What is Zero Liquid Discharge?
Zero Liquid Discharge (ZLD) is a wastewater treatment process developed to completely eliminate all liquid discharge from a system. The goal of a zero liquid discharge system is to reduce the volume of wastewater that requires further treatment, economically process wastewater and produce a clean stream suitable for reuse. Companies may begin to explore ZLD because of ever-tightening wastewater disposal regulations, company mandated green initiatives, public perception of industrial impact on the environment, or concern over the quality and quantity of the water supply.
The first step to achieving ZLD is to limit the amount of wastewater that needs to be treated. Once wastewater generation is minimized and the volume of wastewater that needs to be treated is known, you can then explore what equipment is needed, which depends on the characteristics of the wastewater and its volume. A traditional approach to ZLD is to use filtration technology, funnel the reject waters to an evaporator, and send the evaporator concentrate to a crystallizer or spray dryer. However, the equipment to de-water the concentrated slurry tends to be very large and extremely expensive, which limits the cost effectiveness to only those with very large waste streams.
A common ZLD approach is to concentrate the waste water and then dispose of it as a liquid brine, or further crystallize the brine to a solid. A typical evaporator uses tube-style heat exchangers. The evaporated water is recovered and recycled while the brine is continually concentrated to a higher solids concentration. Concentrated brine is disposed of in a variety of ways, such as sending it to a publicly owned treatment works, using evaporation ponds in areas with net positive evaporative climates, or by treatment in a crystallizing system, such as a circulating-magma crystallizer or a spray dryer. Crystallized solids can be landfilled or applied to land, depending upon the crystal characteristics.
For over 30 years vapor compression evaporation has been the most useful technology to achieve zero liquid discharge. Evaporation recovers about 95 % of a wastewater as distillate for reuse. Waste brine can then be reduced to solids in a crystallizer/dewatering device. However, evaporation alone can be an expensive option when flow rates are considerable. One way to solve this problem is to integrate membrane processes with evaporation. These technologies are nowadays often combined to provide complete ZLD-systems.
The most common membrane processes used so far are reverse osmosis (RO) and electrodialysis reversal (EDR). By combining these technologies with evaporation and crystallization ZLD- systems have become less expensive. They are however combined differently depending on the circumstances. Together with these components, a variety of other well-known water treatment technologies are used in ZLD-systems for pre-treatment and polishing treatment.
These treatments are:
- pH adjustment
- mixed/separate bed
- oil/water separator
- oxidation (uv , ozone, sodium hypochlorite)
- dissolved air flotation (daf)
- carbon adsorption
- anaerobic or aerobic digestion
As environmental, political and public health entities place more focus on waste water management, ZLD strategies are more often being evaluated for feasibility in industrial facilities. The ZLD approach taken, however, greatly depends on the quality of water available for use.
- Reduction or elimination of costly regulatory compliance
- Reliable chemical/physical processes
- Small footprint
- Ease of operation
- Almost 100% water recovery
- Almost 100% metals and chemical recovery
- Modular construction
- Low costs
Well-designed ZLD system will minimize the volume of liquid waste that requires treatment, while also producing a clean stream suitable for use elsewhere in the plant processes.