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Sludge reflux ratio 2-stage influent A / O process heavy oil processing effect

2021-05-28 11:22:03

Nitrogen is the main heteroatom element in petroleum, with a content of about 0.1% ~ 0.6%, mainly in the form of organic nitrogen. In the process of petroleum refining, these organic nitrides will be converted into inorganic nitrogen and dissolved in the aqueous phase to form nitrogen-containing sewage. The total nitrogen treatment load of petroleum refining wastewater is generally 60 ~ 80mg · L − 1, including ammonia nitrogen 70% ~ 90%, nitrate nitrogen 5% ~ 15%, organic nitrogen 5% ~ 10%, and the content of nitrite nitrogen is very low. Most oil refining enterprises adopt traditional A / O process for sewage denitrification, and the total nitrogen concentration of drainage is 30 ~ 40mg · L − 1, which basically meets the requirements of total nitrogen emission limit in the emission standard of pollutants for petroleum refining industry (GB 31570-2015). However, with the improvement of the state's attention to environmental protection, the discharge standard of total nitrogen in sewage is becoming more and more strict. In particular, some regions of China have implemented the discharge limit requirements of total nitrogen less than 15mg · L − 1 or 10mg · L − 1. In the future, it will be the general trend to raise the standard of total nitrogen in the field of sewage treatment of petroleum refining enterprises. How to make use of the existing hardware facilities of sewage treatment, formulate scientific design parameters and operation parameters, simply change the process operation conditions and carry out in-situ enhanced deep removal of total nitrogen is an important task of current research. The segmented influent A / O biological denitrification process does not need the internal circulation of nitrification liquid, with long residence time of sludge, sewage and pollutants, high utilization rate of carbon source and high denitrification efficiency. Compared with the traditional A / O process, it is more conducive to the deep removal of organic pollutants and nitrogen pollutants. From the mechanism analysis, the segmented influent A / O process will be suitable for the treatment of oil refining wastewater with low high-quality carbon source and large treatment load of organic pollutants and nitrogen pollutants, but there are few relevant research reports. Therefore, this study takes the heavy oil processing sewage, which is the most difficult to meet the standard in the field of oil refining sewage treatment, as the research object. Based on the design parameters of the traditional A / O denitrification process, a biochemical denitrification process and parameters based on the two-stage influent A / O process are constructed, and a side line experimental study is carried out on the site to investigate the influence of sludge reflux ratio r on the decarbonization and denitrification capacity of the system, It is expected to provide a basis for the synergistic depth of nitrogen and carbon in heavy oil processing wastewater to meet the standard.

1 materials and methods
1.1 materials and instruments
Experimental sewage: the effluent from the hydrolysis acidification unit of the sewage treatment plant of a typical heavy oil processing enterprise. During the experiment, the average values of COD, TN, ammonia nitrogen, nitrate nitrogen and organic nitrogen were 379, 60, 51, 6.0 and 1.5mg · L − 1 respectively.

Equipment and instruments: UV spectrophotometer, L6, Shanghai Yidian Analytical Instrument Co., Ltd; TN / TOC analyzer, toc-l, Shimadzu; Visible spectrophotometer, 7230g, Shanghai Jingke Instrument Co., Ltd; Portable water quality analyzer, U-50, Horiba, Japan; Electric heating constant temperature blast drying oven, 101-0a, Shanghai Yetuo.

1.2 analysis items and methods

Cod analysis refers to the method in GB / T 11914-1989, NH4 + − n analysis refers to the method in hj636-2012, NO3 − − n analysis refers to the method in HJ / T 346-2007, and the determination methods of MLSS and SVI refer to the methods in the literature. TN is measured by Shimadzu toc-l analyzer, do and temperature are measured by Horiba U-50 analyzer.

1.3 experimental conditions and methods

Experimental process and equipment: simulate the on-site hardware facilities, and construct the on-site side line experimental process according to the preliminary research results, as shown in Figure 1. The total effective volume of two-stage influent A / O reactor is 2160l, and the volume ratio of each section is va1: Vo1: va2: VO2 = 1:2.6:1.4:2.2. The aerobic tank is divided into three compartments, the effective volume of sedimentation tank is 150L, and the effective volume of BAF tank is 150L. Three diaphragm metering pumps respectively control the inlet water and sludge return, the air compressor aerates the aerobic cell, and the mechanical mixer provides mixing for the anoxic area. A1in and a2in are the water inlet sampling points of the denitrification unit respectively. The sampling points of A1, A2, O12, O13, O22 and o23 are 10cm below the liquid level on the outlet side of the corresponding unit. O11 and O21 are sampled at the sampling port set at 20cm from the bottom of the tank on the outlet side.

Start up and operation of the experimental device: take 1.5m3 of activated sludge from Cass tank of sewage treatment plant of a heavy oil processing enterprise and inoculate it into two-stage influent A / O process. Control the experimental temperature to (33 ± 2) ℃, influent flow to 720l · D − 1, hydraulic retention time of two-stage influent A / O reactor to 72h and shutdown time of sludge sedimentation tank to 5h. According to the previous research results, the process parameters of two-stage influent A / O reactor: the flow distribution ratio coefficient A1 is 0.7, A2 is 0.3, the anoxic section adopts 20 R · min − 1 low-speed stirring to control the dissolved oxygen at 0.2 ~ 0.4mg · L − 1, the aerobic section controls the dissolved oxygen at 3.5 ~ 4.5mg · L − 1, the sludge reflux ratio R is 1.0, the sludge age is controlled by regular sludge discharge for 25 ~ 30 days, and the two-stage A / O has no nitrification liquid reflux. On the 7th day of operation of the unit, the effluent quality is stable at the total nitrogen mass concentration of 20 ~ 24 mg · L − 1, the COD mass concentration of 50 mg · L − 1, and the NH4 + − n mass concentration is lower than 1mg · L − 1. The start-up and stability are completed and the experiment is started three days later. Change the R value of sludge reflux ratio (0.60, 0.75, 1.0, 1.25, 1.50), and investigate the impact of different R values on the system operation conditions and treatment effect. Change the sludge reflux ratio every 7 days for a total of 35 days. The R value of sludge reflux ratio operates at 1.0 without investigation.

2 results and discussion
2.1 effect of sludge reflux ratio on operating conditions
2.1.1 impact on hydraulic retention time of each section

The HRT changes of each section under different sludge reflux ratio are shown in Table 1. When the reflux ratio increases, the hydraulic retention time of sewage in each section is reduced, and the pollutant concentration of sewage is also diluted. When the reflux ratio is low, increase the reflux ratio, and the residence time of sewage can meet the removal rate of pollutants by activated sludge. Then the greater the reflux ratio, the better the removal effect. However, when the reflux ratio is high, the residence time of sewage is too short, and the pollutants will be discharged out of the device before they are completely degraded. Therefore, there is an optimal operation range for the reflux ratio of sludge.

2.1.2 impact on MLSS of each section

The influence of sludge reflux ratio on MLSS of each section is shown in Figure 2. If the reflux ratio is too low, the sludge concentration will be high, the HRT will be prolonged, the sludge treatment load will be reduced, and finally the properties of activated sludge will become worse; If the reflux ratio is too high, the sludge concentration will be low, the HRT will be shortened, and the degradation effect of pollutants will become worse. It is found that the reflux ratio has a greater impact on the activated sludge concentration in paragraph 1 by comparing paragraph 1 and paragraph 2. If the activated sludge reflux is too high, a large number of non denitrifying heterotrophic bacteria may compete with denitrifying bacteria for carbon source substrate in section A1, reducing the denitrification efficiency; Similarly, in O1 section, the presence of a large number of heterotrophic bacteria may inhibit the nitrification process and is not conducive to the oxidation of ammonia nitrogen.

2.1.3 effect on sedimentation index of activated sludge in sedimentation tank

The change of sludge return ratio r will affect the hydraulic load and sludge load of sedimentation tank. During the experiment, the SVI of activated sludge was 110 ~ 125ml • g − 1. The effect of R value on activated sludge sedimentation index of sedimentation tank was investigated. The results are shown in Fig. 3. When the R value increases from 0.60 to 1.50, the activated sludge sedimentation index of the sedimentation tank increases slightly, but the sludge sedimentation performance is still good. This phenomenon is basically consistent with the conclusion reached by Wang Wei and others in the process of studying the impact of sludge reflux ratio on the treatment of domestic sewage by segmented influent A / O process, that is, when the sludge sedimentation performance of the system is good, the R value has little effect on the sludge water separation effect of the secondary sedimentation tank.

2.1.4 impact on dissolved oxygen and reflux energy consumption in denitrification area

The increase of sludge reflux ratio r will cause a small increase in the dissolved oxygen concentration of denitrification zone A1 and A2 (the data are not listed). The dissolved oxygen in zone A1 increased from an average of 0.22g · L − 1 to about 0.31mg · L − 1, and the dissolved oxygen in zone A2 increased from an average of 0.27g · L − 1 to about 0.34mg · L − 1. The segmented influent process avoids the backflow of nitrification liquid from the source and is conducive to the dissolved oxygen control in the denitrification area. Moreover, due to the reduction of the return flow of nitrification liquid, the corresponding power consumption can also be reduced. According to the energy consumption accounting of the general submersible sewage pump, the power consumption of about 7 ~ 11kwh can be reduced for every 100m3 of nitrification liquid return. Taking a refinery with an annual processing capacity of 10 million tons as an example, according to the calculation of 0.50m3 of sewage generated per 1t of crude oil, if the nitrification liquid is not refluxed, 350000 ~ 550000 kwh of power consumption can be saved in one year.

2.2 influence of sludge reflux ratio on treatment effect
2.2.1 impact on COD removal effect
The effect of the change of R value on COD removal effect is shown in Figure 4. The two-stage influent A / O process has achieved good removal effect of organic pollutants in heavy oil processing wastewater, and the change of R value has little impact on the final removal effect of organic pollutants. The first stage a / O unit has completed the rapid degradation of 70% organic pollutant load, and the second stage a / O unit has completed the deep degradation of the remaining 30% organic pollutant load and organic pollutants with a long degradation cycle. The COD of the effluent of the unit is 48 ~ 51mg · L − 1.

Comparing the effect of the change of R value on the degradation of organic pollutants in the first and second stage a / O processes, it is found that the change of R value has a great impact on the first stage a / O. The higher the R value, the lower the concentration of organic pollutants in A1 influent due to water dilution, but with the progress of the reaction, the impact becomes smaller and smaller, indicating that the 2-stage influent A / O system has a good adaptability to water load. Compared with the traditional A / O process (nitrification liquid reflux ratio 1 ~ 2), the segmented influent process without nitrification liquid reflux prolongs the residence time of organic pollutants by nearly 50%, which is conducive to the deep degradation of organic pollutants with long degradation cycle in heavy oil processing wastewater.

2.2.2 effect on TN removal

The influence of reflux ratio on TN removal effect of two-stage influent A / O process is shown in Figure 5. The TN of O13 effluent corresponding to R value 0.60 ~ 1.50 were 23.8, 20.1, 17.7, 16.5 and 18.8mg · L − 1 respectively, and the removal rates were 61.61%, 67.58%, 71.45%, 73.38% and 69.67% respectively. When r = 1.0 and R = 1.25, the two-stage influent A / O process achieved a total nitrogen removal rate of more than 70%. Without adding carbon source and nitrification liquid reflux, the denitrification rate has been slightly higher than the conventional A / O Biochemical denitrification process, indicating that this process is suitable for denitrification of heavy oil processing wastewater with low carbon nitrogen ratio. According to the theoretical calculation of denitrification rate, if both A1 and A2 units realize complete denitrification, the denitrification rate corresponding to R value of 0.60 ~ 1.50 is 80% ~ 88%. Compared with the actual denitrification rate, it is found that the TN removal rate still has nearly 20% room for improvement. For details, please contact sewage treasure or More relevant technical documents.

2.2.3 effect on ammonia nitrogen removal

The influence of sludge reflux ratio on the treatment effect of ammonia nitrogen is shown in Figure 6. In the process of increasing R value from 0.60 to 1.50, the two-stage influent A / O process side line pilot plant basically ensures the efficient removal of ammonia nitrogen in a short time. The effluent ammonia nitrogen is less than 1mg · L − 1, and the removal rate is more than 98%. However, for the effluent quality analysis of ammonia nitrogen in each section, it is found that too low reflux ratio and too high reflux ratio are not conducive to the removal of ammonia nitrogen. When r = 1.0 and R = 1.25, the nitrification effect of each section is better, and the nitrification effect of R = 1.0 is slightly better.

Analyze the deamination effect of a / O in the first stage. With the increase of reflux ratio, the ammonia nitrogen concentration of A1 decreases significantly due to dilution. When r = 1.50, the influent ammonia nitrogen concentration of A1 is the lowest, but with the extension of O1 aerobic degradation time, the ammonia nitrogen concentration begins to be higher than the effluent ammonia nitrogen concentration when the reflux ratio is 0.75, 1.0 and 1.25, indicating that under this condition, the ammonia nitrogen oxidation capacity of O1 is declining, which is mainly due to the reduction of sludge concentration and sewage residence time caused by reflux. When r = 0.60, under the same influent ammonia nitrogen treatment load, the O1 sludge concentration is the highest and the residence time of ammonia nitrogen pollutants is the longest, but the O1 effluent ammonia nitrogen concentration is also higher than the conditions with R values of 0.75, 1.0 and 1.25, indicating that the nitrification process is inhibited. The reason may be that there is a competitive relationship between autotrophic nitrifying bacteria and heterotrophic bacteria. The ammonia removal effect of the second stage a / O is analyzed. The increase of reflux ratio has no obvious effect on the ammonia nitrogen removal effect of the second stage. The increase of ammonia nitrogen concentration in A2 influent is mainly caused by the ammonia nitrogen in the first stage a / O which is not fully nitrated.

2.2.4 effect on nitrate nitrogen removal

As shown in Figure 7, increasing the reflux ratio helps to improve the removal effect of nitrate nitrogen in the system when the R value increases from 0.60 to 1.25. When o23 is at r = 1.50, the nitrate nitrogen in o23 effluent rises suddenly. The main reason is that the reflux ratio is too large, resulting in the incomplete degradation of ammonia nitrogen in O13 effluent, which is oxidized to nitrate nitrogen in o23. By analyzing the mass concentration of nitrate nitrogen in the effluent of each section, it is found that too low reflux ratio and too high reflux ratio are not conducive to the removal of nitrate nitrogen. When r = 1.0 and R = 1.25, the denitrification effect of the system is better, and the denitrification effect of R = 1.25 is the best.

The denitrification effect of a / O in the first stage is analyzed. In the process of r increasing from 0.60 to 1.50 in unit A1, the removal amount of A1 nitrate nitrogen is gradually increasing, reaching the maximum value of 0.75g · h − 1 when r = 1.50, indicating that the denitrification efficiency of A1 has not been fully utilized, and the treatment load of A1 nitrate can be moderately increased to make full use of the denitrification capacity of anoxic area in the first stage. The study also found that low reflux ratio will lead to high sludge concentration and vicious competition for carbon sources in denitrification section. When r = 0.60, the ccod reduction / CN reduction of A1 (the mass concentration ratio of degraded cod to degraded nitrate nitrogen) is 3.69 and the removal amount of nitrate nitrogen is 0.51g · h − 1, while when r = 1.0, the ccod reduction / CN of A1 is 3.43 and the removal amount of nitrate nitrogen is 0.62g · h − 1, which proves this.

By analyzing the denitration effect of a / O in the second stage, the removal amount of nitrate nitrogen in unit A2 basically did not change when the R value increased from 0.60 to 1.5. It was found that there was residual nitrate nitrogen of 7.78 ~ 11.0mg · L − 1, which was mainly due to the insufficient carbon source of A2 denitrification. Calculate the ccod drop / CN drop of A2. When r = 0.60, the ccod drop / CN drop of A2 = 3.39; When r = 1.0, the ccod drop / CN drop of A2 = 3.27. The change trend of ccod drop / CN drop in A2 unit is basically consistent with that in A1, indicating that there is also carbon source competition in A2 unit. However, compared with unit A1, the ccod reduction / CN reduction of unit A2 is slightly lower, indicating that the carbon source utilization rate of unit A2 is higher.

3 conclusion
1) In the 2-stage influent A / O process, when the sludge reflux ratio increases from small, it will increase the water load of the system, shorten the residence time of sewage, sludge and pollutants, and reduce the degradation time of pollutants in the influent.
2) Increasing the sludge reflux ratio within a certain range can improve the nitrification and denitrification effect of the system, especially the amount of nitrate nitrogen removal in the anoxic zone of the first stage will increase significantly,