Zero Liquid Discharge (ZLD) Overview
The zero liquid discharge technology market is on a significant upward trajectory, expected to reach a valuation of USD 9.9 billion by 2027, growing at a CAGR of 8.2%. This surge is fueled mainly by increasing adoption in emerging regions like Asia Pacific, the Middle East, and Africa, driven by escalating demands across crucial sectors such as energy & power, food & beverages, pharmaceuticals, and chemicals & petrochemicals.
Driver: Superior Efficiency of ZLD Technology
Zero liquid discharge technology completely eliminates industrial wastewater treatment into the environment. Utilizing advanced treatment methods, ZLD systems efficiently process industrial brine wastewater to dry solids, recovering approximately 95% of liquid waste for reuse. This reduces operational costs and allows for the recovery of valuable byproducts like salts and brines. In contrast to traditional wastewater treatment methods, which often result in brine waste and material loss, ZLD systems offer a more effective and economically viable solution for wastewater management.
Restraint: High Costs Associated with ZLD Systems
Adopting zero liquid discharge systems is a significant investment, encompassing engineering, procurement, and construction (EPC) services for customization to specific industrial needs. Integrating various membranes and components from different manufacturers involves complex processes, including pretreatment, filtration, evaporation, and crystallization. The capital-intensive nature of sourcing materials, system design, and installation, along with considerations for automation level, shipping, regulatory fees, and taxes, contribute to the high initial and operational costs of ZLD systems.
Opportunity: Addressing Environmental Concerns Over Brine Disposal
The environmental impact of discharging brine concentrates into oceans, particularly from desalination processes, has sparked considerable concern. Desalination plants, which often release dense brine into the marine environment, pose potential risks to marine ecosystems. Brine concentrates from the reverse osmosis (RO) process, mainly when used for wastewater and groundwater treatment, contain chemical additives that increase toxicity. This threatens marine life and has broader environmental implications, highlighting the critical role of zero-liquid discharge (ZLD) technology in mitigating such impacts and promoting sustainable water management practices.
Challenge: Overcoming the Awareness Gap in Wastewater Management
The concept of zero-liquid discharge (ZLD) plants is still emerging in various regions, including the Middle East & Africa, Asia Pacific, and South America. Traditional methods like pond evaporation, solar drying, and thermal evaporation dominate, overshadowing the advanced ZLD approach. A significant barrier to ZLD adoption is the need for widespread knowledge about environmental regulations concerning wastewater management. For example, in oil-rich Nigeria, there’s a noticeable absence of regulations for flue-gas desulfurization (FGD) and cooling tower blowdowns. However, as industrial growth accelerates in these areas, awareness and adoption of ZLD plants are expected to rise.
Market Dynamics: Conventional ZLD Plants Lead the Way
In 2021, conventional ZLD plants accounted for the largest market share. These standard systems, preferred for their specific design involving stages like pretreatment, filtration, evaporation, and crystallization, cater to small and medium-sized plants without bespoke requirements.
Evaporation & Crystallization: The Forefront of ZLD Processes
Evaporator and crystallizer processes are pivotal in zero-liquid discharge plants, serving multiple industries, including power, chemicals, textiles, pharmaceuticals, and electronics. Their critical role in ensuring minimal liquid discharge propels the popularity of this process in the ZLD market.
Energy & Power: The Largest Consumer of ZLD Plants
The energy and power sector dominates the ZLD market, driven by the soaring demand for ZLD solutions in applications requiring large installations with high flow rates. The thermal power industry, especially coal-based energy production, significantly contributes to this demand. Initiatives like China’s mandate for ZLD plant installations in the energy sector highlight the critical need for ZLD solutions in managing brine concentrates, especially in non-coastal coal plant locations.
Asia Pacific: Leading Growth in the ZLD Market
As of 2021, the Asia Pacific region held the most outstanding market share and is anticipated to increase at the CAGR throughout the forecast period.This growth is attributed to the increasing demand for ZLD systems because of stringent regulations regarding wastewater treatment, rapid industrialization, and the growing population. In addition, the rising awareness about water treatment for mining and industrial wastewater reuse is expected to increase the demand for ZLD systems in the region.
Recent Developments: Innovations and Partnerships
In September 2022, Veolia Water Technologies partnered with Orange Business Services, enhancing its data collection infrastructure and digital solutions platform, Hubgrade, to optimize water treatment plants remotely. June 2021 saw Aquatech International’s joint venture with Upwell Water, launching AQUIOS, a platform offering comprehensive water treatment services with no upfront cost, merging Aquatech’s technology and operations.
What Is Zero Liquid Discharge (ZLD)?
Zero Water Discharge (ZWD) refers to a treatment method where the facility eliminates the release of liquid by-products into surface waters, thereby removing the environmental pollution associated with the treatment process. In addition to this benefit, a ZLD water treatment system maximizes the efficiency of wastewater treatment, promotes recycling and reuse, and contributes to water conservation by reducing freshwater use.
Why Is Zero Liquid Discharge Important?
In an era where freshwater scarcity escalates, industrial activities pose a dual threat to its preservation unless subjected to zero liquid discharge wastewater treatment. Numerous industries consume vast amounts of water, diminishing its availability for both ecological and alternative industrial uses, or they pollute and discharge water, by no means adversely affecting local ecosystems.
The inception of stringent wastewater regulations can be traced to the US Clean Water Act of 1972. However, history shows that over the past decade, India and China have spearheaded the enforcement of zero-liquid discharge (ZLD) regulations. Faced with the severe pollution of many critical rivers due to industrial effluents, these nations have mandated ZLD to safeguard future water supplies by preventing river and lake contamination.
In regions like Europe and North America, the momentum towards ZLD is propelled by the prohibitive costs associated with wastewater disposal at inland sites. These expenses stem from regulatory constraints on disposal methods and the economic factors affecting the price of disposal technologies. Tong and Elimelech have posited that the growing awareness and public concern over water pollution’s dire impacts will likely result in stricter environmental regulations on wastewater discharge, compelling highly pollutive industries to adopt ZLD.
A compelling argument for ZLD adoption is the opportunity to recover valuable resources from wastewater. Some entities pursue ZLD for environmental reasons and the economic benefit of selling or reusing the byproducts extracted from their wastewater. For instance, lithium levels comparable to those in South American salars have been detected in oil field brines in the USA. Similarly, gypsum, recoverable from mine water and flue gas desulfurization (FGD) wastewater, finds use in drywall production.
Opting for zero liquid discharge wastewater treatment reflects sound economic sense, corporate responsibility, and environmental stewardship. By managing a ZLD plant in-house, companies can slash disposal expenses, enhance water reuse, and reduce greenhouse gas emissions from external transportation, thereby mitigating their ecological footprint and protecting local habitats and the global climate.
How Does a ZLD Treatment System Work?
Pretreatment and Conditioning
This stage aims to minimize Total Suspended Solids (TSS) levels, Chemical Oxygen Demand (COD), and turbidity in the wastewater. Following the significant reduction or removal of TSS, COD, and turbidity, other wastewater treatment processes can be applied to treat the wastewater further.
Phase-One Concentration
In a ZLD system, removing dissolved solids typically employs membrane technologies like Reverse Osmosis (RO), Electrodialysis (ED), or a combination of RO and ED. Utilizing ED or the RO + ED combination can reuse over 98% of the treated water.
Evaporation/Crystallization
The final phase in the ZLD process often involves evaporation and crystallization equipment. This phase produces crystalline salts such as NaCl, Na2SO4, etc., which can be marketed for additional processing. Any solid waste that cannot be used is disposed of in regulated landfills. The evaporated water is collected and can be reused as condensate.
Recycled Water Distribution / Solid Waste Treatment
Treated water after crystallization can be used repeatedly in the plant’s main production, as dissolving, cooling, or as makeup water of open cooling systems.
Produced salt can be utilized as a subproduct on the market depending on the type of solid waste or placed in a landfill.
The Cost of Crystallizing
The implementation cost varies based on customer requirements, treatment level, heat availability, and system volume.
Pros and Cons of Zero Liquid Discharge (ZLD)?
Pros:
Adopting zero water discharge (ZLD) technology is growing globally as an essential strategy for wastewater management, aiming to reduce water pollution and conserve water resources. Key benefits of ZLD include:
- Reduced waste volumes lead to lower waste management costs.
- On-site water recycling reduces water procurement costs and associated risks, potentially reducing the need for extensive treatment compared to meeting strict environmental discharge criteria.
- Minimized reliance on off-site wastewater disposal reduces greenhouse gas emissions and the risk of road incidents involving community traffic.
- Enhanced environmental performance and a more favorable regulatory compliance outlook for future permits.
- Potential recovery of valuable byproducts, such as ammonium sulfate for fertilizers or sodium chloride for de-icing.
Cons:
- High initial and ongoing costs: The installation and maintenance of ZLD systems can be costly, representing a significant investment for businesses. Additionally, the energy-intensive nature of specific ZLD processes can lead to higher operational expenses.
- Operational complexity: Managing ZLD systems requires specialized knowledge, making it challenging to find and keep skilled staff, which could lead to more operational disruptions and downtime.
- Not universally applicable: ZLD may only be suitable for some types of industrial waste, particularly those with high concentrations or toxicity, and might necessitate extra pre-treatment.
- Waste generation: ZLD processes typically produce solid waste or “sludge” that requires proper disposal, incurring further costs.
- Space requirements: Implementing a ZLD system demands considerable space, which may not be feasible for facilities with limited area.
- Significant energy use: The high energy demand of ZLD systems can inflate operational costs, with the evaporation and condensation of water being particularly energy-intensive.
In summary, while ZLD technology offers a viable solution for industries to lessen their environmental footprint and conserve water, it also presents considerable financial and operational challenges. Businesses should thoroughly evaluate the advantages and drawbacks of ZLD before proceeding with its implementation.
Conventional ZLD Systems
Thermal ZLD Systems
Early ZLD systems were typically based on thermal processes. In such systems, the feed wastewater undergoes a pretreatment step that reduces scaling potential and is then concentrated sequentially by two core elements — a brine concentrator and a brine crystallizer (or an evaporation pond). Reusable, clean product water is made from the distillates produced by the crystallizer and brine concentrator units. In contrast, the solids produced are either stored (in evaporation ponds), further processed for landfill disposal, or reused as valuable byproducts.
Brine concentrators, like Mechanical Vapor Compression (MVC), are widely used for water evaporation. Unlike other desalination methods, such as Multieffect Distillation (MED) and Multistage Flash (MSF), MVC’s applications in Zero Liquid Discharge (ZLD) systems are not reported. In MVC, feedwater is preheated by heat exchangers using sensible heat from distillate product water, mixed with recirculating brine slurry at the concentrator’s sump, forming a thin film on heat transfer tubes for water evaporation. Calcium sulfate seeds in the brine prevent scale formation. MVC concentrators, though energy-intensive, achieve high water recovery (90–98%).
However, high capital costs are incurred due to materials like titanium and stainless steel. The concentrated brines are sent to crystallizers with vapor compressors, but small systems may opt for economically favorable steam-driven crystallizers. Crystallizers consume more energy (52–66 kWhe/m3) than MVC concentrators. Evaporation ponds are cost-effective but suitable only for small volumes in areas with high evaporation rates and inexpensive land. A hypothetical ZLD scenario in Las Vegas showed that evaporation pond land acquisition costs are three times higher than brine concentrators and crystallizers combined, with no improvement in water reuse efficiency.
Thermal ZLD with RO Preconcentration
Brine concentrators, like Mechanical Vapor Compression (MVC), are widely used for water evaporation in ZLD processes. Unlike other desalination methods, such as Multieffect Distillation (MED) and Multistage Flash (MSF), MVC’s applications in Zero Liquid Discharge (ZLD) systems are not reported. In MVC, feedwater is preheated by heat exchangers using sensible heat from distillate product water, mixed with recirculating brine slurry at the concentrator’s sump, forming a thin film on heat transfer tubes for water evaporation.
Calcium sulfate seeds in the brine prevent scale formation. MVC concentrators, though energy-intensive, achieve high water recovery (90–98%). However, high capital costs are incurred due to materials like titanium and stainless steel. The concentrated brines are sent to crystallizers with vapor compressors, but small systems may opt for economically favorable steam-driven crystallizers. Crystallizers consume more energy (52–66 kWhe/m3) than MVC concentrators. Evaporation ponds are cost-effective but suitable only for small volumes in areas with high evaporation rates and inexpensive land. A hypothetical ZLD scenario in Las Vegas showed that evaporation pond land acquisition costs are three times higher than brine concentrators and crystallizers combined, with no improvement in water reuse efficiency.
Despite limitations, R.O. technology, a pressure-driven desalination method with superior energy efficiency, has been incorporated into ZLD processes to reduce concentrated brine volume. R.O. doesn’t require phase transition for separation, eliminating irreversible losses associated with thermal processes. Energy consumption by R.O. at 50% recovery is as low as ∼2 kWhe/m3, significantly less than brine concentrators and crystallizers. R.O.’s modular nature allows versatile adaptation into wastewater treatment facilities, enhancing ZLD system efficiencies. A secondary R.O. stage can preconcentrate feedwater, saving 58–75% of energy and 48–67% of treatment costs compared to using only a brine concentrator followed by an evaporation pond.
However, R.O. has inherent membrane fouling/scaling limitations and a salinity range constraint (∼70,000 mg/L). Extensive pretreatment involving chemical use and solid waste generation is required in RO-incorporated ZLD systems. Technologies like ultrafiltration (U.F.) and high-efficiency RO (HERO) with altered operating conditions address some fouling issues. R.O’s salinity limit necessitates its combination with brine concentrators in ZLD systems. Advancing ZLD technology requires developing technologies tolerating higher salinities than R.O. and consuming less energy than brine concentrators.
Electrodialysis
Electrodialysis (E.D.) applies an electric potential to remove ions through selective ion exchange membranes. Unlike R.O. membranes, E.D. membranes permit the transport of counterions, creating salt-depleted dilute and concentrated brine. Electrodialysis reversal (EDR) minimizes fouling by reversing electrode polarity, requiring less pretreatment than R.O. E.D., and EDR can focus feedwaters to higher salinity (>100,000 mg/L), consuming 7–15 kWhe/m3, less than MVC brine concentrators. Despite lower overall costs, ED/EDR effluent has higher salinity (e.g., TDS > 10,000 mg/L), posing a trade-off between product water quality and energy consumption/capital cost.
In ZLD systems, low-salinity product water in ED/EDR increases energy consumption and capital cost. A standalone ED/EDR system is not ideal for ZLD; a multistage configuration is feasible but increases capital cost. In combination with R.O., ED/EDR extends R.O.’s salinity limit and reduces energy consumption, achieving ZLD in several pilot studies by concentrating R.O. brine and effectively removing hardness. The ED/EDR effluent is further desalinated by R.O. or blended with R.O. permeate for desired product water quality.
Unlike hydraulic pressure-driven R.O., <H3> Forward Osmosis F.O. (Forward Osmosis) utilizes osmotic pressure to drive water permeation across a semipermeable membrane. In F.O., water flows from feedwater to a concentrated draw solution, producing brine sent to a crystallizer or evaporation pond. F.O.’s osmotic pressure allows it to treat waters with higher salinity than R.O.
Thermolytic draw solutes, like ammonia–carbon dioxide (NH3/CO2), enable FO-incorporated ZLD systems. This draw solution generates high osmotic pressure and can be regenerated by low-temperature distillation, utilizing low-grade thermal energy. F.O. operates at low pressure, reducing fouling propensity compared to R.O., making it suitable for high-fouling potential wastewater in ZLD. After the R.O. stage, the thermolytic F.O. process can act as a competitive brine concentrator, utilizing volatile draw solutes. F.O.’s modularity results in a smaller footprint and adaptability to feedwater fluctuations.
A recent FO-based ZLD system at the Changxing power plant in Zhejiang Province, China, treats FGD wastewater and cooling tower blowdown. After R.O. concentration, NH3/CO2 FO further concentrates the brine above 220,000 mg/L TDS. The F.O. brine undergoes crystallization for further concentration, producing high-quality product water (TDS < 100 mg/L after secondary R.O.) for reuse as boiler makeup water.
Membrane Distillation
M.D. (Membrane Distillation) is a thermal, membrane-based desalination process using a vapor pressure difference across a hydrophobic membrane. Feedwater heating creates a temperature difference, driving water vapor flux. M.D. variants include DCMD, AGMD, VMD, and SGMD. Due to the liquid-vapor phase transition requirement, M.D. is more energy-intensive than R.O. and ED/EDR. The minimum energy for seawater desalination by DCMD is higher than R.O. Practical MD energy consumption is estimated at 143–162 MJ/m3 for seawater desalination.
Efficient heat recovery is crucial for M.D.’s energy competitiveness compared to electricity-driven technologies. M.D. can treat high-salinity feedwaters unsuitable for R.O., leveraging low-grade thermal energy. It is modular, operates at low pressure, and has a low fouling propensity. However, MD faces challenges like membrane wetting in the presence of volatile pollutants or surfactants, affecting product water quality. M.D.’s potential in ZLD inland desalination is demonstrated at the bench scale. In a near-ZLD system combining M.D. with reverse electrodialysis (RED), M.D. reduced simulated SWRO brine volume by over 80%, and the M.D. brine mixed with seawater generated electrochemical energy in a RED stack.
Large-scale MD applications are hindered by technical immaturity and low water recovery, with no reported pilot-scale ZLD applications.
Environmental Impacts
Despite the main goal of ZLD to reduce water pollution and improve water sustainability, the application of ZLD also results in unintended environmental impacts. One risk stems from the produced solid wastes. For example, solid wastes stored in evaporation ponds have raised concerns about their odors, potentially harmful effects on wildlife, and risk of leakage. Similarly, solid wastes disposed in landfills may lead to chemicals leaching into groundwater. Accordingly, waterproof liners and reliable monitoring systems are typically required to prevent potential contamination from solid wastes.
As discussed earlier, ZLD consumes large amounts of energy, leading to significant emissions of greenhouse gases (GHGs). Some pretreatment methods, such as acidification followed by degasification, release CO2 from the feedwater into the atmosphere. For example, using E.D. in concentrating R.O. brine increases CO2 emission via energy consumption and decarbonization for scaling control. A life-cycle study showed that GHG emissions would increase by 50% if California’s water supply were switched from imported water to BWRO inland desalination. The type of fuel used to generate power impacts the amount of CO2 released into the atmosphere, as the U.S. Energy Information Administration mentioned.
Assuming 939 g of CO2 per kWhe generated by bituminous coal, MVC brine concentrators typically produce 19–23 kg of CO2 per m3 of treated feedwater solely from electricity usage (corresponding to 20–25 kWhe/m3. Incorporating technologies with higher energy efficiency, such as R.O., will significantly reduce the GHG emission. In addition, emerging ZLD technologies that can utilize low-grade or renewable energy (e.g., waste heat, solar power, geothermal energy) enable further reduction of the GHG footprint of ZLD systems.
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Benefits for ZLD Customers
1. Reducing the Cost of Production
Adopting a zero-liquid discharge process can significantly lower production costs. This is achieved through reduced waste management expenses due to minimized waste volumes and decreased water procurement costs thanks to on-site water recycling. Additionally, ZLD can transform waste into valuable products or energy, enhancing the economic efficiency of projects. Ultimately, ZLD aligns profitability with sustainability, making businesses more resilient to external pressures.
2. Water Conservation
Implementing zero-liquid discharge (ZLD) systems is a boon for water conservation, especially in regions where water is scarce or expensive. Recycling and reusing wastewater, ZLD minimizes environmental impacts and reduces dependence on external water supplies. It’s estimated that ZLD can cut water usage by up to 98%, meaning only 2% of water is wasted in industrial processes. Various industries, including power, mining, and food and beverage, have shown that ZLD can effectively recycle and treat wastewater for reuse, promoting sustainable water use and environmental conservation.
3. Reduced Disposal Costs
ZLD systems offer the significant advantage of lowering disposal costs by eliminating the need to discharge wastewater externally. This can lead to substantial savings, particularly in regions with high disposal fees. By recycling water internally, ZLD systems can slash disposal expenses by up to 90%, according to Zero Liquid Discharge Alliance findings.
4. Environmental Sustainability
ZLD systems are implemented to improve environmental sustainability by reducing the adverse environmental effects of industrial processes. Recycling and reusing water within ZLD systems reduce pollution levels, protecting ecosystems and human health. ZLD prevents the release of pollutants into natural water bodies, contributing to cleaner water and air. Moreover, ZLD conserves land and improves energy efficiency in industrial operations. Examples include recycling cooling tower blowdowns in the power sector, wastewater recycling in mining, and wastewater reuse in food processing, showcasing ZLD’s role in preserving freshwater resources and improving water quality.
5. Decrease the Volume of Waste
Zero liquid discharge (ZLD) systems significantly reduce waste volume across various industries. For example, in the power industry, ZLD can repurpose cooling tower blowdown water containing high salts for boiler feedwater or make water for cooling towers. In the mining sector, ZLD facilitates the treatment and reuse of wastewater, cutting down on disposal needs and reducing pollutant levels. In the food and beverage industry, ZLD enables the recycling of wastewater rich in organic content for cleaning and sanitation, thereby reducing the generation of sewage and pollutants.
6. Recycling of treated water
Adopting zero-liquid discharge (ZLD) technology for industrial wastewater treatment plants brings substantial benefits, including significantly reducing water consumption by reutilizing treated water for internal processes. This approach dramatically reduces the need for fresh water in industrial activities, lowering water procurement costs and mitigating external risks. Additionally, recycling water on-site often requires fewer treatments compared to achieving strict environmental discharge criteria. By integrating ZLD, industries can better manage water resources, decrease operational costs, and minimize their ecological footprint through efficient internal water recycling.
7. Improved Public Image
Using ZLD systems can enhance an industry’s reputation by showcasing a dedication to environmental sustainability. This is critical for attracting and retaining customers and ensuring compliance with environmental regulations. By recycling and reusing water, ZLD reduces pollution, positively influencing public perception of the business. Research indicates that ZLD significantly alleviates pressure on limited water resources and reduces the environmental footprint, portraying the company as environmentally responsible. ZLD’s role in promoting energy efficiency and resource conservation further bolsters a business’s eco-friendly reputation. Consequently, ZLD benefits the environment and enhances a company’s appeal to consumers, investors, and regulatory agencies.
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Author of the article: Mykhailo Blazhko
