Zero liquid discharge (ZLD) is an engineering approach applied to wastewater treatment, aiming for the total reuse of wastewater and the complete elimination of liquid discharge into the environment.
ZLD entails a wastewater treatment process where no liquid is discharged into the environment. This means that all wastewater undergoes treatment and is recycled back into the industrial process, creating a closed circuit where the same water is continuously used to fuel industrial operations. This approach significantly contributes to water conservation by dramatically reducing the need for fresh water intake.
How does ZLD work?
ZLD has evolved to offer zero discharge solutions by separating pollutants and salinity from wastewater through various techniques, including membrane-based methods. It concentrates all pollutants in a minimal volume of water called reject, which is then either disposed of or evaporated using multiple-effect evaporation systems.
What are the methods used for Zero Liquid Discharge?
Various technological approaches can be used to achieve Zero Liquid Discharge (ZLD), such as:
- thermal process (evaporation): this method involves the use of heat to evaporate the water from the discharged liquids, leaving behind only the dissolved solids. The condensate can be recovered and reused, while the solids can be disposed of;
- reverse osmosis (RO): this is a membrane-based filtration process that removes impurities from water, including dissolved solutes. Through pressure, water is forced through a semi-permeable membrane, leaving behind contaminants and solids. Know more here;
- electrodialysis: the use of membranes allows the selective passage of ions under the influence of an electric field. This allows the separation of ions present in the waste liquid, producing a concentrate and diluent. The concentrate can be treated or disposed of;
- forward osmosis: Unlike RO, in forward osmosis a solvent is extracted from a solution through a membrane to a more concentrated solution. This process can be used to concentrate waste liquids, reducing the final volume to be treated;
- membrane distillation: This process exploits the difference in vapour pressure between two sides of a membrane to evaporate and condense water. This allows the separation of solutes from waste liquids, producing a permeate and concentrate.
By combining these processes or using them sequentially, an effective ZLD can be achieved, minimising liquid discharge and maximising resource recovery and reuse.
Why is the engineering approach for ZLD important?
The engineering approach involves conducting processes and addressing problems in accordance with engineering principles. Applying an engineering approach to wastewater treatment entails employing typical engineering logic and skills to purify water, developing standardized methodologies for effective water treatment.
What are the benefits of Zero Liquid Discharge?
Zero liquid discharge technology is growing globally as an important wastewater management strategy to reduce water pollution and expand water availability. The main advantages of ZLD are:
- minimisation of wastewater discharge;
- wastewater recovery;
- reduction of environmental impact;
- reuse of water resources in a time of scarcity;
- reduction of the risk of penalties for failure to recover and due to legislative standards;
- potential recovery of resources present in wastewater, which can be destined for reuse or recycling.