What is “zero liquid discharge?” It’s a solution for hard-to-treat or dangerously contaminated wastewater streams, but how does it work, and how does it help make industrial users’ operations more sustainable?
A zero liquid discharge (ZLD) facility is one where no wastewater is discharged into the environment or disposed of by conventional means, such as through a sanitary sewer system.
Zero liquid discharge plants use wastewater treatment technologies that render all of the effluent suitable for reuse, whether it’s for landscape irrigation or for replacing conventional water supplies in an industrial process. Reusing wastewater reduces demand for fresh water.
The increasing popularity of this approach is driven by many considerations, including compliance with stringent regulatory standards, and a desire to be a responsible steward of dwindling water supplies. Adopting the approach can benefit an organization’s bottom line and the perception of its brand. And it can benefit the wider community.
The technologies used in ZLD facilities can include sedimentation, dissolved air flotation, media filtration, activated carbon filtration, ultrafiltration, and reverse osmosis. Exactly which are used can vary widely. For example, in the power industry, salt-heavy wastewater usually requires an evaporation step, which leaves behind solid wastes that are landfilled. Evaporation produces clean water for reuse in cooling or other processes.
In Delhi, India ,where industrial wastes have polluted roughly 543 kilometers of the Ganga River, the Central Pollution Control Board (CPCB) has mandated zero waste disposal for several industries (distillery, tannery, and textile) to curb the release of heavy metals and pesticides into the river.
Mining is another industry that benefits from a ZLD approach to wastewater treatment. Raw mining wastewater is acidic and extremely saline, and often contains a large amount of sediment.A good example is the Collahuasi Copper Mine, a project undertaken by RWL Water of Israel.
The wastewater treatment plant at the facility was sized to accommodate a flow of 216 cubic meters an hour, or about 5 million liters a day. All of the water had to be treated for reuse.
The facility uses a series of treatments, including sedimentation, dissolved air flotation, media filtration, activated carbon filtration, ultrafiltration, and reverse osmosis.
Removing as much sediment as possible from the water is particularly important. It protects the company’s infrastructure investment by preventing fouling or other damage of membranes and other equipment. In the sedimentation process, suspended solids are settled using gravity. Then, dissolved air flotation allows the easy removal of various colloidal materials and dispersed particles.
At this stage, the water is filtered to remove solids that remain after the flotation stage. The removal of dissolved organic material is completed with activated carbon filtration, followed by ultrafiltration. This last step ensures that residual suspended solids, colloids, and organic matter — including bacteria and viruses — are removed before the water is treated using reverse osmosis.
The final polishing step in the water treatment process, reverse osmosis desalination, ensures that dissolved salts and heavy metals are removed from the water before it is reused.
Cost Versus Benefit
Another example is Orlando Utilities Commission’s Stanton Energy Center in Florida, which is sited on the ecologically fragile Floridan Aquifer. In addition to conserving water, the ZLD plant also provides the utility with the operational flexibility — in terms of feed water quality — needed to keep the area’s lights on.
According to Water and Wastewater International, the ZLD system uses four brine concentrator evaporators and four crystallisers to treat blowdown from the cooling tower. It allows 95 to 98% of the wastewater to be reused as high-purity distilled water, which can be used in industrial processes.
Although some may be concerned about the cost of ZLD, Jack Lyons, a project engineer with the OUC, told Water World in 2015:
Pay now or pay later […] You have all types of options out there for water treatment. […] But if you have a need for a high throughput, or extended operations, or high reliability, or great operational flexibility, or confidence in your process, look at ZLD. No one is going to put in a ZLD system and operate it for a year and shut it down […] the benefits justify the cost in the long haul.
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