Salinity greater than how much is high salt wastewater and high salt wastewater biochemical treatment, we have to first understand what is high salt wastewater, and high salt wastewater on the biochemical system!
What is high salt wastewater
High-salt wastewater refers to the total salt mass fraction of at least 1% (equivalent to 10,000mg / L) of the wastewater. It mainly comes from chemical plants and oil and gas gathering and processing. This wastewater contains a variety of substances (including salt, oil, organic heavy metals and radioactive substances). Salt-containing wastewater is generated in a wide range of ways and the volume of water is increasing every year. Removal of organic pollutants from saline wastewater is critical to the environment. When biological methods are used for treatment, the high concentration of saline substances has an inhibitory effect on microorganisms, and physical and chemical methods are used for treatment, which results in large investments, high operating costs, and difficulties in achieving the desired purification effect. The treatment of such wastewater by biological method is still the focus of research at home and abroad.
The organic matter of organic wastewater with high salt content varies greatly in terms of the type and chemical properties of the organic matter contained according to the production process, but the salts contained are mostly Cl-, SO42-, Na+, Ca2+ and other salts. Although these ions are essential nutrients for microbial growth, they play an important role in promoting enzyme reactions, maintaining membrane balance and regulating osmotic pressure in the growth process of microorganisms. However, if the concentration of these ions is too high, it will inhibit and poison the microorganisms, the main performance: high salt concentration, high osmotic pressure, microbial cell dehydration caused by the separation of cell protoplasm; salt precipitation effect of dehydrogenase activity is reduced; chloride ions high on the bacterial toxicity; high salt concentration, the density of wastewater increases, the activated sludge is easy to float and lose, which seriously affects the purification effect of the biological treatment system.
The effect of salinity on biochemical system
1. Lead to the death of microorganism dehydration
The change of osmotic pressure is the main cause in the case of high salt concentration. The interior of bacteria is a semi-closed environment, and must exchange substances and energy with the external environment in its favour in order to maintain its life activity, but it must also prevent most of the external substances from entering in order to avoid interference and obstruction of its internal biochemical reactions.
Salt concentration increases, resulting in the bacterial internal solution concentration is lower than the outside world, and because of the characteristics of water from low concentration to high concentration movement, resulting in the loss of water in the bacterial body caused by a large number of changes in its internal biochemical reaction environment, and ultimately destroy the biochemical reaction process until the interruption of the bacterial body death.
2. The microbial material absorption process is interfered and blocked to death.
The cell membrane has the characteristic of selective permeability, in order to filter the substances that are harmful to the life activities of bacteria and absorb the substances that are beneficial to their life activities. This absorption process is directly affected by the concentration of the solution in the external environment, the purity of the substance, etc. The addition of salt causes the bacterial absorption environment to be interfered with or blocked, which ultimately leads to the inhibition of the bacterial life activity or even death. This situation varies greatly depending on the individual situation of bacteria, species, type of salt and salt concentration.
3.Toxicity and death of microorganisms
Some salts will enter into the bacteria with their life activities, destroying their internal biochemical reaction process, and some of them will interact with the cell membrane of the bacteria, resulting in the change of their properties and no longer play a protective role or no longer absorb certain substances beneficial to the bacteria, which will lead to the inhibition of the life activity of the bacteria or the death of the organisms. Heavy metal salts are representative of this, and some bactericidal methods make use of this principle.
Research shows that the impact of high salinity on biochemical treatment is mainly reflected in the following aspects:
(1) With the increase of salinity, the growth of activated sludge is affected. The change of its growth curve is manifested in: the adaptation period becomes longer; the growth rate of logarithmic growth period becomes slower; the duration of decelerated growth period becomes longer.
(2) Salinity enhances microbial respiration and cell lysis.
3) Salinity reduces the biodegradability and degradability of organic matter. It decreases the removal rate and degradation rate of organic matter.
Biochemical system can withstand how high salt concentration
According to the “Water Quality Standards for Sewage Discharge into Urban Sewers” (CJ-343-2010), the quality of sewage discharged into urban sewers should comply with the provisions of Class B (Table 1) when entering the wastewater treatment plant for secondary treatment, in which 600 mg/L of chloride and 600 mg/L of sulphate are present.
According to “Outdoor Drainage Design Code” (GBJ 14-87) (GB50014-2006 and 2011 version of the salt is not specifically stated) Appendix III “biological treatment structures into the water permissible concentration of hazardous substances””, the permissible concentration of sodium chloride is 4000mg/L.
Engineering experience data show that: when the concentration of chloride ions in wastewater is greater than 2000mg / L, the activity of microorganisms will be suppressed, the COD removal rate will be significantly reduced; when the concentration of chloride ions in wastewater is greater than 8000mg / L, it will result in the expansion of sludge volume, the surface of the water flooded with a large number of bubbles, microorganisms will be killed one after another.
Under normal circumstances, we believe that the chlorine ion concentration is greater than 2000mg/L, salt is less than 2% (equivalent to 20,000mg/L) does not affect the biochemical system treatment effect, you can use the activated sludge method, however, according to if the domestication of a reasonable, salt 3%-4% use of the activated sludge method to achieve the standard of stability has also been encountered (the community has a 5% debugging success). Case), but keep in mind that the salinity of the feed water to ensure stability, can not fluctuate too much, otherwise the biochemical system can not withstand the collapse!
Measures for biochemical system to treat high salt wastewater
1. Domestication of activated sludge
Under the condition that the salinity is less than 2g/L, the saline wastewater can be treated by domestication. By gradually increasing the salinity of biochemical feed water, microorganisms will balance the intracellular osmotic pressure or protect intracellular protoplasm through their own osmotic pressure regulating mechanisms, which include aggregating low molecular weight substances to form a new extracellular protective layer, regulating their own metabolic pathways, and changing their genetic composition.
Therefore, normal activated sludge can be domesticated for a certain period of time to treat high-salt wastewater within a certain salt concentration range. Although activated sludge can increase the salt tolerance range of the system and improve the treatment efficiency of the system through domestication, the microorganisms in the domesticated activated sludge have a limited salt tolerance range and are sensitive to changes in the environment. When the chloride environment changes suddenly, the adaptation of microorganisms will disappear immediately． Domestication is only a temporary physiological adjustment of microorganisms to adapt to the environment and does not have genetic characteristics. The sensitivity of this adaptation is very unfavourable to the sewage treatment.
Activated sludge domestication time is generally 7-10d, domestication can improve the degree of tolerance of sludge microorganisms to salt concentration, domestication of the early activated sludge concentration decreased, is due to the increase of salt solution to produce toxicity to microorganisms, so that some microorganisms die, showing negative growth, in the late domestication of microorganisms adapted to change the environment began to reproduce, so the activated sludge concentration increased. Taking the removal of COD by activated sludge in 1.5% and 2.5% NaCl solution as an example, the removal of COD in the early and late stages of domestication were 60% and 80% and 40% and 60% respectively.
2. Dilution of influent water
In order to reduce the concentration of salt in the biochemical system, the influent water can be diluted so that the salt is lower than the value of the toxic domain, and the biological treatment will not be inhibited. Its advantage is that the method is simple, easy to operate and manage; the disadvantage is that it increases the scale of treatment, infrastructure investment and operating costs.
3.Selection of salt-resistant bacteria
Salt-tolerant bacteria is a kind of bacteria that can tolerate high concentration of salt is a general term, the industry is mostly screened for the enrichment of specialised strains, the highest salt can tolerate about 5% can be stable operation, is also considered to be a kind of high salt wastewater treatment of a biochemical means!
4. Select reasonable process flow
For different concentrations of chloride ions content to choose different treatment processes, appropriate selection of anaerobic process to reduce the range of chlorine ions after the aerobic section of the tolerance concentration.
When the salinity is greater than 5g/L, evaporation and concentration is the most economical and effective way to remove salt. Other methods such as cultivation of salt-containing bacteria have the problem of difficult operation in industrial practice.