What Is the Activated Sludge Process?
The activated sludge process is the main method of wastewater biological treatment with activated sludge as the main body. The activated sludge process is to continuously pass air into the wastewater; after a certain time, due to the reproduction of aerobic microorganisms, a sludge-like floc is formed. On it lives a microbial community mainly composed of zoogloea, which has a strong ability to adsorb and oxidize organic matter.
activated sludge process A method of sewage biological treatment. This method is to continuously mix and cultivate sewage and various microbial communities under artificial aeration conditions to form activated sludge. Using the biological coagulation, adsorption, and oxidation of activated sludge, the organic pollutants in the sewage are decomposed and removed. Then the sludge is separated from the water, most of the sludge is returned to the aeration tank, and the excess part is discharged from the activated sludge system.
The main factors affecting the efficiency (treatment efficiency and economic benefits) of the activated sludge process are the selection of the treatment method and the design and operation of the aeration tank and sedimentation tank.
1 Aeration tank: the main reaction body
2 Secondary sedimentation tank: 1) perform sludge-water separation to ensure effluent quality; 2) ensure return sludge to maintain the sludge concentration in the aeration tank.
3 Return system: 1) maintain the sludge concentration in the aeration tank; 2) change the return ratio to change the operating conditions of the aeration tank.
4 Excess sludge discharge system: 1) one of the ways to remove organic matter; 2) maintain the stable operation of the system.
5 Oxygen supply system: mainly composed of the oxygen supply aeration fan and special aerator, providing sufficient dissolved oxygen to the aeration tank.
a. BOD load rate (F/M), also known as organic load rate, denoted as NS);
In addition to the ordinary activated sludge process, there are also methods such as multi-point inflow, adsorption-regeneration, extended aeration, and high-load activated sludge. The first two methods differ from the basic flow in the number and location of the inlets where the wastewater flows into the aeration tank. In the multi-point inflow activated sludge process, only part of the wastewater enters the tank together with the return sludge at the head end. The rest of the wastewater enters the aeration tank at 2–3 inlets (the spacing of the inlets is generally equal) at a certain distance from the head end, in 2–3 portions. From the flow point of view, it can be said that the adsorption-regeneration activated sludge process (Figure 2) is only a variant of the multi-point inflow process (Figure 3), with only the last of several wastewater inlets used; the latter then becomes the former.
The development of method types is based on the mechanism of the process. The main substances involved in the process are: organic matter, microorganisms, and dissolved oxygen (air). The first two are the main ones, and the dissolved oxygen only needs to maintain a certain concentration.
Throughout the process, the oxygen demand is different. At the beginning, the organic matter concentration is high, the microorganisms reproduce rapidly, and the oxygen demand is large. As the organic matter gradually decreases, 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 improve it. One is to change uniform aeration to gradually decreasing aeration from the perspective of aeration method. The other is the multi-point inflow method. But the multi-point inflow not only reduces the variation range of oxygen demand, but also changes the relative amount of organic matter and microorganisms.
The ratio of organic matter to microorganisms is called the sludge load rate (F:M). It affects the metabolic depth of the process and the settling performance of the sludge, and also affects the stability of operation and capital costs. A lower sludge load rate makes the process easier to operate, the treatment efficiency more stable, and the excess sludge amount less, but the capital construction and operating costs are generally higher. The load rate of the ordinary activated sludge process is often between 0.15 and 0.3 kg BOD/kg sludge. The high-load activated sludge process uses 1 or more, and the return sludge amount and air amount can be greatly reduced, saving costs, but the BOD removal rate is reduced to 60–70%, so it is also called the modified activated sludge process. It is used only for occasions requiring medium treatment degree. The extended aeration activated sludge process is the opposite, with a load rate often less than 0.1, aeration time exceeding 24 hours, deep metabolism, less excess sludge, no need for frequent sludge discharge, stable operation, and simple management, and is often used in occasions with very small flow.
In practice, people found that the efficiency of pollutant transfer to the sludge is fast, but the metabolic rate is slow. When treating urban sewage, the wastewater BOD is often reduced by about 90% in less than 1 hour. But if these sludges are returned to the aeration tank, they cannot reproduce such ability (see aeration), thus creating the adsorption-regeneration method. The regeneration of activated sludge is essentially to give the microorganisms enough time to digest the transferred organic matter. Therefore, some people renamed it the contact-stabilization method.
Aeration tank is the heart of all activated sludge processes; its function is to stir the mixed liquor to fully contact the sludge and water and supply oxygen to the microorganisms. There are two ways of stirring. One is to make the sludge and water that enter the aeration tank at the same time fully mixed and kept until they flow out of the tank, without mixing with the mixed liquor already in the tank to avoid short-circuit phenomenon. The aeration tank uses a long strip shape to ensure that the sludge and water that enter at the same time also flow out at the same time (Figure 4), so that the wastewater entering at the same time has the same aeration time. The other stirring method is to immediately fully mix the entering sludge and water with the mixed liquor of the whole tank, so as to achieve uniform water quality of the mixed liquor, which may make the growth of microorganisms in the best living environment, so that the process runs under the best conditions. There is also a circular aeration long trough, with a shallower depth, and the mixed liquor flows back at a higher velocity in the trough. The aeration time of this aeration trough is close to 24 hours, and it is specifically called an oxidation trough or oxidation ditch. It is actually a kind of aeration tank of the extended aeration activated sludge process.
In addition to designing the geometric shape as required, the aeration method and equipment are also very important. There are two aeration methods: bubble aeration (also known as blower aeration) and surface aeration (also known as mechanical aeration). The deep-shaft aeration that appeared in the late 1970s is also a kind of bubble aeration, which increases the contact time between bubbles and the mixed liquor to improve aeration efficiency.
In the surface aeration method, the aeration device set at the liquid surface makes the tank liquid flow back and makes the liquid surface fluctuate violently to exchange gas closely with the air. The aerator is generally various vertical impellers, and horizontal rotating brushes or propellers are also used. Circular aeration troughs all use horizontal aerators.
To accelerate the dissolution of oxygen, "pure oxygen" aeration began to appear in the 1970s, replacing ordinary air with air of extremely high oxygen concentration. Most use the surface aeration method.
Operation is mainly the control of the amount of activated sludge and oxygen supply. The activated sludge concentration of the aeration tank (called mixed liquor suspended solids) can be adjusted, that is, the amount of activated sludge and the load rate can be adjusted, and attention should be paid to adjustment according to specific conditions during operation. Activated sludge sewage plants are prone to sludge bulking, that is, the sludge water content is extremely high and difficult to settle. This will cause the sludge to flow out of the sedimentation tank with the water, destroying the water quality, and at the same time, the loss of sludge reduces the sludge in the aeration tank, and the whole process gradually fails. When it is found that the sludge has a tendency to bulge, the cause should be analyzed immediately and measures taken.
1 The wastewater contains sufficient soluble and easily degradable organic matter;
2 The mixed liquor contains sufficient dissolved oxygen;
3 The activated sludge is in a suspended state in the tank;
4 The activated sludge is continuously returned, and the excess sludge is discharged in time, so that the mixed liquor maintains a certain concentration of activated sludge;
The principle of the activated sludge process in an image: microorganisms "eat" the organic matter in the sewage, so the sewage becomes clean water. It is essentially similar to the self-purification process of natural water bodies, but with artificial enhancement, the sewage purification effect is better.
A typical activated sludge process consists of an aeration tank, a sedimentation tank, a sludge return system, and an excess sludge discharge system. The sewage and the returned activated sludge enter the aeration tank together to form a mixed liquor. The compressed air sent from the air compressor station enters the sewage in the form of tiny bubbles through the air diffusion device laid at the bottom of the aeration tank, with the purpose of increasing the dissolved oxygen content in the sewage and keeping the mixed liquor in a state of violent agitation, in a suspended state. Dissolved oxygen, activated sludge, and sewage mix with each other and fully contact, so that the activated sludge reaction can proceed normally.
In the first stage, the organic pollutants in the sewage are adsorbed on the surface of the activated sludge particles by the zoogloea, because of their huge specific surface area
and polysaccharide viscous substances. At the same time, some macromolecular organic matter is decomposed into small-molecule organic matter under the action of bacterial extracellular enzymes. In the second stage, under sufficient oxygen conditions, the microorganisms absorb these organic matters and oxidize and decompose them, forming carbon dioxide and water, part of which is supplied
to their own proliferation and reproduction. As a result of the activated sludge reaction, the organic pollutants in the sewage are degraded and removed, and the activated sludge itself
proliferates and grows, and the sewage is purified and treated.
The mixed liquor after the activated sludge purification enters the secondary sedimentation tank, where the suspended activated sludge and other solid substances in the mixed liquor settle down and separate from the water, and the clarified sewage is discharged from the system as treated water. The sludge concentrated after sedimentation is discharged from the bottom of the sedimentation tank, most of which is returned to the aeration tank as seed sludge to ensure the suspended solids concentration and microbial concentration in the aeration tank; the proliferated microorganisms are discharged from the system, called "excess sludge." In fact, the pollutants are largely transferred from the sewage to these excess sludges.
5 No toxic and harmful substances flow in.
