Ⅰ COD of seawater
The COD value in seawater is usually low, generally between 1-10 mg/L. Chemical oxygen demand (COD) is an important water quality indicator used to measure the value of reducing substances in water .
Reducing substances may include organic matter, nitrite, sulfide, etc., which can be measured by oxidants. Seawater usually has a low COD value due to its composition and environment.
This is because seawater contains less organic matter and other reducing substances, the content of which is affected by many factors, including climate, geographical location, and biological species.
Understanding COD values in seawater is important for assessing ocean health and water quality management, especially when considering the protection of marine ecosystems and the impact of human activities on the marine environment.
Ⅱ Ion ratio of seawater
The ratio of various ions in seawater is relatively stable, a property known as the constancy of seawater composition. This constancy provides favorable conditions for studying the physical and chemical properties of seawater.
The concentration ratios of these ions are relatively constant, mainly due to the mixing of seawater, its huge volume, and its long-term historical evolution, which makes it difficult for external influences (such as continental runoff) to cause significant changes in their relative composition.
Ⅲ Mineralization and ion content
The mineralization of seawater refers to the total amount of dissolved salt substances in seawater , which is an important indicator to measure the salt content of seawater.
The average salinity of seawater on Earth is about 35‰ (35 grams of salt per kilogram of seawater), and TDS is 35,000 ppm.
However, the mineralization of seawater varies by region and depth.
The ion content in seawater is determined by its proportion in the seawater.
The major elements in seawater include the following and their average concentrations:
Chloride ion (Cl - ): 19.10 g/kgSodium
ion (Na + ): 10.62 g/kgMagnesium
ion (Mg 2+ ): 1.29 g/
kgSulfate ion (SO 4 2- ): 2.74 g/kgCalcium
ion (Ca 2+ ): 0.412 g/kgPotassium
ion (K + ): 0.399 g/kgBoron
(B): 4.5 mg/kgCarbonate
(CO 3 2- /HCO 3 - ): 27.6 mg/kgFluoride
ion (F - ): 1.3 mg/kgSilicate
(Si): 2.8 mg/LBromide
ion (Br - ): 67 mg/kgStrontium
ion (Sr 2+ ): 7.9 mg/kg
In addition, the salt in seawater mainly exists in the form of sodium chloride (NaCl), accounting for 77.7% of the salt content of seawater, followed by magnesium chloride (MgCl 2 ) accounting for 10.9% , magnesium sulfate (MgSO 4 ) accounting for 4.9%, calcium sulfate (CaSO 4 ) accounting for 3.6%, potassium sulfate (K 2 SO 4 ) accounting for 2.5%, calcium carbonate (CaCO 3 ) accounting for 0.3%, and other salts.
Figure 3 Salt content in seawater
It should be noted that these values are averages, and the actual chemical composition of seawater may vary depending on factors such as geographical location, season, and climate.
Ⅳ The oil content in seawater is very low
The oil content of seawater usually refers to the content of oil substances in seawater, which may come from natural phenomena or human activities.
Every year, approximately 5 to 10 million tons of oil enters water bodies through various channels around the world, of which about 8% comes from natural sources and about 92% comes from human activities.
Sources from human activities include tanker accidents, leaks from offshore oil exploration, oily wastewater discharged from ports and ship operations, oil industry wastewater, and oily wastewater discharged from the catering industry, food processing industry, and car wash industry.
After oil pollutants enter the water environment, they will undergo processes such as migration, transformation and oxidation degradation, resulting in a general decrease in the oil content in the water. There are four main states of oil pollutants in water bodies: floating oil, emulsified oil, dissolved oil and condensed residues.
When the oil content in seawater reaches 0.01 mg/L, it can cause fish, shrimp, and shellfish to have an odor within 24 hours, affecting the edible value of aquatic products. Therefore, monitoring and controlling the oil content in seawater is crucial to protecting the marine ecological environment and human health.
Normal unpolluted seawater contains oil in the microgram range.
In summary, the COD and scaling ion content of seawater are very low, and there is almost no oil. Seawater desalination has become a very mature technology.
Ⅴ Industrial wastewater with a higher salt content than seawater
Industrial wastewater with a higher salt content than seawater mainly comes from a number of industries, which produce wastewater containing a large amount of salt during the production process. The main industries are :
(1)Chemical and petrochemical industries
The chemical and petrochemical industries are one of the main sources of industrial high-salinity water. These industries produce a large amount of wastewater during the production process, which contains a large amount of salt, such as sodium chloride, calcium chloride, sodium sulfate, etc. The salt concentration of these wastewaters is often much higher than that of seawater.
(2)Mining and mineral processing
The mining and mineral processing process produces a large amount of tailings and wastewater, which also contains a lot of salt and is one of the important sources of industrial high-salinity water. The salt content of these wastewaters may also exceed that of seawater.
(3)Food processing
A large amount of wastewater is generated during food processing. In addition to organic matter, these wastewaters may also contain a large amount of salt, such as sodium chloride, potassium chloride, etc. Although the specific salt content varies depending on the processing type and process, some food processing wastewater may also have a high salt content.
(4)Papermaking and pulping
The paper and pulping process generates a large amount of wastewater, which contains not only organic matter but also salts such as sodium chloride and sodium sulfate. Although the salt concentration of these wastewaters may vary depending on the process and raw materials, in some cases, their salt content may exceed that of seawater.
(6)Textile and printing and dyeing
The textile and printing and dyeing processes also generate a large amount of wastewater, which may contain salt substances such as sodium chloride and potassium chloride. Although the salt concentration of these wastewaters may vary depending on the specific process and dye, the salt content of the wastewater may also be high in some printing and dyeing processes.
(7)Other industries
In addition to the above industries, some other industries may also produce high-salt wastewater, such as desulfurization wastewater from the power industry, wastewater from the coal chemical industry, etc. The salt content of these wastewaters may also exceed that of seawater.
It should be noted that the salt content of industrial high-salt water produced by different industries is different, and the specific salt type and concentration are also affected by many factors. Therefore, when treating these high-salt wastewaters, it is necessary to choose appropriate treatment methods and technical means according to the actual situation.
Ⅵ Zero discharge of industrial wastewater is very close to the requirements of seawater desalination (pretreatment + membrane process)
Achieving zero discharge of industrial high-salinity wastewater requires a systematic solution. First, physical or chemical pretreatment methods are generally used to remove suspended solids, colloids and general scaling ions. Then, membrane treatment processes are used to reuse fresh water and reduce wastewater. Finally, the concentrate is evaporated and crystallized to achieve zero discharge of wastewater. This article mainly introduces the commonly used membrane treatment processes.
We can understand it this way: by using physical, chemical, biochemical and other methods to treat high-salt, high-hardness, high-COD industrial wastewater to a composition close to that of seawater, we can also use the idea of seawater desalination to solve the "zero emission" problem.
According to the membrane pore size separation, commonly used membrane technologies can be divided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), etc.
According to the filtration pressure and the final concentration multiple, the reverse osmosis commonly used for zero wastewater discharge can be further divided into low-pressure reverse osmosis (such as BWRO), medium-pressure reverse osmosis (seawater membrane SWRO), high-pressure reverse osmosis (HPRO or DTRO), etc.
At the same time, there are also technologies such as EDI (electrodialysis) and forward osmosis (FO) on the market that have been applied to the high-salt zero-emission industry. Due to their different scope of use and different working conditions, their combined design has been widely used in zero-emission projects.
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