Zero liquid discharge (ZLD) means a plant releases no liquid effluent at all: water is recovered and reused, and the dissolved salts leave as solids. It is expensive to run, but for some industries and locations in India it is a condition of operating.
Who has to achieve ZLD
India's 2015 ZLD guidelines focused on four sectors: textiles, tanneries, distilleries, and pulp and paper. Beyond that, the Central and State Pollution Control Boards can require ZLD for units in sensitive or critically polluted areas. Tamil Nadu was early: dyeing and wet-processing units in Tiruppur have operated under a ZLD requirement since 2008.
Many pharmaceutical and bulk-drug plants also run ZLD systems, either as a consent condition or to secure water supply. Your consent order from the state board is the document that tells you what applies to your unit.
The typical ZLD treatment train
- Pre-treatment: physico-chemical treatment (coagulation, flocculation, settling) and usually biological treatment to remove suspended solids and organics.
- Softening and filtration: removing hardness and silica that would scale the membranes.
- Reverse osmosis: recovers most of the water for reuse. In textile ZLD systems, RO commonly recovers around 80%.
- Evaporation and crystallisation: the RO reject goes to a multiple-effect evaporator (MEE) or mechanical vapour recompression (MVR) system, and the remaining salts are recovered as solids.
Chemicals at each stage
- Coagulants and polyelectrolytes for the primary clarifier (see choosing a coagulant)
- Lime and soda ash for softening and pH control
- Oxidising agents such as hydrogen peroxide, and Fenton chemicals for high-COD streams (see Fenton oxidation)
- Antiscalants and membrane cleaning chemicals for the RO (see RO antiscalants)
- Sodium metabisulphite to remove chlorine before the membranes
- Antifoam for evaporators that foam
Keeping ZLD costs under control
The evaporator is usually the most expensive part to run, so every litre the RO recovers saves money. Good pre-treatment protects the RO, and a steady antiscalant programme lets it run at higher recovery for longer between cleanings.
Where to buy ZLD chemicals
We supply the coagulants, polymers, lime, soda ash, antiscalants and oxidising agents used across ZLD plants. See Pharmaceutical ETP & ZLD Chemicals and Textile Effluent Treatment Chemicals.