |
                          | Evaporator System |                            | Filter Press      |
                          +-------------------+                            +-------------------+
                                    |
                            Concentrated Brine (28-32% TDS)
                                    v
                          +-------------------+
                          | Crystallizer /    | ----> Recovered Pure Salt (*Na_2SO_4* / *NaCl*)
                          | ATFD Solid Dryer  |
                          +-------------------+

---

2. Advanced Pre-Treatment & Color Removal Dynamics

2.1 Biological Treatment & Membrane Bioreactors (MBR)

Raw textile effluent undergoes screening, oil/grease removal, and pH adjustment before entering an Extended Aeration Activated Sludge system or Membrane Bioreactor (MBR). Submerged MBR systems utilize flat-sheet or hollow-fiber PVDF membranes with $0.04 \ \mu\text{m}$ pore size to eliminate Suspended Solids (TSS < 1 mg/L) and reduce biodegradable COD by > 90%.

2.2 Advanced Oxidation Processes (AOP) for Color & Recalcitrant COD

Refractory non-biodegradable dye compounds interfere with RO membrane performance through severe organic fouling. Advanced Oxidation Processes generate highly reactive Hydroxyl Radicals (· OH, E^0 = 2.80 V) to cleave chromophoric double bonds (C=C, N=N azo linkages):

Fenton Oxidation Reactions:

Fe^{2+} + H_2O_2 \rightarrow Fe^{3+} +  ·  OH + OH^-

·  OH + Dye Chromophore \rightarrow Oxidized Intermediates \rightarrow CO_2 + H_2O

The reaction requires strict pH maintenance at pH = 3.0 - 3.5. Post-reaction neutralization with NaOH to pH = 7.0 - 7.5 precipitates ferric hydroxide Fe(OH)_3 floc, carrying down residual suspended organics. Alternatively, catalytic ozonation (O_3 dosage: $30 - 60 \text{ mg/L}$) achieves complete decolorization without generating chemical hydroxide sludge.

---

3. High-Salinity RO Reject Treatment & High-Recovery Membranes

To minimize high-CAPEX, high-OPEX thermal evaporation, the biological and decolorized effluent must be concentrated to maximum osmotic limits via membrane processes.

       MEMBRANE CONCENTRATION SEQUENCE & OSMOTIC PRESSURE RISE
       
   Effluent Feed (TDS: 10,000 mg/L)
       │
       ▼
 ┌───────────┐    Permeate (TDS < 150 mg/L) ──► To Process Reuse
 │ Primary RO│ ─── (75% Water Recovery @ 35 bar)
 └─────┬─────┘
       │ Primary Reject (TDS: 40,000 mg/L)
       ▼
 ┌───────────┐    Permeate (TDS < 300 mg/L) ──► To Process Reuse
 │ High-Press│ ─── (50% Recovery @ 80 bar)
 │ RO (HPRO) │
 └─────┬─────┘
       │ HPRO Reject (TDS: 80,000 mg/L)
       ▼
 ┌───────────┐    Permeate (TDS < 500 mg/L) ──► To Process Reuse
 │ Disc Tube │ ─── (30% Recovery @ 120 bar)
 │ RO (DTRO) │
 └─────┬─────┘
       │ Final RO Reject Brine (TDS: 110,000 mg/L = 11% w/w)
       ▼
   To Thermal Multi-Effect Evaporator (MEE)

3.1 Osmotic Pressure Sizing Calculations

The theoretical osmotic pressure (\Pi) of concentrated saline effluent is calculated via the van 't Hoff equation modified for ionic activity:

\Pi = φ  ·  Σ (i_j  ·  C_j)  ·  R  ·  T

Where:

• φ = Osmotic activity coefficient (≈ 0.92 - 0.95 for sodium salts)
• i_j = van 't Hoff dissociation factor (i = 2 for NaCl, i = 3 for Na_2SO_4)
• C_j = Molar concentration of species j (mol/L)
• R = Universal gas constant ($0.08314 \text{ L}\cdot\text{bar/mol}\cdot\text{K}$)
• T = Absolute temperature (K)

At an RO reject concentration of $80,000 \text{ mg/L}$ ($8% \text{ w/w}$) dominated by NaCl, the operating osmotic pressure reaches:

\Pi = 0.93  ·  ( 2  ·  (80  g/L) / (58.44  g/mol) )  ·  0.08314  ·  303.15  ≈  64.1  bar

Including hydraulic friction losses and flux driving gradient (Δ P - Δ \Pi \ge 15 bar), High-Pressure RO (HPRO) and Disc Tube RO (DTRO) systems operate at working pressure limits of $80 \text{ to } 120 \text{ bar}$.

---

4. Fractional Salt Recovery: Sodium Sulfate (Na_2SO_4) vs. Sodium Chloride (NaCl)

A major economic handicap of basic ZLD plants is the generation of mixed, contaminated hazardous salt cake containing both NaCl and Na_2SO_4. Mixed salt cannot be reused in textile dyeing and must be sent to expensive hazardous waste landfills. Fractional Salt Crystallization solves this by yielding high-purity, reusable industrial salts.

       SOLUBILITY CURVES OF SODIUM SULFATE vs SODIUM CHLORIDE
       
  Solubility
 (g / 100g H2O)
      70 ┬                                         / Na2SO4 (Anhydrous)
         │                                       /
      60 ┼                                     /
         │                                   / 
      50 ┼                                 /
         │                               /
      40 ┼-----------------------------/--------- NaCl (Flat slope: ~36-39g)
         │                           /  |
      30 ┼                         /    | (Transition Point: 32.4°C)
         │                       /      |
      20 ┼                     /        |
         │                   /          |
      10 ┼                 / (Glauber's Salt: Na2SO4·10H2O)
         │               /              |
       0 ┴──────────────┴───────────────┴────────────────────────────
         0             10              32.4          50            100
                                 Temperature (°C)

4.1 Thermodynamic Differential & Separation Logic

1. Nanofiltration (NF) Salt Separation: Loose NF membranes (MWCO 300 - 500 Da) exhibit high rejection toward divalent sulfate ions (R_{SO4^{2-}} > 98%) while allowing monovalent chloride ions (R_{Cl^-} < 15%) to pass freely into the permeate.
   • NF Permeate: Rich in NaCl, virtually zero sulfate.
   • NF Concentrate: Highly concentrated in Na_2SO_4.
2. Chill Crystallization of Sodium Sulfate (Glauber's Salt): Sodium sulfate exhibits an extreme temperature-dependent solubility curve. At $32.4^\circ\text{C}$, it undergoes a phase transition from decahydrate (Na_2SO_4 · 10H_2O, Glauber's salt) to anhydrous form (Na_2SO_4). By cooling the sulfate-rich stream down to $5^\circ\text{C} - 10^\circ\text{C}$ in a surface crystallizer:

Na_2SO_4(aq) + 10 H_2O \xrightarrow{5^\circC} Na_2SO_4  ·  10H_2O \downarrow

Glauber's salt crystallizes rapidly due to steep solubility drop, while residual NaCl remains fully dissolved. 3. Evaporative Crystallization of Sodium Chloride: NaCl solubility remains virtually flat across $20^\circ\text{C} \text{ to } 100^\circ\text{C}$ ($35.7 \text{ g/100g } H_2O$ at $20^\circ\text{C}$ vs $39.1 \text{ g/100g } H_2O$ at $100^\circ\text{C}$). Therefore, NaCl recovery requires pure water evaporation in a forced circulation evaporative crystallizer.

---

5. Thermal Concentration & Evaporation System (MEE + ATFD)

When RO concentration limits are reached (TDS ≈ 70,000 - 110,000 mg/L), thermal processes evaporate the remaining water to achieve final crystallization.

       QUINTUPLE EFFECT FALLING FILM + FORCED CIRCULATION MEE ARCHITECTURE
       
   Feed Brine
       │
       ▼
 ┌───────────┐     ┌───────────┐     ┌───────────┐     ┌───────────┐     ┌───────────┐
 │ 1st Effect│ ──► │ 2nd Effect│ ──► │ 3rd Effect│ ──► │ 4th Effect│ ──► │ 5th Effect│
 │  (FFE)    │     │  (FFE)    │     │  (FFE)    │     │  (FCE)    │     │ (FC Cryst)│
 └─────┬─────┘     └─────┬─────┘     └─────┬─────┘     └─────┬─────┘     └─────┬─────┘
       │                 │                 │                 │                 │
  Steam (120°C)     Vapor (108°C)     Vapor (96°C)      Vapor (84°C)      Vapor (71°C)
       ▼                 ▼                 ▼                 ▼                 ▼
 ┌───────────────────────────────────────────────────────────────────────────────────┐
 │                            Process Condensate Header                              │ ──► Clean Reuse
 └───────────────────────────────────────────────────────────────────────────────────┘     (TDS < 30 mg/L)
                                                                                       │
                                                                                 Brine (300 g/L)
                                                                                       ▼
                                                                               ┌───────────────┐
                                                                               │ Agitated Thin │
                                                                               │ Film Dryer    │
                                                                               └───────┬───────┘
                                                                                       │
                                                                                       ▼
                                                                               Dry Salt Powder
                                                                               (Moisture < 5%)

5.1 Evaporator Selection Matrix: Falling Film vs. Forced Circulation

• Falling Film Evaporators (FFE): Deployed for early concentration stages (from $7% \text{ up to } 16% \text{ TDS}$). Liquids flow as a thin film down inside vertical tubes. FFE provides extremely high heat transfer coefficients (U = 1,800 - 2,800 W/m²·K) with low temperature driving forces (Δ T = 4 - 7^\circC), minimizing thermal stress.
• Forced Circulation Evaporators (FCE): Deployed when brine concentration exceeds $16% \text{ TDS}$ up to saturation ($28 - 32% \text{ w/w}$). High axial flow circulation pumps maintain tube velocities between $2.0 \text{ and } 3.0 \text{ m/s}$. Boiling inside tubes is completely suppressed by hydrostatic head pressure; flash evaporation occurs exclusively inside the flash separator chamber, preventing tube scaling.

5.2 Agitated Thin Film Dryer (ATFD) Solid Recovery

The ATFD represents the absolute final stage of ZLD. Concentrated slurry ($30 - 35% \text{ w/w}$ solids) from the crystallizer separator is fed into the ATFD top inlet.

       AGITATED THIN FILM DRYER (ATFD) MECHANICAL DETAIL
       
                         Vapor Outlet to Condenser
                                  ▲
                                  │
                          ┌───────┴───────┐
                          |  Vapor Hood   |
                          └───────┬───────┘
                     ┌────────────┴────────────┐
                     │   Internal Feed Nozzle  │
                     └────────────┬────────────┘
        Driven Motor              │
          ┌─────┐                 ▼
          │ MOD │ ────── Feed Inlet Slurry (30% Solids)
          └──┬──┘       │                  │
             │          │  ┌────────────┐  │  Steam / Hot Thermal Oil
             ▼          │  │ Scraper    │  │  Heating Jacket (150°C)
       ┌───────────┐    │  │ Blade      │  │    │
       │ Rotating  │    ▼  │            │  ▼    ▼
       │ Shaft /   │ ══════╡ ║        ║ ╞══════▓▓▓▓▓▓▓▓
       │ Rotor     │       │ ║        ║ │      ▓ Heating
       │ Assembly  │ ══════╡ ║        ║ ╞══════▓ Jacket
       └───────────┘    ▲  │            │  ▲    ▲
                        │  │ Scraper    │  │    │
                        │  │ Blade      │  │  Steam Condensate Drain
                        │  └────────────┘  │
                        └─────────┬────────┘
                                  │
                                  ▼
                         Dry Salt Powder Discharge
                         (Moisture < 3-5% w/w)

Mechanical Working Mechanism:

A central rotor equipped with hinged or fixed scraper blades rotates at tip speeds of $6 \text{ to } 12 \text{ m/s}$. The blades continuously clear a liquid/sludge film of $0.5 \text{ to } 2.0 \text{ mm}$ thickness against the steam-jacketed inner shell wall.

• Zone 1 (Liquid Phase): Pre-heating and flash evaporation of liquid.
• Zone 2 (Pasty/Slurry Phase): Violent mechanical shearing breaks up viscous gel structures.
• Zone 3 (Powder Phase): Scrapers crush dried crust into free-flowing salt granules (< 5% moisture), discharging through a bottom rotary airlock valve.

---

6. Mechanical & Process Design Parameters (ASME, TEMA, API)

Executing robust ZLD hardware requires adherence to international pressure vessel, heat exchanger, and tank codes.

          ASME SEC VIII DIV 1 SEPARATOR SHELL MECHANICAL STRESSING
          
                         P (Internal Pressure)
                           │   │   │   │
                           ▼   ▼   ▼   ▼
               ┌───────────────────────────────┐
               │                               │
       t_min   │   Inside Radius R             │  Hoop Stress S_h
      ├───────┤│◄─────────────────────────────►│ ◄==============►
               │                               │
               └───────────────────────────────┘
                               ▲
                               │
                        Corrosion Allowance C_a (3.0 mm)

6.1 ASME Section VIII, Division 1 Pressure Vessel Design

All evaporator shell bodies, vapor-liquid separators, and flash vessels operating under pressure or full vacuum ($0.08 \text{ bar absolute}$) must be designed per ASME Code Section VIII, Div 1.

Shell Wall Thickness Formula (Under Internal Pressure):

t_{min} = (P  ·  R) / (S  ·  E - 0.6  ·  P) + C_a

Where:

• t_{min} = Minimum required shell thickness (mm)
• P = Internal design pressure (MPa) (typically $0.35 \text{ MPa}$ gauge or full vacuum)
• R = Inside radius of shell (mm)
• S = Maximum allowable stress value of metallurgy (MPa) (e.g., $175 \text{ MPa}$ for Duplex 2205 at $100^\circ\text{C}$)
• E = Joint efficiency factor ($1.0$ for $100%$ Radiographic Testing / RT-1)
• C_a = Corrosion allowance ($3.0 \text{ mm}$ minimum for aggressive high-chloride ZLD brine)

Vacuum Design Considerations:

Evaporator bodies working under vacuum must be checked for circumferential buckling stress per ASME Sec VIII Div 1 UG-28, incorporating external stiffening rings where shell length-to-diameter ratio L/D_o > 3.0.

6.2 Vapor-Liquid Separator Sizing: Souders-Brown Equation

Entrainment of brine droplets into distillate streams degrades distillate quality (TDS > 100 mg/L). Vapor separators are sized based on maximum allowable vapor velocity (v_{max}) using the Souders-Brown correlation:

v_{max} = K_{SB}  ·  √((ρ_L - ρ_V) / (ρ_V))

Where:

• v_{max} = Maximum allowable superficial vapor velocity (m/s)
• ρ_L = Liquid brine density (≈ 1,200 kg/m³)
• ρ_V = Water vapor density at operating pressure (kg/m³)
• K_{SB} = Empirical vapor-liquid separation factor (m/s)
  • Without demister pad: K_{SB} = 0.035 - 0.048 m/s
  • With structured SS316L / Titanium demister pad ($150 \text{ mm}$ thickness, mesh density $140 \text{ kg/m}^3$): K_{SB} = 0.107 m/s

Vessel Cross-Sectional Area (A_v) is sized via:

A_v = (\dot{V}_{vapor}) / (v_{actual)} \quad where  v_{actual} \le 0.75  ·  v_{max}

6.3 TEMA Class R & C Standards for Shell & Tube Exchangers

Heat exchangers in MEE units must conform to TEMA (Tubular Exchanger Manufacturers Association) standards:

• TEMA Class R: Heavy-duty specification for severe industrial processing conditions.
• Exchanger Types: TEMA Type BEM (Fixed Tubesheet) for low temperature differentials; TEMA Type BEU (U-Tube) or AKT (Floating Head) for high thermal expansion differentials between shell and tubes.
• Tube Expansion: Tubes are strength-welded to tubesheets followed by two-groove mechanical expansion to prevent chloride leakage into distillate loops.

---

7. Sizing Equations & Thermodynamic / Mass Balance Logic

7.1 Global Mass & Solid Balance Sizing Logic

For an incoming RO reject flow rate \dot{m}_{feed} (kg/h) with solute mass fraction X_{feed}:

\dot{m}_{feed} = \dot{m}_{distillate} + \dot{m}_{sludge}

\dot{m}_{feed}  ·  X_{feed} = \dot{m}_{sludge}  ·  X_{sludge}

Total Required Evaporative Capacity (\dot{m}_{evap}, kg/h):

\dot{m}_{evap} = \dot{m}_{feed} ( 1 - (X_{feed}) / (X_{sludge)} )

7.2 Boiling Point Elevation (BPE) Quantification

Boiling Point Elevation (Δ T_{BPE}) represents the rise in boiling temperature of saline brine over pure water at identical pressure. BPE reduces the effective temperature driving force across evaporator effects.

BPE = i  ·  K_b  ·  m  ·  \gamma_{\pm}

Where K_b = 0.512 \ ^\circC·kg/mol, m is brine molality, and \gamma_{\pm} is the mean ionic activity coefficient.

For high-salinity textile brine approaching saturation ($28% \text{ w/w}$ NaCl + Na_2SO_4), empirical BPE equations are applied:

BPE = 0.58  ·  C_{TDS} + 0.0032  ·  C_{TDS}²

(where C_{TDS} is expressed as weight percentage salt).

At $28% \text{ TDS}$, BPE ≈ 8.7 \ ^\circC.

7.3 Heat Exchanger Surface Area Calculation

The heat transfer area (A_i, m²) for effect i is calculated using the Logarithmic Mean Temperature Difference (LMTD) corrected for BPE and hydrostatic head losses:

Δ T_{effective, i} = (T_{steam, i} - T_{boiling, i}) - BPE_i - Δ T_{hydrostatic}

Q_i = \dot{m}_{evap, i}  ·  \lambda_{v, i}

A_i = (Q_i) / (U_i  ·  Δ T_{effective, i)}

Where:

• Q_i = Thermal duty of effect i (kW)
• \lambda_{v, i} = Latent heat of vaporization at effect pressure (kJ/kg)
• U_i = Overall heat transfer coefficient (W/m²·K)

---

8. Metallurgical Selection & Corrosion Control Matrix

High chloride content (Cl^- > 30,000 mg/L), elevated temperatures (> 80^\circC), and low pH variations create extreme Pitting Corrosion, Crevice Corrosion, and Stress Corrosion Cracking (SCC) risks.

8.1 Pitting Resistance Equivalent Number (PREN)

Material selection is governed by the alloy PREN rating:

PREN = \%Cr + 3.3(\%Mo + 0.5\%W) + 16(\%N)

Materials with PREN > 40 are immune to pitting in hot, concentrated textile brine environments.

       PREN SPECTRUM & METALLURGY APPLICATION RANGES
       
   PREN Value
      50 ┬                                                   Titanium Gr 2 / Hastelloy C-276
         │                                                   (Immune to Hot NaCl Brine)
      42 ┼─────────────────────────────────────────────────  Super Duplex 2507 (PREN ~43)
         │                                                   (Ideal for FCE & ATFD Shells)
      35 ┼─────────────────────────────────                  Duplex 2205 (PREN ~35)
         │                                                   (Low Temp MEE & RO Reject Lines)
      25 ┼─────────────────                                  SS316L (PREN ~25)
         │                                                   (Clean Distillate & Soft Water)
      18 ┼─                                                  SS304L (PREN ~18) - (Unsuitable for Brine)
         ┴─────────────────────────────────────────────────────────────────────────────────

8.2 Comprehensive ZLD Metallurgy Selection Table

---

9. Comparative Analysis Selection Matrices

9.1 Solid Recovery Technologies: ATFD vs. Spray Dryer vs. Filter Press + Dryer

9.2 Salt Recovery Strategies: Mixed Salt ATFD vs. Fractional Crystallization

---

10. Real-World Engineering Case Study: 1,000 m³/day (1 MLD) Textile ZLD Plant

To illustrate design principles, we evaluate an operational 1,000 m³/day (1 MLD) Zero Liquid Discharge system located in an industrial textile dyeing park.

10.1 Feed Effluent Specifications

Daily Flow Rate  Q = 1,000  m³/day = 41.67  m³/h

• TDS: $14,500 \text{ mg/L}$ ($1.45% \text{ w/w}$)
• Total Chloride (Cl^-): $5,200 \text{ mg/L}$
• Total Sulfate (SO_4^{2-}): $3,800 \text{ mg/L}$
• COD: $1,850 \text{ mg/L}$
• Color: $1,800 \text{ Pt-Co units}$

       1,000 m³/day TEXTILE ZLD MASS BALANCE SNAPSHOT
       
   Raw Effluent: 1,000 m³/day (TDS: 14,500 mg/L)
       │
       ▼
 ┌─────────────────────────┐
 │ Pre-Treatment & MBR     │ ──► Sludge Discharge (1.2 tons/day)
 └───────────┬─────────────┘
             │ 995 m³/day
             ▼
 ┌─────────────────────────┐
 │ NF Salt Separation      │ ──► NF Concentrate (180 m³/day, High Na2SO4) ──► Chill Crystallizer
 └───────────┬─────────────┘                                                    │
             │ NF Permeate (815 m³/day, High NaCl)                              ▼
             ▼                                                          Pure Na2SO4 Salt
 ┌─────────────────────────┐                                            (4.8 tons/day)
 │ Primary + HPRO Systems  │ ──► Permeate Water: 845 m³/day (TDS < 150 mg/L) ──► Process Reuse
 └───────────┬─────────────┘
             │ HPRO Reject: 150 m³/day (TDS: 92,000 mg/L)
             ▼
 ┌─────────────────────────┐
 │ Quadruple Effect MEE    │ ──► Distillate Condensate: 136 m³/day (TDS < 30 mg/L)
 └───────────┬─────────────┘
             │ Concentrated Brine: 14 m³/day (TDS: 310,000 mg/L)
             ▼
 ┌─────────────────────────┐
 │ ATFD Crystallizer Unit  │ ──► Distillate Condensate: 9.6 m³/day
 └───────────┬─────────────┘
             │
             ▼
   Pure NaCl Salt Powder: 9.2 tons/day (Moisture 3.8%)

10.2 Stage-by-Stage Performance & Water Mass Balance

10.3 Utility & Energy Consumption Breakdown

       SPECIFIC ENERGY CONSUMPTION (kWh / m³ Clean Water Recovered)
       
   Energy (kWh/m³)
      35 ┬
         │                                                      ATFD Dryer
      30 ┼                                                      (Thermal Steam)
         │
      25 ┼                                  MEE Evaporator
         │                                  (Steam Thermal)
      20 ┼
         │
      15 ┼
         │              HPRO / DTRO
      10 ┼              (High Press Pump)
         │  Primary RO
       5 ┼  (Booster)
         │  ┌───────┐   ┌───────┐           ┌───────┐           ┌───────┐
       0 ┴──┴───────┴───┴───────┴───────────┴───────┴───────────┴───────┴──────
            Primary RO    HPRO Membrane       MEE Evaporator       ATFD Unit

• Electrical Energy Consumption:
  • MBR & Pre-treatment: $0.85 \text{ kWh/m}^3$
  • High-Pressure RO ($80 \text{ bar}$): $4.20 \text{ kWh/m}^3$
  • MEE & ATFD Drive Motors / Pumps: $3.80 \text{ kWh/m}^3$
  • Total Specific Power Consumption: $8.85 \text{ kWh per m}^3$ of raw effluent treated.
• Thermal Energy (Steam) Consumption:
  • Steam economy of Quadruple Effect MEE + TVR: $3.85 \text{ kg evaporation / kg steam}$.
  • Steam consumption for MEE ($136 \text{ m}^3\text{/day}$ evap): $35.3 \text{ tons/day}$ saturated steam at $3.5 \text{ bar(g)}$.
  • Steam consumption for ATFD ($9.6 \text{ m}^3\text{/day}$ evap): $11.5 \text{ tons/day}$ saturated steam at $5.0 \text{ bar(g)}$.

---

11. Conclusion & Engineering Best Practices

Designing a reliable Zero Liquid Discharge plant for textile dyeing unit effluents requires strict adherence to thermodynamic principles, metallurgical safeguards, and process sequence optimization:

1. Prioritize Advanced Pre-Treatment: Eliminate color and refractory organics using MBR and AOP (Fenton/Ozonation) before RO stages to avoid catastrophic membrane organic fouling.
2. Maximize Membrane Concentration: Deploy High-Pressure RO (HPRO/DTRO) up to $120 \text{ bar}$ to push feed TDS to $90,000 - 110,000 \text{ mg/L}$, minimizing high-cost thermal evaporator sizing.
3. Implement Fractional Salt Recovery: Utilize Nanofiltration and Chill Crystallization to separate Sodium Sulfate (Na_2SO_4) and Sodium Chloride (NaCl), generating pure, reusable salt streams and eliminating hazardous landfill liabilities.
4. Adhere to Code-Compliant Metallurgy: Specify Super Duplex 2507 (PREN > 42) or Titanium Grade 2 for high-temperature crystallizer effects and ATFD components subject to hot, saturated chloride brines.
5. Enforce ASME/TEMA Code Rigor: Size pressure vessel shells per ASME Sec VIII Div 1 with minimum $3.0 \text{ mm}$ corrosion allowance, and size vapor-liquid separators via the Souders-Brown equation (K_{SB} = 0.107 m/s with demister) to maintain distillate purity below $30 \text{ mg/L}$ TDS.

---

For customized ZLD process engineering, thermal evaporator calculations, or equipment manufacturing specifications, contact the SEMCO Engineering Team.