Zero Liquid Discharge ( ZLD)
Zero-liquid discharge (ZLD) is a water treatment process in which all wastewater is purified and recycled; therefore, leaving zero discharge at the end of the treatment cycle. Zero liquid discharge is an advanced wastewater treatment method that includes ultrafiltration, reverse osmosis, evaporation/crystallization, and fractional electrodeionization.
Zero Liquid Discharge (ZLD) is a treatment process that its goal is to remove all the liquid waste from a system. The focus of ZLD is to reduce economically wastewater and produce clean water that is suitable for reuse.
ZLD technologies consist traditionally from brine concentrators and crystallizers that use thermal evaporation to turn the brine into highly purified water and solid dry product ready for landfill disposal or for salt recovery. While evaporator/crystallizer systems are the most commonly used in ZLD processes, other promising technologies (ED/EDR, FO and MD which will be explained later) with high recoveries have taken foothold and are used in different combinations in order to lower the cost and raise the efficiency of the systems.
The increasingly tighter government regulations on the discharge of brine due to the environmental effect make ZLD necessary when water is scarce or the local water bodies are protected by law. Thus many industrial facilities and brine effluent contributors that up to now where either discharging brine to nearby available surface water or the sea and to wastewater treatment plants, are trying to find new ways to tackle this issue.
Fresh water scarcity and concerns for environmental impact resulting from industrial wastewater discharges place a high degree of importance on recycling and reuse of water, an increasingly valuable resource.
As regulatory issues, environmental sensitivity, and long term water supply concerns increase, many industrial companies are considering ways to reduce their water discharges with Zero Liquid Discharge (ZLD) processes.
Veolia has decades of innovation using HPD® Evaporation and Crystallization technology in many wastewater treatment applications including inudstry experience in volume reduction and zero liquid discharge:
• Power Generation (fossil and nuclear fuel)
• Oil & Gas field produced water (conventional and unconventional)
• Oil & Gas Refining
• Chemical Processing and Manufacture
• SAGD (Steam Assisted Gravity Drainage, EOR)
• Mining and Ore Processing
• Industrial and Municipal Landfill
• Falling Film Brine Concentrators
• Forced Circulation Crystallizer
• Horizontal Spray Film Evaporator
• Hybrid Systems with Membrane Pre-Concentrators
• Biological Treatment
• Solids Waste Handling
The power industry as well as oil & gas, chemical, petrochemicals, mining and other industries generate large volumes of waste water that must be managed. Commonly these wastewaters are discharged via a plant outfall to a surface water body, an evaporation pond, or in some cases deep well injected. However, there are growing environmental concerns regarding such discharge practices, which has resulted in the development of Zero Liquid Discharge (ZLD) processes.
Zero liquid discharge can be defined broadly as a process for maximum recovery of water from a waste water source that would otherwise be discharged. This water is beneficially reused and the salts, and other solids contained in the waste water are produced and generally disposed in a landfill.
he drivers for zero liquid discharge include a growing concern by the public about the impact of such discharges on the environment, and in many areas of the world, water is a scarce resource. Such concern is resulting in increased regulation and limitation of waste water discharges. Even without regulatory push, many companies in various industries are mandating initiatives for reducing water discharge by recycle reuse, as well as ZLD, to reduce their environmental footprint and improve sustainability.
Zero liquid discharge can be achieved in various ways. There is no “one size fits all” solution, as the optimal system design is site specific. The waste water composition, various streams to be treated, site specific operating costs, foot print availability and other factors are determining factors for an optimal design.
The system objectives for a zero liquid discharge system are to eliminate a liquid waste water discharge, generate solids for landfill disposal or reuse, and to recycle a high-quality water that can be beneficially reused. The design objectives are to minimize the capital investment and system operating cost, while not significantly impacting the manpower required for operation. Further, the system must be designed with operational flexibility to meet the facility needs and be safe and reliable.
Careful consideration of waste water chemistry is needed for the successful design and operation of a zero liquid discharge system. A sound water chemistry design basis is key to successful zero liquid discharge design.
The chemical constituents of concern for a zero liquid discharge system typically are as follows:
Table 1 Typical Chemical Constituents of Concern
In a zero liquid discharge system, the waste water being processed is concentrated to solubility limits of the dissolved salts. When the solubility limits are exceeded, salts crystallize and can then be harvested using an appropriate means.
Evaporation systems generally are more capital and operating cost intensive than membrane systems, with crystallizers the most costly. For that reason, and when possible, membrane systems can be utilized to reduce the capital and operating cost of the evaporation system.
Depending on the waste water composition, preconcentrating using a membrane system can dramatically reduce the sizing requirement of the backend evaporations system and thus the system capital and operating cost. Note that to reach high recoveries in a waste water membrane system, appropriate pretreatment such as softening and pH adjustment is often required.
Vertical tube falling film brine concentrators are generally used to concentrate lower total dissolved solids (TDS) brine solutions up to 12% to as high as 25% total solids and are used to minimize the design capacity of a downstream forced circulation crystallizer. Brine concentrators are specifically designed to manage the scaling of sparingly soluble divalent salts such as calcium sulfate and calcium carbonate, as well as silica that is also commonly present. Forced circulation crystallizers are generally used to concentrate brine blowdown from upstream concentration equipment, although small waste water flows are sometimes treated directly with a forced circulation crystallizer. Crystallizers are designed to manage crystallization of all salts, sparingly soluble as well as highly soluble sodium salts such as sodium chloride and sodium sulfate, without excessive scaling and cleaning frequencies. This robustness comes at the expense of higher specific energy consumption and higher specific capital cost.
The solids generated by a forced circulation crystallizer are generally harvested and dewatered by either an indexing belt filter or by centrifuge. In such case the solids are collected and typically landfilled in a conventional landfill as long as the waste passes Toxicity Characteristic Leaching Procedure (TCLP) testing. However, in some applications involving, zero liquid discharge equipment the highly concentrated brine is discharged to an evaporation pond. Such a configuration reduces the footprint of the evaporation pond, and the labor and expense of operating the dewatering equipment.
PROCESS EXPERIENCE FOR A VARIETY OF EFFLUENT STREAMS
Each effluent stream presents its own unique challenge when designing an entire ZLD water treatment system to efficiently and effectively minimize waste or eliminate the discharge of wastewater.
Veolia’s experience in treating and managing these streams include specification and complete system design utilizing complementary water treatment expertise from Veolia Water Technologies such as deoiling, softening, clarification, and filtration for the following applications:
• Membrane System Reject (NF, MF, UF, RO)
• Cooling Tower Blowdown
• Flue Gas Desulfurization (FGD) Blowdown / Purge
• Produced Water (Conventional, Fracking, SAGD)
• Integrated Gasification Combined Cycle (IGCC) Gray Water
• Mine Drainage
• Refinery, Gas to Liquid (GTL), and Coal to Chemical (CTX) Wastewaters
• Scrubber Blowdown
• Demineralization Waste
• Landfill Leachate
ZLD system with wastewater evaporator and wastewater crystallizer
Veolia offers several options in designing the optimal system to achieve each unique customer requirement for achieving zero discharge or waste reduction objectives, including recovered water quality, energy consumption, minimization of capital and operating costs, and ease of operation. HPD® Evaporation and Crystallization system capacities range from around 10 gpm to greater than 1500 gpm per unit.
Falling film evaporators, sometimes called brine concentrators, are an excellent technology for efficient heat transfer, producing high-purity distillate, and achieving water recovery greater than 90%. Recovered water in a brine concentrator is suitable as cooling tower or scrubber makeup, and can be recycled to other plant processes, including demineralizer feedwater. These wastewater evaporators can be driven by mechanical vapor recompression (MVR) or with live steam depending on the relative costs of electric power and steam.
Crystallizers are designed to handle continuous crystallization of the various dissolved salts, which sometimes can be recovered as valuable by-products. Wastewater crystallizers are used to concentrate the effluent from brine concentrators, and when equipped with solids dewatering, comprise a true zero liquid discharge system. These brine crystallizers typically are driven by live steam but in some cases can use MVR technology to recycle the vapor to reduce energy usage and operating costs.
The industrial involvement with brine is twofold. Many industrial processes require water which they contaminate and releasing it may cause irreversible damages to the local environment.
In India and during the last decade due to heavy contamination of local waters by industrial wastewater was followed by strict regulations that make ZLD necessary in order to ensure the future of their rivers and lakes. In Europe and North America, the drive towards zero ZLD has been applied due to the high costs of wastewater disposal at inland facilities. These costs increase exponentially by government fines and the costs of disposal technologies.
ZLD can also be used to recover valuable resources from the wastewater which can be sold or reused in the industrial process. Some examples are as follows,
• Generation of valuable potassium sulfate (K2SO4) fertilizer from a salt mine
• Concentration of caustic soda (NaOH) to 50 and 99% purity
• Recovery of pure, saleable sodium sulfate (NaSO4) from a battery manufacturing facility
• Reduction of coal mine wastewater treatment costs by recovering pure sodium chloride (NaCl) which can be sold as road salt
• Lithium (Li) has been found in USA oil field brines at almost the same level as South American salars
• Gypsum (CaSO4.2H2O) can be recovered from mine water and flue gas desalinization (FGD) wastewater, which can then be sold to use in drywall manufacturing
Other advantages to the application of ZLD are:
• Decreased volume of wastewater lowers the costs of waste management.
• Recycling water on site thus decreasing the need for water intake and meeting with treatment needs.
• Reduce the truck transportation costs for off-site disposal and the related environmental risks.
Table 1, ZLD Drivers
There is a wide diversity of sources for discharge flow streams that include:
• Cooling tower blowdown in heavy industry and power plants
• Ion exchange regenerative streams particularly in food and beverage processing
• Flue gas desulfurization, wet wastewater stream
• Municipal potable water systems, wastewater streams
• Process water reuse from agricultural, industrial and municipal streams
• Various industrial wastewater streams from the textile, coal-to-chemical, food and dairy or battery industries
More in particular, we can refer to the following applications (Table 2),
Table 2, ZLD Wastewater Stream Applications
Membrane System Reject (NF, MF, UF, RO) Mine Drainage
Flue Gas Desulfurization (FGD) Blowdown / Purge Refinery, Gas to Liquid (GTL), and Coal to Chemical (CTX) Wastewaters
Produced Water (Conventional, Fracking, SAGD) Scrubber Blowdown
NOx Injection Water Demineralization Waste
Integrated Gasification Combined Cycle (IGCC) Gray Water Landfill Leachate
The discharge sources can be further categorized according to volume and complexity. A ZLD solution must take the latter into consideration along with the location of the waste stream.
The most important factors that determine the ZLD design depend on,
1. The specific contaminants in the discharge stream
2. The volume of the dissolved material
3. The required design flow rate
The contaminants of concern are presented in Table 3,
Table 3, Typical Chemical Constituents of Concern
These parameters need to be accurately measured before requesting a quote in order so as to get an accurate estimation of the system’s cost. If the feed is prone to changes in flow and the concentration of the contaminants, inlet buffering tanks regulate the peaks.
Each technology that makes up the ZLD chain has a certain purchasing cost, but an important parameter for calculating the costs and eventually the payback period are the operating costs. The OPEX can change drastically based on what process is selected especially for electrical power and steam-generating facilities. For a long term investment the benefits and drawbacks of each choice have to be weighed as well as what works better for each company and their working staff. This will help to get an initial versus a long-term cost investment.
Table 4, Specific Energy Consumptions (SECs) of Brine Treatment Technologies, Multistage Flash (MSF), Multi-Effect Distillation (MED), Mechanical Vapor Compression (MVC), Electrodialysis (ED/EDR), Forward Osmosis (FO), Membrane Distillation. The energy consumption values are the average of 13 comparative studies on ZLD technologies ranging from 2002 -2017. Clarifications are needed for ED/EDR, FO and MD. 1) ED/EDR SEC depends on the salinity of the feed as higher salinities require higher SECs, 2) FO SEC depends on the Draw Solution and the Regeneration Method. Most papers assume the use of thermolytic salts and their regeneration at a 60oC temperature. 90% of the thermal energy needed can be acquired by waste heat if it’s available, 3) MD SEC depends on the configuration. Most common MD configuration in the studies is Direct Contact MD (DCMD) due to its simplicity. 90% of the thermal energy needed can be acquired by waste heat if it’s available and finally 4) the total electrical equivalent was taken using the following, Total El. Equivalent = El. Energy + 0.45 x Thermal Energy due to modern power plant efficiency (according to relevant paper).
Fig.1 Brine Treatment Technologies SECs graph comparison (see clarifications in the description of table 4)
On a last note for a cost benefit analysis you must always take into consideration factors like,
• Taxes or additional purchasing fees
• Possible utility costs in the installation area
• Environmental regulatory fees or permits
• Regular compliance testing
Basic ZLD Design – ZLD Blocks
Despite the variable sources of a wastewater stream, a ZLD system is generally comprised by two steps which are represented in Figure 1.
Fig.2, ZLD Basic Blocks
1. Pre-Concentration; Pre-concentrating the brine is usually achieved with membrane brine concentrators or electrodialysis (ED). These technologies concentrate the stream to a high salinity and are able to recover up to 60–80% of the water.
2. Evaporation/Crystallization; The next step with thermal processes or evaporation, evaporates all the leftover water, collect it, and drives it for reuse. The waste that is left behind then goes to a crystallizer which boils all the water until all the impurities crystallize and are filtered out as a solid.
The pre-concentration of the liquid waste stream is a very important step due to the fact that it reduces the volume of the waste and downsizes significantly the very costly evaporation/crystallization step. Usually it is achieved with electrodialysis (ED) or membrane processes which consist of Forward Osmosis (FO) and Membrane Distillation (MD) (Figure 3).
Fig.3, Brine treatment technologies, (a) Electrodialysis, (b) Forward Osmosis, (c) Membrane Distillation
ED, FO and MD can function efficiently with a much higher salinity content than RO (150,000 ppm, 200,000 ppm, 250,000 ppm and 70,000 ppm respectively).
Electrodialysis/ Electrodialysis Reversal
Electrodialysis is a membrane process that uses electrodes to create an electric field which pushes negative and positive ions through semipermeable membranes with attached positively or negatively charged species respectively. ED is used in multiple stages to concentrate the brine to saturation levels. It is often used together with RO for very high water recovery. ED differs from RO because it removes the ions and not the water and vice versa for RO. Due to this fact silica and dissolved organics are not removed with ED which is important if the clean stream is to be reused. ED requires solids, as does RO, solids and organics removal from the feed.
Electrodialysis reversal (EDR)
In EDR the polarity of the electrodes is reversed several times an hour and the fresh water and the concentrated wastewater are exchanged within the membrane stack to remove fouling and scaling.
FO is an osmotic membrane process with a semipermeable membrane that unlike RO doesn’t use applied pressure in order to achieve separation of water from dissolved solutes like ions, molecules and larger particles. That means a lot less of energy for the process in comparison to RO. In general FO uses thermal and electrical energy. Thermal energy can be substituted with low grade waste heat which can be found everywhere in most industrial or nearby areas.
MD is a thermally driven transport process that uses hydrophobic membranes. The driving force in the method is the vapor pressure difference between the two sides of the membrane pores, allowing for mass and heat transfer of the volatile solution components (e.g. water). The simplicity of MD along with the fact that it can use waste heat and/or alternative energy sources, such as solar and geothermal energy, enables MD to be combined with other processes in integrated systems, making it a promising separation technique.