Wastewater discharge requirements for salt content:
1. Industrial wastewater: <800-1000 mg/L
2. Domestic wastewater: <150-200 mg/L
According to the TDS, wastewater can be divided into three type:
1. Extremely high salinity wastewater (>30000 mg/L TDS)
2. Medium to high salinity wastewater (2000-10000 mg/L)
3. Low salinity wastewater (500-2000 mg/L)
What are the methods for desalination?
1. Chemical methods: involve adding chemical agents, such as chlorides and hydroxides, to chemically react with the salts in the wastewater to reduce salt content. This method is simple and efficient but can lead to secondary pollution and higher treatment costs.
2. Biological methods: utilize the degradation action of microorganisms to break down salt-containing organic materials into low-salt or salt-free substances. Common biological methods include activated sludge, biofilm methods, and anaerobic biological treatment. This method is environmentally friendly and has low treatment costs, but it has a longer treatment cycle and is sensitive to water quality and environmental factors.
3. Physical methods: include electrodialysis, reverse osmosis, and ion exchange. These methods use membrane separation or ion exchange agents to selectively separate salts from wastewater to achieve desalination. Physical methods can achieve a high desalination effect, but the equipment investment and maintenance costs are high. Sedimentation tanks, anaerobic-aerobic tanks, electrodialysis.
Applications of wastewater desalination
1. Evaporation method: High-salinity wastewater is evaporated by heating, leaving the salt in the solid after evaporation. This method is highly efficient and simple to operate, but requires a large amount of thermal energy and space.
2. Ion exchange method: Uses ion exchange resin to exchange ions in the wastewater with ions on the resin, with a desalting rate of over 90%. However, the resin needs to be replaced regularly, and the concentrated brine after treatment requires further processing.
3. Reverse osmosis method: High-salinity wastewater is subjected to reverse osmosis under high pressure to separate salts and pollutants. The treatment efficiency is extremely high, but the equipment cost is high and operating expenses are expensive.
4. Low-temperature multi-effect evaporation concentration crystallization technology: Utilizes multiple interconnected evaporators where the steam generated by one stage is used as the heat source for the next stage, achieving staged energy utilization and reducing energy consumption. Ultimately, this separates wastewater from solid salts, and the desalinated water can be returned to the production system for reuse.
What are the challenges of desalting sewage?
1. Inhibition of microbial activity due to high salinity: High salinity wastewater significantly suppresses the growth and activity of microorganisms, leading to dehydration or even death of microbial cells, making it impossible to effectively degrade the organic matter in the wastewater. Additionally, high salinity can also cause corrosion and damage to treatment equipment, increasing the cost of maintenance and replacement.
2. High treatment costs: To reduce the salinity of wastewater, it is generally necessary to dilute the wastewater, which increases water consumption and treatment costs in the process. Additionally, treating high salinity wastewater consumes a lot of thermal and electrical energy, further increasing treatment costs.
3. Technology selection and applicability: Different desalination technologies have their own advantages and disadvantages. For instance, membrane separation technology is efficient and environmentally friendly, but the membrane materials are prone to contamination and require regular cleaning and maintenance; electrolytic methods are suitable for treating high-salinity wastewater containing heavy metal ions, but have higher energy consumption; evaporation and concentration methods have high energy consumption and complex equipment; biological treatment requires a longer biological reaction time. Choosing the appropriate technology needs to be weighed against specific circumstances.
4. Complexity of wastewater composition: Oil and gas field wastewater contains various organic substances and inorganic salts, each with different chemical properties and biodegradability, increasing the difficulty of removal. Moreover, impurities such as suspended solids and heavy metal ions in the wastewater can also affect the treatment effectiveness.
What are the hazards of high-salinity wastewater?
The environmental hazards of high-salinity wastewater primarily manifest in the toxicity to microorganisms, corrosion of equipment, and destruction of the ecosystem.
1. Toxicity to microorganisms: High salt concentrations can increase osmotic pressure, leading to dehydration of microbial cells and separation of cell protoplasm. Salting out effects can reduce the activity of dehydrogenase, and increased chloride ion concentration can also be toxic to bacteria. In addition, high salt concentrations increase the density of wastewater, making it easier for activated sludge to float and be lost, seriously affecting the purification effectiveness of biological treatment systems.
2. Corrosion of equipment: The salts in high-salinity wastewater can corrode pipes and equipment, affecting their service life and quality. For instance, excessively high salt concentrations can lead to the corrosion of pipes and equipment, impacting their normal operation and maintenance.
3. Destruction of the ecosystem: Pollutants such as suspended solids, organic matter, heavy metals, and harmful chemicals in high-salinity wastewater can have a huge impact on the environment and ecosystems. These substances not only have toxic effects on microorganisms but can also accumulate through the food chain, ultimately affecting the balance and health of the entire ecosystem.
What’s the significance of wastewater desalination?
1. Protecting the Environment: If the salt content in wastewater is discharged without treatment, it can cause serious pollution to the environment. Salt can alter the chemical properties of water bodies, affecting the survival of aquatic organisms, leading to water hardening and eutrophication. Desalination treatment can effectively remove salts from wastewater, reducing environmental harm.
2. Reuse and Recycling: Many salts in wastewater are valuable resources; for example, salts in certain chemical wastewater can be used to produce sodium sulfate, sodium chloride, and other chemicals. Through desalination treatment, these valuable resources can be extracted, achieving resource utilization of wastewater.
3. Meeting Discharge Standards: Many countries and regions have strict discharge standards for salt content in wastewater. To meet discharge standards, desalination treatment is necessary to reduce the salt content in wastewater to below the specified concentration, ensuring that the wastewater can be safely discharged into the environment.
4. Avoiding Equipment Corrosion and Scaling: The salt content in wastewater can corrode equipment and pipelines, shortening their lifespan. Desalination treatment can remove salts from wastewater, reducing corrosion and scaling on equipment, thus prolonging its lifespan.
5. Preventing Soil and Groundwater Pollution: High concentrations of salts and soluble inorganic substances in wastewater can lead to soil salinization and groundwater pollution if discharged directly or infiltrated into the soil and groundwater. Desalination treatment can reduce the risk of pollution to soil and groundwater.
6. Protecting Ecosystems: Discharging wastewater into natural water bodies can increase salt concentration, disrupting the balance of aquatic ecosystems and affecting the survival and reproduction of aquatic organisms. Through desalination treatment, the negative impact of wastewater on aquatic ecosystems can be reduced, protecting biodiversity and ecological balance in water bodies.
