Table 2- 19 oxidation path and reaction free energy table
(According to Wang Jianlong 1997)
According to Table 2- 18, nitrogen mainly exists in the form of organic nitrogen and ammonia nitrogen, in which organic nitrogen accounts for 37% ~ 4 1% of total nitrogen, ammonia nitrogen accounts for 59% ~ 63% of total nitrogen, and nitrite nitrogen and nitrate nitrogen are very few. The ammonia nitrogen in the seven major water systems of the Yangtze River, Pearl River, Yellow River, Songhua River, Liaohe River, Huaihe River and Haihe River generally exceeds the standard, and nitrite pollution still exists in the Yangtze River and Huaihe River. In order to explore the influence of heavy metals in sewage river on groundwater, lead nitrate was added to the indoor test to increase the content of nitrate nitrogen in the test water. This experiment mainly studied the migration and transformation law of nitrate nitrogen, ammonia nitrogen and total nitrogen, and discussed them in turn by stages.
(A) the migration and transformation of NO3-N
According to the data in the second section of this chapter, from the comparison of effluent concentration and influent concentration of NO3-N, all three columns have gone through the same change process, that is, they have gone through the following three stages: effluent concentration is greater than influent concentration, effluent concentration is slightly less than influent concentration, and effluent concentration is much less than influent concentration. For example, the effluent concentration of 1 ~ 20 days is higher than that of the influent, the effluent concentration of 1 ~ 43 days is slightly lower than that of the influent, and the effluent concentration of 1 ~ 225 days is 5 times lower than that of the influent. 2. The effluent concentration of column 3 at 1 ~ 1 1d is higher than the influent concentration; the effluent concentration of column 2 at 12 ~48d is lower than the influent concentration 1 ~ 3 times; and the effluent concentration of column 3 at 49 ~ 265438 is lower than the influent concentration/kloc-. From the removal rate of NO3-N in the effluent of the three columns, the removal rate of NO3-N in the effluent of the three columns was negative before column 1 was 3 1d, column 2 was 22d, and column 3 was 15d. After that, the removal rate gradually increased, and the removal rate of column 1 was mostly above 70%.
In the first stage, the effluent concentration of NO3-N is higher than the influent concentration, and the removal rate is negative. This stage mainly occurs in the early stage of the experiment. Because the soil column is not blocked, the sewage flow rate is fast, and a small amount of dissolved oxygen is brought into the soil column with the sewage, leading to nitrification, and some ammonia nitrogen is converted into NO3-N. At the same time, it is not excluded that a small amount of NO3-N originally existing in sand is brought out with the sewage.
Nitrification refers to the process that autotrophic microorganisms oxidize NH4-N to NO3-N.. The reaction is carried out in two steps. The first step is to transform NH4-N into NO2-N by nitrifying bacteria, and the second step is to transform NO2-N into NO3-N by nitrifying bacteria, namely
Study on pollution removal mechanism of river infiltration system
Microorganisms involved in nitrification are mainly Mucor nitrosobacteria, nitrosobacteria cocci and nitrifying bacteria, which are collectively called nitrifying bacteria. The growth of nitrifying bacteria is related to the concentration of NH4-N, dissolved oxygen (DO) and carbon source in water, but generally speaking, carbon source is not enough to be the limiting factor for the growth of nitrifying bacteria. Therefore, the growth of nitrifying bacteria is only related to the concentration of NH4-N and DO, and its growth rate conforms to the double Monod model, namely
Study on pollution removal mechanism of river infiltration system
Where: μ is the specific growth rate of nitrifying bacteria, (1/d); μmax is the maximum specific growth rate of nitrifying bacteria, (1/d); SN is NH4-N concentration, mg/L; The same is true for dissolved oxygen concentration, mg/l; KN is the saturation constant of ammonia nitrogen, mg/l; KO is the saturation constant of dissolved oxygen, mg/l.
The factors affecting nitrification mainly include dissolved oxygen, temperature, pH, substrate concentration and poisons. The temperature range of nitrification is 5 ~ 50℃, and the optimum temperature is 30 ~ 35℃. Eh & gt250 ~ 300 mv produces nitrification (Shen et al., 1993). At this stage, the effluent concentrations of NO3-N in the three columns are 1 1.0 ~ 55.2 mg/L, 20.0 ~ 23.0 mg/L and 4.0 ~ 16.5mg/L, respectively. It can be seen that 1 >: column 2 >: column 3. In this experiment, under the same conditions as other influencing factors, the main factor affecting nitrification should be the amount of dissolved oxygen entering the soil column with sewage. At this stage, the flow rate and seepage velocity of sewage passing through the soil column are as follows: 1 ~ 3 1d column flow rate is 3 13 ~ 567 ml/h, and the flow rate is 0.426 ~ 0.770 m/d; The flow rate of column 2 at 1 ~ 22d is 62 ~ 1 14 ml/h, and the flow rate is 0.084 ~ 0.173m/d; On 15d, the velocity of three columns is 46 ~ 120ml/h and 0.063 ~ 0. 163m/d, and the velocity and seepage velocity of sewage flowing through 1 column are higher than those of two columns and three columns, so more oxygen can enter/kloc with sewage. On the other hand, according to the properties of the permeable medium, 1 column is coarse sand, with a large content of coarse particles, a small content of fine particles and a small uneven coefficient. The gap connectivity between particles in the medium is better than that of columns 2 and 3, which creates a smooth passage for oxygen to enter the soil column, so the nitrification of 1 column is stronger than that of columns 2 and 3. The duration of this stage is very short. For example, column 1 is 20d, and columns 2 and 3 are only11d.. The length of nitrification time mainly depends on the water quality of sewage river and the nature of osmotic medium in the lower part of river bed.
In the second stage, the effluent concentration of NO3-N was slightly lower than the influent concentration, and the effluent removal rate began to change from negative to positive, but the removal rate was not large. As pollutants are continuously brought into the soil column with sewage, more and more gaps in the soil column are blocked by the interception, adsorption and precipitation of pollutants, and less dissolved oxygen enters the medium with sewage, and the inside of the medium gradually changes from aerobic environment to micro-aerobic or anaerobic environment, nitrification becomes weaker and weaker, and denitrification begins to occur. This stage is also very short, for example, 1 column is 22d, and 2 columns and 3 columns are 36d.
In the third stage, the concentration of NO3-N in effluent is much less than that in influent, and the removal rate of effluent is much higher than that in the second stage. With the extension of seepage time, the blockage in the soil column becomes more and more serious, and the oxygen that can enter the soil column becomes less, and the interior of the medium basically becomes a micro-oxygen or anaerobic environment. At this time, NO3-N is mainly removed by denitrification. This stage lasted for a long time. The column 1 is 18 1d, and the columns 2 and 3 are 167d.
Denitrification refers to the process that NO3-N is reduced to gaseous nitrogen (N2, N2O) by microorganisms. The microorganisms involved in this process are usually heterotrophic bacteria, and the energy needed for cell synthesis mainly comes from organic carbon, which is suitable for anaerobic environment. The reaction formula is as follows:
Study on pollution removal mechanism of river infiltration system
The above formula can be decomposed into the following three main reactions:
Study on pollution removal mechanism of river infiltration system
If the carbon source is glucose (C6H 12O6), denitrification can be expressed as follows:
Study on pollution removal mechanism of river infiltration system
Therefore, in the process of denitrification, organic matter and organic matter serve as substrates of denitrifying bacteria at the same time, and its reaction kinetics can be described by double Monod model, that is,
Study on pollution removal mechanism of river infiltration system
Where: vDN is the denitrification rate, (mgNO3-n/mgvss h); K is the maximum denitrification rate, (mgNO3-n/mgvss h); K 1 is the Michaelis constant of organic matter, mg/l; K2 is the Michaelis constant of NO3-n, mg/l; S 1 is the concentration of organic matter (BOD5), mg/l; S2 is the concentration of NO3-n, mg/L. ..
The main factors affecting denitrification are the type and concentration of organic carbon source, nitrate concentration, dissolved oxygen, temperature, pH and poisons. The temperature range of denitrification is 3 ~ 85℃, and the optimum temperature is 35 ~ 65℃. Eh< denitrification occurs at 250 ~ 300 mv. Some scholars believe that N2O is the main product of denitrification. Only when pH > 7:00, N2O can be rapidly reduced to N2, pH.
To sum up, because nitrification is very short, in the initial stage of sewage river formation, it only plays a certain role in the removal of NO3-N in the osmotic medium at the lower part of the river bed, and NO3-N is mainly removed by denitrification.
(2) Migration and transformation of ammonia nitrogen
The migration and transformation of ammonia nitrogen in the three columns also went through three stages, among which column 2 and column 3 were discussed separately because of their similar media properties and changing rules.
1 column experienced three stages: before 17d, the effluent concentration was less than the influent concentration, and the effluent concentration gradually increased with time, and ammonia nitrogen basically permeated at 17d; From 18 to 140 d, the effluent concentration is still less than the influent concentration, but the influent and effluent concentrations are very close, and the removal rate is mostly less than 10%. From 14 1 to 225 d, the effluent concentration is greater than the influent concentration. In the first stage, ammonia nitrogen adsorption and nitrification occurred at the same time, which had an opposite effect on the change of ammonia nitrogen concentration in effluent. From the beginning of adsorption to the gradual saturation of adsorption, the concentration of ammonia nitrogen in effluent will gradually increase, while nitrification will reduce the concentration of ammonia nitrogen. Therefore, this stage is mainly the process of ammonia nitrogen reaching adsorption saturation and infiltration, and it will also be accompanied by nitrification, but nitrification does not play a leading role in the change of ammonia nitrogen concentration; In the second stage, from the analysis of the migration and transformation law of NO3-N, it can be seen that nitrification mainly occurs at 1 ~ 20d, and transits from nitrification to denitrification at 2 1 ~ 43d, so there will be weak nitrification in the early stage of this stage, and there may be a small amount of adsorption in the later stage, resulting in a low ammonia nitrogen removal rate at this stage. In the third stage, ammonia nitrogen reaches adsorption saturation desorption, so the effluent concentration of ammonia nitrogen is greater than the influent concentration.
The second column and the third column go through three stages: the second column is at 1 ~ 1~96d, and the third column is at 1~96d. The effluent concentration is very small, which is more than 10 times lower than the influent; The effluent concentrations of two columns with 107 ~ 14 1d and three columns with 97 ~ 13 1d decreased by 1 ~ 2 times and 2 ~ 3 times respectively. At 142 ~ 2 16d, the effluent concentration of column 2 is higher than the influent concentration, while that of column 3 is at 132 ~2 16d+06d, but the increase is not significant. In the first stage, because the No.2 and No.3 columns are medium sand, the clay content is large (see Table 2-4), and the adsorption effect is remarkable. In addition, nitrification in the early stage of this stage makes the concentration of ammonia nitrogen in the effluent very small and the removal rate is very high (> 90%); In the second stage, with the extension of seepage time, the ammonia nitrogen adsorption gradually became saturated, so the ammonia nitrogen concentration in the effluent of Column 2 and Column 3 began to increase until it reached adsorption saturation and infiltration at 13 1d and 14 1d respectively. In the third stage, ammonia nitrogen desorption also occurred.
Study on pollution removal mechanism of river infiltration system
In the process of moving down with water, it may be adsorbed on its surface by osmotic medium, which belongs to cation adsorption (exchange) and is reversible. The adsorption capacity in soil is related to CEC in soil and AAR (ammonia adsorption ratio) in water. The mathematical expression of AAR is as follows: Study on pollution removal mechanism of river infiltration system.
Among them, Ca2 ++ and Mg2 ++ are the concentrations of corresponding ions in water, meq/L.
The relationship between AAR and EAR (exchangeable ammonia ratio) follows the following regression equation:
Study on pollution removal mechanism of river infiltration system
Where: EAR is the exchangeable ammonia ratio, dimensionless; XNH4 is exchangeable ammonia in soil, meq/100g; CEC is the cation exchange capacity, meq/ 100g. The conversion formula (2- 15) is available.
Study on pollution removal mechanism of river infiltration system
Table 2-20 Calculation Table of Ammonia Adsorption Capacity
The author calculated the adsorption capacity of ammonia nitrogen by three columns. According to the test, the average concentrations of Ca2+ and Mg2+ in domestic sewage are 1.955meq/L, 1.475meq/L and 3.5 1meq/L, respectively. Substituting them into formula (2- 13), the ammonia adsorption ratio of water is obtained. Substituting the formula (2- 14) to obtain the exchangeable ammonia ratio EAR of 0.5 19, the exchangeable ammonia xNH4 in soil can be obtained by the formula (2- 16), and the ammonia adsorption capacity of soil column can be obtained by multiplying xNH4 by the soil loading of soil column. Finally, the adsorption saturation of the soil column can be deduced by dividing the ammonia adsorption capacity of the soil column by the concentration in the sewage. As can be seen from Table 2-20, the seepage velocity of sewage in Column 2 and Column 3 is relatively slow, and the hydraulic retention time is relatively long, so the adsorption reaction of ammonia is relatively complete, and the calculated value of influent is not much different from the measured value when the adsorption saturation is reached. However, because the seepage medium of 1 column is coarse sand, the sewage flow rate is fast, and the ammonia adsorption reaction is not fully carried out at the initial stage of the test, and the calculated value is quite different from the measured value. Each medium has a certain adsorption capacity, and when it reaches adsorption saturation, nitrogen removal by adsorption will soon fail, which is also confirmed by previous experimental results. When ammonia penetrates, ammonia desorption occurs. It can be seen that ammonia nitrogen is easy to migrate in both coarse sand and medium sand, which poses a threat to groundwater. However, due to the large particle size of coarse sand and small nonuniformity coefficient, the seepage velocity of sewage passing through coarse sand is fast, and ammonia nitrogen enters groundwater faster. In medium sand, it takes enough time for ammonia nitrogen to enter the groundwater after reaching adsorption saturation, which will pollute the groundwater.
(III) Migration and transformation of total nitrogen
In fact, the migration and transformation law of total nitrogen is a comprehensive reflection of the migration and transformation law of ammonia nitrogen and NO3-N, and it is the result and embodiment of adsorption, nitrification and denitrification. As the migration and transformation of ammonia nitrogen and NO3-N have been discussed in depth, the total nitrogen is not detailed here.
For sewage rivers with a history of more than ten to several decades, the adsorption is very short, and the main mechanism of denitrification should be nitrification and denitrification under the action of microorganisms. The indoor test shows that only at the initial stage of sewage river formation, because the osmotic medium at the lower part of the river bed is not blocked by pollutants and has good permeability, O2 can enter the osmotic medium with sewage, resulting in nitrification, and soon the inside of the osmotic medium becomes a micro-aerobic or anaerobic environment. The longer the sewage river is formed, the thicker the sediment formed at the bottom of the river bed, and the higher the anaerobic degree in the osmotic medium at the lower part of the river bed. Denitrification is the main feature of nitrogen transformation in the process of sewage river infiltration.