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The Role of Reverse Osmosis Membranes in Achieving Zero Liquid Discharge in Chemical Wastewater
The chemical industry is a major source of industrial wastewater, characterized by complex compositions, high salinity, and high concentrations of organic matter, making treatment extremely challenging. As the "15th Five-Year Plan" begins, environmental policies continue to tighten, and Zero Liquid Discharge (ZLD) has become the industry standard for chemical wastewater.
Within the typical ZLD chain of "pretreatment – membrane concentration – evaporation/crystallization", where exactly does the reverse osmosis (RO) membrane stand, and what mission does it carry? Dalton Membrane Technology (Shenzhen) Co., Ltd. has accumulated substantial project experience in chemical wastewater treatment and ZLD projects. Based on engineering practices, this article provides a clear interpretation of the membrane's role.
I. Typical Process Route for Chemical Wastewater ZLD
The standard route for chemical wastewater ZLD can be summarized as three stages: pretreatment → membrane concentration → evaporation/crystallization.
- The pretreatment stage removes suspended solids, hardness, and silica through chemical softening and filtration, creating favorable conditions for stable membrane system operation.
- The membrane concentration stage is the heart of the entire process. Its goal is to "squeeze" water out of the wastewater with minimal energy consumption, thereby significantly reducing the volume of water that enters the evaporation/crystallization stage. The RO membrane is the core element of this stage.
- The evaporation/crystallization stage treats the brine discharged from the membrane system, ultimately producing crystalline salts and condensate.
The position of the RO membrane determines that it must simultaneously shoulder the dual responsibilities of volume reduction and water reuse.
II. Three Core Tasks of RO Membranes in ZLD Systems
Within a ZLD system, the RO membrane performs three indispensable functions:
- Concentration and volume reduction.
It does not simply "purify" the wastewater; rather, it efficiently separates water from salts, compressing the wastewater volume by several times or even dozens of times. A common industry practice is: first, conventional RO concentrates TDS to 20,000–30,000 mg/L, followed by high-pressure RO or disc-tube RO (DTRO) to push the brine TDS up to 80,000–100,000 mg/L. This drastically reduces the treatment load for subsequent evaporation/crystallization, directly lowering system investment and steam consumption.
In several chemical and industrial park wastewater treatment projects, RO membrane elements supplied by Dalton Membrane Technology (Shenzhen) Co., Ltd. have been applied in two-stage concentration processes, achieving stable concentration and permeate reuse through optimized system operating parameters. - Permeate reuse.
RO permeate has extremely low salt content and can be directly reused as make-up water for circulating cooling water or boiler feedwater. In water-scarce regions, the value of this high-quality reclaimed water even exceeds that of mere volume reduction. - Synergy with nanofiltration for salt resource recovery.
Modern ZLD pursues "full wastewater recovery and full salt utilization." Nanofiltration (NF) membranes can separate monovalent salts (sodium chloride) from divalent salts (sodium sulfate), while RO concentrates each fractionated salt solution to near crystallization concentration. The golden combination of "NF for salt fractionation + RO for concentration" transforms mixed hazardous salts into marketable industrial salts, truly achieving a closed-loop system.
III. Selection Differences Across Various Chemical Scenarios
- Coal chemical wastewater has high hardness and high silica content, requiring efficient hardness and silica removal in pretreatment to reduce scaling risks. Subsequent RO systems tend to favor membrane elements with anti-fouling properties and alkali-resistance for cleaning.
- Coking wastewater contains large amounts of refractory organics such as polycyclic aromatic hydrocarbons. The front end requires catalytic oxidation or advanced biological treatment, while the RO membrane emphasizes resistance to organic fouling and ease of cleaning.
- For composite wastewater from electroplating, fine chemical, and similar industrial parks, water quality fluctuates greatly and mixed salts are complex. A "multi-stage RO + DTRO" cascade concentration is often adopted, requiring membrane elements that are high-pressure resistant, have wide feed channels, and can tolerate high TDS.
In the context of ZLD, RO membrane selection has long entered an era of "customized solutions for each water quality" – no single key fits all locks.
IV. Conclusion
In chemical wastewater ZLD systems, the RO membrane is not an isolated product but a pivotal hub that bridges upstream and downstream – it receives pretreated water upstream and dictates the efficiency of evaporation/crystallization downstream. Selection must match the specific water quality; there is no "universal membrane," only the "most suitable membrane."
With precise positioning and science-based selection, the entire ZLD process can run stably and sustainably over the long term.
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