What constitutes high-salinity wastewater, and how does it affect biological treatment? We need to understand what high-salinity wastewater is and its impact on biological systems before addressing salinity levels above a certain threshold.
High-salinity wastewater refers to wastewater with a total salt concentration of at least 1% (equivalent to 10000 mg/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). The sources of saline wastewater are widespread, and the volume is increasing year by year. Removing organic pollutants from saline wastewater is crucial for environmental protection. Biological treatment methods are affected by the inhibitory effect of high concentrations of salts on microorganisms. Physico-chemical methods require high investment and operating costs and may not achieve the desired purification effect. Biological treatment remains a research focus worldwide for this type of wastewater.
The types and chemical properties of organic matter in high-salinity organic wastewater vary greatly depending on the production process. However, the salt content mainly consists of Cl-, SO42-, Na+, and Ca2+. Although these ions are essential nutrients for microbial growth, playing a crucial role in enzymatic reactions, maintaining membrane balance, and regulating osmotic pressure, excessively high concentrations 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 effect of the biological treatment system.
1. 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 that must exchange matter 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 obstruction of its internal biochemical reactions.
Increased salt concentration leads to a lower internal bacterial solution concentration than the external environment. Because water moves from low to high concentration, a large amount of water is lost from the bacteria, changing the internal biochemical reaction environment, ultimately disrupting the biochemical reaction process until it stops, resulting in cell death.
2. Interference and blockage of microbial substance absorption, 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 and substance purity. The addition of salt interferes with or blocks the bacteria's absorption environment, ultimately inhibiting or killing bacterial life activity. This situation varies greatly depending on the individual bacteria, species, type of salt, and salt concentration.
3. 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 changes in its properties, preventing it from protecting or absorbing beneficial substances, thus inhibiting bacterial life activity or causing cell death. 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.
According to the "Standard for Wastewater Discharge into Urban Sewers" (CJ-343-2010), wastewater discharged into urban sewers for secondary treatment must meet the requirements of Class B (Table 1), including 600 mg/L chloride and 600 mg/L sulfate.
According to Appendix III of the "Outdoor Drainage Design Code" (GBJ 14-87) (GB50014-2006 and the 2011 edition do not have specific instructions on salinity), 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 exceeds 8000 mg/L, sludge volume expansion occurs, with a large amount of foam appearing on the surface, and microorganisms die successively.
Normally, we consider that a chloride ion concentration greater than 2000 mg/L and salinity 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, salinity of 3%-4% can achieve stable standards using the activated sludge method (there are cases of 5% successful debugging in the community). It's important to remember that the influent salinity must be stable and not fluctuate excessively, otherwise the biological system will collapse!
1. Acclimation of activated sludge
Under conditions where salinity is less than 2 g/L, saline wastewater can be treated through acclimation. By gradually increasing the salinity of the biochemical influent, microorganisms can balance the osmotic pressure within the cells or protect the protoplasm through their own osmotic pressure regulation mechanisms. These regulatory mechanisms 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 treat high-salt wastewater with a certain salinity concentration through a certain period of acclimatization. Although acclimatization can improve the salt tolerance range and treatment efficiency of the activated sludge system, the salt tolerance range of microorganisms in the acclimatized 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. Acclimatization is only a temporary physiological adjustment of microorganisms to the environment and does not have genetic characteristics. This sensitive adaptability is very unfavorable for wastewater treatment.
The acclimatization time of activated sludge is generally 7-10 days. Acclimatization can improve the tolerance of sludge microorganisms to salt concentration. In the initial stage of acclimatization, the activated sludge concentration decreases due to the toxicity of the salt solution to microorganisms, resulting in the death of some microorganisms and negative growth. In the later stage of acclimatization, microorganisms adapted to the changed environment begin to reproduce, thus increasing the activated sludge concentration. 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 initial and later stages of acclimatization are 60%, 80% and 40%, 60%, respectively.
To reduce the salt concentration entering the biological system, the influent can be diluted to make the salt content lower than the toxic threshold, so that 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.
3. Select salt-tolerant bacteria
Salt-tolerant bacteria are a general term for bacteria that can tolerate high concentrations of salt. In industry, they are mostly screened and enriched obligate strains. Currently, the highest salt tolerance is about 5%, which can operate stably, and it is also a kind of biochemical treatment method for high-salt wastewater!
4. Select a reasonable process flow
Select different treatment processes according to different chloride ion content, and appropriately select anaerobic processes to reduce the chloride ion 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.
