In the daily operation of sewage system, the anomaly of secondary sedimentation tank is the most common phenomenon. For example, the increase of suspended matter in the effluent of the secondary sedimentation tank, the low level of dissolved oxygen, the floating mud of the secondary sedimentation tank and the floating black sludge may have been experienced by everyone. Today, we will talk about the analysis and solution of 4 common abnormal phenomena in the second sedimentation tank.

1. The content of suspended matter in the effluent of secondary sedimentation tank increases

(1) Activated sludge swelling makes sludge settling performance worse, the mud-water interface is close to the water surface, and some sludge fragments overflow through the effluent weir. The countermeasure is to analyze the causes of sludge swelling and eliminate them one by one.

(2) The sudden increase of water intake increases the hydraulic load on the surface of the secondary sedimentation tank, resulting in increased rising velocity, affecting the normal settlement of activated sludge, and the water flow carries sludge fragments through the effluent weir. The countermeasures are to give full play to the function of the regulating pool and make the water intake as balanced as possible.

(3) The activated sludge concentration of the aeration tank is high, the mud-water interface of the secondary sedimentation tank is close to the water surface, and some sludge fragments overflow through the effluent weir. The countermeasure is to increase the discharge of residual sludge.

(4) Activated sludge disintegration causes the flocculation of the sludge to decrease or disappear, and the sludge debris is discharged with the water flow. Countermeasures are to find the causes of sludge disintegration, eliminate and solve one by one.

(5) The mud suction (scraping) machine is in poor working condition, resulting in short flow of sludge or water in the secondary sedimentation tank, local sludge can not return in time, some sludge stays in the secondary sedimentation tank for too long, and the sludge overflows with the water after anoxic decomposition. The countermeasure is to repair the suction (scraping) mud machine in time to restore its normal working state.

(6) The residence time of activated sludge in the secondary sedimentation tank is too long, and the sludge will overflow with the water after decomposition due to hypoxia. The countermeasure is to increase the amount of return sludge and shorten the residence time in the secondary sedimentation tank.

(7) When the water temperature is high and the nitrate content in the water is high, the sludge denitrification and nitrogen removal phenomenon occurs in the secondary sedimentation tank, and the large sludge in the nitrogen coating zone floats to the water surface and overflows with the water flow. The countermeasures are to increase the amount of return sludge and shorten the residence time of sludge in the secondary sedimentation tank.

2. The dissolved oxygen of the secondary sedimentation tank is low or high

(1) The residence time of activated sludge in the secondary sedimentation tank is too long, and the aerobic microorganisms in the sludge continue to consume oxygen, resulting in the decrease of dissolved oxygen in the discharge water of the secondary sedimentation tank. The countermeasure is to increase the amount of return sludge and shorten the residence time.

(2) The working condition of the mud suction (scraping) machine is not good, resulting in the local sludge in the secondary sedimentation tank can not return in time, some sludge stays in the secondary sedimentation tank for too long, and aerobic microorganisms in the sludge continue to consume oxygen, resulting in a decrease in dissolved oxygen in the discharge water of the secondary sedimentation tank. The countermeasure is to repair the suction (scraping) mud machine in time to restore its normal working state.

(3) The sudden rise in water temperature increases the oxygen consumption of aerobic microbial physiological activities and strengthens the anaerobic microbial activities in the local anoxic area, which ultimately leads to the decrease of dissolved oxygen in the effluent water of the secondary settling tank. The countermeasures are to try to extend the residence time of sewage in pretreatment facilities such as homogenization regulation, and make full use of the volume of the regulating tank to circulate high-temperature water. Or by strengthening the pre-aeration to promote water evaporation to reduce the temperature.

(4) If the influent organic load of the aeration pool is low or the oxygen content of the aeration pool is too large, the dissolved oxygen in the effluent of the secondary sedimentation pool is too high but the water quality is very good, the method of transferring more water from the regulating pool can be adopted to increase the influent load, or the method of reducing the number of fans in operation and reducing the oxygen content can be adopted.

(5) The mixture of aeration tank poisoning, microorganisms can not use the dissolved oxygen in the water may also cause the secondary sedimentation tank effluent dissolved oxygen too high. In this way, the phenomenon of excessive dissolved oxygen in the effluent of the secondary sedimentation tank is temporary, followed by the phenomenon of rapid reduction of dissolved oxygen and the deterioration of effluent quality. At this point, the source of the toxic substance should be identified and eliminated.

3. Sludge floating in the secondary sedimentation tank

Sludge floating in secondary sedimentation tank refers to the phenomenon that sludge acidification or denitrification occurs in the secondary sedimentation tank, resulting in sludge floating to the surface of the secondary sedimentation tank. The floating sludge itself does not have quality problems, and its biological activity and settling performance are normal.

The main reason for floating is that these normal sludge stays in the secondary sedimentation tank for too long, and acidification occurs due to the gradual consumption of dissolved oxygen, which produces H2S and other gases attached to the sludge floc, reducing its density and causing the sludge to float. When the sludge age of the system is long, after nitrification, the mixed liquid entering the secondary sedimentation tank will contain a large amount of nitrate. The sludge will undergo denitrification due to the lack of sufficient dissolved oxygen (DO<0.5mg/L) in the secondary sedimentation tank, and the nitrogen generated by denitrification will also adhere to the sludge floc, reducing its density and causing the sludge to float up.

The measures to control sludge floating are: first, to discharge the residual sludge in time and increase the amount of returned sludge, so that the sludge does not stay in the secondary sedimentation tank for too long; The second is to strengthen the oxygen filling at the end of the aeration tank, increase the dissolved oxygen content in the mixed liquid entering the secondary sedimentation tank, and ensure that the sludge in the secondary sedimentation tank is not in an anaerobic or anoxic state. For the sludge rising caused by denitrification, it can also increase the discharge of residual sludge, reduce the age of sludge, and achieve the purpose of controlling denitrification by controlling the degree of nitrification.

4. Black lump sludge appears on the surface of the secondary sedimentation tank

The appearance of black lump sludge on the surface of secondary sedimentation tank is usually caused by sludge decay. Too small aeration rate causes the sludge to lack oxygen in the secondary sedimentation tank, or the sludge generated in the aeration tank is large and the remaining sludge discharge is small, so that the sludge stays in the secondary sedimentation tank for too long, or the mud hopper is unreasonable during the gravity sludge discharge, making it difficult for the sludge to slide down, or part of the suction pipe of the mud scraper is not smooth and there are dead corners that cannot be scraped. It will cause the sludge to stay and deposit in the secondary sedimentation tank for a long time in the local anaerobic metabolism, produce a large number of H2S, CH4 and other gases, wrapped in the mud, and promote the sludge to float up in large chunks, and the color is black. Sludge decay and float up is different from the general sludge float up, when the sludge decay and float up, the sludge will rot and become black, resulting in foul odor.

The solution is to ensure the timely discharge of the remaining sludge, eliminate the failure of the mud discharge equipment, remove the sludge from the inner wall of the sedimentation tank or some dead corners, reduce the degree of nitrification of the sludge in the aerobic treatment system, increase the sludge return flow, and prevent the entry of the rotten sludge from other treatment structures.