Aerobic pool for sewage treatment: Detailed explanation and treatment of six abnormal problems
In the sewage treatment process, the aerobic pool, as the core treatment unit, undertakes the key mission of purifying sewage. However, various abnormal problems frequently occur during operation, seriously affecting the treatment effect and system stability. This article will focus on the common abnormalities of aerobic pools, deeply analyze their causes, and help to accurately solve the problems.
1. Causes of sludge bulking in aerobic pools
(I) Factors of imbalance between microorganisms and the environment
1. Abnormal dissolved oxygen: The dissolved oxygen in the aerobic pool is low for a long time, and the microorganisms cannot obtain sufficient oxygen for aerobic respiration, and normal metabolism is blocked; long-term high dissolved oxygen will aggravate the oxidation of microorganisms themselves, destroy the structure and growth balance of microbial communities, and cause the dominant bacteria such as filamentous bacteria to over-grow, causing sludge bulking. For example, in the aerobic pool of a printing and dyeing factory, the dissolved oxygen was continuously lower than 1mg/L due to the failure of the aeration equipment, and severe sludge bulking occurred after one week.
2. Excessive sulfide content: When the sulfide content of raw water or anaerobic effluent is too high, sulfur bacteria will multiply in large numbers. Sulfur bacteria change the structure and properties of sludge during metabolism. At the same time, their excessive growth inhibits the activity of other microorganisms, breaks the ecological balance of microorganisms, and eventually leads to sludge expansion.
3. Load fluctuation: If the load of the aerobic pool is low for a long time, the microorganisms are in a hungry state, the activity is reduced, and it is easy to cause the proliferation of filamentous bacteria; if the load is high for a long time, it is difficult for microorganisms to completely decompose organic matter in a short time, resulting in substrate accumulation, which also creates favorable conditions for the growth of filamentous bacteria. Due to the surge in sewage volume in the peak production season of a food processing plant, the load of the aerobic pool far exceeded the design value, and the sludge expansion problem became prominent two weeks later.
4. Unsuitable water temperature: Suitable water temperature is the basis for the normal growth and metabolism of microorganisms. If the water temperature of the aerobic pool is high, exceeding the temperature range suitable for the growth of most microorganisms (usually 20-30℃), the enzyme activity of the microorganism is affected, the metabolic pathway changes, and some filamentous bacteria may reproduce in large numbers because they are more adapted to high temperature environments, thereby causing sludge expansion.
(II) Nutrition and water quality factors
1. Nutritional imbalance or deficiency: The nutrients in the sewage are unbalanced or lack key nutrients N and P, and the microorganisms lack the necessary material basis for growth. When the carbon, nitrogen and phosphorus ratio deviates from the classic 100:5:1, the microbial metabolic activity is limited, and the filamentous bacteria have more advantages in the nutrient competition, and the large-scale reproduction leads to sludge expansion.
2. Abnormal pH value: If the influent pH value is too high or too low, it will inhibit the activity of enzymes in the microorganisms, interfere with the normal physiological and biochemical reactions of the microorganisms, affect the ecological balance of the microorganisms in the activated sludge, hinder the growth of microorganisms, and the filamentous bacteria take the opportunity to grow in large quantities, which eventually causes sludge expansion.
(III) Operation and management factors
If the sludge age of the aerobic pool sludge is too long, the microorganisms will enter an aging state, their metabolic activity will weaken but the oxygen consumption will increase, which is easy to lead to insufficient dissolved oxygen. In anoxic environment, filamentous bacteria have a greater survival advantage than other microorganisms, so they reproduce in large quantities and cause sludge expansion.
2. Reasons for sludge disintegration and more fine sludge in the supernatant in the aerobic tank
(I) Improper sludge load
1. Too small load: The sludge load in the aerobic tank is small, and excessive aeration will cause the sludge to oxidize itself. Microorganisms are damaged by excessive oxidation, the flocculation of sludge deteriorates, the structure becomes loose, and they are easy to disintegrate during the sedimentation process, forming fine sludge. At this time, the effluent is clear but has more fine sludge, and the COD is not high.
2. Too large load: When the sludge load in the aerobic tank is too large, the sludge adsorption performance deteriorates, and the microorganisms cannot completely decompose the organic matter within a limited time. Undecomposed organic matter will affect the sedimentation performance of the sludge. Microscopic examination shows that the sludge structure is loose, the effluent is turbid and opaque, and the COD is high.
(II) Sludge age and discharge issues
Excessive sludge discharge from the aerobic tank will result in a too short sludge age in the aerobic tank. When the sludge age is too short, it is difficult for microorganisms to form a mature and stable structure, and the sludge flocs are small and fragile, which easily forms fine sludge in the secondary sedimentation tank. Generally speaking, the SVI value is more suitable in the range of 70-120. Within this range, there is less fine sludge in the secondary sedimentation tank. If it exceeds this range, problems such as sludge disintegration may occur.
(III) Toxic and aging factors
1. Invasion of toxic substances: The influent of the aerobic tank contains toxic substances, such as heavy metal ions, phenolic compounds, etc., which will poison microbial cells, destroy the physiological functions of microorganisms, and cause sludge disintegration. In addition, sludge aging, that is, the sludge age is too long, the microbial activity is reduced, and the cell structure gradually disintegrates, which will also make the sludge turbid, with fine sludge, and the effluent COD is high. A large number of rotifers can be seen during microscopic examination.
2. Nutritional imbalance: Insufficient nutrients in the aerobic pool or an unbalanced nutrient ratio, especially low N and P, lack of necessary nutrients for microbial growth, limited metabolic activity, and inability of cells to maintain normal structure and function, leading to sludge disintegration.
III. Reasons for the appearance of a large amount of foam in the aerobic pool
(I) Raw water and process factors
1. Surfactant pollution: If the raw water contains a large amount of surfactant components, such as detergents, industrial additives, etc., a large number of white bubbles will be generated during the aeration process. These bubbles are small, light and non-viscous. This is because the surfactant reduces the surface tension of the water, making it easier for air to form stable bubbles in the water.
2. Influence of aeration equipment: After the aeration head is newly installed, the tiny bubbles generated by the change in aeration method will cause a large amount of foam in the aerobic pool in a short period of time. This situation is usually only a short-term impact and will gradually improve as the system runs.
(II) Microbial metabolic factors
1. Microbial metabolites: Microorganisms will produce a large amount of lipid substances during the reproduction process, or the growth and reproduction activities of the microorganisms themselves will change the surface properties of the sewage, leading to foam. This type of foam is usually mud-colored, with large bubbles and stickiness.
2. Sludge denitrification: When aerobic sludge stays in the secondary sedimentation tank for too long, denitrification will occur. The nitrogen produced during the denitrification process will entrain the sludge particles to form foam, which is sticky and mud-colored.