Aeration tank is the core link of sewage treatment. It provides living conditions for microorganisms through aeration, promotes their decomposition of organic matter and removal of pollutants, and is crucial to achieving sewage purification and discharge standards. As the core of the entire sewage treatment biochemical unit, the aeration tank is also the most difficult part of the entire biochemical unit and even the entire system to regulate. It is very important to maintain the normal state of the aeration tank.
1. Manifestation of scum problem on the surface of the aeration tank
First of all, in the aeration tank, because there is always aeration by the fan, there is generally no sludge floating after anaerobic operation. Even if the structure is set up unreasonably and there is a dead corner of aeration, when there is sludge floating anaerobically, it will be cut and broken by the gas, and it is difficult to form a large area of anaerobic scum. According to the manifestation, scum may include foam, oil film, biofilm fragments, suspended matter, etc. In addition, there may be color changes, such as white, brown or black, and abnormal odor.
The first type is foam scum: usually white, yellowish brown or black foam layer, may be accompanied by sticky feeling or odor;
The second type is oil film scum: generally there is oily reflective film on the water surface, this phenomenon is usually related to the raw water quality and pretreatment effect;
The third type is biofilm fragment scum: usually fine yellowish brown or gray flocculent suspended matter, may be caused by sludge aging or excessive aeration;
The fourth type is chemical scum: generally feels like colloidal floating matter, most of which are caused by excessive coagulant addition or sudden change in water quality.
The fifth type is mixed scum: a thick floating layer formed by a mixture of multiple substances, often accompanied by a decrease in treatment efficiency. There is also scum caused by filamentous bacteria expansion. This type of scum is caused by the poor sedimentation of sludge, and the color is close to that of activated sludge. Observation under a microscope will immediately reveal the large-scale reproduction of filamentous bacteria.
2. Analysis of the causes of the scum problem on the surface of the aeration tank
First, we analyze the direct cause of the scum from a physical perspective. The basic theory is that the specific gravity of the scum is lower than that of the mixed liquid in the aeration tank, so the scum floats to the surface of the liquid. Then we consider why the specific gravity of the scum is lower than that of the mixed liquid. In practice, we observed the actual difference between the scum and the mixed liquid in the aeration tank. The most intuitive feeling is that there are a lot of bubbles mixed in the scum, and these bubbles cause the scum to float. Once again, we consider the problem of bubbles. The aeration tank is originally a place for aeration, and it is inevitable to produce large bubbles. The cause of the generation of small bubbles will be analyzed later. The main problem is that these scums have the characteristics of entrapping these bubbles. From this point of view, it is still a problem of the state of the activated sludge. When the secretion of mucopeptide substances in the activated sludge is excessive, the viscosity of the sludge will increase significantly. In this way, the tiny bubbles will be wrapped by some viscous sludge, and because the specific gravity is too light, they will float on the liquid surface and become scum. Looking back at the causes of small bubbles, in addition to bubbles caused by physical cutting, the remaining tiny bubbles are mainly the release of carbon dioxide and hydrogen by activated sludge when decomposing organic matter. In addition, if the carbon-nitrogen ratio is unbalanced, denitrification problems will also produce tiny bubbles. We then analyze the causes of these tiny bubbles to determine the root cause of the scum problem on the surface of the aeration tank. When analyzing the specific causes, we can classify the causes and consider them from the aspects of microbial factors, operating parameters, influent water quality, and chemical factors.
Microbial factors: If it is yellow-brown fine scum, accompanied by an increase in the sludge settling ratio (SV30). Then we can consider sludge aging, dead microbial debris and metabolites forming scum. There are also filamentous bacteria expansion and microbial metabolites that may cause scum.
Operating parameters: If the scum is black and the particles are large, it is generally caused by local hypoxia leading to sludge decay, producing gases such as hydrogen sulfide, and the sludge floats to form black scum. Aeration shears the sludge flocs, and small black fragments float with bubbles. Sludge age is also a problem that needs to be considered. When the sludge age (SRT) exceeds the endogenous respiration stage of microorganisms, the decomposition of aged sludge produces too much sticky material, which also aggravates the formation of scum. Disturbance of the return ratio will also affect the formation of scum. The more intuitive reason is the change in the quality of the influent water, such as grease, surfactants, and shock loads, which are common reasons. Chemical factors include excessive coagulants or abnormal pH. In addition, environmental factors such as temperature changes may also have an impact.
3. Adjustment methods and response strategies for the scum problem on the liquid surface of the aeration tank
First, try to confirm the precise control of the aeration tank process parameters, maintain DO (dissolved oxygen) at 2-4mg/L (aerobic stage), and control the anoxic stage at 0.2-0.5mg/L. Sludge age (SRT) control target range: 8-15 days. Others strengthen the front-end water inlet monitoring. If there are oils and surfactants, pretreatment should be added to keep the pH within the neutral range. If there are a large number of inert substances entering the biochemical system, the coagulation and sedimentation effect of the physical and chemical section should be adjusted as much as possible to reduce the total amount of inert substances entering the biochemical system. If the liquid surface scum is caused by sludge aging, the key is to adjust the food-micro ratio of the entire activated sludge system, and it is more convenient and controllable to achieve it by reducing the concentration of activated sludge. If the liquid surface scum problem is caused by filamentous bacteria, the main thing is to find the cause of the filamentous bacteria expansion and solve the problem. In addition to the technical level, it is also necessary to increase the adjustment efforts in management, test DO, pH, SV30 (sludge settling ratio), MLSS (mixed liquor suspended solids concentration) daily, conduct sludge microscopy every week, and monitor the abundance of filamentous bacteria (controlled at <10%).