ZLD Process Flow (Overview)

Below is the typical full train — sequence and exact unit selection depend on feed characteristics and treatability tests.

Note: For low-TDS but high-TOC streams an AOP (Advanced Oxidation) stage can be included before RO to protect membranes and improve recoveries.

Stage 1 – ETP (Pretreatment & Biological)

Objective: Remove suspended solids, oils, FOG, reduce BOD/COD load and stabilise the wastewater before membranes.

• Screening & Grit Removal
• Equalization with pH & flow balancing
• Oil & Grease removal (API / Skimmer) where required
• Coagulation + Flocculation (Ferric / PAC / Polymer)
• DAF or Lamella Clarifier for solids & floatables
• Biological Reactor (MBBR/SBR/ASP) for BOD/COD reduction
• Secondary Clarifier and Sludge Dewatering (Filter Press)

Stage 2 – Ultrafiltration (UF)

Function: Remove colloidal matter, emulsified oils, micro-organisms and suspended solids to protect RO membranes and reduce fouling.

• Hollow fibre or capillary UF modules — automatic backwash & air scouring
• Typical recovery: 90–98% for UF (backwashed periodically)
• Pre-filtration reduces chemical CIP frequency for RO

Stage 3 – RO Stage 1 (Primary RO)

Design objective: Achieve maximum permeate recovery with manageable scaling risk. Use antiscalants, pH correction & controlled recovery. Brine heads to RO2 or evaporator depending on TDS.

RO1 permeate is reused (process or washing); RO1 concentrate is forwarded to RO2 for further concentration or to evaporation depending on economics.

Stage 4 – RO Stage 2 (Concentrator RO / Brine Concentration)

Goal: Increase concentrate TDS to levels suitable for thermal concentration (MEE). RO2 uses high-pressure membranes and often lower recovery per pass but increases overall plant recovery when combined with MEE.

• RO2 can be configured as: two-pass RO, staged RO, or high-recovery RO with energy recovery options
• Typical RO2 recovery: 40–70% of RO1 concentrate (depending on feed & scaling)
• Antiscalant, pH, and aggressive CIP strategies are required

Stage 5 – Mechanical Evaporator / Multiple Effect Evaporator (MEE)

Purpose: Thermally concentrate RO2 brine to near saturation. Choice of MEE / ATFD / Mechanical Vap depends on brine chemistry, required TDS concentration and energy optimisation.

• Multiple-effect evaporator (MEE) for energy-efficient steam economy
• Agitated thin film evaporator (ATFD) or forced-circulation evaporator for viscous or scaling brines
• Evaporation is integrated with vapour recompression (MVR) where economical to reduce steam consumption

MEE Outputs

• Condensate (returned to process or polished for reuse)
• Concentrate (high TDS) → Crystallizer / Dryer

Stage 6 – Crystallizer / Dryer (Solid Recovery)

Final step to produce dry salt or chemical solids for disposal or recovery. Crystallizers separate salts from mother liquor and can be followed by centrifuge or filter press and drying.

• Crystallizer types: forced circulation, draft-tube, or crystallization with seeded crystals
• Drying: spray dryer / rotary dryer / belt dryer depending on product
• Final product: recoverable salts or inert cake for landfill per regulatory norms

Typical Design Considerations & Example Spec

C hemicals & Consumables

• Coagulants & flocculants (ETP stage): Ferric chloride, PAC, polymers
• Antiscalants & dispersants (RO stages)
• Membrane cleaners (alkaline & acidic CIP solutions)
• Biocides for biofouling control (where required)
• Neutralisers and pH adjusters for evaporation liquor handling
• Polymers for sludge dewatering

We supply chemicals, dosing skids and SOPs for CIP and membrane protection.

Energy & OPEX Notes

• Major OPEX drivers: chemical consumption, membrane replacement, energy for pumps & MEE (steam or electricity for MVR)
• Energy saving: use of mechanical vapour recompression (MVR) with MEE, energy recovery on RO brine (where possible)
• Sludge & solid disposal costs must be included in operating budget
• Pilot trials provide best estimation of OPEX for your specific feed

Regulatory & Environmental

• We design ZLD plants to satisfy TNPCB / CPCB / State pollution norms and provide technical packs for submissions
• Hazardous concentrates are handled as per hazardous waste rules; we provide options for co-processing or secure disposal
• Condensate reuse/recycle reduces freshwater demand and easing compliance

FAQ — Common Questions

Q: How do you choose between MEE and ATFD?

A: Based on brine chemistry, scale tendency, viscosity and fouling potential. ATFD / thin film may suit viscous or sticky brines; MEE with MVR fits low-fouling, high-recovery cases.

Q: Will RO membranes survive industrial brines?

A: With correct pretreatment (UF + antiscalant + pH control) and regular CIP, membranes are protected. For very high TDS, staged RO + thermal concentration is recommended.

Q: What solids can be recovered?

A: Depends on feed — salts (NaCl, Na₂SO₄), sulfates, chlorides, and other inorganic salts are commonly recovered; hazardous salts treated per norms.

Q: How long for installation?

A: Typically 8–20 weeks depending on size and civil scope; pilot and design phase adds time up front but reduces commissioning risk.