Zero-Liquid Discharge

Zero-Liquid Discharge (ZLD) is a closed-loop treatment process whereby there is no discharge of wastewater, and all the treated water is thereby reused within the production process. This process reduces the water footprint of manufacturing facilities. Normally dewatered solids and crystallised salts are produced by the Zero Liquid Discharge process and need to be disposed of.

Zero-Liquid Discharge Solutions

Every solution is different, as it depends on the composition of the water or wastewater which is treated. In general, the more polluted the water, the more sophisticated the treatment steps which are required. WEC Projects provides comprehensive ZLD solutions.

  • Physical pre-treatment – such as chemical dosing, clarification, multimedia filtration, dissolved air flotation to remove mainly suspended solids
  • Biological treatment – to remove organic matter which is biodegradable (aerobic or anaerobic). Anaerobic digestion is combined with biogas to energy solutions
  • Membrane treatment technologies – to remove suspended solids (microfiltration and ultrafiltration) and dissolved salts (nanofiltration and reverse osmosis). Reverse osmosis and nanofiltration produce a treated water stream as well as a brine concentrate stream.   The brine concentrate stream contains dissolved impurities and salts, which have to be treated further.
  • All-Membrane Brine Concentration (AMBC) – this is a membrane technology to concentrate brine streams such as from nanofiltration and reverse osmosis systems. This leads to enhanced water recovery and reduces the volumes (and thereby the costs) for “final” treatment of the brine streams using thermal technologies.
  • Thermal evaporation – various configurations of thermal evaporators (electric or steam-powered) are available for recovering more water from brine streams, thereby leading to for further concentration than can be achieved with the AMBC.
  • Thermal Crystallisation – this is normally the “final” brine concentration/treatment step in a zero liquid discharge process.  Remaining brine streams are concentrated to such a degree (by boiling off water) that the concentration of the dissolved salts exceeds the solubility limits.   Salt crystals are formed, and these are either sold, if they contain valuable resources, or disposed of.

Zero-Liquid Discharge Concept

Zero liquid Discharge Diagram
An illustration of the concept of Zero Liquid Discharge (image via WikiCommons)

Zero-Liquid Discharge Benefits

Ensures compliance for extremely strict discharge regulations

Cost savings from the reuse of water and the recovery of valuable compounds or resources which can potentially be sold

Best application in water-scarce areas

Best for minimising the water footprint by reducing water usage in a plant

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