Do you want to talk about the process of aerobic activated sludge treating domestic sewage? Still want to ask the specific data of water?
Activated sludge process is the main process of urban sewage treatment. There are three design and calculation methods: sludge loading method, sludge age method and mathematical model method. The three methods have different operation difficulty and calculation accuracy, which are directly related to the design level, infrastructure investment and processing reliability. Because of this, experts at home and abroad are doing a lot of detailed research to find a more accurate and easy-to-operate calculation method.
1 sludge loading method
This is the most popular design method at home and abroad. For decades, thousands of sewage treatment plants have adopted this method to design, which fully demonstrates its correctness and applicability. On the other hand, this method also has some problems, even serious defects, which affect the accuracy and operability of the design.
The calculation formula of sludge loading method is [1]:
v = 24LjQ/ 1000 fwnw = 24LjQ/ 1000 fr( 1)
Sludge load method is an empirical calculation method, and its basic parameters Fw (sludge load of aeration tank) and Fr (volume load of aeration tank) are set according to the previous experience and the types of aeration. Because of the different requirements of water quality and treatment, the setting of these two basic parameters can only give a wide range. For example, the recommended values of general aeration in China Code are:
fw = 0.2 ~ 0.4kg BOD/(kg mlss·d)
Fr = 0.4 ~ 0.9 kg BOD/(m3 tank volume d)
It can be seen that the maximum value is more than twice the minimum value, and the range is very wide. When other conditions remain unchanged, the volume of aeration tank calculated by selecting the minimum value is twice or more than that by selecting the maximum value, and the capital investment is quite different. It is difficult for designers with less experience to operate the values in this range, which is a big defect of sludge loading method.
Another problem of sludge loading method is that units are easily confused. For example, the unit of Fw in China's design code is KGBOD/(KGMLSS d), but it is KGBOD/(KGMLVSS d) in the design manual, which is quite different. MLSS is the concentration of sludge including inorganic suspended solids, while MLVSS is only the concentration of organic suspended solids. For domestic sewage, MLVSS is generally 0.7 MLSS. If the device is not used properly, the calculated aeration tank volume will be 30% worse. This confusion is not impossible. For example, the Fw of general aeration recommended in China's design manual is 0.2 ~ 0.4 kgbod/(kgmlvss d) [2], which is exactly the same as the design specification, but the unit is different. In design, we often encounter the problem of not knowing which unit to use, especially in the case of insufficient design experience. In addition, in recent years, sewage denitrification has also been put on the agenda. When sewage needs nitrification and denitrification, how much Fw and Fr should be taken?
The most fundamental problem of sludge loading method is that it does not consider the difference of sewage quality. For domestic sewage, the concentrations of SS and BOD are almost the same, and the ratio of MLSS to MLVSS is similar. However, combined with the actual situation in various places, urban sewage generally contains 50% or even more industrial wastewater, so the quality of sewage varies greatly. Some SS and BOD values are as high as 300 ~ 400 mg/L, some as low as 100mg/L, and some sewage SS/BOD ratios are different. Sludge load is based on MLSS, and the proportion of organic matter cannot be reflected. For two factories with the same scale, the same process and the same influent BOD concentration, the volume of aeration tank calculated by sludge loading method is the same, but when the SS/BOD value is very different, the MLVSS is also very different, the actual biological environment is very different, and the treatment effect is obviously different.
To sum up, the sludge loading method needs to be improved. Therefore, IAWQ organized experts from all over the world to launch the first activated sludge model (ASM 1) [3] in 1986, and the second activated sludge model (Asm2) in 1995 [4,5].
2 mathematical model method
The mathematical model method is perfect in theory, but there are many problems in practical application, mainly because of the complexity and diversity of sewage and sewage treatment. Even the simplified mathematical model is difficult to apply, which hinders its popularization and application. So far, the mathematical model method has not become a widely used design method abroad, but it has not been actually applied to engineering in China and is still in the research stage.
The main problem of mathematical model method is that there are many coefficients and constants in the model, including 13 in ASM 1 and 19 in ASM2. Designers need to determine the specific values according to the actual sewage quality and treatment process requirements, and most of them can be obtained through a lot of monitoring and analysis, and different sewage has different values. It is difficult to determine these parameters because of the changeable sewage quality. If these parameters are wrong, it will directly affect the accuracy and reliability of the calculation results. Foreign countries have put forward the values of these parameters, but the composition of sewage in China is very different from that in foreign countries, especially the composition of organic matter in sewage is very different, so we must never blindly apply foreign parameters. Therefore, in order to apply the mathematical model method to the design of sewage treatment in China, it is necessary to organize forces to monitor and analyze all kinds of sewage quality, determine relevant parameters, and put the mathematical model into practice. However, from the domestic situation, data analysis and accumulation is precisely one of the biggest weaknesses. There are hundreds of urban sewage treatment plants in operation in China, so far it is difficult to determine some basic data, let alone all kinds of data needed by mathematical model method. Obviously, it will take a long time for the application of mathematical model method in China.
3 mud aging method
3. Calculation formula of1mud age method
In the design specification, the formula [1] for calculating the volume of aeration tank according to the mud age is put forward:
v =〔24qθcY(Lj-Lch)/ 1 000 nwv( 1+KDθc)(2)
The design specification recommends several key parameters in the formula:
Y = 0.4 ~ 0.8 (20℃, with primary sedimentation tank)
KD = 0.04 ~ 0.075 (at 20℃)
When the water temperature changes, correct it according to the following formula:
Kdt=Kd20(θt)t-20 (3)
Where θt- temperature coefficient, θ t = 1.02 ~ 1.06.
θc—— 0.2 ~ 2.5 for high load, 5 ~ 15 for medium load and 20 ~ 30 for low load.
It can be seen that they have a wide range of values, with the range of Y value reaching 100%, Kd value reaching 87.5%, and θc value ranging from 50% to several times, which is one of the reasons why the mud age method is difficult to popularize in China.
In order to make the calculation method of mud age practical, according to my own design experience, the author suggests to adopt the calculation formula in ATV standard currently used in Germany, and modify the key parameters in the formula according to the specific situation in China. Practice has proved that this formula is a simple, reliable and very effective design and calculation method with clear concept and easy operation, and the calculation results can meet the requirements of Chinese codes. Its basic calculation formula is:
v = 24qθcY(Lj-Lch)/ 1000 NW(4)
Where y is the sludge yield coefficient (kg dry weight/kg dry weight)
The values of Q, Lj and Lch are the initial design conditions, which reflect the requirements of raw water quantity, quality and treatment, and have been determined before design calculation.
Sludge age θc refers to the average residence time of sludge in aeration tank, and its value is:
θc=VNw/W (5)
Where w-excess sludge, kg/day.
W=24QY(Lj-Lch)/ 1000 (6)
According to the above calculation formula, when the activated sludge process is designed and calculated by the sludge age method, the volume V of the aeration tank can be obtained only by determining the sludge age θc, the excess sludge volume w (or sludge yield coefficient y) and the average suspended matter concentration Nw(MLSS) of the aeration tank. Compared with the sludge loading method, it uses the sludge age θc instead of Fw or Fr as the most basic parameter for design and calculation. Compared with the mathematical model method, only one sludge yield coefficient y needs to be determined, and it is not necessary to measure the parameter data of 13 or 19.
3.2 Determination of mud age
Sludge age is determined according to theory and experience accumulation, and different sludge ages are adopted according to different treatment requirements and the scale of treatment plants. See table 1 for the minimum sludge age of German ATV standard single-stage activated sludge process sewage treatment plant.
Table 1 Minimum sludge age of activated sludge process in German standard
Scale of treatment target treatment plant
≤5 000 m3/day ≥25 000 m3/day
Non-nitrification 54
Including nitrification (design temperature: 10℃).
There are nitrification and denitrification (10℃)
VD/V=0.2
VD/V=0.3
VD/V=0.4
VD/V=0.5 12
13
15
18 10
1 1
13
16
Nitrification, denitrification and sludge stabilization are not recommended.
VD/V injection is the ratio of denitrification tank capacity to total tank capacity.
The large value of small sewage plants is selected in the table because the influent quality and operating conditions of small sewage plants change greatly, so a larger safety factor is selected.
Sludge age reflects the average residence time of microorganisms in the aeration tank, and the length of sludge age has two relations with the sewage treatment effect: on the one hand, the longer the sludge age, the longer the microorganisms stay in the aeration tank, the longer the microorganisms degrade organic pollutants, and the more thorough the organic pollutants are, the better the treatment effect is; On the other hand, the length of mud age is selective to microbial populations, because different populations of microorganisms have different generation cycles. If the mud age is less than the generation cycle of a microorganism, the microorganism will be discharged from the pond before it can reproduce, and it is impossible to live in the pond. In order to cultivate a microorganism needed for reproduction, the selected mud age must be longer than the generation cycle of the microorganism. The most obvious example is nitrifying bacteria, which is a microorganism that produces nitrification. It has a long generation cycle and needs an aerobic environment, so it needs a long aerobic sludge age when nitrifying sewage. Denitrifying bacteria are working when sewage is denitrified, and denitrifying bacteria need anoxic environment. In order to carry out denitrification, there must be an anoxic stage (stage or time period). With the increase of denitrifying nitrogen, more denitrifying bacteria are needed, that is, the age of anoxic stage and anoxic sludge will be prolonged. The quantification of the above relationship has been reflected in the table 1.
The minimum sludge age of non-nitrification sewage treatment plant is 4 ~ 5 d. According to the quality of domestic sewage, the treated effluent reaches BOD=30 mg/L and SS = 30 mg/L, which is the accumulation of years of practical experience, just like the value of sludge load.
For sewage treatment plants with nitrification, the sludge age must be longer than the generation cycle of nitrifying bacteria. A safety factor is usually adopted in the design to ensure nitrification, and its calculation formula is:
θc=F( 1/μo) (7)
Where θ c is the design mud age meeting the requirements of nitrification, d
F-safety factor, ranging from 2.0 to 3.0, usually 2.3.
1/μ o-generation cycle of nitrifying bacteria, d
μo- specific growth rate of nitrifying bacteria, d- 1.
μo = 0.47× 1. 103(T- 15)(8)
Where t is the design sewage temperature, which is usually 10℃ in the north and 1 1 ~ 12℃ in the south.
Substitute in equation (8):
μo = 0.47× 1. 103( 10- 15)= 0.288/d
Then substitute it into formula (7) to get:
θc=2.3× 1/0.288=7.99 d
The calculated value is consistent with the table 1.
Table 1 is a German standard, but its theoretical basis and experience accumulation are of universal significance and do not change with the change of water quality, so the author thinks it can be applied in China's design.
In the sludge loading method, sludge loading is the most basic design parameter, and sludge age is the derivative parameter. In the sludge age method, sludge age is the most basic design parameter, and sludge load is a derivative parameter, which is roughly inversely proportional:
θcFw=Lj/Y(Lj-Lch) (9)
Wherein the sludge yield coefficient y is a function of sludge age θ c.
3.3 Determination of sludge yield coefficient
It is very important to accurately determine the sludge yield coefficient y when designing and calculating the activated sludge process with sludge age method. It can be seen from Formula (4) that the volume of aeration tank is directly proportional to the value of y, which directly affects the volume of aeration tank.
Formula (6) gives the relationship between Y value and excess sludge W. Excess sludge W is the amount of sludge discharged from the biological treatment system every day, which includes two parts: one part is discharged with the effluent and the other part is discharged into the sludge treatment system. The calculation formula is as follows:
w = 24 qnch/ 1000+qsn( 10)
Where Nch—— is the concentration of suspended solids in sewage, mg/L.
Qs—— excess sludge discharged to sludge treatment system, m3/d
Ns—— Residual sludge concentration discharged into sludge treatment system, kg/m3.
The amount of excess sludge is best measured. It can be seen from the formula (10) that it is not difficult to determine the values of q, Nch, Qs and Ns for a normal sewage treatment plant, so that the values of w and y can be obtained. The problem is that there is no sewage treatment plant in the design, only referring to the values of other similar sewage treatment plants. Because of the different sewage quality, treatment degree and environmental conditions, the Y value obtained in different places cannot be the same. In particular, many urban sewage treatment plants often run abnormally due to lack of funds, and the value of excess sludge W is not accurate, which will inevitably affect the accuracy and reliability of design.
Theoretically, sludge yield coefficient is related to raw water quality, treatment degree and sewage temperature. First of all, the original meaning of sludge yield coefficient is SS produced after a certain amount of BOD is degraded. Because it is the product of organic matter degradation, SS here should be VSS, that is, volatile suspended matter. However, there are still a considerable amount of inorganic suspended matter and refractory organic suspended matter in sewage, which are not degraded by microorganisms, but are deposited in sludge intact, and the SS produced will be greater than the VSS really produced by BOD degradation, so raw water must be considered when determining the sludge yield coefficient.
Contents of inorganic suspended solids and refractory organic suspended solids. Secondly, with the improvement of treatment degree and the increase of sludge age, the more thorough the degradation of organic matter, the more attenuation of microorganisms, resulting in the reduction of excess sludge. As for the water temperature, it is an important factor affecting the biochemical process. The increase of water temperature and the acceleration of biochemical process will reduce the amount of excess sludge. For the influence of various factors, mathematical equations can be established through experiments according to theoretical analysis, and the calculation results can stand the test of practice and can be used in practical projects. Germany put forward such an equation, and the y value calculated according to this equation has been formally written into ATV standard.
y = 0.6(Nj/Lj+ 1)-0.072×0.6θc×FT/ 1+0.08θc×FT( 1 1)
f = 1.072(T- 15)( 12)
Where NJ refers to the concentration of suspended solids in the influent, mg/L.
Ft- temperature correction coefficient
T—— design water temperature, which is the same as the previous calculation.
It can be seen that Nj/Lj value reflects the proportion of inorganic suspended solids and refractory suspended solids in sewage. If their proportion increases, the amount of excess sludge will naturally increase, and the Y value will also increase. θc value affects the attenuation of sludge. With the increase of θc value, sludge decays more and Y value decreases accordingly. The influence of temperature is reflected in the FT value. With the increase of water temperature, the FT value increases and the Y value decreases, that is, the amount of excess sludge decreases.
This equation is of reference value to our country. Because the living habits in China are very different from those in western countries, the proportion of organic matter in sewage is low, the proportion of fat in organic matter is low, and the proportion of carbohydrate is high, so the sludge output will not be exactly the same. According to the published data in China and the author's experience, the excess sludge output of activated sludge process sewage treatment plants in China is less than that in western countries, so the formula (1 1) must be multiplied by a correction factor k:
y = k×0.6(NjLj+ 1)-と(0.072×0.6θc×FT)/( 1+0.08θc×FT)( 13)
Generally k = 0.8 ~ 0.9.
In the absence of our own formula for calculating Y value, the author thinks that it is feasible to calculate Y value with formula (13).
3.4 determination of mlss
No matter which design calculation method is adopted, MLSS needs to be determined reasonably. Other things being equal, if MLSS is doubled, the volume of aeration tank will be doubled. When MLSS is doubled, the capacity of aeration tank is also doubled. Direct impact on infrastructure investment needs to be carefully determined.
The design specification and manual recommend a selection range of MLSS values, such as ordinary aeration 1.5 ~ 2.5 kg/m3 and extended aeration 2.5 ~ 5.0 kg/m3, which are relatively large and difficult to operate in design. In order to choose the appropriate MLSS value, it is necessary to find out the factors that affect it.
There are three main reasons why MLSS cannot be chosen too low:
① If the ①MLSS is too low, the volume V of the aeration tank will increase accordingly, which is economically unfavorable.
② If ②MLSS is too low, foam will easily be generated in the aeration tank. In order to prevent foaming, it is usually necessary to keep the sludge concentration above 2 kg/m3.
③ When the sludge concentration is very low, the oxygen demand is less. For example, if the MLSS is too low, increasing the tank capacity, the gas supply per unit tank capacity is very small, which may not meet the requirements of mixing in the tank, so it is necessary to increase the mixing equipment. MLSS cannot be chosen too high, mainly because:
① In order to improve MLSS, it is necessary to increase the sludge reflux ratio, reduce the surface load of the secondary sedimentation tank and extend the residence time of the secondary sedimentation tank, which requires increasing the volume of the secondary sedimentation tank and the energy consumption of the returned sludge. Considering the aeration tank, secondary sedimentation tank and return sludge pump house as a whole, in order to minimize the total cost and operation cost, the sludge return ratio is usually limited to 150%. For ordinary municipal sewage, the concentration of sludge returned from the secondary sedimentation tank is usually 4 ~ 8 kg/m3. If the maximum value is about 8 kg/m3, the reflux ratio is 150%, and the MLSS in the aeration tank is 4~8 kg/m3, the maximum MLSS in actual design is generally less than 4.5kg/m3.
② The nature of sewage and the operating conditions of aeration tank have great influence on MLSS. If the composition of sewage or the operating conditions of aeration tank are conducive to sludge bulking, and the value of sludge index SVI remains high (such as SVI > 180ml/g), the concentration of returned sludge will be greatly reduced, so MLSS must be selected at a low value.
According to the above analysis, all aspects should be considered when selecting MLSS:
① The sludge age is long and the sludge load is low, so choose a higher value; The sludge age is short, the sludge load is high, and the lower value is selected; When the synchronous sludge is aerobically stable, choose a high value.
② Choose a lower value when there is a primary sedimentation tank, and a higher value when there is no primary sedimentation tank.
③ When the ③SVI value is low, choose a higher value, and when it is high, choose a lower value.
(4) When the sewage concentration is high, choose a higher value, and when it is low, choose a lower value.
⑤ The combined reactor (such as SBR) does not have the problem of sludge backflow, so choose a higher value or a higher value.
⑥ Check whether the stirring power meets the requirements; if not, make appropriate adjustments.
The German ATV standard stipulates the selection range of MLSS value, and the MLSS values of nitrification and non-nitrification are the same, which is not completely in line with the specific situation in China. The concentration of pollutants in urban sewage in China is usually low. If the MLSS value is too high without nitrification (short sludge age), the retention time may be too short, which is not conducive to biochemical treatment. Therefore, the non-nitrification MLSS value decreased by 0.5kg/m3, and the recommended MLSS value is listed in Table 2.
Table 2 Recommended MLSS value range of aeration tank
Kg/m3 treatment target MLSS
There is a primary sedimentation tank and there is no primary sedimentation tank.
Non-nitrification 2.0 ~ 3.03.0 ~ 4.0
There is nitrification (and denitrification) 2.5 ~ 3.53.5 ~ 4.5.
Sludge stabilization 4.5
3.5 Advantages and disadvantages of mud age method
① Sludge age method is a design and calculation method combining experience with theory. The determination of sludge age θc and sludge yield coefficient y has sufficient theoretical basis and accumulated experience, so it is more accurate and reliable.
② The sludge age method is intuitive, and it is clear at a glance whether the selected process can realize nitrification, denitrification and sludge stabilization according to sludge age.
③ In the calculation of sludge age method, only MLSS value is used instead of MLVSS value. The difference in the proportion of inorganic substances in sludge is reflected in the parameter y value, so it will not cause confusion between the two.
④ The most basic parameters in the sludge age method-sludge age θc and sludge yield coefficient y have recommended values and calculated values with little change, which are more convenient and easier to operate than selecting sludge load value.
⑤ The mud age method does not need to determine many parameters like the mathematical model method, which greatly simplifies the operation.
⑥ The calculation formula of sludge yield coefficient Y is based on the German sewage quality and experiments, and needs to be multiplied by a correction coefficient in combination with the situation in China.
4 conclusion
① It is necessary for the design and calculation method of activated sludge method to gradually transition from sludge loading method to sludge age method and finally to mathematical model method. The mud age method will play an important role before the mathematical model method is put into practical application.
② Formula (4) is used to calculate according to the sludge age method. Compared with the formula in the design code, the conversion between Nw and Nwv and the influence of sludge attenuation are considered in the calculation of Y value, which has clearer theoretical significance and is more convenient to use.
③ The mud age selection data recommended by German ATV standard (see table 1) is based on the combination of organic matter degradation and microbial growth.
The international experience, which does not involve the specific water quality changes of sewage, has practical value in China.
④ The calculation formula of sludge yield coefficient (1 1) has sufficient theoretical basis, but it is obtained from German sewage experiment. In order to fit China, it must be multiplied by the correction coefficient, and the modified calculation formula (13) can be used for actual design calculation.
⑤ The value of ⑤MLSS is stipulated in the design specification, but it is not easy to operate because of its wide range. It is recommended to refer to the data in Table 2 and compare them with each other.
⑥ It is suggested that the representative urban sewage in China should be studied experimentally, and its own formula for calculating Y value should be derived, so as to make the practical foundation of the mud age method more solid and reliable.
Main operation modes of municipal domestic sewage treatment by activated sludge process;
1, plug flow activated sludge process
2. Completely mixed activated sludge process
3. Sectional aeration activated sludge process
4. Adsorption-regeneration activated sludge process
5. Extended aeration activated sludge process.
6, high load activated sludge process
7, shallow well, deep well and deep well aeration activated sludge method
8, pure oxygen aeration activated sludge process
9, oxidation ditch process
10, sequencing batch activated sludge process