The effect of influent suspended solids on sludge concentration
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For municipal domestic wastewater, the designed value for suspended solids in the influent is less than 200 mg/l, but in practice it ranges from 500 to 700 mg/l. Operation remains relatively stable, yet the sedimentation ratio remains low, by 15% to 25%, while the sludge concentration stays high, between 3500 and 4500 mg/l. Every attempt to increase the sedimentation ratio by reducing the amount of sludge discharged affects the quality of the effluent, so operations continue as they are. However, during summer, the situation becomes even more complicated due to wastewater from wool washing plants and slaughterhouses, resulting in very poor performance. I’m not sure how to proceed with operations; the aeration level has already been increased to its maximum and cannot be adjusted any further. Increasing the amount of sludge discharged would lead to another decrease in the sedimentation ratio. Currently, the sedimentation ratio is 19% and the sludge concentration is 4000 mg/l. I would appreciate some advice from the moderators
The reflux ratio of the mixture can be increased, and the DO control range in the aerobic tank can be adjusted; the sludge concentration is generally within the normal range. The sludge concentration of 4000 you mentioned includes both active sludge and aged sludge, and microscopic analysis is needed to determine the actual concentration of active sludge. I’m not sure if this is correct; it’s just for reference.
This post was last edited by zhaolijun on 2015-6-12 at 18:54. Based on your description, there are the following issues: 1. The SVI is too low, 2. The SS level in the influent water is too high, 3. There is an excessive amount of inorganic substances and inert materials in the activated sludge, 4. The dissolved oxygen level is too high. Reasons: 1. Excessively high SS in the influent water leads to an increased proportion of inorganic and inert substances in the activated sludge; maintaining SV30 within a certain range results in insufficient activity of the activated sludge ； 2. To maintain a passive increase in the removal rate by raising dissolved oxygen, excessive oxidation of the activated sludge was exacerbated, resulting in an even greater shortage of the active components in the activated sludge ； 3. Due to excessive oxidation of the activated sludge, its aging degree increases, leading to an increase in inorganic and inert substances within the activated sludge, and the SVI continues to decline ； 4. During low-load and short-duration operation, the system can barely function; over time, especially when the load increases, operational failures occur, and the removal efficiency as well as the quality of the effluent cannot be guaranteed. Solution: 1. Reduce dissolved oxygen to 2–3 or 1.5–2.5 ； 2. Ensure the addition of nutrients ； 3. Increase the return flow to the secondary sedimentation tank by 100–150%, and restore it to the normal level once operation is stable ； 4. Increase the sludge discharge while maintaining SV30 (SV will increase after the DO level drops, so there’s no need to worry about this). 5. Don’t be misled by the apparent value of MLSS; although it may be high, it does not represent the actual value, and in such cases SVI cannot be controlled using the conventional recommended values.
Based on what you’ve described, the high SS level in the influent leads to an increased proportion of the inorganic component in the activated sludge. Keeping SV30 within a certain range results in a shortage of active components in the activated sludge, causing an artificially high sludge concentration; therefore, the MLVSS value should be low. It is recommended to increase the return ratio from the secondary sedimentation tank, and if necessary, active microbial strains can be added.