20 FQAs of Dissolved Oxygen in Sewage Treatment Process Control
Time:2023-02-07 Read:660
Reading guide: There are 20 FQAs of dissolved oxygen in sewage treatment process control. Learn more now.

FQAs of Dissolved Oxygen in Sewage Treatment Process Control

There is a close relationship between dissolved oxygen and sludge concentration, and the demand for dissolved oxygen with high activated sludge concentration is significantly higher than that with low activated sludge concentration; Dissolved oxygen is related to the content of organic matter in raw water, specifically shown in The more organic matter in the water, the more dissolved oxygen that microorganisms need to consume to metabolize and decompose these organic matter, and vice versa; dissolved oxygen is also related to some special components in raw water, such as the presence of detergent in water, so that there is an isolation layer on the liquid surface of the aeration tank that isolates the atmosphere, which affects the improvement of the aeration effect.

Question 1

The sewage plant I operate is a comprehensive industrial wastewater treatment plant. At present, there is a lot of foam in the biochemical area, the conductivity has increased from the original 3ms/cm to 8ms/cm, the SVI value is 200, and the dissolved oxygen has not been increased. Only 0.6mg/l of poisonous substances are suspected to enter. In this situation, what should I do?


1. If it is viscous white and the pile height is obvious, it needs to be considered that the effluent load is too large (for example, the COD influent is too high), so it is necessary to investigate the organic content of the influent and then to judge.

2. As you said, the failure of dissolved oxygen to go up is inversely related to the entry of suspected toxic substances. That is to say, if there are toxic substances entering, dissolved oxygen can go up instead.

Question 2

When the influent MLSS aeration air volume is constant, the influent BOD, and COD increase, why does DO decrease? MLSS is controlled by mud discharge, making it unchanged.


1. As the concentration of organic matter increases, the removal ability of microorganisms increases under the condition that the removal rate remains unchanged. However, when dealing with newly added organic matter, it is natural to consume more DO.

2. Here is a concept to talk about, it is the same MLLSS, but due to the different stages of activated sludge, its activity is different, like 10 elder people and 10 young people, the amount of work is different, naturally, 10 young people eat more than 10 elder people, that is, DO will consume more.

Question 3: 

In the A/O biochemical method, the PH value of the water in the A section and the effluent in the O section are all around 6.7, the influent indicators are normal, the ammonia nitrogen is dozens, and the COD is around 1000 mg/L so that the influent load can only be increased. When it reaches 50 tons, the dissolved oxygen will not be enough. The dissolved oxygen is always around 2. What is the reason?


Please confirm the following possibilities:

1. The reflux ratio is too high;

2. Problems with aeration equipment (such as blocked aeration head, air leakage in other places, equipment failure, etc.);

3. As the temperature rises, the solubility of dissolved oxygen decreases;

4. The sludge concentration control is too high;

5. There is a deviation in the detection of dissolved oxygen.

Question 4: 

What is the relationship between the dissolved oxygen in the aerobic pool?


1. Aeration intensity;

2. The amount of aerated water;

3. Water temperature;

4. Concentration of influent organic matter;

5. The number of microorganisms (sludge concentration).

Question 5: 

Recently, the aerobic MLSS is around 3500, and the sludge contains some black particles, and the overall aerobic DO is also low, so I increased the sludge discharge. After a few days of discharge, the MLSS dropped rapidly, and the COD of the effluent was short-term 40, basically reaching the normal level, but today, the COD has risen back, and MLSS2600, DO is still at the same level, the first 7 pools are lower than 1, the final effluent DO3~4 mg/L, the effluent sludge color depth, And the water is cloudy. What I can't figure out is: the fan has not changed, and the MLSS has dropped so quickly (SV30 has also dropped, the SVI value before the mud discharge is 72, and after the mud discharge is 88, basically in a compressed state), but why does the DO not see a recovery?


1. Whether DO has rebounded, the first analysis is to ensure that your detection value is correct, otherwise subsequent analysis is meaningless. The consumption of dissolved oxygen is related to the activity of microorganisms, that is, the decomposition of organic matter requires the consumption of dissolved oxygen, and the maintenance of normal activities of microorganisms requires the consumption of dissolved oxygen.

2. It is normal that the dissolved oxygen at the head end of the biochemical pool is low, because there is water inflow at the head end, and the dissolved oxygen in the influent water is low. The high dissolved oxygen in the latter stage is caused by the water body being aerated for a long time.

3. Why did you say that the dissolved oxygen does not increase after the microbial biomass decreases? It is necessary to judge whether the concentration of the influent water increases. The microorganisms work overtime to decompose the organic matter without increasing the total amount, and the consumption of dissolved oxygen does not decrease with the microorganisms. And reduce.

4. Factors that affect DO not increase. 1) Whether the water intake has increased (or the return flow has increased). 2) Whether there is a problem with the aeration equipment, such as partial blockage 3) Whether the concentration of influent water is reduced, resulting in a decrease in the dissolved oxygen required for the decomposition of organic matter. 4) The water temperature rises, resulting in a relative decrease in the dissolved oxygen in the water. 5. Problems with the detection equipment (accuracy), such as whether the probe has not been cleaned for a long time.

Question 6:

I bought a dissolved oxygen analyzer and measured the DO in the contact oxidation tank (65 cubic meters) to be 4mg/l. After shutting down at night, it was 0.39mg/l the next morning. The COD of the influent water was 1200 mg/l. From 750 mg/l before the film to 690 mg/l now, although there has been a decrease, the range is not large. What is the reason? After the film is installed, the influent water has been reduced to 3 tons per day, and the COD of the influent water is 1200 mg/l. The process flow is regulating tank, primary sedimentation tank, UASB, contact oxidation tank, and secondary sedimentation tank. If the dissolved oxygen measurement is correct, are there too few microbes? So what to do?


The low removal rate may be related to the following reasons. The biofilm has not been effectively formed. 

1. The cultivation time is not enough. 

2. The biodegradability of the wastewater is not strong (B/C) .

3. The wastewater has antibacterial components (see the design wastewater components) .

4. Unbalanced nutrition (see if there is a lack of nitrogen and phosphorus, and confirm with the nitrogen and phosphorus value in the effluent). 

5. Excessive aeration will wash away the biofilm, resulting in the ineffective formation of the film.

Question 7: 

In the biochemical treatment, the DO at the back end of the aeration tank is higher than that at the front end. Is it because of the load?


It should be caused by the water body of the biochemical pool moving backward, and the aeration time of the latter part of the water is relatively prolonged.

Question 8:

It is stated in the book that after sludge poisoning, it is necessary to aerate and increase aeration and sludge discharge. What is the principle of aeration after poisoning? Also, after sludge poisoning, the dissolved oxygen has been higher than usual, and then increase aeration Will the amount of dissolved oxygen be too high?


After the sludge is poisoned, this part of the sludge needs to be removed, and the system is restored by adding new sludge. Sludge removal is to remove the poisoned sludge and create a substrate concentration environment for the new sludge. Poor exposure is beneficial to the activation of the activated sludge. Activate new sludge activity in a short time to promote proliferation. Mud discharge and stuffy exposure need to be used in combination, first mud discharge and then stuffy exposure.

Question 9:

Our factory adopts the improved A2O process (the anaerobic section is preceded by a pre-anoxic section). At present, the influent COD is around 600 ppm, the pH is around 7, the conductivity is 2300us/cm, and the pool capacity is 28000m3. At present, when the aeration rate is 12,000 cubic meters per hour, the DO value of the 6# corridor in the aerobic section (the order of water intake is 6, 5, 4) has not been able to come up, and it is generally around 0.1 ppm. 

The air is in good condition (a microporous aeration disk is used, and one aeration disk is damaged near the probe, but after investigation, this is not the main reason affecting the DO value near the probe), which is relatively uniform. At present, it often occurs that there is no problem with the influent water quality, but the DO value in the two corridors of the aerobic sections 6 and 5 drops rapidly. Ammonia nitrogen has been on the rise throughout the process and has no removal effect. I would like to ask: 1. What aspects should be considered for the rapid decline of DO value in the aerobic segment? (There are tanneries, pipe-drawing factories, and some electroplating factories near our factory) 2. How to control the process to ensure the removal effect of ammonia nitrogen?


The low head end of dissolved oxygen is related to the rapid adsorption of influent organic matter on the surface of activated sludge and the rapid decomposition of microorganisms to consume a large amount of dissolved oxygen. You don't have to worry too much about the drop in dissolved oxygen. Dissolved oxygen needs to be ensured in the latter stage. The removal of ammonia nitrogen is not ideal. Check the denitrification in the anaerobic section, such as the release of air bubbles. How is the alkalinity and whether it is satisfied?

Dissolved Oxygen in Sewage Treatment

Question 10: 

A printing and dyeing wastewater plant with a design flow rate of 12,000 tons/day had an abnormal production in May this year. At that time, the water limit was more than 8,000 tons/day, and the third backup fan was also turned on. The dissolved oxygen in the aerobic pool was still insufficient. It lasted for nearly a month, and then the system returned to normal. Two fans were turned on to process 12,000 tons of water, and the dissolved oxygen was still sufficient. Why can't the oxygen be filled when the production is abnormal? (If the water is not mixed, it means that the load cannot be increased, and if it is lifted, there will be severe hypoxia).


The purpose of aeration is to ensure that aerobic microorganisms provide oxygen when decomposing organic matter. If the load is high, microorganisms will increase their horsepower to decompose organic matter, and naturally need more dissolved oxygen. This is a normal phenomenon.

Question 11:

When the load is high, the oxygen consumption is less than when the load is low, how to explain this?


When the sludge concentration is low, the relative sludge load is high, and if the relative load is high, it will appear as you said. That is to say, when the influent concentration is the same and the sludge concentration is high, the oxygen consumption will be higher than that when the sludge concentration is low.

Question 12:

What is the effect of pH on the dissolved oxygen in the aeration tank? How does it work? In this incident, it was found that the pH was high and the dissolved oxygen was relatively low. I wonder if it was caused by the increase in pH.


There is no direct relationship, but the PH value leads to the activity of microorganisms, and then indirectly affects the utilization rate of oxygen, which is reflected in DO fluctuations, as you said.

Question 13:

If the pH of this kind of influent water is too high, the acid-adding equipment cannot be used, and if it can only be added manually, which kind of medicine is better, sulfuric acid, bleaching powder, or other better medicines? In the past few days, the dissolved oxygen has not come up. The water volume is 500, the air volume is 8400, and the dissolved oxygen is only 1-2 (online and actual measurement). What may affect the dissolved oxygen?

Today I found that the water in one of the aeration pools has turned grayish-white, like porridge. This is not the case in other pools. It has never happened before. What's going on? How to solve it? The biofilm in this pond is a kind of transparent and sticky substance. I don’t know what it is. It also turned gray today. What is it?


1. It is better to add hydrochloric acid, which is less dangerous to the human body.

2. There are many factors affecting dissolved oxygen. If the water load is high, the dissolved oxygen will not go up. Of course, whether the detection method is correct is also a problem. If the temperature is affected, there will be no sudden change, it is a gradual change process.

3. If the biofilm peels off, the treated water is not continuous, and the aeration is too much, there may be such a situation.

4. Let the fungi change, especially after the pH rises, some microorganisms need to protect themselves, and there will be such changes.

Question 14:

Why in the orbital oxidation ditch process, the dissolved oxygen in the outer ditch reaches 0.5 mg/l, the middle ditch reaches 0.8 mg/l, and the dissolved oxygen in the inner ditch is only 0.9 mg when the aeration equipment in the inner ditch is fully opened /l, can't you come up? If the dissolved oxygen in the outer ditch and the middle ditch is reduced, will the middle ditch be lower?


1. In addition to the intensity of aeration, DO is also related to the concentration of sludge, the content of organic matter in the influent, and the location of the aeration.

2. The oxidation ditch process is a low-load process, and a little low dissolved oxygen is not a big problem.

the dissolved oxygen in the aerobic pool

Question 15:

A few days ago, the dissolved oxygen in the aerobic pool has been low and inconsistent (upper high, and lower). After replacing the large fan these days, the dissolved oxygen is high and consistent up and down. Due to the phenomenon of sludge poisoning some time ago, the sludge has been discharged and the sludge grows quickly. Finally, the COD dropped to more than 500 mg/L (from 1300 mg/L), the ammonia nitrogen dropped to about 250 mg/L (from 330), and the sludge concentration dropped to 20 %. 

For unknown reasons, the dissolved oxygen suddenly dropped from 5.0 mg/L to about 1.5 mg/L (same aeration). In order to increase the dissolved oxygen, two fans were turned on. There was a lot of foam, and the dissolved oxygen could reach 5.0 mg/L. It is all foam, and it also takes away a lot of sludge. When a fan is stopped, the dissolved oxygen quickly drops below 1.8-1.5 mg/L, and the foam decreases slightly. Now COD and ammonia nitrogen have started to rise again (more than 700 mg/L, more than 300 mg/L), and the sludge concentration has also risen uncomfortably (about 17%). What is the problem? Is it enough to run two fans or one? We are chemical wastewater, with a salt content of about 1% and a BOD of about 200 mg/L.


1. It should be said that your wastewater is not biodegradable. Activated sludge needs to be better adapted (trained) before it can resist load shocks.

2. For the control of dissolved oxygen, 5 is too high. When the influent concentration is high and the sludge concentration is low, the activated sludge is easy to be too active, resulting in sticking to the foam.

3. If sludge poisoning has been alleviated by sludge discharge, it is better not to discharge sludge excessively. After the sludge concentration gradually stabilizes, the system will gradually recover.

Question 16:

Insufficient dissolved oxygen in the aerobic pool causes the filamentous bacteria to swell rapidly. Now that we want to increase the dissolved oxygen, the fan has been fully turned on. According to the calculation of dissolved oxygen, we can reduce the concentration of sludge and reduce the oxygen demand of microorganisms. However, there is such a passage in the article "The Causes and Control Methods of Filamentous Bacteria Sludge Bulking" on China Water Supply and Drainage 2004Vo1.20: "It can be prevented by increasing the DO concentration and MLSS (reducing F/M) in the aeration tank. In sludge bulking with low DO, it is wrong to think that fewer microorganisms require less oxygen to reduce MLSS concentration, because oxygen demand increases while MLSS decreases.


I personally think that high DO and high load are beneficial to inhibit the expansion of filamentous bacteria, but it does not mean that the expansion of filamentous bacteria will be completely improved after doing so. Because the reasons for the expansion of filamentous bacteria are far more than these. These reasons are not the main reason.

Question 17:

Does the water temperature have a great influence on the dissolved oxygen efficiency? In the treatment of industrial wastewater, how much does the air volume need to increase when the water temperature rises from 30 degrees to 40 degrees?


The specific change has not been confirmed, but as you said, the water temperature has a great impact on dissolved oxygen, and it is estimated that there will be an impact of about 15% to 20%.

Question 18:

The current situation is that the dissolved oxygen has been unable to increase, and it is around 0.5. According to previous experience, it should be enough. I don’t know how much MLSS control is appropriate. The SS entering the oxidation ditch has been at 500mg/L


The concentration of organic matter in the influent is not low, so there is no problem with a high MLSS, but if the influent is SS500, it is still high, which may easily cause your biochemical pool MLVSS to be too low. Therefore, it is necessary to increase the sludge discharge and calculate the MLSS value you need by controlling the ratio of food to micron at 0.1. The failure of dissolved oxygen to go up is still related to the high concentration of your sludge and the high organic matter in the influent.

Question 19:

1. Process hydrolysis acidification-intermittent aeration IBR-flocculation sedimentation-clo2 sterilization. 

2. The original design treatment capacity is 150t/h COD250 mg/L, but now due to the imported water source COD400mg/L, the effluent can only reach 80t/h. 

3. Activity Sludge load F/M is about 0.06-0.1. 

4. The minimum concentration of activated sludge MLSS is 1200, and the maximum is 5000, but it is generally controlled at 1500-2000 through current adjustments. 

5. Dissolved oxygen control level DO wants to be controlled at 1-3, but continuous aeration can only reach about 1 or even lower. 

6. The sedimentation ratio of the activated sludge SV30 is about 17%. 7. No nutrients are added, the ammonia nitrogen in the effluent is about 5 mg/L, and the total phosphorus is 0.5 mg/L. 


1. It is reasonable that your pollutant concentration and sludge load are not so high that the whole process of aeration cannot go up. Therefore, it is necessary to confirm whether the whole process of aeration can meet the requirements during design. In addition, whether the aeration equipment is faulty (such as blocked), of course, it is also necessary to confirm whether the detection equipment is abnormal and whether the detection position is correct (dissolved oxygen value at the outlet).

2. It is mainly to increase the sludge concentration to correspond (according to the increase of influent load, the corresponding sludge concentration will also gradually increase).

3. the COD doesn't have to be almost the same. What is important is to improve the degradability of the organic matter in the later biochemical system.

Question 20: 

DO1mg/L or so, the first two or two years have been controlled between 0.3-2 mg/L and the operation index is very good. During the debugging in 2009, the DO value has been controlled at around 4 mg/L, but the ammonia nitrogen exceeded the standard, slowly Just controlling the dissolved oxygen at this value. PH has been around 9.5. Now it is certain that the sludge has expanded, and the foam is also caused by the sludge expansion. I adjusted with 30 ppm iron salt and it didn't get better, it seems to be getting worse.


Dissolved oxygen cannot be lower than 2 mg/L. If the pH is 9.5, it is difficult for ordinary microorganisms to grow normally. It needs to be controlled below 9, and then see how the effect is.