The turbidity of the water leaving the secondary sedimentation tank increases suddenly, the settling ratio in the aerobic tank drops abruptly, and there are many bubbles on the surface of the water in the aerobic tank – what’s the cause?
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The wastewater from our company’s sewage treatment plant originates mainly from the acidic and alkaline waters produced during the cleaning of resin columns in the workshops; the COD level is between 1800 and 2300, while the ammonia nitrogen level is below 80； The treatment process at the sewage treatment plant is: equalization tank – pre-acidification tank – anaerobic tank – anoxic tank – aerobic tank (three stages) – secondary sedimentation tank – outlet ； The daily treatment capacity is 300 cubic meters; after treatment, the COD level can be reduced to below 100, and the ammonia nitrogen level to below 10. After the workshop stopped production for maintenance on July 15, the wastewater treatment plant supplies water at a rate of 30–40 cubic meters per day (water supply is only arranged for 3–4 hours during the day shift; no water supply is provided during the night shifts). The aeration period has been adjusted to from 7 a.m. to 12 a.m., with no aeration taking place during the later night shifts. 25 KG of nutrients (glucose) are added daily, and operations remained normal until July 24. Starting from the day shift on July 24, the turbidity of the water exiting the secondary sedimentation tank increased significantly, while the settling ratio dropped sharply to below 10 (previously, the settling ratio in the aerobic tank was always between 35 and 40). Additionally, there were many bubbles on the surface of the water in the aerobic tank during aeration. What could be the cause?
The microorganisms die off to some extent; as a result, the suspended solids in the effluent from the secondary sedimentation tank decrease, meaning that the sludge settling ratio drops. The bubbles on the surface of the biological tank are biological foam that does not break easily – they arise from the bubbles generated during aeration after the sludge breaks down into smaller particles. It is necessary to check the nutrient ratios to see if they are appropriate; not only should a carbon source be added, but nitrogen and phosphorus sources as well must be supplied, with the ratio being crucial
BOD5 was measured, the aeration rate was adjusted, nitrogen and phosphorus were added in a ratio of BOD:N:P=100:5:1, and flour was used to adjust the C source
In simple terms, too low a load and excessive aeration lead to sludge aging and disintegration, resulting in a significant drop in sludge concentration, turbidity in the effluent, and a deterioration in the quality of the effluent water.