Zero Liquid Discharge: Advanced Solutions for Industrial Wastewater

Zero liquid discharge (ZLD) desalination eliminates wastewater discharge by extracting valuable salts and minerals from brine streams. Unlike conventional systems producing high-salinity wastewater, ZLD converts waste into valuable resources. KISR is developing ZLD technology for Kuwait, which will generate economic returns through domestic reuse or export of extracted minerals. The institute has also developed a proprietary device for extracting magnesium from wastewater, including desalination brine and industrial wastewater. This technology combines thermal and chemical processes to selectively extract rare elements with high global demand. The system can be adapted for extracting other minerals like calcium, demonstrating flexibility in resource recovery applications.

After initial crystallization, part of the mother liquor is purged and processed for additional products. Potassium chloride is recovered through fractional crystallization. Sodium sulfate is recovered by cooling in scraped surface coolers. Bromine is removed through chlorine stripping. From the polyvalent stream, drinking water, calcium sulfate, and magnesium hydroxide are produced, which is then converted to magnesium oxide. Since no additives are used in this process, all recovered materials achieve marketable quality specifications.

A zero liquid discharge (ZLD) crystallizer operates by heating concentrated brine above its boiling point in a shell-and-tube heater, causing the brine to flash and evaporate water as steam, which concentrates the solution and forms crystals; the flashed vapor passes through a mist eliminator and compressor, condenses in the heater shell side for recovery, while crystallized solids are filtered or centrifuged to achieve complete solid-liquid separation with no liquid discharge.

Two approaches address mixed base salt disposal challenges: (1) Selective crystallization using brain chillers to remove sulfates completely, yielding high-purity sodium chloride (98%) and sodium sulfate, both sellable in open markets; (2) Nano-filtration membranes separating divalent sulfate salts from monovalent chloride salts, achieving 97% salt recovery and eliminating waste salt generation. These technologies, developed in collaboration with research institutions, enable zero-waste ZLD implementations where all recovered salts find industrial applications or market outlets.

Achieving comprehensive mineral recovery requires managing antiscalants in evaporation systems—these must be removed before crystallization as they inhibit scaling. Calcium sulfate recovery demands concentrations below saturation threshold. Magnesium recovers as hydroxide, then calcines to oxide. High-purity sodium chloride (99.99%) requires stringent specifications: 25 ppb sulfate, 50 ppm potassium, 25 ppm bromide—necessitating recrystallization. Potassium chloride crystallizes separately at different temperatures. Bromine removes from purge streams. This integrated approach achieves full recovery of all seawater constituents, transforming desalination reject into valuable products.

Zero Liquid Discharge (ZLD) projects recover 97.5%+ of inlet water for reuse, but must be evaluated case-by-case. If operational costs exceed tap water prices, ZLD becomes economically unviable. Water mapping and cost-benefit analysis are essential. Case studies demonstrate high performance: jam production achieved 8,200 mg/L to 200 mg/L COD reduction; chocolate manufacturing reduced 5,000 mg/L to 125 mg/L; cosmetic industry achieved 75% water reuse. Sugar production recovered molasses and brown color, saving 70-80% of sodium chloride costs.

The textile industry has shifted from developed to developing countries (China, India, Bangladesh, Pakistan, Vietnam), causing severe pollution. Major environmental problems include toxic chemicals, high water consumption, high energy consumption, heavy transportation, and excessive packaging. Life Cycle Assessment (LCA) evaluates sustainability by analyzing inputs, outputs, and improvement points. China has 70% of rivers classified as polluted, with textile factories releasing hazardous chemicals. Major brands like Nike, Adidas, H&M source from polluting suppliers. In Tirupur, India, court orders required Zero Liquid Discharge (ZLD) systems, achieving 90%+ water reuse through treated brine technology. The industry is unsustainable due to toxic chemicals, high resource consumption, waste generation, and environmental degradation.

Cold crystallization is an acronym for crystallization of high-solubility salts at low temperature under deep vacuum, enabling treatment of FGD wastewater containing calcium chloride and magnesium chloride that resist conventional crystallization. Using calcium chloride phase diagrams, atmospheric pressure prevents crystallization as boiling and solubility curves don't intersect; however, under deep vacuum (~0.5 psia), the boiling point curve drops sufficiently for intersection at ~60% concentration and 176°F. This eliminates the need for chemical softening and reduces total waste volume. Economic analysis shows similar capital costs but significantly lower operating costs compared to softening plus crystallizer approaches due to elimination of chemical and sludge disposal expenses. Cold crystallizers use steam-driven forced circulation crystallizers with deep vacuum systems. Equipment life expectancy for two-bundle falling film evaporators is approximately 30 years, with titanium tubes forming protective oxide coatings though fluorides can penetrate this protection.

In December 2015, water consumption standards were tightened to 3.5 m³/MW for existing plants and 2.5 m³/MW for new plants. ZLD evolved from MLD to achieve zero discharge. ZLD systems have three components: pre-treatment (geochemical and biological processes), RO (Reverse Osmosis), and evaporator/crystallizer. RO is the more reliable technology for water recovery. Recovery rate depends on osmotic pressure (increases with effluent concentration) and delta pressure. Challenges include coagulants causing scaling, ineffectiveness of anti-scalants, calcium magnesium sulfates, and silica limitations. Phosphate and silica treatment is particularly challenging as no anti-scalant is available for silica.

ZLD implementation creates more problems than it solves due to significant environmental trade-offs. Studies comparing ZLD against meeting standard discharge requirements show: carbon emissions increase by approximately 4 times; hazardous solid waste generation increases by about 2.5 times; and overall operating costs rise by nearly 3 times. ZLD is only justified in areas with no proper discharge infrastructure or extreme water scarcity conditions, such as coastal regions with deep-sea discharge facilities.

Stage 4 represents the core of the reverse osmosis system, where water passes through a semi-permeable membrane capable of filtering particles as small as 0.0001 microns. This membrane effectively removes 99% of undesirable substances from the water. Continuous water circulation prevents any material from depositing on the membrane surface, ensuring efficient operation and gradual evacuation of impurities to the drain.

A eletrodiálise reversa (EDR) é uma tecnologia eletroquímica que remove sais e íons de águas e efluentes através de membranas iônicas e campos elétricos, oferecendo vantagens significativas em comparação com outras tecnologias como osmose reversa e troca iônica, incluindo menor consumo energético (aproximadamente 0,07 kWh por kg de sal removido), maior recuperação de volume (até 95%), menor volume de rejeito (5-40% versus 20-60% na osmose reversa), e capacidade de operar com influentes de maior turbidez e orgânicos. A tecnologia é especialmente eficaz para aplicações de reuso industrial, dessalinização de águas salobras, e tratamento de efluentes com altas concentrações de sais, sendo uma alternativa viável para reduzir custos operacionais e mitigar impactos ambientais no setor de saneamento e indústria.

When selecting MLD technologies, important benefits to consider include: technology maturity and relevant project references (ultra-high pressure RO, CFRO, and OARO are mature technologies with established project references operating at various chemistries and high salinities), more water recovery (reduces source water consumption and increases recovery), environmental sustainability (minimal environmental impact in terms of generating least waste and requiring least energy), optimized life cycle costs (capacity, energy consumption, membrane replacement, and chemical usage heavily impact costs), and compliance with discharge regulations. Limitations include managing the MLD brine stream through appropriate disposal methods (evaporation ponds, injection wells, outfall to local water bodies, small crystallizer if discharge not allowed, or brine sewer line, confirmed with local regulations), fouling and scaling issues requiring adequate pre-treatment, and operational considerations including cleaning frequency, chemical consumption, membrane warranties, and replacement rates. ZLD typically has much higher CAPEX and OPEX than MLD, but may be more cost-efficient when there is excess steam available, when solids can be sold as products, or when zero liquid discharge is mandatory by regulation. A hypothetical case study compared four options for a plant producing 1 million gallons/day of demineralized water: Option A (no recovery) at $88 million; Option B (MLD 95% recovery) at $94 million; Option C (ZLD 100% recovery) at $206 million; Option D (partial MLD 75% recovery) at $68 million (lowest). ZLD costs are extremely high due to nearly double the CAPEX for evaporator and crystallizer plus up to seven times higher OPEX. The 75% MLD option yields the lowest life cycle cost and is preferred.

Forward osmosis with thermolytic draw solutions represents a promising alternative to energy-intensive reverse osmosis desalination, utilizing low-grade heat (waste heat from power plants, geothermal sources, or solar thermal energy) to regenerate the draw solution and recover fresh water, potentially achieving energy consumption as low as 0.5 kWh/m³ compared to 3-4 kWh/m³ for conventional reverse osmosis, while addressing the fundamental thermodynamic limits that constrain desalination efficiency.

Electrodialysis uses ion-selective membranes under electromotive force to separate charged particles, with applications in water desalination, organic compound purification, and industrial wastewater treatment. Liquid membranes use liquid phases (bulk, supported, or emulsion types) as selective barriers, offering very high membrane-specific surface areas (1000-3000 m²/m³). Applications include heavy metal removal, phenol extraction, amino acid separation, and enzymatic bioconversions.