i. short-cut nitrification and denitrification
1, introduction
biological denitrification includes nitrification and denitrification. the first step is the nitrification process in which nitrite bacteria oxidize nh4 -n to no2-n; the second step is the process in which nitrifying bacteria oxidize no2-n to no3-n; and then the nitrate-n is transformed into n2 through no2-n through denitrification, and no2-n is the intermediate product of nitrification and denitrification. in 1975, voets et al. discovered the accumulation of no2-n in the nitrification process in the treatment of high concentration ammonia-nitrogen wastewater. the concept of short-cut nitrification and denitrification was proposed for the first time. as shown in figure 1.
comparing the two approaches, it is obvious that short-cut nitrification and denitrification can reduce the two-step reactions of no2-, no3-and no3-, no2-than full-course nitrification and denitrification, which makes short-cut nitrification and denitrification biological denitrification have the following advantages:
1. oxygen supply can be saved by 25%. the amount of oxygen that no2-oxidizes to no3-is saved.
2. carbon source can be saved by 40% in denitrification stage. the removal rate of tn was increased under certain c/n ratio. and it can save the amount of alkali.
3. because the generation cycle of nitrosobacteria is shorter than that of nitrifying bacteria, controlling the nitrification stage can increase the nitrification reaction speed and microbial concentration, shorten the nitrification reaction time, and because of the short hydraulic retention time, the volume of the reactor can be reduced by 30%~40% in general.
4. short-cut nitrification and denitrification process can reduce sludge production by 25%~34% in nitrification process and 50% in denitrification process.
because of the above advantages, the short-cut nitrification-denitrification process is especially suitable for wastewater with low c/n ratio, i.e. high ammonia nitrogen and low cod, which can save both power cost and supplementary carbon source cost, so the process is very feasible in coal chemical wastewater.
2. factors affecting short-cut nitrification and denitrification
effect of 2.1 temperature
temperature has a great influence on microorganisms. the optimum temperature of nitrite bacteria and nitrite bacteria is different. short-cut nitrification and denitrification can be realized by adjusting temperature to inhibit the growth of nitrite bacteria without inhibiting nitrite bacteria. the domestic research shows that the short-cut nitrification and denitrification process can be carried out stably only when the temperature of the reactor exceeds 28 c.
2.2 effect of ph value
when the ph is low, ammonia ion and nitrite are more in the water, which is conducive to the nitrification process, and there is no nitrite accumulation at this time; but when the ph is high, nitrite can be accumulated. therefore, the appropriate ph environment is conducive to the growth of nitrosobacteria. ph also affects the concentration of free ammonia and the activity of nitrite bacteria. the results show that the optimum ph value of nitrite bacteria is around 8.0 and that of nitrifying bacteria is around 7.0. therefore, the best ph value to realize the accumulation of nitrite bacteria is about 8.0.
2.3 effect of dissolved oxygen (do)
do plays an important role in controlling nitrite accumulation. nitrification and nitrification are both aerobic processes, and there are differences in kinetics between nitrite bacteria and nitrate bacteria: under low do conditions, nitrite bacteria have stronger affinity for do than nitrate bacteria. by controlling do, the nitrification process can be carried out only to the stage of ammonia-nitrogen oxidation to nitrite-nitrogen, thus eliminating nitric acid bacteria and achieving the purpose of short-cut nitrification.
effect of 2.4 sludge age
the nitrification rate of ammonia nitrogen is faster than that of nitrite nitrogen, and the generation cycle of nitrite bacteria is shorter than that of nitrite bacteria. therefore, the sludge age can be controlled between the minimum residence time of nitrite bacteria and nitrite bacteria by controlling hrt, so that nitrite bacteria can become dominant strains and gradually eliminate nitrite bacteria.
ii. simultaneous nitrification and denitrification
1, introduction
according to the traditional biological denitrification theory, the denitrification process generally includes two stages: nitrification and denitrification. the two processes need to be carried out in two isolated reactors, or in the same reactor which causes alternating anoxic and aerobic environments in time or space. in fact, in the earlier stage, in some activated sludge processes without obvious anoxic and anaerobic stages. nitrogen loss due to non-assimilation has been observed many times in the layer, and nitrogen loss has also been observed many times in the aeration system. in these treatment systems, nitrification and denitrification often occur under the same treatment conditions and in the same treatment space. therefore, these phenomena are called simultaneous nitrification/denitrification (snd).
snd phenomena in various treatment processes have been reported extensively, including biological rotating disc, continuous flow reactor and sequencing batch sbr reactor. compared with traditional nitrification-denitrification process, snd has the following advantages:
1. it can effectively keep the ph of the reactor stable and reduce or cancel the addition of alkalinity.
2. reduce the volume of traditional reactor and save the cost of capital construction.
3. for a sequencing batch reactor consisting of only one reactor cell, snd can reduce the time required to realize nitrification-denitrification.
4. the saving of aeration can further reduce energy consumption.
therefore, snd system provides the possibility of reducing investment and simplifying biological nitrogen removal technology in the future.
2. study on the mechanism of simultaneous nitrification/denitrification
2.1. macro-environment
do in bioreactor is mainly obtained by aeration equipment. no matter what aeration device, the oxygen in the reactor can not be fully mixed in sewage. finally, the anoxic and aerobic sections in different areas of the reactor were formed, which provided an advantageous environment for denitrifying bacteria and nitrifying bacteria respectively, resulting in simultaneous nitrification and denitrification in fact. in addition to the heterogeneity of dissolved oxygen in different space of the reactor, the change of dissolved oxygen in the reactor at different time points can also lead to simultaneous nitrification/denitrification. hyungseok yoo studied the phenomenon of simultaneous nitrification/denitrification in sbr reactor during the aeration reaction stage. do concentration in sbr reactor increased gradually after decreasing.
2.2. microenvironment theory
the theory of anoxic microenvironment has been widely accepted as one of the main reasons for simultaneous nitrification/denitrification. the basic viewpoint of this theory is that in activated sludge flocs, oxygen is present at different levels, from the surface of flocs to the core, due to the limitation of oxygen transfer.
2.3 biological explanation
traditional theory holds that nitrification can only be accomplished by autotrophic bacteria and denitrification can only be carried out under anoxic conditions. in recent years, the existence of aerobic denitrifying bacteria and heterotrophic nitrifying bacteria has been confirmed.
3. influencing factors of simultaneous nitrification and denitrification
the key to realize snd lies in the cultivation and control of nitrifying and denitrifying bacteria. at present, domestic and foreign studies believe that the factors affecting nitrifying and denitrifying bacteria are as follows.
3.1. dissolved oxygen
the effect of do on simultaneous nitrification and denitrification is very important. the research shows that the optimal effect can be achieved by controlling do concentration and making the nitrification rate basically consistent with the denitrification rate.
3.2. organic carbon sources
the influence of organic carbon sources on the whole simultaneous nitrification and denitrification system is particularly important. the results showed that denitrification could not meet the requirement if the content of organic carbon source was low, while the removal of ammonia nitrogen was unfavorable if the content of organic carbon source was high.
3.3. microbial floc structure
the structure of microbial flocs not only affects the diffusion of do in bioflocs, but also affects the distribution of carbon sources. the proper size and density of flocs are beneficial to simultaneous nitrification and denitrification. the results showed that the simultaneous nitrification and denitrification ability of microbial flocs increased with the increase of the size of activated sludge flocs.
3.4, ph value
the optimum simultaneous nitrification and denitrification value is about 7.5. the optimum ph of nitrifying bacteria was 8.0-8.4, while that of denitrifying bacteria was 6.5-8.0.
3.5 temperature
the optimum temperature for simultaneous nitrification and denitrification is 10-20 c. the performance of nitrifying bacteria decreases at 20-25 (?) c, whereas nitrosation decreases. nitrification performance is the highest at 25 c. nitrite bacteria were significantly inhibited by free ammonia at 25 c.
source: polaris water treatment network
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