What is considered high-salinity wastewater in terms of salt content? Below what salt content can biological treatment be performed?
What constitutes high-salinity wastewater, and how does it affect biological treatment systems? To address this, we must first understand what high-salinity wastewater is and its impact on these systems.
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What is high-salinity wastewater?
High-salinity wastewater refers to wastewater with a total salt concentration of at least 1% (equivalent to 10 g/L). It primarily originates from chemical plants and the extraction and processing of oil and natural gas.
This wastewater contains various substances (including salts, oil, organic heavy metals, and radioactive materials).
High-salinity wastewater is generated through various means, and its volume increases yearly. Removing organic pollutants from saline wastewater is crucial for environmental protection.
Biological treatment methods are hindered by high concentrations of salts, which inhibit microbial activity. Physicochemical methods require significant investment and high operating costs, and may not achieve the desired purification results. Therefore, biological treatment remains a key focus of research worldwide.
The organic matter in high-salinity organic wastewater varies greatly in type and chemical properties depending on the production process, but the salts present are mostly Cl - 、SO 4
2- 、Na + 、Ca 2+ and other salts.
Although these ions are essential nutrients for microbial growth, playing a crucial role in enzymatic reactions, maintaining membrane balance, and regulating osmotic pressure during microbial growth,
excessively high concentrations of these ions can inhibit and harm microorganisms. This is mainly manifested as: high salt concentration, high osmotic pressure, and microbial cell dehydration causing plasmolysis; salting-out effects reduce dehydrogenase activity; high chloride ion concentration is toxic to bacteria; high salt concentration increases wastewater density, causing activated sludge to float and be lost, thus severely affecting the purification efficiency of the biological treatment system.
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Impact of Salinity on Biological Systems
a. Leads to microbial dehydration and death
At higher salt concentrations, changes in osmotic pressure are the primary cause. The interior of a bacterium is a semi-closed environment that must exchange beneficial substances and energy with the external environment to maintain its life activity. However, it must also prevent most external substances from entering to avoid interference with or disruption of its internal biochemical reactions.
Increased salt concentration causes the internal solution concentration of bacteria to be lower than the external environment. Because water moves from low to high concentration, this leads to a large loss of water from the bacterial cells, causing changes in their internal biochemical reaction environment, ultimately disrupting the biochemical reaction process until it stops, resulting in cell death.
b. Interferes with and blocks the microbial substance absorption process, leading to death
The cell membrane has selective permeability, filtering out substances harmful to bacterial life and absorbing beneficial substances.
This absorption process is directly affected by the external environment's solution concentration, substance purity, etc. The addition of salt interferes with or blocks the bacteria's absorption environment, ultimately inhibiting or killing bacterial life activity. This varies greatly depending on the individual bacteria, species, type of salt, and salt concentration.
c. Causes microbial poisoning and death
Some salts enter bacteria during their life activities, disrupting their internal biochemical processes. Others interact with the bacterial cell membrane, causing its properties to change so that it no longer provides protection or can no longer absorb certain beneficial substances, thus inhibiting or killing bacterial life activity. Heavy metal salts are a prime example, and some sterilization methods utilize this principle.
Studies show that, the impact of high salinity on biological treatment is mainly reflected in the following aspects:
1. As salinity increases, the growth of activated sludge is affected. Changes in the growth curve are manifested as: a longer adaptation phase; a slower growth rate in the logarithmic growth phase; and a longer deceleration growth phase.
2. Salinity enhances microbial respiration and cell lysis.
3. Salinity reduces the biodegradability and degree of degradation of organic matter, decreasing the removal rate and degradation rate of organic matter.
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What salinity concentration can a biological system tolerate?
According to the "Standard for Wastewater Discharge into Urban Sewers" (CJ-343-2010), wastewater entering a sewage treatment plant for secondary treatment must meet the requirements of Class B, including 600 mg/L chloride and 6000 mg/L sulfate.
According to the "Code for Outdoor Drainage Design" (GBJ 14-87) (GB50014-2006 and the 2011 edition do not specify salinity) Appendix III "Permissible Concentrations of Harmful Substances in the Influent of Biological Treatment Structures," the permissible concentration of sodium chloride is 4000 mg/L.
Engineering experience data shows that when the chloride ion concentration in wastewater exceeds 2000 mg/L, microbial activity is inhibited, and the COD removal rate decreases significantly;
when the chloride ion concentration in wastewater exceeds 8000 mg/L, sludge volume expansion occurs, a large amount of foam appears on the surface, and microorganisms die successively.
Normally, we consider that a chloride ion concentration greater than 2000 mg/L and a salt content less than 2% (equivalent to 20000 mg/L) does not affect the treatment effect of the biological system and can be treated using the activated sludge method. However, with proper acclimation, we have encountered cases where a salt content of 3%-4% can achieve stable compliance using the activated sludge method (there are successful cases with 5% in the community). However, it is important to remember that the influent salt content must be stable and not fluctuate excessively, otherwise the biological system cannot withstand it.
Successful cases with 5% salt content have been observed in the community, but influent salt concentration must remain stable to avoid overwhelming the system.
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Measures for Biological Systems to Treat High-Salinity Wastewater
Acclimation of activated sludge
Under salinity conditions less than 2g/L, saline wastewater can be treated through acclimation. By gradually increasing the salinity of the biochemical influent, microorganisms will balance the osmotic pressure inside the cells or protect the protoplasm inside the cells through their own osmotic pressure regulation mechanism. These regulation mechanisms include aggregating low-molecular-weight substances to form a new extracellular protective layer, regulating their own metabolic pathways, and changing gene composition, etc.
Therefore, normal activated sludge can treat high-salinity wastewater through acclimation for a certain period of time within a certain salinity concentration range. Although acclimation of activated sludge can improve the system's salt tolerance range and improve the system's treatment efficiency.
However, the salt tolerance range of microorganisms in acclimated activated sludge is limited, and they are sensitive to environmental changes.
When the chloride ion environment suddenly changes, the adaptability of microorganisms will immediately disappear. Acclimation is only a temporary physiological adjustment of microorganisms adapting to the environment and does not have genetic characteristics. This sensitivity of adaptability is very unfavorable for wastewater treatment.
The acclimation time of activated sludge is generally 7-10 days. Acclimation can improve the tolerance of sludge microorganisms to salt concentration. In the early stage of acclimation, the concentration of activated sludge decreases because the increase in salt solution is toxic to microorganisms, causing some microorganisms to die, resulting in negative growth. In the later stage of acclimation, microorganisms that have adapted to the changed environment begin to reproduce, so the concentration of activated sludge increases.
Taking the COD removal of activated sludge in 1.5% and 2.5% sodium chloride solutions as an example, the COD removal rates in the early and late stages of acclimation are: 60%, 80% and 40%, 60%, respectively.
In order to reduce the concentration of salt in the biochemical system, the influent can be diluted to make the salt content lower than the toxic threshold, and the biological treatment will not be inhibited. Its advantages are simple methods, easy operation and management; the disadvantages are increased treatment scale, infrastructure investment and operating costs.
c. Selection of halophilic bacteria
Halophilic bacteria are a general term for bacteria that can tolerate high concentrations of salt. In industry, they are mostly screened and enriched obligate strains. At present, the highest salinity can tolerate about 5% and can operate stably, which is also a kind of biochemical treatment method for high-salinity wastewater.
d. Selection of reasonable process flow
Different treatment processes are selected according to different chloride ion content, and anaerobic processes are appropriately selected to reduce the chloride ion concentration tolerance range of the subsequent aerobic section.
When the salinity is greater than 5g/L, evaporation and concentration desalination is the most economical and effective feasible method. Other methods, such as culturing salt-containing bacteria, have problems in industrial practice.
