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.
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.