In the daily operation of wastewater systems, secondary sedimentation tank abnormalities are the most common phenomenon. For example, increased suspended solids in the secondary sedimentation tank effluent, low dissolved oxygen, floating sludge and floating black sludge in the secondary sedimentation tank, these are likely experiences for many. Today, let's talk about the analysis and solutions for four common abnormal phenomena in secondary sedimentation tanks.

1. Increased suspended solids content in secondary sedimentation tank effluent

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

(2) A sudden increase in influent flow increases the surface hydraulic load of the secondary sedimentation tank, resulting in an increased upward flow velocity, affecting the normal settling of activated sludge, and the water flow carrying sludge fragments overflowing through the effluent weir. The solution is to make full use of the equalization tank to make the influent as even as possible.

(3) The activated sludge concentration in the aeration tank is too high, the sludge-water interface in the secondary sedimentation tank is close to the water surface, and some sludge fragments overflow through the effluent weir. The solution is to increase the amount of waste sludge discharged.

(4) Activated sludge disintegration causes the flocculation of sludge to decrease or disappear, and sludge fragments flow out with the water. The solution is to find the cause of sludge disintegration and eliminate and solve it one by one.

(5) Poor working condition of the sludge suction (scraping) machine causes short-circuiting of sludge or water flow in the secondary sedimentation tank, local sludge cannot be returned in time, some sludge stays in the secondary sedimentation tank for too long, and after the sludge is anoxically decomposed, it overflows with the water flow. The solution is to repair the sludge suction (scraping) machine in time to restore its normal working condition.

(6) The activated sludge stays in the secondary sedimentation tank for too long, and the sludge decomposes due to lack of oxygen and overflows with the water flow. The solution 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, denitrification of sludge occurs in the secondary sedimentation tank, and nitrogen gas carries large pieces of sludge to the surface and overflows with the water flow. The solution is to increase the amount of return sludge and shorten the sludge residence time in the secondary sedimentation tank.

2. Low or high dissolved oxygen in secondary sedimentation tank effluent

(1) Activated 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 secondary sedimentation tank effluent. The solution is to increase the amount of return sludge and shorten the residence time.

(2) Poor working condition of the sludge suction (scraping) machine causes local sludge in the secondary sedimentation tank to not be returned 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 secondary sedimentation tank effluent. The solution is to repair the sludge suction (scraping) machine in time to restore its normal working condition.

(3) A sudden increase in water temperature increases the oxygen consumption of aerobic microorganisms' physiological activities, strengthens the activities of anaerobic microorganisms in local anoxic zones, and ultimately leads to a decrease in dissolved oxygen in the secondary sedimentation tank effluent. The solution is to try to extend the residence time of wastewater in pretreatment facilities such as homogenization and equalization, make full use of the volume of the equalization tank to circulate high-temperature water, or enhance pre-aeration to promote water evaporation to reduce the temperature.

(4) The organic load of the aeration tank influent is too low or the oxygenation capacity of the aeration tank is too high. In this case, the dissolved oxygen in the secondary sedimentation tank effluent is too high, but the water quality is good. Measures can be taken to increase the influent load by adjusting more water from the equalization tank, or to reduce the oxygenation capacity by reducing the number of operating blowers.

(5) The mixed liquor in the aeration tank is poisoned, and microorganisms cannot utilize the dissolved oxygen in the water, which may also cause excessive dissolved oxygen in the secondary sedimentation tank effluent. The excessive dissolved oxygen in the secondary sedimentation tank effluent formed in this way is temporary, and it will be followed by a rapid decrease in dissolved oxygen and deterioration of effluent water quality. At this time, the source of toxic substances should be identified and eliminated.

3. Sludge floating in the secondary sedimentation tank

Sludge floating in the secondary sedimentation tank refers to the phenomenon that sludge in the secondary sedimentation tank undergoes acidification or denitrification, causing the sludge to float to the surface of the secondary sedimentation tank. These floating sludge themselves do not have quality problems, and their biological activity and settling performance are normal.

The main reason for floating is that these normal sludge stay in the secondary sedimentation tank for too long, and acidification occurs due to the gradual consumption of dissolved oxygen, and gases such as H2S are attached to the sludge flocs, reducing their density and causing sludge to float. When the sludge age of the system is long and nitrification occurs, the mixed liquor entering the secondary sedimentation tank will contain a large amount of nitrate. Due to the lack of sufficient dissolved oxygen (DO<0.5mg/L) in the secondary sedimentation tank, denitrification occurs, and the nitrogen gas produced by denitrification will also be attached to the sludge flocs, reducing their density and causing sludge to float.

Measures to control sludge floating include: firstly, timely discharge of excess sludge and increase the amount of return sludge to prevent sludge from staying in the secondary sedimentation tank for too long; secondly, strengthen the oxygenation at the end of the aeration tank to increase the dissolved oxygen content in the mixed liquor entering the secondary sedimentation tank, ensuring that the sludge in the secondary sedimentation tank is not in an anaerobic or anoxic state. For sludge floating caused by denitrification, the discharge of excess sludge can also be increased to reduce the sludge age, and the degree of nitrification can be controlled to control denitrification.

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

Black lumpy sludge appearing on the surface of the secondary sedimentation tank is usually caused by sludge decomposition. Too little aeration causes sludge to be anoxic in the secondary sedimentation tank, or the sludge production in the aeration tank is large while the discharge of excess sludge is small, causing the sludge to stay in the secondary sedimentation tank for too long, or the sludge hopper is unreasonable during gravity sludge discharge, making it difficult for the sludge to slide down, or some sludge suction pipes of the scraper are not smooth and there are dead corners that cannot be scraped, all of which can cause sludge to stay and accumulate locally in the secondary sedimentation tank for a long time, resulting in anaerobic metabolism, producing a large amount of H2S, CH4 and other gases, which are wrapped in sludge blocks, causing the sludge to float in large blocks, and the color is black. Sludge decomposition and floating is different from general sludge floating. When sludge decomposes and floats, the sludge will rot and turn black, producing a foul odor.

The solution is to ensure the timely discharge of excess sludge, eliminate faults in sludge discharge equipment, remove sludge from the sedimentation tank walls or dead corners, reduce the nitrification level of sludge in the aerobic treatment system, increase the sludge return flow, and prevent the entry of decayed sludge from other treatment structures.