Introduction to Activated Sludge Process–Sewage Treatment

In order to accelerate the dissolution of oxygen, “pure oxygen” aeration began to appear in the 1970s, replacing air with air with extremely high oxygen concentration. Most use surface aeration.


The operation is mainly to control the amount of activated sludge and oxygen supply. The concentration of activated sludge (referred to as mixed liquid suspended solids) in the aeration tank can be adjusted, that is, the amount and load rate of activated sludge can be adjusted. During operation, attention should be paid to adjusting according to specific conditions. Activated sludge sewage treatment plant is prone to sludge swelling, that is, the sludge has a very high moisture content and is not easy to settle. This will cause the sludge to flow out of the sedimentation tank with water and damage the water quality. At the same time, the loss of sludge will reduce the sludge in the aeration tank, and the entire process will gradually fail. When the sludge is found to have a tendency to swell, the cause should be analyzed immediately and measures taken.

In the biological treatment of wastewater, microorganisms are suspended in water. The suspended microbial community is named for its mud-flower state (flocculation). Generally refers to the aerobic activated sludge process.
When activated sludge comes into contact with wastewater in the basic process, it not only absorbs some pollutants as nutrients, but also absorbs and traps other pollutants (such as colored substances and suspended solids), which has a good treatment effect. The basic mechanism is manifested in the biological treatment of water. The basic flow of the activated sludge process (Figure 1) is as follows.

The characteristics of activated water not only meet the basic requirements of national drinking water hygiene standards, but also have some special functions that are beneficial to human health. According to different processing technologies and additional functions, the products currently on the market include mineralized water, magnetized water, electrolytic ion water, natural backwater and so on.


The wastewater in the aeration tank is fully mixed and contacted with the accumulated activated sludge, and the pollutants are transferred to the sludge. The microorganisms use the pollutants as nutrients and obtain oxygen from the water for growth and reproduction (biomass). Some metabolites ( (Such as carbon dioxide and ammonia) are present in water to enter. Then, the mixed liquid flows through the sedimentation tank, the mud and water are separated, the effluent is treated, the sludge flows back to the inlet of the aeration tank, and the process is repeated to enter. Organic matter that cannot be metabolized by microorganisms is more or less left in the water and cannot be removed. Due to lack of contact or time, some metabolizable soluble substances will also be left, and incomplete separation of mud and water will also reduce the quality of the effluent. However, the stabilization of activated sludge process and the efficiency of sewage purification are generally high, and the removal rate of BOD5 (see Biochemical Oxygen Demand) and suspended matter can reach more than 90%. If further filtration is performed using a quick filter, BOD5 and suspended solids in the sewage can be reduced to about 1 to 2 mg / liter. In order to reduce the load of the aeration tank, a primary clarifier is set up before the aeration tank, and the latter is usually called a secondary clarifier.

The main factors affecting the working efficiency (treatment efficiency and economic benefits) of activated sludge process are the choice of treatment method and the design and operation of aeration tank and sedimentation tank.

Method design

Methods In addition to the ordinary activated sludge method, there are a multi-point water inlet method, an adsorption regeneration method, an extended aeration method, and a high-load activated sludge method. The first two methods are different from the basic process, and the amount and location of sewage entering the aeration tank are also different. In the multi-point inflow activated sludge method, only a part of the wastewater and the returned sludge enter the pond together at the head end. The rest of the wastewater is divided into 2-3 times at a distance of 2-3 inlets (the inlet spacing is generally equal) from the head end into the aeration tank. From the point of view of the process, it can be said that the adsorption and regeneration of activated sludge method (Figure 2) is only a deformation of the multi-point water intake process (Figure 3). Only the last of several wastewater inlets was used, the latter becoming the former.





The development of method types is based on a process mechanism. The main substances involved in this process are: organic matter, microorganisms and dissolved oxygen (air). The first two are the main ones. Dissolved oxygen only needs to maintain a certain concentration.
Throughout the process, the oxygen requirements are different. The initial organic matter concentration is high, the microorganisms multiply quickly, and the oxygen demand is large. With the gradual reduction of organic matter, the oxygen demand also gradually decreases. In the ordinary activated sludge process, the oxygen supply of the aeration tank is uniform. This is obviously unreasonable. There are two ways to chat

The ratio of organic matter to microorganisms is called the sludge loading rate (F: M). It affects the metabolic depth of the process and the sedimentation performance of the sludge, as well as the operational stability and infrastructure costs. The lower the sludge loading rate, the easier the process operation, the more stable the treatment efficiency, and the smaller the remaining sludge ratio, but the higher the infrastructure and operating costs. The load rate of ordinary activated sludge process is usually between 0.15 and 0.3 kg BOD5 / kg sludge. The high-load-rate activated sludge method is greater than 1, the amount of returning sludge and air can be greatly reduced, saving costs, but the removal rate of BOD5 is reduced to 60 ~ 70%, so it is also called modified activated sludge method. For situations where only moderate treatment is required. On the other hand, in the extended aeration, the load rate is usually less than 0.1, the aeration time is more than 24 hours, the metabolic depth is large, the remaining sludge is small, and frequent sludge removal is not required. The operation is stable and the management is simple. Occasions with very small flows.

In practice, it has been found that pollutants are transferred to sludge very quickly and at a slower metabolic rate. When treating urban sewage, the BOD5 of wastewater is often reduced by about 90% in less than an hour. However, if these sludges were returned to the aeration tank, this ability could not be reproduced (see aeration), thus creating an adsorption regeneration method. The regeneration of activated sludge essentially gives the microorganisms sufficient time to digest the transferred organic matter. Therefore, it was renamed the Contact Stability Act.

Aeration tank

The aeration tank is the heart of all activated sludge processes, and its role is to stir the mixed solution to make the mud and water fully contact and to supply oxygen to the microorganisms. There are two ways of stirring. One is to make the mud and water mixed into the aeration tank fully mixed and kept until they flow out of the tank without mixing with the mixed liquid phase already in the tank to avoid short circuit. The long shape of the aeration tank is to ensure that the mud and water entering the pool are discharged simultaneously (Figure 4), so that the wastewater entering the pool at the same time has the same aeration time. Another stirring method is to make the mud and water entering the pond immediately and fully mixed with the mixed liquid in the whole pool to achieve a uniform water quality of the mixed liquid, which may make the growth of microorganisms in the best living environment and make the process at the most. Run under good conditions. There is also a long annular aeration tank with a shallow depth, and the mixed liquid flows back at a higher flow rate in the tank. The aeration time of this aeration tank is close to 24 hours, which is specially called oxidation tank or oxidation ditch. In fact, it is a kind of aeration tank of delayed aeration activated sludge method.

In addition to designing the geometry as required, aeration methods and equipment are also important. There are two types of aeration methods: bubble aeration (also known as blast aeration) and surface aeration (also known as mechanical aeration). The deep well aeration introduced in the late 1970s is also a kind of bubble aeration, in order to increase the contact time of bubbles and the mixed liquid to improve the aeration efficiency.
In the surface aeration method, the liquid in the pool is returned by an aerator installed on the liquid surface, and the liquid surface is violently fluctuated to exchange gas in close contact with the air. Aerators are generally various vertical impellers, and there are also horizontal rotary brushes or propellers. The annular aeration tanks all use horizontal aerators.

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