I have a question regarding the sharp drop in dissolved oxygen at wastewater treatment plants every afternoon!
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Introduction to the wastewater treatment plant: It is a domestic wastewater treatment plant that uses the A2/O process. Its designed capacity is 20,000 m³/day, but the structural facilities are actually designed for a capacity of 30,000 m³/day. The current actual flow rate of wastewater entering the plant is 13,000 m³/day. The COD level in the incoming wastewater is around 70 mg/L, ammonia nitrogen is around 20 mg/L, total nitrogen is around 25 mg/L, and total phosphorus is around 2 mg/L. In the treated wastewater, the COD level is around 20 mg/L, ammonia nitrogen is below 0.5 mg/L, total nitrogen is around 20 mg/L, and total phosphorus is around 0.7 mg/L. The sludge concentration in the aerobic tank is 2500-3000 mg/L ； The dissolved oxygen level in the anaerobic tank is strictly controlled below 0.2 mg/L, while it is kept around 0.5 mg/L in the anoxic tank. The dissolved oxygen level in the aerobic tank is slightly higher, at 3–4 mg/L. To increase the carbon source in the wastewater, 2 bags of glucose are added to the anoxic tank every four hours, and 1 bag (25 kg per bag) is added to the anaerobic tank. As this is a newly built facility, it has been under commissioning for 50 days so far, during which no sludge has been removed. Problem: Over the past week, the dissolved oxygen level in the aerobic tank of this wastewater treatment plant has remained at around 3–4 mg/L during the day. It starts to decline around 5 p.m. each day, dropping to about 0.5 mg/L by around 8 p.m. If the frequency of the fans is not adjusted, the dissolved oxygen level will continue to fall. Upon observation, it was found that the quality of the water entering the system remained unchanged, nor was the frequency of the fan adjusted (an air suspension fan imported from South Korea); yet the dissolved oxygen level dropped significantly, which is quite puzzling. I would appreciate it if someone with expertise could help analyze the reasons. Thank you! Additionally, the total nitrogen level in this plant remains high, with a removal rate of only around 20%; whereas the removal rate of ammonia nitrogen is quite high, at around 97% ； The BOD/TKN ratio is around 1, while the BOD/TP ratio is approximately 10. In my opinion, the lack of carbon sources is responsible for the high levels of total nitrogen, which in turn hinders the removal of total phosphorus. Am I correct?
My personal analysis is that the lack of carbon sources results in high levels of total nitrogen, which in turn hinders the removal of total phosphorus. Is this correct? It is a correct judgment; denitrification can proceed properly only when there is a supply of carbon source for the denitrifying bacteria, with a C/TN ratio generally greater than 4 being the minimum requirement. Additionally, phosphorus-accumulating bacteria have a weaker ability to absorb carbon compared to denitrifying bacteria, and since denitrifying bacteria cannot grow and reproduce properly, phosphorus-accumulating bacteria are even less able to absorb carbon sources in order to release phosphorus. Suggestions: 1. Increase the carbon source according to the C/N ratio. 2. The system must discharge sludge; otherwise, the sludge age will become infinitely large, affecting the proper operation of the biological system.
The return flow rate, as well as the mixed return flow rate, have a significant impact on DO; in addition, water inflow volume and aeration volume also play an important role. Another factor is whether the fan frequency changes at fixed intervals, or whether the voltage drops due to increased power consumption at that time.