Why is high-salt wastewater so difficult to treat? We must first understand what high-salt wastewater is and the impact of high-salt wastewater on the biochemical system! This article only discusses the biochemical treatment of high-salt wastewater!
1. What is high-salt wastewater?
High-salt wastewater refers to wastewater with a total salt content of at least 1% (equivalent to 10,000mg/L). It mainly comes from chemical plants and the collection and processing of oil and natural gas. This wastewater contains a variety of substances (including salts, oils, organic heavy metals and radioactive materials). Salty wastewater is produced through a wide range of sources, and the amount of water is increasing year by year. Removing organic pollutants from salty wastewater has an important impact on the environment. Biological methods are used for treatment. High-concentration salt substances have an inhibitory effect on microorganisms. Physical and chemical methods are used for treatment, which requires large investment and high operating costs, and it is difficult to achieve the expected purification effect. The use of biological methods to treat such wastewater is still the focus of research at home and abroad.
The types and chemical properties of organic matter in high-salt organic wastewater vary greatly depending on the production process, but the salts contained are mostly salts such as Cl-, SO42-, Na+, Ca2+. Although these ions are essential nutrients for the growth of microorganisms, they play an important role in promoting enzymatic reactions, maintaining membrane balance and regulating osmotic pressure during the growth of microorganisms. However, if the concentration of these ions is too high, it will have inhibitory and toxic effects on microorganisms. The main manifestations are: high salt concentration, high osmotic pressure, dehydration of microbial cells, causing cell protoplasm separation; salting out reduces dehydrogenase activity; high chloride ions Bacteria are toxic; the salt concentration is high, the density of wastewater increases, and activated sludge easily floats and is lost, thus seriously affecting the purification effect of the biological treatment system.
2. Effect of salinity on biochemical systems
1. Lead to dehydration and death of microorganisms
At higher salt concentrations, changes in osmotic pressure are the main cause. The interior of a bacterium is a semi-closed environment. It must exchange beneficial materials and energy with the external environment to maintain its vitality. However, it must also prevent most external substances from entering to avoid damaging the internal biochemistry. Interference and obstruction of response.
The increase in salt concentration causes the concentration of the solution inside the bacteria to be lower than the outside world. Furthermore, due to the characteristic of water moving from low concentration to high concentration, a large amount of water is lost in the bacteria, causing changes in their internal biochemical reaction environment, ultimately destroying their biochemical reaction process until it is interrupted. , the bacteria die.
2. Interfering with the absorption process of microbial substances and blocking their death
The cell membrane has the characteristic of selective permeability to filter substances harmful to bacterial life activities and absorb substances beneficial to its life activities. This absorption process is directly affected by the solution concentration, material purity, etc. of the external environment. The addition of salt causes the bacterial absorption environment to be interfered with or blocked, eventually causing the bacterial life activity to be inhibited or even die. This situation varies greatly due to individual bacterial conditions, species conditions, salt types and salt concentrations.
3. Poisoning and death of microorganisms
Some salts will enter the interior of the bacteria along with their life activities, destroying their internal biochemical reaction processes, and some will interact with the bacterial cell membrane, causing their properties to change and no longer protect them or no longer be able to absorb certain harmful substances to the bacteria. Beneficial substances, thereby causing the vital activity of bacteria to be inhibited or the bacteria to die. Among them, heavy metal salts are the representative ones, and some sterilization methods utilize this principle.
Research shows that the impact of high salinity on biochemical treatment is mainly reflected in the following aspects:
1. As salinity increases, the growth of activated sludge is affected. The changes in its growth curve are as follows: the adaptation period becomes longer; the growth rate in the logarithmic growth period becomes slower; and the duration of the deceleration growth period becomes longer.
2. Salinity strengthens microbial respiration and cell lysis.
3. Salinity reduces the biodegradability and degradability of organic matter. Reduce the removal rate and degradation rate of organic matter.
3. How high salt concentration can the biochemical system withstand?
According to the “Water Quality Standard for Sewage Discharged into Urban Sewers” (CJ-343-2010), when entering a sewage treatment plant for secondary treatment, the quality of sewage discharged into urban sewers should comply with the requirements of Grade B (Table 1), among which chlorine Chemicals 600 mg/L, sulfate 600 mg/L.
According to Appendix 3 of the “Code for Design of Outdoor Drainage” (GBJ 14-87) (GB50014-2006 and 2011 editions do not specify salt content), “Permissible concentration of harmful substances in the inlet water of biological treatment structures”, the allowable concentration of sodium chloride is 4000mg/ L.
Engineering experience data shows that when the chloride ion concentration in wastewater is greater than 2000mg/L, the activity of microorganisms will be inhibited and the COD removal rate will be significantly reduced; when the chloride ion concentration in wastewater is greater than 8000mg/L, the sludge volume will be increased. Expansion, a large amount of foam appears on the water surface, and microorganisms will die one after another.
Under normal circumstances, we believe that chloride ion concentration greater than 2000mg/L and salt content less than 2% (equivalent to 20000mg/L) can be treated by the activated sludge method. However, the higher the salt content, the longer the acclimation time. But remember one thing, The salt content of the incoming water must be stable and cannot fluctuate too much, otherwise the biochemical system will not be able to withstand it.
4. Measures for biochemical system treatment of high-salt wastewater
1. Domestication of activated sludge
When the salinity is less than 2g/L, salty sewage can be treated through domestication. By gradually increasing the salt content of the biochemical feed water, microorganisms will balance the osmotic pressure within the cells or protect the protoplasm within the cells through their own osmotic pressure regulation mechanisms. These regulatory mechanisms include the accumulation of low molecular weight substances to form a new extracellular protective layer and regulate themselves. Metabolic pathways, changes in genetic composition, etc.
Therefore, normal activated sludge can treat high-salt wastewater within a certain salt concentration range through domestication for a certain period of time. Although activated sludge can increase the salt tolerance range of the system and improve the treatment efficiency of the system through domestication, domestication of activated sludge Microorganisms have a limited tolerance range for salt and are sensitive to changes in the environment. When the chloride ion environment changes suddenly, the adaptability of microorganisms will disappear immediately. Domestication is only a temporary physiological adjustment of microorganisms to adapt to the environment and has no genetic characteristics. This adaptive sensitivity is very detrimental to sewage treatment.
The acclimation time of activated sludge is generally 7-10 days. Acclimation can improve the tolerance of sludge microorganisms to salt concentration. The reduction in activated sludge concentration in the early stage of acclimation is due to the increase in salt solution poisoning microorganisms and causing the death of some microorganisms. It shows negative growth. In the later stage of domestication, microorganisms that have adapted to the changed environment begin to reproduce, so the concentration of activated sludge increases. Taking the removal of COD by activated sludge in 1.5% and 2.5% sodium chloride solutions as an example, the COD removal rates in the early and late acclimation stages are: 60%, 80% and 40%, 60% respectively.
2. Dilute the water
In order to reduce the concentration of salt in the biochemical system, the incoming water can be diluted so that the salt content is lower than the toxic limit value, and the biological treatment will not be inhibited. Its advantage is that the method is simple and easy to operate and manage; its disadvantage is that it increases processing scale, infrastructure investment and operating costs.
3. Select salt-tolerant bacteria
Halotolerant bacteria are a general term for bacteria that can tolerate high concentrations of salt. In industry, they are mostly obligate strains that are screened and enriched. Currently, the highest salt content can be tolerated at around 5% and can operate stably. It is also considered a kind of high-salt wastewater. A biochemical method of treatment!
4. Choose a reasonable process flow
Different treatment processes are selected for different concentrations of chloride ion content, and the anaerobic process is appropriately selected to reduce the tolerance range of the chloride ion concentration in the subsequent aerobic section.
When the salinity is greater than 5g/L, evaporation and concentration for desalination is the most economical and effective method. Other methods, such as methods for cultivating salt-containing bacteria, have problems that are difficult to operate in industrial practice.
Lianhua company can provide fast COD analyzer to test high salt wastewater because our chemical reagent can shield tens of thousands of chloride ion interference.
Post time: Jan-25-2024
