Increase in SS in the effluent from the biological unit

1. When the processing load on biological systems (in terms of water volume and concentration) increases, sludge loss can occur. This usually happens when the water volume rises, resulting in a shorter retention time in the secondary sedimentation tank; as a result, the activated sludge does not have enough time to settle before flowing out of the tank, thereby causing sludge loss. At the same time, an increased influent concentration leads to enhanced activity of the activated sludge, which is unfavorable for sedimentation. The effluent is turbid and shows sludge carryover. 2. Operating at too low a load causes the sludge to age, leading to self-oxidation of microorganisms and flocculation breakdown. Silt runoff will also occur. 3. When filaments swell, sludge leakage can occur if it does not have enough time to settle. 4. Additionally, low temperatures, excessive aeration, large pH fluctuations, and the entry of toxic and inert substances into the biological system can also cause sludge loss. First, it is necessary to figure out what caused the problem, and then take appropriate corrective actions.

The last edit to this post was made by zhaolijun on 2016-5-2 at 20:19. What are the signs of sludge leakage from the secondary sedimentation tank? 1. The effluent is clear, but it contains sludge particles; this is what is referred to as \"sludge floating\"“； 2. The effluent is clear, with sludge accumulating in layers and overflowing; the overflow becomes more severe when the water volume is high ； 3. The effluent is turbid, containing sludge particles ； 4. The effluent is turbid, with both the effluent and fine sludge particles overflowing outward ； 5. In the secondary sedimentation tank, lumps of sludge rise from below the water surface; after being disturbed, some of this sludge sinks while another part flows away with the water. The color of this sludge is the same as that of biochemical sludge ； 6. In the secondary sedimentation tank, lumps of sludge rise from beneath the water surface; after being disturbed, some of this sludge sinks while some flows away with the water, and the sludge is dark in color ； 7. The scum on the surface of the secondary sedimentation tank flows away with the water ； 8. Foam on the surface of the secondary sedimentation tank; sludge particles adhered to the foam flow away with the water ； 9. In some areas of the secondary sedimentation tank, sludge flows out with the water, and this phenomenon becomes more severe when the water volume is high ； 10. The deposits attached to the weir of the secondary sedimentation tank enter the effluent stream from the secondary sedimentation tank. Reasons and influencing factors for sludge loss in the secondary sedimentation tank: 1. The effluent is clear but contains sludge particles; this is what is referred to as \"sludge floating\"“ ； •Low F/M ratio, insufficient bacterial nutrients ； •The DO level is too high, indicating an excessive degree of oxidation ； •Slight aging of sludge ； •The c/n/p ratio is not appropriate. 2. The effluent is clear, with sludge accumulating in layers and overflowing; the overflow becomes more severe when the water volume is high ； •The surface load on the secondary sedimentation tank is too high ； •Filamentous sludge bulking occurs in the aerobic tank ； •The sludge level in the secondary sedimentation tank is too high ； •Sludge poisoning. • The P H fluctuates too much. 3. The effluent is turbid, containing sludge particles ； •In the aerobic tank, organic matter is not completely degraded, and the organic load is too high ； •Excessively high or low DO in the aerobic tank ； •Excessive aeration or abnormal pH ； •Decomposition of aged sludge ； •The aerobic tank has an excessively high level of mineral oil ； •Sludge poisoning ； •Severe imbalance of C/N/P ； •The water temperature in the aerobic tank is below 5°C or above 38°C. 4. The effluent is turbid, with both the effluent and fine sludge particles overflowing outward ； •The sludge poisoning is quite severe ； •The high mineral oil content in the aerobic tank severely affects sludge flocculation ； •Denitrification occurs in the secondary sedimentation tank. • The pH is greater than 10 or less than 5.5. Lumps of sludge rise from below the water surface in the secondary sedimentation tank; after being stirred, some of this sludge sinks while some flows away with the water. The color of this sludge is the same as that of biochemical sludge ； Denitrification or partial denitrification occurs in the secondary sedimentation tank. 6. In the secondary sedimentation tank, lumps of sludge rise from beneath the water surface; after being disturbed, some of this sludge sinks while some flows away with the water, and the sludge is dark in color ； Anaerobic conditions occur in the secondary sedimentation tank, or there are local anaerobic dead zones. 7. The scum on the surface of the secondary sedimentation tank flows away with the water ； A fault in the aerobic tank causes foam or scum to flow into the secondary sedimentation tank, where it forms scum and is carried away with the effluent. 8. Foam on the surface of the secondary sedimentation tank; sludge particles adhered to the foam flow away with the water ； Failure in the aerobic tank; foam flows into the secondary sedimentation tank. 9. In some areas of the secondary sedimentation tank, sludge flows out with the water, and this phenomenon becomes more severe when the water volume is high ； •The mud surface is too high ； •The load on the secondary sedimentation tank exceeds the designed load by a significant amount ； •Filamentous fungus swelling ； •The outlet weir is uneven or there is misalignment in the secondary sedimentation tank. 10. The deposits attached to the weir of the secondary sedimentation tank enter the effluent stream from the secondary sedimentation tank. The effluent from the secondary sedimentation tank is rich in N, P, etc., which causes algae and other deposits to form at the weir opening, and these are carried away by the flow of the effluent. 11 Effects of the secondary sedimentation tank itself on sludge migration: • Faults in the sludge skimmer; • Uneven water outlet weir; • Deviation in flow within the inlet tank that operates with water entering from the center and exiting around the edges, as well as uneven distribution of water flow; • Delayed sludge removal resulting in an excessively high sludge level in the secondary sedimentation tank ； •Inlet flow velocity and flow rate spike or exceed the design load ； •Local dead zones or prolonged sludge retention lead to anaerobic fermentation or denitrification ； •The sludge scraper is rotating too fast. Solutions and preventive measures for sludge leakage in the secondary sedimentation tank: 1. Strengthening the monitoring of aeration volume and influent water quality is a prerequisite for ensuring the stable operation of the biological treatment system. Once any fluctuations in the quality of the incoming water are detected, appropriate adjustments can be made promptly to prevent high-concentration wastewater from causing damage to the biological treatment system. 2. To maintain sludge activity, long-term operation of the system at low load should be avoided. 3. The dosage of PAC chemical in the air flotation system should be appropriate; excessive PAC can cause the sludge to disintegrate into flocs, and may even lead to sludge poisoning. 4. Low F/M ratio, insufficient bacterial nutrients ； The bacteria become smaller, the pellicle particles are smaller and their sedimentation capacity decreases, causing them to flow away with the water ； Increase nutrients or reduce MLSS. 5. High DO levels, excessive reaction depth ； Excessive oxidation of sludge causes bacteria to undergo auto-oxidation and become smaller ； Reduce dissolved oxygen or shorten aeration time. 6. Slightly age the sludge ； Enhance sludge removal, reduce DO levels, and supply nutrients. 7. The C/N/P ratio is not appropriate; adjust it to be at least 100:5:1. 8. After the sludge breaks down into floccules, it accumulates on the surface of the secondary sedimentation tank as scum. The reasons for this are: 1) Excessive aeration in the aerobic tank leads to the auto-oxidative decomposition of the sludge, and frequent shearing results in smaller sludge particles, thereby reducing the sludge’s flocculation ability or preventing flocculation altogether ； 2). Sludge aging can occur due to a low F/M ratio, excessive sludge age, insufficient addition of nutrients, over-aeration, and high sludge concentration ； Solutions: 1. Reduce DO, 2. Add nutrients ； 3. Increase F/M; 4. Improve sludge removal – dead sludge that does not flocculate cannot be converted back into active sludge ； 5. Add PAC or PAM to promote flocculation. Denitrification sludge floating phenomenon: Bubbles rise and burst in the secondary sedimentation tank, and lumps of sludge float to the surface along with bubbles. In the sludge sedimentation test, clear bubbles could be seen attached to the sludge in the measuring cylinder, indicating normal sedimentation properties; however, after 1–2 hours, the sludge clumps floated to the surface. After stirring with a glass rod, the sludge resumed normal flocculation and sedimentation. Microscopic examination confirmed that biological activity and the microbial community were normal. Remedial measures: 1. Increase the dissolved oxygen at the biochemical outlet (this step may accelerate the nitrification process) ； 2. Increase the recirculation flow in the secondary sedimentation tank to reduce the sludge retention time there; combined with an increase in biochemical DO, this will keep the dissolved oxygen level in the secondary sedimentation tank above 0.4 mg/L ； 3. Strengthen sludge discharge from the secondary sedimentation tank ； 4. Intermittent aeration is possible if conditions permit; when aeration is stopped, a large amount of sludge from the secondary sedimentation tank is returned to the biochemical tank ； 5. Increase the internal recirculation flow rate and strictly control the DO level in the anoxic zone to below 0.5 mg/L, so as to ensure that denitrification can take place smoothly in this zone. Sludge aging and sludge floating phenomenon: During sedimentation tests, almost no sedimentation of flocs is observed; the active sludge flocs are small in size, and there is poor layered sedimentation as well as poor compressibility of the sludge. Microscopic examination of the sludge in the aeration tank and the scum in the secondary sedimentation tank revealed a normal biological community; no Nocardia or filamentous organisms were found, and the floating sludge contained small bubbles enclosed within it. Reason: Excessive aeration or prolonged operation at low load levels causes the sludge to undergo aerobic endogenous respiration continuously, resulting in a high number of dead bacteria. This leads to a reduction in the active components of the activated sludge, preventing the formation of proper flocs. The activated sludge has poor flocculation properties, and there are many inert residues left behind from the dead bacteria; these residues are light in weight and tend to float to the surface. The microbial flocs become dispersed and small; the inert components of dead bacteria, combined with the aeration bubbles, result in a reduced density that causes them to rise to the surface. Meanwhile, the solid load on the surface of the secondary sedimentation tank is much higher than the empirical values, the sludge flocs are small and their settling speed is reduced, which leads to a large amount of sludge attaching to the aeration bubbles and rising together with them. This can be fully demonstrated by reducing the solid surface load in the secondary sedimentation tank, which leads to a decrease in the amount of sludge. Solutions: 1. Reduce the aeration volume, or stop aeration for a short period of time ； 2. Increase the load or add more carbon source, paying special attention to the supplementation of nutrients and ensuring that the C:N:P ratio is greater than 100:5:1 (to prevent organic load shocks caused by too rapid an increase in load) ； 3. Appropriately increase the sludge discharge volume; (due to the aging of the sludge, its compressibility increases and the amount of sludge decreases, so be careful not to discharge too much sludge.) ； 4. Increase the return flow to the secondary sedimentation tank in order to reduce the sludge retention time in the aerobic tank and prevent denitrification in the secondary sedimentation tank ； 5. Appropriately increase the water exchange volume in the aerobic tank. The impact of water temperature on the aerobic tank: When the temperature in the aerobic tank exceeds 35°C, the sludge flocs begin to break down, resulting in a decline in their settling capacity ； When the temperature exceeds 40°C, the protozoa disappear and the effluent becomes turbid ； When the temperature exceeds 45°C, dispersed flocs become dominant, and the sedimentation performance deteriorates severely. A large amount of sludge floats on the surface of the secondary sedimentation tank, forming a thick layer of sludge. Low water temperatures also lead to a decrease in sludge activity, resulting in longer times required to decompose organic matter; this is manifested by an increase in the size of the activated sludge flocs in the secondary sedimentation tank, as well as the release of smaller particles through the outlet weir ； The organic matter is not completely decomposed, resulting in turbid effluent. Analysis of secondary sedimentation tank failures shared by Haiyou: Since the end of November 2011, sludge leakage has occurred frequently in the secondary sedimentation tank, 2 to 3 times per week on average. The activated sludge flowed out in large quantities along with the water discharged from the tank, causing the effluent to become turbid, which severely affected the operation of the subsequent wastewater reuse system. As a result, the COD level in the effluent increased from below 60 mg/L to around 100 mg/L, exceeding the specified limits. Additionally, a large amount of sludge was lost from the biochemical system, impacting its performance. Upon comparison with the test data from January to March 2011, it was found that there was no significant change in the quality of the water entering the A/O aeration tank; however, there was a notable change in the volume of water entering the tank. The average water flow rate from January to March was around 350 t/h, while after November it dropped to about 190 t/h – almost half of the previous level. This reduction in water volume led to a decrease in the load on the biochemical system, and there was not enough “food” available for the microorganisms’ metabolism, resulting in their own oxidation and decomposition, sludge flocculation breakdown, and a decline in their activity. This is the main reason for sludge leakage in the secondary sedimentation tank. At the same time, the flocculant PAC used in the two-stage air flotation pretreatment was not adjusted according to the decrease in water volume; as a result, the dosage was too high. The excess PAC had a toxic effect on the activated sludge, reducing its stability, and in severe cases it could cause the sludge to die due to poisoning. The air volume in the A/O aeration tank was not adjusted in a timely manner; as a result, the aerobic section maintained a relatively high dissolved oxygen concentration for an extended period. This accelerated the metabolism of microorganisms and was also a cause of the self-oxidation and decomposition of the sludge. Solutions: ① Due to the limited water supply from upstream, it is not possible to increase the water volume; therefore, the only option is to raise the concentration of the incoming water. After consideration, it has been decided to integrate the recycled water from the homogenization tank of another salt-containing wastewater system into the oil-containing wastewater treatment system; after pretreatment, this water enters the A/O biological treatment system, thereby raising the COD level of the water fed into the biological system to 500 mg/L–600 mg/L and increasing the load on the biological system as a result. ②The dosage of PAC in the two-stage air flotation tanks was optimized. Through multiple experiments, it was found that when the COD level of the water entering the flotation process is between 400 mg/L and 600 mg/L, and the oil content is between 15 mg/L and 25 mg/L, the optimal dosage is 150–200 g of PAC per ton of water (with a solid content of PAC of around 15%). This measure not only prevents excessive PAC from affecting the activated sludge but also saves on chemical usage, achieving two benefits at once. ③Adjust the blower volume of the A/O aeration tank in a timely manner to keep the dissolved oxygen concentration in the aerobic zone between 2 mg/L and 4 mg/L, thereby preventing excessive air flow from causing the sludge to become dispersed. ④Nutrients are added to the biochemical system; based on the amount of nitrogen, this is sufficient, so phosphates (disodium hydrogen phosphate) are added at a rate of 25 kg per day to provide adequate nutrition for the microorganisms. Following adjustments and recovery, the 30-minute sedimentation ratio of the sludge decreased to 50–55, and the sludge concentration remained stable at 2.5 g/L to 3.5 g/L. Sludge leakage from the secondary sedimentation tank ceased, and the effluent became clear again. At the same time, the amount of excess sludge discharged was reduced, thereby alleviating the burden on the sludge dewatering system.