Why is high-salinity wastewater treatment so difficult?
High-salinity wastewater (such as that discharged from the chemical, printing and dyeing, pharmaceutical, petrochemical, and pesticide industries) is indeed considered one of the most difficult types of industrial wastewater to treat. The difficulties primarily arise from the following aspects:
1. Inhibitory Effects on Microorganisms
High-salinity environments (especially those with NaCl > 3% or containing sulfates, chlorides, or heavy metal ions) can lead to:
Excessive osmotic pressure → microbial dehydration and cell membrane damage;
Reduced activity → Significantly reduced biodegradation rates;
Imbalanced bacterial flora → Susceptibility to “system collapse.”
Therefore, traditional activated sludge, A/O, and SBR processes struggle to directly treat high-salinity wastewater.
2. Recalcitrant Organic Matter Degradation
High-salinity wastewater is often accompanied by high COD and recalcitrant organic matter (aromatic hydrocarbons, phenols, pesticides, surfactants, etc.).
Salt has a protective effect on organic matter, increasing its stability and making it more difficult to oxidize or biodegrade.
This results in poor biodegradability (BOD/COD < 0.1-0.2), making it difficult to meet standards through biological methods alone.
3. High Corrosiveness & Scaling
High salt content → Pipes and equipment are susceptible to corrosion, increasing maintenance costs.
During high-temperature evaporation and crystallization, salt scaling is severe, easily leading to heat exchanger blockage and affecting long-term stable operation.
4. High Treatment Costs
Common “end-of-line” treatment methods, such as membrane separation (RO/NF) + evaporation and crystallization, have very high investment and operating costs.
High-salinity wastewater requires large amounts of chemicals (neutralization, coagulation, oxidants), further increasing operating costs.
5. Strict Discharge and Reuse Requirements
High-salinity wastewater often contains ammonia nitrogen, total nitrogen, total phosphorus, and heavy metals, making it difficult to meet standards through desalination alone.
Even if salt is removed through evaporation and crystallization, the mother liquor may still contain high concentrations of organic pollutants, requiring a multi-stage process combination to meet discharge or reuse standards.
6. Difficulty in Process Selection
Physicochemical methods: Limited desalination effect, suitable for pretreatment;
Biological methods: Limited by salinity, require the acclimation of salt-tolerant bacteria, but carry significant operational risks;
Membrane methods: Prone to scaling, difficult to treat brine;
Evaporation and crystallization: High investment and energy consumption, and difficult to utilize by-product salts.
✅ Summary:
High-salinity wastewater treatment is difficult because salt is toxic to microorganisms, contains a high concentration of recalcitrant organic matter, causes severe equipment corrosion and scaling, and is costly. A multi-stage process combination, including “pretreatment → biochemical (halotolerant bacteria/anaerobic) → membrane separation/evaporation and crystallization,” is often required to meet standards.