influence mechanism of chloride ion
the toxic effect of high concentration chloride ion on biological treatment of wastewater is mainly to destroy the cell membrane and enzymes of microorganisms by increasing environmental osmotic pressure, thus destroying the physiological activities of microorganisms. microorganisms grow well under isoosmotic pressure, such as microorganisms in 5-8.5g/l nac1 solution; at low osmotic pressure (p (nac1) = 0.1g/l), a large number of water molecules infiltrate into microorganisms, causing the expansion of microbial cells, serious rupture, leading to microbial death; at high osmotic pressure (p (nac1) = 200g/l), micro-organisms. a large number of water molecules in the body infiltrate into the body (i.e. dehydration), resulting in cell wall separation. the unit structure of microorganism is cell, cell wall is equivalent to semi-osmotic membrane. when the concentration of chloride ion is less than or equal to 2000mg/l, the osmotic pressure of cell wall is 0.5-1.0 atmospheric pressure. even if the cell wall and plasma membrane have certain toughness and elasticity, the osmotic pressure of cell wall can not be greater than 5-6 atmospheric pressure. however, when the concentration of chloride ion in the aqueous solution is above 5000mg/l, the osmotic pressure will increase to 10-30 atmospheric pressure. under such a large osmotic pressure, water molecules in the microorganism will permeate into the solution in vitro, resulting in cell dehydration and plasmolysis, and serious microbial death. engineering experience data show that when the chlorine concentration in wastewater is greater than 2000mg/l, the activity of microorganism will be restrained and the cod removal rate will obviously decrease. when the concentration of chloride ion in wastewater is greater than 8000mg/l, it will cause sludge volume expansion, and the water surface will overflow a large number of bubbles, and the microorganism will die one after another.
inhibition of sludge activity
when the chloride ion concentration in the biochemical system changes dramatically, the carbonization and nitrification performance of the sludge will soon weaken or even disappear, leading to a significant decline in cod removal rate, nitrite accumulation in the nitrification process, even if the dissolved oxygen in the sewage is increased, the effect is not obvious. that is to say, activated sludge has a certain tolerance for chloride concentration, when the chloride concentration exceeds a certain value, the degradation capacity of the system will decline until the system loses treatment capacity. the sudden change of chloride ion is more disturbing than that of chloride ion. with the increase of chloride ion, the degradation rate of organic matter decreases, so low f / m (the ratio of nutrient to activated sludge in quality) is more suitable for the treatment of wastewater containing chloride ion. chloride ions change the composition of microorganisms in sludge, change the sedimentation of sludge and effluent ss, resulting in serious sludge loss, the concentration of activated sludge decreased, sludge index increased, 30 minutes sedimentation rate decreased. the results of microscopic examination of activated sludge showed that there were abundant organisms in the activated sludge at low salinity. there were many kinds of filamentous bacteria, bacterial micelles and protozoa. the activated sludge particles were very large, the bacterial micelles were closed and the flocs had a certain degree of compactness. with the increase of chloride ion concentration, when the chloride ion mutation increased from 150 mg/l to 1000 mg/l, filamentous bacteria and protozoa basically did not exist, and the bacterial micelles became more dense, at this time the flocs became small and abnormally tight. the degradation of organic matter in sewage mainly depends on the interaction of a large number of microorganisms in the sewage. the increase of chloride ion leads to the decrease of the number of microorganisms in the activated sludge, which makes the degradation rate of organic matter decrease.
how to eliminate the influence of chloride ion?
1. domestication of activated sludge
by gradually increasing the chloride content in biochemical influent, microorganisms can balance the osmotic pressure or protect the protoplasm of cells through their own osmotic pressure regulation mechanisms, which include aggregating low molecular weight substances to form a new extracellular protective layer, regulating their own metabolic pathways, and changing the genome composition. therefore, normal activated sludge can be domesticated to treat high chloride ion wastewater within a certain range of chloride ionicity for a certain period of time. although domesticated activated sludge can improve the chloride ion tolerance range and improve the treatment efficiency of the system, the tolerance range of microorganisms in domesticated activated sludge to chloride ion is limited. and sensitive to environmental changes. when the chloride environment suddenly changes, the adaptability of microorganisms will disappear immediately. acclimation is only a temporary physiological adjustment for microorganisms to adapt to the environment, without genetic characteristics. this sensitivity of adaptability is unfavorable to sewage treatment. the acclimation time of activated sludge is usually 7-10 days. acclimation can improve the tolerance of sludge microorganisms to salt concentration, and decrease the concentration of activated sludge at the initial stage of acclimation. it is because the increase of salt solution produces toxicity to microorganisms, which causes some microorganisms to die and shows negative growth. in the late stage of acclimation, microorganisms adapted to the environment begin to multiply. therefore, the concentration of activated sludge increased. taking the removal of cod by activated sludge in 1.5% and 2.5% nacl solution as an example, the removal rates of cod in the early and late domestication stages were 60%, 80% and 40%, 60% respectively.
2. dilute high chloride concentration wastewater.
in order to reduce the concentration of chloride ion in the biochemical system, the influent water can be diluted so that the chloride ion is lower than the toxic threshold value, and the biological treatment will not be inhibited. it has the advantages of simple method, easy operation and management, but the disadvantages are increased treatment scale, infrastructure investment and operating costs. for yangli sewage treatment plant, because of the large inflow and continuous operation, even though the chloride concentration is high at a certain time measured by on-line instruments, the operability of targeted dilution is poor. therefore, the method is more suitable for the factories and enterprises that produce high chloride concentration wastewater.
3. choose reasonable technological process to choose different treatment process according to different concentration of chloride ion content, and select appropriate anaerobic technological process to reduce the tolerance of aerobic section in the post-sequence.
