Breaking Through the Crystallization Dilemma of High-Salinity Wastewater: The Rise and Reform of Zero Liquid Discharge (ZLD) Technologies
- Gu Zhouying
- Jul 24
- 4 min read
As industrial development surges forward, the rational use and protection of water resources have become increasingly critical. With stricter discharge regulations for industrial wastewater, Zero Liquid Discharge (ZLD) technologies are emerging as a "golden key" to tackling pollution challenges thanks to their environmental and resource recovery benefits. However, the complex composition of high-salinity wastewater—much like the hidden reefs beneath calm waters—poses significant hurdles for crystallization treatment. This article takes a deep dive into the key pain points of high-salinity wastewater crystallization and explores multidimensional solutions to accelerate the path toward resource recovery.
Industry Context: Challenges and Policy Drivers in High-Salinity Wastewater Treatment
Sources and Hazards of High-Salinity Wastewater
High-salinity wastewater is an inevitable byproduct of various industrial processes, especially in sectors like chemicals, pharmaceuticals, and textiles. It is characterized by three major challenges:
High Total Dissolved Solids (TDS): Often exceeding 10,000 mg/L, comprising both inorganic salts like NaCl, Na₂SO₄, and complex organic compounds.
High Organic Load: COD levels can reach several thousand or even tens of thousands mg/L, with toxic substances like benzene derivatives and PAHs that are difficult to degrade.
Complex Composition: Heavy metals, sulfides, and chelating agents are commonly present, creating a highly intricate wastewater matrix.
If discharged untreated, such wastewater severely harms the environment. In soil, salt accumulation inhibits plant growth and causes hardening. In water bodies, high COD levels consume oxygen during decomposition, disturbing aquatic ecosystems and causing eutrophication.
Policies and Technologies Driving Zero Liquid Discharge
China’s 14th Five-Year Plan emphasizes "near-zero discharge of industrial park wastewater," setting clear targets for high-salinity wastewater treatment. The prevailing ZLD process follows this route: Pre-treatment → Nanofiltration Salt Separation → Membrane/Thermal Concentration → Crystallization. However, crystallization remains the weakest link in this chain, acting as the "short plank" that limits system performance.
According to industry data, over 30% of failures in ZLD systems are due to issues in the crystallization section. Solving this bottleneck is now a top technical priority.
Core Technical Challenges in the Crystallization Process
Scaling and Blockage: The Efficiency Killers
In evaporators operating at high temperatures (>80°C), some organic compounds undergo thermal decomposition and polymerization, forming coke or tar-like substances. These combine with salts to form composite scale layers, reducing heat transfer efficiency by 30%–50%.
Furthermore, complexation between salts and organics results in uneven crystallization and clogged flow channels. For instance, one coal chemical plant neglected to remove lignin during pre-treatment, leading to a 45% drop in heat exchange efficiency after three months, forcing a system shutdown for cleaning.
Corrosion and Material Degradation: Equipment at Risk
Chlorinated organics decompose at high temperatures, producing HCl that lowers local pH to as low as 1–2, accelerating pitting and stress corrosion cracking in carbon steel and stainless steel. Sulfides form galvanic cells with metal ions, intensifying corrosion on internal surfaces.
At one power plant, hydrogen sulfide from flue gas desulfurization wastewater reduced heat exchanger lifespan to just one year.
Foaming and Energy Overload: Rising Costs
High-COD wastewater is highly viscous, leading to excessive foam during evaporation. This foam contaminates the condensate and decreases heat exchange performance. Additionally, viscosity increases boiling points and lowers efficiency, boosting energy consumption by 30%–50%.
Stability Issues in System Operation
In multi-effect evaporators, non-condensable gas accumulation leads to lower vacuum levels and reduced evaporation efficiency. In MVR systems, insufficient secondary steam can cause compressor surging, threatening the overall system stability.
Crystal Salt Quality and Resource Recovery Limitations
Pigments embed in salt crystals, reducing purity. Chelating agents suppress crystal growth, resulting in abnormal crystal structures that hinder downstream utilization.
Technological Optimization Strategies and Future Outlook
Process Design Optimization
- Use Computational Fluid Dynamics (CFD) to optimize internal flow within crystallizers.
- Adopt nanofiltration + electrodialysis hybrid systems to improve salt separation and reduce mixed salt content to below 5%.
Equipment Selection and Material Innovation
- Promote corrosion-resistant materials like duplex stainless steel and titanium alloys.
- Integrate online sensors to enable early fouling detection and automated cleaning systems.
Enhanced Operational Management
- Strengthen pre-treatment using advanced oxidation to reduce COD levels.
- Establish real-time dynamic control to adjust system parameters based on water quality fluctuations.
Resource Recovery Approaches
- Upgrade salt recovery to produce high-purity industrial salts for food-grade and chlor-alkali applications.
- Apply cement solidification and high-temperature melting to stabilize mixed salts and reduce hazardous waste disposal costs.
Conclusion
High-salinity wastewater crystallization is a complex, multi-step engineering challenge. Achieving stable, efficient ZLD requires comprehensive improvements:
Strengthen pre-treatment to lower organic content
Upgrade system materials to extend equipment life
Optimize crystallization processes to enhance salt purity
Improve policies to support salt resource recovery
Only with a holistic, system-wide approach can industries achieve both zero discharge and resource utilization—paving the way for sustainable industrial growth and environmental protection.
FAQs
1. Why is crystallization the biggest challenge in high-salinity wastewater ZLD systems?
Because it’s highly sensitive to organics, scaling, and fouling—leading to frequent system failures and high operational costs.
2. What industries produce high-salinity wastewater?
Main contributors include chemical manufacturing, pharmaceuticals, textiles, and coal processing.
3. Can crystallized salts from wastewater be reused?
Yes, with proper purification, some can be reused in industrial applications like chlor-alkali production, but many require further treatment.
4. What is the role of nanofiltration in ZLD systems?
It separates different types of salts and reduces the complexity of the crystallization process, improving overall system efficiency.
5. Are there emerging technologies to replace traditional crystallizers?
Hybrid technologies such as membrane-crystallization, improved heat exchangers, and AI-based process controls are emerging to enhance performance.
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