When discussing the technical aspects of domestic wastewater treatment, it is essential to grasp numerous critical concepts. The entire treatment process comprises multiple interconnected stages; a minor anomaly in any single step can compromise the integrity and efficiency of the whole system. This article presents a brief analysis of common queries regarding domestic wastewater treatment, developed in collaboration with manufacturers of online water quality monitoring .
To illustrate, let us examine a specific operational scenario through a question-and-answer format:
Case Study:
A wastewater treatment plant (WWTP) has a design capacity of approximately 30,000 m³/d. Currently, the influent flow rate is 18,000 m³/d. The average Chemical Oxygen Demand (COD) concentration in the influent is roughly 180 mg/L, while the Mixed Liquor Suspended Solids (MLSS) concentration is maintained at approximately 3,600 mg/L. The Dissolved Oxygen (DO) level is observed to be around 2 mg/L. Under these operating conditions, two potential issues arise:
Question 1:
When operating two blowers, the MLSS concentration measures 3,600 mg/L. However, when reducing operation to a single blower, the measured MLSS drops to 2,400 mg/L. If the second blower remains off after initially running both, the MLSS continues to decline. Over a prolonged period, could operating with only one blower lead to sludge putrefaction (anaerobic decomposition) within the aeration tank?
Answer:
This phenomenon is primarily attributed to differences in hydraulic mixing and suspension capabilities. Increasing the number of active aeration units enhances turbulence and mixing energy, thereby keeping more solids in suspension and resulting in higher measured MLSS values. Conversely, reduced aeration leads to increased settling velocity of solids within the bioreactor, causing the sampled MLSS concentration to appear lower due to stratification or incomplete homogenization during sampling.
Question 2:
During the treatment process, a small quantity of sludge floats in the secondary clarifier, causing the effluent Total Suspended Solids (TSS) to reach approximately 120 mg/L. Online monitoring indicates that despite rising phosphorus levels, the effluent COD remains low (~15 mg/L), and Ammonia Nitrogen (NH₃-N) is minimal (~0.1 mg/L). While the water surface near the connection between the secondary clarifier and the aeration tank appears clear, the water near the effluent weir is notably turbid. What explains this discrepancy?
Answer:
The turbidity near the weir is likely caused by sludge aging. This occurs when the organic loading rate of the influent is too low relative to the high MLSS concentration being maintained. Such imbalances trigger endogenous respiration, leading to the disintegration of aged flocs and the release of fine particulate matter into the supernatant. To mitigate this, excess sludge should be discharged to reduce the MLSS concentration, thereby restoring the balance between biomass activity and substrate availability.
