How much salt does sewage contain that can enter the biochemical system?

According to the Water Quality Standard for Sewage Discharged into Urban Sewers (CJ-343-2010), when entering the sewage treatment plant for secondary treatment, the water quality of the sewage discharged into the urban sewers should meet the provisions of Class B (Table 1), including 600mg/L chloride and 6000 mg/L sulfate. According to the "Outdoor drainage Design Code" (GBJ 14-87) (GB50014-2006 and 2011 versions do not specify salt) Appendix 3 "Allowable concentration of harmful substances in the water of biological treatment structures", the allowable concentration of sodium chloride is 4000mg/L.

Under normal circumstances, we believe that salt less than 2% (equivalent to 2000mg/L) does not affect the treatment effect of the biochemical system, it can use the ordinary activated sludge method, but if the domestication is reasonable, the salt of 3%-4% using the activated sludge method to stabilize the standard has also been encountered, but remember one point, the influent salt should be stable, not fluctuate too much. Otherwise the biochemical system can't withstand a breakdown!

Effect of high salt wastewater on activated sludge

1. When the concentration of death salt is high, the change of osmotic pressure is the main cause. The interior of bacteria is a semi-closed environment, and it must exchange favorable substances and energy with the external environment to maintain its life activity, but it must also prevent the entry of most of the external substances to avoid interference and obstruction of its internal biochemical reactions. The increase of salt concentration leads to the lower concentration of the solution inside the bacteria than the outside world, and due to the characteristics of water moving from low concentration to high concentration, a large amount of water loss in the bacteria causes changes in its internal biochemical reaction environment, and eventually destroys its biochemical reaction process until it is interrupted and bacteria die.

2, so that the absorption process of microbial substances is disturbed by blocking the dead cell membrane has the characteristics of selective penetration, in order to filter the substances harmful to the life activities of bacteria and absorb the substances beneficial to their life activities. This absorption process is directly affected by the concentration of solution and the purity of substance in the external environment, and the addition of salt leads to the interference or blocking of the bacterial absorption environment, and ultimately leads to the inhibition of bacterial life activity or even death. This situation varies greatly due to the individual conditions of bacteria, varieties, types of salt and salt concentration.

3, the microbial poisoning death Some salts will enter the bacteria with the life activities of the bacteria, destroy its internal biochemical reaction process, some will interact with the bacterial cell membrane, resulting in its nature change and no longer play a protective role or can no longer absorb some beneficial substances for bacteria, resulting in the suppression of bacterial life activity or cell death. Among them, heavy metal salt is the representative, and some sterilization methods use this principle.

Studies have shown 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 changes of the growth curve are as follows: the adaptation period becomes longer; The growth rate slows down in the logarithmic growth period. The duration of deceleration growth period becomes longer.

2. Salinity enhances microbial respiration and cellular lysis.

3. Salinity reduces the biodegradability and degradability of organic matter. The removal rate and degradation rate of organic matter decreased.

Process selection of high salt wastewater

1. Acclimation of activated sludge Under the condition of salinity less than 2g/L, salt-containing sewage can be treated by acclimation. By gradually increasing the biochemical influent salinity, microorganisms will balance the osmotic pressure within the cell or protect the protoplasm within the cell through their own osmotic pressure regulation mechanisms, including aggregation of low molecular weight substances to form a new extracellular protective layer, regulation of their own metabolic pathways, and change of genetic composition. Therefore, normal activated sludge can be acclimated to treat high-salt wastewater within a certain salt concentration range for a certain period of time. Although the acclimation of activated sludge can improve the salt tolerance range of the system and improve the treatment efficiency of the system, the microbial tolerance range of domesticated activated sludge is limited and it is sensitive to environmental changes. When the chloride environment suddenly changes, the microbial adaptation will immediately disappear. Domestication is only a temporary physiological adjustment of microorganisms to adapt to the environment, and does not have genetic characteristics. This adaptive sensitivity is detrimental to wastewater treatment. The acclimation time of activated sludge is generally 7-10d, and acclimation can improve the tolerance of sludge microorganisms to salt concentration. In the early stage of acclimation, the activated sludge concentration decreased because the increase of salt solution poisoned microorganisms, causing some microorganisms to die and showing negative growth. In the later stage of acclimation, microorganisms adapted to the changed environment began to multiply, so the activated sludge concentration increased. Taking the COD removal of activated sludge in 1.5% and 2.5% sodium chloride solution as an example, the COD removal rates in the early and late acclimation stages were 60%, 80% and 40%, 60%, respectively.

2, dilute influent In order to reduce the salt concentration of the biochemical system, the influent can be diluted so that the salt is lower than the toxic domain value, 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 processing scale, infrastructure investment and operating costs.

3, the selection of salt-tolerant bacteria Salt-tolerant bacteria is a general term for bacteria that can tolerate high concentration of salt. In the industry, most of them are specialized bacteria that are screened and enriched. At present, the highest salt can tolerate about 5% and can run stably, which is also a biochemical means of treating high-salt wastewater!

4, Select a reasonable process according to different concentrations of chloride ion content to select different treatment processes, appropriate selection of anaerobic process to reduce the tolerance of chloride ion concentration range of the subsequent aerobic section. 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 the cultivation of salt-bearing bacteria, are difficult to operate in industrial practice.