Fine chemicals are closely related to our lives, affecting our necessities, food, housing and transportation, and are the main source of innovation in today’s chemical industry.
However, in the production process of fine chemicals, various chemical raw materials and solvents are used. In addition, the added value of their products is relatively high and the synthesis route is cumbersome. Therefore, a large amount of wastewater containing various types of pollutants will be generated. Moreover, fine chemical wastewater has the characteristics of high COD concentration, high toxicity, poor biodegradability and unstable water quality. If it is not treated and discharged, it will bring serious damage to the environment and human health.
Upflow Anaerobic Bioreactor (UASB) has the advantages of compact process structure, high anaerobic sludge concentration, large processing capacity, no mixing and stirring equipment, short hydraulic retention time, and good impact resistance. This paper studies the use of UASB treatment process to treat the waste water of a fine chemical enterprise after expansion.
Test materials and methods
1.1 Test water
Select the mixed water sample after pretreatment of wastewater from a fine chemical enterprise as the research object, and its water quality indicators are shown in Table 1.
1.2 Main instruments and equipment
1) Electronic balance, MS104TS.
2) Airflow dryer, HG-3.
3) pH meter, Thunder Magnetic pHSJ-4F.
4) Blast drying oven, DHG-9203A.
5) UV spectrophotometer, UV1902PC.
6) Constant flow pump, LABV6.
7) Constant temperature water bath, HH-4.
8) Magnetic heating mixer, HWCL-3.
9) Heater, WLD15S.
1.3 Determination method (see Table 2)
1.4 UASB test device
The UASB test device is divided into three parts, namely the top, middle and bottom. Among them, the three-phase separator at the top effectively classifies gas, solid and liquid, and is the core and most crucial component of the entire UASB device. It can be said that only with the core component of the three-phase separator can it be called a UASB and ensure the normal operation of the UASB.
The UASB reactor of this test device is mainly made of plexiglass. The device height is 1.2m, the diameter is 50mm (outer diameter), and the total volume is about 8L. Considering that the reaction temperature has a great influence on the test, the cylinder outside the device was insulated. In addition, in order to prevent the generation of methane gas in the later stage, which may cause danger, a collection device is installed. The test flow device is shown in Figure 1.
UASB reaction start-up device and test operation
The start-up process of the UASB reaction device is divided into initial start-up and re-start. Among them, the initial start-up is mainly to cultivate and domesticate the inoculated sludge, and the re-start is mainly to shorten the overall start-up time of the UASB reaction device. According to research and practical experience, the following six points should be paid attention to when starting the UASB reaction:
1) Keep the pH in the reactor at 6-8 to promote the formation of methanogens.
2) In order to prevent the failure of domestication, it is necessary to dilute the influent with high COD.
3) When starting, the organic load of wastewater should be increased first, and then the hydraulic load should be gradually increased.
4) For wastewater with too high suspended solids, pretreatment methods such as sedimentation and air flotation should be used to reduce the content of suspended solids. 5) Control the influent concentration between 500mg/L and 1000mg/L. 6) No circulating sludge is used, and the excess sludge will no longer enter the UASB reactor. In addition, attention should also be paid to the UASB reaction initiation device in this experiment.
2.1 Startup plan
The cultivation and domestication of sludge generally have two methods: synchronous method and asynchronous method. By comparing the advantages and disadvantages of the two methods, combined with the specific situation, this experiment chooses to add an appropriate amount of wastewater, so that the bacteria can synchronously adapt to the wastewater during reproduction and growth. And in the early stage of the test, in order to make the granular sludge more efficient, the device needs to be controlled to a certain extent, and the control parameters and measures are shown in Table 3.
2.2 Sludge inoculation run
Through the trial operation of the UASB reaction unit to treat the fine chemical wastewater of the enterprise, we can obtain:
1) The initial stage of sludge inoculation (0d ~ 30d). In the initial stage of sludge inoculation, almost no gas is produced in the UASB device, the removal efficiency of COD is not high, less than 50%, and the effluent COD is unstable. It may be because the microorganisms need to have an adaptation process to the substrate in the initial stage of sludge inoculation. Most of the microorganisms have not yet adapted to the environmental conditions, so they cannot reach the normal metabolism level and cannot play their due role.
2) Inoculation sludge formation period (31d～60d). At this stage, anaerobic bacteria have been effectively grown and reproduced, and a small amount of gas has been produced, the COD removal rate has been continuously improved, reaching 60% to 70%, and the effluent COD has gradually stabilized. It may be due to the fact that after the domestication of the sludge in the previous stage, the sludge with small particles and poor sedimentation performance in the system is basically eliminated, and the anaerobic bacteria have been effectively grown and reproduced. Decompose and reduce COD.
3) The maturity period of the inoculated sludge (61d～94d). On the 80th day of the mature period of the inoculated sludge, under the combined decomposition of the original VFA in the wastewater and produced by acid-producing bacteria, the removal efficiency of COD is high, reaching 75% to 85%, and the effluent COD has basically Stablize.
It may be due to the continuous domestication and growth and reproduction of the sludge in the first two stages, the methane-producing bacteria and the acetic acid-producing bacteria in the system reach an equilibrium state, and in this state, the treatment effect of wastewater is the best.
2.3 Analysis of factors affecting COD removal
During the test period, factors affecting COD removal were investigated and the results are as follows.
1) Hydraulic retention time (HRT).
In this experiment, the removal rate of COD was investigated when HRT was 12h, 18h, 24h, 30h and 36h.
The results showed that in the initial 12h, the COD removal rate was low, about 60%. When the HRT was increased from 12h to 24h, the COD removal rate increased rapidly to 75%.
After that, with the continuous increase of HRT, the COD removal rate did not change much. It may be because when the hydraulic retention time is short, the contact between the pollutants and the activated sludge is insufficient, and the methanation has not been completed, so the COD removal rate is low. Considering the time efficiency and combining various factors, the optimal hydraulic retention time HRT was determined to be 24h.
In this experiment, the removal rate of COD was investigated at the temperature of 20℃, 30℃, 35℃ and 40℃. The results showed that the removal rate of COD increased gradually with the increase of temperature at 20℃～35℃, and the change was not obvious after that. It may be because we are studying anaerobic bacteria in the mesophilic region. When the temperature is too low, the anaerobic bacteria are in a gel state and lose their activity. When the temperature rises to 35 °C, the acid-producing bacteria and methanogens in the reaction are relatively balanced, and the system is effective. In operation, the COD removal rate is about 75%.
Therefore, the optimum temperature was finally determined to be 35°C.
In this experiment, the removal rate of COD was investigated when the alkalinity was 500, 800, 1100, 1400, 1700 and 2000 mg/L.
The results showed that the removal rate of COD showed a “convex” parabolic shape with the increase of alkalinity. When the alkalinity was 1100 mg/L, the removal rate of COD was the largest, about 75%. It may be because when the alkalinity is low, not only the growth of methanogens is inhibited, the ratio of methanogens and acidogens is unbalanced, but also the value of the digestate in the reactor is low due to insufficient buffer capacity, so the COD removal rate is low. lower.
When the alkalinity is high, the pH value also increases synchronously, which is not conducive to the growth of acid-producing bacteria, resulting in an imbalance of the system and affecting the removal rate. Therefore, the optimal alkalinity was determined to be 1100 mg/L.
4) pH value.
In this experiment, the removal rate of COD was investigated at pH values of 6, 6.5, 7, 7.5 and 8.
The results showed that the removal rate of COD also showed a “convex” parabolic shape with the increase of pH value. When the pH value was 7, the removal rate of COD was the largest, about 75%. It may be because the change of pH value will affect the microbial growth environment in the system, and then affect the activity of biological enzymes, resulting in abnormal metabolism in microbial cells, resulting in low COD removal rate.
Since the right end of the parabola is higher and gentler when the pH value is 7-8, the most suitable pH value is 7-8.
Project operation results
After the end of this test, after debugging, the optimal operating parameters are that the hydraulic retention time (HRT) is 24h, the temperature is 35°C, the alkalinity is 1100mg/L, and the pH value is 7-8, the system is running normally, UASB After operation, the COD removal rate remains above 70%, with an average of 74.3%, and the COD of the aerobic effluent is below 300mg/L (national wastewater discharge standard), which can be discharged up to the standard.
In this experiment, the UASB process was adopted on the basis of the original process of a fine chemical enterprise, and the following conclusions were drawn:
1) The optimal parameters found are that the hydraulic retention time (HRT) is 24h, the temperature is 35°C, the alkalinity is 1100mg/L, and the pH value is 7-8.
2) The UASB process constructed in this experiment, the effluent COD<300mg/L (national wastewater discharge standard), can be discharged up to the standard.
3) This test is reliable and can be further practiced, researched and promoted.