Q&A on the Essentials of Concentrated Wastewater Treatment Technology (1)

Thread Content

1. Question: In the CAST process, the mixture resulting from sludge dewatering is directly discharged into the feedwater pump room, which leads to high levels of COD and SS in the feedwater and affects the denitrification reaction in the treatment tanks (since some of the carbon sources have already been degraded in the preceding aeration grit chamber). How can this problem be solved? Answer: This is an issue that is often overlooked in wastewater treatment plants – namely, the impact of the filtrate resulting from sludge dewatering when it is returned to the biological treatment tanks. Since conditioning agents such as PAC and PAM are added before sludge dewatering, and some of these agents are toxic, they can return to the biochemical reaction tank along with the filtrate during sludge dewatering. There is no technical issue with treating these filtrates; it’s purely a cost matter. By using appropriate sludge conditioning agents and controlling the dosage of these agents as well as the amount of sludge fed into the dewatering machine, it will not have a significant impact on the previous biochemical treatment processes. It is still emphasized that the sludge dewatering efficiency depends on the overall management of the entire sludge treatment process, including the management of the sludge thickening tank. 2. Question: How is the \"sludge age\" determined? How is it controlled? Is it determined by the sludge discharge volume, or is something else used to determine the sludge discharge volume? Answer: Sludge age, F/M, etc., are more accurately considered design parameters rather than control parameters for operation; they serve only as reference values in process control. In actual operation, the sludge discharge rate is usually controlled based on the MLSS value along with empirical considerations; when SVI remains relatively stable, SV30 can also be used as a reference. 3. Question: Our factory uses the Carrousel oxidation ditch process. Sometimes the ammonia nitrogen level in the water exiting the device is even higher than that in the incoming water; the TP level in the incoming water is around 2.5 mg/L, while it is only about 0.2 mg/L in the outgoing water, with 3 aeration units operating at full capacity. No cause could be identified; what’s going on? Answer: A preliminary analysis can only be done based on the information you’ve provided. It’s possible that the wastewater contains a high amount of nitrogen-containing organic compounds, the reaction time is insufficient, and the rate of ammonification of organic nitrogen is greater than the rate of nitrification of ammonia nitrogen. Additionally, it’s possible that there isn’t enough phosphorus, which affects the efficiency of removing ammonia nitrogen through assimilation processes. 4. Question: During operation, a thick layer of sludge accumulates on the surface of the oxidation ditch; the sludge particles, with a diameter of around 1 mm and a yellowish color, often cause a large amount of sludge to float in the secondary sedimentation tank. The sludge turns white, and flocs are carried away with the effluent, resulting in a rapid decrease in SV30 and a loss of treatment efficiency. The accumulated sludge then thins out and disappears. It repeats over and over again; what are the causes and control measures? Answer: This indicates that the sludge has lost its activity, resulting in an increase in ESS. There are two possibilities: one is the oxidation of the sludge itself; the other is sludge poisoning. Based on the phenomena you described, the former possibility is greater; this can be determined by measuring the specific oxygen consumption rate, that is, the ratio of endogenous oxygen consumption to substrate-induced oxygen consumption, so that targeted measures can be taken. 5. Question: How is Section A of the AB method controlled? Is it through continuous recirculation to Section A from the primary sedimentation tank at an equal flow rate? What should the SV30 value be? Is it 5%-10%? Answer: The recirculation ratio for Section A should be relatively high, but the residence time of the sludge in the primary sedimentation tank must not be too short. Although Section A is primarily used for adsorption, it also has a certain degree of biological degradation capability; most of this biological degradation occurs within the sedimentation tank. Only by decomposing the organic matter adsorbed on the surface of the sludge can its adsorption capacity be restored. MLSS should be used for control; SV30 can also be employed when the sludge sedimentation performance is stable. The decision should be based on actual conditions, as a sedimentation ratio of 5%-10% is too low. 6. Question: If a wastewater treatment plant does not achieve optimal performance after being in operation for one or two years, should one consider re-colonizing it with bacteria (replacing the sludge)? What is the difference between replacing the sludge and carrying out bacterial colonization at the beginning? Answer: There is no need to replace it! If the operating conditions remain unchanged, replacing the sludge will have no effect; even if advantageous bacterial strains are added, their effect will only last for a while. What’s important is to maintain proper operating conditions, and if there are design-related issues, they should be addressed promptly. 7. Question: I am debugging industrial wastewater. The process consists of hydrolysis + anaerobic tank + aerobic tank 1 + aerobic tank 2 + sedimentation. Due to installation issues, the aeration in the aeration tank is uneven (using circular aeration nozzles). At each aerator, there is a phenomenon similar to water splashing up and down (with a diameter of about 1 meter), resulting in uneven aeration. What impact does this have on the treatment efficiency? It was also observed that there is little biofilm on the fillers in the aeration area; under microscopic examination, large protozoa were found, but no other organisms were detected. The surface of the biofilm on the fillers is light yellow, while the biofilm outside the aeration area is up to 3 cm thick. Could you please explain this to me? Answer: What you’re describing cannot be considered uneven aeration; it’s a normal phenomenon. You also mentioned that the biofilm is not thick; I wonder what the exact thickness is. It would be good if the biofilm covers the filler basically entirely. If the thickness of the biofilm outside the aeration zone reaches 3 cm, it represents severe balling, and measures such as flushing with large amounts of air or anaerobic depiling need to be taken. 8. Question: I have the following questions regarding the contact oxidation tank. (1) When the contact oxidation tank is drained, how long can the sludge on the packing survive? (2) When the treatment capacity of the contact oxidation tank declines, is it necessary to add nutrients? (3) Regarding foam, do you think using kerosene to eliminate it is effective? If so, how much should typically be used? Answers to the three questions are as follows: (1) After draining the contact oxidation tank, it’s not a matter of how long the biofilm sludge can survive; rather, it’s important to prevent the soft packing from drying out and becoming compacted. Once compacted, it becomes difficult for the packing to expand again once it is submerged in water, so this situation must be avoided. (2) The decline in the treatment capacity of the contact oxidation tank should be considered from multiple perspectives, with controlling the thickness of the biofilm being particularly important. An overly thick biofilm can significantly affect the treatment capacity. It’s also important to drain the tank slowly, as otherwise the soft packing covered with a large amount of biofilm may collapse or deform. (3) Spraying water is more effective for eliminating chemical foam (direct watering is not suitable); I do not approve of using methods such as kerosene to eliminate foam. 9. Question: The average values for the inlet water, outlet water, and various parameters in the biological treatment tank over the past week are as follows: Inlet water: BOD: 253; COD: 810; pH: 7.9; SS: 286; Color intensity: 32 times; Ammonia nitrogen: 28; Total nitrogen: 64; Total phosphorus: 6.0. Outlet water: BOD: 4.8; COD: 74; pH: 8.1; SS: 12; Color intensity: 8 times; Ammonia nitrogen: 7.6; Total nitrogen: 22.8; Total phosphorus: 1.02. Biological treatment tank: MLSS: 4200; MLVSS: 2340; SV%: 47.2; Sludge index: 118.9. The sludge age is 35 days. An improved activated sludge process is used for treatment. At present, the volume of inlet water is approximately 25,000 tons per day (the design capacity is 50,000 tons per day); over 80% of this water is industrial wastewater, with a small amount of high-concentration landfill leachate as well. The process flow consists of an aeration grit chamber, a post-Biological treatment tank, and a post-secondary sedimentation tank; there are no contact tanks or hydrolysis tanks. The biochemical tank is supplied with air by a blower and uses deep-water disc aerators; when water is supplied continuously, the dissolved oxygen level does not reach 1 mg/L, but after the supply of water is stopped, the dissolved oxygen level rises gradually to around 4-5 mg/L. Severe excess inflow water and defects in the structures resulted in a high load on the biochemical tank, as well as a very small sludge thickening tank (180 cubic meters); a considerable amount of excess sludge was sent back to the inlet pump room. The problems encountered at present are: (1) After water is introduced into the secondary sedimentation tanks, suspended activated sludge particles are often found there. Is this due to insufficient settling time or difficulty in sedimentation? (2) Aggregated water fleas have been observed in all three secondary sedimentation tanks; water fleas seem to indicate good water quality, but could their excessive proliferation be caused by a high sludge concentration? (3) Sometimes a thin layer of floating sludge can be seen in the secondary sedimentation tanks. Is this due to poor sedimentation properties of the sludge, insufficient aeration in the biological treatment tank, or delayed sludge return? (4) Moss or algae tend to grow on the triangular weir plates in the secondary sedimentation tanks. What methods can be used to address this issue? (5) I believe the sludge has become severely aged, and it is necessary to keep the MLSS level between 3000–3500, or even lower, while increasing the amount of excess sludge discharged in order to reduce the sludge age. Will this lead to a decrease in the resilience of the biological treatment tank, or will the quality of the effluent improve? Answer: The sludge is indeed somewhat aged, but not to a severe extent. The sludge age is already 35 days, which means the sludge load is less than 0.03. It is sufficient to control 2/3 of the current sludge concentration; the sludge concentration should be gradually reduced. Daphnia have no impact on the effluent, and daphnia should not be included in the samples taken for analysis. Attention must also be paid to controlling the sludge layer in the sedimentation tank; moss and algae on the triangular weir plates in the secondary sedimentation tank can only be removed manually. 10. Question: We use two-stage biological treatment for petrochemical wastewater; the first stage consists of a circular fully mixed aeration tank, while the second stage is a plug-flow aeration tank. The DO level in the first stage is 0.2 mg/L, and that in the second stage is 5.0 mg/L. During this period, the pH of the feed water in the first stage of biological treatment was 8.0, while that of the effluent from this stage was 6.5. After the second stage of biological treatment, the pH was 5.78, which is above the specified range of 6–9. What’s causing this? Answer: A low DO level in the first stage is normal, as it results from a high sludge load. The decrease in pH in the first stage may be due to acidification caused by an excessive load, whereas the drop in pH of the effluent from the second stage is likely due to the consumption of alkalinity during the nitrification process. Since your introduction was too brief, I can only provide a simple analysis and inference as well! To be continued