Zero Liquid Discharge
Alexey Sergeychik @ stock.adobe.com
Zero Liquid Discharge (ZLD) systems are designed to efficiently treat wastewater from production lines in such a way that clean water, suitable for reuse, is produced and no liquid waste is discharged to water bodies. As there are many types of contaminants present in wastewater, the technical specifications of these systems have to be tailor-made to match each site, likely increasing the implementation costs. The complexity of the wastewater composition determines the intricacy of the solutions required, ranging from evaporation systems to physiochemical membrane filtration, as well as crystallization, and reverse osmosis. ZLD systems need to be able to process variations in water and waste flow, recover up to 95% of liquid waste for reuse, and recover byproducts from the waste treatment process. According to researchers, the primary challenge to be addressed with ZLD is to "increase the recovery of the membrane installation to its limits, without increasing the costs of water produced."
ZLD systems typically comprise a series of processes to treat and concentrate waste water to near saturation, using clarifiers or reactors to precipitate out metals, hardness, and silica; evaporators to vaporize water in the final phases of waste concentration; and crystallizers to boil off any remaining liquid, leaving a dry, solid cake for disposal.
Commonly adopted in Europe, ZLD is especially useful in the power generation sector. In this region, ZLD programs are also becoming more popular in industries that are inherently highly regulated or that require a steady water supply, such as chemical, food and beverage, pharmaceutical, steel, and pulp and paper, as well as specific remediation projects. Implementation and running costs can be high, so even with the possibility of integral reuse of water, solid waste separation, and nutrient recovery, today's industrial giants choose these treatments as last-resort scenarios; usually only in locations where the availability of water is low or the cost of acquisition and disposal is too high.
Key Emerging Technologies
While ZLD systems have become relatively mainstream, new technologies are emerging to increase efficiencies and enable treatment of higher salinity feedwaters, significantly reducing the operational costs and increasing feasibility for advanced wastewater treatment and zero liquid discharge.
The challenge is to adequately treat water in order for it to be re-used within the closed loop of ZLD systems. Nanofiltration is an emerging technology being employed to separate wastewater from contaminants. Ceramic Membranes are being increasingly used in ZLD because they are highly durable, can withstand high pressure and high heat, and can be used to remove contaminants that cannot be removed by standard treatment processes. As such, they are particularly suited to wastewater treatment in industrial processes, for example, separating oil and water, or removing precipitated metals from any precipitation process, or treating wasted tailing ponds in mining processes. Their versatility, durability, and high performance make them particularly cost-effective as Ceramic Membranes are able to replace a series of conventional treatment stages such as separators and filters.
Ceramic Membranes can also be used as a pre-treatment for Reverse Osmosis (RO). RO is a well-established method for removing dissolved solids from wastewater, but until recently, limitations including cost and the ability to handle concentrations of contaminants such as grease and silica have proved a barrier to implementation. Emerging methods of High-Efficiency Reverse Osmosis give substantially higher recoveries with minimal waste discharge at lower energy consumption, which makes wastewater recovery much more efficient than previous conventional RO methods. Thus High-Efficiency Reverse Osmosis can now reduce conventional RO brine volumes by 50%, as well as treat water containing high levels of organics and solvents while maintaining membrane integrity.
Forward Osmosis is another low energy technology for desalinating and cleaning wastewater to obtain clean water suitable for reuse within the industrial process as needed. This is being demonstrated at an oil and gas pilot plant in Canada to minimize the reverse salt flux in a ZLD of oil and gas flow back water and produced water, effectively cutting out the brine scrubbing process.
Once wastewater has been treated for re-use within the industrial process, the final stage of the ZLD process can then be carried out: recovering waste byproducts and removing them from the ZLD system for disposal. This can be done via ultrafiltration and brine concentrators that further concentrate the water stream to reduce waste volume even further. Brine crystallization, a Selective Compound Removal method, is performed in a reactor to produce a dry cake of soluble salt which can then be disposed of easily. Eutectic Freeze Crystallization is a new technique for simultaneous crystallization of ice and salt that is proving to be energetically more efficient than conventional evaporative crystallization.
Decreased capital investment requirements for implementation and increased quality of recovery are critical factors for technology dissemination. Consolidation of ZLD systems into industrial environments could also drive the adaptation of these systems for smaller-scale use, even within the home, thus assisting efforts to decentralize water treatments around the world.
Governments have begun to understand that if unregulated and not pressured, industries will choose to dispose of their wastewater in the cheapest way possible for the least negative impact on their bottom line, and most hardly invest in technologies that increase operational costs, such as the ZLD. Growing industrial regulation and consequent social accountability for their polluting practices, along with advances in technology, are expected to accelerate the widespread implementation of ZLD systems across the globe.