RO Reject Treatment for ZLD

Reverse Osmosis is the backbone of industrial Zero Liquid Discharge (ZLD) systems — but RO alone cannot achieve ZLD. An industrial RO plant recovers 65–75% of its feed as purified permeate and rejects 25–35% as a high-TDS brine concentrate. Under CPCB ZLD mandates — applicable to food processing, pharmaceutical, textile, distillery, and tannery industries in water-stressed zones — this RO reject cannot be discharged. It must be further concentrated and dried to zero liquid using thermal evaporation and crystallisation technology.

Spans Envirotech designs and commissions complete RO reject treatment systems as part of integrated ZLD plants. Our approach begins with a detailed analysis of the RO reject stream — TDS, specific ions (sulphates, chlorides, silica, calcium, magnesium), organic content, and temperature — to select the appropriate evaporation technology and materials of construction. Silica scaling, calcium sulphate fouling, and chloride-driven corrosion are the three dominant failure modes in RO reject treatment systems; each is addressed through anti-scalant dosing, softening pre-treatment, and careful material selection (316L SS, duplex steel, or titanium for high-chloride streams).

The standard Spans ZLD technology stack for RO reject treatment begins with pre-treatment softening or anti-scalant dosing to prevent mineral scaling in evaporator tubes. The primary evaporation stage uses either MEE (Multiple Effect Evaporator) for steam-available or lower-volume applications, or MVR (Mechanical Vapour Recompressor) for high-volume continuous operation. MVR is significantly more energy-efficient at 25–35 kWh/tonne evaporated versus 65–90 kWh/tonne for double-effect MEE. The evaporator concentrates the reject TDS from 8,000–20,000 mg/L up to 150,000–200,000 mg/L (15–20% solids).

The concentrated liquor from the evaporator then feeds an Agitated Thin Film Dryer (ATFD) or spray dryer for final drying to a semi-solid or dry cake. Where the dissolved solids are predominantly sodium chloride or sodium sulphate, a crystalliser can produce a crystalline salt product — which may have commercial value or can be disposed of as non-hazardous solid waste. The condensate from all evaporation stages is collected as distillate with TDS typically <20 mg/L, suitable for boiler feed or cooling tower makeup water.