Problems and solutions in the operation of activated sludge process

What problems exist in the operation of the activated sludge method?

The organic pollutant load at the head of the aeration tank is high, and the aerobic rate is also high. In order to avoid the formation of anaerobic state due to lack of oxygen, the organic load of the influent should not be too high. In order to achieve a certain decontamination capacity, the aeration tank needs to be large, so more land is occupied and the infrastructure cost is high; the aerobic rate varies along the length of the tank, and the oxygen supply rate is difficult to match and adapt to it. The aerobic rate may be higher than the oxygen supply rate in the front section of the tank, and the dissolved oxygen may be excessive in the back section of the tank. In this regard, the use of a gradually decreasing oxygen supply method can solve these problems to a certain extent; in addition, the activated sludge has a low adaptability to changes in influent water quality and water volume, and the operation effect is easily affected by changes in water quality and water volume.

The problems existing in the operation of the activated sludge process are:

① Abnormal biological phase; ② Abnormal sludge SVI value; ③ Sludge expansion; ④ Sludge disintegration; ⑤ Sludge corruption; ⑥ Sludge floating; ⑦ Foam problem; ⑧ Abnormal effluent from the secondary sedimentation tank is mainly manifested in reduced transparency, increased SS and BOD values, and increased coliform counts.

1. What are the concepts of sludge bulking and what are the solutions?

(1) Causes of sludge bulking

① Filamentous bacteria bulking: filamentous bacteria in activated sludge flocs overgrow, leading to bulking. The contributing factors include low organic matter in the influent, too low F/M, insufficient microbial food, insufficient nitrogen and phosphorus in the influent, low pH, too low dissolved oxygen in the mixed liquor, and insufficient demand; too large fluctuations in the influent, causing impact on microorganisms.

② Non-filamentous bacteria bulking: due to the large amount of dissolved organic matter in the influent, the sludge load is too high, while the influent lacks sufficient N and P, or DO (dissolved oxygen) is insufficient. Bacteria quickly absorb a large amount of organic matter into their bodies, but cannot metabolize and decompose it, and secrete excessive amounts of polysaccharides. These substances contain hydroxyl groups in their molecules and have strong hydrophilicity, making the bound water of activated sludge as high as 400% (normally around 100%), presenting a viscous gel and unable to be separated in the secondary sedimentation tank. Another type of non-filamentous bacterial bulking is that the influent contains too much toxins, which leads to bacterial poisoning, and the inability to secrete enough sticky substances, form flocs, and separate.

(2) Solution: The imbalance of the various components that make up the wastewater can also cause sludge bulking. For example, if the C/N ratio in the wastewater is imbalanced, if the carbohydrate content is too high, urea, ammonium carbonate or ammonium monoxide can be appropriately added. If the system influent concentration is too high, the influent volume can be reduced. As for the environment of the aeration tank (such as pH, temperature, dissolved oxygen, etc.), it also has a certain impact on the properties of the activated sludge. Other factors such as the presence of a large amount of organic matter or petroleum in the wastewater, as well as a large amount of corrupt substances, can cause bulking. Excessive or insufficient oxygenation or insufficient stirring in the aeration tank can cause bulking. From this, we can see that in order to prevent sludge swelling, we should first strengthen management operations, regularly test sewage quality, dissolved oxygen in the aeration tank, sludge settling ratio, sludge index and conduct microscopic observations. If any abnormal situation is found, timely measures should be taken, such as increasing the air volume, discharging sludge in time, and taking staged water intake when possible to reduce the load on the secondary sedimentation tank.

2. What are the concepts of sludge floating and what are the solutions?

(1) Sludge floating mainly refers to sludge denitrification floating. After sewage stays in the secondary sedimentation tank for a long time, it will cause hypoxia (DO is below 0.5mg/L), then denitrifying bacteria will convert nitrate into ammonia and nitrogen. When ammonia and nitrogen escape, the sludge absorbs ammonia and nitrogen and floats, which reduces the sludge sedimentation.

 (2) Solution The sludge floating phenomenon has nothing to do with the properties of activated sludge. It is only because bubbles are generated in the sludge, making the sludge density lower than that of water. Therefore, sludge floating should not be confused with sludge expansion. Specific solutions are: ① Reduce the salt concentration of the influent and control the impact of high-load COD. ② Accurately control the COD load in the aeration tank. Therefore, the water inflow of the aeration tank should be controlled in operation. By accurately controlling MLSS (recommended 6~8g/L) and the water inflow of the aeration tank, the COD load is controlled within the appropriate range of 0.2~0.4kg/(m3 · d) to reduce the impact of sewage. If the COD concentration of the sewage after passing through the homogenization tank still exceeds the design standard, the sewage should be introduced into the accident tank for later treatment. ③ Improving the new sewage pretreatment process and controlling the sewage anaerobic and facultative aerobic acidification hydrolysis tank are key steps to ensure the normal operation of the subsequent aeration tank. After the difficult-to-degrade organic matter in the sewage is degraded here, the effluent requirements of the aeration tank sewage can be guaranteed, and the sedimentation performance of the secondary sedimentation tank is also improved. The following measures should be taken: complete the transformation of the submersible mixer power distribution system, pump the sludge to the acidification tank as soon as possible, and debug the acidification tank and acclimate the acidified sludge. The residual sludge added at one time is about 1/5 of the tank capacity, and the addition amount is about 100m3, so that the concentration of the mixed liquid in the tank is 4~6g/L. ④Control the dissolved oxygen concentration in the oxygen aeration tank, appropriately reduce the MLSS of the oxygen aeration tank, basically control it within 10g/L, and the corresponding dissolved oxygen concentration control should be adjusted in time according to the organic load of the influent. ⑤Increase the sludge return volume, remove the excess sludge in time, reduce the sludge concentration of the mixed liquor, shorten the sludge age, and reduce the dissolved oxygen concentration, but it cannot enter the digestion stage.

What are the concepts of foam problems and their solutions?

(1) Foam problems Foam is generally divided into three forms: ① Startup foam. In the initial stage of the activated sludge process, surface foam is easily caused due to the presence of some surfactants in the sewage. However, as the activated sludge matures, these surfactants are degraded by microorganisms and the foam phenomenon will gradually disappear. ② Denitrification foam. If the sewage treatment plant undergoes nitrification, denitrification will occur in the sedimentation tank or in places where aeration is insufficient, generating nitrogen bubbles and driving part of the sludge to float, resulting in foam. ③ Biological foam. Due to the abnormal growth of filamentous microorganisms, the foam mixed with bubbles and flocculent particles is stable, continuous and difficult to control. Biological foam is very detrimental to the normal operation of the sewage treatment plant: a large number of filamentous microorganisms appear in the aeration tank or secondary sedimentation tank, floating on the water surface and accumulating a large amount of foam, causing the concentration of effluent organic matter and suspended solids to increase, generating odor or harmful gases, reducing the oxygen transfer efficiency of the mechanical aeration method, and may cause a large amount of surface foam to be generated during the later sludge digestion.

(2) Solutions ① Spraying water This is the most commonly used physical method. By spraying water or water droplets to break the bubbles floating on the water surface, the foam can be reduced. The broken sludge particles partially regain their sedimentation performance, but the filamentous bacteria still exist in the mixed liquid, so the foam phenomenon cannot be fundamentally eliminated. ② Adding defoamers can be used. Bactericides with strong oxidizing properties, such as chlorine, ozone and peroxide, etc. There are also commercially available agents produced using polyethylene glycol and silicone, as well as mixed agents of ferric chloride and copper pickling liquid. The effect of the agent can only reduce the growth of foam, but cannot eliminate the formation of foam. The widely used bactericides generally have side effects, because excessive or improper addition positions will greatly reduce the number of flocculants and the total amount of organisms in the reaction tank. ③ Reduce the sludge age. Generally, the residence time of sludge in the aeration tank is reduced to inhibit the growth of actinomycetes with a longer growth period. ④ Reflux the supernatant of the anaerobic digester. Experiments have shown that the formation of bubbles on the surface of the aeration tank can be controlled by refluxing the supernatant of the anaerobic digester to the aeration tank. ⑤ Add special microorganisms. Some studies have shown that some special bacteria can eliminate the vitality of Nocardia bacteria, including protozoa Nephrodiella. In addition, increasing the predatory and antagonistic microorganisms can control some foam bacteria.